Strategies for carbon and sulfur tolerant solid oxide fuel cell materials, a: Department of Earth Science & Engineering, Imperial College London

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1 Strategies for carbon and sulfur tolerant solid oxide fuel cell materials, incorporating lessons from heterogeneous catalysis Paul Boldrin a*, Enrique Ruiz-Trejo a, Joshua Mermelstein b, Jose Bermudez Menendez c, Tomas Ramirez Reina d, Nigel P. Brandon a a: Department of Earth Science & Engineering, Imperial College London b: The Boeing Company c: Department of Chemical Engineering, Imperial College London d: Department of Chemical and Process Engineering, University of Surrey * Corresponding author: p.boldrin@imperial.ac.uk Abstract Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low carbon world, providing high efficiency, potential to use carbonaceous fuels and compatibility with carbon capture and storage. However, current state-of-the-art materials have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to deactivation due to carbon deposition when using carbon-containing compounds. In this review we first study the theoretical basis behind carbon and sulfur poisoning, before examining the strategies towards carbon and sulfur tolerance used so far in the SOFC literature. We then study the more extensive relevant heterogeneous catalysis literature for strategies and materials which could be incorporated into carbon and sulfur tolerant fuel cells. Contents 1. Introduction 2. Scope of the review 3. Fundamentals of carbon poisoning 1

2 3.1 Theoretical studies on carbon deposition in catalysts and fuel cell anodes 4. Fundamentals of sulfur poisoning 4.1 Theoretical studies on sulfur poisoning of catalysts and SOFC anodes 5. Systems approaches to carbon and sulfur tolerance 6. Materials design strategies for carbon tolerance in SOFC anodes 6.1 Ni-YSZ cermets 6.2 Alloying with noble metals 6.3 Alloying or replacement of nickel with base metals 6.4 Replacement of nickel with non-metal electronic conductors 6.5 Increasing alkalinity 6.6 Use of ceria and other oxygen storage materials 6.7 Replacement of cermets with mixed ionic-electronic conductors (MIECs) Single phase MIECs Addition of catalytic metal nanoparticles to MIECs 6.8 Regeneration of SOFC anodes deactivated by carbon 7. Materials design strategies for sulfur tolerance in SOFC anodes 7.1 Replacement of YSZ with ceria 7.2 All-ceramic anodes 7.3 Alloying of nickel with other metals 8. Strategies from conventional catalysis 8.1 Carbon tolerance in conventional catalysis Sulfur passivation Alloying and bimetallic systems Promoters 2

3 8.1.4 Regeneration of Catalysts Deactivated by Carbon Deposition 8.2 Strategies against sulfur poisoning Noble metal-based catalysts Alloys, bimetallic and promoters Support and structural modifications Regeneration of sulfur poisoned catalysts 9. Conclusions and Perspectives 9.1 Alloying of nickel 9.2 Alkaline promoters and supports 9.3 Ceria, doped ceria and oxygen storage 9.4 Preferential sulfur binding sites 9.5 Non-metal electronic conductors 9.6 Infiltration of nanoparticles 9.7 Regeneration 9.8 Theoretical and computational studies 9.9 Reflections on experimental work 10. Acknowledgements 11. References 1. Introduction to solid oxide fuel cells Solid oxide fuel cells (SOFCs) are electrochemical devices for the direct conversion of fuels into electricity. Because they operate by the conduction of oxide ions they are capable of using a wide variety of fuels including hydrocarbons, syngas, biogas and ammonia, as well as hydrogen. The oxidation of fuel takes place at the anode, which needs to be active for electrochemical oxidation of the fuel species and possess both electronic and ionic 3

4 conductivity. Typically anodes are made either from ceramic-metallic composites (cermets) where each component provides one aspect of the conductivity, or from a mixed ionicelectronic conductor (MIEC), a ceramic which provides both ionic and electronic conductivity. There are a number of other properties that any materials to be used in SOFC anodes need to possess, including stability towards high temperatures and highly reducing conditions, chemical compatibility with other materials such as electrolytes and interconnect materials, and thermal expansion coefficients matched to the other components during operation and manufacture. The need for these properties places a limitation on which materials can be used, for example there are materials with high ionic conductivity which are not stable in reducing atmospheres, or which have a large thermal expansion mismatch compared to common electrolyte materials. As well as the direct electrochemical oxidation of fuel species, other relevant reactions which take place in an SOFC anode are water-gas shift, steam reforming, dry reforming, Boudouard reaction, methanation and hydrocarbon decomposition and cracking, among others. The development of SOFCs has reached an important phase, with rapid technological advancement over the last decade resulting in multiple programs run by governments and/or companies testing systems greater than 100 kw, and installed commercial products in the low kw range combined heat and power market. An initial understanding of the recent progress of multi-kw-scale SOFC development can be gained by studying the US Department of Energy s SOFC program (Solid State Conversion Alliance, SECA) which is interested in systems of 100 kw and upwards operating on syngas from coal or natural gas. For the period the SOFC targets were for 1500 hour tests on fuel cell stacks with performance degradation targets at steady state of <4%/1000 hours, while the latest 4

5 target is for >25 kw stacks with >4 years lifetime and degradation of <0.2%/1000 hours, with cumulative operation times of between 5,000 and 10,000 hours for this generation of SOFCs by The financial year 2015 funding round supported Fuel Cell Energy and Versa Power to produce a 400 kw system 1. Other large projects include Mitsubishi Heavy Industries demonstrating a 200 kw combined SOFC-gas turbine system operating on syngas at 900 C, with a degradation rate of 0.13%/1000 hours 2, while Bloom Energy, based in California, have a commercially-available SOFC capable of generating kw aimed at the commercial market, especially data centres, with an installed base of over 30 MW 3. Figure 1 shows a diagram of a combined cycle SOFC with integrated gas turbine. Figure 1 Diagram of a combined cycle SOFC system with integrated gas turbine The focus on degradation rates clearly seen above in the large scale and commercial programs is a reflection that one of the key issues facing SOFCs is degradation and its effect on lifetime costs. Performance degradation can be caused by thermal gradients or thermal cycling, oxidation cycling and long-term incompatibility of components. For SOFCs to continue to become successful commercially, they will need to operate on carbonaceous fuels, and be tolerant to common contaminants in those fuels. Two of the most common 5

6 poisons are carbon and sulfur, and current anodes based on composites of nickel and yttrium-doped zirconia (yttria-stabilised zirconia, YSZ) are not tolerant to them, resulting in long term degradation and a need for regeneration, which has additional effects on degradation relating to thermal and/or oxidation cycling. For this reason carbon and sulfur tolerance is a vital area of research for the next generation of SOFCs to compete with conventional power plants at the grid scale, and with boilers and combustion engines at smaller scales. The application of catalysis in fuels processing has been an important research topic for several decades. In catalytic processes involving fuels, carbon (from the fuel itself) and sulfur (present as contaminant) critically affect the performance of the catalyst. Under certain conditions, this effect can be extremely important and the catalyst is deactivated quickly, leading to unpractical and/or costly processes. For these reasons, huge research efforts have focused on the design of catalysts resistant to carbon deposition and sulfur poisoning. As a result of this, a vast knowledge of possible alternatives to address these issues has been generated. This literature could provide insights into improving the carbon and sulfur tolerance in SOFC materials. 2. Scope of the review This review discusses all aspects of carbon and sulfur tolerance in SOFC anodes, from mechanistic and theoretical studies to strategies for materials design. In addition, we have studied the catalysis literature, focussing on fundamental studies and catalysts used in reactions under conditions similar to those in an SOFC anode (e.g. steam reforming and partial oxidation). Since, in the end, poisoning by carbon and sulfur may be inevitable, we have also included sections on regeneration. We have chosen to put these at the end of the relevant materials design section (e.g. regeneration of SOFCs after carbon deposition is at 6

7 the end of the carbon tolerant SOFC section). In the final section, we have summarised the various strategies used in catalysts and SOFCs to provide carbon and sulfur tolerance with lessons learned from each. Certain aspects of this review have been covered in other reviews in the last decade: Ni-based anodes in hydrocarbon fuels 4, sulfur poisoning of Nibased anodes and catalysts 5-6, anode performance in hydrocarbons 7-8, catalyst deactivation and regeneration 9, steam reforming for fuel cells 10, internal reforming in fuel cells 11 and sulfur tolerance in hydrogen production catalysts Fundamentals of carbon poisoning Deposition of carbon-containing species on metal catalysts is one of the main causes of catalyst deactivation and is virtually inevitable in any reaction involving hydrocarbons9, It should be clarified that carbon and coke, although often used interchangeably, refer to different species. Carbon refers to the product of CO disproportionation whereas coke is produced by decomposition or condensation of hydrocarbons 9, However, for the sake of clarity and readability, only the term carbon will be used in this work. In reactions involving carbon-containing fuels, the principal reactions leading to carbon deposition can be summarized as follows 13 : 2 CO (g) C (s) + CO2 (g) CnHm (g) n C (s) + m/2 H2 (g) CO (g) + H2 (g) C (s) + H2O (g) The first reaction is the disproportionation of carbon monoxide and is commonly known as the Boudouard reaction, after its discoverer Octave Leopold Boudouard, a French chemist of the late 19 th and early 20 th century. It is exothermic at all temperatures but due to the reduction in entropy becomes more favourable at lower temperatures. The second 7

8 reaction is the decomposition of hydrocarbons and conversely is endothermic with an increase in entropy, so is favoured at high temperatures. The final reaction is the reverse of the original water-gas reaction used to produce water gas (now known as syngas) from coke using steam. It is distinct from the water-gas shift reaction, which was originally used to reduce (or shift) the carbon monoxide content of the water gas, so that it could be more safely used. It has similar thermodynamics to the Boudouard reaction and so is more favoured at lower temperatures. Carbon deposition is strongly affected by the presence of sulfur and aromatic compounds in the fuel 13, 18. Sulfur deactivation can either promote or reduce carbon deposition depending on the conditions 19, and the ability of sulfur to potentially improve carbon tolerance is discussed later on in section The presence of aromatics in the fuel tends to increase carbon deposition far more than would be expected from their concentration in the fuel. This is likely because carbon deposition is thought to proceed through a mechanism involving the formation of aromatics. Once formed, these aromatics are less reactive than other compounds in the fuel and serve as nucleation sites for the formation of polynuclear carbon compounds 9, 13. The mechanism of carbon formation varies with material (e.g., if it is a metal or metal oxide/sulfide) 9, 16. This is important because the effect of the structure and location of carbon on deactivation can be more relevant than the total quantity of carbon deposited on the catalyst 9, 20. In the case of metals, the rate of carbon deposition is a function of the type of metal, the crystal size, the promoters and the interaction between the metal and the support 9, Formation of solid carbon is favoured thermodynamically in a large proportion of the potential operating space of SOFCs 30. Figure 2 shows the region in which carbon deposition is favoured at different temperatures, showing that all common carbon containing fuels are 8

9 in the carbon deposition region below 1000 C, including CO and CH3OH. This indicates that oxygen-containing species need to be added to make carbon deposition thermodynamically unfavourable. Factors which increase the thermodynamic favourability of carbon deposition include lower temperatures, higher carbon:oxygen ratios and low oxygen fluxes. In addition to this, carbon deposition is strongly influenced both inside and outside this thermodynamic window by kinetic factors, especially the relative rates of the forward and reverse Boudouard and methane decomposition reactions, and the presence of aromatic and polyaromatic compounds. Figure 2 Carbon deposition limit lines in the C-H-O phase diagram. Reproduced by permission of The Electrochemical Society from J. Electrochem. Soc. 150 (7) A885-A888 (2003). Copyright 2003 The Electrochemical Society When carbon deposition takes places on metal particles, several situations can lead to deactivation (Figure 3) 9 : Strong chemisorption as a monolayer or physical adsorption in multilayers blocking access to metal surface sites. Encapsulation of metal particles, deactivating them completely. 9

10 Plugging of micro- and mesopores blocking access to the active sites inside them. Growth of carbon filaments (whisker carbon) that can stress and fracture the support or push the metal particles off the support. In the case of SOFC anodes the growth of this carbon can destroy the structure of the fuel cell. Dissolution of carbon atoms into the metal, causing a volume expansion. This is mainly a problem for SOFC anodes, where the metal may have a structural role and therefore these volume changes can destroy the structure of the anode. By blocking active sites for catalytic and electrocatalytic reactions, carbon can reduce the performance of both catalysts and SOFCs. This type of deactivation can occur even at low levels of carbon deposition, but is generally fully reversible by oxidation of the carbon. Techniques for achieving this are discussed in sections 5.8 (for SOFC anodes) and (for catalysts). Structural deactivation, where carbon deposition causes structural failure, tends to be the most serious problem caused by carbon poisoning in SOFCs. This mode of deactivation is caused by longer term running under conditions favourable to carbon deposition, or when using materials such as nickel which catalyse carbon deposition. In SOFCs, because the metal component can have some structural role, failure can also occur by dissolution of carbon into components of the anode, causing a volume expansion which can result in dusting, where the anode becomes pulverised. This tends to occur when carbon is repeatedly dissolved and removed from the anode materials. Different types of carbon can be formed in these reactions 9, 13. These types of carbon have different reactivities and morphologies, which affect their potential for deactivation. In addition, they can react to be transformed in a different type of carbon, thus varying during the reaction their potential to deactivate the catalyst 9, 13,

11 In the case of metal oxides and sulfides, the formation of carbon is the result of cracking reactions catalysed by acid sites. The rate of carbon deposition depends on the acidity of the catalyst and its porous structure. In this case deactivation can be caused by chemical or physical effects. In the case of chemical deactivation, carbon can strongly adsorb on the acidic sites while physical deactivation is the result of the pore plugging which blocks access to some catalytic sites. Figure 3. Different situations in which carbon deposits can lead to deactivation: a) Carbon layers chemisorbed on metal particles (reprinted from J. Power Sources 2010, 195 (2), , with permission from Elsevier); b) encapsulation of metal particles by carbon deposits (Reprinted with permission from J. Am. Chem. Soc. 2006, 128 (35),

12 Copyright (2006) American Chemical Society); c) growth of carbon nanofilaments that push metal particles off the support (reprinted from Int. J. Hydrogen Energy 2014, 39 (24), , with permission from Elsevier); and d) pore blockage by carbon deposits (reprinted from Chem. Eng. J. 2010, 163 (3), , with permission from Elsevier). An ideal carbon tolerant cell would be able to run on hydrocarbons without any added oxidant and would therefore not require high temperatures, steam generators or other extra modules which are currently used to mitigate carbon poisoning in SOFC-based power generators. Generally, there are two ways for suppressing (or at least minimizing) the rate of carbon deposition: changing process conditions, such as increasing steam to carbon ratio or increasing temperature; or developing carbon resistant materials 9, 13, 15, The rate of deactivation is related to the balance between the rates of formation and gasification/oxidation of the carbon, which are strongly influenced by the reaction conditions and the catalytic activity of the materials towards the different reactions involved 9, 13, 15, 31. In catalysis, the range of variation of the reaction conditions is often quite limited since the conditions need to be designed to optimise the yield of the desired product rather than protect the catalyst. In SOFCs, there is more scope to alter reaction conditions, with compromises made to cost, power and flexibility. Since Ni is such an effective catalyst for hydrocarbon decomposition, use of reforming to convert hydrocarbons into syngas can be effective. This reforming can be done internally or externally. External reforming requires the extra cost of a separate reforming unit, but has the advantage that the reformer has a protective effect on the fuel cell. Internal reforming with steam, CO2 or O2 can be effective due to the high activity of Ni for reforming reactions but certain conditions such as periods 12

13 at open circuit voltage (OCV) and low oxygen:carbon ratios can result in carbon deposition Alternatively, separate reforming layers have been investigated, but these could complicate fabrication and would need to be composed of a carbon tolerant catalyst Both internal and external reforming have problems with the separation of endothermic reforming and exothermic oxidation reactions in external reforming there is a need for heat exchangers while in internal reforming the proximity of exothermic and endothermic reactions causes thermal gradients. Both types of reforming reduce the power output of the cell. Because of how SOFCs operate, increasing the current density also mitigates against carbon deposition (at least in sulfur-free fuels), due to the increased flow of oxygen into the anode side of the cell. This has the advantage of encouraging reforming reactions without reducing the power output of the cell. Because of the protective effect of oxygen flow across the electrolyte, SOFCs can be started up under hydrogen with the carbon-containing fuel being switched on once the cell is already under load, if it is feasible to have a dedicated hydrogen supply for this purpose. Running the fuel cell at high temperature can move the conditions outside the region where carbon deposition is thermodynamically favoured, although this does not guarantee there will be no carbon deposition. The higher temperatures increase the cost of components other than the anode, which need to be designed to withstand higher temperatures, for example above 800 C, the most suitable alloys for interconnects have high levels of chromium, which can cause problems with formation of resistive phases 39 and cathode degradation 40.The higher temperatures may also reduce the overall lifetime of the system. Alternatively, with Ni-YSZ anodes, it has been shown that decreasing the temperature reduces carbon deposition in a cell operating under load in humidified 13

14 methane as it slows the methane cracking reaction more than the electrocatalytic oxidation, although high currents and thus high oxygen fluxes into the anode were still required to eliminate carbon deposition entirely Theoretical studies on carbon deposition in catalysts and fuel cell anodes The formation of carbon deposits in catalytic reactions involving hydrocarbons is the consequence of the dehydrogenation of these hydrocarbons. Methane is the simplest hydrocarbon and therefore provides the simplest model for understanding the fundamentals of carbon deposition. The dissociation of CH4 over metal surfaces occurs in four steps : CH4(g)a *CH3a+ *Ha *CH3a *CH2a + *Ha *CH2a *CHa + *Ha *CHa *Ca + *Ha Considering Ni as the active metal surface, the dissociation of methane can take place on two different kinds of active sites: those associated with the planar surfaces (or terraces) and those associated with stepped and defect sites on the metal surfaces Considering the planar sites, theoretical studies have shown results that can be surprising at a first view, as can be seen in Figure The most stable intermediate in planar surfaces is *CH and the last step of methane dissociation from *CH to produce carbon is an endothermic process with high activation energy (Table 1) These data suggest that carbon deposition should not take place on those Ni surfaces, something that contradicts what has been widely reported experimentally. However, observing the results from the stepped sites, the phenomenon of carbon deposition is easily explained. Stepped surfaces are more reactive than planar, due to electronic and geometrical defects that take place in 14

15 these low-coordinated surface geometries 41-42, As a consequence of this, the production of carbon on stepped surfaces is exothermic and thermodynamically favoured, creating the driving force for the formation of graphitic carbon deposits 42. A similar situation occurs in the case of other metals and alloys, as can be seen in Table 1. In all the cases, the formation of carbon is thermodynamically more feasible on stepped than planar surfaces. Figure 4. Thermodynamic pathway for the dissociation of methane (CH4) on planar (111) and stepped (211) Ni surfaces. Reprinted from J. Catal. 2007, 247 (1), , with permission from Elsevier. The process starts with the activation of the first C H bond in methane. As can be seen in Figure 5, in both cases (planar and stepped surfaces), this takes place over the top of a surface Ni atom. However, in the case of planar surfaces, the energy barrier is higher than in the case of stepped (Table 1). This is due to the higher stability of the adsorbed CH3 on the stepped surface, which gives rise to a stronger bond 44. Similarly, the subsequent steps of the dissociation of methane give rise to species that are more stable on stepped than on planar surfaces. Finally, whereas CH is the most stable species in planar surfaces, C is the most stable species on the stepped, favouring its deposition in these sites

16 Table 1. Activation barriers (Ea) and reaction energies ( E) of the different steps in the dissociation of methane and adsorption energies (Eadsorption) of the different species involved in the process reported on different metal surfaces. Stepped surfaces are shaded, exothermic steps are in bold. All values are in kj/mol. CH4 *CH3+*H *CH3 *CH2+*H *CH2 *CH+*H *CH *C+*H Ea E Ea E Ea E Ea E Ni (1 1 1) a 54 a 75 a 17 a 29 a -29 a 130 a 63 a Ni (1 1 1) Ni (2 1 1) a 42 a 88 a 8 a 42 a -33 a 88 a -29 a Cu (1 1 1) Cu (1 1 1) Cu (2 1 1) Fe (1 1 1) Co (1 1 1) Cu-Ni (1 1 1) Cu-Ni (2 1 1) 38 n.a. 29 n.a. 63 n.a. -54 n.a. 21 Fe-Ni (1 1 1) Co-Ni (1 1 1) Rh (1 1 1) Rh(1 1 0) Rh(1 0 0) a - Approximate values extracted from Figure 2 in 41 16

17 (a) (b) Figure 5. From left to right, initial, transition and final state for the dissociation of methane on: a) Ni (1 1 1); and b) Ni(2 1 1). C, H and Ni atoms are represented by dark grey, black and white colours respectively. Reprinted from Surf. Sci. 2005, 590 (2 3), , with permission from Elsevier. Once carbon has been deposited in the metallic sites, two different processes that lead to carbon deposits formation can take place. Either C-C bonds can be formed and then graphitic planes grow parallel to the planar surfaces of the Ni. Graphene is more stable on planar surfaces because carbon atoms are organized in hexagonal structures that can lie parallel to the Ni atoms 41-42, Alternatively, those isolated atom carbons, once adsorbed, can dissolve into the bulk Ni forming carbides (Figure 6). As a result of this diffusion, carbon atoms can reach facets on the support side of the metal particle. These facets are suitable for the eventual growth of carbon nanotubes 42, 47, 49. (a) (b) 17

18 Figure 6. (a) Transition and (b) product states of the diffusion of one C atom from an fcc hollow site to a sublayer octahedral site of the Ni(111) surface. Green (light) spheres represent Ni atoms and black (dark) spheres represent C atoms. Reprinted with permission from ACS Catalysis 2011, 1 (6), Copyright 2012 American Chemical Society. However, as stated by Abild-Petersen et al. 44, the fact that stepped surfaces are more active than planar ones does not mean that steps control the activity of the catalyst. So, if the steps could be blocked, side reactions like carbon deposition would be eliminated, while only moderately reducing the activity of the catalysts for methane processing. This can explain the effect that the addition of Au to Ni catalysts has on coke deposition. Au preferentially binds to low-coordinated Ni sites (like those present on steps). Consequently, it increases the effective coordination number of adjacent Ni atoms and lowers the Ni surface energy due to electronic interaction with gold Another strategy for decreasing carbon deposition is to increase the reaction rate of C-O bond formation relative to C-C bond formation. C atoms can be removed from the surface of the catalyst by oxidation to form CO and CO Thus, if carbon diffusion and C-C formation rates are decreased and oxidation rate increased, carbon deposition can be avoided 49. Following these ideas, the use of different promoters 41, 51, partial passivation and alloys 41, 43, 49-50, 52 have been proposed. Table 1 shows that in all cases alloys have higher thermodynamic barriers to carbon deposition than the metals which make them up, 41, 45 18

19 meaning they are obvious targets. Two clear examples of this can be the effect of alloying Ni with Rh or Sn The studies by Guo et al. 49 and Nikolla et al. 50 showed that when Ni is alloyed with Rh or Sn both carbon and oxygen diffusion in the metal lattice and the C-C and C-O bond formation are hindered, but to a different extent, as shown in Table 2 and Figure 7. Consequently, the overall carbon deposition rate was diminished. These studies have supported their theoretical studies with experimental findings that point in the same direction as the DFT results. Table 2. Activation barriers (Ea), and reaction energies ( E) of C-C bond and C-O formation over different surfaces of Ni(1 1 1) and Ni-Rh (1 1 1). All energies are shown in ev. C-C formation C-O formation Ea E Ea E Ni(1 1 1) Ni2Rh1(1 1 1) Rh(1 1 1) Figure 7. (a) DFT-calculated potential energy surfaces for C-C bond formation on Ni(1 1 1) and Sn/Ni(1 1 1). Inserts show the lowest energy pathways for the attachment of a C 19

20 atom to a carbon nucleation center (modelled as a chain of carbon atoms) on the two surfaces shown in the insert, (b) C-O bond formation on Ni(1 1 1) and Sn/Ni(1 1 1). Inserts show the lowest energy pathways for the two surfaces shown in the insert. Ni is depicted as large blue (light) atom, Sn as a large purple atom, carbon chain as a chain of small black atoms. Reprinted from Catal. Today 2008, 136 (3 4), , with permission from Elsevier. While the metal particles are regarded as the main sites for carbon deposition, this is also possible on oxide surfaces, for example both CO and CH4 will form carbon on Y2O3, YSZ and ZrO2, with the amount of carbon decreasing in that order 53-54, so clearly there is a mechanism for carbon deposition on oxides which is controlled by the surface chemistry. DFT studies on ceria and doped ceria show that carbon deposition should be extremely unfavourable on a ceria surface as long as there are oxygen ions available to react with the carbon atom, which will desorb as CO or CO2 55. The presence of Ni does not affect the favourability of this process, indicating that the activity of the ceria in cermets should be similar to the activity of pure ceria 56. DFT studies on Ce2O3, show that surface vacancy formation is as energetically unfavourable as on YSZ, indicating a low activity towards oxidation reactions. Combined with experimental measurements showing that the ceria surface was more active in a more reduced state, this indicates that Ce2O3 is not formed at the surface 57. In fact, DFT modelling shows that it is energetically favourable for CeO2 to have two oxygen vacancies, providing the explanation for these results and the high oxidation activity of ceria. A study on BaCeO3 perovskites found that CeO2-terminated surfaces had much stronger interactions with CH4 than BaO-terminated surfaces, although they did not link this to carbon deposition but to methane oxidation 58. Unfortunately the thermodynamically- 20

21 favoured termination under SOFC anodes conditions is BaO, meaning that BaCeO3 should be inactive for methane oxidation. 4. Fundamentals of sulfur poisoning In addition to the tolerance to carbon, tolerance to sulfur is required to make an SOFC or a catalyst flexible to fuels. The interaction of sulfur with anodes in SOFC has been reviewed in the last few years 5, Sulfur, contained in all fuels originated from natural sources (fossil or biogas), can be minimised but will always be present in a wide range of concentrations, for example from 85 to 5000 ppmw for diesel 61. If degradation is unavoidable, at least a certain degree of regeneration must exist in order to guarantee long term operation. Several studies have addressed the influence of sulfur poisoning mainly on Ni/YSZ anodes operating on H2/H2O In recent years, the interest has grown to include carbon fuels and H2S, again mainly on Ni/YSZ The reactivity of sulfur is related to the number of electron pairs available for bonding, therefore, from the chemical point of view, toxicity decreases in the order H2S, SO2 and SO Other compounds of sulfur may exist in the fuels but it is expected that in the majority of conditions occurring in an SOFC anode all sulfur compounds are transformed into H2S 77. Following the notion of chemical reactivity and electrons available for bonding, a non-noble metal with electrons available for bonding will be more affected by sulfur than a ceramic 78. In terms of SOFCs, H2S itself is a fuel that can be oxidised electrochemically and the obvious choice to oxidise the sulfur is the oxygen ion that is being transported through the electrolyte. This happens in the same way that hydrogen is oxidised but it should be noted that three times more electrons are being used per mole in the electrochemical oxidation of H2S. 21

22 H2S + 3O 2- H2O + SO2 + 6e - H2 + O 2- H2O + 2e - Examples of the use of SOFCs with H2S as a fuel have been given in the literature but are rather limited H2S can and has been used as a fuel and it has been shown that SO2 is the product of utilisation in a fuel cell with Pt as the catalyst 85, 87 or a highly conductive and catalytically active thiospinel 86. It is not clear however, if these thiospinels could operate in hydrogen rich fuels, are ionically conductive or are resistant to redox cycling. To facilitate this electrochemical reaction the supply of electrons and oxygen ions must be a continuous process and therefore, as in the case of hydrogen oxidation in a classic Ni/YSZ anode, the triple phase boundary (TPB), i.e. the interface between Ni, YSZ and gas phase, is critical to the performance. Anything that hinders or slows down this supply of oxygen and electrons to the TPB will have a detrimental effect; examples of hindrance are carbon deposition or agglomeration of the Ni phase. Similarly, anything that blocks the reaction sites for hydrogen oxidation or internal reforming will be equally detrimental. In the case of nickel anodes, sulfur poisoning is one of the reasons for the decreased electrochemical activity. In what follows, the scope is more concentrated on the presence of H2S in the fuels as pollutant rather than as fuel. 4.1 Theoretical studies on sulfur poisoning of catalysts and SOFC anodes Considering H2S as the source of sulfur, the depletion of the anode performance under H2S containing gas mixtures at elevated temperatures originates from H2S dissociation leading to the adsorption of atomic sulfur (S*) on the anode surface (i.e. adsorbed on Ni atoms when a model anode Ni/YSZ is considered) The strongly adsorbed S* species block the active sites of the anode surface, decreasing the electrochemical oxidation performance. Experimental studies have shown that sulfur coverage fits a Temkin 22

23 isotherm on nickel surfaces in catalysts, where the enthalpy of adsorption of sulfur varies linearly with coverage 90. In solid oxide fuel cell anodes, performance degradation is proportional to sulfur coverage at constant current density 91. Figure 8 shows the coverage of sulfur on Ni and phase equilibria, highlighting that sulfur coverage is high even at low sulfur concentrations, with a very strong dependence on temperature. Bulk sulfidation of Ni does not occur until much higher sulfur concentrations 92. DFT calculations clearly illustrate the situation 60. Figure 9 pictures a Ni based anode built as infinite slabs with an adequate vacuum space (around 15 Å). Under these circumstances four types of active sites can be imagined including atop, bridge, and threefold fcc- and hcp-hollow sites. As schematically illustrated in Figure 9 (c), the mechanism of S* formation could be described as an interfacial reaction of adsorbed H2S* with the Ni surface via two elementary steps of S H bond cleavages (i.e., H2S* /HS* + H* and HS* / H* + S*). 23

24 Figure 8 Chemisorption equilibria plotted in the chemical potential diagram for the Ni S H system, log[p(h2s)/p(h2)] vs 1/T plot. Dotted and dashed lines for θs = 0.6 and 0.8, respectively, are isocoverage lines calculated from the equation given in literature 91. Reproduced by permission of The Electrochemical Society from J. Electrochem. Soc. 157 (6) B802-B813 (2010). Copyright 2010 The Electrochemical Society The associated energy barriers for the subsequent steps of dissociation and adsorption of H2S are presented in Table 3. For sake of comparison, Table 3 includes analogue calculations for several noble metals. The calculated energies evidence that sulfur adsorption is clearly a favourable process on Ni surfaces with large exothermic reaction energies (ΔE) and low activation energies Ea. Furthermore, these computational results suggest that replacing Ni with noble metals is not a viable solution to mitigate sulfur poisoning since energy-wise H2S dissociation and S* adsorption also take place on noble metal surfaces 60. The adsorption energies summarized in the table also show that H2S * and HS * bind to metallic surfaces weaker than S*. Hence in principle greater sulfur resistance could be achieved by avoiding H2S dissociation on the anode surface, although the latter is difficult to achieve given that a stronger S* adsorption energy involves a redistribution of the electronic density that reduces the energy demand for H-S bond breaking. 24

25 Figure 9 (a) Schematic representation of a slab model with a proper vacuum space for periodic DFT calculations. (b) Four active sites on a (111) plane. (c) Schematic energy profile of gas-phase H2S dissociation on Ni (111) forming atomic S* and H*. * denotes surface species. TS1 and TS2 are the transition states. Extracted from Energy and Environmental Science 2011, 4 (11), with permission of The Royal Society of Chemistry Table 3 Activation barriers (Ea) and reaction energies (ΔE) for the elementary steps in a H2S dissociative adsorption process and adsorption energies (Eads) of sulfur species (S*, HS* and H2S*). All values in ev. metal Ea1 a ΔE1 a Ea2 b ΔE2 b EadsS* EadsHS* EadsH2S* Pt(111) Pd(111) Rh(211) Ni(100) Ni(111) a - Ea1 and ΔE1 correspond to H2S* HS* + H* b - Ea2 and ΔE2 correspond to HS* H* + S* Alternatively to noble metals, alloying Ni with base metals as Cu may result in an improved sulfur tolerance 96. Indeed, DFT calculations evidenced that Cu based anode materials display better tolerance to carbon deposition and sulfur than Ni based anodes 97. Figure 10 shows the evolution of sulfur adsorption energies with the Ni-Cu alloy composition. It seems very clear that the alloying approach increases Ni resistance to sulfur poisoning but the bimetallic system never reaches lower sulfur adsorption energy than monometallic Cu. 25

26 A B Figure 10 a) Supercell models of homogeneous Ni1 xcux as a function of the alloy composition. Ni and Cu are in grey and in brown, respectively B) Comparison of the predicted adsorption energies of atomic sulfur on Ni1 xcux(1 1 1) at PAW GGA DFT ( ) and GGA DFT (ο). Adapted from J. Alloys Compd. 2007, 427 (1-2), with permission of Elsevier The smaller adsorption energy exhibited by the Ni-Cu alloy can be explained in terms of the density of states (DOS) analysis, as detailed in Norskov s d-band theory As shown in Figure 11, the antibonding orbitals in Ni-S are higher energy than the Cu-S antibonding orbitals, meaning that it is easier to excite electrons into the Cu-S antibonding orbital to break the Cu-S bond. This favourable situation has motivated a number of studies focusing on Cu 89, 97, 100,Ni-Cu alloys 96, 101 and other alloys such as Ni-Sn targeting weaker sulfur interaction with the fuel cell anode. Nevertheless, the main problem of these alternative materials is their poor catalytic activity for the hydrogen oxidation reaction (HOR). Simultaneously improving activity for the HOR and reducing poisoning by H2S is difficult as both are related to the affinity of the material towards hydrogen-containing species 104. In other words, the alloys can effectively enhance the tolerance towards sulfur poisoning but cell performance is sacrificed in turn. 26

27 Figure 11 (a) DOS analysis of S* on Ni(111) and Cu(111) in red and blue curves, respectively. A circle represents the antibonding states around the Fermi level. (b) A scheme of the energies of bonding and antibonding states corresponded to those of metal d bands. Adapted from Energy and Environmental Science 2011, 4 (11), with permission of The Royal Society of Chemistry According to thermodynamics, sulfur poisoning of traditional Ni based anodes is largely unavoidable under a wide range of conditions at very low concentrations of H2S (e.g. below 0.1 ppm H2S at 800 C and below 10 ppm at 1000 C under dry hydrogen) However, further DFT calculations 106 have demonstrated that Ni anodes could be regenerated through a two-step treatment: (1) addition of H2 to reduce sulfur coverage; (2) oxidation with oxygen realising S as SO2. Galea et al. described the sulfur removal pathways via oxidation 106. They described a two-step mechanism. In the first step sulfur concentration is reduced from 0.5 to 0.25 monolayers and in the final stage surface cleaning from 0.25 monolayers of sulfur to complete sulfur removal is achieved (Figure 12). 27

28 A B Figure 12. Energy profiles of the regeneration process via sulfur oxidation. A) Gibbs free energy (ΔG at 800 C, black line) and enthalpy (ΔH, red line) profile illustrating relative thermodynamic energy and kinetic pathways of O2 adsorption and SO2 desorption on S Ni(111) surface with initial coverage θs = 0.50 ML B) Gibbs free energy (ΔG at 800 C, black line) and enthalpy (ΔH, blue line) profile illustrating relative thermodynamic energy and kinetic pathway of O2 adsorption and SO2 desorption on S Ni(111) surface with initial coverage θs = 0.25 ML. Adapted from J. Catal. 2009, 263 (2), with permission of Elsevier. Although this oxidative treatment is effective it has an associated drawback which is a high likelihood for Ni oxidation. Therefore, ideally this approach could be improved if the oxygen is supplied by oxygen ion flux through the electrolyte and interacts selectively with sulfur. In response to this problem, YSZ could be total or partially substituted by other ceramic phases with higher oxygen conductivity as CGO showing greater sulfur tolerance 60. The presence of a highly oxygen conductive phase in the anode permits a certain degree of electrochemical oxidation of S* to SO2 facilitating sulfur removal. This strategy of using mixed oxides with high oxygen mobility seems to mitigate (but not fully eliminate) sulfur poisoning in both SOFCs and catalytic processes. 5. Systems approaches to carbon and sulfur tolerance 28

29 As discussed above, carbon and sulfur represent a technical challenge for SOFC technology. Although this review focuses on anode materials design strategies, currently the main method for mitigating carbon and sulfur poisoning is processing of the fuel externally to the SOFC stack, so it is worthwhile briefly reviewing these aspects of SOFC-based power generation systems which are intended to achieve carbon and sulfur tolerance. Haldor Topsøe have been involved in gas cleaning for SOFCs for many years, and John Bøgild Hansen has very helpfully reviewed the company s experience in this area 107. The main strategy used in relatively clean fuel (e.g. consumer grade natural gas, LPG etc.) is fuel reforming. This converts most of the hydrocarbons to H2 and CO. The reformer can be provided with oxidising gas as fresh steam or as recycled anode gas. CO2 can also be used in so-called dry reforming. The use of reformers has been demonstrated practically in a number of systems. Reformers can add significantly to the cost of the system, with the cost reported as being similar to the fuel cell module itself 108. For dirtier fuels, such as gasified biomass or coal, or biogas, fuel processing becomes more complicated and hence expensive 109. The feedstock may contain up to several percent of sulfur compounds as well as other contaminants such as alkali metals, halides and phosphorus compounds. For solid fuels, the gasification process which converts the feedstock into a gaseous form suitable for fuel cells can produce significant amounts of aromatic compounds, including smaller molecules such as toluene, and larger polyaromatic compounds which can cause carbon deposition in SOFCs. All of the feedstocks mentioned above contain methane and/or short chain hydrocarbons, which again can cause carbon deposition. These feedstocks need several layers of treatment, from desulfurisation to particulate filtering, although most of this is not exclusive to SOFCs, so may not impact on the economics of the process compared to competing technologies. 29

30 For SOFC-based systems using these fuels, the level of desulfurisation required is crucial to the cost and complexity of the system. For example, to reach levels below 10 ppm, deep desulfurisation is needed, which is normally carried out at 40 bar of pressure or higher , necessitating gas compression and increasing safety issues. More recent work has reduced sulfur below 1 ppm at 10 bar, but this pressure is still too high 112. A final intermediate case is provided by liquid transport fuels, which may be an important market for SOFCs in future. These have largely been cleaned of contaminants such as tars and metallic impurities, but may still contain varying levels of sulfur. In general the level of sulfur in these fuels is being driven downwards due to legislation. Ultra-low sulfur diesel (ULSD) standards are normally around ppm, although the actual content of sulfur may be as low as 2 3 ppm. For aviation fuels, the sulfur levels are up to 3000 ppm, with an average of around 600 ppm. As discussed above, some of these levels of sulfur may be too high for Ni/YSZ anodes, although there are examples of SOFC stacks being run on reformed ULSD without desulfurisation. A Topsøe SOFC stack was run on steam reformed ULSD (<10 ppm S) for 1200 hours 113. After an initial 150 h period of rapid degradation there was only 0.2%/1000 h voltage degradation over the rest of the test. Delphi tested a 5-cell stack with simulated reformate containing 2.5 ppm sulfur, and also found a rapid initial degradation followed by stable performance 114, indicating that SOFCs may be able to operate stably with ULSD reformate without desulfurisation, albeit with a performance drop caused by sulfur poisoning. If SOFCs can tolerate ULSD-levels of sulfur, they should be economically attractive for truck auxiliary power units (APUs) 115, and higher sulfur tolerance would allow them to be used in aircraft APUs). 30

31 Crucially, both the examples above had no hydrocarbons in the reformate. Even low levels of compounds such as ethylene are capable of causing carbon deposition on Ni, even in thermodynamic regimes which do not encourage carbon formation 116. In the first example above, a secondary reformer was used to remove the low levels of hydrocarbons produced by the first reformer 117, while the second example, in common with most studies, used a simulated reformate without these problematic molecules. From this summary, several points relating to fuel processing become clear. The first is that since producing a suitable feed gas for an SOFC from almost any starting material is technically feasible, then the driver for carbon and sulfur tolerance in the anode itself is almost entirely economic. The costs of reforming and desulfurisation are each of a similar order of magnitude to the cost of the fuel cell itself, and become more important for lower power and/or more portable systems. That being the case, it becomes clear that the key targets for carbon and sulfur tolerance in SOFC anodes are related to either eliminating or reducing the specifications for the reforming and desulfurisation units. So for carbon tolerance, some important targets could be: to be able to operate directly on methane, propane, ethanol or biogas (methane-carbon dioxide mixtures), preferably without steam generation or off-gas recycling; or to be able to tolerate low levels of species such as ethene and tars. Meanwhile, for sulfur tolerance, important targets are tolerance to the low sulfur levels in ULSD or natural gas (<10 ppm), then for fuels with higher levels of sulfur, tolerance to the levels of sulfur after hydrodesulfurisation catalysis (i.e. without deep desulfurisation at high pressure, or ZnO or other sorbents), then finally tolerance to the levels in those fuels themselves. 6. Materials design strategies for carbon tolerance in SOFC anodes 31

32 6.1 Ni-YSZ cermets Cermet-based anodes are the most widely used anodes in SOFCs. Traditionally they have the advantage that the best oxide ion conductors can be used, while the metal can provide the catalytic activity and the electronic conductivity. The industry standard material is yttria-stabilised zirconia (YSZ), so-called because the addition of yttrium ions to the zirconia stabilises the cubic form of the material under a wide range of temperatures. The presence of the Y 3+ ions also creates oxygen vacancies, which allows oxygen ion transport, with the maximum conductivity being with 8 mol% of yttria added (Known as 8YSZ, (ZrO2)0.92(Y2O3)0.08). YSZ is very stable towards high temperatures and a wide range of oxidising conditions. It is also the most widely used electrolyte, having extremely low electronic conductivity, meaning that issues of compatibility between the anode and electrolyte are eliminated by using YSZ in both the anode and electrolyte. The industry standard metal is nickel. Nickel is relatively cheap and highly active towards various reactions involving carbon, as well as being active towards electrochemical oxidation. It is also more stable than other base metals towards high temperatures, and unreactive towards common electrolytes such as YSZ. Both components of Ni/YSZ have problems relating to carbon and sulfur tolerance. Nickel easily dissolves both carbon and sulfur, leading to volume expansions which can cause structural failure of the anode. Nickel is also an extremely good catalyst for solid carbon formation, meaning that carbon filaments can be formed, potentially destroying the structure of the anode and blocking gas diffusion pathways as discussed in section 3. As well as causing failure of the cell, this propensity towards carbon formation also renders nickel a poor catalyst for direct oxidation of hydrocarbons, meaning that high quantities of steam need to be used for cells running on methane or higher hydrocarbons. It also makes nickel 32

33 susceptible to poisoning by aromatic or polyaromatic compounds which may be present in gasified coal or biomass The problems of YSZ relate to its inertness and consequent inability to mitigate any of the failings of nickel. It has little activity towards electrochemical oxidation or any of the other important catalytic reactions and possesses extremely low electronic conductivity, meaning that once the nickel has deactivated the cell is useless. It also has no oxygen storage capacity and no ability to absorb sulfur, either of which could help improve carbon or sulfur tolerance. Strategies to mitigate the issues with Ni and YSZ are described schematically in figure 13. Figure 13. Schematics of the most common materials strategies to improve carbon tolerance. The diagram shows a strategy and does not imply a specific mechanism. As shown above, the propensity of Ni/YSZ anodes towards carbon deposition is largely a function of nickel s ability to catalyse carbon formation. Thus it is natural to look at partially or entirely replacing the nickel. Since nickel is an exceptional electrocatalyst, many efforts to replace this have focused on substituting some other potentially active material for some of the nickel rather than replacing the nickel entirely. The rationale behind this is two-fold: firstly, heteroatoms could break up large continuous nickel surfaces which are predisposed towards carbon deposition; and secondly, to enhance the rates of reactions which compete with carbon deposition, such as carbon oxidation and steam reforming. 33

34 6.2 Alloying with noble metals The so-called noble metals (roughly the second and third row transition metals in groups 8 11) may offer enhanced catalysis as well as reducing carbon deposition, and are known from conventional catalysis to be active in very small quantities. The earliest example was gold, which causes a reduction in carbon deposition under oxygen-methane mixtures, at the expense of methane reforming activity. Carbon deposition was reduced by up to 8000 times with one-fifth of the nickel replaced with gold SOFCs using Au doping have been tested in dry and humidified methane atmospheres, where they showed no carbon deposition after 200 hours 127. A stabilisation of CHx surface species leading to a reduction in the rate of graphite formation was found to be responsible 128. Impregnation of Pd into Ni/YSZ anodes showed a marked decrease in polarisation resistance in hydrogen, methane and ethanol, with suppression but not elimination of carbon deposition under the carbonaceous fuels 129. The same was found for impregnation of Pd into Ni on Ce0.9Gd0.1O1.95 (CGO) 130. Carbon deposition was primarily found to occur in Pd-poor regions. Likewise impregnation of Ru, also into Ni/CGO anodes, was found to improve stability under methane, ethane and propane under load and short periods at open circuit voltage (OCV), with the caveat that a µm CGO electrolyte was used CGO possesses a relatively high electronic conduction under reducing conditions, meaning that there would be a significant oxygen flux even at OCV 131. Carbon deposition was not seen, as measured from carbon balance analysis. This study also noted one of the problems with the use of Ru, which is its tendency towards vaporisation during synthesis. A comparative study looking at Ru, Pt, Pd and Rh on Ni/YSZ found that Ru, Pt and Pd suppressed carbon deposition under dry methane compared to the unpromoted material, while Rh actually increased carbon deposition 132. In addition, Ru and Pt improved the power 34

35 density in fuel cell tests. Rh has however been shown in other tests to reduce carbon deposition on Ni-CGO in microreactors and give more stable performance in button cells in humidified butane, although the butane used in this paper contained sulfur compounds, so the improved performance may be due to improved sulfur tolerance 133. A further difference could be due to the high activity of Rh-ceria for water-gas shift compared to Rh on other supports 134. Silver has been shown to be a good catalyst for CO oxidation, with no propensity towards carbon deposition 135. Co-doping of Ni/YSZ with Ag and Cu was found to reduce carbon deposition by a factor of three or four relative to samples doped with Cu or Co, with the carbon deposited being more amorphous 136. Silver can also be deposited electrolessly, and this appears to reduce carbon deposition in dry methane and ethane 137. Cells produced in this way were stable over a period of 100 h in dry methane 138. Noble metals are also used in so-called catalyst or barrier layers in anodes where a layer is placed between the active anode and the gas supply. This serves to reduce the hydrocarbon content in the anode by blocking hydrocarbons from entering or water and CO2 from leaving. If reforming catalysts are used they can also increase the reforming rate. This is at the cost of power density, due to the increased resistance to diffusion to the electrochemically-active layer. In the original work showing this effect, Ru supported on CeO2 was used in a catalytic reforming layer over a Ni-YSZ anode which showed good stability in iso-octane-air-co2 and propane-air mixtures 139. Ir-CGO has also been used successfully 140, but more recent work has shown that barrier layers made from materials which show less reforming activity such as Ni-Cu on Zr-doped ceria 141, Ni-doped ceria 142, La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM)-CeO2 143, partially-stabilised zirconia and zirconium-doped ceria 144 and even Ni/Al2O3 145 can also give low or no carbon deposition in the Ni-based 35

36 anodes underneath, indicating that the main effect may be the barrier layer effect rather than the reforming activity. A further drawback to practical use of barrier layers is their nonconductivity, which may hinder current collection. This could be combatted by incorporation of reforming catalysts into a mainly metallic composite 146. Nevertheless, further work on barrier layers may be informed by section 7 which discusses sulfur and carbon tolerant catalyst materials. 6.3 Alloying or replacement of nickel with base metals A similar rationale is behind the use of top row transition metals Co, Cu and Fe, which also act to break up the continuous nickel sheets. In comparative studies with Ni/YSZbased anodes, all of these elements, while reducing the carbon deposition, also reduce the electrocatalytic activity compared to pure Ni/YSZ 147. Despite this, the benefits of reduced carbon deposition may outweigh the reduced performance, so these systems have been extensively studied, including using different fabrication techniques such as impregnation, microwave irradiation 148, and electroless deposition 149. Impregnation was used to produce a series of Ni-Cu alloys on a porous YSZ substrate which was also impregnated with CeO While no carbon was detected on the pure Cu anode, anodes with a Cu-Ni ratio of 9:1 and lower displayed significant weight gain due to carbon deposition, although the deposition seemed to be self-limiting at 4:1 and higher, and the cell structure was not destroyed. Interestingly, a higher reduction temperature resulted in lower carbon deposition, and it was suggested that this is caused by copper enrichment at the surface of the alloy. Cell tests on the 4:1 Cu:Ni anode showed a large increase in performance caused by carbon deposition improving electronic percolation. A catalytic study of Ni-Cu/YSZ+CeO2 with the Ni, Cu and CeO2 impregnated into the YSZ also found significant carbon deposition in the 50:50 Ni:Cu sample after exposure to a 2:1 CH4:O2 mixture for 20 h at 800 C 151. The amount 36

37 of carbon was also not reduced by addition of Pd to the composite. A sample with a 25:75 Ni:Cu mixture in contrast showed no carbon deposition. Electroless deposition 149 produced an inhomogeneous distribution of copper, leading to carbon deposition in copper-poor areas. When microwave irradiation was used to deposit copper nanoparticles on a Ni-YSZ anode, the effect was similar to cells produced using impregnation of a Ni-Cu solution, indicating that alloying during synthesis may not be necessary to reduce carbon deposition 148. Tests using copper alone have shown very low activities compared to nickel, with Cu/YSZ anodes showing very low OCV with a dry methane fuel, indicating that it has little activity towards methane oxidation 147, This highlights the importance of the ceria used in a number of the above studies, which will be discussed later. Iron has also been tested. In a series of studies it was found that iron could reduce carbon deposition in quantities as low as 10%, in both Ni-Fe/La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) 154 and Ni-Fe/CGO 155 anodes. One study compared Ni and Ni0.9Fe0.1 as supports in metalsupported cells under humidified methane at 650 C 156. They found that while carbon was deposited in both supports, the carbon in the Ni-Fe support was amorphous, did not retard the rate of the methane reforming reaction in the support, and prevented carbon deposition in the Ni-CGO anode layer (from SEM). In contrast, carbon on the Ni support was highly graphitic, completely deactivated the reforming reaction and led to cell failure due to carbon deposition in the anode in less than 10 h. Cobalt, similarly to nickel, has known catalytic activity towards carbon-containing compounds, so has been investigated in anodes. It seems promising for electro-oxidation of CO, with alloys with Cu producing higher performance in syngas than an equivalent Ni or Cu only cell 157 and Ni alloys with Co producing higher exchange current densities in syngas than 37

38 in hydrogen 158. Cobalt is expected to have less tendency to carbon deposition than nickel, but in tests where nickel is entirely replaced with cobalt in a YSZ cell, carbon deposition was still observed after 15 hours in dry methane. No performance loss was observed however, indicating that the carbon is not poisoning the activity of the Co/YSZ cell, although it could still eventually cause structural failure 159. Under syngas, cells based on Ni-Co alloys became completely delaminated in CO:H2 ratios above 60:40, indicating that Ni-Co alloys are still vulnerable to carbon deposition 158. In a catalytic study, Co-Cu/YSZ+CeO2 with a 50:50 mix of Co and Cu produced by impregnation into YSZ showed very little carbon deposition after exposure to a 2:1 CH4:O2 mixture for 20 h at 800 C, much lower than a comparative Ni-Cu sample 151. A similar study conducted with dry butane found that the amount of carbon deposition increased with increasing metal loading, indicating that the metal is still encouraging the formation of carbon, despite the lack of nickel 160. This carbon was amorphous, and did not cause any short term degradation of the anode performance, although metal particles were seen encapsulated in the carbon fibres formed, indicating that the carbon deposition would cause long term disruption of the anode structure. Tin is another metal which has been used to reduce the tendency of nickel to form carbon. Tin has the advantage that it alloys easily with nickel, and the tin segregates to the surface of the particles, meaning that a large improvement in the stability in dry methane and isooctane while under load can be achieved with only 1% of tin with respect to nickel 161. The effect of 1% tin in reducing carbon deposition was also seen in ethanol-fuelled SOFCs 162. There has been some debate about the role and effect of tin. One study replicated some of the testing conditions in reference 161 as well as other conditions with dry and wet methane at different temperatures, but failed to observe improvements in carbon tolerance 38

39 under most conditions 163. They ascribed this to their use of electrolyte-supported cells (compared to anode-supported cells in reference 161 ). Further work by the group suggested that the tin appears to cause the formation of less stable carbon species, meaning that any carbon deposited in the electrochemical region is oxidised, but that carbon can still form outside of this region 164. TPO experiments agree that the stability is due to a reduced rate of graphitic carbon formation rather than total elimination of carbon formation, and that keeping the cell under load is still necessary 165. Another paper suggests that hydroxyl groups formed at the tin atoms on the surface are responsible for the effect (figure 14) 166, while a study using DFT and microreactor tests showed that the effect is due to an increase in formation energy of carbon nucleation sites with no increase in energy for CO formation 167. One further study failed to show any improvement when using 1% tin, with increased carbon deposition on Ni-CGO in microreactor tests on humidified butane, which they ascribed to a low operating temperature of 600 C 133. Figure 14 - Polarization curves and power densities of (a) Ni CGO and (b) Sn/Ni CGO anode-supported single cell SOFCs operating at 650 C with H2 and CH4, and (c) their voltage variations measured at 650 C in CH4 as a function of time. Reproduced from Reference 166 with permission of The Royal Society of Chemistry. 6.4 Replacement of nickel with non-metal electronic conductors 39

40 It is also possible to use electronically-conducting non-metals, and these should have intrinsically less tendency towards carbon deposition. They can also have the advantage that nanostructured catalysts, especially precious metal catalysts, can be used without the loss of activity or function caused by alloying with base metals like copper. The carbon deposited due to hydrocarbon cracking is conductive, and one study exploited this. Porous YSZ scaffold was exposed to dry butane, depositing a conductive carbonaceous layer. This was then impregnated with ceria and/or noble metals to improve the catalysis. Pd showed the best activity out of Pd, Pt or Rh in these cells 168 (Figure 15). The cells performances in butane showed a much smaller improvement through adding a catalytic metal, which was suggested to be due to saturation of carbon on the active metal surfaces 169. Figure 15. Potentials (open symbols) and power densities (closed symbols) as a function of current density at 973 K for H2 (diamonds), n-butane (triangles), and CH4 (circles). In (A), the cell had a C-ceria-YSZ anode; in (B), the anode also contained 1 wt% Pd. Reproduced by permission of The Electrochemical Society from Electrochem. Solid-State Lett. 2003, 6 (11), A240-A243. Copyright 2003, The Electrochemical Society. A longer term test of the Pt/C-CeO2-YSZ cell in dry methane showed a 15% drop in performance over 100 h. The impedance spectra showed an increase in the Ohmic resistance, so the loss in performance was attributed to a loss in carbon. An earlier paper by the same group had shown that for a Cu/C-CeO2-YSZ cell, the OCV in C4H10 settles over time 40

41 to a value of 0.85 V, implying that an equilibrium is reached between partial oxidation products 170. The authors also observed gradual changes in the performance under load, implying changes in the carbonaceous layer over time. These results taken together suggest that the carbonaceous layer will reach an equilibrium over time depending on the fuel, oxygen flux, presence of catalytic metals and other factors. A combined thermodynamic and experimental investigation looked at the stability of a range of electronically-conducting carbides, borides, nitrides and silicides in humidified hydrogen with a partial pressure of CO of either 10-1 or 10-6 at 950 C 64. Of these, only the tungsten carbides and molybdenum carbides were stable, and then only at the higher concentration of CO. Since carbides should have an intrinsic carbon tolerance, as well as having been investigated in catalysis for various reforming reactions, this marks them out as potential anode materials. Despite this, tungsten carbide has only recently been investigated in actual anodes, in a WC-YSZ anode 171. The performance with pure WC-YSZ was poor, but could be improved several times by impregnation of a Ru-CeO2 catalyst. The cell was stable under dry methane, with low carbon deposition which was not detrimental to the performance, but careful balancing of the fuel utilisation is required to prevent oxidation of the WC. A follow-up study tested fuel cells in humidified methane and methane-hydrogen mixtures, with maximum power densities of ~80 and ~250 mw/cm 2 respectively at 900 C with a 300 µm YSZ electrolyte 172. In a further study, the Ru was replaced with Ni, and this cell showed stable performance over a week under humidified methane at 850 C with no evidence of carbon deposition from SEM 173. Removal of the ceria reduced the performance, but the high stability remained, indicated that it is the tungsten carbide which is preventing carbon deposition on the nickel particles. 41

42 Molybdenum carbide has been tested in a proton-conducting cell with a BaCe0.7Zr0.1Y0.2O3 δ (BCZY) electrolyte operating on ethane 174. The carbide was stable and showed very low levels of carbon deposition both in the cell and when exposed to ethane as a powder as assessed by thermogravimetry and XPS. Under hydrogen at 0.55 V there was degradation of 5% over 100 h which was attributed to reduction of the carbide to metallic molybdenum. One study looking at molybdenum as a dopant in a Ni-YSZ anode observed extremely good performance under humidified methane in steam reforming activity, low carbon deposition and fuel cell tests, especially in materials which were reduced in the humidified methane rather than in hydrogen. This was suggested to be due to the formation of highly active molybdenum carbide, but no further investigations were conducted to test this hypothesis 125. The ability of tungsten and molybdenum to form carbides means they may be able to Tungsten and molybdenum have also simply been used as promoters in a similar way as the other base metals described above. In a combined mass spectrometrythermogravimetric study, Mo-Cu, W-Cu and Cu doped Ni-YSZ were exposed to dry methane at 800, 650 and 500 C. The samples were produced so as to retain Mo and W in their metallic state. The Mo and W doped samples showed improved tolerance to carbon deposition, which may have been due to the formation of carbides Increasing alkalinity A third strategy involves increasing the basicity of the material, especially by using an alkaline earth. This strategy is known to reduce carbon deposition in conventional catalysis. All of the alkaline earths have been tested except Be, which can be highly toxic. They have strong basicity, and this modifies the electronic state of nearby nickel to make it less active 42

43 for carbon deposition 176. In the particular case of MgO, (Ni,Mg)O solid solutions are formed, from which the nickel can be reversibly exsolved 177. This may help when regeneration is required. Microreactor tests using Ni/YSZ cermets doped with small amounts of MgO, CaO and SrO (0.2, 1 and 2% of anode mass) showed that CaO and SrO suppressed carbon deposition even at the lowest loadings, while MgO increased the rate of carbon deposition 176. A separate study looking at the same elements plus BaO and La2O3 made similar findings, but also noted large changes in microstructure and conductivity depending on which promoters were used, highlighting the need to take into account all the effects of promoters 178. They found that CaO-promoted cells had the highest performance in humidified methane, due to a combination of good steam reforming activity, high conductivity and low carbon deposition. The alkaline earths can also dissociatively uptake water, which is then able to oxidise nearby carbon. First principles studies indicate that BaO is the best alkaline earth for water adsorption 58 and its effect is shown by a study on the effect of Ba on Ni/YSZ anodes fuelled with dry propane 179. Using thermogravimetic analysis (TGA) and Raman spectroscopy they observed water incorporation (weight gain and O-H stretching modes) in the anode. This water uptake may assist the oxidation of carbon on the Ni particle and was further evidenced using DFT calculations 179. This type of carbon elimination process occurs preferentially at the BaO/Ni interfaces. The catalyst works synergistically: the water splitting takes place on barium oxide, the carbon deposition occurs on Ni sites of BaO/Ni and the subsequent steps occur near the BaO/Ni interfaces. Figure 16 summarizes the proposed mechanism for carbon mitigation in a BaO/Ni-YSZ composite anode of a fuel cell fed with propane. A combined 43

44 microreactor and fuel cell study on Ni-Cu/CGO anodes doped or not doped with Ba showed that the Ba does not reduce the rate of carbon deposition in microreactor tests in dry methane, but does reduce the rate of carbon deposition in fuel cell tests under load 180. It was suggested that Ba assists in oxygen transfer from the electrolyte to the metal surface, although other explanations were not ruled out. Impregnating Ni-CGO with BaO also greatly reduces carbon deposition in humidified CO 181. Because of the mechanisms of carbon suppression of the alkaline earths, the nanostructure of the anode plays a large role in the results for these elements as the oxide must be very near to the nickel without completely covering it. Incorporation of CaO by solid state methods was found to decrease the performance of the cell 176, while doping with Ba by impregnation was found to eliminate carbon deposition while only lowering power density by around 10% 182. Experiments using vapour deposition of Ba on Ni/YSZ showed remarkable stability in dry C3H8 with a sustained power output of 0.4 W/cm² over 100 hours compared to complete deactivation after less than 1 hour for a cell without Ba 179. Figure 16 Proposed mechanism for water-mediated carbon removal on the anode with BaO/Ni interfaces. Large balls in bright blue, green, red, blue-grey and purple are Ni, Ba, O of BaO or YSZ, Zr and Y, respectively, whereas small balls in red, white and grey are O 44

45 from H2O, H and C, respectively. D1 is the dissociative adsorption of H2O, whereas D2 is the dehydrogenation of hydrocarbons or the CO disproportionation reaction. TPB is the triple phase boundary. Reprinted by permission from Macmillan Publishers Ltd: Nat. Commun. 2, 357, copyright 2011 An expansion of this technique has been to use Ba-containing proton conductors, which have the dual ability to store water and provide some ionic and electronic conductivity. Impregnation of yttrium-doped barium zirconate (BYZ) 183 reduced carbon deposition at the same time as improving the performance, but only if the BYZ was present in the electrochemically active zone. This indicates that the improvement may be due to the increased electrochemical oxidation activity. A DFT study on Ni on yttrium-doped barium cerate (BYC) or YSZ indicated that the termination of the surface of the oxide is important BaO-terminated surfaces adsorb water much more strongly than ZrO2 or CeO2-terminated surfaces, and are thus more able to oxidise carbon at the triple phase boundary 58. The water storage capacity of one material Ni- BaZr0.4Ce0.4Y0.2O3 (BZCY) was actually measured, and found to be four to five times higher than a selection of other anode and catalyst materials 184. These materials included Ni-BaZrO3, indicating that the water storage capacity is not solely due to the barium, but may also have some contribution from the proton or electron conductivity, the other elements present or, as mentioned above, differences in the preferred surface termination. The Ni-BZCY showed much lower levels of carbon deposited in microreactor tests at all temperatures and ethanol-steam mixtures, and anodes based on Ni-BZCY were stable under ethanol-steam for 180 h at 750 C, in contrast to Ni-SDC and Ni-YSZ anodes which failed after less than 2 h. BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) is a MIEC with proton conductivity, but on its own it has poor performance due to low electronic conductivity 185. When impregnated with metals 45

46 the performance improves markedly, and with Ni-Cu impregnated there is no carbon deposition as measured by Raman under dry methane at OCV at 750 C, while Niimpregnated cells showed no carbon deposition under humidified methane at OCV at 750 C. Ni-BZYYb composites have also been used with ethanol as the fuel, in this case there was significant carbon deposition but it was limited to the outside of the anode and was amorphous in nature, indicating that these materials may also reduce the amount of graphitic carbon when carbon deposition does occur 186. Lastly, an in situ Raman study on BaO and barium zirconates showed that, as well as the water adsorption effect on carbon deposition, there was also a reverse Boudouard effect in the barium zirconates, where CO2 adsorbed as CO3 2- ions was able to react directly with deposited carbon at the triple phase boundary. This effect was not seen in BaO, as the BaCO3 formed was too stable 187. In theory the alkali metals should also reduce carbon deposition, and this has been shown for Li in reforming layers in SOFC anodes, where doping with Li or La and Li reduced carbon deposition in the Ni-Al2O3 layer under an 11.5:1 CH4:O2 mixture. It should be noted that in this study, Ca was more effective at reducing carbon deposition, but Li (on its own or combined with La) also showed an improvement in the methane reforming reaction 188. The main concern with use of alkali metals, especially Li, is their volatility. One study used a Liion conducting material, Li0.33La0.56TiO3, as the support rather than Al2O3, and found that this improved long term stability 189. The authors measured the lithium loss, and found that the lithium content reduced from 4.68 to 4.63 wt% after 100 hours at 800 C in air, compared to 0.42 to 0.20 wt% for a sample of Li-doped Al2O3. The fact that this study was carried out at 800 C indicates that the volatility at lower temperatures may be less of a problem. 46

47 Other than the oxides of alkali metals and alkaline earths, there are a few other basic oxides which have been tested. Under the conditions of an SOFC anode Mn occurs in the form of MnO which is a basic oxide. Under wet methane at 800 C, Ni-YSZ doped with 2 or 5% of the NiO replaced with MnO, the cells lasted less than 1 h, similar to the performance of an undoped cell. With 10% MnO the cell showed dramatically improved performance, with no degradation over 40 h 190. Microreactor tests showed that the amount of carbon decreased by over 150 times compared to undoped Ni-YSZ, and this was shown to be due to a relative decrease in the rate of methane cracking compared to steam reforming. Conversely, increasing the acidity can worsen carbon deposition. Adding 2.7% aluminium oxide, an acidic oxide, to a Ni-YSZ anode reduced the amount of carbon deposition in a simulated CH4-CO2 biogas mixture, due to an improvement in the dry reforming rate 191. However, when the amount of aluminium oxide was increased to 10%, the carbon deposition was increased due to the increased acidity. 6.6 Use of ceria and other oxygen storage materials The concept of oxygen storage materials was first used in three-way catalysts, where a partially reducible oxide is able to supply oxide ions during periods of fuel-rich conditions, and is reoxidised during fuel-lean conditions 192. In SOFCs this may help to prevent carbon deposition by increasing the rate of supply of oxide ions for oxidising carbon on the surface of the anode. In reality, for SOFCs the only oxygen storage material of note is ceria and doped ceria. Although there are potentially other oxygen storage materials relevant to anode conditions, only one, MnO 190, has been used (as discussed above), and the effect of its oxygen storage capacity was implied rather than confirmed through experiments. Ceria is particularly attractive due to its high ionic conductivity arising from the creation of oxygen vacancies on its fluorite lattice when exposed to reductive atmospheres 47

48 becoming a mixed conductor These structural defects are known to improve the oxygen mobility of surface and bulk oxygen of ceria resulting in an enhanced oxygen storage capacity (OSC) which at the same time benefits the oxidation processes 196. These boosted redox features might be useful to eliminate C and S adsorbed species via oxidation and release them as CO2 or SO2. The oxygen vacancies population and consequently the ionic conductivity of ceria may be enhanced using promoters In particular, acceptor-dopants (e.g., Sm2O3 or Gd2O3) are used to substitute some cerium ions in the fluorite structure resulting in the formation of oxide-ion vacancy site to compensate the charge-balance. Furthermore, the well-known activity of ceria based catalysts for soot combustion in automobiles makes this material interesting for SOFC anodes 194, 199. For instance, Gd-doped ceria mixed oxide was employed for methane and hydrogen oxidation exhibiting high current density and good tolerance towards carbon deposition 200. Initial work used impregnation to introduce ceria into porous Ni-YSZ anodes. It was reported by some papers to eliminate carbon deposition while using dry methane 201, but others disagree, showing deactivation over only 30 mins. 152 Doped cerias can also be impregnated. Doped cerias are oxide ion conductors, and have some electronic conductivity under the conditions in a fuel cell anode. This has the effect of extending the triple phase boundary region, which outweighs the fact that pure ceria is a better catalyst for direct oxidation than doped ceria 153, 202. Impregnation of samarium-doped ceria (CSO) nanoparticles into a Ni-YSZ electrode produced a cell with stable performance under dry methane over 1000 h, which was attributed to suppression of nickel sintering and carbon deposition observed in separate catalytic reactions with methane-air mixtures. 203 Impregnation of gadolinium-doped ceria (CGO) into anodes of nickel/scandia-stabilised 48

49 zirconia (Ni/ScSZ) showed relatively stable performance under humidified methane, although carbon deposition could still be observed, and had the effect of improving the performance initially (due to formation of conducting carbon networks), before eventually degrading. 204 It should be noted that Ni/ScSZ cells without CGO also showed relatively stable but inferior performance, indicating that the main effect of the CGO was to improve performance rather than reduce carbon deposition. The ceria-zirconia system is well known in catalysis for its high oxygen storage capacity as the seven coordinate zirconium ions serve to stabilise Ce 3+. Ce0.9Zr0.1O2 was impregnated into Ni-YSZ anodes and was found to greatly reduce carbon deposition in methanol at OCV, and almost eliminate it under load, as measured by EDX and TPO 205. Cu/CeO2-YSZ anodes show performance in cell tests under dry methane close to that of Ni/CeO2-YSZ, but with no carbon deposition 152. Importantly, the ceria needs to be impregnated before the Cu, showing that the catalysed step is the oxidation of hydrocarbons on ceria using oxide ions from the electrolyte. 206 The non-catalytic nature of the copper was reinforced by a study which showed that the replacement of copper with gold showed very little change in performance 207. These results indicate that ceria is active towards electrochemical oxidation, while copper simply acts as a current collector, meaning that the use of ceria allows the complete replacement of Ni with Cu. The advantage of using copper rather than nickel is that copper does not catalyse carbon formation, but the low melting point of copper oxide (1326 C) means that traditional electrode fabrication methods cannot be used. The above papers use impregnation of copper nitrate into porous YSZ substrates. To improve the conductivity of this type of cell, Fe was added to Cu/CeO2-YSZ anodes, causing carbon deposition, which 49

50 initially improved the performance by improving the conductivity before causing it to decline slowly 208. Cells based on Cu produced by impregnation may be limited by their electronic conductivity at low Cu contents. In this case, small (<2 wt%) amounts of carbon deposited from exposure of the anode to dry butane were found to improve performance, again because of an increase in electronic conductivity 170. The rate of carbon deposition was the same on YSZ and Cu/YSZ-CGO, implying that Cu and CGO are not catalysing the carbon deposition. In addition, oxidation and re-reduction returned the cell to its original performance, suggesting that the carbon deposits caused no permanent changes in the structure of the anode. Further improvements to these Cu/CeO2-YSZ anodes can be made by using CSO rather than pure ceria 209. The developed system was suitable for several type of fuels and conserves high power densities after switching from one fuel to another. Figure 17 presents the effect of switching fuel type on the cell with the Cu-(doped ceria) composite anode at 973 K. As shown in the plot, 1-butene and ethane leads to the higher power density while toluene generates a current sensitive current drop. Toluene as an aromatic compound increases C formation, however they observed that the anode was self-cleaning upon switching to n-butane. Use of a porous doped ceria interlayer can also reduce carbon deposition with humidified methane as fuel

51 Figure 17. Effect of switching fuel type on the cell with the Cu-(doped ceria) composite anode at 973 K. The power density is shown as a function of time. The fuels were: n-butane (C4H10), toluene (C7H8), n-butane, methane (CH4), ethane (C2H6), and 1- butene (C4H8). Reprinted from Nature 2000, 404 (6775), , with permission from Nature Publishing Group. Since doped cerias are oxide ion conductors in their own right, it is possible to dispense with the YSZ altogether. Cells produced using Ni-CGO synthesised via a Pechini method showed no carbon deposition from Raman under humidified methane at 600 C for 50 hours, although it should be noted that the cells, which used a 20 µm CGO electrolyte, showed extremely low OCVs, and no attempt was made to find out whether this low OCV was due to oxygen leaks into the anode or to the non-zero electronic conductivity of CGO 211. High levels of carbon deposition were still observed under humidified propane. A Ni-CSO anode was operated on dry methane at 600 C for 72 hours under a current load of 300 ma with very low levels of carbon detected by FIB-SEM and TPO post-test analysis, although again thin CSO electrolytes were used and OCVs of ~0.9 V obtained 212. Ni on Mo-doped ceria showed less than 0.04 wt% carbon deposition after exposure to a methane-oxygen mixture (5:2 molar ratio) 213. Cells based on this material using 400 µm LSGM electrolytes showed 51

52 reasonable stability over 10 h under load and wet methane, although unfortunately the amount of carbon deposition was not quantified. There is strong evidence that CSO has high enough electronic conductivity under hydrogen that the limiting factor is not the triple phase boundary length but the surface area of the CSO, intimating that an optimal strategy may be to optimise the surface area of the ceria and improving the catalysis for hydrocarbon oxidation, utilising the minimum amount of current collecting metal necessary 214. This electronic conductivity, along with the reforming activity of CGO, has been used in the current-collecting layer, where CGO-coated Ni was used at the top of the anode to reduce exposure of Ni to unreformed methane. The cells were stable in dry methane at 610 C over 1000 h, compared to cells without the CGOcoated Ni layer which failed after <200 h 215. While work on metal-ceria composites has understandably focused on the doped cerias with the highest ionic conductivities (CSO, CGO etc.) some work has been done on materials with higher oxygen storage capacities. Ce0.9Zr0.1O2-based (CZO) impregnated anodes were found by EDX to reduce carbon deposition in humidified methane compared to CeO2-based anodes 216. A larger effect was seen by replacing Ni with Cu, but unlike replacement of CeO2 with CZO this had a large negative effect on performance for total replacement. A partial replacement of Ni with Cu on CZO was found to be the best compromise between carbon tolerance and performance. A further advantage of ceria and doped ceria is that the methane cracking reaction is extremely slow. Undoped ceria or ceria doped with varying amounts of Nb or Gd showed between 0.07 and 0.9 monolayer coverage of carbon after 150 minutes exposure to methane at 900 C 217, compared to 142 monolayers deposited on Ni/YSZ at the same temperature 218. The electrochemical oxidation of hydrocarbons over doped ceria is still 52

53 relatively low however 219, with higher activities caused by the activity of either Pt current collectors or Ni towards steam reforming. In theory, cells based on Cu and ceria could be doped with noble metals to improve their activity, but the noble metals alloy with Cu forming less active phases Replacement of cermets with mixed ionic-electronic conductors (MIECs) Single phase MIECs At the time of the resurgence in interest in SOFCs in the late 1980s, a concurrent area of interest was direct hydrocarbon oxidation catalysts, for removal of hydrocarbons from car and power plant exhausts. It had been established that a number of oxides were active towards this reaction, and they came to the attention of groups working on SOFCs, with particular attention paid to the Perovskite family of oxides. Perovskites are defined as a family of materials, which present the same structure as the face-centered cubic calcium titanium oxide CaTiO3. The structure of these compounds of general formula ABO3 may be described as a combination of the oxygen and A-site cations that form the cubic close-packed (ccp) framework, the oxygen atoms occupy three quarters of the sites of the cubic close packed layer and the A-site cation, the larger one, the remaining quarter. The B-site cations occupy one fourth of the octahedral holes of the ccp arrangement. This structure can be also viewed as the B-site cations occupying the center of the cubic structure while A and O ions are located at the corners and half edges, respectively. Perovskites have a high degree of structural and electronic flexibility, with many different elements and oxidation states able to be incorporated into the structure. The A-site cation can be a low valence rare earth, alkali or alkaline earth ion, for example La, Na, Ca, Sr or Ba, while the B-site is a transition metal, such as Ti, Zr, Fe, Co, Ni or Cu. Both sites are able to accept multiple different ions simultaneously, and this produces 53

54 possibilities for variable oxidation states In addition, if there is more than one different element occupying the B-site, these can become ordered, and Perovskites displaying this behaviour are known as double Perovskites (materials with only one B-site element or two or more unordered B-site elements are single Perovskites). Their defect chemistry gives the potential for them to exhibit MIEC properties under a wide range of partial pressures of O2 at elevated temperatures 199. Early work centred on perovskites of lanthanum with top row transition metals, for example La0.8Sr0.2FeO3 (LSF), nowadays more familiar as a cathode material, which while it showed better activity than Pt electrodes and no carbon deposition under dry methane, was not stable under relevant anode overpotentials (<-0.3 V) Attention quickly focused on substituted lanthanum chromites which were already used as interconnects in SOFCs due to their stability in very low po2 224, despite the fact that the base material exhibited one of the lowest activities for methane oxidation 225. While lanthanum chromites are not expected to catalyse carbon formation to the same extent as nickel, carbon deposition has been observed. When exposed to dry methane in a fixed bed catalytic reactor, at temperatures above 600 C calcium-doped lanthanum chromite was observed to catalyse methane decomposition, resulting in an average of half a monolayer coverage of carbon, compared to 112 for Ni/YSZ under the same conditions 226. While this amount of carbon is small, it was found to have a deleterious effect on the catalytic reactions. Addition of 3% steam to the 5% methane feed prevented this carbon build-up. A study of various strontium and manganese-doped lanthanum chromites (LSCM) containing varying amounts of Cr and Mn found that larger amounts of carbon deposition for Cr-rich compounds and/or exposure to methane at higher temperatures were linked to lower selectivities towards the total oxidation of methane relative to partial oxidation

55 Due to the low activity of lanthanum chromites, the immediate focus for improvement was on the activity, rather than on further reducing carbon deposition. Notwithstanding this, several authors did measure the tendency of doped lanthanum chromites towards carbon deposition. One such study tested various first row transition metal dopants to improve the activity as well as alkaline earth dopants to improve the conductivity. All dopants produced carbon deposition of less than four monolayers at 800 C. The exception was the Fe-doped material under conditions representing internal reforming, which produced 69.4 monolayers of carbon 228, which is similar to levels which would be expected from a nickel-based cermet 218. Materials which produced no carbon under any conditions were the Sr and Mg double-doped material and the Co-doped material. The two Ni-doped materials (singly doped and co-doped with calcium) surprisingly showed no extra carbon deposition compared to most other dopants, but did produce considerably higher conversions between 3 and 5 times higher than any other materials for the reactions representing partial oxidation and dry reforming, and 1 2 orders of magnitude higher for the reaction representing steam reforming. In fact, although not known at the time, it is likely that the nickel-doped samples were producing nickel metal nanoparticles under reducing conditions, which helps explain the vastly improved catalysis 229. This is discussed further below in section Composites of LSCM with doped ceria show better activity, as well as increased carbon deposition. In one study, carbon deposition after exposure to dry methane for 6 h at 750 C increased from less than 0.1 wt% in pure LSCM to 1.5 wt% in 33 wt% LSCM:67 wt% lanthanum-doped ceria 230. An increase in the amount of the doped ceria improved performance in fuel cell tests in methane, although above 50 wt% ceria the performance dropped, probably due to lower electronic conduction. Doping lanthanum chromites with 55

56 ceria may also help. Iron-doped lanthanum chromite co-doped with 5% Ce showed much lower carbon deposition at 800 C in syngas in a microreactor, while symmetrical cells showed less drop in performance under the same conditions and were able to be regenerated by 24 h under load in H Other perovskite-based anode materials have been tested, for example lanthanum aluminates 232, and barium titanate 233, but the most studied single perovskite other than lanthanum chromite is strontium titanate, which is stable and when doped is a MIEC under reducing conditions. To induce electronic conductivity, the base material can be doped with La 3+ on the A-site (known as LST) or Nb 5+ on the B-site 236, with the stoichiometry controlled to produce either Ti 3+, Nb 4+ or oxygen deficiency or excess, meaning that this system is compositionally very flexible. A possible hindrance to using this material is the high temperature (>1000 C) reduction needed to induce a suitable degree of electronic conductivity, and the fact that this conductivity is lost under oxidation. LST does possess very low propensity towards carbon deposition, with less than 1 wt% of carbon deposited after 6 h under dry methane at 800 C in a microreactor 237. Composites of LST and CGO in the same study showed increased carbon deposition, although still less than 2 wt% carbon with 40 vol% CGO. The increase in carbon deposition was related to the greater degree of interaction between CGO and methane, but in fuel cells the carbon was not shown to have any detrimental effect on the performance, with a direct correlation found between polarisation resistance in the impedance spectrum and propensity towards carbon deposition in the microreactor tests. More recently Sr2MMoO6 (where M is a small 2+ cation such as Mg or Ni) double perovskites (perovskites in which the B-site cations are ordered) have also been used as SOFC anodes. The ordering occurs as a consequence of the very different charges on the B- 56

57 site cations, but currently no advantage for carbon or sulfur tolerance of using a double perovskite rather than a single perovskite related specifically to this ordering has been suggested. Sr2MgMoO6 showed good activity for CH4 oxidation and stability under short term testing of 15 hours 238. The power density dropped under wet CH4 compared to dry CH4, indicating that direct oxidation was the main route for CH4 conversion. Materials where the Mg was partially or fully replaced with Mn performed worse, with a power density of 838 mw/cm² for the pure Mg sample reducing to 650 mw/cm² for the pure Mn sample. Materials using Co or Ni rather than Mn showed similar performance decreases compared to the Mg sample 239. Co was seen to exsolve from the perovskite as Co metal, although initial performance was similar. Co and Ni showed different catalytic behaviours Co acted mainly through steam reforming, with low performance in dry CH4, while Ni showed no steam reforming activity. It is important to note that all the above studies were carried out with Pt current collectors and a doped ceria barrier layer, which later work has suggested could be responsible for most of the methane oxidation 240. A study doping the Mo site with Nb which did not use a barrier layer or Pt current collector agreed with this poor activity towards methane oxidation, and suggested that similarly to other MIECs studied, the catalysed reactions between the methane and the MIEC were likely to be limiting in pure MIEC-based systems 241. The study did find that the amount of carbon deposited was very low, however this would be expected from a system with poor activity towards methane conversion. Due to the structural flexibility of perovskites, they are able to form reduced compounds while maintaining the perovskite structure, and one promising material which illustrates this is the A-site layered double perovskite PrBaMn2O5+δ (PBMO) 242. This material is stable across a good po2 and temperature range, and unlike many of the perovskites 57

58 described above appears to have some activity towards hydrocarbon oxidation (with silver used for current collection). Although this material was not tested for carbon deposition or stability under operation in hydrocarbons, a calcium-doped version, PrBa0.8Ca0.2Mn2O5 (PBCMO) was, and was stable for 50 h in humidified iso-octane followed by 150 h in humidified propane, with currents of A/cm² achieved at 0.6 V at 700 C Addition of catalytic metal nanoparticles to MIECs Since MIECs by definition are electronically percolating, a percolating metal phase is not necessary, but dispersed metals can still be added to promote the catalysis. However, these metal nanoparticles can also be prone to carbon deposition. Impregnation of metal salts (typically nitrates) into the anode is a technique borrowed from catalysis, where it is an extremely widely-used method for producing catalysts. Ni, Pd and Ni-Pd were added to Srdoped LaCrO3, with Ni-Pd showing a synergistic effect for methane oxidation in dry methane, with little or no carbon deposition observed using a carbon balance approach during testing at 0.5 V and 800 C 244. A short period at OCV was sufficient to completely deactivate the electrode towards methane decomposition, while returning the cell to 0.5 V could only recover 60% of the activity. Addition of hydrogen to the cell was necessary to fully reactivate the cell through methanation of the carbon. Further work on Ni-Pd and Pd nanoparticles dispersed on an LSCr-CSO anode suggested that the reaction mechanism for the oxidation is fundamentally different comparing Ni-Pd alloys with pure Pd particles 245. The work suggested that the reaction on Pd was close to direct electrochemical oxidation, while the reaction on Ni-Pd alloys was likely through methane cracking followed by electrochemical oxidation of hydrogen, steam reforming of carbon and electrochemical oxidation of the CO produced. These alloys were found to be resistant to carbon deposition, and it was proposed that doping of Ce, Sr, La or 58

59 Sm into the alloy was preventing the formation of carbon fibres, as highlighted by the fact that the Ni-Pd nanoparticles contained trace amounts of these elements 246. Improvements to the stability of Pd nanoparticles can be achieved by impregnation of Pd-core/CeO2-shell nanoparticles, which are able to operate on dry methane without carbon deposition and survive heat treatments in air up to 900 C with only 9% loss in performance compared to 40% loss in performance with impregnation of just Pd 247. These results highlight some advantages of using a MIEC combined with dispersed metal particles compared to cermets: although the cell can still be deactivated through carbon deposition under certain conditions, since the metal is not load bearing, complete structural failure does not occur and the cell can be regenerated. in cells based on cermets, the only economically feasible method for adding expensive elements such as Pd is via impregnation into an already formed cermet anode. This results in segregated Pd and Ni particles, which rules out this synergistic alloying effect and does not prevent carbon deposition in Pd poor regions 130. An interesting approach towards decoration of MIECs with catalytic nanoparticles is exsolution, where a reducible metal is incorporated into the oxide structure during synthesis, and exsolved forming remarkably stable nanoparticles under reducing conditions 248. A feature of these systems is that the nanoparticles can be cyclically readsorbed and exsolved from the structure. This method has the advantage compared to traditional impregnation that it produces stronger particle-support interactions and so less sintering occurs and the particles are more stable. There are two main disadvantages compared to impregnation: higher reduction temperatures and less control over the 59

60 composition of the particles currently there are no reports of alloy nanoparticles deliberately produced by exsolution. Exsolution was first (deliberately) tested in SOFC anodes with Ru-doped LSCM 249. The Ru exsolved forming particles up to 5 nm in diameter over 50 hours under hydrogen at 800 C, doubling the cell performance and reducing the polarisation resistance by a factor of three. Only 15% of the Ru was found to have exsolved, and the authors suggest that this is due to a combination of slow diffusion and energetic barriers towards removal of too much Ru from the perovskite structure. Later studies found that the exsolved Ru particles acted to hinder carbon deposition on the LSCM 250. Aside from improving the performance of cells running on dry ethanol, it was found in fixed bed tests that carbon deposition was eliminated compared to around 1 wt% for the Ru-free material 250. Co particles can also be exsolved, and these showed <1% weight gain due to carbon deposition when exposed to dry methane for four hours, compared to >100% weight gain for Co/CeO2 prepared by impregnation 251. As mentioned previously, nickel doped into LSCM can exsolve out as nickel nanoparticles, and depending on the particle size produced these can be resistant to carbon deposition. Pulse reaction studies on Ni-doped LSCM indicated that essentially all the methane was converted to carbon dioxide until oxygen stoichiometries below 2.7, where the methane conversion continues to increase despite CO2 conversion reducing, indicating that methane decomposition (and consequent carbon deposition) was taking place 229. This was considered to be due to the greater degree of nickel exsolution implied by lower oxygen stoichiometries and potentially larger particle size. Co particles were also found to exsolve from Co-doped LSCM, and it was found that exsolved Ni and Co particles have a large effect on the methane oxidation rate with only a small increase in the rate of carbon deposition 60

61 compared to LSCM. Exsolved Ni showed far better carbon resistance than impregnated Ni 252 (Figure 18). While exsolved Ni and Co can still show raised levels of carbon deposition, addition of Cu to form nano-alloys can mitigate this. Ni and Co exsolved from Ce0.8(Co,Ni)0.2VO3 showed significant amounts of carbon deposition on exposure to dry methane at 700 C, with 10% weight gain caused by carbon deposition for Co and 27% for Ni, compared to <1% for the undoped material. 253 However, double doping Cu and Co reduced the weight gain to 2% 254. Double doping Cu and Ni did not reduce the amount of carbon deposition, probably because the 50:50 mix of Cu and Ni used is still prone to carbon deposition as discussed in section 5.3. Likewise, LSC double-doped with Ni and Fe showed less carbon deposition after exposure to syngas at 850 C than singly-doped Ni-LSC. Singly-doped Fe-LSC showed less carbon deposition than either, but performed worse in fuel cell tests, while Ni-Fe doublydoped cells performed best 255. XRD and SEM analysis showed that the exsolved Ni and Fe formed alloy particles of around nm. The promising symmetrical electrode material Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (PSCFN) forms Co-Fe nano-alloys at 900 C under hydrogen, with stable performance for 50 h under dry methane and 100 h under dry butane at 800 C 256. Microreactor tests on reduced PSCFN showed considerable carbon deposition (30 wt%) under methane at 850 C and also high activity for methane cracking 257. The stable performance under methane could be explained by the fact that on initial exposure to methane CO2 (and presumably water) is produced rather than hydrogen, indicating that there are species active for methane oxidation. In addition, the carbon was able to be oxidised at 450 C, implying that it was dispersed and amorphous, and therefore may be oxidised by oxygen flux under SOFC anode conditions. 61

62 Figure 18 - Carbon production rates averaged per pulse for LSCM, LSCMCo, LSCMFe, LSCMNi and LSCM+Ni. Note that the latter is shown on the right axis. Reprinted with permission from Chem. Mater. 2010, 22 (21), Copyright 2010 American Chemical Society. The exsolution of nanoparticles could be limited by energetic barriers towards removing B-site cations from a stoichiometric perovskite. This can be combatted by synthesis of A-site deficient materials, which allow B-site cations to be removed much more efficiently, allowing even metals such as iron to be exsolved from LST 258. Control of the stoichiometry in this way also allowed Ni metal and CeO2 to be exsolved from lanthanum cerium titanate 259. Exsolution of Ni from Ni and Ce double-doped LST was found to greatly 62

63 reduce the amount and ease of removal of deposited carbon in microreactor tests in methane compared to Ni-doped LST 260. SrMoO3 is a MIEC stable under anode conditions, but doping with Ca allowed Mo nanoparticles to exsolve under reducing conditions. These particles had a small beneficial effect under hydrogen, but under methane the Ca-doped materials allowed carbon deposition, in contrast to the undoped material which did not 261. While the formation of carbon implies a greater ability to interact with methane, strangely both the undoped and doped materials showed very low OCVs which indicates a lack of ability to convert methane. Nanoparticles of oxides can also be produced through exsolution-type processes. LSCF impregnated with nickel was used as an anode, where it decomposed into strontium cobalt iron oxide perovskite with La2NiO4 finely dispersed over the surface 262. The La2NiO4 was presumed to be the electrocatalytically-active phase, and the cells exhibited good performance in dry propane with only a few carbon whiskers observed in the SEM after 100 h of use. 6.8 Regeneration of SOFC anodes deactivated by carbon As can be seen above, carbon deposition can occur, to varying degrees, on all materials so far studied. In some circumstances the carbon deposition is not detrimental to performance, or can even be positive in small amounts as it can improve the electronic conductivity of the anode. As the amount of carbon increases in an SOFC operating over potentially tens of thousands of hours deleterious effects such as pore blocking and risk of structural failure will inevitably increase, so it may be desirable from time to time to remove this carbon. Clearly it is always possible to remove carbon by heating the cell to high temperatures in air, but a number of studies have investigated the possibilities for removing carbon without damaging the cell. 63

64 Kirtley et al. studied carbon removal from Ni-YSZ using 3% H2O, 10% O2 or 11% CO2 in nitrogen, and found that the carbon was removed fastest in H2O and slowest in CO2, with times ranging from s 263. Through examining the OCV and in situ Raman, the authors were able to identify the stages of carbon removal. First the OCV increased to accompanied by the disappearance of carbon peaks in the Raman. This was attributed to the formation of a CO/CO2 gas mixture. This is followed by the appearance of NiO peaks in the Raman and an OCV reflecting the thermodynamic equilibrium of the Ni/NiO couple in the regenerating gas. O2 leads to complete oxidation of Ni to NiO, while H2O and CO2 lead to partial oxidation. The above study induced carbon deposition from dry methane at OCV, but carbon induced using diesel reformate under load was able to be removed and the cell fully regenerated using dry and wet hydrogen, albeit over a time period of 44 h 264. Regeneration via this method was not possible under conditions where the cathode had also degraded, indicating that the carbon is removed largely by oxygen flux through the electrolyte 265. It is theoretically possible to regain performance without changing the gas mixture by moving from an operating regime where carbon deposition is favoured to one where it is not. Ni-YSZ cells were found to completely regain their initial performance after 24 hours under load at 850 C in a simulated partial oxidation reformate feed, having previously had carbon deposited in the same gas mixture at 650 C under OCV 266. Symmetrical cells (where the electrode material used during fabrication is the same for anode and cathode) offer interesting theoretical potential for regeneration, given that if carbon deposition occurs they can simply be reversed, whereupon the deposited carbon will be exposed to air and thus oxidised. In a recent review on symmetrical electrode materials, the authors conclude that little work has been done on regeneration of these materials after carbon deposition

65 Table 4 Selected papers reporting improved carbon tolerance in SOFC anodes through materials strategies. Noble metals Metal Fabrication Performance Ref Comments Au 1.5 mol% Au Tested under CH4-H2O, 127 Microreactor and deposited onto 850 C, compared to no mechanistic studies Ni/CGO powder, Au: 0.15 V higher OCV, reinforce effect of screen-printed onto 0.2 V higher at 500 gold , 128 electrolyte ma/cm² (both S/C = 3/2); no V degradation under dry methane vs. 0.3 mv/h degradation under S/C = 1/2 (no Au); less C deposition (visual) Pd mg/cm² Pd Tested in wet CH4 and 129 Microreactor tests impregnated into EtOH, 800 C, OCV, suggest carbon slurry-painted compared to no Pd: Rp suppression Ni/CGO electrode decreases by 2x in CH4 effect 132 and 4x in EtOH in loadings above 0.07 mg/cm²; C deposition still observed (SEM, EDX) 65

66 Ru 0 9 wt% RuO2 Tested in wet CH4, Another paper mixed into Ni/CGO, C, compared to no Ru: agrees that Ru has a formed into a pellet current density increased beneficial effect 132, with ~30 µm CGO by 2x at 0.4 V; also, stable but there are few electrolyte performance for 20 h, no papers on Ru due to C deposition (from problems with its carbon balance) (not oxides volatility. Ru compared to no Ru) doped in ceramic anodes may be more feasible Ag wt% Ag Tested in dry CH4, 750 C, 138 A further paper by impregnated into 0.3 A/cm², compared to the same group Ni/YSZ anode no Ag (at 0.6 A/cm²): 0.9 showed similar supports, 20 µm YSZ and 2.5 wt% Ag failed at results for C2H6 137, electrolyte 12 and 81 h respectively, while microreactor 1.6% Ag showed no results also show C degradation to 100 h. tolerance 136. The Control failed at 5 h. Very high mobility of Ag little carbon observed by at ~600 C and EDX above must be noted. Base metals Metal Fabrication Performance Ref Comments 66

67 Cu 0 100% Cu Powders and cells tested 150 There are many impregnated with Ni in dry CH4: at 700 C, papers on Cu with (Cu + Ni = 20 wt%) powders with Cu:Ni of widespread and CeO2 (10 wt%) 9:1 or 10:0 showed no C agreement that it into 400 µm porous deposition. 4:1 gave <0.1 reduces C YSZ support with 60 g C/g. In a cell at 800 C, deposition. Activity µm YSZ electrolyte performance of 4:1 is poor so normally improved from ~0.1 CeO2 or doped ceria A/cm² to >0.6 A/cm² over is used h due to C deposition. Fe Anode supports Cells tested in dry CH4, 155 There is some were prepared from 650 C, 0.2 A/cm²: Cells evidence that Ni:Fe Fe2O3, NiO and CGO, Fe:Ni up to 30:70 gave alloys at 10% Fe are powders (Fe:Ni similar power densities more active for 0:100 50:50 w/w), (~0.3 A/cm²), 50:50 gave methane CGO electrolyte <0.2 A/cm². All Fe- oxidation 155 and containing cells were reforming 156. This stable over 50 h at 0.2 level of Fe gives A/cm², Ni only cell stable cells with a stopped after ~12 h. No variety of oxide ion carbon observed on Fe:Ni conductors :90 by SEM after test 67

68 Co Ni/YSZ and Co/YSZ Cells tested in dry CH4, 159 Other papers were prepared by 850 C, OCV: anodic confirm that Co is coprecipitation then overpotential remained less vulnerable to C coated on a 500 µm stable over 15 h in Co deposition, but still electrolyte support anode, Ni anode failed. C vulnerable 158, 160. deposition still observed Activity for CO in Co anode by SEM. oxidation looks promising Sn 1% Sn was Cells tested in dry CH4 161 Despite some impregnated into and C8H18-air mixtures at papers showing Ni/YSZ anode- 740 C and 0.6 V and 0.5 little impact of supported cells with V respectively. Stable tin 133, 163, the bulk of a 20 µm YSZ performance was papers studying electrolyte. obtained in both fuels performance and over 6 h (CH4) or 13 h mechanisms in (C8H18). Ni-only cells SOFCs , completely deactivated. and catalysts (see section 8.1.2) suggest that the effect of tin is real. Several papers have examined 1% and 68

69 5% loading, with 1% being the best. Non-metal conductors Phase Fabrication Performance Ref Comments C Porous YSZ scaffold Tested in dry CH4 and 168 Several papers in impregnated with C4H10 at 700 C: the early 2000s 10 wt% CeO2 and Maximum power looked promising optionally 1 wt% Pt, densities of 0.1 W/cm² in for this Pd or Rh. ~4 wt% C4H10 and 0.02 W/cm² in technique , carbon is then CH4, similar to Cu/CeO2- however there have deposited in dry YSZ cells. Performance in been no papers C4H10 at 700 C, 100 all fuels was greatly since by this group µm YSZ electrolyte. increased by adding 1% or others. Pd. A 100 h test of Pt/C- CeO2-YSZ in CH4 showed large increase in Ohmic resistance due to loss of carbon. WC Porous YSZ scaffold Tested in humidified CH4 173 This strategy is quite impregnated with at 850 C at OCV: no unexplored, but the 25 vol% WC, then 5 carbon observed visually ability of WC to wt% CeO2 and 5 after 36 h; at 0.7 V stable protect Ni from C wt% Ni performance of 50 69

70 mw/cm² over 24 h with no carbon observed deposition could be interesting. visually. Increasing alkalinity Phase Fabrication Performance Ref Comments BaO Vapour deposition Tested in dry C3H8 at While CaO and SrO of BaO onto an C at 0.5 A/cm²: Cell also reduce C anode-supported voltage stable at 0.8 V for deposition176, 178 NiO/YSZ cell with 15 over 100 h compared to BaO appears to be µm YSZ electrolyte BaO-free cell which failed the best prospect. after <1 h. Similar results Microstructure for wet CO and gasified appears to be vitally carbon. No C deposition important. observed by SEM. BZCY Sol-gel synthesis of Tested in wet C2H5OH at 184 Numerous Ba-based NiO/BZCY 600 C at 0.3 A/cm². perovskites have composites co- Voltage stable at 0.75 V now been tested pressed with CSO to for 180 h compared to including BYZ 183, form an anode- Ni/YSZ and Ni/CSO which BYC 58 and supported cell with failed after <2 h due to C BZCYYb 185. The 20 µm electrolyte deposition. No carbon efficacy of these detected or morphology perovskites seems clear, and 70

71 changes detected by SEM after testing. microreactor and modelling studies both confirm this and elucidate the mechanisms. Ceria-based oxygen storage materials Phase Fabrication Performance Ref Comments CGO Ni-coated CGO by Tested in dry CH4 at The benefits of ceria hydrothermal C at 1.2 A/cm². Voltage and doped ceria synthesis, 2 µm stable at 0.6 V for 1000 h with regards to layer deposited at compared to Ni/CGO cell carbon tolerance the top of a without Ni-coated CGO are so long conventional layer. No carbon established as to be Ni/CGO anode detected by SEM after beyond doubt. Still support, 5 µm CGO test or by Raman after work is continuing electrolyte microreactor tests. with recent studies including use of high OSC cerias 205, 216, microstructuring and the electrocatalytic performance of ceria itself

72 Mixed ionic-electronic conductors Phase Fabrication Performance Ref Comments SMMO SMMO powders by Cells tested in dry CH4 at 240 It is becoming clear solid state method, 800 C: OCVs of all cells that most MIECs slurry painted onto were stable over 100 h, appear to lack 250 µm LSGM very low OCVs were catalytic activity electrolyte with CLO observed for Au and Ag towards methane buffer layer. Pt, Au, current collectors. At 0.5 and other Ag and LST current V the cell with Pt current hydrocarbons. In collectors used. collector was stable for this paper, the 80 h at ~0.3 A/cm² after a activity was large initial drop. dominated by the current collector. PBCMO PBCMO prepared by Cells tested in wet C8H This class of reduced Pechini method, and C3H8 at 700 C and perovskites may screen printed on a 0.6 V: stable at hold some promise 250 µm LSGM A/cm² in C8H18 for 50 h regarding activity electrolyte with CLO and C3H8 for 150 h. C towards buffer layer. deposition was not hydrocarbon measured. oxidation, but much more work is needed to confirm this. 72

73 LST+Ni+ LSCNT powders Cells tested in dry CH4 at 260 Exsolution of metal CeO2 made by sol-gel, 900 C and 0.5 V: The cell nanoparticles from screen printed onto without CeO2 gave an perovskites has a 300 µm YSZ initial current of 0.4 been of intense electrolyte, with Ni A/cm² which declined to interest recently, and CeO2 exsolved 0.35 A/cm² over 80 h, however long term in situ while the cell with studies in operating exsolved Ni and CeO2 fuel cells are lacking. increased in current from In this study, the A/cm². performance is Microreactor tests increasing after 80 showed little carbon h, perhaps deposition in the material indicating that Ni is with Ni and CeO2. still exsolving, a process which is slow in stoichiometric compounds259, Materials design strategies for sulfur tolerance in SOFC anodes There seems to be sufficient consensus in the literature that sulfur will be adsorbed at the surface of nickel blocking the reaction sites for oxidation or reforming reactions 72, 269 ; although initially an unwelcome feature, it can be used as an advantage to minimise carbon deposition 21, 71. It is also accepted that absorption is more dramatic at lower temperatures 73

74 and at higher concentration of sulfur 62, 66. It is also known that two stages of sulfur poisoning have been observed, one is the surface absorption of sulfur that blocks the reaction sites but that can be reversed and a second one related to an in-depth formation of nickel sulfide that changes the microstructure of nickel and is therefore irreversible 71, 270. Figure 19 shows a possible mechanism for sulfur poisoning in hydrogen and carbon fuel environments 62. There is, however, no consensus in the effect of the current densities on sulfur poisoning. Some authors reported that increasing current densities leads to a decrease in sulfur coverage because of its conversion to SO2 63, 65, 271. On the contrary, other authors have indicated that sulfur coverage increases with current density 60, A very good agreement with the latter view is concluded in the recent modelling work of Riegraf et al 274, where the model involves all gas and solid chemical reactions coupled with electrochemistry. When operating in methane, the adsorbed sulfur supresses the reforming reaction by blocking the catalytically active sites, and these sites become available if sufficient hydrogen is present to unblock the sites 274. Again there is agreement that full recovery can be achieved if H2S is removed completely from the fuel stream but there is a limit beyond which damage is irreversible. Concentration and temperatures where recovery is possible vary from article to article but reversibility has been reported independently. This may be related to desorption of sulfur from the nickel surface and reaction with H2 from the clean stream. Some good examples of this recovery are the work of Rasmussen and Hagen 68, Sasaki 62 and Zha 63. It is also generally 74

75 accepted that the whole surface of nickel is covered and not only the TPB region 270. Figure 19. Possible mechanisms of degradation by sulfur poisoning. Taken from J. Power Sources 2011, 196 (22), Reprinted with permission from Elsevier. Whatever change takes place in the anode during poisoning, it must be reversible and provided that oxygen is migrating to the anode via the electrolyte, it is of paramount importance that this process is not stopped and that oxidation or removal of adsorbed sulfur is favourable. To provide sulfur tolerance, the materials and structure of the anode should therefore be capable of adsorbing sulfur and then react with any of the gaseous species present H2, H-C or even O 2- to form SO2. From the point of view of the materials modification, the strategies more frequently used for the development of sulfur tolerant anodes can be summarised as follows: 75

76 1) High oxygen transport to increase sulfur oxidation (Figure 20a). In similar conditions, ScSZ working under H2S/H2 atmospheres shows a higher tolerance to H2S than YSZ, indicating the importance of a higher oxygen supply through the electrolyte 62. 2) Incorporation of additives or partial substitution of nickel (Figure 20b). Substituting Ni for a more sulfur tolerant metal without compromising H2 activity has been behind much of the work on alloys 89, 96. Some of the earlier work attempted copper97, 275 while the most recent use of additives has been aiming at using these as preferential sites of sulfur incorporation 276, and this is reinforced by a thermodynamics studies showing that oxides such as BaO and CeO2 reduce the coverage of sulfur on Ni 277, which is strongly linked to performance 91. Catalytic activity for hydrogen oxidation reaction and H2S dissociation seem to follow analogous trends, maintaining the catalytic activity while simultaneously improving sulfur tolerance difficult via this route ) Use of all-ceramic anodes (Figure 20c). Perovskites are favoured as they can be tailored on the A and B site to improve ionic conductivity, electronic conductivity, catalytic activity, and resistance (e.g. Mg 2+ more resistance to sulfide formation than Cr 3+ or Mn 2+ ) 278. Additionally, the reactivity of a ceramic material is expected to be smaller than that of a metal surface

77 Figure 20. Schematics of the most common materials strategies to improve sulfur tolerance. The diagram shows a strategy and does not imply a specific mechanism of desulfurization. 7.1 Replacement of YSZ with ceria A few papers have compared Ni-YSZ and Ni-CGO electrodes and, for example, Zhang has shown that degradation in Ni-CGO is lower than in the Ni-YSZ under similar conditions of operation 279. This may not be surprising considering that sulfur can also accumulate in the surface of the CGO forming Ce0xSy-type phases which can react with O 2- to produce SO2. Recent studies on the adsorption and removal of H2S from fuel streams by rare earth oxides again suggest that CGO is one of the most promising anodes for operation under H2S poisoned fuels. Elimination of the adsorbed sulfur can take place in ceria and other rare earth oxides using a reducing, oxidising, or inert gas or even steam This may explain the tolerance to H2S of an anode that has been infiltrated with ceria 282 and the minimised potential drop in anodes with lanthanides as additives 62. The tolerance of ceria-based materials to sulfur environments has been known for some years

78 Donor-dopants of ceria have been studied to a lesser extent compared to acceptor species. Nevertheless some of them are interesting for improving sulfur tolerance, for example Mo. This dopant is especially desirable for sulfur tolerance goals since it can trap S forming MoS2. In this sense, Li et al. investigated the electrical properties of the Mo-doped CeO2 (CMO) as potential anodes for SOFCs. Mo and rare-earth-co-doped Ce0.9-x RExMo0.1O x (x=0.2, 0.3) (CRMO) oxides were found to retain their fluorite-type structure under H2 at elevated temperatures 284. The same team demonstrated the remarkable stability of these Mo-doped CeO2 anodes in wet H2 and wet CH4 mixtures 285. As mentioned above Mo is a key element to incorporate sulfur resilience. In a recent publication, Chen and co-workers developed a sulfur-resistant SOFC anode by impregnation of Mo0.1Ce0.9O2+δ into a typical Ni- YSZ material 286. Figure 21 shows the successful performance of this material when submitted to 50 ppm of H2S. Figure 21. Sulfur tolerance test for a CMO-impregnated cell under a current density of 0.60 A cm 2 at 750 C using H2 and H2 with 50 ppm H2S as the fuel, respectively. Reprinted from J. Power Sources 2012, 204, with permission of Elsevier. 78

79 The system allows power densities of 440 mw cm 2 and 420 mw cm 2 using H2 with 50 ppm H2S and methane as fuel, respectively under a current density of 0.60 A cm 2 at 750 ⁰C. 7.2 All-ceramic anodes A number of all-ceramic electrodes have shown promising performance in SOFCs or SOECs , As mentioned before it is expected that an oxide is less prone to adsorb sulfur than a metal. The classic perovskite SrTiO3 can be doped both in the A and B site or even have A site deficiency 278. Some work has been performed on Y-doped SrTiO3 doped with Ru and CeO2 showing a limited tolerance to H2S (up to 40 ppm) and especially reversibility when the H2S stream is removed 289. The Sr0.6La0.4TiO3/YSZ (50/50 wt %) anode showed no degradation in the presence of up to 5000 ppm of H2S in a hydrogen fuel 290 and it has even suggested that the presence of H2S can promote the oxidation of methane In general it seems that perovskites are stable towards operation in sulfur, with many examples being reported, including double perovskites 242, lithium-ion conducting perovskites 294. It is generally recognised that Perovskite-based materials lack the catalytic activity of nickel. As discussed in section 5.7.2, one method to improve the catalytic activity has been to dope the perovskite with transition metals which then exsolve out as catalytically-active nanoparticles on reduction. Little work has been done on the tolerance of these nanoparticles towards sulfur, but one study showed that Fe nanoparticles ex-solved out of Sr2Fe1.5Mo0.5O6 forms FeS under 50 ppm H2S in H2, with a decline in activity of around 20% from around 0.1 to around 0.08 W/cm² at 600 C over a period of 46 hours, followed by stable operation for a further 200 hours

80 It has also been reported that the presence of H2S improves the performance of the fuel cell when methane is used as the fuel for La0.4Sr0.6Ba0.1TiO3-d 291, 296 but the oxidation of H2S to SO2 does not seem to be the main reaction as suggested previously 269 in SOFCs but rather a gas reaction with methane and potentially an increase in the conductivity of the perovskite by some as yet unclear mechanism 296. Although doped SrTiO3 has been independently shown to be stable 289, 291 in H2S, the high concentrations used need to be independently confirmed. Barium-based perovskites have also shown promise for sulfur tolerance. For instance, Kan et al. prepared the proton-conducting Ba3CaNb2O9 doped with Mn, Fe and Co and checked the stability of these materials towards H2S 297. They used a 5000 ppm H2S/H2 stream to evaluate whether the investigated samples can be used as electrodes in contaminated fuels (e.g. natural gas with ppm levels of H2S). Their XRD study indicated that the samples preserved the double-perovskite structure at 600 ⁰C for 12 h. No secondary phase was detected due to the formation of sulfides such as MnSx, FeSx, or CoSx. The SEM study also confirmed that the particle sizes and shapes did not change after H2S treatment. This result suggests that their materials are physically and chemically stable in the SOFC working environments. The same group reported enhanced stability of perovskite-type BaZr0.1Ce0.7Y0.1M0.1O3-δ (M=Fe, Mn and Co) with a substantial chemical stability in 30 ppm H2S/ H2 at elevated temperature during 24 h 297. Another Ba-based Perovskite prepared by Yang et al. seems to be a promising material 276. In a very complete paper they reported outstanding sulfur and coking resistance of a barium zirconate-cerate co-doped with Y and Yb (BaZr0.1Ce0.7Y0.2-xYbxO3-δ) anode. The terminal voltages of the same cells (with BZCYYb and SDC as electrolyte) at 750 C were recorded as a function of time when the fuel was contaminated with different 80

81 concentrations of H2S. The Ni-BZCYYb anodes for both cells showed no observable change in power output as the fuel was switched from clean hydrogen to hydrogen contaminated with 10, 20, or 30 ppm H2S. XRD data corroborated the chemical stability of the designed anodes. A study on Ni-BZCY anodes featured even higher levels of H2S (up to 1000 ppm), used EIS to show that as well as reducing the anode polarisation losses compared to Ni-CSO, the BZCYbased anodes showed little increase in Ohmic losses even at 200 ppm H2S, while the Ni-CSO cell showed severe increases in Ohmic resistance at 100 ppm H2S (figure 22). Post-test EDX showed large decreases in Ni content in the Ni-CSO anode, which were not seen in the Ni- BZCY anode 298. This indicates that these materials may be hindering restructuring of the Ni at high sulfur levels. Figure 22. I V, I P curves and EIS for the fuel cells with the Ni+SDC (a, c) and Ni+BZCY (b, d) anodes operating on different fuels at 600 C. Reproduced from Environ. Sci. Technol., 2014, 48 (20), pp , copyright 2014 American Chemical Society 81

82 The role of barium in the improved tolerance may be related to the reduction of sulfur chemisorption on nickel. Da Silva and Heck have calculated that the incorporation of oxides, in particular BaO, reduces the sulfur chemisorption on Ni by minimizing the sulfur chemical potential and favouring the formation of BaS. This sulfide can be reconverted to BaO in the presence of water and additional BaO, leading to an in situ regeneration 299. It was also predicted that the addition of BaO enables the anode to tolerate 100 ppm in humidified H Alloying of nickel with other metals The incorporation of additives or secondary phases has been known in metallurgy for many years. The extraction of nickel (or cobalt) metal from ores involve roasting or oxidation of the sulfide to the oxide followed by in situ reduction with CO. It should be noted that all the key elements necessary for oxidation and elimination of sulfur used in metallurgical processes are present in a fuel cell anode and the analysis of roasting may provide the key to achieve tolerance to sulfur in SOFC. In roasting, the sulfide minerals are treated with very hot air and the sulfide is converted to an oxide while sulfur is released as sulfur dioxide; typical examples being ZnS, FeS2, PbS2 and Cu2S. Roasting is usually carried out between 500 and 1000 C 300, the same range of operation of SOFC. Improvement of the roasting process is achieved by adding pyrite (FeS) with the highest rest potential among sulfide minerals, therefore acting as a cathode which accelerates the oxidation of the other sulfides. Finally, reduction with CO leads to the formation of the metal although a few metals can be obtained directly by oxidation of their sulfides since their oxide is less stable than SO2, well known examples being: Cu, Ag and Hg. Finally, it is worth mentioning the idea of decomposing H2S into hydrogen and sulfur, both valuable products; several routes have been explored in the past 301. The most 82

83 straightforward suggestion of relevance to SOFC is that H2S be decomposed thermally according to: 2H2S 2H2 + 1 /4S8, ΔH = 79.5 kj/mol This decomposition has been performed in the presence of MoS2 302 between C and more recently, it has been reported H2S can be decomposed in the C temperature range using the Perovskite oxide LaSr0.5Mo0.5O The presence of Mo and a possible decomposition of H2S may be behind the activity and the reported stability of Sr2Mg1-xMnxMoO6-δ in these complex perovskites anodes 238, 304. Mo-containing catalysts are commonly used in hydrodesulfurisation processes 305. The interaction of molybdenum with sulfur can be modified with the presence of a second metal with direct consequences for the hydrodesulfurisation properties. In particular the synergistic effects of Ni-Mo bonding has been proved to be active towards the hydrodesulfurisation 306. In contrast, the effects of Zn, Cu, and Fe on the Mo-S interactions and hydrodesulfurisation activity are less pronounced. The Ni-Mo and Ni-S-Mo interactions increase the electron density on Mo making it more chemically active in two key steps for the reactions: the adsorption of S- containing molecules and the dissociation of H. It is therefore not unthinkable that molybdenum plays a key role in the sulfur tolerance in anodes containing this metal. Most studies within the literature regarding the tolerance to sulfur report the effect of sulfur poisoning on the electrochemical properties as it the most direct way of in-situ degradation analysis. Therefore, cell voltage changes, power output and area specific resistances are commonly used to describe the changes to the anode upon a modification. Comparison between the different reports is difficult and therefore we shall provide the overall change observed in the very same paper when the anodes is modified with the 83

84 intention to improve sulfur tolerance. Table 5 presents results from selected papers where there has been a variation in the anode with the intention to improve the tolerance to sulfur. Table 5 Selected papers reporting improved sulfur tolerance in SOFC anodes through materials strategies. Cell Modification Sulfur tolerance and figures of merit Reference Ni-CGO in YSZ High porosity Two stage degradation with area specific 307 scaffold anode (not quantified) resistance of 0.35 Ω cm 2 at 650 C, 0.25 A ScSZ cm -2. Full regeneration possible. electrolyte Low porosity Two stage degradation area specific (La0.6Sr0.4)0.99Co (not quantified) resistance of 0.70 Ω cm 2 at 650 C, 0.25 A O3-δ cathode cm -2. Full regeneration possible. Ferritic (FeCr) stainless steel supported Ni/8YSZ and Ni/CGO anode Two stage poisoning. Stack voltage 308 Ni/CGO decrease only to 98.7 % of initial value 3YSZ upon addition of 2 ppm H2S to fuel at 850 electrolyte C, A cm -2 after 15 h. Fuel mixture: support 43.8% H2, 6.2% H2O and 50% N2. LSM cathode Ni/8YSZ anode One stage poisoning. Stack voltage decrease to 86.5 % of initial value upon addition of 2 ppm H2S to fuel at 850 C, 84

85 0.319 A cm -2 after 15 h. Fuel mixture: 43.8% H2, 6.2% H2O and 50% N2. Ni/YSZ or Ni/CGO anode Polarization resistance of anode is 4.3 Ω 279 Ni/CGO anodes cm 2 in 700 ppm H2S in H2, 200 ma/cm 2 at YSZ or CGO 800 C after 2 h. Regeneration possible. electrolyte Ni/YSZ anode Polarization resistance of anode is 1.2 Ω supported cm 2 in 700 ppm H2S in H2, 200 ma/cm 2 at Pt cathode 800 C after 2 h. Ni-CGO anode Ni/CGO+ Cell voltage 0.74 V in pure H2 at 650 C 309 supported BaCe0.9Yb0.1O3 δ with 640 ma/cm 2 goes immediately to 0.7 CGO V stable over 20 h upon introduction of electrolyte 500 ppm H2S. Full regeneration possible. NdBa0.75Ca0.25C o2o5+δ-cgo cathode Ni/CGO Cell voltage 0.63 V in pure H2 at 650 C with 640 ma/cm 2 goes immediately to 0.61 V and decreases continuously for 6 h. Regeneration not possible. Ni1 xcox-ysz Ni0.69Co0.31O- Current exchange density A/cm 2 in 101 anodes YSZ pure H2 improving to A/cm 2 in 10% YSZ electrolyte H2S in CH4 after 15 h at 850 C. 85

86 LSM cathode Ni-YSZ Current exchange density A/cm 2 in pure H2 improving to A/cm 2 in 10% H2S in CH4 after 15 h at 850 C. Ni YSZ or Sr0.85Ce0.15Co0.2F Current density is A/cm 2 in H2 at Sr1 xcexco0.2fe0 e0.8o3 δ anode V..8O3 δ anodes Lowered to 88 % of initial value upon ScSZ addition of 20 ppm H2S at 800 C after 500 electrolyte min at constant 0.6 V. supported LSM cathode Ni-YSZ Current density is A/cm 2 in H2 at 0.9 V. Immediate drop in current. After 500 min, current lowered to 81 % initial normalized value in 50 ppm H2S at 800 C at constant 0.6 V. Ni YSZ anode Ni-YSZ + Cell voltage remains above 0.72 V even 311 YSZ electrolyte infiltrated upon addition of 30 ppm H2S to pure H2, at support BaZr0.1Ce0.7Y0.1Y 700 C, A/cm 2. V. slow degradation. SDC/LSCF. b0.1o3 δ (BZCYYb) Ni-YSZ Cell voltage from 0.72 V in H2 to 0.62 V to 20 ppm H2S in H2 within a few minutes at 700 C, A/cm 2. 86

87 Ni-doped Ni-ScSZ anodes Two stage poisoning mechanisms, one 312 zirconia anode ScSZ electrolyte fast, reversible and one slow and supported cell irreversible. Poisoning of methane Doped zirconia reforming. LSM cathode Cell voltage 0.7 V in 2 ppm H2S, 13% H2, 58 % H2O, 29% CH4, 850 C, 1 A/cm 2 after 500 h Ni-YSZ Cell voltage 0.45 V in 2 ppm H2S, 13% H2, YSZ electrolyte 58 % H2O, 29% CH4, 850 C, 1 A/cm 2 after 500 h. Two stage poisoning mechanisms. Poisoning of methane reforming. Ni-doped Ni-ScSZ anode Cell voltage: 0.55 V 62 Zirconia anode at 200 ma/cm 2, 800 C, 100 ppm H2S in H2 YSZ electrolyte after 1000 s with stable performance. LSM cathode Ni-YSZ anode Cell voltage: 0.18 V 200 ma/cm 2, 800 C, 20 ppm H2S in H2 after 1000 s. Null voltage after 1650 s. Two stage poisoning: initial one is fast and reversible, second is slow and irreversible Ni-YSZ anode Ni-YSZ anode Voltage drop 0.12 V, 200 ma/cm 2 62 Doped zirconia ScSZ electrolyte in 5 ppm H2S in H2 at 850 C. electrolyte Ni-YSZ anode Voltage drop 0.52 V, 200 ma/cm 2 87

88 LSM cathode ScSZ electrolyte in 5 ppm H2S in H2 at 850 C. Ni-YSZ Anode- Ceria-modified Cell voltage = 0.6 V at 0.3 A/cm 2 in H supported Ni-YSZ 200 ppm H2S at 700 C YSZ electrolyte Ni YSZ anode Cell voltage = 0.4 V at 0.3 A/cm 2, 700 C in LSM cathode H ppm H2S Ni- Ni-BZCY OCV = 1.01 V in pure H2 298 BaZr0.4Ce0.4Y0.2 O3-δ (BZCY) and Stable 148 mw/cm 2 in 100 ppm of H2S at Ni- 200 ma/cm 2, 600 C for 700 min Sm0.2Ce0.8O1.9 (SDC) anodes SDC electrolyte Ni-SDC OCV = V in pure H2 Ba0.5Sr0.5Co0.8F e0.2o3-δ (BSCF) From 137 mw/cm 2 to 81 mw/cm 2 in 100 and ppm of H2S at 200 ma/cm 2, 600 C for 150 Sm0.5Sr0.5CoO3- min δ (SSC) cathodes 88

89 Ni YSZ anode Bimetallic Peak power densities 1.4 W/cm 2 in H supported coating Ni- Decreases to 1.0 W/cm 2 in 500 ppm H2S- YSZ electrolyte Cu/Co/Fe on H2. LSCF-GDC anode Enhanced dry reforming of methane. cathode No extra Peak power densities 1.4 W/cm 2 in H2. coating NiSn-YSZ Infiltrated NiSn Cell voltage decreases from 0.72 V in pure 314 anode + reformer H2 to 0.63 V on addition of 500 ppm H2S at supported NiSn/Al2O3 at 850 C and 1.25 A/cm 2. Complete YSZ electrolyte regeneration. LSM-YSZ No infiltration Cell voltage decreases continuously from cathode and without 0.5 V to 0.45 V in 48 h at 1.25 A/cm 2, 850 reformer C, 200 ppm in CO2:CH4 8. Strategies from conventional catalysis The problems faced by fuel cell anodes regarding carbon and sulfur poisoning are similar in many ways to those faced by conventional catalysts. In fact, in one very important respect carbon and sulfur tolerance is more challenging in conventional catalysis there is no equivalent of the oxygen flux through the electrolyte which occurs in SOFCs, which tends to reduce the problems with carbon and sulfur. Because of this, it is instructive to look at solutions for tolerant catalysts. This issue has been studied over a much longer period of time, and more intensively, than for SOFCs, and many of the findings have not yet been incorporated into SOFC research. 89

90 8.1 Carbon tolerance in conventional catalysis Several strategies have been studied for minimizing the carbon deposition in catalyst used in reactions involving hydrocarbons, such as steam reforming, dry reforming, partial oxidation or water gas shift. The use of noble metals, instead of Ni, as the active phase is the best option in terms of carbon resistance. However, similarly to SOFCs, the high cost and low availability of noble metals mean that Ni-based catalysts are favoured, and strategies for minimizing carbon deposition in these catalysts have been developed in the last decades 16, Sulfur passivation Sulfur passivation was one of the first strategies developed to diminish carbon deposition. The first published works, in steam and dry reforming of methane, appeared in the mid-80s 18, The approach consists of partially passivating the active centers of Ni catalysts with sulfur, normally using H2S 18, Lately the use of alkanethiols for the passivation has also been proposed with promising results Hydrogen sulfide chemisorbs on the nickel surface and blocks access to the catalytic centres. This blockage decreases the carbon deposition rate more than the methane reforming rate 316. At complete coverage, carbon atoms cannot be dissolved into the nickel crystal and the whisker growth mechanism is inhibited. However, the complete coverage of the nickel surface with sulfur results in total deactivation 316. Using coverage ratios of around 0.7 it is possible to diminish carbon deposition without compromising reforming activity At this coverage ratio it is not possible to inhibit carbon formation. Nevertheless, the usual whisker structure is replaced by more amorphous structures, which are less deactivating , 319. Hence, at this coverage, the reaction still takes place. This is due to 90

91 the number of active nickel surface sites needed for each process. Carbon nucleation needs larger sites, which are almost completely blocked at high coverage rates, whereas methane reforming reactions can proceed in the smaller sites which are still available 18, However, reforming of larger molecules, like toluene and tars, requires large active sites, so the sulfur passivation can deactivate reforming reactions as well 318. This technology, in the case of the dry reforming of CH4, has been industrially implemented by Haldor Topsøe in the SPARG process 21, 32, 322. Sulfur passivated catalysts have been also applied to the dehydrogenation of isobutane, with sulfur passivation improving both the selectivity of the process and inhibiting carbon formation Alloying and bimetallic systems The introduction of additional metals that can modify the ability of carbon to assemble or to dissolve in the bulk metal of the catalyst can drastically reduce the potential for carbon deposition. A vast number of bimetallic combinations can be found in the literature. Focusing only on Ni catalysts, bimetallic systems like Ni-Co , Ni-Fe 324, Ni- Cu , , Ni-Mn 324, Ni-Sn 167, 325, and Ni-NM (being NM a noble metal: Rh, Pt, Pd, Ir, Ru, Au, Ag) 9, 49, 325, 330, , have been studied, showing in some cases very promising results. When one or more other metals are introduced into the system, different structures can be originated depending on the metals properties, interactions with the support, atmosphere, temperature, etc 325, 336. A schematic representation of these structures can be seen in Figure 23. Among these structures, interest in alloys is increasing. The use of alloys of different metals as the active phase has been deeply developed in the last years. This is 91

92 due to the superior performance of alloys in terms of conversion and resistance to carbon deposition Figure 23. Possible structures shown by bimetallic nanoparticles: a) core-shell; b) heterostructure; c) nano-alloy; d) segregation; e) ensembles (adapted from Catal. Today 2012, 197 (1), , with permission from Elsevier). The interaction between two or more metals can give rise to geometric and electronic effects, which could affect carbon deposition 325. The geometric effect is the result of the dilution of the atoms of one metal in the other. Thus, surface ensembles are reduced in size. This can dramatically affect the catalyst performance, since many reactions depend on the size of the ensembles, as was explained for the partial sulfur passivation 325, 341. The electronic effect is the result of the difference in electronic affinity between the metals, that can produce an electronic density increase or a decrease in the main metal depending on whether the secondary metal has a lower or higher electronic affinity. These modifications in the electronic density alter phenomena such as adsorption or desorption of species during the reaction process, affecting activity and selectivity 325. Noble metals are well known to be more resistant to carbon deposition than Ni, as well as possessing other features such as improved catalytic activity, suppression of Ni oxidation or sustainability in daily start-stop operations , 340, 343. Among the noble metals, the most common used in bimetallic systems with Ni is Rh. In this type of catalyst Rh 92

93 atoms enrich the Ni surface, forming a surface alloy Ni Rh, rather than dissolving into Ni particles and forming a bulk alloy. However, the formation of the alloy strongly depends on the support used and its interaction with the metallic particles 324, 340. In addition, preparation conditions can also affect the carbon resistance of the bimetallic system. Thus, if the catalysts are calcined in oxygen at high temperatures, metal segregation can occur, giving rise to lower carbon resistance 339. The presence of Rh increases the energy barriers of C diffusion and C C bond formation whereas the O diffusion and C O bond formation are not significantly affected. As a consequence, the global rate of carbon deposition is decreased 49, 325. Similar behaviour has been found in the case of Ni-Pt systems 325, 336. The presence of Pt has been found to promote the formation of small NiO crystals, which facilitates the reduction to Ni 0 and improves Ni dispersion 340. The versatility of Ni-Pt system allows the creation of different surface structures (core shell, monolayers, alloys) that need to be controlled to minimize carbon deposition 336, 340. Although less studied, Au and Ag have given rise to interesting results in terms of carbon resistance 5, 330. Particularly in the case of the Ni Au alloys, it has been found that the presence of a small amount of gold on a supported nickel catalyst can induce a significant effect on the carbon formation process during the steam reforming of methane 5. Au makes the diffusion of the CHx species (intermediates in carbon growth) significantly difficult, preventing carbon nucleation 330. In the dry reforming of methane, the presence of gold promotes the formation of carbonaceous species which have high reactivity with CO2, thus facilitating gasification 340. However, the elevated cost of noble metals makes it more practical from the industrial point of view to develop noble metal-free catalysts 49, , 344. Ni-Co might be a more affordable option. Ni-Co bimetallic catalysts show a synergetic effect that makes the 93

94 catalyst more active and resistant to carbon deposition than Ni and Co monometallic catalysts 324, Ni and Co benefit from the electronic effects that appear in bimetallic systems. They present different oxidation states depending whether they are used in monometallic or in bimetallic catalysts, indicating an electronic transfer between Ni and Co in the bimetallic catalyst 324. This protects metal from oxidation during the reaction and confirms the near-distance interaction between the two metal atoms, making it easier to form Ni Co alloy on the catalyst surface 324. In addition to the synergetic effect, the formation of various spinel-type solid solutions with the supports improves the metalsupport interaction and therefore the carbon resistance 324, 327. Cu-Ni system stability has been found to be dependent on temperature and Cu/Ni ratio 329, 331. Copper seems to stabilize the structure of the active site on Ni surface, thus preventing sintering or loss of nickel crystallites. Adding Cu into Ni catalyst system can fine-tune the catalytic activity, so that carbon formation and removal can be balanced, preventing deactivation by carbon accumulation , 332. However, an excessive load of Cu could give rise to a Cu-rich alloy that can increase carbon deposition 331. Sn/Ni alloys seem to be the most promising alternative, not only for their high resistance to carbon deposition but also for the low price of Sn compared to noble metals 167, 325, Sn/Ni alloys have shown up a huge potential for improving the carbon resistance in steam reforming processes by modifying the relative kinetics of C-O and C-C bond formation. Once again, in this case the formation of the surface alloy is favoured over the bulk alloy, especially at low Sn loadings 167, 333. DFT studies showed that the presence of Sn, which is mainly located at the surface of the alloy particles, imposes an important barrier to carbon diffusion in Ni crystallites, thus hindering carbides formation and the subsequent 94

95 nucleation of carbon 167, 334. These theoretical results were confirmed in steam reforming of various hydrocarbons 167, Promoters The addition of different promoters can affect the interaction between the metal and the support or the acid-basic nature of the support, which modifies its tendency to give rise to carbon deposition 27, 29. The use of several promoters can be found in the literature, including Li 22, Na , K 22, 29, , Mg 15, 17, 22, 346, , Ca 346, 351, , La 22, 26-27, Zr 25, 326, 335, 337, , Mn 29, 348, Ce 29, 348, 370. Alkali (Li, Na, K) 22, 29, or alkali earths (Mg, Ca) 17, 22, 346, 351, , are usually introduced in catalyst formulations with the aim of accelerating carbon removal from the catalyst surface due to their basic nature 371. K2O can reduce the carbon deposition rate in reforming processes, but occasionally it also compromises the catalytic activity 22, 29, This reduction of the carbon deposition is a consequence of an improvement of the gasification rate of the carbon deposits, thus improving the stability of the catalyst. However, the interaction of K with Ni gives rise to large NiO crystalline particles. This species has high mobility, thus promoting aggregation of particles and decreasing the activity, while larger Ni particles also promote carbon deposition 21-24, 31. K2O has also been used as a promoter in Ni catalysts for the water gas shift reaction 345 and in bimetallic Ni-Mo catalysts for the dry reforming of propane 372, presenting in these cases improvements both in carbon resistance and catalytic activity. Similar behaviour to K, although less pronounced has been observed in the case of Na2O and CaO 346, 351, , In the case of CaO some researchers have suggested that while the amount of carbon deposits increases, the reactivity of the deposits also increases, leading to a higher stability of the catalyst 351, 360,

96 The use of Mg has also been shown as an interesting option for minimizing carbon deposition, although in this case the preparation of the catalyst and the interaction of Mg with the support play a critical role 17, 22, 345, 352, , If Mg is used as a promoter of Ni/Al2O3, it interacts with Ni, leading to a NiO-MgO solid solution, but when Mg is used as a dopant of the Al2O3 support, it can react to produce MgAl2O4 spinel. In both cases, Ni sintering is prevented and the carbon deposition is reduced whereas the catalytic activity increases, but the effect achieved by the spinel has been shown to be quantitatively better than that from the solid solution. Mn is used as a promoter, especially in dry reforming of methane, to reduce carbon deposition, both in Ni and Co based catalysts , MnOx forms patches that partially cover the Ni surface giving a similar effect to that from sulfur passivation Moreover, Ni dispersion is improved 376, 378 and the moderate basicity of the MnOx improves CO2 adsorption and increases carbon gasification rate by forming reactive carbonate species Lanthanide oxides have also been thoroughly studied, with La and Ce oxides the most promising promoters La2O3 has been found to affect positively both activity and resistance to carbon deposition 22, Two different effects are promoted by the presence of La2O3. On one hand, its basicity promotes the absorption of CO2, giving rise to lanthanum oxycarbonates 22, 26-27, These species play a role in conserving the stability of the catalyst, since they promote the CO2 decomposition to CO and O, which can increase the carbon gasification rate 22, 27. On the other hand, the interaction between La and Ni forms a mixed oxide (NiLa2O4) in the same way as happens with Al2O3. This phase prevents the sintering of Ni particles, thus reducing carbon deposition 27, 382. It has also been found that the performance can be improved by the addition of alkaline oxides 384 or other lanthanides 96

97 like Ce or Pr 383, 385. In the case of the combination of La-Ce, it has been found that after reduction of the catalysts, particles of a mixed oxide appear on top of nickel particles. This decoration of Ni particles reduces the ensemble of Ni similarly as in the case of sulfur passivation, thus lowering the probability of the nucleation of carbon precursors 383. CeO2 constitutes another interesting promoter for minimizing and even suppressing carbon deposition 29, 348. It has been used as support as well, but the conversions were lower probably due to the strong metal-support interaction. However the results showed that its use as promoter is much better, giving rise to high conversions and resistance to carbon deposition 370, 386. It should be noted that the amount of CeO2 used as a promoter should not exceed certain value to avoid compromising the catalytic activity 338, 387. The high resistance to carbon deposition comes from the oxygen storage capacity and oxygen mobility that ceria presents 192, 388. CeO2 can store and release reversibly a large amount of oxygen, thus increasing its availability for gasifying the carbon deposits 29, 348, 353. As discussed in section 5.6 CeO2 exhibits excellent redox properties with a Ce 3+ -Ce 4+ equilibrium and the coexistence of CeO2 and Ce3O4 192, 353, 388. This behaviour can influence the oxidation state of atoms on the surface of the active metal particles (for example Rh 0 /Rh δ+ ) favouring the activation of reacting molecules 338, 386, 389. Other features that can improve catalyst performance are that CeO2 gives rise to a better dispersion of the active phase, enhancing the catalyst performance and inhibiting the transition of the γ-al2o3 used as support to the low-surface-area α-al2o3 at high temperatures 387. When CeO2 is used as dopant of the Al2O3, CeAlO3 species are formed. This species completely inhibits the growth of filamentous carbon, although amorphous carbon is still deposited (Figure 24) 380, 383, 387. It has been suggested that these species suppress the growth of this filamentous carbon by 97

98 chemical blocking rather than by gasifying them after they have been deposited in the catalyst 387. Figure 24. Carbon deposition models in the steam reforming of hydrocarbons over Ni/Al2O3 and Ni-Ce/Al2O3 catalyst: a) not doped; and b) doped with Ce (adapted from J. Catal. 2005, 234 (2), , with permission from Elsevier). Finally, ZrO2 is also able to enhance carbon deposition resistance 25, 335, 362, , 368. This promoter enhances the dissociation of CO2, forming oxygen intermediates near the contact between ZrO2 and Ni. These intermediates increase the rate of gasification of the carbon deposits 362. In addition, ZrO2 has both basic and weak acidic sites, which improves its resistance to carbon deposition 326, 367. However, the main interest in ZrO2 seems to be its use in combination with CeO2, since ZrO2 enhances the oxygen storage capacity and oxygen ion mobility of CeO2. This enhancement of CeO2 properties results in an improvement of the resistance against carbon deposition 335, 337, , 367, 369, Regeneration of Catalysts Deactivated by Carbon Deposition Under certain conditions catalyst deactivation due to carbon deposition can be inevitable even in the most resistant catalysts. For this reason the regeneration of the 98

99 catalysts is extremely important to maximize the benefit obtained from them 9, 13. Catalysts deactivated by carbon deposition can be regenerated using different gasifying agents (in order of gasification rate): oxygen, steam, carbon dioxide or hydrogen. The reactions involved in the regeneration processes with these gasifying agents are shown below 9, 13, C + O2 CO2 C + H2O H2 + CO C + CO2 2CO C + 2H2 CH4 As discussed above, different types of carbon can deposit on the catalyst surface 13. Thus, they will behave differently during the regeneration. The carbons formed on Ni catalysts involved in reactions with hydrocarbons can be monoatomic carbon, polymeric amorphous films, vermicular fibres or whiskers, nickel carbide and graphitic films 9, 13. Both, preparation of the catalyst (metal loading, calcination temperature, particle size, use of promoters) and reaction conditions (temperature, H/C ratio, presence of carbon precursors) can affect the type and amount of carbon deposited. Due to the differences in reactivity between the different types of carbon deposits, different conditions should be applied. Thus, less ordered and more reactive carbons (monoatomic carbon or amorphous) need lower temperatures and weak gasifying agents (about 400 C in H2 or H2O) whereas graphitic carbon needs higher temperatures and strong gasifying agents ( C in air) 9, 392. However, as O2 is the strongest and cheapest gasifying agent, in industry catalysts are usually regenerated in air at about 600 C 9. Although the catalytic activity can be recovered almost completely under certain conditions, catalysts lose activity after each recovery cycle due to different reasons 13, 394. For 99

100 example, regeneration in air is a very exothermic process that can lead to hot spots. These hot spots can lead to metal reorganization or sintering, thus deactivating the catalysts in the attempt of recovering the catalytic activity lost due to carbon deposition 9, 13, 392. The main reasons for losing catalytic activity during regeneration are: Loss of metal particles that were pulled out from the support due to the formation of carbon filaments 9 Oxidation of the metals 9, 13, 26, 390, 395. Although it can be reversed by subsequent reduction of the catalyst, it sometimes gives rise to the irreversible formation of different structures that can be inactive. 13, Sintering9, 26, 393, Damage of the support 328, 400. However, in the same way that the addition of promoters or the formation of alloys can enhance catalytic activity and resistance against carbon deposition, the regeneration can be positively affected by the presence of these promoters 27, 361 and alloys , 328, 395. Thus, the presence of small amounts of noble metals can improve reducibility of the main metal , 340. Moreover, in some cases, after a few regeneration cycles the performance of the catalyst can be enhanced 325. These strategies for improving carbon resistance can help to facilitate carbon removal during regeneration of the spent catalyst. Table 6. Strategies to minimize carbon deposition in catalysts Strategy Catalysts Process Ref. Sulfur passivation Ni(S) Steam reforming of CH4 100

101 Ni(S)/Al2O3 CO2 reforming of 316 Ni(S)/MgAl2O4 CH4 CO2 reforming of 316 CH4 Dehydrogenation of isobutane 323 Bimetallic catalysts Ni-Au/MgAl2O4 Ni-Co/Al2O3 Ni-Co/CeO2-Al2O3 Steam reforming of n-butane Steam reforming of glycerol CO2 reforming of , Ni-Co/MgAl2O4 CH4 CO2 reforming of 324, 327 Ni-Co/MgO-ZrO2 CH4 CO2 reforming of 326 Ni-Cu/Al2O3 CH4 CO2 reforming of 331 Ni-Cu/SiO2 CH4 CO2 reforming of 329 CH4 101

102 Ni-Cu/MgO-SiO2 Ni-Cu/CaO-SiO2 Ni-Cu/ZnO-Al2O3 Ni-Mn/MgAl2O4 Steam reforming of ethanol Steam reforming of ethanol Steam reforming of methanol Steam reforming of ethanol CO2 reforming of Ni-Mo/Al2O3 Ni-Mo-K2O/Al2O3 Ni-Mo/Al2O3 Ni-Pt/Al2O3 CH4 Steam reforming of gasoline CO2 reforming of propane Steam reforming of gasoline CO2 reforming of Ni-Re/Al2O3 Ni-Rh/SiO2 CH4 Steam reforming of gasoline CO2 reforming of CH4 102

103 Ni-Rh/CeO2-Al2O3 CO2 reforming of 49, 338 Ni-Rh/CeO2-ZrO2 Ni-Sn/YSZ Co-Rh/CeO2-ZrO2 Co Rh/SiO2 Co Ru/SiO2 CH4 Steam reforming of ethanol Methanation of carbon dioxide Steam reforming of methane Steam reforming of propane Steam reforming of isooctane Steam reforming of ethanol Oxidative steamreforming of ethanol Oxidative steamreforming of ethanol , , , Promoters Ni-K2O Water gas shift

104 Ni-CaO/Al2O3 CO2 reforming of 346, 351, 360, 374 Ni/CeO2-Al2O3 CH4 CO2 reforming of 348, 353, 370, 387, 389 Ni-K2O/Al2O3 CH4 Steam gasification of polypropylene Steam reforming of propane Oxidative reforming of hexadecane CO2 reforming of , Ni/La2O3-Al2O3 CH4 CO2 reforming of 22, 27 Ni/CeO2-ZrO2-Al2O3 CH4 Steam reforming of ethanol Steam reforming of propane Steam reforming , 397 of CH4 104

105 Ni/CeO2-La2O3-Al2O3 Ni-Li2O/Al2O3 Oxidative reforming of hexadecane CO2 reforming of Ni-MgO/Al2O3 Ni-MnO/Al2O3 CH4 CO2 reforming of CH4 Steam reforming of ethanol Steam gasification of polypropylene CO2 reforming of 22, , 365, Ni-Na2O/Al2O3 CH4 CO2 reforming of 346 Ni/ZrO2-Al2O3 CH4 CO2 reforming of 362 Ni/ CeO2-ZrO2 CH4 Methanation of carbon dioxide Autothermal reforming of isooctane 337,

106 Ni-CaO/La2O3 CO2 reforming of 384 Ni-SrO/La2O3 CH4 CO2 reforming of 384 CH4 Partial oxidation 367 Ni-MgO/SiO2 of CH4 CO2 reforming of 359 Ni-MnO/SiO2 CH4 CO2 reforming of 378 Ni-ZrO2/SiO2 CH4 CO2 reforming of 378 Ni-K-Ca/ NaZSM-5 CH4 CO2 reforming of 361 Ni-MgO/Zeolite HY CH4 CO2 reforming of 375 Ni-MnO/Zeolite HY CH4 CO2 reforming of 375 Ni-Co/CeO2-Al2O3 CH4 CO2 reforming of 379 Ni-Co/MgAl2O4 CH4 CO2 reforming of 324, 327 CH4 106

107 Ni-Co/MgO-ZrO2 CO2 reforming of 326 Ni-Cu-MgO/SiO2 Ni-Cu-CaO/SiO2 Ni-K/CeO2-Al2O3 CH4 Steam reforming of ethanol Steam reforming of ethanol CO2 reforming of Ni-Mo-K2O/Al2O3 Ni-Rh/CeO2-Al2O3 CH4 CO2 reforming of propane CO2 reforming of , 338 Ni-Rh/CeO2-ZrO2 Co/CeO2-ZrO2 CH4 Methanation of carbon dioxide Steam reforming of ethanol CO2 reforming of Co-Rh/CeO2-ZrO2 Pt/CeO2-Al2O3 CH4 Steam reforming of ethanol Oxidative reforming of hexadecane

108 Pt/ZrO2-Al2O3 CO2 reforming of 368 CH4 Partial oxidation 368 Pt/CeO2-ZrO2 Pt/CeO2-La2O3-Al2O3 Rh/CeO2-ZrO2 of CH4 Partial oxidation of n-tetradecane Oxidative reforming of hexadecane Partial oxidation of n-tetradecane Strategies against sulfur poisoning Sulfur containing molecules are frequent impurities in fuels and oil-derived feedstock. These impurities, even at very low concentrations, are responsible for heterogeneous catalyst deactivation. Millions of dollars are lost in chemical and oil industries as a result of sulfur poisoning Generally speaking, two different approaches have been intensively studied to face this problem. The first consists of the sulfur removal from the fuel via hydrodesulfurization and involves a thoughtful catalyst design to achieve high efficiencies (see section 5). In industrial reactors, sulfur is removed to levels below 0.1 ppm by a multiple step process, finishing with adsorbents normally based on ZnO However, a balance has to be struck between cost, convenience and effectiveness, and significant savings can be made if higher levels of sulfur can be tolerated. In this sense, the second strategy is to develop sulfur- 108

109 tolerant catalysts able to operate in sulfur-rich reaction mixtures 78. This second approach is in line with the aim of this review, which is to provide an overview of the current status in carbon and sulfur tolerant systems. At this point, a brief reiteration of the fundamental basis of sulfur poisoning given in detail in section 4 may help to understand the developed strategies. Sulfur poisoning takes place due to sulfidation of the active catalytic species, namely metallic particles and/or metal oxides 13. In the case of a metallic particle (Me) and considering H2S as the source of sulfur the process could be simplified as follows: Me 0 + H2S MeS + H2 At high T its effect should decrease, because sulfidation is thermodynamically unfavored. However, its kinetics is favoured and the result can be different to the expected, depending on the metal used (for example, with Ni ΔG is not positive even at 1000 C). Sulfur as a poison causes a multifold effect in the catalytic activity. Firstly, sulfur adsorption physically blocks the catalyst active sites limiting accessibility for the reactants and reducing the probability of reactant molecules encountering each other. Secondly, by virtue of its strong chemical bond it electronically modifies the neighbour metal atoms thus modulating their ability to adsorb and/or dissociate reactant molecules 9. In addition, the catalyst surface could be reconstructed due to the strong chemical adsorption. Finally, the presence of strongly bonded sulfur species on the surface of the catalyst hinders the diffusion of both product and reactant species. Figure 25 schematizes the multiple effects caused by sulfur in a metal supported catalyst. 109

110 Electrons withdrawal M A S Site blockage M Hindered reactants encounter S B S A S M M support limited products diffusion M C A M B Figure 25 Simplified representation of the multifold poisoning effect due to sulfur chemisorption (M represents an active metal. A and B represent reactant molecules and C the reaction product) In this scenario, catalyst deactivation must be overcome and/or the poisoned catalysts must be regenerated. It must be always kept in mind that the degree of poisoning depends on the studied reaction, process conditions and the involved catalysts, among other factors. Consequently, a specific catalyst and/or strategy is required for each process. In response to these needs, intensive research has been carried out in the field of heterogeneous catalysis in the last decades generating a wide variety of multicomponent catalysts with different natures and different features aimed at sulfur poison mitigation. Herein, a summary of the most conventional approaches and proposed materials are discussed Noble metal-based catalysts Nickel-based catalysts are still the most preferred materials for reforming reactions due to their good performance, low cost, relatively simple preparation and wide availability However, apart from the well-known Ni deactivation due to sintering and carbon deposition, this metal is one of the most sensitive active phases towards sulfur poisoning The chemical equilibrium of sulfidation at 900 C for Ni is much more favourable 110

111 compared to the values obtained for Ru, Pt, Rh or Co underlining that Ni is the most sulfur sensitive metal among the conventional reforming active phases 13. In this sense, the use of noble metals based catalysts is a good choice from an activity and sulfur tolerance point of view, although the cost must be considered Mono and bimetallic Pt-based catalysts developed by Farrauto et al. were stable under continuous operation when exposed to sulfur-containing streams in reforming reactions 411. Pt/CGO was successfully employed in the steam reforming of isooctane to produce hydrogen demonstrating complete sulfur tolerance 413. In this study the authors compared the performance of this material with a similar Ni/CGO and a conventional Pt/Al2O3. Only Pt supported on ceria resisted the effect of sulfur. The latter indicates that not only the active phase matters but also the support plays a crucial role in sulfur resistance. Apparently, the Pt atoms in the Pt/CGO are more electron-deficient than Pt atoms in Pt/Al2O3 limiting the interaction with S species 413. However, Pt tolerance towards S poisoning also depends on the considered reaction. For example, in the WGS reaction many high performance Pt based catalysts suffer from severe deactivation when exposed to sulfur 9, Furthermore, this adverse effect seems to be proportional to the amount of sulfur. For example for a Pt/ZrO2 catalyst, Xue and co-workers reported that the conversion went from 44% in the absence of H2S to 25% (50 ppm H2S) to 14 % (200 ppm H2S) and finally 12% when 1000 ppm H2S were introduced into the reactant mixture 414. Not only the considered reaction, but also the nature of the noble metal influences the sulfur tolerance capacity of the catalysts. In other words, not all the noble metals exhibit the same sulfur resistance. For example, McCoy et al. and Azad et al. demonstrated in different papers that Rh is remarkably less sensitive than Pd towards sulfur poisoning The combination of metals (Rh-Pd) enhanced the tolerance, conserving high and stable 111

112 conversion during 12 h of reaction (50 ppm of H2S were used as a sulfur source). The unsuitability of Pd for sulfur tolerance was also shown in the work of Goud et al. 25 Their results show the deactivation of a Pd/ZrO2 catalyst on the reforming of hexadecane after a few hours of operation. The inefficiency of Pd was also evidenced in the WGS this time using ceria as a support and SO2 as a source of S 423. As mentioned above sulfur poisoning can be envisaged as a steric and an electronic effect. From the electronic point of view, sulfur ligands withdraw electron density from the metals. For instance, the differences among Rh, Pt and Pd can be explained in terms of electronic effects. Theoretical calculations for model clusters S/M12 (M = Rh, Pt and Pd) indicate that the tendency of a metal to lose d electrons increases in the following order: Rh < Pt < Pd 9, 103, 424. This agrees well with the relative occupancy of the d shell in the isolated elements: Rh: d 8 s 1 < Pt d 9 s 1 <Pd d 10 s 0. This tendency correlates with the decrease of density of states around the Fermi level for these elements (25 % reduction for Rh, 50 % for Pt and approximately 55 % for Pd). According to the latter, and strictly considering electronic effects, Pd is the most vulnerable to sulfur poisoning among the three mentioned noble species, in good agreement with the observed behaviour in reforming reactions 425. In summary, noble metals may constitute an alternative to mitigate the sulfur poisoning effects in heterogeneous catalysts. Nevertheless, there is no guarantee that these precious metals will completely tolerate sulfur and indeed they frequently fail depending on the reaction conditions and the sulfur concentration. In addition, the nature of the noble metal is a factor to take into account. In this sense, Rh seems to be one of the most promising Alloys, bimetallic and promoters 112

113 Many efforts have been made aiming to improve the sulfur tolerance capacity of the traditional Ni based catalysts for reforming reactions 12. The use of promoters and bimetallic combinations (whether alloys or not) have been a frequent strategy in recent years. For example, Xie et al. investigated the behaviour of Ni, Rh, and Ni-Rh supported on CeO2-Al2O3 catalysts in the steam reforming of hydrocarbons, introducing sulfur into the reactant mixture None of the Ni-containing catalysts was stable to sulfur-laden mixtures, although the Ni-Rh catalyst requires more time before deactivation; over 60 h on-stream. Moreover enhanced carbon deposition due to sulfur was observed, especially for Ni-based materials, but also noble metal combinations, for example in a commercial Pt-Rh/ZrO2 catalyst for the steam reforming of ethanol/gasolines 428. A small amount of sulfur (5 ppm) was enough to deactivate this catalyst after 22 h on stream. Other Ni based bimetallic combinations have been tried. For example, Wang et al. carried out screening of catalysts for liquid hydrocarbon reforming using Ni-Mo, Ni-Co and Ni-Re supported on Al2O3 and introducing 20 ppm of sulfur in the reactant mixture 429. All the bimetallic samples exhibit superior performance to the primary monometallic Ni with Ni-Re/Al2O3 being the most active sample. Indeed, this Ni-Re/Al2O3 sample showed an outstanding performance maintaining hydrocarbon conversions around 90% during a 300 h test in a sulfur-containing stream and at relatively low reforming temperatures (580 C). A similar positive effect due to the addition of Re in a Ni/Zeolite ZSM5 system was reported elsewhere highlighting the ability of Re to mitigate sulfur poisoning 430. In addition, Re can be employed not only as a part of bimetallic systems but also as a promoter of an already active catalyst. For example, Murata et al. developed a very active Ni/Sr/ZrO2 catalyst but with poor sulfur tolerance 431. In order to improve sulfur resistance a series of dopants were added including Re, La, Nd, Sm, Ce, Yb, Eu and Mo. Among all the dopants only La and 113

114 especially Re enhanced sulfur tolerance. Actually the best sample in this study was Ni- Sr/ZrO2 with 5 wt% Re, which was able to remain stable during 30 h processing a commercial premium gasoline. It can be argued that rhenium seems to be the most promising metal to diminish Ni sulfur poisoning with the extra benefit of enhanced catalytic activity, although the exact mechanism ascribed (sulfur tolerant alloy formation or sacrificial phase) varies between different studies. Some other traditional bimetallic systems are Ni-Mo and Ni-W. As indicated in the paper of González et al., the addition of Mo and W to Ni-based catalysts reduces deactivation in steam reforming 432. The idea is to use Mo as a sacrificial agent given its facility to be sulfidized. In the presence of any sulfur species Mo would tend to form MoS2, Ni atoms would not be affected and so in principle the active sites should be available. Indeed, the electronic interaction between Ni and Mo in the Ni-Mo ensemble increases Mo electron density easing its interaction with electronegative ligands such as S 78. In other words, Ni promotes the formation of MoS2 and in some particular applications, for example hydrodesulfurization reactions, Ni is considered a promoter while Mo is the metal that carries out the sulfur removal. In reforming reactions the classic paper of Bartholomew proved that a Ni-catalyst doped with Mo was more sulfur resistant than the Ni catalyst alone in a feed containing 10 ppm sulfur 433. The combination of an active metal for reforming reactions such as Ni or Pt with Sn is another widely explored alternative 16, Dumesic et al. obtained very promising results in hydrogen production from biomass reforming using Ni-Sn catalysts 434. In principle, bimetallic Ni-Sn phases were designed to avoid Ni deactivation due to C deposition. As proposed by Trimm the similar electronic structure of carbon and elements of groups IV and V of the periodic system may favor the interaction of these metals with Ni 3d electrons 114

115 thereby reducing the chance of nickel interactions to carbon 16. Further, as explained by Rodriguez and Hrbek, the addition of tin to platinum is a good strategy to prevent sulfur poisoning 78. Tin may act as a site blocker to platinum avoiding the noble metal interaction with sulfur and improving the stability of the reforming catalysts 78. Tin and platinum form well defined alloys that are very stable 103. When compared to pure Sn and Pt, these alloys exhibit a lower chemical reactivity towards sulfur-containing species such as SO2, H2S, S2 and thiophene Figure 26 adapted from Rodriguez s paper underlines the superiority of the Pt-Sn alloy in terms of sulfur uptake compared to the monometallic systems. Among the typical site blockers (Cu, Au, Ag, Zn, Sn) tin is the best choice to promote sulfur tolerance of Pt based catalysts 78. The electronic perturbations arising from the Pt-Sn bond produce a system which has remarkably low reactivity towards sulfur poisoning 78. Other types of bimetallic systems and alloys involving noble metals have been proposed aiming to gain sulfur resistance Bimetallic Pt-Pd and Pt-Ni catalysts were significantly higher sulfur tolerant compared to the monometallic Pt based catalysts during 50 h of a stability test 440. A commercial catalyst from BASF based on Pt-Rh was also tested for the ATR of JP The addition of 125 ppm of sulfur in the stream slightly deactivated the catalysts on the first 250 h of operation. A more demanding stability test based on startup/shutdown operations strongly affected the catalysts performance with these series of start/stop cycles the main reason for the catalysts deactivation. 115

116 Total sulphur coverage (ML) 0,40 Sulphur uptake K 0,35 Polycrystalline Sn 0,30 0,25 0,20 Pt (111) 0,15 Sn/Pt (111) 0,10 0,05 0, SO 2 exposure (L) Figure 26. Total sulfur uptake for the adsorption of SO2 on polycrystalline Sn, Pt(111), and a Sn/Pt(111) alloy. Adapted from Acc. Chem. Res. 1999, 32 (9), Copyright 1999 American Chemical Society. As mentioned in the previous section, among the noble metals Pd seems to be the least sulfur tolerant. Nevertheless, bimetallic combinations also open up a route to improve Pd-based catalysts sulfur resistance 78. Metal-metal interactions reduce the electron donor capacity of Pd limiting its tendency to form strong bonds with sulfur-like ligands 443. In particular, Pd-Rh, Pd-Ni and Pd-Mn may present a good catalytic behaviour and be notably less sensitive to the presence of sulfur-containing molecules in the reactant mixtures than pure Pd 78. Briefly, it can be concluded that most of the bimetallic systems proposed in the literature exhibit superior performance (higher catalytic activity and enhanced carbon and sulfur resistance) compared to their individual counterparts. Several reasons account for the positive results obtained with the bimetallic materials: (i) a change in the number of active sites (cooperative effects); (ii) the sacrificial role played by one of the species forming 116

117 the bimetallic system leaving free and available the second metal; (iii) an electronic effect coming from the metal-metal interactions resulting in less sensitive materials towards sulfur poisoning (the bimetallic bonding modifies the chemical reactivity of the metal towards sulfur-containing molecules, "ligand effect") The addition of promoters is an alternative to the bimetallic systems. Special attention has been devoted to alkali metals in this regard. Due to their electropositive behaviour they can easily donate electrons to sulfur ligands thus shielding the interaction between sulfur species and the actual active phase of the catalyst. Apart from the electronic effect, these types of promoters may act as a site blocker species, physically hindering the arrival of sulfur to the catalytic active centre. Ferrandon and co-workers demonstrated that the addition of potassium to a Rh/Al2O3 catalyst in gasoline steam reforming appreciably increased sulfur tolerance 444. They pointed out that sulfur adsorption on the Rh/Al2O3 was limited due to site blockage attributed to K. In turn, they found a drawback: alkali inclusion increased the temperature in the catalyst bed by inhibition of the endothermic steam reforming reaction more than the partial oxidation processes. At the same time, this effect enhanced the sulfur tolerance beyond the initial expectations when K was intended to be a mere sorbent since the stability of sulfide species decreases with temperature Support and structural modifications So far all the discussed approaches are focused on the metallic active phase of the catalysts. However, similarly to SOFC anodes, conventional catalysts are composed of metal/oxide mixtures and therefore the role of the support and its behaviour towards sulfur poisoning should not be disregarded. Indeed, on the surface of a metal oxide sulfur can interact with the metal oxygen sites, producing species that have different electronic properties (i.e. sulfides and sulfates) and maybe responsible for catalyst deactivation. 117

118 In this sense, one of the most widely used strategies to alleviate sulfur poisoning is to select supports with high oxygen mobility 13. It is well established that oxygen mobility mitigates the carbon deposition which can accompany sulfur poisoning and presumably helps avoid the formation of inactive metal sulfides. As mentioned in previous sections, ceria is one of the most desirable supports when oxygen mobility is required In this way, a rather sulfur tolerant catalyst was developed by Xue et al. using Pt supported on alumina impregnated with ceria and gadolinia 450. In this report the Pt/CGO-alumina catalysts were compared vs. a conventional Pt/Al2O3 sample. Only the ceria based materials resulted in immunity to sulfur attack, with significant differences depending on the order of addition of ceria and gadolinia. Interestingly, the sample where the ceria was impregnated first was the most stable, which the authors ascribe to an improved Pt-CeO2 interaction. This catalyst presented good activity in commercial-gasoline reforming with relatively high sulfur concentration ( ppm provided by thiophene). The authors argue that Pt possesses different electronic properties when supported on bare alumina compared to the ceriaalumina based support. Pt metallic sites in alumina are unable to resist sulfur poisoning. A valuable point of this paper is the redox mechanism that the authors proposed for sulfur elimination. Under steam reforming conditions, thiophene was transformed to H2S which is released and eliminated from the cycle via reduction and re-oxidation of the ceria-doped support 450. Azad and Duran obtained also some interesting results using Rh/CeO2 based materials 420. In this work, the presence of 50 ppm of H2S activates the catalysts increasing H2 yields in the steam reforming of toluene. They suggested that such positive effect could be due to the formation of Ce2O2S which presumably promotes the activity of the supported Rh. Actually, in this particular situation ceria is acting as a sulfur sorbent and the redox properties of ceria are useful since the reduced oxide (Ce2O3) is more prone to trap sulfur. 118

119 It is well known that the redox properties of ceria can be boosted by the use of promoters resulting in materials with enhanced oxygen storage capacity 196, 198, 453. Laosiripojana et al. investigated the steam reforming of biomass tar using Ni-Fe supported on MgO-Al2O3, coated with CGO 454. The results indicated that the formation of various Ce-O- S phases influences the catalytic activity with the sulfates having a positive effect in the oxygen mobility and therefore increasing the activity and the sulfides producing an activity drop. Some other examples using Pd/CeO2 samples and CuO and Y2O3 as metal oxide additives benefit the reforming performance. These dopants increase H2 yield due to an increase in metal surface area available for reaction. In addition, CuO increased the stability against sulfur poisoning due to the oxide acting as a sacrificial sulfidation site, taking the sulfur species away from the active metal and/or the ceria support 421. Some groups proposed other types of support modifications in order to improve sulfur tolerance. For instance, incorporation of the active metal into the crystal structure of the oxide phase, followed by exsolution of metal particles on reduction with the aim of stabilizing the particles and at the same time increasing metal dispersion. Smaller, more stable particles should improve sulfur tolerance since the sintering of Ni particles leads to larger crystallites that are more easily poisoned 76, 455. For example, Ni particles can be stabilized on hexaaluminate structures Smith et al. prepared nickel hexaaluminate dispersed on zirconia doped ceria catalysts obtaining rather good sulfur tolerance in the partial oxidation of methylnaphthalene 458. Pyrochlore-like structures are also interesting to avoid sulfur poisoning. Pyrochlores are a class of ternary metal oxides based on the fluorite structure with a cubic unit cell with a general formula of A2B2O7. An important property of these materials is that catalytically 119

120 active noble metals can be substituted isomorphically on the B site to form a crystalline catalyst 457. In particular, metals like Ru, Rh and Pt can be introduced into the B site of the pyrochlore structure because they meet ionic radius constraints and have the required oxidation state. In this situation, the metal is included in the solid network and somehow protected towards sulfur species. The group of Spivey has done intensive research on this type of materials 457, For example, they found that a La/Sr/Zr/Ni-pyrochlore loses some activity with 50 ppm of dibenzothiophene at the initial stages of the reforming reactions. However, the deactivation was not continuous with time on stream. This suggests that the poisoning species are adsorbed on the catalyst surface in the initial steps but are not accumulated on the surface itself. In addition, almost complete activity was recuperated when sulfur was removed from the stream. Similar results were obtained when Rh instead of Ni was introduced in the pyrochlore lattice 462. The pyrochlore structure, although it experienced some deactivation, was more tolerant to sulfur compared to a reference Rh/Al2O3 catalyst. In a similar way to the carbon tolerance section, Table 7 summarizes the developed catalytic formulations following a given strategy to mitigate sulfur poisoning. Table 7. Strategies to minimize sulfur poisoning in catalysts 120

121 Strategy Catalysts Process Ref. Bimetallic catalysts Partial oxidation Ni-Co/MgO-Al2O3 Ni-Co/ Al2O3 Ni-Mo/ Al2O3 Ni-Re/ Al2O3 Ni-Fe/MgO-Al2O3 Ni-Rh/ CeO2-Al2O3 Ni-Sn/MgO-Al2O3 Ni-Sn/CeO2-MgO-Al2O3 reforming of isooctane Steam reforming of liquid methylcyclohexane Steam reforming of liquid methylcyclohexane Steam reforming of liquid methylcyclohexane Steam reforming of gasoline Partial oxidation reforming of isooctane Steam reforming of liquid hydrocarbons Steam reforming of glycerol Steam reforming of glycerol ,

122 Rh-Pd/ Gd2O3-CeO2 Rh-Pd/ ZrO2-CeO2 Rh-Pt/ ZrO2 Rh-Pt/ Gd2O3-CeO2 Rh-Pt/ ZrO2-CeO2 Ni-Re/SrO-ZrO2 Pt-Pd/ Al2O3 Pt-Rh/ Al2O3 Steam reforming of toluene Steam reforming of toluene Steam reforming of ethanol/gasoline Steam reforming of toluene Steam reforming of toluene Autothermal reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons

123 Pt-Rh/ La2O3-Al2O3 Rh-Pt/ SiO2 Rh-Pt/ ZrO2-CeO2-SiO2 Rh-Pt/ TiO2 Rh-Pt/ MgO-TiO2 Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Promoters Ni-Co/MgO-Al2O3 Ni-Fe/MgO-Al2O3 Partial oxidation reforming of isooctane Partial oxidation reforming of isooctane

124 Ni-Sn/MgO-Al2O3 Ni-Sn/CeO2-MgO-Al2O3 Pt/ Gd2O3- CeO2 Pd/ Gd2O3-ZrO2-CeO2 Pd-Y2O3/ Gd2O3-ZrO2-CeO2 Pd-CuO/ Gd2O3-ZrO2-CeO2 Rh/ Gd2O3-CeO2 Rh/ ZrO2-CeO2 Rh-CuO/ Gd2O3-CeO2 Rh-CuO/ ZrO2-CeO2 Rh-Pt/ Gd2O3-CeO2 Steam reforming of glycerol Steam reforming of glycerol Steam reforming of isooctane Steam reforming of toluene Steam reforming of toluene Steam reforming of toluene Steam reforming of toluene Steam reforming of toluene Steam reforming of toluene Steam reforming of toluene Steam reforming of toluene , , , , ,

125 Rh-Pt/ ZrO2-CeO2 Rh-Pd/ Gd2O3-CeO2 Rh-Pd/ ZrO2-CeO2 Ni-Rh/ CeO2-Al2O3 Ni/ CeO2-Al2O3 Rh/ CeO2-Al2O3 Ni/CeO2-ZSM-5 Ni/SrO-ZrO2 Ni-Re/SrO-ZrO2 Ni/La2O3-SrO-ZrO2 Steam reforming of toluene Steam reforming of toluene Autothermal reforming of liquid hydrocarbons Steam reforming of toluene Steam reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons Steam reforming of liquid hydrocarbons 420,

126 Pt-Rh/ La2O3-Al2O3 Rh-Pt/ ZrO2-CeO2-SiO2 Rh-Pt/ MgO-TiO2 Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Autothermal reforming of liquid hydrocarbons Support modification Pt/CeO2-Gd2O3 Ni-Fe/MgO-Al2O3-CGO commercialgasoline reforming Steam reforming of biomass tar Pd/CeO2-MOx (M = Cu and Y) Jet fuel reforming 420 Ni/hexaaluminates La/Sr/Zr/Ni-pyrochlore partial oxidation of methylnaphthalene Several reforming processes Regeneration of sulfur poisoned catalysts Since most of catalysts are expensive and industrial productivity in many cases depends on the catalysts performance, there is a need to reactivate or regenerate them. 126

127 Although the regeneration method is rather catalyst-specific, generally, they involve thermal treatments in oxygen, hydrogen or steam atmospheres. As indicated by Bartholomew the toxicity of the sulfur species depends on how many electron pairs are available for the interaction with the metals 9. In general toxicity decreases as follows: H2S > SO2 >SO4 2- etc. in the order of increased shielding by oxygen. Therefore oxidation treatment to eliminate sulfides is an alternative to recover activity. Ideally, the main goal of oxygen treatments is to remove all the sulfur species as SO2 (Ssolid + O2 gas SO2 gas) at high temperature. For example, Choudhary et al. managed complete activity recovery of a Ni-ceria based catalyst after being exposed to 7400 ppm of thiophene by thermal treatment at 800 C in an O2/N2 50:50 mixture 463. An inherent drawback of this procedure is the oxidation of the active phase (Ni) during the recovery thus making necessary a reduction step before re-running the reaction. Apart from the active phase oxidation, this type of oxidative treatment involves other disadvantages that limit its application and therefore it cannot be considered as a general regeneration procedure. More precisely, the exothermicity associated with this process may produce irreversible catalyst deactivation by thermal degradation and/or phase transformation of the active components 13. For instance, some authors reported irreversible formation of the inactive NiAl2O4 spinel when they tried to re-activate a Ni/Al2O3 reforming catalyst using diluted oxygen at high temperatures 396. In this scenario, oxidative treatment is only useful in some specific cases when the oxidation at high temperatures would not risk modifying the catalyst structure. Alternatively to oxygen, thermal treatments in steam can be applied to re-activate sulfur contaminated catalysts. One of the seminal works in this area was carried out by 127

128 Rostrup-Nielsen dealing with Ni-based catalysts deactivated upon H2S exposure 464. This indicated that steam can remove sulfur as hydrogen sulfide via: Ni-S + H2O NiO + H2S Although H2O may produce also some oxidation of reduced Ni: Ni + H2O Ni-O + H2 At temperatures between 800 and 900 C up to 90% of the sulfur can be removed from the catalyst surface. In the same paper, the positive role of alkali promoters such as Ca and Mg in the steam regeneration was discovered. The catalysts doped with small amounts of calcium and magnesium were easier to re-activate. In turn, some other dopants like K or Na did not improve the regeneration process, most likely because sulfur is converted into a form that is retained in the catalysts in the presence of K and Na. Complete recuperation of reforming activity for bulk Ni catalysts was found by Hassini et al. using Ar/steam mixtures 465. Regenerated catalysts were characterized by means of XPS indicating complete sulfur removal from the catalyst surface after the steam treatment. Nevertheless, as indicated above, Ni oxidation occurred and some oxidised Ni species were identified by infrared spectroscopy underlining again the risk of altering the catalysts structure when an oxidative treatment is applied. Reducing atmospheres do not present the catalyst oxidation drawback observed when the spent samples are treated with steam or oxygen. In this sense, this alternative is currently viewed as the most desirable way to remove sulfur from catalysts. Typically, sulfur is released as H2S by the direct reaction of adsorbed sulfur species and H2 (Ssolid + H2 gas H2S gas). According to the thermodynamics of sulfide formation, this process is essentially reversing the metal sulfide equilibrium formation

129 H 2 S concentration (a.u.) Cheekatamarla et al. observed complete regeneration of a molybdenum carbide catalyst deactivated upon exposure to 500 ppm of benzothiophene using a sequential thermal treatment: first one hour in He and later one hour in hydrogen at 900 C for both processes 466. The authors claimed that the heating step in He may remove weakly adsorbed sulfur species while for the chemisorbed species (likely forming metal sulfides) heating in hydrogen was required. The effectiveness of the hydrogen thermal treatment depends as expected on the sulfur concentration used in the catalytic test. For example Hepola et al. 318, 467 demonstrated through temperature programmed hydrogenation (TPH) that complete sulfur removal from a commercial Ni based catalysts was achieved when 500 ppm of H2S was used. On the contrary, when the H2S concentration was increased up to 2000 ppm the hydrogen treatment was not sufficient to eliminate all the chemisorbed sulfur. Figure 27 represents the TPH profiles discussed in ref 318. It is clear that 2000 ppm provoked a strong adsorption of sulfur on the catalyst s surface making complete sulfide removal almost impossible. 500 ppm H 2 S 2000 ppm H 2 S Temperature ( o C) 129

130 Figure 27. TPH profiles (70% Ar/30% H2) of Ni based catalysts after exposure to 500 and 2000 ppm of H2S in N2 at 2 MPa, 900 C and 4-6 h. (reprinted from Appl. Catal. B 1997, 14 (3-4), , with permission from Elsevier) Other reducing mixtures have been successfully employed to regenerate sulfur poisoned catalysts. For example Arosio et al. demonstrated that CH4-reductive pulses can partially recuperate a sulfur-contaminated Pd/Al2O3 catalyst spent in methane combustion 468. A small increase of the temperature up to 600 C using short time pulses (2 min.) gave almost complete catalyst regeneration. Such a treatment combines extensive sulfate decomposition with a PdO reduction/oxidation cycle. The authors claimed that the reductive regeneration of sulfur-poisoned catalysts with CH4-containing atmospheres could be more effective than the analogous treatment in H2, possibly due to the milder reducing action resulting in minor formation of sulfide species on the catalyst surface 468. In summary, there are routes to regenerate sulfur-poisoned catalysts via thermal treatments in different atmospheres. However, the success of this process depends on several factors such as sulfur concentration, strength of sulfur interaction with the catalyst surface, catalyst composition and its susceptibility to be affected by the recovery treatment, etc. This complex situation makes necessary a careful choice of the thermal treatment and may not ensure complete activity recuperation. 9. Conclusions and Perspectives Solid oxide fuel cells (SOFCs) generate electricity and heat electrochemically from hydrogen and/or carbon-based fuels. The electrodes in SOFCs need to exhibit electronic conductivity, oxygen ion conductivity and catalytic activity. The fuel oxidation takes place at the anode, where the deactivation by carbon and/or sulfur is one of the key challenges in SOFC technology. 130

131 We have reviewed the approaches used in catalysis to prevent or minimise the effects of carbon or sulfur on catalysts. Carbon and sulfur poisoning are much more challenging in conventional catalysis since, as opposed to SOFCs, normally there is no oxygen flux that could help to minimise their deleterious effect. Some strategies have been shown to work in both catalysis and SOFCs, we can have confidence that the effect is real and the basic knowledge is in place to expand or refine those strategies. It is clear that the search for carbon and sulfur tolerance in catalysts and solid oxide fuel cells is exemplified by the properties of one element, nickel. Its unrivalled propensity to catalyse carbon-carbon bond formation is matched by superiority to other base metals in a variety of other useful reactions. It is also extremely vulnerable to the electron withdrawing effects of sulfur. The search, then, has focused on two different goals first to mitigate carbon deposition and sulfur poisoning in nickel-based catalysts, and second to find catalysts which approach the activity (and cost) of nickel without vulnerability towards sulfur poisoning or catalytic activity towards carbon formation. Several strategies to achieve these goals have emerged in the SOFC and catalysis literature. 9.1 Alloying of nickel Alloying of nickel is a strategy that can have an effect on both carbon and sulfur tolerance. Alloying can improve carbon tolerance by reducing the rate of carbon-carbon bond formation, reducing the amount of the most destructive and deactivating graphitic carbon, and/or increasing the rate of competing reactions, such as carbon oxidation. For sulfur tolerance, nickel is the element most vulnerable to sulfidation, so alloying with almost anything improves sulfur tolerance. Conversely, since nickel is an excellent catalyst for many of the reactions in an SOFC anode, alloying may reduce the activity for these reactions. 131

132 This strategy has been implemented in a number of different ways in both catalysis and SOFC studies. The classic example in catalysis is addition of noble metals such as Rh and Au, and these have been used in SOFCs with some success. The use of noble metals in SOFCs is complicated by the larger total amount of metal, meaning that proportionally more of the expensive noble metals need to be used. For this group of elements, the developments in MIECs and non-metal electronic conductors for SOFC anodes may allow more realistic amounts of these metals to be used, and nanoalloys of Ni with Au, Rh or Re may be promising for carbon and sulfur tolerance. In both fields the issue of cost has driven a search for cheaper alternatives. For obvious reasons the top row transition metals from Fe through to Cu have been explored extensively. These seem to be effective in reducing the overall carbon deposition, and decreasing the amount of graphitic carbon. In the case of these promoters, research could switch to other issues affecting SOFCs, for example tolerance to redox cycling or compatibility with electrolytes and other components, as their ability to mitigate carbon deposition seems largely agreed upon. Outside of the top row transition metals, there are some other candidates for carbon and sulfur tolerance, the most promising of which is tin. Tin has been trialled in both catalysis and SOFC anodes and appears to confer both carbon and sulfur tolerance. A further advantage of tin is that the mechanism by which it works is reasonably well known, meaning this is a good target for further testing in terms of long term stability and compatibility. Another promising element is molybdenum, which is widely used in catalysis. It has complicated chemistry, with different carbide, sulfide and oxide phases being stable under possible regimes in an SOFC anode, meaning that further work to clarify its behaviour is 132

133 needed. However, it is potentially a promising electrocatalyst in its own right, so further investigation may be fruitful. 9.2 Alkaline promoters and supports It is well known in catalysis that basic oxides reduce carbon deposition by increasing the carbon oxidation rate. This is thought to work as the basic sites act as stores for highly reactive hydroxyl radicals. The strategy has found use in the SOFC literature, with elements such as Ba and La looking the most promising for further investigations. The use of alkali metals is underexplored compared to catalyst science, due to the higher mobility of these elements, and also their potential for poisoning the catalytic reactions. The vapour pressures of their oxides approaches that of Ni at 1000 C (~10-10 bar) at temperatures ranging from ~800 C for Li2O down to ~500 C for K2O, while the melting points of Na2O (1132 C) and K2O (740 C) are also a concern. The move to intermediate temperature fuel cells may bring at least Na and Li into play. One interesting strategy which currently appears to be unique to the SOFC literature is the use of basic cationic conductors such as Li + and H + conductors for carbon tolerance (although the latter have recently begun to be used as catalysts for the reverse water-gas shift reaction, the link to carbon tolerance has not been made ). These maintain the basicity of the materials promoted solely with simple non-conducting alkali and alkaline earth oxides but add in some extra conductivity to improve both the carbon tolerance and electrochemical performance. In the case of Li + conductors the Li is also stabilised so less volatile. One aspect which needs exploration regarding these basic promoters is their sulfur tolerance, especially regarding their ability to mitigate carbon deposition in a sulfurcontaining gas feed. 133

134 9.3 Ceria, doped ceria and oxygen storage It is fair to say that the discovery of the redox properties of ceria and doped cerias has revolutionised both catalysis and SOFC science, especially given the oxide ion and electronic conductivity of doped cerias. These materials work both by acting as a store for oxygen which is then able to react with carbon species, and by their ability to trap sulfur species. It has also been shown conclusively that doped cerias are both electrocatalysts and catalysts in their own right for important reactions such as electrooxidation of hydrogen and reforming of hydrocarbons. It is clear that doped cerias will continue to be incorporated into the current and next generations of SOFC anodes. With such a useful and varied class of materials there are obviously many fruitful avenues for research. One of the most obvious is the use of the extremely high oxygen storage capacity materials found in three-way catalysts and other catalytic systems. The earliest of these, the Ce-Zr system, has been somewhat investigated, but it does not appear that other ceria-based systems have been used at all in SOFCs. It is also worth noting that although ceria-based oxygen storage materials are favoured because of their relative structural stability on redox cycling, the use of impregnation and MIECs may allow the use of less stable oxygen storage materials. Other possible routes for further investigation include trying to improve the catalytic activity of doped cerias. Current work in SOFCs has focused on the doped cerias with the highest ionic conductivity, but a focus on the activity towards electrooxidation and reforming of hydrocarbons may be useful, especially where ceria is not the only ionic conducting species. Work to improve the sulfur storage capacity could also be important. 9.4 Preferential sulfur binding sites 134

135 Phases which preferentially bind sulfur, and thereby lower the sulfur coverage on Ni or other active metals have been used in both SOFCs and catalysis. In catalysis species such as Cu, Zn and Mo are known to act as sulfur sorbents in preference to Ni, while in SOFCs this effect has been noted in ceria and Ba-containing compounds. In addition, there is literature on sulfur sorbents for gas cleaning which may be useful 405. The deposit of a barrier layer (i.e. the first point of contact with the fuel) that protects the most electrochemically active area of the anode (i.e. close to the electrolyte layer) is known to protect against carbon deposition in SOFCs, but has not yet been investigated for sulfur poisoning. 9.5 Non-metal electronic conductors Removal of the metal electronic-conducting phase solves many of worst effects of carbon and sulfur poisoning, and there are two possible solutions for this. Non-metal conductors in a cermet such as carbides or carbon retain the benefits of cermets, such as the ability to independently optimise the electronic and oxide conducting phases, as well as the disadvantages, such as having to match thermal expansion coefficients and more complicated microstructure optimisation. MIECs lose both the advantages and disadvantages of cermets. Both non-metal conductors and MIECs as potential solutions have deficiencies in different areas. Non-metal conductors are generally under-researched and in particular more work on stability is needed. MIECs are lacking in either electronic or ionic conductivity, and some of the more widely used materials, such as the strontium titanates, require high processing temperatures and are difficult to fabricate into anodes. Both solutions are lacking in catalytic activity and will likely require a further catalytic phase. 9.6 Infiltration of nanoparticles In catalysis, infiltration of porous structures with metal nanoparticles is a common practice to maximise the active surface while simultaneously hindering carbon deposition by 135

136 decreasing the area of graphitic growth. In SOFCs, this approach was first used because the low melting point of copper oxide meant that Cu-YSZ anodes could not be produced by the conventional solid state route. Since then it has been used to add a variety of metals (including nickel) and now has been proved that it can improve important parameters such as the triple phase boundary length. There are many issues to be resolved with infiltration, especially relating to long term stability and feasibility of scaling up the process to industrial-sized anodes, but the reason it is interesting for carbon and sulfur tolerance is that it allows much greater control over the chemistry and structure of the electrode. The exploration of the possibilities in SOFC anodes is only just beginning but already we have seen that the infiltration of barium or ceria allows fine dispersion of the promoter over the surface of the material, enhancing carbon tolerance by ensuring that any given nickel particle is close to a particle of the promoter. In the future we could see more complex oxygen storage materials or more advanced catalysts incorporated into the anode structure by this method. The advances in MIECs and possibilities of non-metal conducting phases such as carbon and carbides should allow designed catalyst nanoparticles (whether containing nickel or not) to be added without their effect being destroyed by alloying into the percolating metal phase. 9.7 Regeneration Catalysts deactivated by carbon deposition are commonly regenerated by stopping the process and then passing a stream of cleaning gas (hydrogen, steam, carbon dioxide or oxygen). All of these gases are eventually present in a SOFC anode: as fuel, as a product of oxidation, as a permeant gas, etc. The literature on regeneration of SOFC anodes is very sparse, and modifications of the anode to aid regeneration are non-existent. Nevertheless, it 136

137 has already been shown that it is possible to remove carbon deposits from Ni-YSZ anodes by a variety of gases, and also by oxygen flux through the electrolyte. Ideally anodes would be designed so that they can be regenerated without use of alternative feedstocks or extensive downtime, but failing this they need to be designed to be regenerated at the minimum temperature for as short a time as possible, and be able to withstand any changes which take place during regeneration. From the catalysis literature it is probable that many promoters which prevent carbon deposition in the first place are also effective in aiding regeneration, whereas for sulfur tolerance where sacrificial phases are used, these might bind more strongly to sulfur, requiring harsher conditions for regeneration. Exsolution of nanoparticles from MIECs and symmetrical SOFCs also provide interesting alternatives to conventional cermet anodes in terms of regeneration. While this potential benefit has been noted in the literature on these materials there is little published experimental work proving it. 9.8 Theoretical and computational studies It has become clear in the last five years that theoretical and computational chemistry is finally becoming able to provide accurate insights into the chemical behaviour of materials and even interfaces 471. Very recently accurate predictions have even been made as to the structure and properties of previously unknown materials relevant to SOFCs 472. A historical problem in SOFC research (and scientific research in general) is the lack of coordination between groups working in different fields. Groups working in fields such as materials chemistry and detailed in situ and ex situ characterisation would surely benefit from incorporating insights from theoretical chemistry in the future, and making sure their work is relevant to the challenges in SOFCs. 137

138 9.9 Reflections on experimental work Many in the literature have claimed experimental results of tolerance against carbon and sulfur. However, caution needs to be taken in the techniques used to analyze these results, and during the research and writing of this review we have noted some points relating to this: Claiming carbon tolerance by lack of performance degradation. It is certainly true that an anode is carbon tolerant if it maintains performance over a long period of time regardless of whether or not carbon is actually present. However, it cannot be claimed that a lack of degradation means that there is no carbon, as it has been shown many times that cells can operate without performance degradation for significant periods despite carbon being deposited. Certainly a testing period of a day or a week as used in many papers is not long enough to claim carbon tolerance in the absence of other data proving that either there is no carbon or that the carbon has reached some kind of steady state. Claiming carbon tolerance by lack of carbon in SEM. While SEM is clearly a useful technique for assessing microstructure, the lack of carbon whiskers in an SEM image is not proof of a lack of carbon. Even EDX needs careful sample preparation for accurate quantitative analysis, for example polishing. Low measured OCVs. This applies to two different systems thin doped ceria electrolytes and air leakage into the anode chamber. It has been shown that leaks causing an increase in measured OCV of less than 0.02 V from V to V in dry methane results in a decrease in implied water content (as calculated from the H2/O2 equilibrium) of 30% from 0.24% to 0.17% 473. This is enough to result in a dramatic decrease by over half in the amount of carbon deposition over a period of one hour. Likewise, many groups using µm CGO and CSO electrolytes report OCVs below 0.9 V, which implies a substantial 138

139 oxygen flux at OCV through the electrolyte caused by the electronic conductivity of doped ceria (or through a slightly permeable electrolyte). This oxygen flux or leakage should be extremely effective at preventing carbon deposition and improving sulfur tolerance, but if the aim is to study the carbon and sulfur tolerance of the electrode materials then care should be taken to account for this. Current collectors. The paper referred to above also showed that the coverage of silver current collector paste can have a large effect on carbon deposition, even completely preventing it if the electrode is completely covered 473. Presumably this works in a similar way to the barrier layer concept discussed in section 5.2. There has also been considerable controversy over the use of platinum current collectors, with claimed high power densities in dry methane for some MIECs being shown to be almost entirely due to the use of platinum current collectors and doped ceria electrolytes 240. Humidification. As shown above, the exact level of humidification can have a profound effect on carbon and sulfur tolerance. Many studies report 3% humidification levels, which if using a bubbler, implies a water temperature of just under 25 C (25 C would actually be 3.1%). This seems quite warm for a lab temperature, although the authors of this review are based in Britain so maybe used to cooler temperatures than many. The humidity level at 20 C is 2.3%, and at 16 C is 1.8%, care should be taken that the correct humidity levels are being reported. It should also be borne in mind that these are 100% relative humidity levels, whereas it is known that bubblers are not necessarily effective in reaching 100% relative humidity 474. Interaction between carbon and sulfur. Many papers claim carbon and sulfur tolerance, with separate experiments done to prove each using model gas feeds (e.g. one experiment with dry methane and another with H2S in H2). However, it is well known from 139

140 both catalysis and SOFC literature that there is strong interaction between carbon and sulfur, with each having the possibility of hindering or promoting the other under different operating conditions. While there are undoubtedly advantages in simplifying the system by separately studying carbon and sulfur tolerance, it does not necessarily follow that a system which is separately carbon and sulfur tolerant will be simultaneously carbon and sulfur tolerant. This could especially be the case where sulfur may react with promoters which are present to reduce carbon deposition, for example with BaO. More work needs to be done on sulfur-containing carbonaceous fuel in SOFC anodes. In this review, we have seen the different materials solutions to carbon and sulfur tolerance in catalysis and solid oxide fuel cells, but there is also an aspect of different experimental techniques in the two fields. The foundational techniques of catalysis are well established over decades, with a focus on gas phase techniques such as chemisorption measurements and temperature-programmed reactions. Some of these have started to be incorporated into SOFC studies, for example temperature-programmed oxidation. In SOFCs, electrochemical impedance spectroscopy (EIS) has long been a key feature of the investigations, and although it has not been used in catalysis, with advances in impedance analysers, analysis techniques such as distribution of relaxation times (DRT) and modelling, it is possible that EIS could start to be used in model catalysis studies. Because of the more widespread nature of catalysis science, newer techniques have generally been adopted in catalysis first, and later in solid oxide fuel cells. An example of this is in situ Raman, which has been known in catalysis since the early 1990s, but is now being used to investigate SOFCs. Other techniques such as high-resolution TEM, XPS and XRD are following a similar path. A notable counter-example is FIB-SEM and tomography in general, 140

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196 2009 he moved to Denmark as Senior Scientist at Risoe National Laboratories for Sustainable Energy followed by a position as Research Fellow at the University of St Andrews. Since 2012 he is Research Associate in Fuel Cells and Materials Processing at Imperial College. His areas of interest include materials for energy applications and gas separation membranes, the development of electrodes for fuel cells and the manufacture of metal-ceramic composites. Joshua Mermelstein is a fuel cell systems engineer at the Boeing Company in Huntington Beach, CA with an expertise in solid oxide fuel cell (SOFC) and proton exchange membrane (PEM) fuel cell systems. He is currently the lead scientist for fuel cell system development within Boeing s Electronic and Information Solutions Advanced Technology Programs (ATP). Joshua is currently leading efforts as the chief engineer for Boeing s development of a 50 kw reversible solid oxide fuel cell (RSOFC) system used for microgrid energy storage. Joshua also provides technical support for the development of other SOFC and PEM based fuel cell systems throughout Boeing. Joshua earned his Bachelor s degree in Chemical Engineering from the University of Arizona in 1999, Masters from the University of Southern California in 2000, and Ph.D. from Imperial College of London in 2010 with the Department of Chemical Engineering and Fuel Cell Research Group of the Energy Futures Lab, after working in industry as a chemical engineer for 7 years. His research at Imperial College focused on the impact and mitigation of carbon formation on SOFC anodes arising from biomass gasification tars through steam reforming, partial oxidation, and dry reforming technologies. Joshua has published over 10 publications related to his work in this field. His career background spans 10+ years of industry experience in chemical/process engineering, cryogenic and compressed gases, hydrogen 196

197 and fuel cell technology, fuel cell electric vehicles, plug-in hybrids and BEVs, alternative and renewable energy for stationary power, hydrogen production, and combined heat and power (CHP) for energy efficiency. Jose M. Bermúdez graduated in Chemical Engineering (2008) and got a MSc in Process and Environmental Engineering (2010) from the University of Oviedo, Spain. He obtained his PhD in Chemical Engineering (2013) from the same university. His PhD Thesis deals with the CO2 reforming of coke oven gases to produce syngas for methanol synthesis and was developed in the National Institute of Coal-CSIC (Spain). He worked in this research centre for more than 5 years, where he was involved in the development of microwave-assisted processes in the field of energy, mainly focusing on pyrolysis, gasification and catalytic heterogeneous reactions. He gained a postdoctoral position in Imperial College London in 2014, where he is working on the thermochemical stability of mixed ionic-electronic conductors for oxygen transport membranes. He is also involved in the development of thermochemical processes like supercritical water upgrading or catalytic hydrocracking of heavy oils and biomass. He has co-authored more than 30 peer-reviewed papers and 2 patents on these topics and has been finalist of the Best Young Researcher Award 2015 of the Spanish Group of Coal. Tomas Ramirez Reina received his PhD in Chemistry from the University of Seville (Spain) in 2014 under the supervision of Prof. Odriozola and Dr. Ivanova. For his PhD work, he was awarded best PhD thesis 2014 by the Spanish Society of Catalysis (SECAT). He worked as visiting researcher in 2011 in the Brookhaven National Laboratory (NY, USA) and in 2012 in the Institute of Chemical Engineering ICE-HT (Patras, Greece). In 2014 he moved to UK as Research Associate in the Chemical Engineering Department at Imperial College London. Currently, Dr. Reina is a lecturer in the Department of Chemical and Process Engineering in 197

198 the University of Surrey. His research interests include the development of advanced heterogeneous catalysts for energy and sustainability. In particular, his work is focused on clean hydrogen production, selective oxidation and hydrocarbon upgrading. Nigel Brandon's research is focused on electrochemical devices for energy applications, with a particular focus on fuel cells, electrolysers, and batteries. He is Director of the UK Research Council Energy programme funded Hydrogen and Fuel Cells SUPERGEN Hub, and Co- Director of the SUPERGEN Energy Storage Hub. He was the founding Director of the Energy Futures Lab at Imperial College, and a founder of Ceres Power, an AIM listed fuel cell company spun out from Imperial College in In 2014 he was appointed to the BG Chair in Sustainable Gas and as founder Director of the Sustainable Gas Institute at Imperial College. Table of contents graphic The TOC 100% original and created by the authors, free of copyright or publication elsewhere. 198