(Registration number: 2001EF004) Efficient and Flexible SFC system Research Coordinator Harumi Yokokawa National Institute of Advanced Industrial Science and Technology : JAPAN Research Team Members Truls Norby University of slo : NRWAY Koichi Eguchi University of Kyoto : JAPAN Ellen Ivers-Tiffée University of Karlsruhe : GERMANY DurationApril, 2001 March, 2004 Abstract Scandia stabilized zirconia (ScSZ), iron doped calcium titanate (Ca(T i0.9 Fe 0.1 ) 3 ) and (Zr 0.5 Ce 0.5 ) 0.65 Y 0.35 1.825 were selected as common anode components inside FLEXSYS team. Tests on cell performance revealed that the Ni/ScSZ cermet anode exhibited the best performance for the hydrocarbon fuels among the presently tested anodes, namely, Ni/YSZ, Ni/ScSZ, Ni/(Ce,Gd) 2, Ni/(Zr 0.5 Ce 0.5 ) 0.65 Y 0.35 1.825, Ni/Ca(Ti 0.9 Fe 0.1 ) 3. The surface reaction rate and the proton solubility of ScSZ were essentially the same as of YSZ despite high activity of Ni/ScSZ; the high ability of supplying oxygen seems crucial in avoiding carbon deposition. For Ca(T i0.9 Fe 0.1 ) 3 having the proton conductivity, high isotope exchange reaction rate and high hydrogen dissociation ability, the activity was not good but the stability against carbon deposition was better than Ni/YSZ. The FLEXSYS cell consisting of ScSZ electrolyte with Ni/ScSZ was tested and demonstrated to show better performance than the conventional Ni/YSZ. Keywords: Solid oxide fuel cell, Fuel flexibility, Energy conversion, Hydrocarbon fuels, Carbon deposition 1. Introduction Solid oxide fuel cells (SFCs)[1,2] have many advantages against the polymer electrolyte fuel cells (PEFCs). Particularly, higher energy-conversion efficiency can be achieved without using expensive precious metals. In addition, the long-term stability can be also achieved by selecting appropriate materials combinations. n the other hand, SFCs have a week point that it is technologically difficult to construct stacks because of the chemical and mechanical instability during fabrication and thermal cycles[2]. Even so, well designed stacks have been constructed and have survived for frequent thermal cycles or operations longer than 70,000 h. This makes it clear that SFCs are suitable for large power generators and also for many other applications in a rather small size. It should be noticed, however, that in a moderate size, it is not easy to obtain high efficiency and fuel flexibility simultaneously because gas
circulation and heat transfer could not be made V + 0.5 2 (g) = + 2h effectively in a small system. Instead, the present V + 0.5 2 (g) + 2e - = FLEXSYS team aims at development of flexible but Air Electrode 2 H e- (g)+ V 2 =2-+2 H i still efficient SFC systems in a moderate size H (several kw class) by adopting advanced methods of (g) 2 Electrolyte h H + feeding fuels to small SFCs. The FLEXSYS team 2-2- V V thus decided to make the primary focus on the anode[3] side of the cells and the reforming[4] Fuel H 2 Electrode e- H process. 2 CH 4 The main focuses in the FLEXSYS project are Fig. 1 Schematic electrochemical reaction mechanism placed to investigate the direct feeding/oxidation of with strong interaction with water vapors. fuels (methane, propane and butane) in the FLEXSYS cells consisting of alternative as well as standard anodes. The direct oxidation [5] is the interdisciplinary subjects covering catalyst, electrochemistry, solid state ionics etc. In the FLEXSYS project, this will be investigated with emphases on the metal/oxide/gas interactions; that is, possible effects of water vapors [6,7] on oxygen exchange reaction rate and electrode activity as illustrated in Fig. 1; water vapor is one of the reaction product of direct oxidation of hydrocarbons in SFCs so that reactions between water vapor with hydrocarbons on metal/oxide interfaces will become crucial. So far, the acid-base theory provides basis of understanding the chemical behavior in catalytic reactions and proton affinities. The FLEXSYS project aims at bridging among different treatments on basicity in catalyst and solid-state electrochemistry. The FLEXSYS project does not attempt only tests of new ideas in a small size of experimental cells but also will test the FLEXSYS concept in rather large cells that can be applicable to one kw class system. Another focus will be placed on modeling for a small but efficient SFC system by analyzing the FLEXSYS cells and the energy flow of those systems containing the FLEXSYS cells. 2 Materials and Methods The followings are the important decisions of FLEXSYS teams on the interesting but wide-spread research areas: 1) Since Fuel Flexibility is the most important concept in the FLEXSYS project, anodes will be focused but effects on cathode will not be focused; 2) Model electrodes and model fuels are selected by considering the followings: Karlsruhe will test the FLEXSYS cells in a large size (10 x 10 cm) with a focus on well characterized materials, whereas Kyoto and slo will use smaller cells to test a wide range of materials. Tsukuba will focus mainly on the SIMS investigation on the anode related materials and on the thermodynamic considerations on basicity. 3) As common materials, scandia-stabilized zirconia (ScSZ), iron-doped calcium titanate and (Zr 2 ) 0.325 (Ce 2 ) 0.325 (Y 1.5 ) 0.35 were selected. Investigation subjects of the respective groups are summarized in Fig. 2. Tsukuba (AIST) will make measurements on proton solubility, surface reaction rate and electrical properties of the oxides in relation with alternative anodes and reforming catalysts to be investigated in Kyoto, slo and Karlsruhe. slo also focuses on the fundamental properties related with electrocatalytic and catalytic behaviors as functions of water vapors, temperature,
Karlsruhe Univ. Single cell Test CH 4,; HC;Liquid Microstructure multilayer anodes AIST Model Fuels with/without water System Model Electrodes standard : alternative? Correlation Fuel Processing Exergy analysis Surface Exchange Kinetics Transport Parameters Thermodynamics of Water Solubility Kyoto Univ. Catalytic reforming // Carbon deposition. Cermet for internal reforming Single cell peration slo Univ. Fig 2. Schematic illustration to show the research topics and their mutual relations in the FLEXSYS project. oxygen partial pressure etc. Kyoto analyzes effects of oxides in cermet (ceramics and metal) anodes and reforming catalysts. Since carbon deposition is the key phenomena when direct fuel feeding/oxidation is attempted, Kyoto clarifies the determining factor of carbon deposition in thick cermet anodes. Some additives are selected among the basic oxides. In the FLEXSYS project, the basicity can be correlated with the proton solubility, the thermodynamic activity of basic oxides, surface reaction rate etc. The surface property, surface reaction rate, and the mass transport in solids will be considered on the basis of many accumulated and newly created knowledge in the FLEXSYS project. In order to clarify the system image of the FLEXSYS project, the modeling of cell performance will be made in Karlsruhe, exergy analysis being in Kyoto. Test on the FLEXSYS cells will be tested in Karlsruhe using gas and liquid fuels on the long term behavior, degradation on fuel direct oxidation etc. This test will become the key experiments in the FLEXSYS project. 3. Results 3.1 Water/xide Interaction In order to understand the electrochemical performance, it is essential to know the interactions of the oxide component in electrodes and gaseous species such as oxygen, hydrogen, water vapor etc. Particularly, the interaction of water vapor with zirconia or ceria has been examined in Tsukuba with an emphasis on the proton solubility, surface reaction rate of oxygen isotope exchange, effects of water vapor on the surface reaction rate etc. The thermodynamic analysis has been also made on the doped ceria or zirconia-ceria solid solutions and has clarified that the geometrical arrangement of oxide ion vacancies relative to the dopant lattice position governs the thermodynamic properties and also behavior of protons and holes. In slo, literature survey has been made on the proton conduction in a large number of oxides, and the empirical correlations were derived. Experimental investigation showed no effects of protons in La 2 Ni 4 and LaFe 3. These investigations give the physicochemical basis of understanding the roles of
oxides in anodes. 3.2 Design for Reforming Catalysts and Anodes Scandia-stabilized zirconia (ScSZ), which exhibits the higher oxide ionic conductivity than YSZ, is interesting from the oxide/metal interactions in cermet anodes, because Ukai et al.[8] have already shown that Ni/ScSZ cermet is stable in slightly humidified methane whereas Ni/YSZ cermet is not. Tsukuba revealed that the surface oxygen isotope exchange reaction rate, α, is essentially the same as YSZ as shown in Fig. 3; the surface exchange reaction rate is enhanced in the presence of the water vapor and the magnitude of the enhancement can be correlated with the surface coverage of water molecule on the ScSZ. In addition, no significant effects of Fe doping to ScSZ were observed. A similar insignificant effect was observed on Mn-doped YSZ, although the electronic conductivity is enhanced. The present results lead to the following important considerations that not the surface property, but the high ionic conductivity in ScSZ is important in avoiding carbon deposition inside cermet anodes. In other wards, the high oxygen flow rate is critical. In degradation behavior of the ionic conductivity of ScSZ was measured in Karlsruhe as in an order of 4% / 1000 h. The second common material, Ca(Ti 0.9 Fe 0.1 ) 3, exhibits the proton conductivity as confirmed in slo. In addition, slo also observed that hydrogen or hydride ions can exist in this material in the same manner as in SrTi 3. This material in the same batch has been distributed to respective groups. In Tsukuba, the surface exchange reaction rate and the oxide ion diffusivity, D *, were measured; as given in Fig. 4, the surface reaction rate is high, whereas the oxide ion diffusivity is low when compared with other perovskite oxides such as lanthanum cobaltites. In slo, the dissociation rate on oxide materials has been measured for the oxygen as well as hydrogen dissociation reactions log (D * / m 2 s -1 ), log (α / m s -1 ), -6-7 -8-9 -10-11 -12-13 α D * -14 2 3 4 log(p(h 2 ) / Pa) 0-1 -2-3 -4 log θ Fig. 3 xygen isotope diffusivity (D *) and surface exchange rate constant (α) of ScSZ (open symbols) and Fe-ScSZ (closed symbols) as functions of water partial pressure. Solid line was derived from the concentration of water chemisorbed on YSZ quoted from Ras et al.) α / m s -1 D* / m 2 s -1 600 500 400 10-8 10-9 10-10 10-11 10-12 10-13 10-14 CaTi 0.9 Fe 0.1 3 Praxair p(h 18 2 ) = 2 kpa, p( 18 2 ) = 7 kpa 1.1 1.2 1.3 1.4 1.5 1.6 kk / T Fig. 4 xygen isotope diffusivity, D *, and surface exchange rate constant, α, of CaTi 0.9 Fe 0.1 3-δ in humid atmosphere: ; D *, ; α
Dissociation rate / mol 2 s -1 g -1 10-4 10-5 10-6 10-7 La 0.9 Sr0.1Fe3-δ La 2 Ni4+δ La0.9Sr0.1Fe3-δcalc Ce 0.9 Gd 0.1 2-δ Ce 2-δ Dissosciation rate / mol X 2 cm -2 s -1 10-6 10-7 10-8 Fe dopedti 2 CFT Nd-doped Ce 2 Ti 2 Ce 2 Zr 2 10-8 10-9 1.0 1.2 1.4 1.6 1.8 0.8 1.0 1.2 1.4 1000K/T 1000/T (K -1 ) Fig. 5. (a) The oxygen dissociation rate and (b) the hydrogen dissociation rate over different oxides at a total pressure of 20 mbar (2 kpa) as a function of the inverse temperature. CFT=Ca(Ti.9 Fe.1 ) 3. (Fig. 5). As shown in Fig. 5(b), Ca(Ti 0.9 Fe 0.1 ) 3 exhibits very high values for the hydrogen dissociation rate; this should be compared with the rather high surface reaction rate given in Fig. 4. Kyoto examined the catalytic activity for steam reforming and shift reactions; the catalytic activity of Ca(Ti 0.9 Fe 0.1 ) 3 is found to be quite low. As a Ni/Ca(Ti,Fe) 3 cermet anode, several characteristic properties have been investigated in slo, Kyoto and Karlsruhe. The first attempts on Ni/Ca(Ti,Fe) 3 cermet in Kyoto revealed the followings: (1) their anode activity is generally lowered than Ni/YSZ for H 2 and CH 4 fuels; (2) heat treatment at a higher temperature than 1300 ºC caused some degradation. Similar phenomena associated with heat treatment were also observed in slo and Karlsruhe. During the experiments with CH 4, low CV has been observed in Kyoto (Fig. 6). This gave rise to some discussions within the team, mainly because with hydrogen fuels, it is rare to observe low CV and as a result, low CV indicates some inappropriateness in experimentation. However, with CH 4 fuels, methane is not the directly electrochemical active species but hydrogen or C will be active. This implies that the CV value depends on water vapor/c 2 vapor pressure as well as hydrogen vapor pressure. This makes the situation complicated. The carbon deposition rate on the Ni/Ca(Ti 0.9 Fe 0.1 ) 3 cermet was observed to be smaller than that on the Ni/YSZ. This better performance against the carbon deposition is due partly to the proton solubility in the cermet anode, which is consistent with the mechanism proposed from AIST Tsukuba. Although Ni/Ca(Ti 0.9 Fe 0.1 ) 3 cermet is not better than Ni/YSZ, the durability is better particularly when the Ni to Ca(Ti 0.9 Fe 0.1 ) 3 ratio of 1/1 is adopted as shown in Fig. 7. The metal component of cermet anodes has been investigated in Karlsruhe. Copper was the first candidate for current collector because copper is inactive to methane decomposition reactions. However, cooper was found to be
0.70 1.4 1.2 Ni/CaTi 0.9 Fe 0.1 xide calcined at 1200 Ni/CaTi 0.9 Fe 0.1 xide calcined at 1300 0.65 Ni/CaTi 0.9 Fe 0.1 xide calcined at 1400 Terminal voltage / V 1.0 0.8 0.6 0.4 Ni/YSZ calcined at 1400 Terminal voltage / V 0.60 0.55 0.50 0.2 (b) 0.45 Ni-FCT (4:1) Ni-FCT (1:1) 0.0 0 0.1 0.2 0.3 0.4 0.5 0.6 Current density / Acm -2 0.40 0 5 10 perating time / h 15 20 Fig.6 Current-voltage characteristics of CH 4 -fueled SFCs with Ni- Ca(Ti 0.9 Fe 0.1 ) 3 oxide. Fig.7 Terminal voltage vs. operating time for CH 4 -fueled SFCs at 1000ºC with Ni-Ca(Ti -0.9 Fe 0.1 ) 3 anodes at under a constant current load of 200 ma/cm 2. instable in a cell due to the evaporation as CuH vapors. This is consistent with the thermodynamic prediction made in Tsukuba. In Karlsruhe, tests were made on Ni-Cu, Ni-Fe, Ni-Al. Among them, Ni-Al was found to be promising as current collector. Redox stability of cermet anodes has been tested in Karlsruhe for conventional anodes with different firing temperature. It has been found that the microstructure seriously affects the redox stability and the multilayer-structure anode improves the adhesion of anodes to electrolyte substrate and as a result, long term stability. Furthermore, Sc stabilized zirconia has been fabricated to produce the electrolyte-support cells after optimizing sintering process by adopting new control techniques; that is, the rate controlled mass loss and the rate controlled sintering for binder burnout processes. Fuel flexibility has been examined by Kyoto and Karlsruhe. Kyoto first investigated the effects of precious metals added to Ni/YSZ cermets for steam reforming reaction and anode reactions. They found that the presence of Ru or Pt is effective in prompting the reforming reaction and suppressing the carbon deposition. As a result, this is also effective in improving anode activity for methane and water under the internal reforming condition. Discussion has been made on the roles of oxides and metals in cermet anodes. As shown in Fig. 5, slo observed that the hydrogen dissociation rate is quite high for metals such as Pt and Ni, but quite low for oxides such as YSZ or ceria. This strongly suggests that as one of anode components, metals are needed. Furthermore, the selection of oxides should be crucial to obtain those anodes which can be applied to various fuel situations. Tsukuba proposed one material (Zr 2 ) 0.325 (Ce 2 ) 0.325 (Y 1.5 ) 0.35. To examine the appropriateness as the anode component, the electron conductivity, the chemical volume expansion and other properties related to the anode behavior have been examined in Tsukuba. The phase stability is found to be improved by increasing the content of Y 1.5 compared with the corresponding solid solution with Y 1.5 content of 0.2. However, the cell test in Tsukuba showed the worse anode activity compared with ScSZ cermet anodes. This material was sent to Karlsruhe to make cell tests.
3.3 Characterization of FLEXSYS Cells in Experiments and Simulation In Karlsruhe, the FLEXSYS single cells have been tested with conventional fuels, propane and butane. The main point of the test with conventional fuel (hydrogen) is to examine the degradation rate at the high current density and at high fuel utilization. Some improvement has been obtained by adopting the two layer anode, leading to the decrease in degradation from 5%/1000 h to 3%/1000 h. The first attempt on methane without adding water vapor (S/C=0) using the conventional Ni/YSZ cermet revealed that the cell performance showed no degradation for more than 1000 h. This is, however, due to the oxygen or water vapor that was transported as a result of electrochemical reactions. Under the CV condition where no such a gas will be available, the anode had serious damage on the nickel part due to carbon depositions. When S/C ratio was changed, the best performance was obtained at S/C=0.5. For propane and butane fuels, carbon deposition becomes severer and operation with S/C=0 could not be made. Thus, steam reforming, partial oxidation and their combination were adopted as fuel processing system. Under the condition where carbon deposition can be avoided, namely, S/C (steam to carbon ratio) =2 to 3 or lambda value (air to fuel ratio) = 0.2 to 0.3, good performance was obtained without serious problem associated with carbon deposition. Furthermore, no difference was observed between fuels, indicating that the fuel processing system worked very well. Figures 8 and 9 show the experimental results on the common materials in Karlsruhe. Figure 8 is for the operation for hydrocarbon fuels with Ni-ScSZ anode, whereas Fig. 9 is for the methane operation with different S/C ratios on Ni/CaTi 0.9 Fe 0.1 3 Table 1 summarizes the results of cell tests made in Karlsruhe. From the results for 5% humidified hydrogen, the standard anode activity can be judged. From the results at different S/C ratios, it can be derived information on durability against carbon depositions. When comparison is made between Ni/YSZ and Ni/ScSZ, the performance of Ni/ScSZ is better for hydrocarbon fuels. However, the operation at S/C=0 was failed due to carbon deposition at 950 C. This is different from Ukai et al.[8]. The Ni/CaTi 0.9 Fe.1 3-d cermet anode exhibited poor performance when compared with other fluorite-type oxides. Even so, this anode can be used for S/C=0 at 950 and 800 C. The Ni/CZY B2704AK.047, B2704AK.048, B2704AK.049, B2704AK.051 1.0 b0901bk.002, b0901bk.004, b0901bk.005 Voltage V 0.8 0.6 0.4 0.7 V H 2 = 200 ml/min (28 A) + 60 % H 2 C 4 H 10 = 15 ml/min (28 A) λ = 0,1 S/C = 3 1.5 W Power Voltage 1.0 V 0.7 1.0 0.5 S/C = 0 S/C = 1 0.2 C 3 H 8 = 20 ml/min (28 A) λ = 0,1 S/C = 3 0.5 S/C = 2 S/C = 3 0.0 0.0 0.00 0.05 0.10 0.15 0.20 0.25 A/cm² 0.30 Current density 0.0 0.00 0.05 0.10 A/cm² 0.15 current density Fig. 8 Hydrocarbon operation of a Ni/SSZ-anode at T=800 C Fig. 9 Ni/Ca(Ti 0.9 Fe.1 ) 3 anode operated with methane at different S/C ratios at T=800 C
{(Zr 2 ) 0.325 (Ce 2 ) 0.325 (Y 1.5 ) 0.35 } cermet showed about the same performance with other anodes Ni/YSZ, Ni/ScSZ. This is slightly different from the result in Tsukuba probably because of better powder processing. Gas analysis has been made on the internal steam reaction along the anode gas channel at 800 and 950 C. At 950 C, essentially the same reforming activity was obtained for Ni/YSZ and Ni/CG, whereas some dependence on anode materials appears at 800 C; a higher methane conversion is obtained for Ni/CG. This information is needed to model FLEXSYS cell performance. Modeling the temperature distribution and the local gas composition at the anode side has been carried out by computational fluid dynamics (CFD) with the software FLUENT. This model already took into account the gas flow, chemical reactions (methane reforming, shift), heat transfer and diffusion in porous media. Furthermore, the electrochemical oxidation for different species such as hydrogen and C was incorporated in the model. 3.4 Demonstration of FLEXSYS Cells and Stacks and System Analysis Conceptual design of the FLEXSYS system and evaluation of efficiency attained by improved fuel flexibility has been made by Kyoto on the basis of planar cells with taking into account the reforming and shift reactions. After incorporating this model to the commercial process simulator ASPEN Plus, process simulation was made to clarify the difference between the internal and the external reforming of methane at the S/C ratio = 2 at 1000ºC and U fuel = 0.8. Results showed that the internal reforming leads to the conversion efficiency of 54.7%(HHV) compared with the 49.8% for external reforming. This is due to better utilization of heat evolved from the cells. In Karlsruhe, a five-cell-stack test bench, which was installed initially as a fuel cell diagnosis system, was modified in the fuel processing system and the fuel supplying system (evaporator for liquid fuel and water). Single cells for the FLEXSYS stack have been prepared; cells consist of ScSZ electrolyte with a strontium lanthanum manganite cathode and a Ni/ScSZ anode. Stack tests were carried out in June/July 2004. 4. Discussions 4.1 Role of Proton Solubility/Conductivity in Anodes with Hydrocarbon Fuels: The present FLEXSYS cell is based on the idea that protons in oxides may play critical roles in determining the anode reaction mechanism as illustrated in Fig. 1. The present investigation revealed that no difference was observed Temperature Fuel Ni/YSZ j at 0.7 V Standard Table 1: Performance of different anodes Ni/ScSZ j at 0.7 V Common I Ni/CG j at 0.7 V Ni/CZY j at 0.7 V Common III Ni/CFT j at 0.7 V Common II 950 C H 2, 5% H 2 0.64 A/cm 2 0.72 A/cm 2 0.63 A/cm 2 0.68 A/cm 2 0.49 A/cm 2 CH 4 S/C=2 0.35 A/cm 2-0.41 A/cm 2-0.3 A/cm 2 CH 4 S/C=1 0.49 A/cm 2-0.54 A/cm 2 0.53 A/cm 2 0.36 A/cm 2 CH 4 S/C=0 0.51 A/cm 2 * - * 0.43 A/cm 2 800 C H 2, 5% H 2 0.17 A/cm 2 0.35 A/cm 2 0.2 A/cm 2 0.25 A/cm 2 0.16 A/cm 2 CH 4 S/C=2 0.11 A/cm 2 0.22 A/cm 2 0.1 A/cm 2-0.06 A/cm 2 CH 4 S/C=1 0.14 A/cm 2 0.22 A/cm 2 0.13 A/cm 2-0.09 A/cm 2 CH 4 S/C=0 0.13 A/cm 2 0.12 A/cm 2 0.17 A/cm 2-0.11 A/cm 2 * failure: carbon deposition at S/C=0 CG=(Ce 0.8 Gd 0.2 ) 1.9 ; CZY=((Zr 2 ) 0.325 (Ce 2 ) 0.325 (Y 1.5 ) 0.35 ; CFT=Ca(Ti 0.9 Fe 0.1 ) 3
between ScSZ and YSZ in the effect of water vapors on the surface reaction rate whereas apparent difference is observed in the electrochemical performance as listed in Table 1. This indicates the importance of supplying oxide ions in the electrolyte on the basis of the proposed mechanism of transferring oxygen from electrolytes to nickel metal surface in the form of water vapor; this may enable direct electrochemical oxidations of hydrocarbons on the nickel surfaces. The present results on the CaTi 0.9 Fe 0.1 3 anodes also revealed that stability against carbon deposition is improved in this anode partly because of its high proton conductivity. The poor performance of Ni/ CaTi 0.9 Fe 0.1 3 can be ascribed to its peculiar microstructure caused during the high temperature treatment and also to the poor electron and oxide ion conductivity. This makes it necessary to compare with those oxides having higher proton solubility and higher electron conductivity. In view of this, the present results on the Ni/CG and Ni/(Zr 2 ) 0.325 (Ce 2 ) 0.325 (Y 1.5 ) 0.35 seem to be interesting because of their mixed conductivity and proton solubility. Against this expectation, these anodes showed worse performance than Ni/ScSZ. ne possible explanation is that both anodes exhibit still large chemical volume expansion associated with the reduction of tetravalent cerium ions, another being that both may have some chemical or mechanical instability with ScSZ electrolyte. 4.2 Microstructure of FLEXSYS Cell Anodes The present results revealed that although no degradation was observed in the operation with dry methane for more than 1000 h, carbon deposition became severe for the CV condition. This strongly suggests that anodes should consist of several components having their respectively specified functions in the anode-current collection assembly. In many cases, nickel current collector is used. However, this nickel current collector is weakest against carbon deposition because this is far from the anode reaction sites from which water vapors will be emitted and this is exposed to fuels before reforming. The present results showed that Ni-Al alloys will be one of possible current collectors. From a similar reason, the oxide current collector will be also interesting. ne possible candidate is Y 1.5 doped CaTi 3 ; this is analogous oxide to La 1.5 doped SrTi 3 that has been intensively investigated in USA. Even so, these materials have poor chemical stability. Further thermodynamic considerations lead to Zr 2 -Ce 2 -M 1.5 (M=Y or Sc) solid solutions. As described above, these materials showed worse or comparable performance as the oxide component in cermet anodes. This is because the chemical volume expansion associated with cerium reduction is still large even when the cerium content in solid solutions is reduced by keeping some level of electron conductivity. 5. Conclusions The systematic approach has been adopted to clarify the soundness of the FLEXSYS concept for achieving the efficient and fuel flexible SFC system in a moderate size. Experimental analyses have been made on the catalytic activity, electrical properties, dimensional stability and other characteristics for the oxide components and the metallic components. For the oxide components, several common oxides are selected in addition to the standard oxides such as YSZ or CG from the view point of oxide ion conductivity, proton conductivity, and electron conductivity. For the metallic components, Cu and other alloys are examined in addition to Ni. In the present investigation, the best cell performance in the operation for hydrogen as well as hydrocarbons at various S/C ratios was achieved for Ni/ScSZ that was selected as the first common material. Although the second common material, Ni/CaTi 0.9 Fe 0.1 3, exhibited worse performance, its stability against carbon deposition is remarkable, suggesting that some role of proton conduction in anode mechanism. The present results on the third common material, Ni/(Zr 2 ) 0.325 (Ce 2 ) 0.325 (Y 1.5 ) 0.35 implies that
as the oxide component in cermet anodes, the chemical volume expansion associated with cerium reduction should be reduced to provide better performance. To demonstrate the feasibility of the FLEXSYS cells, the 5-cell stack based on the ScSZ electrolyte with Ni/ScSZ anode was operated in June/July 2004. 6. References [1] S. C. Singhal and K. Kendall ed. High Temperature Solid xide Fuel Cells: Fundamental, Design and Application, Elsevier, (2003). [2] H. Yokokawa and N. Sakai, Chapter 13 History of high temperature fuel cell development, Volume 1, pp.219-266, W. Vielstich et al. ed. Handbook of Fuel Cells Fundamentals Technology and Applications, Wiley (2003). [3] E. Ivers- Tiffée, A. V. Virkir, Chapter 9 Electrode Polarizations, in ref (1), pp229-260. [4] K. Eguchi, Chapter 75 Internal Reforming, pp. 1057-1069, in ref(2). [5] S. A. Barnett, Chapter 78 Direct hydrocarbon SFCs, pp. 1098-1108, in ref(2). [6] N. Sakai et al. PCCP 5, 2253-2256 (2003). [7] T. Norby, Nature 410, 877-878 (2001). [8] K. Ukai, Y. Mizutani, Y. Kume Current Status of SFC development using Scandia Doped Zirconia, pp. 375-383, in Solid xide Fuel Cells VII, H. Yokokawa and S. C. Singhal ed.. ECS PV-2001-16, (2001) The list of the most important papers from the project [1] N. Sakai et al. Effect of water on oxygen transport properties in electrolyte surface in solid oxide fuel cell, J. Electrochem. Soc. 150, A689-A694 (2003). [2] A. C. Müller, A. Weber, E. Ivers-Tiffée, Intrinsic Degradation of Solid Electrolytes based on Zirconia, Solid State Ionics in press. [3] T. Takeguchi, T. Yano, R. Kikuchi, K. Eguchi, Effect of precious metal addition to Ni-YSZ cermet on reforming of CH 4 and electrochemical activity as SFC anode, Catalyst Today 84, 217-222 (2003). [4] H. Yokokawa, T. Horita, N. Sakai, K. Yamaji, M. E. Brito, Y.P. Xiong and H. Kishimoto, Protons in Ceria and their Roles in SFC Electrode Reactions from Thermodynamic and SIMS Analyses, Solid State Ionics in press. [5] T. Norby, High temperature proton conductors properties and applications, British Ceramic Proc. 63, 1-8(2001). Presentations [1] H. Yokokawa, et al. Study on an Efficient and Flexible SFC System, Solid xide Fuel Cells VIII, Paris, (2003). [2] N. Sakai et al. Significant effect of water on surface reaction and related electrochemical properties of mixed conducting oxide (invited), 14 th Int. Conf. Solid State Ionics, Monterey, (2003). [3] T. Norby, Hydrogen in oxides, 78 th Bunsen discussion meeting, Vaals, (2002). [4] T. Takeguchi et al. Effect of Additive to NI-YSZ cermet on reforming CH 4 and electrochemical activity for SFC, SFC VIII, Paris, (2003). [5] D. Fourquet et al. SFC Single Cell Test Setup For the Use of Various Hydrocarbons, SFC VIII, Paris (2003). Awards [1] Harumi Yokokawa, utstanding Achievement Award of High Temperature Materials Division, The Electrochemical Society, 2002.10.21, in recognition of his contributions to the practical applications of thermochemistry to high temperature materials research and technology, especially in the area of solid oxide fuel cells.