6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends

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1 j265 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Ekaterina V. Tsipis and Vladislav V. Kharton Following Chapter 5 where state-of-the-art fuel cell technologies are reviewed, this chapter presents a short survey on the novel electrode materials for the solid oxide fuel cells (SOFCs) with oxygen ion- and proton-conducting electrolytes. Attention is primarily drawn to most recent research studies published during the past few years. Numerous literature data on the electrode performance and key properties of the electrode materials are systematically compared, including the total electrical conductivity, thermal expansion, and reactivity with solid electrolytes. A comparison of the cell performance with various fuels and cell components is also presented. In addition to the fuel cells, related applications such as high-temperature electrolyzers of steam and carbon dioxide are briefly considered. 6.1 Introduction As illustrated by many examples discussed in the first volume [1] and in the preceding chapters, the progress in the field of solid-state electrochemical devices is significantly limited by the materials science-related factors. Particularly, the development of high- and intermediate-temperature (IT) solid-state electrochemical devices, such as solid oxide fuel cells (SOFCs), electrolyzers (SOEC) of carbon dioxide and water vapor, oxygen and hydrogen pumps, and various ceramic reactors and sensors are associated with a search for novel cathode and anode materials with superior electrocatalytic activity, optimization of their fabrication and processing technologies, and efforts to deeper understand the electrochemical reaction mechanisms (see Refs [1 8] and references therein, Chapters 12 and 13 of the first volume and Chapters 5 and 9 11 of this book). For example, commercialization of SOFCs requires reducing costs and enhancing their reliability and long-term stability. These problems can be partially solved by decreasing the SOFC operation temperatures down to the so-called intermediate-temperature range, K, which makes it possible to use less expensive construction materials, to suppress degradation caused by high operating temperatures and by thermal cycling, to facilitate miniaturization, and to improve efficiency of the kw-scale generators [5 10]. On the other hand, Handbook of Solid State Electrochemistry: Volume 2: Electrodes, Interfaces and Ceramic Membranes. Edited by Vladislav V. Kharton. Ó 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 266j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends lowering operation temperature also leads to a greater role of electrode polarization that may become critical for the overall performance. In fact, approaches elaborated to improve the electrode properties are similar for SOFC systems and other electrochemical devices based on oxygen ion- or proton-conducting solid electrolytes. This chapter is centered on the comparative analysis of electrochemical behavior and properties relevant to the electrode applications, such as thermal expansion, stability, and mixed conductivity of the oxide and composite materials, recently reported as promising for the electrochemical devices, primarily fuel cells and electrolyzers. In this chapter, no attempt has been made to provide a complete overview of all novel electrode materials, relevant phase equilibria, and microstructural design, or a thorough analysis of the microscopic electrode reaction mechanisms. Furthermore, since the major emphasis is on novel rather than on wellestablished materials and approaches, the scarce information and preliminary reports available in the literature may lead to a need for additional studies and validation employing complementary experimental methods. The references in this chapter were selected in order to show typical relationships between the properties of various electrode materials reported during the past 2 4 years and, in general, to reflect state of the art in this attractive field. 6.2 General Comments The anode and cathode are supposed to possess a high electronic conduction and catalytic activity toward oxidation and reduction processes, respectively, and to have an appropriate microstructure minimizing mass transport limitations (see Chapter 12 of the first volume [1] and Chapter 5 of this book). Moreover, all the components of high-temperature electrochemical cells should be chemically and thermomechanically compatible with each other, and be stable under the operation and fabrication conditions. These stringent and often conflicting requirements have resulted in a continuous search for optimum electrode materials and their preparation methods. In addition to the key role of electrode microstructure and the effects of solid oxide electrolyte on the electrode performance, the performance is primarily governed by the properties of electrode materials determined, in turn, by their chemical and phase compositions and crystal structure. The most widely studied materials for potential use as oxygen electrodes are based on the perovskite-related oxide compounds (Chapters 9 and 12 of the first volume [1] and Chapter 5 of this book). These are often more tolerant to extensive cation substitution and possess better transport properties with respect to other known families. The lower overpotentials (g) and polarization resistances (R g ) are usually observed for the electrode materials made of mixed electronic ionic conductors (MIECs) owing to effective spatial expansion of the electrochemical reaction zone beyond the triple-phase boundary (TPB) (see Refs [5, 7, 8] and references cited therein). In this respect, the cobaltite- and nickelate-based phases are of primary interest for the IT range, although many Fe- and Cu-containing materials attract

3 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j267 serious attention as well. Although the electronic and oxygen ionic conductivities of manganites, still considered as state-of-the-art cathode materials for SOFCs operating at K, are lower compared to their Fe-, Co-, and Ni-containing analogues, the latter families exhibit other serious disadvantages, including excessively high thermal and/or chemical expansion and limited thermodynamic stability. For operation in reducing environments, ceramic metal composites (cermets) containing stabilized zirconia and/or doped ceria fluorites and Ni metal are widely employed [8, 10]. The presence of nickel, however, provokes carbon deposition at the anode surface of hydrocarbon-fueled cells and may induce various types of microstructural degradation, all leading finally to the electrode destruction; therefore, a number of alternative metal and oxide materials, which may be used without metallic phase, are being considered. The major groups of ceramic compositions for the fuel electrodes include, in particular, perovskite-related titanates, chromites, and molybdates, such as (Sr,Ln)TiO 3d (Ln ¼ rare earth cations or Y), LaCrO 3 -based solid solutions, or Sr 2 MoMO 6 d (M ¼ Mg, Mn, Cr). Note that under reducing conditions, many transition metal-containing oxide materials exhibit a limited thermodynamic stability, a poor electronic transport, or insufficient catalytic activity, limiting their use. 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions The perovskite-related cobaltites and their derivatives show a relatively high electrochemical activity compared to other groups of the cathode materials (Tables 6.1 and 6.2 and Figures ). Consequently, these materials are being widely appraised as promising IT SOFC cathodes, despite the excessively high volume changes that may be induced by temperature changes (thermal expansion) and/or oxygen chemical potential and overpotential variations (chemical expansion) [11 19]. For the relevant information, readers may refer to Chapter 3 of the first volume [1] and Chapter 9 of this book; Table 6.1 and Figure 6.2 present several examples. The thermal and chemical expansivity limit compatibility with common solid oxide electrolytes and long-term cell operation. For instance, the polarization resistance of porous Sm 0.5 Sr 0.5 CoO 3 d cathode in contact with Ce 0.8 Sm 0.2 O 2 d (CSO20) electrolyte was found to undergo fast degradation both on thermal cycling and during an isothermal operation [11]. Partial substitution of cerium for cobalt in Sm 1 x Sr x CoO 3 d was shown to slightly suppress the lattice expansion, decrease the conductivity, and improve the cathode performance; however, Ce solubility limit is quite low, 5 mol% [12]. The highest electrochemical performance of singlechamber solid oxide fuel cells (SC-SOFCs), fabricated with R 0.6 Sr 0.4 Co 1 x Fe x O 3 d (R ¼ La, Nd; x ¼ 0, 0.5) perovskite cathodes, Ce 0.9 Gd 0.1 O 2 d (CGO10) interlayer, thin 8 mol% yttria-stabilized zirconia (8YSZ) electrolytes, and Ni 8YSZ anodes by tape casting, cofiring, and screen printing, was achieved for La 0.6 Sr 0.4 CoO 3 d cathode providing a maximum power density (P max )of550 mwcm 2 at 1073 K [13]. The

4 268j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Table 6.1 Properties of selected cobaltite- and ferrite-based cathode materials in air. Composition s 1073 K (S cm 1 ) Average TECs Chemical interaction R 1073 g K (V cm 2 ) References T (K) a (10 6 K 1 ) Electrolyte T (K) SrCo0.95Sb0.05O3 d CNO20 > [14] Sr 2 Co 0.9 Fe 0.1 NbO 6 d 5.7 CGO10 > a) [16] La0.5Sr0.5Co0.8Fe0.2O3 d LSGM(10 20) > [20] PrBaCo 2 O 5 þ d CSO [26] PrBa0.5Sr0.5Co2O5 þ d 1173 CGO [28] SmBa 0.5 Sr 0.5 Co 2 O 5 þ d CGO [28] YSZ > YSZ/CGO10 interlayer 0.01 Sm0.5Sr0.5CoO3 d LSGM(10 20) > [20] Sm0.5Sr0.5Co0.8Fe0.2O3 d LSGM(10 20) > [20] GdBa0.5Sr0.5Co2O5 þ d 1173 CGO [28] Ba2CoMo0.5Nb0.5O6 d CSO20 > a) [25] Ba0.5Sr0.5Co0.8Fe0.2O3 d CSO [25] LSGM(10 20) > [20] Ba 1.2 Sr 0.8 CoO 4 þ d CGO10 > [29] YBaCo4O7 þ d LSGM(9.8 20) 0.08 [33, 34] YBaCo 3.2 Fe 0.8 O 7 þ d [33] YBaCo3ZnO7 þ d YSZ 1273 [37] CGO20 > CGO [36] LSGM(20 20) 1273 TbBaCo3ZnO7 þ d CGO [36] Ca 3 Co 4 O 9 d YSZ 1073 [38] CGO10 > a)

5 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j269 Sr 0.9 K 0.1 FeO 3 d 26 LSGM(20 17) 0.2 b) [45] LaSrFeO4 þ d CSO10 >1273 [47] (La 0.8 Sr 0.2 ) 0.95 FeO 3 d 44 8YSZ 1373 [52] (La0.8Sr0.2)0.95Fe0.8Cu0.2O3 d [52] (La 0.8 Sr 0.2 ) 0.95 Fe 0.8 Ni 0.2 O 3 d YSZ 1273 [52, 53] CGO >1373 [53] (La 0.8 Sr 0.2 ) 0.95 Fe 0.8 Ni 0.2 O 3 d CGO [50] Notes: 8YSZ: 8 mol% Y 2 O 3 -stabilized ZrO 2 (Zr 0.85 Y 0.15 O 1.93 ); CGO10: Ce 0.9 Gd 0.1 O 1.95 ; CGO20: Ce 0.8 Gd 0.2 O 1.9 ; CSO20: Ce 0.8 Sm 0.2 O 1.9 ; CNO20: Ce 0.8 Nd 0.2 O 1.9 ; LSGM(10 20): La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 d ; LSGM(20 20): La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 d ; LSGM(20 17): La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3 d ; LSGM(9.8 20): (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d.; LSAO: La 10 Si 5 AlO s 1073 K is the total electrical conductivity at 1073 K; a is the apparent thermal expansion coefficient averaged in the given temperature range; R 1073 g K is the electrode polarization resistance at 1073 K. a) Extrapolated. b) Estimated from polarization data.

6 270j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Table 6.2 Properties a) of selected Ni- and Mn-containing cathode materials in air. Composition s 1073 K (S cm 1 ) j 1173 (10 8 mol s 1 cm 2 ) Average TECs Polarization resistance References T (K) a (10 6 K 1 ) Electrolyte R 1073 g K (V cm 2 ) La2NiO4 þ d LSGM(10 20) 2.0 [55, 56] [57, 66] La 3 Ni 2 O 7 d 52 b) LSGM(10 20) 1.5 [55, 56] La4Ni3O10 d 86 b) LSGM(10 20) 0.9 [55, 56] LSAO 2.2 [66] La4Ni2.9Cu0.1O10 d LSAO 1.6 [66] La 3.95 Sr 0.05 Ni 2 CoO 10 d LSAO 2.7 [66] La2Ni0.9Co0.1O4d LSGM(10 20) 0.6 [55] La 2 Ni 0.8 Cu 0.2 O 4 þ d LSGM(9.8 20) 0.62 [61, 65] LSGM(10 20) 2.8 [60] 8YSZ 6.1 [60] LSAO 1.54 [65] La 2 Ni 0.5 Cu 0.5 O 4 þ d LSGM(9.8 20) 1.6 [66] LSAO 2.2 Pr 2 Ni 0.8 Cu 0.2 O 4 þ d LSGM(9.8 20) 0.29 [57] La1.9Sm0.1NiO4d 82 LSGM(20 20) 3.1 [55] La 0.95 Sr 0.05 NiO 4d CGO [62] LaSr2Mn1.6Ni0.4O7 d LSAO 24 [66]

7 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j Gd0.6Ca0.4Mn0.9Ni0.1O3 d LSAO 20 [66] Sr0.7Ce0.3Mn0.9Cr0.1O3 d LSAO 10.2 [66] SrMn0.6Nb0.4O3 d LSAO 31 [66] a) j is the steady-state oxygen permeation flux through 1.0 mm thick membrane under oxygen partial pressure gradient of 0.21/0.021 atm. Other symbols are explained in the notes to Table 6.1. b) Porous samples.

8 272j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends La 0.19 Pr 0.21 Sr 0.26 Ca 0.34 FeO 3- CSO SSZ La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3- -CGO20 (50-50 wt%) 8YSZ Ba 1.2 Sr 0.8 CoO 4- -CGO10 (70-30 wt%) CGO10 Pr 1.6 Sr 0.4 NiO 4- -8YSZ (80-20 wt%) 8YSZ -, mv Sr 2 Co 0.9 Fe 0.1 NbO 5+ CGO10 LaSrFeO 4+ CSO10 SrCo 0.95 Sb 0.05 O 3- CNO20 La 2 Ni 0.8 Cu 0.2 O 4+ LSGM(9.8 20) La 2 Ni 0.8 Cu 0.2 O 4+ La 10 Si 5 AlO 26.5 Pr 2 Ni 0.8 Cu 0.2 O 4+ LSGM(9.8 20) T = 973 K p(o 2 ) = 0.21 atm i, ma/cm 2 Figure 6.1 Comparison of the overpotentials of various cathode layers at 973 K in air [14, 16, 29, 43, 47, 57, 61, 65, 241, 242]. 8YSZ is 8 mol% Y 2 O 3 -stabilized ZrO 2 ; SSZ is Sc 2 O 3 -stabilized ZrO 2 ; CGO10 and CGO20 correspond to Ce 0.9 Gd 0.1 O 1.95 and Ce 0.8 Gd 0.2 O 1.9, respectively; CSO10 is Ce 0.9 Sm 0.1 O 1.95, CNO20 is Ce 0.8 Nd 0.2 O 1.9, and LSGM(9.8 20) is (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d.a4 6 mm thick samaria-doped ceria interlayer between the electrode and the electrolyte was used in Ref. [43]. substitution of Nd for La and Fe for Co, which might provide better compatibility of the materials, reduces cell performance, and the effect of iron doping is much more pronounced. The thermal and chemical expansion of these materials remains, however, very high [8]. Among the SrCo 1 x Sb x O 3 d (x ¼ 0 0.2) cathodes applied onto Ce 0.8 Nd 0.2 O 2 d (CNO20) electrolyte, the composition with x ¼ 0.05 displays the highest conductivity and lowest polarization resistance (Figure 6.2), ranging from to 0.23 V cm 2 at K [14]. Stabilization of tetragonal (x ¼ ) or cubic (x ¼ 0.2) structure was reported to suppress abrupt changes in the lattice expansion at elevated temperatures, observed for the parent cobaltite due to phase transitions associated with thermally induced disordering. The apparent average linear thermal expansion coefficient (TEC) tends, however, to increase with x, reaching K 1 at K and x ¼ 0.15 [14]. Similar effects were found for the SrCo 1 x Nb x O 3 d system considered for cathode materials in lanthanum gallate-based cells [15]. For the tetragonal double perovskite Sr 2 Co 1 x Fe x NbO 5 þ d, the total conductivity and cathode performance decrease on increasing x [16]. Doping strontium cobaltite with iron, stabilizing the perovskite structure, does not reduce the lattice expansion on heating

9 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j273 (a) TEC 10 6, K -1 (b) log (S/cm) T = K, p 2 (O) = 0.21 atm La 2-x Sr x NiO 4 La 2 Ni 1-x Cu x O 4+ SrCo 1-x Sb x O 3- YBa(Co 1-x Zn x ) 4 O 7+ T = 1073 K p(o 2 ) = 0.21 atm (c) 0.8 log R η (Ω cm 2 ) T = 1073 K p(o 2 ) = 0.21 atm x Figure 6.2 Composition dependences of the average linear thermal expansion coefficients (a), total electrical conductivity (b), and cathode polarization resistance (c) of SrCo 1 x Sb x O 3 d [14], YBa(Co 1 x Zn x ) 4 O 7 þ d [37], La 2 x Sr x NiO 4d [62], and La 2 Ni 1 x Cu x O 4 þ d [60, 61, 66]. R g values were obtained in contact with ceria-based solid electrolytes, except for La 2 Ni 1 x Cu x O 4 þ d cathodes applied onto LSGM. In the case of YBa (Co 1 x Zn x ) 4 O 7 þ d and La 2 x Sr x NiO 4d, the polarization resistance values correspond to porous (YBa(Co 1 x Zn x ) 4 O 7 þ d Ce 0.8 Gd 0.2 O 1.9 (50 50 wt%) composite and cone-shaped electrodes, respectively [37, 62]. down to the level appropriate for electrode applications [8]. In addition to thermomechanical strains, Sr(Co,Fe)O 3 d exhibit typical ordering processes in the oxygen sublattice on cooling, which often result in the formation of brownmillerite-type phases and cause significant decrease in the oxygen ionic conductivity. As the partial substitution of Ba for Sr suppresses vacancy ordering and lowers the oxygen content variations, a significant attention was drawn to the (Ba,Sr)(Co,Fe)O 3 d system,

10 274j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends 4 3 SrCo 0.95 Sb 0.05 Sr 2 Co 0.9 Fe 0.1 NbO 5+δ Ba 2 CoMo 0.5 Nb 0.5 O 5+δ PrBa 0.5 Sr 0.5 Co 2 O 5+δ Electrolyte: doped ceria log R η (Ω cm 2 ) SmBa 0.5 Sr 0.5 Co 2 O 5+δ GdBa 0.5 Sr 0.5 Co 2 O 5+δ GdBaCo 2 O 5+δ La 2 Ni 0.9 Co 0.1 O 4+δ LSGM(10 20) -1 Ca 3 Co 4 O 9-δ La Ba Sr 0.4 Fe 0.8 Co Pr Ba Sr 0.4 Fe 0.8 Co 0.2 Sm Ba Sr 0.4 Fe 0.8 Co Sm 0.3 Sr 0.7 Co 0.95 Ce /T, K -1 Figure 6.3 Temperature dependences of the polarization resistance of Co-containing cathode layers in contact with ceria-based solid electrolytes in air [12, 14, 16, 25, 27, 28, 38, 41]. The data on La 2 Ni 0.9 Co 0.1 O 4 d applied onto La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 d electrolyte [55] are shown for comparison. R η, Ω cm Er Y R Ba Sr 0.4 Fe 0.8 Co 0.2, 923 K RBa 0.5 Sr 0.5 Co 2 O 5+δ, 973 K RBaCo 3 ZnO 7+δ, 973 K R 2 Ni 0.8 Cu 0.2 O 4+δ, 973 K p(o 2 ) = 0.21 atm Pr Sm La La (CN=6) Tb Gd Sm Pr (CN=9) R 3+ radius, Å Figure 6.4 Comparison of the polarization resistances of ferrite-, cobaltite- and nickelate-based cathodes with various A-site cations, in contact with CGO (gadolinia-doped ceria) [28, 36, 41] and (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d [57, 61] solid electrolytes.

11 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j275 particularly Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 d (BSCF) [17, 18, 20 24]. At the same time, problems of interaction with CO 2 and water vapor important in the IT range increase on barium incorporation [21]. Moreover, the cubic BSCF perovskite phase was demonstrated to undergo gradual decomposition into a perovskite phase mixture on cooling in air below 1173 K [22 24], making the use of BSCF in the IT SOFCs questionable. The apparent TECs of Ba 0.5 Sr 0.5 Co 1 x Fe x O 3 d (x ¼ ) contributed by chemical expansivity may achieve K 1 at K and p(o 2 ) ¼ 10 5 atm, depending on sample prehistory [24]. This may be responsible for the minimum polarization resistance observed at rather low (1173 K) sintering temperature of such electrodes applied onto CGO10 electrolyte [18]. Substantially lower TECs, relatively high mixed conductivity, and fast oxygen exchange kinetics drew a significant interest to the layered cobaltites where the state of Co cations is often more stable with respect to disordered perovskite analogues. Important compositional series are A 2 CoBO 5 þ d (A ¼ Sr, Ba; B ¼ Nb, Mo), RBaCo 2 O 5 þ d (R ¼ Pr, Gd Ho or Y), and RBaCo 4 O 7 þ d (R ¼ Dy Yb or Y) [8, 16, 25 37]; the electrochemical performance of selected cathodes is illustrated in Table 6.1 and Figures Compared to BSCF, the B-site cationordered double perovskite Ba 2 CoMo 0.5 Nb 0.5 O 5 þ d shows a considerably lower total conductivity, but a similar electrochemical activity owing to the effect of Mo doping, higher structural stability, and reduced reactivity with CSO20 [25]. The electrode layers of the A-site cation-ordered PrBaCo 2 O 5 þ d double perovskite applied onto CSO20 exhibit an area-specific resistance of 0.4 V cm 2 at 873 K in air; the corresponding SOFC with thin-film electrolyte was reported to have P max ¼ 620 mwcm 2 [26] La 0.19 Pr 0.21 Sr 0.26 Ca 0.34 Fe CSO SSZ Ba 1.2 Sr 0.8 CoO 4-δ -CGO10 (70-30 wt%) CGO10 Electrolyte: LSGM Sr 0.9 K 0.1 Fe YBaCo 4 O 7+δ YBaCo 3.2 Fe 0.8 O 7+δ La 2 Ni 0.8 Cu 0.2 O 4+δ Pr 2 Ni 0.8 Cu 0.2 O 4+δ -η, mv Electrolyte: La 10 Si 5 AlO 26.5 La 2 Ni 0.8 Cu 0.2 O 4+δ La 2 Ni 0.5 Cu 0.5 O 4+δ La 4 Ni 3 O 10-δ La 4 Ni 2.9 Cu 0.1 O 10-δ La 3.95 Sr 0.05 Ni 2 CoO 10-δ 50 T = 1073 K p(o 2 ) = 0.21 atm i, ma/cm 2 Figure 6.5 Comparison of the cathodic overpotentials of various half-cells with porous mixed conducting electrodes at 1073 K in air [29, 33, 34, 43, 45, 57, 61, 65, 66].

12 276j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Electrolyte: (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 YBaCo 4 O 7+δ [33, 34] YBaCo 3.2 Fe 0.8 O 7+δ YBaCo 4 O 7+δ -10 wt% Ag log R η (Ω cm 2 ) Electrolyte: Ce 0.8 Gd 0.2 O 2-δ YBaCo 3 ZnO 7+δ [37] YBaCo 3 ZnO 7+δ [36] ErBaCo 3 ZnO 7+δ [36] TbBaCo 3 ZnO 7+δ [36] /T, K Figure 6.6 Temperature dependences of the polarization resistances of porous RBa(Co,M) 4 O 7 - based cathode layers in contact with (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d and Ce 0.8 Gd 0.2 O 1.9 solid electrolytes, in air [33, 34, 36, 37]. 700 YBaCo 4 O 7+δ YBaCo 3.2 Fe 0.8 O 7+δ YBaCo 3 ZnO 7+δ T = 1073 K -η, mv wt% composites YBaCo 3 ZnO 7+δ -CGO20 YBaCo 2.5 Zn 1.5 O 7+δ -CGO20 YBaCo 2 Zn 2 O 7+δ -CGO i, ma/cm Figure 6.7 Cathodic polarization curves for YBa(Co,Fe) 4 O 7 þ d LSGM(9.8 20) [33, 34], YBaCo 3 ZnO 7 þ d CGO20 LSGM(20 20) [37], and YBaCo 1 x Zn x O 7 þ d YBaCo 1 x Zn x O 7 þ d CGO20 (50 50 wt%) CGO20 LSGM(20 20) [37] cells in air at 1073 K. CGO20 is Ce 0.8 Gd 0.2 O 1.9, LSGM(20 20) is La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 d, and LSGM(9.8 20) corresponds to (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d.

13 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j277 High oxygen surface exchange rate and a reasonable level of the oxygen ion diffusion in the ITrange were also reported for GdBaCo 2 O 5 þ d, leading to an excellent cathode performance [27] (Figure 6.3). The electrochemical properties of LnBa 0.5 Sr 0.5 Co 2 O 5 þ d (Ln ¼ Pr, Sm, Gd) as cathode materials for IT-SOFC were analyzed in Ref. [28]. A very low R g value, V cm 2 at 973 K, was observed for SmBa 0.5 Sr 0.5 Co 2 O 5 þ d electrode sintered on CGO10 at 1273 K to form symmetrical cells (Figures 6.3 and 6.4). Ba 1.2 Sr 0.8 CoO 4 þ d with K 2 NiF 4 -type structure [29] shows a worse performance compared to the perovskite-type cobaltites in contact with ceriabased electrolytes. Note that in many literature reports on the behavior of novel cobaltite-based electrode materials, their phase stability and/or expansivity critical for the practical applications were not assessed. At the same time, the apparent TECs of double-perovskite cobaltites are still high; the cobaltites with other layered structures usually possess a lower stability compared to the perovskite-type analogues and often undergo various phase transitions [8, 31 35]. In most cases, solid electrolyte additions to the cathode layers lower the strains and enable to enlarge the electrochemical reaction zone, resulting in better performance (Figure 6.8). Oxide materials derived from recently discovered RBaCo 4 O 7 (R ¼ lanthanide or Y), whose crystal structure consists of closely packed alternating triangular and Kagome layers of corner-sharing Co tetrahedra [30, 31], also attract a growing interest due to their unusual oxygen sorption and transport properties [32 37]. Porous YBaCo 4 O 7 - based cathodes show very high electrochemical activity in contact with LaGaO 3 - and ceria-based solid electrolytes at K (Table 6.1 and Figures 6.2 and ). However, at atmospheric oxygen pressure, YBaCo 4 O 7 -based compounds appear thermodynamically stable only above approximately K and metastable at lower temperatures. A slow oxygen uptake at K causes complete decomposition of YBaCo 4 O 7 þ d into a mixture of perovskite-like phases and binary 0.0 SmBa 0.5 Sr 0.5 Co 2 O 5+δ Ba 1.2 Sr 0.8 CoO 4-δ YBaCo 3 ZnO 7+δ 30 log R η (Ω cm 2 ) TEC 10 6, K T = 973 K 15 Electrolyte: (Ce,Gd)O 2-δ wt% of (Ce,Gd)O 2-δ Figure 6.8 Polarization resistances of composite cobaltite CGO cathodes in contact with Ce 0.9 Gd 0.1 O 1.95 (CGO10) [28, 29] and Ce 0.8 Gd 0.2 O 1.9 (CGO20) [37] solid electrolytes, and TECs of SmBa 0.5 Sr 0.5 Co 2 O 5 þ d CGO10 composite ceramics [28], at 973 K in air.

14 278j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends oxides when the average cobalt oxidation state approaches þ 3; decomposition is accompanied by a conductivity jump and dramatic volume contraction presenting a serious drawback to electrochemical applications. In the RBa(Co,M) 4 O 7 þ d (R ¼ Y, Er, Tb, Ca, In; M ¼ Zn, Fe, Al) series, several compositions such as YBaCo 4 x Zn x O 7 þ d (1 x 2) were found stable at the SOFC operating temperatures and thermomechanically compatible with the electrolyte materials [36, 37]. Although the substitution of Zn for Co decreases the electronic conductivity and cathode performance (Figures 6.2 and 6.6), under open-circuit conditions the YBaCo 3 ZnO 7 þ d and composite YBaCo 3 ZnO 7 þ d CGO20 (50 50 wt%) cathodes exhibit low R g values [37], comparable to those of materials derived from the perovskite-type cobaltites, while the cathodic overpotentials in air seem rather high (Figure 6.7). The electrochemical activity of porous RBaCo 3 ZnO 7 þ d layers increases in a sequence Er < Y < Tb [36] (Figures 6.4 and 6.6). The misfit compound Ca 3 Co 4 O 9 d, where the crystal structure built of alternating CdI 2 -type and rock salt slabs, also shows moderate expansion and was proposed as a promising cathode material [38]. The performance of porous Ca 3 Co 4 O 9 d electrodes is similar to that of layered nickelates (Figures 6.1 and 6.3) and may be further improved by CGO10 additions. As for the cobaltite-based materials, perovskite-type ferrites exhibit very high thermal and chemical expansion limiting their electrode applications. In general, however, incorporation of Fe cations into the cobaltite perovskites decreases both the thermal and chemical induced strains and the electrochemical activity. In the most widely studied perovskite-type ferrite system, La 1 x Sr x FeO 3 d, the highest level of electronic and oxygen ionic transport is observed at x 0.5 [39], while further acceptortype doping promotes vacancy ordering and hole localization processes, with a negative impact on the transport properties. Increasing difference of the R 3 þ and A 2 þ cation radii in R 0.5 A 0.5 FeO 3 d (R ¼ La, Pr, Nd, Sm; A ¼ Sr, Ba) results in higher oxygen deficiency and lower oxygen ionic and p-type electronic conductivities [40]. Contrary to the ionic conductivity variations, the role of surface exchange kinetics in the oxygen permeation processes tends to decrease on Ba 2 þ doping and on decreasing R 3 þ size in R 0.5 Sr 0.5 FeO 3 d series. In correlation with this behavior, the incorporation of Ba 2 þ reduces polarization resistance of R 0.58 x Ba x Sr 0.4 Fe 0.8 Co 0.2 O 3 d (R ¼ La, Pr, Sm) electrodes in contact with ceria-based electrolyte [41]; the best performance achieved for x ¼ and R ¼ Pr is close to the level of undoped cobaltites (Figures 6.3 and 6.4). The total electrical conductivity and cathode performance of rhombohedral R 0.5 A 0.5 FeO 3 d (R ¼ La, Pr; A ¼ Sr, Ca, Ba) was reported to increase with average A-site cation radius [42]. Very interesting results [43] were obtained on the mixed solid solutions R 1 x A x FeO 3 d (R ¼ La, Pr, Nd; A ¼ Ca, Sr), synthesized fixing the average A-site cation radius and cation size mismatch in order to isolate the effect of divalent dopant concentration from the steric effects. The maximum total conductivity, predominantly electronic in air, was found for La 0.19 Pr 0.31 Sr 0.26 Ca 0.24 FeO 3 d ceramics, while the La 0.19 Pr 0.21 Sr 0.26 Ca 0.34 FeO 3 d cathode layer showed minimum overpotentials in contact with scandia-stabilized zirconia (SSZ) and CSO [43], thus indicating correlations with the oxygen ionic conduction and/or oxygen vacancy ordering processes rather than with electronic transport. For La 0.8 A 0.2 FeO 3 d

15 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j279 (A ¼ Ca, Sr, Ba), the highest ionic conductivity and lowest polarization resistance of the composite cathodes calcined at 1373 K were found for La 0.8 Sr 0.2 FeO 3 d [44]. For porous layers formed of Co-free Sr 0.9 K 0.1 FeO 3 d perovskite [45], quite low overpotentials (Figure 6.5) and high power densities (Table 6.3) were observed in the cells based on La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3 d (LSGM(20 17)) solid electrolyte. The effects of potassium doping on the phase stability, interaction with solid electrolytes, and thermal expansion, and also potential volatilization require, however, further evaluation. The polarization resistances of dense thin-film SrTi 1 x Fe x O 3 d (x ¼ ) layers in contact with single-crystal YSZ solid electrolyte were reported comparable to those of LSFC and BSCF thin films [46]. It should also be mentioned that as for cobaltite-based materials, attractive combinations of physicochemical and thermomechanical properties may also be expected for layered ferrites. As a particular example, at 973 K the LaSrFeO 4 þ d porous electrode layers sintered on CSO10 at 1273 K show the open-circuit polarization resistance of 3.95 V cm 2 and cathodic overpotential of 57 mv at current density of 55 ma cm 2 [47]. These values are close to the other materials with K 2 NiF 4 -type structure (Table 6.1 and Figure 6.1). The incorporation of Ni or Cu cations in the perovskite materials is considered as a possible strategy to optimize their properties relevant for electrode applications. In general, such doping often makes it possible to improve the thermal expansion and electronic conductivity, but worsens phase stability; its effects on ionic transport are more complex [8]. As an example, the total polarization resistance of porous (La 0.6 Sr 0.4 ) 0.99 Co 0.9 Ni 0.1 O 3 d and (La 0.8 Sr 0.2 ) 0.99 Co 0.8 Ni 0.2 O 3 d electrodes in contact with CGO10 electrolyte is approximately 0.1 V cm 2 at 973 K in air [48]. The incorporation of Sr 2 þ in perovskite-type La 1 y Sr y Fe 1 x Ni x O 3 d increases the oxygen deficiency and ionic transport at elevated temperatures, but leads to a lower thermodynamic stability as reflected by narrowing the solid solution domain at 1373 K down to x 0.25 at y ¼ 0.10 and x 0.12 at y ¼ 0.20 in air [49]. The average thermal expansion coefficients of La 1 y Sr y Fe 1 x Ni x O 3 d (x ¼ , y ¼ ) ceramics vary in the ranges ( ) 10 6 K 1 at K and ( ) 10 6 K 1 at K, rising with strontium and nickel content. These additives also lower the temperature of orthorhombic! rhombohedral phase transition and increase the total conductivity. Doping with nickel was found to have a weak negative effect on the ionic transport, probably due to defect cluster formation involving oxygen vacancies and Ni 2 þ [49]. (La 1 x Sr x ) z Fe 1 y M y O 3 d (x ¼ ; y ¼ ; z ¼ ; M ¼ Ni, Cu) perovskites were proposed as alternative SOFC cathode materials [50 52]. However, the initial performance and long-term stability of (La 0.8 Sr 0.2 ) 0.95 Fe 0.8 Ni 0.2 O 3 d were found to be inferior to LSFC cathodes [50]. For (La 0.8 S 0.2 ) 0.95 Fe 1 y M y O 3 d system, the substitution level is limited by dopant solubility, close to 20 mol% Ni or Cu. Doping with copper and increasing z in (La 1 x Sr x ) z Fe 1 y M y O 3 d lower the melting point and consequently the sintering temperature [52]. These factors may not only enable to decrease interaction with solid electrolyte ceramics but also promote microstructural instability of the porous electrodes at the SOFC operating temperature, particularly their sintering. Similar to many other perovskite electrodes [8], the A-site cation deficiency in

16 280j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Table 6.3 Examples of the maximum power density in various single SOFCs with alternative anodes. Anode Electrolyte/thickness Cathode T (K) Fuel Pmax (W cm 2 ) Reference Ni 9.6 Fe (10 wt% Fe2O3-coated NiO) Ni CGO10 (50 wt% NiO), Ni6Fe7 support Ni 3.8 Fe CSO20 (50 50 wt%) CoFe CSO20 (50 50 wt%) LSGM(10 20), 6 mm CSO20, 0.5 mm CGO10/25 mm La 0.6 Sr 0.4 CoO 3 d CGO10 (50 50 wt%) LSGM(20 17)/0.3 mm, CSO20 interlayer LSGM(20 17)/0.3 mm, CSO20 interlayer Sm 0.5 Sr 0.5 CoO 3 d 973 Wet H [83] SrCo 0.8 Fe 0.2 O 3 d H [84] Wet H SrCo 0.8 Fe 0.2 O 3 d 1073 Wet H [87] Ce 0.8 Fe 0.2 O 2 d LSGM/0.88 mm Sm 0.5 Sr 0.5 Fe 0.8 Co 0.2 O 3 d 1073 Wet CH [88] La0.2Sr0.7TiO3 d 8YSZ/75 mm La0.6Sr0.4CoO3 d 1023 Wet H [90] impregnated with CGO20 and Cu Sr 0.88 Y 0.8 TiO 3 d 8YSZ (50 50 wt%), infiltrated with CeO 2 d and Ru La0.4Sr0.6Ti0.4Mn0.6O3 d 8YSZ (65 35 vol%) 8YSZ/10 mm La 0.65 Sr 0.3 MnO 3 d 8YSZ (50 50 wt%) 8YSZ/0.2 mm La0.65Sr0.3MnO3 d 8YSZ (50 50 wt%) LSM La0.8Sr0.2Cr0.82Ru0.18O3 d LSGM(10 20)/0.4 mm La0.6Sr0.4Fe0.8Co0.2O3 d CGO10 LSFC [86] 1073 H [91] 10 ppm 0.47 H 2 S H Wet H a) [94] Wet CH Wet H [116] (La0.75Sr0.25)0.95Cr0.5Mn0.5O3 d LSGMCo/0.6 mm Gd0.4Sr0.6CoO3 d 1123 Wet H a) [99] La0.75Sr0.25Cr0.5Mn0.5O3 d LSGM(20 17)/0.3 mm, SrCo0.8Fe0.2O3 d 1123 Dry H [109] CLO interlayer Dry CH La0.75Sr0.25Cr0.5Mn0.5O3 d LSGM(20 17)/0.25 mm SrCo0.8Fe0.2O3 d 1073 Dry H [102] 1123 Dry CH La0.75Sr0.25Cr0.5Mn0.5O3 d LSGM(10 20)/1.5 mm La0.75Sr0.25Cr0.5Mn0.5O3 d 1073 Wet 5% H [20] Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 d 1073 Wet 5% H [20]

17 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j281 La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 d LSGM(10 20)/0.12 mm La 0.8 Sr 0.2 MnO 3d 1073 Wet H [100] La0.75Sr0.25Cr0.5Mn0.5O3 d 8YSZ/0.25 mm (La,Sr)MnO3d 1173 Wet H [98] La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 d YSZ/83 mm La 0.8 Sr 0.2 MnO 3d impregnated with CSO20 La0.75Sr0.25Cr0.5Mn0.5O3 d impregnated with CSO20 La0.75Sr0.25Cr0.5Mn0.5O3 d impregnated with Ni La0.75Sr0.25Cr0.5Mn0.5O3 d impregnated with CSO20 þ Ni La0.75Sr0.25Cr0.5Mn0.5O3 d YSZ graded La0.75Sr0.25Cr0.5Mn0.5O3 d YSZ (50 50 wt%) La0.75Sr0.25Cr0.5Mn0.5O3 d YSZ (50 50 wt%), impregnated with Pd La 0.8 Sr 0.2 Cr 0.5 Mn 0.5 O 3 d YSZ (45 65 wt%) La 0.8 Sr 0.2 Cr 0.5 Mn 0.5 O 3 d YSZ (45 65 wt%), impregnated with Pd 8YSZ impregnated with Cu La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 d (35 65 wt%) Cu La0.75Sr0.25Cr0.5Mn0.5O3 d (20 80 wt%) Cu La0.75Sr0.25Cr0.5Mn0.5O3 d (20 80 wt%), sputtered Pt 1123 Dry H [96] Dry CH Dry H Dry CH Dry H Dry CH Dry H Dry CH YSZ/0.3 mm, CGO20 La0.8Sr0.2MnO3d YSZ 1173 Wet H [95] interlayer graded Wet CH YSZ/1 mm Pt 1073 CH [105] C 2 H 5 OH 0.01 YSZ/1 mm Pt 1073 CH YSZ/60 mm La 0.8 Sr 0.2 FeO 3 d YSZ (40 60 wt%) YSZ/60 mm La 0.8 Sr 0.2 FeO 3 d YSZ (40 60 wt%) 8YSZ/50 mm La0.8Sr0.2MnO3d YSZ (50 50 wt%) C2H5OH Wet H [107] 973 Wet H Dry H [112] CH C 4 H LSGM(20 17)/0.3 mm, SrCo0.8Fe0.2O3 d 1123 Dry H [109] CLO interlayer Dry CH LSGM(20 17)/0.25 mm SrCo0.8Fe0.2O3 d 1123 Dry H [102] Dry CH (Continued )

18 282j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Table 6.3 (Continued ) Anode Electrolyte/thickness Cathode T (K) Fuel P max (W cm 2 ) Reference Pr0.7Ca0.3Cr0.6Mn0.4O3 d 8YSZ/0.37 mm Pr0.7Ca0.3Cr0.6Mn0.4O3 d 1223 Wet H [111] 1223 Wet CH La0.8Sr0.2Sc0.2Mn0.8O3 d 10SSZ/0.3 mm La0.8Sr0.2Sc0.2Mn0.8O3d 1173 Wet H [119] 1173 Wet CH LaSr2Fe2CrO9 d CGO10 (50 50 wt%) CGO10/0.3 mm La0.6Sr0.4Fe0.8Co0.2O3 d CGO10 (50 50 wt%) 1073 Wet H [118] LSGM(10 20)/0.4 mm 1073 Wet H Sr 2 MgMoO 6 d LSGM(20 17)/0.3 mm Sr 0.9 K 0.1 FeO 3 d 1073 H [45] Sr2MgMoO6 d LSGM(20 17)/0.3 mm, CLO4 interlayer SrCo0.8Fe0.2O3 d 1073 Dry H [120] Wet H Dry CH Wet CH Sr2MgMoO6 d LSGM(20 20)/0.6 mm La0.6Sr0.4Fe0.2Co0.8O3 d 1073 Wet H [122] With CGO20 interlayer Wet H Sr2MnMoO6 d LSGM(20 17)/0.3 mm, SrCo0.8Fe0.2O3 d 1073 Dry H [120] CLO4 interlayer 5 ppm 0.57 H2S H2 Sr2Mg0.9Cr0.1MoO6 d SrCo0.8Fe0.2O3 d 1073 Dry H Sr 1.2 La 0.8 MgMoO 6 d LSGM(20 17)/0.3 mm, CLO5 interlayer 5 ppm 0.61 H2S H2 SrCo 0.8 Fe 0.2 O 3 d 1073 Dry CH [121] Wet CH Wet C 2 H Wet C3H Notes: In most cases, wet H 2 and wet CH 4 refer to the gas humidified under ambient conditions (3 %H 2 O); 10SSZ is 10 mol% Sc 2 O 3 -stabilized ZrO 2 ; CLO5 and CLO4 are Ce 0.4 La 0.5 O 2 d and Ce 0.6 La 0.4 O 2 d, respectively; LSGMCo is La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 3 d. a) Extrapolated.

19 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j283 (La 1 x Sr x ) z Fe 1 y M y O 3 d was found favorable to decrease thermal expansion and reactivity with 8YSZ. The Ni-substituted materials exhibit the highest conductivity in these compositional series and also have a greater reactivity with YSZ if compared to the parent ferrites, a result of reduced thermodynamic stability of the doped perovskite phases; no indication of chemical reaction or cation interdiffusion between (La 0.8 Sr 0.2 ) 0.95 Fe 0.8 Ni 0.2 O 3 d and gadolinia-doped ceria (CGO) was revealed by X-ray diffraction (XRD) analysis [52, 53]. At the same time, despite the lower electrical conductivity, La 0.8 Sr 0.2 FeO 3 d porous layers were reported to possess higher performance than La 0.7 Sr 0.3 Fe 0.8 Ni 0.2 O 3 d and LaNi 0.6 Fe 0.4 O 3 d applied onto YSZ electrolyte with CSO20 interlayer [51]. The effects of A-site nonstoichiometry on the electrochemical activity of La 1 x Fe 0.4 Ni 0.6 O 3 d were investigated using model cone-shaped electrodes and CGO10 electrolyte [54]; increasing x results in a separation of NiO, inhibiting oxygen reduction reaction. During the past decade, materials with Ruddlesden Popper (RP) structure built of n perovskite-like layers alternating with one rock salt sheet, in particular the nickelate series (Ln,A) n þ 1 Ni n O 3n þ 1, have attracted significant attention among most promising groups of the cathode materials (Figures 6.1, 6.2, and 6.9 and Table 6.2). Their advantages include a relatively high level of oxygen ionic transport, reasonable electronic conductivity, and moderate thermal expansion [8, 9, 55 62]. Increasing the number of perovskite layers in the RP nickelates (n) usually leads to higher ionic and electronic conduction [55 57]. These effects are primarily associated with increasing concentration of Ni O Ni bonds responsible for the electronic transport, progressive delocalization of the p-type electronic charge carriers, and increasing vacancy migration contribution to the oxygen ion diffusivity. Note that except for oxygen hyperstoichiometric La 2 NiO 4 þ d and its analogues, where anion diffusion is essentially dominated by the interstitial migration, most RP nickelates are oxygen deficient at elevated temperatures. As a particular consequence, the electrochemical activity of La n þ 1 Ni n O 3n þ 1 d cathodes becomes substantially higher when n increases from 1 to 3 [55, 56]. Analogous results were also obtained for praseodymium nickelate electrodes where Pr 4 Ni 3 O 10 d phase is formed due to oxidative decomposition of undoped [58] or Cu-doped [57] Pr 2 NiO 4 þ d (Figures and 6.9 and Table 6.2). Modest A-site deficiency was found to decrease the polarization resistance of Nd 1 x NiO 4 þ d porous layers in contact with 8 mol% 8YSZ solid electrolyte [58]. An opposite trend was, however, reported in Ref. [59] where the La 3 Ni 2 O 7 d cathode displayed higher overpotentials than La 2 NiO 4 þ d in contact with CSO20 2 mol% Co-doped ceramics. Another necessary comment is related to the higher concentration of A-site cations in the RP phases in comparison with their perovskite-type analogues. As for the other factors including thermodynamic stability of the electrode material phase or the presence of high-diffusivity additives, the concentration of A-site cations may substantially affect reactivity with the solid oxide electrolytes. For example, chemical reaction between La 2 Ni 1 x Cu x O 4 þ d (x ¼ 0 1) and 8YSZ leads to the formation of the insulating La 2 Zr 2 O 7 pyrochlore phase and is observed at relatively low temperatures, such as 1173 K [60]. This effect becomes more pronounced with increasing x, leading to a worse cathode performance. Therefore, the layered nickelate cuprate electrodes

20 284j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends La 2 Ni 0.8 Cu 0.2 O 4+δ Electrolyte: 8YSZ LSGM(9.8 20) La 10 Si 5 AlO 26.5 La 2 NiO 4+δ Electrolyte: 8YSZ CGO10 LSGM(10 20) log R η (Ω cm 2 ) p(o 2 ) = 0.21 atm La 2 Ni 0.9 Co 0.1 O 4+δ La 2 Ni 0.8 Cu 0.2 O 4+δ La 4 Ni 3 O 10-δ Electrolyte: LSGM(10 20) La 10 Si 5 AlO 26.5 Electrolyte: LSGM Pr 2 Ni 0.8 Cu 0.2 O 4+δ 2 1 La 2 NiO 4+δ La 1.9 Sm 0.1 NiO 4+δ 0 La 0.9 Sm 1.1 NiO 4+δ La 3 Ni 2 O 7-δ -1 La 4 Ni 3 O 10-δ /T, K Figure 6.9 Temperature dependences of the polarization resistance of porous [55 58, 60, 61, 65, 66] and cone-shaped [62] nickelate cathodes in contact with various solid electrolytes in air. should be used in contact with doped ceria or lanthanum gallate electrolytes rather than with YSZ (Figure 6.9). At K, porous Pr 2 Ni 0.8 Cu 0.2 O 4 þ d electrodes deposited on (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d (abbreviated as LSGM(9.8 20)) electrolyte substrates exhibit lower polarization resistances compared to La 2 Ni 0.8 Cu 0.2 O 4 þ d (Figures 6.4 and 6.9 and Table 6.2), while cathodic reduction decreases their performance [57, 61]. Under oxidizing conditions, the extensive oxygen uptake at temperatures below K leads to reversible decomposition of Pr 2 NiO 4 - based solid solutions into Pr 4 Ni 3 O 10 and praseodymium oxide phases [57]. The substitution of copper for nickel decreases the oxygen content and phase transition temperature, while the incorporation of iron cations has an opposite effect. Both types of doping tend to decrease stability in reducing and even inert environment. The steady-state oxygen permeability of Pr 2 NiO 4 þ d ceramics at K, limited

21 6.3 Novel Cathode Materials for Solid Oxide Fuel Cells: Selected Trends and Compositions j285 by both surface exchange kinetics and bulk ionic conduction, is similar to that of La 2 NiO 4 þ d. The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation and is also associated with significant volume changes revealed by dilatometry [57]. The area-specific resistance of coneshaped La 2 x Sr x NiO 4 þ d electrodes in contact with CGO10 electrolyte [62] is close to that of perovskite electrodes, being yet much higher than for the best ferrite cobaltite materials. The lowest value, 23.8 V cm 2 in air at 873 K, was found for La 1.75 Sr 0.25 NiO 4 þ d [62]. Apatite-type La 10 x (SiO 4 ) 6 O 2d silicates and their derivatives possess a substantially high oxygen ionic conductivity, moderate thermal expansion, low electronic transport in a wide range of oxygen chemical potentials, and relatively low costs, and may thus be considered for potential use as SOFCs electrolytes [7, 63, 64]. The performance of mixed conducting cathodes in contact with silicate ceramics is, however, usually lower compared to other solid electrolytes; the high polarization resistance originates primarily from the surface diffusion of silica, partially blocking the electrochemical reaction zone, without formation of secondary phases detectable by XRD [65, 66]. This explains very poor performance of La 0.75 Sr 0.25 Mn 0.8 Co 0.2 O 3 d - based cathode layers cosintered with La 9 SrSi 6 O 26.5 electrolyte at 1673 K, while no diffusion of the electrode components into the apatite ceramics was detected [67]. The electrochemical activity of porous La 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3 d Ce 0.8 Gd 0.2 O 2 d (CGO20), La 0.7 Sr 0.3 MnO 3 d CGO20, SrMn 0.6 Nb 0.4 O 3 d,sr 0.7 Ce 0.3 Mn 0.9 Cr 0.1 O 3 d, Gd 0.6 Ca 0.4 Mn 0.9 Ni 0.1 O 3 d,la 2 Ni 1 x Cu x O 4 þ d (x ¼ 0.2, 0.5), La 2 Ni 0.8 Cu 0.2 O 4 þ d Ag, LaSr 2 Mn 1.6 Ni 0.4 O 7 d,la 4 Ni 3 x Cu x O 10 d (x ¼ 0 0.1), and La 3.95 Sr 0.05 Ni 2 CoO 10 d electrodes was studied at K in contact with apatite-type La 10 Si 5 AlO 26.5 electrolyte [65, 66]; note that the level of oxygen ionic conductivity in the latter electrolyte is higher than that of YSZ in the intermediate temperature range [64]. In all cases, however, the polarization resistances and overpotentials of nickelatebased cathodes applied onto silicate electrolyte are substantially higher compared to similar layers applied onto LSGM(9.8 20); see Figures 6.1, 6.5, 6.9 and 6.10 and Table 6.2. The electrochemical activity of porous nickelate-based layers was found to correlate with the concentration of mobile ionic charge carriers and bulk oxygen transport, thus decreasing in the series La 4 Ni 2.9 Cu 0.1 O 10 d > La 4 Ni 3 O 10 d > La 3.95 Sr 0.05 Ni 2 CoO 10 d and on copper doping in K 2 NiF 4 -type La 2 Ni 1 x Cu x O 4 d (Figure 6.5). Compared to the intergrowth nickelate materials, the manganite-based electrodes exhibit substantially worse electrochemical properties (Figure 6.10), in correlation with the level of oxygen ionic and electronic conduction in Mn-containing phases [66]. Due to the effects of cation interdiffusion between the cell components, the performance of mixed conducting cathodes applied onto La 10 Si 5 AlO 26.5 can be improved by reducing electrode fabrication temperature [65]. Qualitatively similar results were obtained in [68] for La 0.8 Sr 0.2 MnO 3d, La 0.7 Sr 0.3 FeO 3 d, La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 d, and La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 d cathodes and for La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 d, NiO CGO, and Sr 2 MgMoO 6 d anodes applied onto La 10 Si 5.5 Al 0.5 O solid electrolyte. The information on electrochemical behavior of various electrode materials in contact with apatite-type solid electrolytes is, however, still scarce and nonsystematic.

22 286j 6 Electrodes for High-Temperature Electrochemical Cells: Novel Materials and Recent Trends Sr 0.7 Ce 0.3 Mn 0.9 Cr 0.1 Gd 0.6 Ca 0.4 Mn 0.9 Ni 0.1 SrMn 0.6 Nb 0.4 LaSr 2 Mn 1.6 Ni 0.4 Ce 0.3 O 7-δ Electrolyte: La 10 Si 5 AlO 26.5 T = 1073 K -η, mv (La 0.75 Sr 0.25 ) 0.95 Mn 0.5 Cr 0.5 LSGM(9.8 20) wt% composites La 0.8 Sr 0.2 Fe 0.8 Co 0.2 -CGO20 La 0.7 Sr 0.3 Mn -CGO i, ma/cm Figure 6.10 Cathodic polarization curves of Fe-, Cr-, and Mn-containing electrode layers in contact with apatite-type La 10 Si 5 AlO 26.5 solid electrolyte [65, 66]. The data on (La 0.75 Sr 0.25 ) 0.95 Cr 0.5 Mn 0.5 O 3 d applied onto (La 0.9 Sr 0.1 ) 0.98 Ga 0.8 Mg 0.2 O 3 d [104] are shown for comparison. Pt mesh current collectors were used in all cases. The electrochemical performance of Ca-, Ag-, and Sr-doped pyrochlore-type Bi 2 Ru 2 O 7 and their composites with 20 mol% erbia-stabilized d-bi 2 O 3, which exhibits a very high level of the oxygen ionic transport (Chapters 2 and 9 of the first volume), was evaluated in contact with ceria-based electrolyte [69]. At 973 K, the area-specific resistances of undoped, 5 mol% Ca-doped, and 5 mol% Sr-doped bismuth ruthenate electrode were 1.45, 1.24, and 1.41 V cm 2, respectively. Addition of erbia-stabilized bismuth oxide up to wt% made it possible to further reduce the electrode resistance down to V cm 2 [69]. Due to high reactivity and thermal expansion of Bi 2 O 3 -containing materials, this family of electrode compositions may be considered for potential use in the electrochemical cells based on doped Bi 4 V 2 O 11 (BIMEVOX) solid electrolytes; their stability in ceria-based cells is rather problematic. 6.4 Oxide and Cermet SOFC Anodes: Relevant Trends Systematic information on the anode performance available in the literature is essentially limited to the electrochemical cells with zirconia-, ceria-, and lanthanum gallate-based electrolytes (see Chapters 9 and 12 of the first volume and Chapter 5 of this book). The latter group of solid oxide electrolytes refers primarily to La 1 x Sr x Ga 1 y Mg y O 3 d (LSGM); the long-term stability of transition metal cationdoped gallates under anodic conditions still requires further investigations. The two

23 6.4 Oxide and Cermet SOFC Anodes: Relevant Trends j287 major approaches, which can be identified in the developments of SOFC anode materials, are an optimization of cermet compositions and a search for novel oxide phases with high electrochemical activity and sufficient thermodynamic stability both under oxidizing and reducing conditions [5, 8, 10, 70 72]. Continuous attention is drawn to the Ni-ceria-based cermets. The advantages of ceria as the anode component originate, first of all, from a very high catalytic activity for the combustion reactions involving oxygen, particularly to carbon oxidation beneficial for the fuel cells operating on hydrocarbons and biogas. In addition, reduced CeO 2 d and its derivatives possess a substantial mixed oxygen ionic and n-type electronic conductivity; the transport properties and reducibility can be enhanced by acceptor-type doping (Chapter 9 of the first volume). Nevertheless, the use of ceria-based compositions without any metallic phase results in quite poor anode performance, even with current collectors formed of Au ink and Pt paste [20, 88]. The electrocatalytic activity of Ni Ce 0.8 Ti 0.2 O 2 d (60 wt% NiO) was found higher compared to Ni CeO 2 d cermet anode under 10% CH 4 fuel, a result of the higher conductivity of doped ceria [73]. A Ni Cu CeO 2 d ( vol%) anode fired at reduced temperature, 1173 K, onto CGO10 electrolyte, had R g < 0.1 V cm 2 at 873 K under wet 50% H 2 [74]. However, as the cermets with ceria-based phases suffer from dimensional instabilities caused by the local p(o 2 ) variations and minor TEC mismatch, the presence of one redox-stable phase with moderate thermal expansion, such as YSZ, is still desirable [8, 75]. Taking into account the durability issues related to carbon deposition and sulfur poisoning, efforts are being undertaken to use other transition metal components or lower Ni concentrations (Figures and Table 6.3). The anode assemblies consisting of one Ni CGO10 functional layer and a Ni La 0.9 Mn 0.8 Ni 0.2 O 3 d contact layer were proposed to reduce nickel 400 Electrolyte: LSGM 320 T = 1073 K, 10% H 2 -N 2 ( wt%) composites Ni - 8YSZ - CGO20 Ni - 8YSZ - Ce 0.8 Ca 0.2 V Ni - Gd 1.86 Ca 0.14 Ti 2 O 7 - CGO η, mv Ni - Gd 1.86 Ca 0.14 Ti 2 O 7 - CGO CeO 2-δ -modified Electrolyte: YSZ 80 T = 1223 K, 100% H 2 La 0.6 Sr 0.4 V -YSZ (50-50 wt%) i, ma/cm T = 1123 K, 100% H 2 Ni 2.2 Co-YSZ (65-35 wt%) Figure 6.11 Anodic overpotentials of La 0.6 Sr 0.4 VO 3 d YSZ composite [125] and various Nicontaining cermets with current collectors made of Pt mesh [75] and Pt Rh mesh embedded in the electrode layer [82] in H 2 -containing environments. Experimental conditions are given in the legend.