SOFC Cathode

Oxygen Reduction The overall reaction for the oxygen reduction at a SOFC cathode can be written as... 

Simner SP, Anderson MD, Pederson LR, and Stevenson JW. Performance Variability of La(Sr)Fe03 SOFC Cathode with Pt, Ag, and Au Current Collectors. J Electrochem Soc 2005 152 A1851-A1859. 

The impedance polarization performance of LSM electrode is closely related to the mechanism and kinetics of the oxygen reduction reactions. 02 reduction at SOFC cathodes is the most heavily studied subject, and this subject is sufficiently broad and complex to warrant its own review. Interested readers should consult the recent excellent articles by Adler [1] and Fleig [55], Here, only the polarization performance and its influencing factors are discussed. 

An alternative to the Co-rich perovskites is the Sr-doped LaFe03 which has a lower thermal expansion coefficient and a superior chemical compatibility with doped Ce02 electrolyte. LaFe03 is expected to be more stable than Ni- and Co-based perovskites because the Fe3+ ion has the stable electronic configuration 3d5. It is, therefore, expected that compositions in the system (La,Sr)(Co,Fe)03 will have desirable properties for intermediate temperature SOFC cathode applications. 

The suitability of lanthanum nickelate as an SOFC cathode has been examined by Virkar s group [138], They showed that LN performed poorly as a single-phase cathode in an anode-supported YSZ cell. However, with an SDC/LN composite interlayer the performance of the LN cathode increased substantially and the maximum power density of the cell with a YSZ thin electrolyte (-8 pm) was -2.2 Wear2 at 800°C, considerably higher than 0.3 to 0.4 Wcm-2 of similar cells with only LN or SDC interlayer. The results are significant as it shows that the composite MIEC cathodes perform much better than single-phase MIEC in the case of LN despite its mixed ionic and electronic conductivity. 

The perovskite oxides used for SOFC cathodes can react with other fuel cell components especially with yttria-zirconia electrolyte and chromium-containing interconnect materials at high temperatures. However, the relative reactivity of the cathodes at a particular temperature and the formation of different phases in the fuel cell atmosphere... 

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

Single-phase perovskite MIECs such as Sr-doped lanthanum cobaltite (LSC), lanthanum ferrite (LSF), lanthanum cobalt ferrite (LSCF) and samarium cobaltite (SSC), and Ca-doped lanthanum ferrite (LCF) [13] are sometimes used alone in SOFC cathodes, as depicted in the lower right-hand comer of Figure 6.1, but combining an... 

Tubular SOFC cathode supports with diameters or distance between flat faces on the order of 1 to 2 cm are commonly prepared by extrusion. Extrusion is a wet-ceramic process used to prepare tubes, and one which facilitates the formation... 

Hung MH, Madhava RMV, and Tsai DS. Microstructures and electrical properties of calcium substituted LaFe03 as SOFC cathode. Mater. Chem. Phys. 2007 101 297-302. 

Hansen KK, Spgaard M, and Mogensen M. Gd06Sr04Fe08Co02O3 5 A novel type of SOFC cathode. Electrochem. Solid State Lett. 2007 10 B119-B121. 

Hagiwara A, Hobara N, Takizawa K, Sato K, Abe H, and Naito M. Preparation and evaluation of mechanochemically fabricated LSM/ScSZ composite materials for SOFC cathodes. Solid State Ionics 2006 177 2967-2977. 

Simner SP, Anderson MD, Templeton JW, and Stevenson JW. Silver-perovskite composite SOFC cathodes processed via mechanofusion. J. Power Sources 2007 168 236-239. 

Bebelis S, Kotsionopoulos N, Mai A, Rutenbeck D, and Tietz F. Electrochemical characterization of mixed conducting and composite SOFC cathodes. Solid State Ionics 2006 177 1843-1848. 

Huang Y, Vohs JM, and Gorte RJ. SOFC cathodes prepared by infiltration with various LSM precursors. Electrochem. Solid State Lett. 2006 9 A237-A240. 

Hence, catalysis related challenges for SOFC cathode are the development of cathode specifications, i.e., material and microstructure, having high catalytic activity for oxygen reduction at 600 °C, high electron and ion conductivity, and a low sensitivity for poisoning by volatile Cr species. Again, as for the anode, cost and compatibility related requirements have to be considered. 

Summary Platinum as a Framework for Understanding Other SOFC Cathodes... 

Summary Importance of the Bulk for Mixed-Conducting SOFC Cathodes... 

Factors Complicating our Understanding of SOFC Cathode Mechanisms... 

This review focuses on the factors governing SOFC cathode performance—advances we have made over... 

Figure 2. Common strategies for SOFC cathodes (a) porous single-phase electronically conductive oxide such as (La,Sr)Mn03 (LSM) (b) porous single-phase mixed conductor (c) porous two-phase composite. The SEM micrograph of LSM on YSZ in a is adapted from ref 84. (Adapted with permission from ref 84. Copyright 1997 Swiss Federal Institute of Technology.)...

Figure 4. Some mechanisms thought to govern oxygen reduction in SOFC cathodes. Phases a, and y refer to the eiectronic phase, gas phase, and ionic phase, respectiveiy (a) Incorporation of oxygen into the buik of the electronic phase (if mixed conducting) (b) adsorption and/or partial reduction of oxygen on the surface of the electronic phase (c) bulk or (d) surface transport of or respectively, to the oJy interface, (e) Electrochemical charge transfer of or (f) combinations of and e , respectively, across the aJy interface, and (g) rates of one or more of these mechanisms wherein the electrolyte itself is active for generation and transport of electroactive oxygen species.

In the case of SOFCs, a large volume of work shows that for many SOFC electrodes, overall performance scales with the ID geometric length of this three-phase boundary. As such, the TBP concept and electrode performance models based on it have proven to be some of the most useful phenomenological concepts for guiding design and fabrication of SOFC cathodes, particularly the microstructure. 

Although the SOFC community has generally maintained an empirical approach to the three-phase boundary longer than the aqueous and polymer literature, the last 20 years have seen a similar transformation of our understanding of SOFC cathode kinetics. Few examples remain today of solid-state electrochemical reactions that are not known to be at least partially limited by solid-state or surface diffusion processes or chemical catalytic processes remote from the electrochemical—kinetic interface. 

The literature reviewed in sections 2—3.6 has shown that oxygen reduction on Pt is quite complex, involving several possible rate-limiting (or co-limit-ing) steps. As we will see in sections 4 and 5, this complexity is a universal feature of all SOFC cathodes, with many of the same themes and issues reappearing for other materials. We therefore highlight below several general observations about the mechanism of Pt that frame the discussion for other solid-state gas-diffusion electrodes involving O2. These observations are as follows. 

One of the first such kinetic studies of a perovskite mixed conducting electrode was reported by Ohno and co-workers in 1981, who found Lai /la/IoOs-a to have better kinetic properties than Pt as an SOFC cathode at 1000—1100 °C. °° A number of other JiepomOKizeo of general formula Lai jSr rMOs a (M = Cr, Mn, Fe, Co) were later studied by Takeda et al. ° To avoid reaction of the perovskites with the YSZ... 

Indeed, in the spirit of this latter comment, the purpose of this review is to consolidate our understanding of SOFC cathodes so that it becomes easier to identity and propose new avenues of research. 

In this section we saw that the active region of a SOFC cathode can be significantly enhanced by incorporating a mixed conductor (a material which conducts both ions and electrons). While electrodes of this type have proven challenging to implement in a SOFC operating environment (see section 6), they nonetheless have taught us a lot about what factors can limit electrode performance and opened the realm of possibilities for future materials development. To summarize, some of the salient points of our current understanding are listed. 

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