SOFC Cathode Electrode

When the electrode material and electrol)de material possess only electronic and ionic conductivity, respectively, such as Sr-doped LaMnOg (LSM) electrode and YSZ electrolyte, ese criteria are fulfilled in the vicinity of TPB. 

Strontium-doped lanthanum manganite, La1 3.Sr3.MnO3 (LSM) is developed by doping lanthanum manganite with a small fraction (10%-20%) of strontium in order to enhance electronic conductivity of the cathode electrode. Lanthanum strontium cobalt ferrite (LSCF) is a mixture of lanthanum oxide, strontium oxide, cobalt oxide, and iron oxide with typical composition given as Lao eSro iCoo Fco sOs. 

While the thickness of a typical cathode electrode in an SOFC is around 50 gm, research is also in progress to develop a cathode-supported SOFC cell that supports a thinner electrolyte deposited over a cathode electrode of thickness on the order of 350-1500 gm. Table 9.7 lists the properties of anode, cathode, and electrolyte materials. 

Properties of SOFC Anode, Cathode, and Electrolyte Materials 

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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. 

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. 

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... 

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. 

As we have seen in the previous sections, our understanding of SOFC cathode mechanisms often hinges on interpretation on the magnitude and time scale of electrochemical characteristics. However, these characteristics are often strongly influenced by factors that have nothing to do with the electrode reaction itself but rather the setup of the experiment. In this section we point out two commonly observed effects that can potentially lead to experimental artifacts in electrochemical measurements (1) polarization resistance caused gas-phase diffusion and (2) artifacts related to the cell geometry. As we will... 

Moreover, despite the many advances in electrochemical measurement and modeling, our understanding of SOFC cathode mechanisms remains largely circumstantial today. Our understanding often relies on having limited explanations for an observed phenomenon (e.g., chemical capacitance as evidence for bulk transport) rather than direct independent measures of the mechanism (e.g., spectroscopic evidence of oxidation/reduction of the electrode material). At various points in this review we saw that high-vacuum techniques commonly employed in electrocatalysis can be used in some limited cases for SOFC materials and conditions (PEEM, for example). New in-situ analytical techniques are needed, particularly which can be applied at ambient pressures, that can probe what is happening in an electrode as a function of temperature, P02, polarization, local position, and time. 

Matsuzaki, Yasuda (2001), Dependence of SOFC Cathode Degradation by Chromium-containing Alloy on Compositions of Electrodes and Electrolytes ,). Electrochem. Soc., 148, A126. 

Odgaard, M., and Skou, E. (1996). SOFC cathode kinetics investigated by the use of cone shaped electrodes the effect of polarization and mechanical load. Solid State Ionics 86-88 1217-1222. 

There is an increasing move from LSM to mixed ionic-electronic conductors for use in the SOFC cathode. These mixed conductors can remove theoretically provide all of the electrode material requirements in a single phase. This can open the entire electrode surface for reaction by... 

Perovskites as SOFC cathode material electrode-electrolyte reactions and electrochemical characterisation. Solid Oxide Fuel Cells 10 (Sofc-X), Pts 1 and 2 7 1015... 

Matsuzaki Y, Yasuda I (2001) Dependence of SOFC cathode degradation by chromium-containing alloy on compositions of electrodes and electrolytes. J Electrochem Soc 148(2) A126-A131... 

Materials suitable for an SOFC cathode have to satisfy the following requirements high electronic conductivity stability in oxidizing atmospheres at high temperature thermal expansion match with other cell components compatibility and minimum reactivity with different cell components sufficient porosity to allow transport of the fuel gas to the electrolyte/electrode interface [148-150]. 

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