The most widely used Solid Oxide Fuel Cell (SOFC) anode materials, Ni-cermets, are susceptible to coking in hydrocarbon fuels as well as many coal- and bio-derived syngas compositions. Ni-based anodes are also poisoned by fuel impurities such as hydrogen sulfide, and can be structural vulnerable during repeated redox cycling. These problems can be largely avoided by implementing fuel processing measures, but not without sacrificing the overall power plant cost effectiveness and efficiency.

A number of different conducting oxide materials have been proposed as anodes that can avoid the above-mentioned problems with Ni-based anodes. However, these oxide anodes exhibit much higher polarization resistance than anodes containing Ni or other catalytic metals. The mechanisms that limit hydrogen oxidation rates in these oxide anodes have not been determined. Several studies of chromite, titanate-manganite, chromite-manganite, and doped ceria anodes have suggested that hydrogen adsorption was a possible rate-limiting step, along with hydrogen electrochemical oxidation. Recent electrochemical impedance spectroscopy (EIS) measurements on SOFCs with SrTi_0.3Fe_0.7O_3-δ (STF)-Gd_0.1Ce_0.9O₂ (GDC) anode as a function of hydrogen partial pressure provided direct evidence that a H₂adsorption mechanism was an important rate-limiting step in the anode reaction.

Here we present new results on (La,Sr)(Ga,Mg)O₃ (LSGM) electrolyte-supported SOFCs with STF and (La,Sr)(Cr,Fe)O_3-δ (LSCrFe) based oxide anodes. The results show clear evidence that adsorption becomes the rate-limiting step at low temperature and low fuel hydrogen partial pressure. Figure 1 shows an example of the current-voltage results for an STF anode cell, where there is a limiting current that decreases with decreasing hydrogen partial pressure.  Figure 2 shows that the limiting current decreases with decreasing temperature, indicating that it is an activated process and hence not a gas diffusion limitation.  A model accounting for hydrogen adsorption and electrochemical oxidation as possible rate-limiting steps is developed and used to fit current-voltage and EIS results from the oxide anode cells. The fits, shown in Figures 1 and 2, are good for cell voltages from open circuit down to ~ 0.5 V.  Possible reasons for the deviation at lower cell potentials will be discussed in the talk.  It is also shown that the model is consistent with results for other oxide anodes, suggesting that the dissociative adsorption of H₂can be an important rate-limiting step for various oxide anodes under SOFC operating conditions, especially lower temperature and hydrogen partial pressure. Moreover, both the experimental data and model fitting results suggest that the precipitation of metal particles at the anode surface, like Ru in LSCrFe based anodes and Ni in STF based anodes, effectively promote hydrogen adsorption.

Figuer captions:

Fig. 1 Current-voltage data and fitting results for LSGM electrolyte supported cell with STF anode at 800 ^oC versus the hydrogen partial pressure

Fig. 2 Current-voltage data and fitting results for LSGM electrolyte supported cell with STF anode at various temperatures under wet H₂