Carbonate Selective Ca₂Ru₂O7−yPyrochlore Enabling Room Temperature Carbonate Fuel Cells I. Synthesis and Physical Characterization

The electrochemical selectivity of a Ca 2 Ru 2 O 7−y pyrochlore catalyst towards carbonate formation was investigated and compared to platinum. During constant current operation, an increase in CO 2 content was observed at the anode effluent when Ca 2 Ru 2 O 7−y was used as the cathode catalyst, compared to Pt/C. Carbonate selectivity for Pt/C was 1.78, while for Ca 2 Ru 2 O 7−y it was 7.33, a 4.1X increase, which confirmed the preferential formation of carbonate on the pyrochlore. This elevated selectivity was attributed to the high surface basicity of the pyrochlore catalyst, which led to preferential CO 2 adsorption. During constant voltage operation, a 134% CO 2 concentration increase was measured at the anode effluent when 10% CO 2 was supplied to the cathode inlet. Optimal fuel cell performance on the carbonate cycle was obtained with a 90% O 2 / 10% CO 2 cathode concentration, where maximum currents where 4X compared to operation with no CO 2 . Higher CO 2 concentrations were matched with a gradual decrease in performance. This was attributed to a relatively low electrochemical activity, which combined with high surface basicity, led to O 2 site blocking. Thin-film electrode experiments showed excellent electrochemical stability on the oxygen reduction region. An increase in the current and a positive shift in the half-wave potential were observed when CO 2 was added to the electrolyte in ex-situ three electrode experiments.