Effect of carbonate anions on Bi-doped Ca2Ru2O7 pyrochlores that are potential cathode catalysts for low temperature carbonate fuel cells

Abstract

This ex situ electrochemical study investigates how the oxygen reduction reaction (ORR) on Bi-doped Ca₂Ru₂O₇ pyrochlore catalysts is affected by the addition of carbonate to the aqueous KOH (1 mol dm⁻³) electrolyte. The parent Ca₂Ru₂O₇ catalyst has been previously reported to be selective towards the generation of CO₃²⁻ on the reaction of O₂ with CO₂ (in the presence of H₂O) at the cathode of low temperature alkaline polymer electrolyte fuel cells containing alkaline anion-exchange membranes (AAEM): a target is to develop low temperature carbonate fuel cells involving CO₃²⁻ conduction through the AAEM (for potential CO₂ utilisation). Rotating ring disk electrode (RRDE) voltammetry was used to probe the ORR behaviours of Ca₂Ru_2−xBi_xO_7−y catalysts with x = 0.25, 0.5, 0.75, and 1. The results show that as more Bi was doped into the pyrochlore catalysts, the poorer the on-set potentials compared to the parent Ca₂Ru₂O₇ (which itself yielded a poorer on-set potential to a benchmark Pt black catalyst). Higher levels of Bi-doping tended to reduce n values with higher levels of peroxide generated: all of the pyrochlore catalysts tested gave higher peroxide yields compared to the Pt black benchmark. However, the presence of CO₃²⁻ in the O₂-saturated KOH (1 mol dm⁻³) electrolyte appeared to improve kinetic performance of the Bi-doped pyrochlore catalysts (the effect being greatest with the x = 0.75 catalyst).