Designing bifunctional perovskite catalysts for the oxygen reduction and evolution reactions

Abstract

The development of unified regenerative fuel cells (URFCs) necessitates an active and stable bifunctional oxygen electrocatalyst. The unique challenge of possessing high activity for both the oxygen reduction (ORR) and oxygen evolution (OER) reactions, while maintaining stability over a wide potential window impedes the design of bifunctional oxygen electrocatalysts. Herein, two design strategies are explored to optimize their performance. The first incorporates active sites for the ORR and OER, Mn and Co, into a single perovskite structure, which is achieved with the perovskites Ba_0.5Sr_0.5Co_0.8Mn_0.2O_3−δ (BSCM) and La_0.5Ba_0.25Sr_0.25Co_0.5Mn_0.5O_3−δ (LBSCM). The second combines an active ORR perovskite catalyst (La_0.4Sr_0.6MnO_3−δ (LSM)) with an OER active perovskite catalyst (Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF)) in a physical mixed composite (BSCF/LSM). The success of the two strategies is investigated by measuring the catalysts’ catalytic performance and response to alternating reducing and oxidizing potentials to mimic the dynamic conditions experienced during the operation of URFCs. Additionally, the continuous, potentiodynamic change in Mn, Co, and Fe oxidation states during the ORR and OER is elucidated with operando X-ray absorption spectroscopy (XAS) measurements, revealing key insights into the nature of the active sites. The results reveal important catalyst physiochemical properties and provide a guide for future research and design principles for bifunctional oxygen electrocatalysts.