Incorporating high acidity cation on Co-free BiFeO3-based air electrodes for enhancing electrocatalytic activity and durability in reversible solid oxide cells

Abstract

Developing high catalytic activity and outstanding durability air electrodes for oxygen reaction and evolution reactions (ORR and OER) are crucial for the commercialization of reversible solid oxide cells (RSOCs). Co-free BiFeO3-δ-based perovskite oxides are considered as promising air electrodes materials due to the high polarizability of Bi3+ and low oxygen vacancy migration energy. In this work, we propose Bi0.8Ca0.2Fe1-xTixO3-δ (BCFTix, x=0, 0.05, 0.1 and 0.15) perovskites as efficient air electrodes for RSOCs via a A/B-site co-doping engineering. Ca and Ti co-doping significantly improves the electrochemical performance and operational stability of BiFeO3-based air electrodes. The Bi0.8Ca0.2Fe0.9Ti0.1O3-δ (BCFTi0.1) exhibits the best electrocatalytic activity with a polarization resistance of 0.064 Ω cm2 in air at 700 ℃ in symmetrical cells, decreasing approximately 48% compared to the BCF (0.123 Ω cm2). In addition, BCFTi0.1 possesses excellent CO2 tolerance and remains stable electroactivity in 3% CO2-air condition at 700 ℃ for 100 h. Fuel electrode-supported single cell with BCFTi0.1 air electrode shows remarkable performance at 700 ℃, achieving a power density of 1.03 W cm-2 in fuel cell mode, which is about 88% higher than that of BCF (0.6 W cm-2). In electrolysis mode, a current density of 0.9 A cm-2 is obtained at 700 ℃ and 1.3 V with 70%H2O-30%H2. The single cell with BCFTi0.1 air electrode demonstrates good cycling durability under humidified H2 (10% H2O). The reduced activation energy for oxygen ion migration and increased oxygen vacancy concentration via Ca and Ti co-doping promote the surface oxygen exchange and bulk transport kinetics, leading to enhanced electrocatalytic activity. At the same time, the high acidity of the Ti4+ and the large average bonding energy enhance the CO2 tolerance of BCFTi0.1. This study provides a collaborative strategy for A/B-site cations regulation to design novel air electrodes with high activity and chemical stability for RSOCs.