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

Abstract

Developing air electrodes with high catalytic activity and outstanding durability for the oxygen reduction and evolution reactions (ORR and OER) is crucial for the commercialization of reversible solid oxide cells (RSOCs). Co-free BiFeO_3−δ-based perovskite oxides are considered promising air electrode materials owing to the high polarizability of Bi³⁺ and low oxygen-vacancy migration energy. In this work, we report the synthesis of Bi_0.8Ca_0.2Fe_1−xTi_xO_3−δ (BCFTi_x, x = 0, 0.05, 0.1 and 0.15) perovskites as efficient air electrodes for RSOCs via A-/B-site co-doping engineering. Ca and Ti co-doping significantly improved the electrochemical performance and operational stability of BiFeO₃-based air electrodes. Bi_0.8Ca_0.2Fe_0.9Ti_0.1O_3−δ (BCFTi_0.1) exhibited the highest electrocatalytic activity with a polarization resistance of 0.064 Ω cm² in air at 700 °C in symmetrical cells, with a decrease of approximately 48% compared to BCF (0.123 Ω cm²). In addition, BCFTi_0.1 possessed excellent CO₂ tolerance and exhibited stable electroactivity in 3% CO₂-air at 700 °C for 100 h. A fuel electrode-supported single cell with the BCFTi_0.1 air electrode demonstrated remarkable performance at 700 °C, achieving a power density of 1.03 W cm⁻² in fuel cell mode, which was about 88% higher than that of BCF (0.6 W cm⁻²). In electrolysis mode, a current density of 0.9 A cm⁻² was obtained at 700 °C and 1.3 V with 70% H₂O–30% H₂. The single cell with the BCFTi_0.1 air electrode demonstrated 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 promoted surface oxygen exchange and bulk-transport kinetics, leading to enhanced electrocatalytic activity. At the same time, the high acidity of Ti⁴⁺ and large average bonding energy enhanced the CO₂ tolerance of BCFTi_0.1. This study provides a collaborative strategy for the regulation of A/B-site cations to design novel air electrodes with high activity and chemical stability for RSOCs.