Highly promoted electrocatalytic activity of spinel CoFe 2 O 4 by combining with Er 0.4 Bi 1.6 O 3 as a bifunctional oxygen electrode for reversible solid oxide cells

Solid oxide cells (SOCs) allow the eco-friendly and direct conversion between chemical energy ( e.g. , hydrogen) and electric power, effectively mitigating the environmental issues associated with excessive fossil fuel consumption. Herein, we report the development of a highly active bifunctional oxygen electrode by combining the spinel-type CoFe 2 O 4 and the cubic fluorite structured Er 0.4 Bi 1.6 O 3− δ (CFO–ESB) for applications in reversible SOCs at reduced temperatures. X-ray diffraction analysis showed that CFO had a good chemical compatibility with the ESB phase. X-ray photoelectron spectroscopy and O 2 -temperature-programmed desorption results revealed that the incorporation of ESB into CFO increased the number of chemisorbed oxygen species, thereby promoting their catalytic activity in the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). These results were consistently confirmed by electrochemical impedance spectroscopy, wherein the low polarization resistance of the CFO–ESB electrode in the YSZ electrolyte was observed ( e.g. , ∼0.29 Ω cm 2 at 700 °C). Consequently, the reversible SOC prepared using the CFO–ESB oxygen electrodes showed excellent performances in both the fuel cell (∼1.0 W cm −2 peak power density) and electrolysis cell (∼1.5 A cm −2 at 1.3 V) modes at 700 °C. These results are one of the best values among the spinel-based oxygen electrode adopted cells reported to date, demonstrating highly efficient ORR and OER bifunctionality of the CFO–ESB. Thus, our findings suggest that CFO–ESB has high potential for use in oxygen electrodes for reversible SOC applications.