Efficient reversible CO/CO2 conversion in solid oxide cells with a phase-transformed fuel electrode

The reversible solid oxide cell (RSOC) is an attractive technology to mutually convert power and chemicals at elevated temperatures. However, its development has been hindered mainly due to the absence of a highly active and durable fuel electrode. Here, we report a phase-transformed CoFe-Sr₃Fe_1.25Mo_0.75O_7−δ (CoFe-SFM) fuel electrode consisting of CoFe nanoparticles and Ruddlesden-Popper-layered Sr₃Fe_1.25Mo_0.75O_7−δ (SFM) from a Sr₂Fe_7/6Mo_0.5Co_1/3O_6−δ (SFMCo) perovskite oxide after annealing in hydrogen and apply it to reversible CO/CO₂ conversion in RSOC. The CoFe-SFM fuel electrode shows improved catalytic activity by accelerating oxygen diffusion and surface kinetics towards the CO/CO₂ conversion as demonstrated by the distribution of relaxation time (DRT) study and equivalent circuit model fitting analysis. Furthermore, an electrolyte-supported single cell is evaluated in the 2:1 CO-CO₂ atmosphere at 800°C, which shows a peak power density of 259 mW cm⁻² for CO oxidation and a current density of −0.453 A cm⁻² at 1.3 V for CO₂ reduction, which correspond to 3.079 and 3.155 mL min⁻¹ cm⁻² for the CO and CO₂ conversion rates, respectively. More importantly, the reversible conversion is successfully demonstrated over 20 cyclic electrolysis and fuel cell switching test modes at 1.3 and 0.6 V. This work provides a useful guideline for designing a fuel electrode through a surface/interface exsolution process for RSOC towards efficient CO-CO₂ reversible conversion.