Ce Promotion of In₂O₃ for Electrochemical Reduction of CO₂ to Formate

In₂O₃ is a promising electrocatalyst for CO₂ electroreduction (CO₂ER) to formate. In₂O₃ nanoparticles doped with Pd, Ni, Co, Zr, and Ce promoters using flame-spray pyrolysis were characterized and evaluated in a gas diffusion electrode for the CO₂ER. Doping results in slight shifts of the In binding energy as probed by XPS, which correlates with a change of the Faradaic efficiency to formate (FE_formate) in the order Ce-doped In₂O₃ > Zr-doped In₂O₃ > In₂O₃ > Pd-doped In₂O₃ > Ni-doped In₂O₃ > Co-doped In₂O₃. However, the differences in CO₂ER performance are caused mainly by the different extent of In₂O₃ reduction. Co-doped In₂O₃ is prone to complete reduction to a stable Co-In alloy with a low FE_formate due to a high hydrogen evolution activity. The stabilizing effect of Ce on In₂O₃ is further demonstrated by an X-ray absorption spectroscopy study of a set of Ce-doped In₂O₃ samples (10, 50, 90 at%), highlighting that reduction of In₂O₃ is suppressed with increasing Ce content. Optimum performance in terms of FE_formate is obtained at a Ce content of 10 at%, which is attributed to the stabilization of In₂O₃ under negative bias up to -2 V. At higher Ce content, less active CeO₂ is formed. The highest FE_formate of 86% observed for In₂O₃ doped with 10 at% Ce, at a current density of 150 mA/cm², compares favorably with a FE_formate of 78% for In₂O₃.