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 (FEformate) 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 FEformate 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 FEformate 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 FEformate of 86% observed for In2O3 doped with 10 at% Ce, at a current density of 150 mA/cm2, compares favorably with a FEformate of 78% for In2O3.