Ce Promotion of\nIn<sub>2</sub>O<sub>3</sub> for Electrochemical\nReduction of CO<sub>2</sub> to Formate

In<sub>2</sub>O<sub>3</sub> is a promising electrocatalyst\nfor\nCO<sub>2</sub> electroreduction (CO<sub>2</sub>ER) to formate. In<sub>2</sub>O<sub>3</sub> nanoparticles doped with Pd, Ni, Co, Zr, and\nCe promoters using flame-spray pyrolysis were characterized and evaluated\nin a gas diffusion electrode for the CO<sub>2</sub>ER. Doping results\nin slight shifts of the In binding energy as probed by XPS, which\ncorrelates with a change of the Faradaic efficiency to formate (FE<sub>formate</sub>) in the order Ce-doped In<sub>2</sub>O<sub>3</sub> >\nZr-doped In<sub>2</sub>O<sub>3</sub> > In<sub>2</sub>O<sub>3</sub> > Pd-doped In<sub>2</sub>O<sub>3</sub> > Ni-doped In<sub>2</sub>O<sub>3</sub> > Co-doped In<sub>2</sub>O<sub>3</sub>. However,\nthe\ndifferences in CO<sub>2</sub>ER performance are caused mainly by the\ndifferent extent of In<sub>2</sub>O<sub>3</sub> reduction. Co-doped\nIn<sub>2</sub>O<sub>3</sub> is prone to complete reduction to a stable\nCo–In alloy with a low FE<sub>formate</sub> due to a high hydrogen\nevolution activity. The stabilizing effect of Ce on In<sub>2</sub>O<sub>3</sub> is further demonstrated by an X-ray absorption spectroscopy\nstudy of a set of Ce-doped In<sub>2</sub>O<sub>3</sub> samples (10,\n50, 90 at%), highlighting that reduction of In<sub>2</sub>O<sub>3</sub> is suppressed with increasing Ce content. Optimum performance in\nterms of FE<sub>formate</sub> is obtained at a Ce content of 10 at%,\nwhich is attributed to the stabilization of In<sub>2</sub>O<sub>3</sub> under negative bias up to −2 V. At higher Ce content, less\nactive CeO<sub>2</sub> is formed. The highest FE<sub>formate</sub> of 86% observed for In<sub>2</sub>O<sub>3</sub> doped with 10 at%\nCe, at a current density of 150 mA/cm<sup>2</sup>, compares favorably\nwith a FE<sub>formate</sub> of 78% for In<sub>2</sub>O<sub>3</sub>.