Electrocatalytic Oxidation of Glycerol to Formic Acid by CuCo₂O₄ Spinel Oxide Nanostructure Catalysts

The electrochemical oxidation of abundantly available glycerol for the production of value-added chemicals, such as formic acid, could be a promising approach to utilize glycerol more effectively and to meet the future demand for formic acid as a fuel for direct or indirect formic acid fuel cells. Here we report a comparative study of a series of earth-abundant cobalt-based spinel oxide (MCo₂O₄, M = Mn, Fe, Co, Ni, Cu, and Zn) nanostructures as robust electrocatalysts for the glycerol oxidation to selectively produce formic acid. Their intrinsic catalytic activities in alkaline solution follow the sequence of CuCo₂O₄ > NiCo₂O₄ > CoCo₂O₄ > FeCo₂O₄ > ZnCo₂O₄ > MnCo₂O₄. Using the best-performing CuCo₂O₄ catalyst directly integrated onto carbon fiber paper electrodes for the bulk electrolysis reaction of glycerol oxidation (pH = 13) at the constant potential of 1.30 V vs reversible hydrogen electrode (RHE), a high selectivity of 80.6% for formic acid production and an overall Faradaic efficiency of 89.1% toward all value-added products were achieved with a high glycerol conversion of 79.7%. Various structural characterization techniques confirm the stability of the CuCo₂O₄ catalyst after electrochemical testing. These results open up opportunities for studying earth-abundant electrocatalysts for efficient and selective oxidation of glycerol to produce formic acid or other value-added chemicals.