Cobalt Carbonate as an Electrocatalyst for Water Oxidation

Co^II salts in the presence of HCO₃ ⁻/CO₃ ²⁻ in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at E_pa≈0.71 V (vs. Ag/AgCl) owing to the Co^III/II redox couple. (ii) A second wave is observed at E_pa≈1.10 V with a considerably larger current. In which the Co^III undergoes oxidation to form a Co^IV species. The large current is attributed to catalytic oxidation of HCO₃ ⁻/CO₃ ²⁻ to HCO₄ ⁻. (iii) A process with very large currents at >1.2 V owing to the formation of Co^V(CO₃)₃ ⁻, which oxidizes both water and HCO₃ ⁻/CO₃ ²⁻. These processes depend on [Co^II], [NaHCO₃], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Coⁿ(CO₃)₃ ⁿ⁻⁶, n=4, 5 are attainable at potentials similar to those experimentally observed.