Cobalt carbonate as an electro‐catalyst 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 Epa approximately 0.71 V (vs. Ag/AgCl) owing to the Co(III/II) redox couple. (ii) A second wave is observed at Epa approximately 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 Co(n) (CO3 )3 (n-6) , n=4, 5 are attainable at potentials similar to those experimentally observed.