Controlled electrodeposition of cobalt oxides from protic ionic liquids for electrocatalytic water oxidation

Amorphous thin films of cobalt oxides with controlled nanostructures and compositions have been prepared by electrodeposition from a room temperature protic ionic liquid for water oxidation electrocatalysts. The electrochemical behaviour of Co2+ has been studied in ethylammonium nitrate (EAN) using cyclic voltammetry and chronoamperometry. The electrodeposition of Co2+ in EAN occurs at -0.95 V vs. Fc(0/+), followed by hydrogen evolution reaction. The electro-oxidation of Co2+ to Co3+ is found to be a reversible process in dried EAN at E-1/2 = 1.47 V vs. Fc(0/+). The presence of water significantly affects the voltammetric behaviour of Co2+ and the subsequent electrodeposition processes. Cobalt oxides nanostructures have been deposited onto glassy carbon and indium tin oxide (ITO)-coated glass substrates by two routes, (i) controlled-potential deposition of Co, followed by spontaneous air oxidation and (ii) controlled-potential deposition of Co followed by anodic oxidation in the presence of water. SEM, EDX, XRD and XPS reveal that cobalt oxides of different morphologies and chemical compositions are formed on the surface of electrodes. The cobalt oxides films exhibit catalytic activity towards water oxidation reaction in neutral phosphate buffered solution with low overpotentials.