Preparation of thin Co3O4 films on Ni and their electrocatalytic surface properties towards oxygen evolution

Thin films of Co3O4 of varying thicknesses have been prepared on an Ni substrate by spray pyrolysis and sequential solution coatings and investigated for their physicochemical and electrochemical properties towards O-2 evolution. The study indicated that the oxide roughness factor tended to level off with increasing thickness. Further, with an increase in oxide loading, the apparent electrochemical activity enhanced considerably at low range of loadings and then became relatively constant over a range of loadings. The higher loadings were found to be harmful. The layered oxide films (2.4-4.2 mg cm(-2)) prepared in sequential solution coatings exhibited greatest electrocatalytic activity in 1M KOH. However, the oxide electrode (loading: 3-5.7 mg cm(-2)) prepared by rotating spray pyrolysis produced the lowest oxygen overpotential-360 +/- 3 mV against a current density of 1 A cm(-2) in 30 w/o KOH at 70 degrees C. The oxygen evolution reaction at these film electrodes exhibited two Tafel slopes: 51-68 mV decade(-1) at low overpotentials and 120-140 mV decade(-1) at high overpotentials. The reaction orders with respect to OH- concentration were found to be similar to 1 for the layered and similar to 2 for the sprayed films. Based on the results, a suitable mechanism is also suggested for oxygen evolution.