A Comparative Study of Cathodic Electrodeposited Nickel Hydroxide Films Electrocatalysts

This paper describes the cathodic electrodeposition of nickel hydroxides from NiCl2 at constant potentials. In a single-compartment electrochemical cell, the procedure yields black films that have the appearance of glassy carbon. XPS analyses reveal that the films consist of a mixture of Ni(OH)2 and NiOOH in a proportion that depends on the deposition potential; furthermore, SIMS analyses indicate that the coatings are richer in NiOOH in the inner regions towards the support. The electrochemical behavior has been compared with data obtained on electrodes based on pure Ni(OH)2 prepared by cathodic electrodeposition from Ni(NO3)2, following literature procedures. This comparison confirms the composite nature of black nickel hydroxide (BN) coatings, adding that Ni(OH)2 and NiOOH are present in nearly equal amounts. The electrocatalytic activity of BN electrodes was tested in the oxidation of methanol in alkaline solutions. Experimental results are consistent with a mechanism previously proposed in literature for the oxidation of organic compounds on nickel electrodes. Comparison with electrodes based on pure Ni(OH)2 shows that, in the low potentials region, the presence of a high amount of NiOOH causes a decrease of activity, while an opposite effect is observed when the potential is increased. The black nickel electrodes show a remarkable stability in repeated cycles of oxidation of methanol and O2 evolution; under similar conditions, conventional Ni(OH)2 films show an appreciable decrease in thickness, as evidenced by the decrease of peak charge in cyclic voltammetry experiments.