Insight into the water oxidation mechanism on nickel hydroxide electrocatalysts : Density Functional Theory calculations and Electrochemical experiments

Abstract: Hydrogen production is an interesting way to store solar energy and to diversify the range of applications for indirect solar power. A promising production method is to use electric power from solar photovoltaic cells to split water in an electrolysis setup. To efficiently run such a setup, one must however have an efficient catalytic material on the two electrodes. This work presents a study of a catalytic material for the oxygen evolution electrode; nickel hydroxide. The study is performed both experimentally and theoretically. In the experimental part, an electrode material was synthesized by growing nitrogen doped carbon nanotubes (NCNTs) on a carbon paper (CP) and then decorating the NCNTs with the catalytic material. Scanning electron microscopy (SEM) images of the electrode material showed that the NCNTs were individually coated with a spiky nickel hydroxide nanostructure, with a very large surface area. Electrodes with both as-prepared catalytic material and catalytic material first treated in an alkaline solution were then tested in a three-electrode electrolysis setup, in alkaline conditions. It was found that the overpotential for onset of the oxygen evolution reaction (OER) was roughly 0.27 V, which is in the range of previous reports. In contradiction to other reports, the data of this work however indicated that aging the catalytic material decreased its activity and hence that the phase often stated as the more active, was in fact the less active phase. The overall efficiency of the electrodes was found to be low, most likely due to overloading of active material in the electrode structure. The theoretical part of the work focused on using Density Functional Theory (DFT) simulations to analyze the OER pathway on three different surfaces of the catalytic material. To simulate the effect of an alkaline environment these surfaces were also passivated with hydroxyl groups in some of the simulations. The lowest overpotential for OER onset found in the calculations was 0.68 V. The calculations further showed that, for the pathways with the smallest overpotentials, the limiting reaction step was a step where an adsorbed hydroxyl group was deprotonated by a hydroxide ion from the solution and oxidized to an adsorbed oxygen atom. In addition, the calculations also indicated that passivation of the surfaces had the important effect of lowering the overpotentials for two of the three studied surfaces.