Electrochemical Studies of the Oxygen Evolution Reaction at Mn, Ni/Co Mixed Oxide Electrodes in Aqueous Alkaline Solution

Abstract

There has been an abundance of publications over the last twenty years on electrode materials proposed to be suitable catalysts for the oxygen evolution reaction (OER) in alkaline solutions. Although it is widely accepted that RuO2 and IrO2 anodes exhibit the most efficient electrocatalyst performance towards the reaction, their high cost renders prohibitive the widespread adoption of this material for industrial applications. Amongst the more costeffective materials that have been proposed as OER anodes are, various inter-metallic alloys, typically containing Ni, Co or Fe, and transition mixed oxides, including spinels (particularly nickelites, cobalites and ferrites) and perovskites. In view of this, the present work investigates the redox, the electrocatalytic and the mechanistic properties of two types of electrodes based on transition metal oxides were examined for the oxygen evolution reaction. The oxide materials were repetitive potential multi-cycled prepared hydrous manganese oxide electrodes and thermally prepared nickel cobalt mixed oxide electrodes. The electrocatalytic properties of both were described for potential use in the industrially and commercially significant OER, which is a rate determining step for hydrogen production via alkaline water electrolysis. Thus, the “backbone” of the present work is to be found in the steady-state polarisation and electrochemical impedance measurements. The aim of these measurements was to obtain experimental values for the OER Tafel slope and the OH- ion reaction order for these electrodes in aqueous alkaline solution, with a view to perform classical kinetic mechanistic analyses, to identify the operative oxygen evolution pathway for each of these anodes. In addition, steady-state polarisation measurements were used to quantify and compare the OER catalytic activity of electrodes at a given applied potential. Electrochemical impedance spectroscopy (EIS) measurements have demonstrated that the OER Tafel slope, derived from dc or ac measurements are truly characteristic of the kinetics of the reaction. Physical, elemental and morphological characterisation gave further insight into the nature of the modified film surface and enabled one to relate the electrocatalytic activity of these anodes with physical aspects such as surface morphology and surface composition. It was also noted that knowledge of the chemical and structural properties of the underlying oxide phase is crucial in understanding the OER; the amphoteric character of the anodic oxides implies that it is more realistic to investigate the oxygen evolution active sites in terms of anionic surface complexes rather than using the traditional viewpoint of stoichiometric units of the bulk oxide material. We noted that the chemistry of the surfaquo group determines the chemistry of the OER active catalytic site. Concluding, this work has demonstrated that manganese and nickel cobalt mixed oxides display very promising potential as inexpensive materials for electrochemical oxygen evolution in alkaline solutions due to their relatively low overpotential values and high stability in strong alkaline solutions. Particularly, the nickel cobalt mixed oxide electrode with 60 % M Co content, annealed at 400°C in air, exhibit the best OER catalytic activity in alkaline solutions among all the other electrodes studied in this thesis.