Water Oxidation Catalysis at RuO2/NiO Mixed Oxide Electrodes
Authors: Godwin Ian , Lyons Micheal E.G 
Published: February 2020

Electrochemical water splitting via alkaline water electrolysis is currently an extremely active research area of intense topical international interest. This is due to the need for the development of a clean, reliable and sustainable method for large scale production of high purity hydrogen gas for use as a fuel in a potential hydrogen economy. [1–3] However, one of the grand challenges fully utilising alkaline water electrolysis for hydrogen production is in the large anodic overpotential associated with the oxygen evolution reaction (OER). Over the past 30 years considerable research effort and resources have been focused on the development and improvement of novel anode materials, with the aim of achieving useful rates of the OER at the lowest possible overpotential and cost in order to improve the economic viability of this technology. Dimensionally stable anode (DSA®) electrodes, based on RuO2 and IrO2 currently exhibit the lowest overpotential for the OER at practical current densities [4] Despite their excellent OER performance, the relative high cost of these materials, in particular iridium, combined with their poor long term chemical stability in alkaline media renders their long term use as anode materials for water electrolysers impractical. Because of this problem we have attempted to overcome this problem by using oxides/hydroxides/ oxyhydroxides of first row transition metals which offer comparable OER performance but at significantly lower cost. [5,6] In this work we have studied RuO2/NiO mixed oxide electrodes prepared by thermal decomposition for use as potential water oxidation catalysts. Addition of just 10 mol% RuO2 to a NiO electrode was found to decrease the oxygen evolution reaction (OER) onset potential by 20% with increasing additions having significantly diminishing returns. The OER current densities for the 25/75 mol% RuO2/NiO electrode was found to significantly increase when preconditioned by application of prolonged polarisation regimes with the Tafel slope also decreasing from ca. 75 mV dec-1 to ca. 50 mV dec-1. NiO prepared by thermal decomposition was found to behave in a similar manner to other nickel oxides [7-13] prepared using different methodologies and we propose a similar OER mechanism based on the kinetic data obtained using the surfaquo group concept.
Acknowledgements
This publication has emanated in part from research conducted with the financial support of Science Foundation Ireland (SFI) under Grant Number SFI/10/IN.1/I2969.
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Fig . 1: Oxygen evolution reaction onset overpotential as a function of oxide composition.