The Mechanism of Oxygen Reactions at Porous Oxide Electrodes

Abstract

RuO 2 /NiO mixed oxide electrodes prepared by thermal decomposition were examined as potential water oxidation catalysts. Addition of just 10 mol% RuO 2 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 RuO 2 /NiO electrode were found to increase when preconditioned by application of prolonged polarization regimes with the Tafel slope also decreasing when conditioned. NiO prepared by thermal decomposition was found to behave in a similar manner to other nickel oxides prepared using different methodologies and we propose a similar OER mechanism based on the kinetic data obtained using the surfaquo group concept. A dual barrier model was used to rationalize the fractional reaction order of ca. 0.5 observed for RuO 2 .Alkaline water electrolysis has become an area of intense international research due to the need for a method to efficiently produce large amounts of molecular hydrogen which would be required for a potential hydrogen economy. Currently one of the main drawbacks limiting the widespread use of this technology is in the large anodic overpotential associated with the oxygen evolution reaction (OER). Dimensionally Stable Anodes (DSA) are one such technology which can effectively catalyze the OER thus increasing the overall efficiency of the water electrolysis reaction. DSA electrodes consist of an underlying electrochemically inert valve metal substrate such as titanium or tantalum, coated usually with an oxide or mixed oxides of platinum group metals (PGM). To date, they have been employed in many industrial processes such as the chlorine evolution reaction, 1 electrowinning of metal ores 2 and as an anode for use in industrial water electrolyzers. 3 Herein, we focus attention on the latter use in an alkaline environment. DSA electrodes based on IrO 2 /RuO 2 currently exhibit one of the lowest overpotential for the oxygen evolution reaction (OER), which is said to be the 'bottleneck' or rate determining step in a water electrolyzer cell at standard operating currents. PGM oxide mixtures such as IrO 2 /RuO 2 as studied by Lyons, 3,4 Trasatti 4-6 and others on Ti exhibit extremely low OER overpotentials (ca. 0.20-0.25 V) and display long term stability in strong alkaline conditions required for an alkaline water electrolyzer setup. However, the main drawback in fully utilizing PGM oxide based DSA electrodes is in the high cost associated with these metals, and thus, significant international research is being currently focused on finding cheaper alternatives which still display relatively low overpotentials. This paper focuses on minimizing the amount of expensive PGM oxides, RuO 2 in this case which currently costs $2.1 million/ton, required by combining them with low cost NiO, currently $14 000/ton, which displays relatively low overpotentials for the OER 7-9 but at a significantly lower cost. A range of RuO 2 /NiO compositions was studied herein and the e...

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“…(32) to various cases are given in Table 1. Expressions similar to some of those in Table 1 have been previously formulated by Bockris and others [12,34,35,36,37] for an alkaline context. The first entry of Table 1,…”

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