Cobalt Oxide Nanoflake Modified Electrodes and Their Application As Supercapacitors and Oxygen Evolution Catalysts

Abstract

Finding alternative ways to access and store energy has become a major issue recently. Transition metal oxides have shown promising behaviour as catalysts and supercapacitors1. Liquid exfoliation2 of bulk metal oxides has been recently shown to provide ready access to 2-dimensional nanoflakes, the size of which can be readily varied. These 2D materials exhibit excellent electrochemical, charge storage and catalytic activity, with potential use in the areas of energy storage and conversion. The production of hydrogen from the electrolysis of water is not energetically efficient because of the high anodic overpotential associated with the oxygen evolution reaction (OER). The latter is a kinetically demanding and inefficient step compared with the hydrogen evolution reaction (HER) itself. As seen in eqn.1 to minimise the cell potential the overpotential of the OER, ηa, has to be minimised. E cell = E0 + iR + ηa + ηc (1) Currently anodes used in industrial electrolysers which exhibit low oxygen over-potentials are based on platinum group metal oxide materials such as RuO2 and IrO2. However because of materials scarcity issues and high cost , coupled with long term stability issues (especially in alkaline media) their use is limited. The use of less noble metal oxides or hydroxides as OER catalysts can significantly reduce the cost and demonstrate promising enhancements both in electrocatalytic activity and in anode stability3. One of the key parameters for a good OER catalyst is the overpotential value recorded at 10 mA.cm-2, which should be as low as possible and should be less than or equal to that observed with existing anode materials such as RuO2 and IrO2 4. In the present study we examine the redox behavior and catalytic activity for OER of exfoliated nanostructured cobalt oxide Co(OH)2, in aqueous base5. Cobalt oxide is well known as a good catalyst for oxygen evolution and also exhibits excellent charge storage capability during redox switching6. The liquid exfoliation and dispersion process used to fabricate the cobalt oxide nanoflake material is time efficient and highly reproducible. The modified electrode was fabricated by spraying the metal oxide flake suspension in air, onto a porous conductive support electrode foam using a spray gun under a nitrogen flow. High surface area Nickel and glassy carbon foams were the support materials examined. The effect of metal oxide nanoflake mass loading was then carried out to evaluate the overpotential at 10 mA.cm-2 and the capacitance values of each systems. The chemical composition of the Co(OH)2 films coated on the support foams was analysed using Raman spectroscopy (fig.1) and the nanoflake morphology was observed via scanning electron microscopy (SEM) (fig.2). During this study we have noted that the oxide material performance metrics such as the oxygen overpotential at 10 mA.cm-2 and the redox capacitance are dependant on the support used (figs.3). Moreover, the nanoflake mass loading was found to have a significant effect on the recorded value of these parameters (figs.3 (a) and (b)).