Cyclic voltammetry investigation of borohydride oxidation at a gold electrode

Sodium borohydride (NaBH₄) is receiving increasing attention during the last decade regarding its possible application in energy systems. NaBH₄ has the dual potential of generating hydrogen on demand or being directly oxidised in a direct borohydride fuel cell (DBFC). Progress on DBFCs relies on the development of systematic studies to allow a more comprehensive characterisation of the borohydride (BH₄⁻) oxidation process. In this paper, cyclic voltammetry (CV) is applied to study systematically the BH₄⁻ electrooxidation on a gold (Au) disc macroelectrode in 2 mol l⁻¹ NaOH solutions. Voltammograms are obtained for various NaBH₄ concentrations [0.03-0.12 mol l⁻¹], working temperatures [25-65 °C], and potential scan rates [0.02-20 V s⁻¹], over a wide potential range [−1.0-0.8 V vs. SCE]. Modelling of CV data indicates that BH₄⁻ oxidation on Au electrode follows a first irreversible electrochemical pathway via the direct BH₄⁻ oxidation reaction, involving nearly 8 mol of exchanged electrons per mole of BH₄⁻. A second pathway, at higher potentials, concerns a yet undetermined oxidation mechanism in the partially oxidised Au surface which, in a third pathway, is reactivated, allowing an electrochemical-adsorption mechanism to take place. Relevant parameters such as transfer coefficient, kinetic rate constant, standard rate constant, charge transfer activation energy, and number of exchanged electrons are estimated. The BH₄⁻ oxidation reaction on Au is found to be first order with respect to BH₄⁻.