Determination of Kinetic and Diffusional Parameters for Sodium Borohydride Oxidation on Gold Electrodes

Direct borohydride fuel cells are among the latest fuel cell types under considerable research and development. There is a clear need for further elucidation of the borohydride electrochemical oxidation. From this perspective, potential step experiments were carried out with a Au disk electrode in alkaline sodium borohydride solutions of varying concentrations and temperatures. Analysis of the current transients for the full time scale enabled the delimitation of the domains where diffusion, electron transfer, and mixed control prevail. Chronoamperometric measurements in the diffusion-controlled region led to current responses exhibiting Cottrellian behavior, from which accurate diffusion coefficients, D, were estimated. Arrhenius plots of D allowed determination of the diffusion activation energies and maximum diffusion coefficients. Chronocoulometric measurements generated Anson plots from which the kinetic rate constants and charge transfer activation energies were calculated. Analysis of the current densities in conditions of infinite mass transfer led to Tafel plots from which exchange current densities, transfer coefficients, standard rate constants, and standard activation energies for charge transfer were determined. Purely charge-transfer regimes were not recognized in the studied system under the imposed potentials. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3082371] All rights reserved.