Enhanced borohydride oxidation kinetics at gold-rare earth alloys

Abstract

Gold-rare earth (Au-RE) alloys with equiatomic compositions are prepared by arc (RE = Dy, Ho, Y) or induction (RE = Sm) melting. Morphology and phase composition is assessed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDXS), while X-ray powder diffraction (XRPD) is used to confirm crystal structures. The Au-RE electrodes are evaluated for borohydride oxidation reaction (BOR) in alkaline media employing cyclic voltammetry and chronoamperometry. The obtained data allows calculation of kinetic parameters that characterize the borohydride (BH4-) oxidation at Au-RE alloys, including the number of exchanged electrons, n, and the anodic charge transfer coefficient, α. n values range from 2.4 to 4.4, while α values are found to be in the 0.60–0.83 range. The BOR apparent activation energy, Eaapp, and the reaction order, β, are also determined from CV data obtained at different temperatures and different BH4- concentrations, respectively. Low Eaapp values range from 16.4 (Au-Sm) to 20.2 kJ mol−1 (Au-Y) and β values suggest that BOR at the examined alloys is a 1st order reaction with respect to BH-4 concentration. A small-scale direct borohydride-peroxide fuel cell (DBPFC) operating with Au-Y anode at 25 °C reaches a peak power density of 150 mW cm−2. The cell performance is enhanced when increasing the temperature to 45 °C, with a maximum power density of 215 mW cm−2 being attained.