Rotating disc electrode studies of borohydride oxidation at Pt and bimetallic Pt–Ni and Pt–Co electrodes

The electrochemical oxidation of borohydride has been studied by means of voltammetry at Pt and Pt–Ni, Pt–Co bimetallic rotating disc electrodes (RDEs). The bimetallic catalysts are prepared by means of a galvanic replacement method (whereby electrodeposited Ni and Co layers are partially replaced by Pt when immersed in a chloroplatinic solution) and are shown to have a Pt shell–bimetallic alloy core. The effects of electrode history, potential scan direction, rotation speed and electrode material on borohydride oxidation have been investigated. For all Pt-based catalysts tested a gradual decrease of the voltammetric current from its initial value to a steady state response is observed in the kinetic control potential region, irrespective of scan direction and rotation speed. The initial deactivation of the catalyst at low overpotentials as well as the shape of the initial and steady-state voltammograms in that region point to the heterogeneous hydrolysis of borohydride and the subsequent oxidation of its products. As the catalytic activity for the hydrolysis reaction decreases the voltammograms shift to more positive potentials where direct borohydride oxidation dominates. The bimetallic Pt–Ni and Pt–Co catalysts exhibit a more negative open circuit potential and higher oxidation currents at low overpotentials than Pt, but lower apparent number of electrons transferred in the mass transport control region. These findings can be interpreted by the lowering of the d-band energy level of Pt in the presence of Ni and Co and the associated decrease in its adsorption affinity.