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.

Figure optionsDownload high-quality image (234 K)Download as PowerPoint slideResearch highlights▶ Cyclic voltammetry experiments at a Pt RDE, positive to the rest potential in a 0.01 M NaBH4 + 1 M NaOH solution, revealed a gradual decrease in current response with number of experiment until a steady-state picture is achieved, associated with a change in heterogeneous borohydride hydrolysis and/or oxidation rates of thus produced molecular hydrogen. A significant shift of the voltammograms upon increased rotation rates to more positive potentials, indicative of a highly irreversible reaction, was also observed. ▶ Cyclic voltammetry experiments at Pt(Ni)/GC and Pt(Co)/GC RDEs, prepared on glassy carbon, GC, electrodes by spontaneous galvanic replacement of Ni and Co electrodeposits upon their immersion in chloroplatinic acid, allowed access to lower overpotentials where borohydride oxidation appears to proceed via the oxidation of hydrogen produced by heterogeneous hydrolysis at the electrode surface. This is due to the rest potential of Pt(Co) and Pt(Ni) being more negative than that of Pt as a result of cathodic hydrogen evolution suppresion. ▶ The catalytic activity of Pt(Co) and Pt(Ni) in the kinetic and mixed control potential range is higher than that of pure Pt and this is attributed to a higher activity for oxidation of molecular hydrogen produced by heterogeneous hydrolysis. ▶ The apparent number of electrons exchanged during borohydride oxidation in the limiting plateau-mass transfer control potential range is higher at Pt (ca. 5.5) than at Pt(Ni) and Pt(Co) (ca. 4).