Borohydride Electrooxidation on Carbon-Supported Noble Metal Nanoparticles: Insights into Hydrogen and Hydroxyborane Formation

Abstract

Borohydride anions (BH4–) are interesting as fuel for low-temperature alkaline fuel cells, owing to their high hydrogen content and low theoretical potential of oxidation. However, the borohydride electrooxidation mechanism and the potential dependence of the undesirable parallel hydrolysis pathway are not completely understood. In this study, by using a dual thin-layer flow-cell online coupled with a mass spectrometer and a rotating ring-disk electrode, the electrocatalytic activity and the dependence of the molecular hydrogen and hydroxiborane (BH3OH–) formation were investigated for carbon-supported Au, Ag, Pt, and Pd nanoparticles. For Au/C and Ag/C, the H2 and BH3OH– production presented a peak in the potential region of the first branch of the BOR wave and another increase in the metal oxide region. Pt/C and Pd/C showed accentuated H2 detection at the OCP, with a sharp decrease to practically zero after the BOR onset. Interestingly, and contrarily to what was observed for Au/C and Ag/C, the RRDE measurements showed BH3OH– production only at higher potentials (Pt- or Pd-oxides). These results were explained on the basis of the higher reactivity of Pt/C and Pd/C for the BOR, in which BHx-like species remain adsorbed and hydrogen is consumed via electrooxidation on their surfaces, at low potentials. On the other hand, Au/C and Ag/C, possessing lower reactivity (lower d-band center), the BH3-like species, produced in the first BOR steps, desorb from their surfaces and are detected at the ring. Concomitantly, at the BOR onset, H2 is formed, via recombination of adsorbed hydrogen atoms and can be detected by the mass spectrometer because these materials are relatively inactive for the hydrogen oxidation reaction.