Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations

The borohydride oxidation reaction (BOR) was characterized on the three most-studied noble metals (Au, Pt and Pd) in a range of NaBH₄ concentration and temperature that enables to bridge model studies of the BOR (low concentration) to more practical ones, relevant to the direct borohydride fuel cell (DBFC) operation. BOR mechanistic insights were unveiled using the complementary techniques of rotating disk electrode cyclic voltamperometry, rotating ring-disk electrode measurements of the BH₃OH⁻ production and differential electrochemical mass spectrometry detection of H₂ escape. When the concentration of sodium borohydride is brought to DBFC-like operating conditions, the H₂ escape is more severe and the poisoning effect of the metal surfaces by the BOR intermediates (BH_ads or BH_3,ads) is more significant, and stronger on Pt surfaces compared to Au and Pd ones. Even at high NaBH₄ concentrations, Pd exhibits promising BOR kinetics, making of this material an interesting candidate for DBFC anode electrocatalysis. These data enabled to complement our previous kinetic model and to confirm the BH₃ species oxidation pathways for NaBH₄ concentrations of 5 mM and 50 mM. However, this model is incomplete for high borohydride concentrations; it does not take into account possible local pH variations and cannot explain the origin of the important reduction currents measured at high potential on the Au ring electrodes. Finally, it is shown that the BOR mechanism at Pd electrodes must take into account PdH formation and oxidation.