Ubiquitous Borane Fuel Electrooxidation on Pd/C and Pt/C Electrocatalysts: Toward Promising Direct Hydrazine–Borane Fuel Cells

摘要

Carbon-supported platinum and palladium nanoparticles were studied toward the oxidation of several boranes (namely ammonia–borane (AB), dimethylamine–borane (DMAB), hydrazine–borane (HB), and hydrazine–bis-borane (HBB)); only palladium is capable to oxidize directly and efficiently these fuels, as platinum first decomposes the boranes and then valorizes the evolved H_(2) and adsorbed H_(ad). Changing the nature of the borane fuel enables modulation of the borane oxidation performances at palladium electrodes; the best compromise is reached with HB (HBB suffers safety issues, and AB and DMAB are poisoned by the “counter-fragment” and/or its electroinactivity for any electrooxidation reaction). As a result, with a Pd/C electrode, HB oxidation is possible at low potential (close to the theoretical value), which holds promise for direct alkaline fuel cell applications. The temperature, HB concentration, and palladium nanoparticle loading on the electrode have remarkable effects, which shows that the “direct” electrooxidation of the borane fuel (BH_(3)OR) or of its adsorbates may compete with its spontaneous catalytic decomposition/hydrolysis into H_(2) followed by electrooxidation of H_(2) (HOR). The study also highlights that the reactant time of residence influences the pathway and completion of the reactions. These results demonstrate that, using suitable electrocatalysts, well-structured electrodes, and adequate borane fuel, the BH_(3)OR thermodynamic onset potential value and the theoretical number of electrons per fuel moiety (n _(e~(–)) = 10 in the case of HB, 6 for the borane fragment and 4 for the hydrazine one) can nearly be reached, at reasonably low anode potential, which paves the way toward optimization of direct HB fuel cell systems.