Electrooxidation of Sodium Borohydride at Pd, Au, and PdxAu1−x Carbon-Supported Nanocatalysts

Abstract

Carbon-supported palladium, gold, and bimetallic Pd−Au nanocatalysts with different compositions were synthesized by a “water-in-oil” microemulsion method. Their catalytic activity toward borohydride electrooxidation was evaluated in alkaline medium. Physical and electrochemical methods where applied to characterize the structure and surface of the synthesized catalysts. It was shown that PdxAu1−x/C catalysts were alloys, which present an increase of crystallite (X-ray diffraction) and particle (transmission electron microscopy) sizes with increasing Au atomic fraction. Their surfaces were palladium-rich whatever the Pd atomic ratios. The onset potential of NaBH4 oxidation was close to −0.2 V versus reversible hydrogen electrode (RHE) on Pd/C. PdxAu1−x/C catalysts presented lower onset potential for BH4− oxidation than Au and Pt (in the range from −0.2 to −0.1 V vs RHE against 0 and 0.3 V vs RHE for Au/C and Pt/C, respectively). The NaBH4 oxidation on Pd/C catalyst was found to be a first-order reaction with respect to borohydride concentration. From voltammetric measurements, rotating disk electrode experiments, and hydrogen production estimations it was proposed that NaBH4 oxidation on palladium-based nanocatalysts followed two pathways. The first one, at negative potentials, involved the formation of BH3OH− intermediate with H2 generation. The second one, at higher overpotentials, occurred mainly via the direct BH4− oxidation reaction, involving 6 mol of exchanged electrons per mole of borohydride. However, it was shown that addition of gold to palladium leads to increase significantly the hydrogen evolution rate. At last, comparison of the activity of the different catalysts toward the borohydride oxidation reaction showed that up to 50% of the palladium atoms can be replaced by a noncatalytic foreign metal like gold, while keeping identical catalytic activity than that of the monometallic Pd/C catalyst.