Computational Mechanistic Study of Borohydride Electrochemical Oxidation on Au3Ni(111)

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Au-3d metal alloys have recently gained interest for use as anode catalysts for direct borohydride fuel cells since these are less expensive than pure Au and exhibit desirable properties for borohydride oxidation. In this paper, a mechanistic study on the electrochemical oxidation of borohydride on Au3Ni(111) using first principles calculations based on spin-polarized density functional theory is presented. A reaction energy diagram showing the free energies of possible elementary surface-bound species on Au3Ni(111) as a function of electrode potential is constructed to show the favorable reaction path for a complete eight-electron oxidation of borohydride. As compared to pure Au, the adsorption of borohydride is favorable on Au3Ni(111) at lower potential due to the greater stability of borohydride on this surface, which is attributed to the upshift of the derived antibonding states of the BH4-sp and Au3Ni-d interaction with respect to pure Au. At a potential of −0.44 V vs NHE and T = 300 K, all subsequent elementary reaction steps for the complete oxidation of borohydride are downhill in energy with B(OH)3,ads as the highly favored final adsorbed species. The overall oxidation is limited by the initial adsorption of borohydride on the surface since it requires the highest electrode potential requirement among all electrochemical steps considered. Calculation of the dehydrogenation barrier of borohydride provides significant evidence that a more favorable elementary reaction on Au3Ni(111) as compared to Au(111) also leads to a lower reaction barrier.