Mechanistic Insight into the Au-3d Metal Alloy-Catalyzed Borohydride Electro-Oxidation: From Electronic Properties to Thermodynamics

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Recent theoretical and experimental findings have motivated the use of 3d transition metals as alloying elements to improve the performance of Au for the electro-oxidation of borohydride. In this paper, we provide mechanistic insights into the electrochemical oxidation of borohydride on pure Au and Au-3d alloys (Au3M with M = Cr, Mn, Fe, Co, Ni) using first principles calculations. We found that the initial oxidative adsorption of borohydride is the least exothermic among the elementary reactions considered for the complete eight-electron oxidation process. Interestingly, Au-3d metal alloy surfaces promote this oxidation step at a lower electrode potential compared to pure Au due the enhanced stability of borohydride on these alloy surfaces. The most negative borohydride oxidation potential is achieved by M = Co, followed by Fe, Ni, Mn, and Cr in order of increasing electrode potential. Subsequent to the initial oxidative adsorption of borohydride, the dehydrogenations of BH3*, BH2OH*, and BH(OH)2* are endothermic on pure Au at very low potentials. However, these activated and possibly limiting elementary reaction steps are more exothermic on Au-3d alloys than on pure Au. Following the adsorption of borohydride on the surface, all elementary reaction steps for the complete electro-oxidation process proceed downhill in energy at a lower electrode potential on Au-3d alloys than on pure Au.