(457d) Design of Electrocatalysts for Direct Borohydride Oxidation

Direct Borohydride Fuel Cells (DBFCs) have the potential to generate high power densities for use in portable power applications. Current applications are limited, in part, by the lack of an effective anode electrocatalyst. Though a number of pure metals have been tested as anodes, no previously tested electrocatalyst demonstrates both the required activity and selectivity to direct oxidation over hydrolysis reactions. The complexity of the 8 electron oxidation reaction challenges experimental mechanism determination and therefore limits the rational design of improved electrocatalysts. We will describe the use of computational catalysis methods, specifically density functional theory, to drive catalyst design. Electrokinetic studies and in situ electrochemical Raman characterization of reactive intermediates is used to corroborate mechanisms proposed based on computational work and to evaluate the activity of improved alloy catalysts.

Pure metal electrocatalysts for direct borohydride oxidation suffer from either large overpotentials (Au, Ag) or low Coulombic efficiency due to non-selective hydrolysis reactions (Pt, Ni, Pd). DFT calculations examine the mechanism of borohydride electro-oxidation over Au(111) and Pt(111) surfaces. Stable surface intermediates and limiting steps are identified. The initial oxidative adsorption of borohydride and the dissociation of O-H bonds of boron containing intermediates are proposed as key reaction sequences that dictate activity and selectivity. These key energetic parameters are evaluated for pure and bimetallic electrocatalysts. Au-Cu alloys are determined from DFT calculations to be encouraging for improved performance. Electrodeposition methods were used to synthesize Au-Cu bimetallics. Alloying of Au and Cu metals is substantiated by X-Ray diffraction. Cyclic voltammetry and rotating disk electrode studies were used to evaluate the activity and stability of these alloy catalysts, with comparison to pure Au, Pt, and Cu electrodes.