(487d) A First Principles Study Of Direct Electrooxidation Of Aqueous Borohydride On Au And Pt Surfaces

Direct borohydride fuel cells (DBFCs) offer the potential for direct chemical to electrical energy conversion from a high specific energy, water soluble fuel. Power densities similar to those attained from direct methanol fuel cells have been demonstrated [1], despite substantially less research focus. However, the lack of effective anode materials for the electrocatalysis of borohydride oxidation is the major limitation in advancing the application of these devices. This paper discusses the application of electronic structure modeling to determine the electrocatalytic reaction mechanism and design metal alloy catalysts for increasing the power density and efficiency of DBFCs. The development of optimal electrocatalysts for these processes could enable the use of aqueous borohydride solutions as a chemical energy storage vehicle for portable power applications. This presentation will discuss the use of density functional theory based electronic structure calculations to elucidate the elementary steps of borohydride oxidation at the metal-solution, electrochemical interface. Auxiliary benefits of mechanism determination are the elucidation of catalytic paths for hydrogen evolution from aqueous borohydride solutions and for reduction of the borate ion to produce/regenerate the borohydride fuel. The use of aqueous borohydride solutions (NaBH4) as an anode fuel in an alkaline fuel cell was demonstrated in the 1960's, however, the work of Amendola et al. in 1999 re-stimulated research in the area [2]. The ideal anode will oxidize BH₄^- (with 8OH^- delivered from the alkaline oxygen reduction cathode) to BO₂^- at low overpotentials producing 8 electrons per BH₄^- reactant molecule. However, high overpotentials and reduced Coulombic efficiency is observed due to both slow kinetics and non-selective hydrolysis reactions. We will present a mechanistic investigation of BH₄^- oxidation over the Au(111) surface, identifying stable surface species and rate limiting steps to motivate improved catalyst design. Periodic density functional theory calculations are used to determine elementary reaction energies and activation barriers. Multiple approaches to including the electrode potential dependence of both ion adsorption and surface reaction will be discussed, employing both metal-vapor and metal-solution model systems. Initial reaction steps will be compared over Pt(111) to determine the mechanism of non-selective hydrogen evolution, and determine the competing pathways which must be considered in electrocatalyst design.

References [1] C. Ponce de Leon, F. C. Walsh, D. Pletcher, D. J. Browning, J. B. Lakeman, J. Power Sources 155 (2006) 172. [2] S. C. Amendola, P. Onnerud, M. T. Kelly, P. J. Petillo, S. L. Sharp-Goldman, M. Binder, J. Power Sources 84 (1999) 130.