Borohydride Oxidation over Au(111): A First-Principles Mechanistic Study Relevant to Direct Borohydride Fuel Cells

Abstract

Direct borohydride fuel cells (DBFCs) offer the potential for direct chemical to electrical energy conversion from a high-specific-energy, water-soluble fuel. The lack of effective anode materials for the electrocatalysis of borohydride has been the major limitation in advancing the application of direct borohydride fuel cells. In this study, we apply electronic structure calculations to elucidate the mechanism of borohydride oxidation over the Au(111) surface. Reaction free energies computed as a function of electrode potential are used to identify stable surface bound intermediates and likely rate-limiting steps. The results suggest that the weak adsorption of BnormalH4− over Au(111) may limit the coverage of reactive intermediates at low overpotentials. Breaking O–H bonds on the Au(111) surface is highly endothermic and occurs over substantial reaction barriers at low overpotentials, leading to stable B(OH)2* and BOOnormalH* species on the surface. B–O bond formation on the Au(111) surface is facile. B–H bond cleavage has a relatively low barrier, suggesting that Au(111) is effective in breaking B–H bonds. These results indicate that effective anode catalysts with stronger BnormalH4− adsorption and greater activity for O–H dissociation compared to gold are necessary for improving the efficiency and power density of DBFCs.

Pd–Ir alloy as an anode material for borohydride oxidation

“…The free energy of adsorption of borohydride ions on the Pd 2 eIr 1 (111) and Pd 2 eIr 2 (111) surfaces can be calculated using Equations (8) and (9): (9) where G BH À 4;aq is the free energy of the borohydride ions in aqueous solution [15], G * is the free energy of the bare surface, G BH * 3 þH * is the free energy of the dissociatively adsorbed borohydride species,…”

“…The reaction energy and activation energy for each step of the oxidation of borohydride were calculated using the lowest energy adsorbed states [15,16]. The harmonic vibrational modes were calculated to determine the zero point vibrational (ZPVE) corrections to the total energy of adsorbates.…”

“…Experimental solvation free energies and concentration adjustments were included to consider solution conditions, assuming an ideal solution. The activation energy for each step of the borohydride oxidation reaction on PdeIr surfaces was calculated by locating transition states between the lowest energy adsorbed states, presuming that each step follows the minimum energy path [15,16]. The transition states of elementary reactions on the surfaces were isolated with the climbing image nudged elastic band method (CI-NEB) [34], which finds saddle points and minimum energy paths between the known reactants and products.…”

“…A symmetry factor is used to linearly relate the activation barriers to the equivalent electrochemical reaction [15]:…”

Electrocatalyst Design for Direct Borohydride Oxidation Guided by First Principles

“…13 Our mechanistic studies [10][11] of the BOR over the Au(111) surface concluded that the unfavorable borohydride adsorption and low activity for breaking B-H bonds causes low activity and large overpotentials. Alternatively, very exothermic, dissociative adsorption of BH 4 -ions over the platinum surface produces a large surface coverage of hydrogen.…”