Direct borohydride oxidation: mechanism determination and design of alloy catalysts guided by density functional theory

Abstract

Direct borohydride fuel cells (DBFCs) convert an aqueous soluble, high specific energy density borohydride fuel directly to electrical energy. The lack of effective anode electrocatalysts for the anodic oxidation of borohydride limits the efficiency and power density attainable in these devices. The complexity of the eight electron reaction makes experimental determination of the reaction mechanism extremely challenging, thereby hampering the development of a rationale for optimizing catalyst composition. Computational quantum mechanical methods provide a unique tool for evaluating elementary step reaction kinetics in this system, and can be applied to guide a rational catalyst design procedure. In this perspective, we review the experimental literature on borohydride oxidation catalysis and discuss the usefulness of quantum mechanical methods towards electrode design. Mechanistic insights provided by these computational methods are discussed as well as the prospects of applying a computationally guided design procedure towards developing novel catalyst compositions.

Developments in direct borohydride fuel cells and remaining challenges

“…Density functional theory (DFT) is widely used to analyse the reactivity of metal and metal-oxide surfaces. Rostamikia and Janik [36] applied a range of DFT approaches to evaluate aspects of the reaction mechanism of borohydride oxidation on Au (111) and Pt (111) crystalline surfaces [90,91]. The authors found that the molecular adsorption of borohydride ions on Au(111) is only favourable at high overpotentials, whereas initial adsorption over the Pt(111) surface is dissociative and highly favourable at all potentials of interest.…”

“…The reactions of oxidation or hydrolysis of borohydride on these metals must produce the breaking of four BeH bonds. Density functional theory (DFT) calculations suggest that these bond breaking reactions are slower on gold than on platinum surfaces, indicating a higher overpotential for BH 4 À oxidation at Au surfaces [36].…”

Gold–Copper Aerogels with Intriguing Surface Electronic Modulation as Highly Active and Stable Electrocatalysts for Oxygen Reduction and Borohydride Oxidation

“…The moderate upshift in the d-band center might lead to stronger molecular adsorption of BH 4 À on the AuCu aerogelss urface and then facilitateB ÀHd issociation, [36] and moreover,s imultaneous stronger adsorption of HO À speciesw ould further favor direct BH 4 À oxidation. [37] Previous DFT calculations suggested that the combination of Au and Cu sites rather than Au-Au or Cu-Cus ites provided higherc atalytic activities. [37,38] Herein, the atomic ratio of Au/Cu % 52:48 in the AuCu aerogels is believed to reflect an optimal atomic-scale alloying effect of the Cu atoms surrounding the Au atoms.…”

Electrocatalytic performance of environmentally friendly synthesized gold nanoparticles towards the borohydride electro-oxidation reaction

“…Despite Chatenet et al [10] not considering gold as a faradaic-efficient electrocatalyst for the borohydride oxidation reaction (BOR), numerous works [11e14] obtained high coulombic efficiency with Au electrocatalyst, which is attributed to its low activity towards hydrolysis of the BH 4 À anion. The hydrolysis or electro-oxidation of BH 4 À require the break of the BeH, which is fast on Pt(111) and slow on Au(111) [15,16]. The polyol [17e19] and microwave-assisted [20e24] methods have been extensively employed in the preparation of noble nanoparticles.…”