High-throughput discovery of anode catalysts for direct sodium borohydride fuel cells

Abstract

Combinatorial synthesis and high-throughput characterization and electrochemical screening methods have been used to accelerate the design and optimization of binary alloy catalysts for the anode of the Direct Borohydride Fuel Cell. Single metal catalysts are shown to have limited effectiveness, in agreement with previous literature. In principle, the direct oxidation of the BH4ˉion is accompanied by the transfer of eight electrons. Competing electrochemical oxidation reactions1-3 , and the spontaneous catalytic hydrolysis of BH4ˉ hinder the exploitation of the eight electron reaction. The order of the catalytic activity for the direct oxidation of the BH4ˉ ion in alkali environment on pure metals as reflected in the onset potential vs. SHE for the reaction was found to be Au (-0.43 VSHE) > Ag (-0.21 VSHE) > Pt (0.04 VSHE) > Ni (+0.62 VSHE) > Cu (+0.7 VSHE). Pt hydrolysed BH4ˉ producing H2 which was electro-oxidised at -0.7 VSHE, competing with direct BH4ˉ oxidation. The catalytic activity of the Ni and Cu towards the direct BH4 ̄oxidation reaction (BOR) was found to be sharply affected by the passivation layer formed by the adsorption of O2/OH. The intent was to modify the catalytic properties of the pure metals by alloying through both electronic and ensemble effects, lowering the overpotential for the eight-electron process and reducing the parasitic effects of competing catalytic pathways, and hindering O2/OH adsorption. A wide gradient of compositions of Au-Ni, Au-Cu, and Au-Pt alloy catalysts (unannealed and annealed) were synthesized by High Throughput Physical Vapor Deposition (HT-PVD). High Throughput bulk/surface methods including Energy Dispersive X-ray Spectroscopy (EDX), X-ray Diffraction (XRD) and X-ray Photon Spectroscopy (XPS) were used to characterise the alloy catalysts as a function of composition. To efficiently determine the best alloy catalysts for the direct BOR, High Throughput ii Electrochemical Screening method was used. Additionally, a High Throughput Hydrogen Probe method was developed to monitor H2 evolution on the Au-Pt alloy catalysts. This method was found to be effective in determining the lowest/highest H2 evolution as a function of composition. New anode alloy catalysts were discovered and can catalyse the BOR at low over potential. The optimum Au-Ni alloy composition was found to be at composition Ni35-Au65 offering a higher electrode activity for the direct BOR among other binary alloy catalysts. The Au-Cu alloy catalysts were found to be affected by the addition of Cu on the Au-Cu alloy catalysts. The activity of the Cu78.3-Au21.7 (unannealed) was found to be higher than pure Au. In order to evaluate the electrocatalytic activity on the Au-Pt alloy catalysts, the H2 evolution was successfully detected as a function of composition using High Throughput Hydrogen Probe. This experimental approach proved that the BOR on the AuPt alloy catalysts was strongly influenced by the competing H2 oxidation reaction produced by the spontaneous catalytic hydrolysis reaction.

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