Abstract
Pt-group metal (PGMs) catalysts are widely employed for the borohydride oxidation reaction (BOR) in direct borohydride fuel cell anodes, however, they have significant drawbacks. PGMs are not efficient for the BOR; they promote non-negligible hydrogen evolution reaction (HER) for E < 0 V vs RHE, allow associated H2 escape that is detrimental to cell operation, and suffer non-negligible poisoning by BHx,ad intermediates (1, 2). Better BOR catalysts should combine poor HER activity and high BOR activity, which seems unfeasible with PGMs. Nickel-based electrodes, widely-used for hydrogen reactions in alkaline environment (3), were recently demonstrated very active for the BOR at low overpotential values, if their state of the nickel surface is tightly controlled and maintained as metallic (4).
The present contribution will present BOR mechanism investigation at reduced (metallic) Ni electrodes, using a combination of various electrokinetic measurements, differential electrochemical mass spectrometry (DEMS), Fourier-transform infrared spectroscopy (FTIR), Density Functional Theory (DFT) calculations and microkinetic modelling. The BOR mechanism differs between metallic Ni and PGM surfaces: metallic Ni is less active for the HER (due to a stronger H adsorption on Ni compared to Pt) and less prone to BHx,ad poisoning (due to a weaker adsorption on Ni compared to Pt) without compromising its BOR activity. Cumulatively, these effects account for the higher performance of reduced Ni surfaces at low potential values. The contribution will also highlight how electrodeposited Ni electrodes have been successfully prepared, using various substrates compatible with their implementation in a direct borohydride fuel cell (DBFC). Very open and carbon-free structures like Ni-felts (NFT), have enabled outstanding performance in DBFC conditions, should it be with a cation-exchange membrane (in Na+ form) (5) or a bipolar interface (6), or even anion-exchange membranes. The key in such developments is the proper preparation of the Ni-felt support, so that it essentially presents a high-surface-area metallic surface (Figure 1), stable long enough to be mounted in the fuel cell fixture.
Acknowledgments
This work has been performed in the frame of the MobiDiC project, funded by the French National Research Agency (ANR, grant # ANR-16-CE05-0009-01) and the DGA.
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Figure 1