Insights into Vanadium Electrocatalysis with High-Resolution In-Situ X-Ray Spectroscopy: Bi Nanoparticles on Graphene for Enhanced V Redox Flow Batteries

Aqueous redox flow battery systems present notable benefits such as affordability, safety, and superior ionic conductivity. However, a central obstacle across all applications lies in effectively functioning within the limited electrochemical stability range of aqueous electrolytes, preventing their decomposition. Our research here is centred on investigating the side reaction of hydrogen evolution (HER) in the context of an all-Vanadium redox flow battery (VRFB), using it as a representative case [1]. VRFB is a crucial technology for large-scale energy storage on the grid, although they presently suffer from performance challenges and suboptimal coulombic efficiency due to detrimental side reactions. The hurdle lies in finding suitable catalysts for VRFBs—ones that promote essential redox reactions without concurrently promoting hydrogen evolution.<br/>Leveraging a novel synthesis approach employed in creating robust Pt/graphene catalysts for fuel cells [2], we have successfully engineered a novel Bi-based catalyst tailored for VRFB. A precise and gentle synthesis method ensures the uniform dispersion of bismuth metal nanoparticles on a highly conductive graphene support, resulting in a substantial boost to battery performance that demonstrates an impressive energy efficiency of 75% over 100 cycles at a relatively high current density of 200 mA cm^-2. To unravel the catalytic mechanism, we conducted in situ X-ray absorption near edge structure (XANES) experiments, employing high energy resolution fluorescence detection (HERFD) and extended X-ray absorption fine structure (EXAFS) at the Bi L_III and L_I edges within electrolytes containing H⁺ or both H⁺ and V³⁺. The XANES experimental data, enriched by HERFD, were meticulously compared with ab initio calculations to elucidate the relationship between electronic structure and potential. Utilizing density functional theory-based theoretical simulations, we unveiled a concentration-dependent relationship between H-ions and V-ions on Bi crystal surfaces. Given their inherent high V-ion concentration in the electrolyte to boost energy density, Bi holds promise for significantly suppressing HER side reactions. This work also provided insights into the role of ionized water molecules (H*) and their implications on electrochemical processes, which is of utmost importance for advancing the development of efficient electrocatalytic systems in aqueous environments.<br/><br/>References:<br/>[1] Y. Wen, T.P. Neville, A. Jorge Sobrido, P.R. Shearing, D.J.L. Brett, R. Jervis, Bismuth concentration influenced competition between electrochemical reactions in the all-vanadium redox flow battery, J Power Sources. 566 (2023) 232861. https://doi.org/10.1016/J.JPOWSOUR.2023.232861.<br/>[2] G.M.A. Angel, N. Mansor, R. Jervis, Z. Rana, C. Gibbs, A. Seel, A.F.R. Kilpatrick, P.R. Shearing, C.A. Howard, D.J.L. Brett, P.L. Cullen, Realising the electrochemical stability of graphene: Scalable synthesis of an ultra-durable platinum catalyst for the oxygen reduction reaction, Nanoscale. 12 (2020) 16113–16122. https://doi.org/10.1039/d0nr03326j.