Redox flow batteries (RFBs) are seen as a potentially integral technology for grid scale energy storage but currently suffer from performance issues and low coulombic efficiency due to side reactions such as hydrogen evolution. For this reason, catalysis in RFBs is tricky as the potential catalyst must enhance the redox reactions of interest without also promoting hydrogen evolution. Adapting a novel synthesis method used to produce highly dispersed and durable Pt/graphene catalysts for fuel cells¹, we have developed a novel Bi based catalyst for RFBs. The precise control of a mild synthesis method succeeds in producing evenly distributed bismuth nanoparticles on highly conductive graphene support and has exhibited tremendous enhancement of vanadium redox flow battery (VRFB) performance. To understand its catalysing mechanism as well as the electrolyte additive impacts, in situ x-ray absorption near edge structure (XANES) with high energy resolution fluorescence detection (HERFD) and extended X-ray absorption structure (EXAFS) were studied at the Bi L_III and L_I edges in the acid electrolytes containing Bi-ion, V-ion or both Bi- and V- ions respectively. The XANES experimental data with detailed features thanks to HERFD were studied with ab initio calculations to understand its electronic structure variation. The EXAFS results were quantified via fitting in the R-space to obtain bond distance and oxygen coordination numbers of the Bi-O shell, where its oxidation states under real-time operation were found to be not only dependent on the potential but also the electrolyte. With the assistance of the high surface area catalysts and in-situ XES study, the dynamic mechanism of the Bi catalytic process under influence of potential and electrolyte compositions is able to be elucidated.

Figure 1,(a) XANES spectra of Bi₂O₃/Graphene catalyst at the Bi L_III edge in 0.1 M vanadium electrolyte containing 100 ppm Bi³⁺ and holding at a different voltage (-0.1, -0.2, -0.3, -0.4, -0.5 and -0.6 V); (b) Comparison with Bi and Bi₂O₃ standard; (c) Experimental (black line) and simulated (red dots) (r) versus r plots at the Bi L_III edge in vanadium electrolyte and holding at a different voltage; (e) Low-resolution TEM image (inset: histogram of particle size) and (f) High-resolution TEM image of Bi₂O₃/Graphene catalyst.

1. G. M. A. Angel et al., Nanoscale, 12, 16113–16122 (2020).