Computational investigation of coordinating electrolytes with vanadium ions in redox flow batteries

The solvation environments of the vanadium ions central to vanadium redox flow battery (VRFB) operation (V²⁺, V³⁺, VO²⁺, and VO₂⁺) in the presence of common supporting electrolytes: sulfate, bisulfate, chloride, dihydrogen phosphate, and methanesulfonate were investigated via a combined classical molecular dynamics (MD) and density functional theory (DFT) thermodynamic framework. Classical MD simulations, validated by ab initio MD, showed that vanadium-electrolyte coordination increases with supporting electrolyte concentration and varies with vanadium species, with V³⁺ exhibiting the highest coordination and VO₂⁺ the lowest. Chloride and dihydrogen phosphate coordinated readily with all vanadium ions even at low concentrations, while methanesulfonate exhibited minimal coordination even at 6 mol kg^-1. DFT energetic analysis aligned with these trends, indicating favourable complexation for sulfate, chloride, and dihydrogen phosphate, but unfavourable complexation for bisulfate and methanesulfonate. Additionally, peripheral (non-coordinated) electrolyte molecules were found to shift half-cell reduction potentials negatively-more significantly in the negative half-cell-resulting in an overall increase in cell voltage. This was found to occur even if the electrolyte molecule was not directly coordinated with the vanadium centre. Together, these results clarify the atomistic coordination behaviour of supporting electrolytes with vanadium ions and their implications for VRFB performance.