A computational investigation into the solvation and thermodynamic processes within vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) present a promising solution for large-scale renewable energy storage in grid power systems. A defining advantage of VRFBs lies in their decoupled design; energy storage capacity is governed by the volume of external reservoirs containing redox-active vanadium ions, while power output is determined by the electrode surface area or cell stack size. Despite these benefits, the energy density of VRFBs remains constrained by the solubility limits of the vanadium ion redox pairs, which can be modulated through the choice of supporting electrolytes. However, the mechanisms by which supporting electrolytes interact with vanadium ions to enhance their solubility and stability in aqueous environments remain poorly understood. Similarly, the specific active sites within carbon-based electrodes responsible for redox activity are not well characterised, hindering rational electrode design. This thesis addresses both of these knowledge gaps through atomic-scale modelling aimed at clarifying the roles of electrolyte coordination and electrode surface features in VRFB operation.

Our atomic-scale modelling of VRFBs begins by utilising a combined classical molecular dynamics (MD) and density functional theory (DFT) study of the solvation environments of vanadium ions (V²⁺, V³⁺, VO²⁺, and VO₂⁺) in the presence of common supporting electrolytes: sulfate, bisulfate, chloride, dihydrogen phosphate, and methanesulfonate. 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 thermodynamic analysis aligned with these trends, indicating favourable complexation for sulfate, chloride, and dihydrogen phosphate, but unfavourable binding 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. Together, these results clarify vanadium–electrolyte coordination behaviour and reveal, for the first time at the atomistic level, that methanesulfonate likely influences VRFB performance through indirect rather than direct coordination.

Building on the understanding that supporting electrolytes influence vanadium solvation in VRFBs, the next study examines their less-explored role in modulating electrochemical activity at carbon-based electrode surface. Using periodic DFT, we modelled VRFB half-cell reaction mechanisms of the vanadium redox couples at both edge and basal plane sites, coordinated by various electrolytes. The results revealed that electrolyte coordination significantly impacts reaction thermodynamics, with basal planes—typically exhibiting the flattest profiles—which were further improved by coordination of supporting electrolytes over solely water hydrated complexes. In contrast, edge site responses were more variable, strongly dependent on both the electrolyte identity and adsorption site. Among the species studied, methanesulfonate consistently offered the most beneficial coordination, enhancing thermodynamic profiles across nearly all sites and redox states. These findings suggest that supporting electrolytes play a dual role: stabilising vanadium ions in solution and tuning reaction energetics at the electrode. As such, thoughtful selection of electrolyte composition could be key to optimising VRFB efficiency and performance.

Extending the investigation of half-cell reaction mechanisms, the next chapter shifts focus to how structural modifications of carbon-based electrodes influence VRFB performance. Experimental electrode treatments often introduce multiple changes—such as doping, defects, and functional groups—making it difficult to isolate the origins of improved activity. Through periodic DFT modelling, we systematically assessed the impact of these features. For the negative half-cell, edge-site modifications generally improved reaction thermodynamics by weakening vanadium-electrode interactions, whereas basal plane doping and vacancy defects impaired performance. In contrast, positive half-cell reactions benefited from both edge and basal plane modifications but similarly suffered poor activity from vacancy defects. These findings underscore the importance of selective electrode design, as different surface features affect each half-cell reaction differently. Furthermore, post-treatment strategies that interconvert inactive groups (e.g., carboxyls) to catalytically active motifs (e.g., pyrazoles) could enable more effective VRFB electrodes.

Finally, we evaluate the stability of modified carbon electrodes in the negative half-cell of VRFBs by assessing their susceptibility to the parasitic hydrogen evolution reaction (HER) and hydrogen-induced degradation. Using DFT hydrogen adsorption studies modelled on pristine, doped, defected, and functionalised graphene-based surfaces, the study identifies features that increase HER activity or compromise long-term electrode stability. Basal planes were found to offer high stability and HER resistance, whereas single-atom defects and certain edge sites (e.g. P-doping and some N- and O-functional groups) were found to be vulnerable to HER, while other electrode edges (pristine, B-doping, N-doping, and certain O-functionalities) were susceptible to degradative hydrogenation. These findings inform rational electrode design strategies that balance electrocatalytic activity with long-term durability, highlighting the importance of avoiding defects or other features while introducing active VRFB active sites in the form of heteroatom doping or targeted selective edge functionalisation. Future studies to explore whether hydrogenated sites may facilitate alternative mechanisms relevant to VRFB operation are proposed.