Multiscale Simulation of Concentration-Dependent Molecular Clustering and its Role in the Air-Stability of Viologen Anolytes for AORFB

Molecular dynamics simulations combined with quantum-chemical analysis elucidate how electrolyte concentration governs aggregation, solvation, and air stability of viologen anolytes (BVCl₄, BVCl₃, and BVCl₂) in aqueous organic redox-flow batteries. Increasing viologen concentration drives the evolution from well-hydrated, dispersed viologen species to disordered molecular clusters with reduced local hydration. Radial distribution functions and coordination number analyses show that such clustering significantly reduces hydration of redox-active pyridinium nitrogen sites. For BV²⁺, low-concentration electrolytes favor parallel π–π stacked dimers that partially suppress oxygen reduction, whereas higher concentrations disrupt orientational order, weaken local water hydrogen-bonding networks, and induce disordered aggregates. Importantly, disordered clustering sterically hinders water access to pyridinium sites, suppressing water-mediated protonation and subsequent reactive oxygen species formation. These molecular-level insights establish disordered clustering as an intrinsic stabilization mechanism in concentrated viologen electrolytes, highlighting concentration as a key design parameter for achieving air-tolerant, high-performance aqueous organic redox-flow batteries.