Diffusion Coefficient and Viscosity of Methyl Viologen Electrolyte Estimation Based on a Kinetic Monte Carlo Computational Approach Coupled with the Mean Square Displacement Method

Abstract

Methyl viologen (MV) and its derivatives are emerging as promising candidates within the organic redox flow battery community due to their commendable reversibility and rapid reaction kinetics. However, experimental observations reveal the influence of solute concentration on the diffusion coefficient and the tendency of MV⁺ to form dimers or multimers, affecting electrolyte viscosity. Traditional characterization methods may not fully capture these properties. To explore concentration and state of charge effects on diffusion coefficient and viscosity, a kinetic Monte Carlo (kMC) model coupled with mean square displacement analysis is introduced. The kMC model offers a 3D simulation space with expandable periodic boundary conditions, enabling realistic ion movement. The mean square displacement (MSD) algorithm extracts diffusion coefficients, followed by the estimation of the electrolyte viscosity using the Stokes-Einstein equation. Validation with NaCl solutions precedes adaptation to simulate MV⁺⋅diffusion coefficients at 1.5 M with varying states of charge (SoC), aligning with experimental data. Simulation results indicate increased multimerization at higherSoCs. The diffusion coefficient of fully charged MV⁺⋅decreases with electrolyte concentration due to dimer and multimer formation. This modeling approach provides insights into MV⁺⋅behavior, crucial for organic redox flow battery development.