Investigating the Effects of Operation Variables on All-Vanadium Redox Flow Batteries Through an Advanced Unit-Cell Model

Next-generation redox flow batteries will benefit from the progress of macroscopic continuum models that enable the optimization of new architectures without the need of expensive fabrication and experimentation. Despite previous attempts, there is still need for robust and thoroughly validated models. Here, a steady-state two-dimensional unit-cell model of an all-vanadium redox flow battery is presented. The model integrates state-of-the-art descriptions of the fundamental physical phenomena, along with new features such as local mass transfer coefficients for each active species, precise sulfuric acid dissociation kinetics, and experimental data of the electrochemical parameters and electrolyte properties. The model is validated at different states of charge and flow rates using polarization, conductivity and open circuit voltage measurements. Then, the contribution of operating conditions on battery performance is studied by analyzing its separate effect on the various phenomena that affect cell performance, such as local pore mass transfer limitations, parasitic hydrogen evolution reactions, crossover and self-discharge fluxes. The resulting model is a reliable tool that can be used to assess the relevance of these coupled phenomena that take place simultaneously within the reaction cell. This important information is critical to optimize cell components, reactor design and to select optimal operating conditions.