Inter-compartment concentration monitoring of key species in a vanadium redox flow cell operating with and without polarization: Experiments and construction of a predictive model

Abstract

The usually encountered phenomenon of vanadium crossover in Vanadium Redox Flow Batteries (VRFB), leading to energy capacity losses, is specifically studied without (diffusion measurements) and with the application of an electric current (battery operating conditions). The experimental set-up is an electrochemical filter-press reactor integrating a NafionTM 424 membrane and two storage tanks. The electrolytes composition on both sides was determined using potentiometric titration (VO2+ and H+) and UV–Vis analysis (V2+, V3+ and VO2+). First, experiment is conducted without polarization and a model based on mass balance equation incorporating diffusion flux and electrolyte volumes change is derived to predict electrolytes composition on both sides. The effective diffusion coefficient of VO2+ in the membrane is obtained by fitting the model with experimental data. The experiment is then carried out under galvanostatic polarization (10 mA/cm2) and a model embedding, simultaneously, electrolyte volumes variation, diffusion and migration across the membrane and the electrochemical and self-discharge reactions, is proposed to predict electrolytes composition. The partitioning of the applied current between all possible electrochemical reactions is addressed using a specific algorithm based on logical tests on limiting currents. The resulting system of coupled ordinary differential equations is solved numerically using standard python functions. The results indicate a satisfactory agreement between predicted and experimental data and highlight the requirement of considering all phenomena for an accurate prediction of electrolytes composition in VRFB. The interest of applying the developed model to simulate and predict composition evolution in VRFB (State of Charge (SoC) and acidity) is discussed.