Crossover analysis in a commercial 6 kW/43kAh vanadium redox flow battery utilizing anion exchange membrane

Abstract

The volumetric and species transport is a significant source of capacity decay in vanadium redox flow batteries (VRFBs). However, even with the prevalent use of anion exchange membranes (AEMs) in commercial systems, the understanding of the crossover mechanisms in this context remains limited. The primary objective of this study was to gain a deeper comprehension of these mechanisms through experimental investigations conducted on a 6 kW/43kAh VRFB system using AEMs. It is concluded that protons serve as the primary charge carriers, owing to the high ionic concentrations of the electrolyte. During normal operation, there was a consistent pattern of volume transfer from the positive to the negative half-cell (∼0.5 % of the total electrolyte per cycle/day). Experimental assessments performed at different states-of-charge (SoCs) revealed that the volumetric transport towards the anolyte increases with SoC. The osmotic effects are concluded to be the main contributors to the volumetric transport. The osmotic pressure difference is hypothesized to arise from the asymmetric diffusion coefficients of the vanadium ions, changes in pH affecting sulfate and bisulfate equilibrium alongside their different diffusion coefficients, and a likely predominance of the bisulfate concentration gradient in the water transport through the membrane.