Direct Observation of Crossover and Current Distribution in All-Vanadium Redox Flow Batteries

Abstract

One of the critical ways to improve the performance of all-vanadium redox flow batteries (VRFBs) is through improved cell design and material selection 1. However, there are certain issues yet to be addressed, including overcoming rapid capacity decay over long-term cycling, and deeper understanding of the contributions to this decay is needed. The primary reason for capacity loss during cycling is undesired vanadium and water transport through the ion-exchange membrane (usually labeled crossover) 2-5. One common technique for mitigating the capacity loss is utilization of thicker membranes along with periodic electrolyte rebalancing. However, these techniques are only partially effective and each introduces additional performance losses in the system. Accordingly, the focus of active research is to develop new ion exchange membranes with reduced crossover and improved ionic conductivity. Recently, we studied the effect of the electric field on vanadium crossover and deduced interaction coefficients for quantifying vanadium crossover as a function of state of charge (SoC), focusing on the through-plane, cumulative crossover effect. Thus, the transport parameters for vanadium ions with and without the effect of electric field and as a function of SoC are now known for Nafion ® membranes 6. Here we present an in-depth experimental approach for assessing the parameters affecting vanadium ion and water crossover through ion-exchange membranes, primarily via through-plane current distribution measurement and in-plane crossover analysis. However, to the best of our knowledge, there is no published work focused on the investigation of in-plane distribution of vanadium ion crossover. In this talk we will report detailed experimental data focused on the through-plane vanadium crossover distribution. A unique facility has been designed and built, equipped with UV/Vis spectroscopy to assess vanadium crossover under different operating conditions for through-plane, cumulative crossover analysis. We have also incorporated localized current distribution to assess in-plane non-uniformity in current density via inclusion of a printed circuit board 7. The localized current distribution is not solely a function of electrolyte transport in the electrode due to the combined effects of diffusion, advection, and migration; it is also affected by the vanadium ion crossover. We will show examples of membrane properties, commonly linked to vanadium ion crossover and ohmic losses, impacting instantaneous, in-plane current distribution.