Multicomponent Mass Transfer in All-Vanadium Redox Flow Battery Membrane Separator

Redox flow batteries (RFB) are promising candidate for energy storage systems (1). One of the attractive types of redox flow batteries is the all-vanadium redox flow battery (VRFB) as originally conceived by Skyllas-Kazacos and coworkers (2). Among different components of the VRFB, the ion-exchange membrane directly affects the voltage and coulombic efficiencies of the VRFB. Increased voltage efficiency is obtained with a trade-off of increased cross-over of reactant species through the membrane. Understanding the mechanism of water and reactive species transport through the ion-exchange membrane has been the focus of recent modeling and experimental works (3-5).  

A number of experiments by several research groups have been conducted for the measurement of vanadium cross-over. These experiments have either used an ex-situ static dialysis cell or focused on the long term accumulated effect of the vanadium ion transport (6-10). Also, some recent work have conducted to measure the cross-over of active species in an operating VRFBs (11-13). However, these measurements have either been performed in a very low current densities or the measurements have had qualitative value due to the system leakage or operational problems.

In this talk, we will address the modeling and experimental work that has been performed to quantify the cross-over of species through ion-exchange membrane within an operational VRFB. We have designed and built a novel 6 cell test system that utilizes the UV-Vis spectrometry technique to measure the cross-over of species and this system enables us to quantify the cross-over of active species with and without the passage of current (diffusion and diffusion-migration). We have also developed a model to account for ion-ion interactions within the membrane. The model is based upon the Stefan-Maxwell multicomponent diffusion equation where the fluxes of the species are fully coupled. The driving force for species transport has been modeled in terms of concentration and electrostatic potential gradients. The results of this study can be used to develop concentrated solution diffusion relationships which can be used to increase the fidelity of computational simulations and optimize VRFB system design and operation. 

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