Effect of State of Charge on Physical Properties of Vanadium Redox Flow Battery Electrolytes

Abstract

Vanadium redox flow batteries (VRFB)s consist of two half-cells separated by an electrolyte/membrane through which ions migrate in order to maintain charge balance.1 Nafion, a perfluorinated polymer with sulfuric acid functional groups that facilitate proton transport, is a widely utilized membrane for both flow battery and fuel cell technologies. Parasitic vanadium crossover measurements can take into account the correlation between the composition of electrolyte solutions and the membrane properties. In the case of VRFB based on vanadium sulfate, vanadium concentration, sulfuric acid concentration, and state of charge (SOC) are three main factors describing electrolyte composition. Electrolyte solutions with higher H2SO4 concentration have been found to corresponded to lower VO2+ membrane permeability.2 In previous crossover-diffusion experiments,3 the permeability of VO2+ with respect to the counter ion followed the trend H+ > VO2 + > VO2+. Absorption of vanadium species in the membrane followed the order V3+ ≈ VO2+> VO2 + at all concentrations of H2SO4. The state of charge (SOC) of the electrolyte, expressed as a percentage from 0% to 100%, describes the extent of oxidation that has occurred in each electrolyte. In the case of this study, 0% SOC consists of all VO2+ and 100 % SOC consists of all VO2 + while the intermediate SOCs consist of mixtures of the two and probe the electrochemical reactions of the positive electrode in the VRFB. The characterization of mass transfer in the operating VRFB has yet to fully investigate the role of SOC on diffusion and other properties. Here, we study the relationship between SOC and properties of the electrolyte solutions using electron paramagnetic resonance spectroscopy4,5 6, cyclic voltammetry,7 and gravimetrics. References M. Skyllas-Kazacos, L. Cao, M. Kazacos, N. Kausar, and A. Mousa, ChemSusChem, 9 (2016).J. S. Lawton, A. Jones, and T. Zawodzinski, J. Electrochem. Soc., 160, 697–702 (2013).J. S. Lawton, A. M. Jones, Z. Tang, M. Lindsey, and T. Zawodzinski, J. Electrochem. Soc. , 164, A2987–A2991 (2017) http://jes.ecsdl.org/content/164/13/A2987.abstract.J. S. Lawton, E. S. Smotkin, and D. E. Budil, J. Phys. Chem. B, 112, 8549–8557 (2008).J. S. Lawton and D. E. Budil, J. Phys. Chem. B, 113, 10679–10685 (2009).J. S. Lawton and D. E. Budil, Macromolecules, 43, 652–661 (2009).J. S. Lawton, S. M. Tiano, D. J. Donnelly, S. P. Flanagan, and T. M. Arruda, Batteries, 4, 1–14 (2018).