Redox flow batteries are a promising utility scale energy storage technology, recognized for the safe storage of surplus energy harnessed by wind and solar. Of particular promise is the all vanadium redox flow battery (VRFB) which relies on two redox couples of VO₂⁺ (V⁵⁺)/VO²⁺(V⁴⁺) on the cathodic side and V³⁺/V²⁺ on the anodic side. Significant efforts in recent years have focused on the optimization of operating conditions of VRFBs required for widespread commercialization. However many of these endeavors rely on incomplete or inappropriate data applied to specific systems, (e.g. wrong diffusion coefficients under specific conditions) which is necessary for a more complete and robust model.

One such phenomena inherent to the operation of a VRFB is proton and vanadium ion crossover.^1–4 Proton transport is a requirement for the electrochemical reactions and to maintain charge balance. However, in practice vanadium ions also transport across the membrane causing parasitic and other unknown effects. For example, it is difficult to monitor changes to proton concentration (e.g. pH) as chemical treatment (titration) is clouded by the precipitation of VO(OH)₂, and pH measurements under operational conditions are impossible or impractical.

This presentation will report our fundamental measurements of VRFB electrolytes by first investigating concentration effects of SO₄^2-/HSO₄^- to gain insight on the discharged (VO²⁺) electrolyte and the chemical equilibrium with a goal of being able to identify proton concentration. This understanding will allow for the determination of proton concentrations of the charged electrolytes and shed light on vanadium crossover which has occurred during charging. Those data will be compared to our experimental electron paramagnetic spectroscopy EPR vanadium crossover data.

1. 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.
2. J. S. Lawton et al., J. Electrochem. Soc., 163, A5229–A5235 (2016).
3. R. A. Elgammal, Z. Tang, C.-N. Sun, J. Lawton, and T. A. Zawodzinski, Electrochim. Acta, 237, 1–11 (2017) http://linkinghub.elsevier.com/retrieve/pii/S0013468617306138.
4. R. M. Darling, a. Z. Weber, M. C. Tucker, and M. L. Perry, J. Electrochem. Soc., 163, A5014–A5022 (2015) http://jes.ecsdl.org/cgi/doi/10.1149/2.0031601jes.