State of Charge Effects on Vanadium Crossover in Vanadium Redox Flow Batteries
Authors: Lawton Jamie S. , Chace Wyndom , Arruda Thomas M 
Published: May 2020

Large-scale energy storage technologies play a pivotal role in the global clean energy transition, enabling intermittent renewable energy sources such as solar and wind to serve as feasible replacements for fossil fuels. The vanadium redox flow battery (VRFB) is a promising candidate for renewable energy storage applications due to its high energy efficiency, low toxicity, and long lifespan. [1] The half-cells of the battery are separated by a membrane through which ions migrate in order to maintain charge balance. Nafion, a perfluorinated polymer with sulfuric acid functional groups that facilitate proton transport, is the most widely used cation-exchange membrane due to its high proton conductivity and chemical and thermal stability. However, the low ion selectivity of Nafion permits positively charged vanadium ions to also cross through the membrane into the opposite electrolyte resulting in self-discharge of the battery, reducing efficiency.
The characterization of vanadium crossover should consider the correlation between electrolyte composition and membrane properties. Vanadium concentration, sulfuric acid concentration, and state of charge are the three main factors that determine electrolyte composition. Electrolyte solutions with higher H2SO4 concentration corresponded to lower VO2+ membrane permeability.[2] A higher initial VO2+ concentration also corresponded to lower VO2+ membrane permeability. The low permeability at high H2SO4 and VO2+ concentrations could only be partially attributed to increased viscosity; other contributing factors may include dehydration of the membrane from interactions with sulfuric acid.
In previous crossover-diffusion experiments,[3] the permeability of VO2+ with respect to the counter ion followed the trend H+ > VO2
+ > VO2+, while the permeability of V3+ with respect to the counter ion was found to be VO2
+ > VO2+. Uptake of vanadium species in the membrane followed the order V3+ ≈ VO2+> VO2
+ at all concentrations of H2SO4 (0.5 M to 5 M), suggesting that permeability behavior of each vanadium species depends on the presence or absence of competitive partitioning resulting from particular vanadium counter ions. For all vanadium species, increased acid concentration decreased both the permeability and uptake of vanadium ions, likely due to the dehydration of the membrane in more acidic conditions (Lawton, JES, 2017).
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 catholyte, for example, a 0% SOC solution consists of entirely VO2+, while a 100% SOC solution is entirely VO2
+. The characterization of mass transfer in the operating VRFB has yet to fully investigate the role of SOC on electrolyte properties. Here, we study the relationship between SOC and physical constants of the catholyte using electron paramagnetic resonance spectroscopy.[4,5][6] Our work seeks to elucidate some of the questions that remain in characterizing Nafion membrane permeability in the VRFB catholyte, in particular: how vanadium ion permeability varies with SOC; how the presence of VO2+ and VO2
+ together in the catholyte influences each ion’s permeability; and how H2SO4 concentration affects all of these factors.

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