Abstract
The VO2+/VO2+ redox couple commonly employed on the positive terminal of the all-vanadium redox flow battery was investigated at various states of charge (SOC) and H2SO4 supporting electrolyte concentrations. Electron paramagnetic resonance was used to investigate the VO2+ concentration and translational and rotational diffusion coefficient (DT, DR) in both bulk solution and Nafion membranes. Values of DT and DR were relatively unaffected by SOC and on the order of 10−10 m2s−1. Cyclic voltammetry measurements revealed that no significant changes to the redox mechanism were observed as the state of charge increased; however, the mechanism does appear to be affected by H2SO4 concentration. Electron transfer rate (k0) increased by an order of magnitude (10−6 ms−1 to 10−8 ms−1) for each H2SO4 concentrations investigated (1, 3 and 5 M). Analysis of cyclic voltammetry switching currents suggests that the technique might be suitable for fast determination of state of charge if the system is well calibrated. Membrane uptake and permeability measurements show that vanadium absorption and crossover is more dependent on both acid and vanadium concentration than state of charge. Vanadium diffusion in the membrane is about an order of magnitude slower (~10−11 m2s−1) than in solution (~10−10 m2s−1).
Highlights
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 [1]
The VO2+ /VO2 + redox couple was studied as a function of state of charge (SOC) at 1, 3 and 5 M
Vanadium species crossover through Nafion membranes was studied by Electron Paramagnetic Resonance (EPR), diffusion cell measurements and gravimetric methods
Summary
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 [1]. 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 [2,3]. Like all redox flow batteries, the VRFB utilizes two tanks of electrolyte that circulate through the battery cell stack, where the redox couples in each electrolyte react at the electrodes to generate an electrical current. Because energy storage capacity scales with the volume of the electrolyte, redox flow batteries are conducive to large-scale applications such as electrical grid storage. The VRFB in particular is unique in that a single chemical species, vanadium, is employed on both the positive and negative half-cells of the battery: the catholyte contains the redox couple VO2+ /VO2 + , while anolyte contains V2+ /V3+ , in a supporting electrolyte, typically sulfuric acid. The half-cells of the battery are separated by a proton exchange membrane to allow for proton transport while minimizing
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