The vanadium redox flow battery (VRFB) is a tool for energy storage that has been promising in applications of large grid storage. Two cells are separated by an ion exchange membrane, which allows proton conduction to preserve electrical balance during the charge/discharge cycles¹. The battery is composed of positive and negative cells operating with VO²⁺/VO₂⁺ and V(II)/V(III) redox couples, respectively. The membrane also allows water, vanadium and acid to crossover from one cell to the other, this can lead to concentration imbalances and self-discharge, ultimately decreasing the efficiency of the battery.^2,3

The focus of this investigation is on measurements of diffusive properties of ionomer films. Perfluorinated ionomers (e.g. Nafion®) have been extensively studied owing to the application of Nafion in fuel cells and electrolyzers. Electron paramagnetic resonance (EPR) enables the measurement of diffusion in of a spin probe or EPR active metal. Rotational diffusion, R, is obtained from analyzing the EPR line shape of the probe.^4,5 This illustrates the local viscosity in the diffusion pathway, i.e. the hydrated channels. In turn, Rmay be related to the effective local viscosity by the Stokes-Einstein equation (see attached image), where a is an effective hydrodynamic radius of the probe, s is an anisotropy factor, and i labels the axis. These values are probe dependent, for the Tempone spin probe, a = 4.2 Å, s_|| = 0.402 and s_^ = 0.543. EPR has also previously been used to further understand the environment of absorbed species in the membrane, including alternative hydrocarbon membranes.^6,7 Previous studies have shown the VO²⁺ ion permeability to have a dependence on acid concentration in solution.⁸

In addition, translational diffusion in hydrated membranes can be measured with UV/vis or EPR by allowing the vanadium permeants to diffuse through the membrane in a cell designed for this purpose where concentrated solution is in contact with one side of the membrane and concentration increase is monitored in a blank solution in contact with the opposite side. A recent paper has shown a more significant discrepancy in comparisons of rotational diffusion and translational diffusion in aromatic hydrocarbon membranes than in perfluorinated ionomer membranes, suggesting a different diffusion pathway in the hydrocarbon based membranes. ⁹

To further characterize the diffusion of vanadium ions in ionomer membranes electrochemical methods are be used. Two separate measurements determine the diffusion of the redox active species through a membrane cast onto a glassy carbon rotating disc electrode (RDE). By taking detailed voltammetric RDE and chronoamperometric measurements at varying film thicknesses and diffusant concentrations, considerable information about the diffusion of different species in the membrane. The effects of backbone architecture and ionization level on the diffusion and solubility of migrant species can be further elucidated.

References

(1) Rychcik, M.; Skyllas-Kazacos, M. Journal of Power Sources 1988, 22, 59.

(2) Xi, J.; Wu, Z.; Teng, X.; Zhao, Y.; Chen, L.; Qiu, X. Journal of Materials Chemistry 2008, 18, 1232.

(3) Weidmann, E.; Heintz, E.; Lichtenthaler, R. N. J. Mem. Sci 1998, 141, 207.

(4) Lawton, J.; Smotkin, E.; Budil, D. J. Phys. Chem. B 2008, 112, 8549.

(5) Lawton, J.; Budil, D. J. Phys. Chem. B 2009, 10679.

(6) Lawton, J.; Budil, D. J. Mem. Sci 2010, 357, 47.

(7) Lawton, J.; Budil, D. Macromolecules 2010, 43, 652.

(8) Lawton, J. S.; Jones, A.; Zawodzinski, T. Journal of The Electrochemical Society 2013, 160, A697.

(9) Lawton, J.; Jones, A.; Tang, Z.; Lindsey, M.; Fujimoto, C.; Zawodzinski, T. J. Electrochem. Soc. 2016, 163, A5229.