(229d) Importance of Ion-Ion Interactions in Membranes for All-Vanadium Redox Flow Batteries

Redox Flow Batteries (RFB) have the potential to provide the high capacity energy storage needed to improve power grid efficiency through peak shaving, as well as aid in the integration of renewable energy sources, such as wind energy and solar energy. The high cost of traditional ion selective membranes has motivated the exploration of the many different membranes available for use in RFBs¹. In order to facilitate this exploration it is necessary to accurately characterize multi-component mass transport in various membranes, so the impact of using different membranes on RFB performance can be modeled.

Treatment of binary diffusion of species present in all-vanadium RFBs is ubiquitous in the literature², however, for these measurements to be useful in accurate mass transport models, the effects of ion-ion interactions on multicomponent transport across ion-selective membranes must be negligible. Heintz et al³ have investigated these effects for sodium and hydrogen diffusing across neosepta membranes and have seen that the magnitude of the ion-ion interactions is comparable with the magnitude of the ion-solvent interactions. This shows that the importance of including ion-ion interactions in models of RFBs should be tested, so we have measured ion-ion interactions for VOSO₄ and H₂SO₄ in porous, cation-exchange, and anion-exchange membranes.

This talk will briefly discuss how Barnes's transient diffusion model⁴ can be used to quickly measure binary diffusion coefficients and sorption equilibria. These results will then be used to measure ion-ion interactions through application of the Stefan-Maxwell formalism given in Equation 1.

 (1)

For n diffusing species, this system of n force explicit equations can be reduced to a system of n-1 flux explicit equations by taking the membrane velocity as the reference and applying the Gibbs-Duhem equation⁵. Taylor and Krishna⁶ show that a batch dialysis cell will decay exponentially toward equilibrium at long times, with decay constants that are the eigenvalues of the system of transport equations. Therefore, if the binary diffusion interactions are known, the ion-ion interactions can be measured with an interdiffusion experiment in a batch dialysis cell. The values of the binary diffusion coefficients as well as the ion-ion interactions will be given for a system composed of VOSO₄ and H₂SO₄ diffusing through porous, cation exchange, or an anion exchange membranes. The importance of ion-ion interactions in each membrane class will be discussed.

1. Skyllas-Kazacos, M., M. H. Chakrabarti, et al. (2011). J. Eletrochem Soc 158(8): R55-R79
2. Wiedemann, E., A. Heintz, et al. (1998). J. Membrane Sci. 141(2): 215-221.
3. Heintz, A., E. Wiedemann, et al. (1997). J. Membrane Sci. 137(1-2): 121-132.
4. Barnes. (1934). Physics 5(1): 5.
5. Meyers, J. P. and J. Newman (2002). J. Electrochem. Soc. 149(6): A718-A728.
6. Taylor, R. and R. Krishna (1993). Multicomponent mass transfer, Wiley.