The Influence of Current Density on Transport of Vanadium Cations through Three Exemplary Membranes

Transport of a redox active molecule through the separator in a flow battery is an important source of inefficiency and electrolyte imbalance. This transport occurs by diffusion, migration, and convection, with driving forces that change with state of charge. Typically, only diffusive permeability and conductivity are measured to characterize transport in membranes, because they are straightforward experiments. The Nernst-Planck-Einstein equation for transport in electrolyte solutions suggests that this may be sufficient because the transference number of an ion can be calculated from its diffusion coefficient and conductivity. We measured transport of four vanadium cations through Nafion (cation exchange), Fumasep FAPQ-330 (anion exchange), and polybenzimidazole as a function of current density. The measurements were made using a cell with three membranes and four flow compartments designed to collect vanadium passing through sample separators and into receiver compartments. These experiments were augmented with independent sorption and conductivity measurements to assist interpretation in the context of Nernst-Planck-Einstein. The figure shows the sum of the partial current densities of V 2+ and V 3+ versus current density for the three membranes. Partial currents at zero total current are proportional to diffusive permeability and slopes at currents greater than zero approximate transference numbers. Transport of vanadium cations is hindered by an anion exchange membrane more than a cation exchange membranes as expected. The gaps in transport observed at zero current widen as current increases. Further data, not shown here, taken with VO 2+ and VO 2 + ions on the same membranes indicate that transport is strongly influenced by the nature of the cation. All experimental results are compared to predictions from Nernst-Planck-Einstein. Acknowledgements This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1