Ion-exchange membranes (IEM) are an essential component of electrochemical systems such as reverse electrodialysis (RED) and flow batteries. For RED, the membranes separate alternating concentrate and dilute solutions, creating emf. Stacks of such repeating units in series can be used as flow batteries [1] or to produce hydrogen [2] given appropriate electrode selection. Using non-equilibrium thermodynamics, the coupled solute and solvent transport through IEM may be understood in terms of transport numbers [1]. This work highlights the coupling between salt and water fluxes through membranes in RED, and the implications for the efficiency of various technologies which rely upon IEM in contact with aqueous solutions. In general for a mixture of one solute and solvent, the water transported together with ions across the IEM decreases the permselectivity, and thus the emf, of the membrane.

[1] D. Bedeaux, S. Kjelstrup, Non-Equilibrium Thermodynamics of Heterogeneous Systems, World Scientific, 2008.

[2] K. W. Krakhella, M. Morales, R. Bock, F. Seland, O. S. Burheim, K. E. Einarsrud, Electrodialytic Energy Storage: Permselectivity Stack Measurements and Life-Cycle Analysis, Energies 13, 2020.