Fluxes of negatively charged ligand-coordinated iron, Fe(CN)63/4-, and chromium, CrPDTA1/2-, through two promising commercial cation-exchange membranes, Aquivion E87-05S and Fumasep E-620(K), were measured as functions of current density. The magnitude of the partial current density reached a maximum of |-43| A cm-2 at the maximum applied current density magnitude of |-43| mA cm-2 for Fe(CN)63/4- transport through Aquivion, or 0.1% of the total current density. Fumasep E-620(K) blocks

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... ly all crossover of both compounds. Both membranes sorb more Fe(CN)63/4- and CrPDTA1/2- than predicted by Donnan equilibrium, and low crossover rates can be attributed mainly to slow diffusion, not charge-based rejection of co-ions. The magnitude of the diffusion coefficient appears to correlate with hydraulic permeability. Although Aquivion E87-05S and Fumasep E-620(K) have significant and observable differences in membrane crossover rates, cells built with the DI-soaked membranes offer similarly high coulombic efficiency, indicating the relatively small contribution that crossover makes to inefficiency over a single cycle.