Transport Characteristics of Perfluorinated Sulfocation-Exchange Membrane with Respect to Vanadium(IV) and (V) Cations

Abstract

Ion-exchange membrane in contact with acidic vanadium salt solution represents an object of study by many research teams in the context of its indispensable presence as an element of membrane–electrode assemblies in both all-vanadium redox flow batteries and hybrid flow power sources, which use vanadium compounds in one of the half-cells. In this work, a new method is employed for assessing transport characteristics of the Nafion211 membrane with respect to vanadium ions of high oxidation degrees, i.e., vanadyl (VO²⁺) and vanadate (\({\text{VO}}_{2}^{ + }\)) cations, in aqueous sulfuric-acid solutions of varying acidity. The method is based on the measuring of chronoamperograms, i.e. current transients after the applying of potential step to electrode contacting a membrane (pressed up to its surface) under conditions when the current of the vanadyl–vanadate electrochemical conversion at the electrode/membrane interface in the forward or backward direction is limited by the transport of these species from an external solution across the membrane. The short-time segments of the transients were found to be described by the Cottrell dependence (I ~ t^–0.5); both the Cottrell coefficients and the stationary currents are proportional to the reactant concentration at the membrane outer side. Measurement of chronoamperograms as well as their interpretation aimed at the calculating of both the vanadyl and vanadate cations’ diffusion coefficients inside the membrane and their distribution coefficients at the membrane/solution boundary are carried out for varied sulfuric-acid concentration (from 2.2 to 5 M) and the [VO²⁺]/[\({\text{VO}}_{2}^{ + }\)] external-solution concentration ratio from 0 to 1. The increase in the acid content was found to lead to the growth of the vanadyl diffusion coefficient in the membrane from 1.76 to 0.84 × 10^–11 m²/s; the vanadate diffusion coefficient falls from 1.89 to 0.8 × 10^–11 m²/s. Meanwhile, the distribution coefficients decreased for both cations: from 0.27 to 0.13 and from 0.21 to 0.12, respectively. It is concluded on the applicability of the method in the analyzing of the ion transport and the equilibrium composition of ion-exchange membranes in contact with a sulfuric-acid–oxovanadium-cation mixed solution.