Transport Characteristics of Perfluorinated Sulfocation Exchange Membrane With Respect to Vanadium (Iv) and (V) Cations

Ion-exchange membrane in contact with acidic solution of vanadium salts represents an object of study by many research teams in the context of its indispensable presence as an element of membrane-electrode assemblies of both all-vanadium redox flow batteries and hybrid flow power sources which use vanadium compounds inside one of the half-cells. In this work, a new method has been employed for assessing the transport characteristics of the Nafion211 membrane with respect to vanadium ions of high oxidation degrees, i.e. vanadyl (VO) and vanadate (VO) cations, inside aqueous sulfuric acid solutions of varying acidities. The method is based on measuring chromoamperograms, i.e. current transients after a potential step of the electrode in contact with a membrane (pressed to its surface) under conditions where the current of the electrochemical conversion of vanadyl/vanadate at the electrode/membrane interface in the forward or backward direction is limited by the transport of these species across the membrane from an external solution. It has been found that the short-time segments of the transients can be described by the Cottrell dependence (I ~ t); both the Cottrell coefficients and the steady-state currents are proportional to the reagent concentration at the outer side of the membrane. Measurement of chromoamperograms as well as their interpretation to calculate both the diffusion coefficients of vanadyl and vanadate cations inside the membrane and their distribution coefficients at the membrane/solution boundary have been carried out for varying the sulfuric-acid concentration (from 2.2 M to 5 M) and the concentration ratio [VO]/[VO] from 0 to 1 inside the external solution. It has been found that the increase of acid content leads to the growth of vanadyl diffusion coefficient in membrane from 1.76 до 0.84·10 m/s, and vanadate diffusion coefficient falls from 1.89 to 0.8·10 m/s. Meanwhile, distribution coefficients decrease for both of them: from 0.27 to 0.13 and from 0.21 to 0.12 correspondingly. Conclusion has been made on the applicability of the method to analyze the ion transport and the equilibrium composition of ion-exchange membranes in contact with a mixed sulfuric-acid oxovanadium-cations solution.