Bromide crossover through various perfluorinated cation-exchange membranes in the presence of two cationic components in solution

Abstract

Chronoamperometry and steady-state voltammetry data for a specially designed working electrode composed of a Pt disk covered mechanically with various perfluorinated cation-exchange membranes in contact with an external 2 M sulfuric acid solution with addition of various NaBr concentrations have been employed to estimate crossover parameters of bromide ions in relation to redox-flow battery applications. This technically simple but efficient approach has allowed us to determine the values of the diffusion coefficient of the electroactive Br⁻ co-ion inside each membrane and of its equilibrium distribution coefficient between the membrane and the outer solution via an express experimental procedure and subsequent simple calculations. These crossover parameters of bromide co-ions have been found for Nafion NR211, Nafion XL, Nafion NR212, Nafion N115, Nafion N117 as well as GP-IEM-103, GP-IEM-105 membranes. Correlation of the steady-state diffusion-limited current due to the bromide oxidation both with its concentration in the outer solution and with the membrane thickness has been analyzed. It has been established that the transport characteristics of the bromide anion are close to each other for all homogeneous membranes under study in contact with a mixed X M NaBr +2 M H₂SO₄ solution (the value of X varies between 0.125 and 0.75): their values belong to the range from 2.6 10⁻⁶ cm² s⁻¹ to 3.4 10⁻⁶ cm² s⁻¹ for its diffusion coefficient inside membrane and to the range from 0.13 to 0.18 for its distribution coefficient at the membrane/solution boundary. Compared to the homogeneous membranes, the Br⁻ anion diffusion inside the heterogenous (Nafion XL) membrane is slower and there is a tendency to its accumulation to a larger amount. Comparison of these results for Nafion NR212 in contact with the NaBr+H₂SO₄ solution with those previously obtained for the same membrane in contact with the HBr + H₂SO₄ solution has allowed us to conclude that the applied approximate treatment of experimental data based on the theory of molecular-diffusion transport of this co-ion inside the membrane (without taking into account the migration contribution to the bromide flux owing to the suppression of the electric field by highly mobile H⁺ cations) remains applicable for systems where the NaBr concentration inside the external solution does not exceed 0. 75 M.