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membranes Article Electrochemical Measurement of Interfacial Distribution and Diffusion Coefficients of Electroactive Species for Ion-Exchange Membranes: Application to Br2/Br− Redox Couple Dmitry V. Konev 1,2,* , Olga I. Istakova 1 and Mikhail A. Vorotyntsev 2,* 1 2 * Correspondence: dkfrvzh@yandex.ru (D.V.K.); mivo2010@yandex.com (M.A.V.) Abstract: A novel method has been proposed for rapid determination of principal transmembrane transport parameters for solute electroactive co-ions/molecules, in relation to the crossover problem in power sources. It is based on direct measurements of current for the electrode, separated from solution by an ion-exchange membrane, under voltammetric and chronoamperometric regimes. An electroactive reagent is initially distributed within the membrane/solution space under equilibrium. Then, potential change induces its transformation into the product at the electrode under the diffusion- limited regime. For the chronoamperometric experiment, the electrode potential steps backward after the current stabilization, thus inducing an opposite redox transformation. Novel analytical solutions for nonstationary concentrations and current have been derived for such two-stage regime. The comparison of theoretical predictions with experimental data for the Br2/Br− redox couple (where only Br− is initially present) has provided the diffusion coefficients of the Br− and Br2 species inside the membrane, D(Br− ) = (2.98 ± 0.27) 10−6 cm2 /s and D(Br2 ) = (1.10 ± 0.07) 10−6 cm2 /s, and the dis- tribution coefficient of the Br− species at the membrane/solution boundary, K(Br−) = 0.190 ± 0.005, for various HBr additions (0.125–0.75 M) to aqueous 2 M H2SO4 solution. This possibility to deter- mine transport characteristics of two electroactive species, the initial solute component and its redox product, within a single experiment, represents a unique feature of this study. Keywords: membrane electrode assembly; crossover of molecule/co-ion; co-ion distribution coeffi- cient between membrane and solution; solute electroactive component; voltammetric and chronoam- perometric techniques; diffusion permeability of membranes; diffusion coefficient inside membrane; bromide anion; bromine 1. Introduction In rechargeable chemical power sources such as redox flow batteries (PRBs), which use electroactive components dissolved in the electrolytes of the positive and/or negative electrodes, the crossover, i.e., the problem of their transmembrane penetration into the opposite chamber, is of particular importance [1,2]. In addition to a decrease in the battery’s capacitance as a result of redox reactions of the penetrating reagent with “local” substances, the crossover can cause poisoning of the electrode surface and the associated slowdown in the target half-reaction, as well as lead to a change in the composition of electrolytes which seriously affects the characteristics and service life of the device as a whole [3,4]. Therefore, the crossover of redox components through the membrane, for example, in relation to vanadium PRBs, should be considered in the course of simulations of the functioning of both individual membrane-electrode assemblies [5–7] and their stacks [8,9]. Both novel membrane materials [10–12] and novel methods for modification of membranes [13–15] are studied to minimize these harmful effects. Federal Research Center for Problems of Chemical Physics and Medicinal Chemistry of the Russian Academy of Sciences, Chernogolovka 142432, Russia Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, Moscow 119071, Russia Citation: Konev, D.V.; Istakova, O.I.; Vorotyntsev, M.A. Electrochemical Measurement of Interfacial Distribution and Diffusion Coefficients of Electroactive Species for Ion-Exchange Membranes: Application to Br2/Br− Redox Couple. Membranes 2022, 12, 1041. https://doi.org/10.3390/ membranes12111041 Academic Editor: Liang Ge Received: 1 October 2022 Accepted: 18 October 2022 Published: 26 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Membranes 2022, 12, 1041. https://doi.org/10.3390/membranes12111041 https://www.mdpi.com/journal/membranesPDF Image | Electrode Surface Area Affect the Yield of Hydrolysis
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