How do proton-transfer reactions affect current-voltage characteristics of anionexchange membranes in salt solutions of a polybasic acid?

Authors

Semyon A. Mareev, Andrey D. Gorobchenko, Olesya A. Rybalkina, Kseniya A. Tsygurina, Victor V. Nikonenko, Natalia D. Pismenskaya

Affiliation

Kuban State University, Krasnodar, Russian Federation

Corresponding Author

Semyon A. Mareev, mareev-semyon@bk.ru

Presenting Author

Semyon A. Mareev

Abstract

The complexity of the behavior of anion-exchange membranes (AEMs) in ampholyte-containing solutions has been repeatedly confirmed in various experimental studies that have revealed the nontriviality of the electrochemical characteristics of such systems: the presence of two or more limiting current densities (j_lim) on the current-voltage characteristics (CVC); two Gerischer arcs on electrochemical impedance spectra; several transition times of chronopotentiograms. Insufficient understanding of ion transport mechanisms of ampholytes through AEMs hinders the widespread use of electrodialysis for processing solutions containing such salts. In this work we present experimental and simulated CVCs and partial fluxes of H₂PO₄⁻ and HPO₄²⁻ across an AEM and discuss practical applications of results obtained.

We used a mathematical model based on the Nernst-Planck-Poisson equations coupled with the kinetic equations for chemical reactions. The model describes the nonstationary transport of phosphate acid species through an AEM and adjacent solution diffusion layers. Experimental and simulated CVCs of the AMX membrane in 0.02 mol/L KH₂PO₄ solution have good quantitative agreement at wide range of current densities 0< j <1.5j_lim. We showed that at such currents the membrane behavior is governed by electrodiffusion of ions accompanied by dissociation of acid species upon their entrance in the membrane. The latter affects greatly the ion partial currents through the membrane and the shape of the CVC. The H⁺ ions, which are released during the dissociation of acid species, move into the depleted diffusion layer, and contribute to the charge transfer. This decreases the system resistance, and, when the current sweep rate is relatively high, causes a negative differential resistance at low potential drops.

The finite rate of acid species dissociation leads to a significant difference in membrane system behavior simulated under the often-used condition of the local chemical equilibrium. The dissociation of H₂PO₄ ⁻ anions entering the membrane from the side of the depleted solution occurs within the interfacial region can essentially slow down the formation of ion concentration profiles.

The shape of the CVC of AEM in the KH₂PO₄ solution is greatly influenced by the current sweep rate. The role of this parameter is much higher than in the case of strong electrolyte solutions, since the formation of quasi-stationary concentration profiles of acid species in AEMs takes much more time than the formation of concentration profiles of ions in diffusion layers. For membrane systems in strong monobasic electrolyte solutions (e. g. NaCl), the time to obtain a quasi-stationary CVC is about 40 minutes), while for the system considered in this work with KH₂PO₄ this model-calculated time is 10 hours.

This investigation was realized with the financial support of Russian Science Foundation, grant No. 21-19-00087.