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Effect of Surface Modification of Heterogeneous Anion-Exchange Membranes on the Intensity of Electroconvection at Their Surfaces

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Abstract—

Electroconvection is the principal mechanism that allows markedly increasing the rate of ion transfer through ion-exchange membranes in intensive current regimes. In this work, we investigated the possibility of intensifying electroconvection in solution near heterogeneous MA-41 anion-exchange membrane (Shchekinoazot production) by the modifying of its surface. The use of weakly crosslinked ion-exchange resin (MA-41P) in the course of the membrane manufacturing, with subsequent chemical modification of its surface (MA-41PM), is shown to make it possible to increase the limiting current density almost twice. The value of the reduced potential drop (after subtracting the ohmic contribution), at which significant generation of H+ and OH– ions begins, is shifted from 0.8 V in the case of MA-41 to 1.7 V in the case of MA-41PM. The current density related to the onset of water splitting is equal to 0.9\(i_{{{\text{lim}}}}^{{{\text{Lev}}}},\) in the case of MA-41; 2\(i_{{{\text{lim}}}}^{{{\text{Lev}}}},\) in the case of MA-41PM (where \(i_{{{\text{lim}}}}^{{{\text{Lev}}}}\) is the theoretical value of the limiting current density). The special feature of the modified membrane behavior is the presence of a range of potential drop (between 50 and 80 mV in the reduced scale), in which the system with the MA-41PM has negative differential resistance: in this range, the potential drop decreases when the current density increases. This behavior occurs when measuring quasi-stationary I–V curves; correspondingly, in the chronopotentiogram there is a time interval, where the potential drop decreases with time. The electroconvection is intensified near a modified membrane due to a higher fraction of conductive areas on the surface of the modified membrane and the redistribution of these areas via formation of their agglomerates in the centers of the cells formed by the reinforcing mesh. Mathematical modeling shows the concentration polarization of the modified membrane being less than that of the pristine one. Meanwhile, the structure of electroconvective vortices is optimized: the vortices near the modified membrane are larger; they do not extinguish each other, unlike the case of MA-41.

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ACKNOWLEDGEMENTS

Authors are grateful to the core facilities center “Ecology and analytical center” of Kuban State University and Laboratory “Metals and Ceramics with Controlled Microstructures,” Metallurgy and Inorganic Chemistry Department at the East Paris Institute of Chemistry and Materials Science, CNRS, France.

Funding

The work is carried out in the Russian–French International associated laboratory “ion-exchange membranes and processes” under the sponsorship of Ministry of Science and Higher Education of the Russian Federation (Ref. no. RFMEFI58617X0030) and CNRS, France (project no. 38200SF).

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Authors and Affiliations

  1. Kuban State University, Krasnodar, Russia

    N. D. Pismenskaya, S. A. Mareev, E. V. Pokhidnya & V. V. Nikonenko

  2. Institut de Chimie et des Matériaux Paris-Est, UMR7a82 CNRS – Université Paris-Est, Paris, France

    C. Larchet & L. Dammak

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  1. N. D. Pismenskaya
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  2. S. A. Mareev
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  3. E. V. Pokhidnya
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  6. V. V. Nikonenko
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Translated by Yu. Pleskov

APPENDIX

APPENDIX

Let us examine thin solution layer with thickness of ∆x\( \ll \) δ adjacent to a membrane surface. We write the counter-ion flux (for definiteness, of single-charged cations in the case of cation-exchange membrane) entering this layer from the solution bulk as

$${{j}_{{1\,{\text{in}}}}} = D\frac{{c_{1}^{0} - {{c}_{{{\text{1}}\,{\text{s}}}}}}}{\delta } + \frac{{i{{t}_{1}}}}{F},$$
((А1))

where \(D{{\left( {c_{1}^{0} - {{c}_{{{\text{1s}}}}}} \right)} \mathord{\left/ {\vphantom {{\left( {c_{1}^{0} - {{c}_{{{\text{1s}}}}}} \right)} \delta }} \right. \kern-0em} \delta }\) is the finite-difference estimate of the flow diffusion component, \(c_{1}^{0}\) and \({{c}_{{1\,{\text{s}}}}}\) is the counter-ion concentration in the solution bulk and near the membrane surface, respectively. We write the flux outward from this layer to the membrane bulk by using the definition of the effective transport number for ions 1 in the membrane, T1, as the portion of charge carried by the ions of this kind under the conditions of possible involvement of the migration and diffusion transfer mechanisms:

$${{j}_{{1\,{\text{out}}}}} = \frac{{i{{T}_{1}}}}{F}.$$
((А2))

Because the contribution of the diffusion transfer in highly selective ion-exchange membranes in dilute solutions is insignificant, the value of T1 approaches the electromigration transport number \({{\bar {t}}_{1}}\) [69].

We write the differential equation of the ion 1 conservation in the layer under examination (per unit membrane surface area, mol m−2) as:

$$\frac{{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)}}{{\partial t}} = D\frac{{c_{1}^{0} - {{c}_{{1\,{\text{s}}}}}}}{\delta } + \frac{{i{{t}_{1}}}}{F} - \frac{{i{{T}_{1}}}}{F}.$$
((А3))

The rate of increment \({{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} \mathord{\left/ {\vphantom {{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} {\partial t}}} \right. \kern-0em} {\partial t}}\) gain depends on the current density i and time t. While in the initial state we have \({{c}_{{1\,{\text{s}}}}}\) = \(c_{1}^{0}\) (the system is at equilibrium) and there is no ion diffusion delivery to the membrane surface, upon the current switching-off the quantity \({{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} \mathord{\left/ {\vphantom {{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} {\partial t}}} \right. \kern-0em} {\partial t}}\) becomes negative (because T1 > t1) and has its maximal absolute value in time. As time goes by, \(\partial {{\left( {{{c}_{{1\,{\text{s}}}}}\Delta x} \right)} \mathord{\left/ {\vphantom {{\left( {{{c}_{{1\,{\text{s}}}}}\Delta x} \right)} {\partial t}}} \right. \kern-0em} {\partial t}}\) decreases in absolute value because \({{c}_{{1\,{\text{s}}}}}\) decreases, while \({{D(c_{1}^{0} - {{c}_{{1\,{\text{s}}}}})} \mathord{\left/ {\vphantom {{D(c_{1}^{0} - {{c}_{{1\,{\text{s}}}}})} \delta }} \right. \kern-0em} \delta }\) increases. When the current density remains not too high (less than its limiting value, ilim), the diffusion delivery can compensate the difference of migration fluxes in the solution and membrane, and the system reaches its steady state when \({{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} \mathord{\left/ {\vphantom {{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} {\partial t}}} \right. \kern-0em} {\partial t}}\) = 0. When i > ilim, then, upon the reaching of some sufficiently small threshold value of , an additional mechanism of the current flowing becomes developing in the system (a current-induced convection or the generation of Н+ and ОН− ions, the new charge carriers). The appearance of the new transfer mechanism causes a decrease in the intramembrane potential drop growth rate and formation of inflection in the chronopotentiogram. The same mechanism (along with the diffusion from solution bulk) provides compensation of the difference in the ion migration fluxes and the achieving of the steady state.

It is not too difficult to see from equation (А3) that the less the diffusion layer thickness, the more intensive is the electrolyte diffusion delivery to the membrane surface, all other things being equal. This means that with the decreasing of δ the absolute value of \({{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} \mathord{\left/ {\vphantom {{\partial ({{c}_{{1\,{\text{s}}}}}\Delta x)} {\partial t}}} \right. \kern-0em} {\partial t}}\) will decrease (remaining negative); a longer time will be required for the achieving of the critically low concentration near the surface.

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Pismenskaya, N.D., Mareev, S.A., Pokhidnya, E.V. et al. Effect of Surface Modification of Heterogeneous Anion-Exchange Membranes on the Intensity of Electroconvection at Their Surfaces. Russ J Electrochem 55, 1203–1220 (2019). https://doi.org/10.1134/S1023193519120139

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