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==In electrochemistry==
In [[electrochemistry]], '''concentration polarization''' denotes the part of the [[Electrical polarity|polarization]] of an [[electrolytic cell]] resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface.<ref>S.P. Parker, McGraw-Hill Dictionary of Scientific & Technical Terms 6E, 2003.</ref> Here ''polarization'' is understood as the shift of the [[Electrochemical potential]] difference across the cell from its equilibrium value. When the term is used in this sense, it is equivalent to “[[Overpotential#Concentration overpotential|concentration overpotential]]”.<ref>A.J. Bard, G.R. Inzelt, F. Scholz (Eds.), Electrochemical Dictionary, Springer, Berlin, 2012.</ref><ref>J. Manzanares, K. Kontturi, In: Bard A.J., Stratmann M., Calvo E.J., editors. In Encyclopedia of Electrochemistry, Interfacial Kineticsand Mass Transport, VCH-Wiley, Weinheim; 2003.</ref>
The cause of the changes in concentration (emergence of concentration gradients in the solution adjacent to the electrode surface) is the difference in the rate of electrochemical reaction at the electrode and the rate of ion migration in the solution from/to the surface. When a chemical species participating in an electrochemical electrode reaction is in short supply, the concentration of this species at the surface decreases causing diffusion, which is added to the migration transport towards the surface in order to maintain the balance of consumption and delivery of that species.
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Thus, the selectivity of separation and the membrane lifetime are deteriorated.
Generally, to reduce the concentration polarization, increased flow rates of the solutions between the membranes as well as spacers promoting turbulence are applied [5, 6]. This technique results in better mixing of the solution and in reducing the thickness of the diffusion boundary layer, which is defined as the region in the vicinity of an electrode or a membrane where the concentrations are different from their value in the bulk solution.<ref>IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.</ref> In electrodialysis, additional mixing of the solution may be obtained by applying an elevated voltage where current-induced convection occurs as gravitational convection or electroconvection. Electroconvection is defined <ref>R.F. Probstein, Physicochemical Hydrodynamics, Wiley, NY, 1994.</ref> as current-induced volume transport when an electric field is imposed through the charged solution. Several mechanisms of electroconvection are discussed.<ref>I. Rubinstein, B. Zaltzman, Electro-osmotically induced convection at a permselective membrane, Physical Review E 62 (2000) 2238.</ref><ref>N.A. Mishchuk, Concentration polarization of interface and non-linear [[electrokinetic phenomena]], Advances in Colloid and Interface Science 160 (2010) 16.</ref><ref name=victor11>V.V. Nikonenko, N.D. Pismenskaya, E.I. Belova, P. Sistat, P. Huguet, G. Pourcelly, C. Larchet, Intensive current transfer in membrane systems: modeling, mechanisms and application in [[electrodialysis]], Advances in Colloid and Interface Science 160 (2010) 101.</ref><ref>Y. Tanaka, Ion Exchange Membranes: Fundamentals and Applications, Elsevier, Amsterdam, 2007.</ref> In dilute solutions, electroconvection allows increasing current density several times higher than the limiting current density.<ref name=victor11/> Electroconvection refers to [[electrokinetic phenomena]], which are important in [[microfluidic]] devices. Thus, there is a bridge between membrane science and micro/nanofluidics.<ref>J. De Jong, R.G.H. Lammertink, M. Wessling, Membranes and microfluidics: a review, Lab on a Chip—Miniaturisation for Chemistry and Biology 6 (9) (2006) 1125.</ref> Fruitful ideas are transferred from [[microfluidics]]: novel conceptions of electro-membrane devices for water desalination in overlimiting current range have been proposed.<ref>S.-J. Kim, S.-H. Ko, K.H. Kang, J. Han, Direct seawater [[desalination]] by ion concentration polarization, Nature Nanotechnology 5 (2010) 297.</ref><ref>M.Z. Bazant, E.V. Dydek, D. Deng, A. Mani, Method and apparatus for desalination and purification, US Patent 2011/0308953 A1.</ref>
==References==
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