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*S Supporting Information Article pubs.acs.org/Langmuir Overlimiting Current and Shock Electrodialysis in Porous Media Daosheng Deng,† E. Victoria Dydek,†,§ Ji-Hyung Han,† Sven Schlumpberger,† Ali Mani,†,∥ Boris Zaltzman,†,⊥ and Martin Z. Bazant*,†,‡ †Department of Chemical Engineering and ‡Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 United States ABSTRACT: Most electrochemical processes, such as electrodialysis, are limited by diffusion, but in porous media, surface conduction and electroosmotic flow also contribute to ionic flux. In this article, we report experimental evidence for surface-driven overlimiting current (faster than diffusion) and deionization shocks (propagating salt removal) in a porous medium. The apparatus consists of a silica glass frit (1 mm thick with a 500 nm mean pore size) in an aqueous electrolyte (CuSO4 or AgNO3) passing ionic current from a reservoir to a cation-selective membrane (Nafion). The current−voltage relation of the whole system is consistent with a proposed theory based on the electroosmotic flow mechanism over a broad range of reservoir salt concentrations (0.1 mM to 1.0 M) after accounting for (Cu) electrode polarization and pH-regulated silica charge. Above the limiting current, deionized water (≈10 μM) can be continuously extracted from the frit, which implies the existence of a stable shock propagating against the flow, bordering a depleted region that extends more than 0.5 mm across the outlet. The results suggest the feasibility of shock electrodialysis as a new approach to water desalination and other electrochemical separations. ■ INTRODUCTION Electrochemistry is playing an increasingly important role in sustainable world development. Besides energy conversion and Figure 1. Steady ion concentration polarization from a reservoir to an ideal cation-selective membrane through a bulk electrolyte (a), where the salt vanishes at the membranes (b) at the diffusion-limited current (c) analogous to the diode or through a negatively charged porous medium with thin double layers (d), where surface transport enables a broad depleted region (e) and a nearly constant overlimiting conductance (f), acting as a shunt resistance. storage, electrochemical systems also provide unique capa- bilities for desalination and other separations. The availability of fresh water may soon exceed that of energy as a global concern, which will require advances in water purification technolo- gies.1−4 Water treatment is also a key challenge for energy- related industrial processes, such as hydraulic fracturing (“fracking”) for shale gas extraction.5 The most difficult step is the removal of dissolved salts, especially multivalent ions.6−8 Reverse osmosis (RO) driven by mechanical pressure is widely used for large-scale seawater desalination but is costly and energy-intensive. Electrochemical methods, such as electro- dialysis9,10 (ED) and capacitive deionization11 (CD), can be attractive for brackish or wastewater treatment and for compact, portable systems. The rate-limiting step in electrochemical separations, including ED10 and CD,11 is typically diffusion. The classical diffusion-limited current arises when cations are completely removed at a membrane or electrode surface as anions are rejected to maintain neutrality (Figure 1a). For a dilute z/z electrolyte, ambipolar diffusion leads to a linear concentration profile at steady state (Figure 1b), and the current−voltage relationship,9,12 ⎡ ⎛ zeV⎞⎤ I = Ilim⎢1 − exp⎜−k T⎟⎥ ⎣ ⎝B⎠⎦ (1) is equivalent to that of an ideal diode (Figure 1c), where © 2013 American Chemical Society 16167 dx.doi.org/10.1021/la4040547 | Langmuir 2013, 29, 16167−16177 2zeDc0 Ilim= L A (2) is the diffusion-limited current, A is the area of the current collector, I is the measured current, V is the voltage across the electrolyte, kB is Boltzmann’s constant, T is the temperature, e is Received: January 31, 2013 Published: December 9, 2013PDF Image | Overlimiting Current and Shock Electrodialysis in Porous Media
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