Diffusion-bonded CNT carpets for fundamental CDI studies
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Diffusion-bonded CNT carpets for fundamental CDI studies R. Enright1,2, R. Mitchell1, H. Mutha1, C. Lv1,3, M. Christiansen1, C. V. Thompson1, and E. N. Wang1 1 Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts, 02139, U.S.A. 2 Stokes Institute, University of Limerick, Limerick, Ireland 3 Tsinghua University, Beijing, China ABSTRACT Uncertainty about future energy and water supplies suggests a pressing need to develop efficient technologies for water desalination. Capacitive deionization (CDI), a method that captures ions in the electrical double layer (EDL) of an electrochemical capacitor, is a promising technology that can potentially fulfill those requirements. Similar to supercapacitors, ideal CDI electrodes should have a large electrolyte-accessible specific surface area available for ion adsorption with rapid charging/discharging characteristics. Unlike supercapacitors, CDI electrodes are required to operate in aqueous electrolytes with low ionic concentrations in a nonlinear charging regime. To explore this practically and theoretically important regime, we developed robust, electrochemically-compatible carbon nanotube (CNT) carpet electrodes that posses a well-defined and uniform pore structure that is more readily analyzed in comparison to the random and multi-scale pore structure of typical carbon electrodes. The fabricated electrodes were characterized using cyclic voltammetry and potentiostatic charging in aqueous NaCl solutions (no = 20 - 90 mM) using a three electrode setup. Examination of the CV and potentiostatically-measured capacitances were consistent with EDL behavior dictated by the Stern layer. However, some deviations from the expected behavior were observed with increasing salt concentration during potentiostatic testing. INTRODUCTION The increasing use of water resources for human consumption, industry, and irrigation has resulted in a shortage of fresh water supply in many parts of the world. Only 0.5% of the 1.4 billion km3 of water in the world is accessible fresh water that is poorly distributed across the globe. With nearly 98% of the world’s available water supply being sea or brackish water, desalination has become an important alternative source of clean water [1]. One promising technology for desalinating brackish waters (10 – 1000 mg/l) is capacitive deionization (CDI). CDI uses high-surface-area electrodes to adsorb significant quantities of ions from water in an electrochemical double layer formed at a solid/electrolyte interface, thereby resulting in desalination (see Fig. 1) [2]. Carbon materials (aerogel, CNT) are ideal electrode materials because of their low electrical resistivity (
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