Water Desalination Using Capacitive Deionization with Microporous Carbon Electrodes S. Porada,* ,†,‡ L. Weinstein, § R. Dash, § A. van der Wal, ⊥ M. Bryjak, † Y. Gogotsi, || and P.M. Biesheuvel ‡,⊥ † Department of Polymers and Carbon Materials, Faculty of Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ‡ Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, 8934 CJ Leeuwarden, The Netherlands § Y-Carbon, Inc., 900 First Ave, King of Prussia, Pennsylvania 19406, United States ⊥ Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands || Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States ABSTRACT: Capacitive deionization (CDI) is a water desalination technology in which salt ions are removed from brackish water by flowing through a spacer channel with porous electrodes on each side. Upon applying a voltage difference between the two electrodes, cations move to and are accumulated in electrostatic double layers inside the negatively charged cathode and the anions are removed by the positively charged anode. One of the key parameters for commercial realization of CDI is the salt adsorption capacity of the electrodes. State-of-the-art electrode materials are based on porous activated carbon particles or carbon aerogels. Here we report the use for CDI of carbide-derived carbon (CDC), a porous material with well-defined and tunable pore sizes in the sub-nanometer range. When comparing electrodes made with CDC with electrodes based on activated carbon, we find a significantly higher salt adsorption capacity in the relevant cell voltage window of 1.2-1.4 V. The measured adsorption capacity for four materials tested negatively correlates with known metrics for pore structure of the carbon powders such as total pore volume and BET-area, but is positively correlated with the volume of pores of sizes <1 nm, suggesting the relevance of these sub-nanometer pores for ion adsorption. The charge efficiency, being the ratio of equilibrium salt adsorption over charge, does not depend much on the type of material, indicating that materials that have been identified for high charge storage capacity can also be highly suitable for CDI. This work shows the potential of materials with well-defined sub-nanometer pore sizes for energy-efficient water desalination. KEYWORDS: capacitive deionization, carbide-derived carbons, water desalination, electrostatic double layer theory, porous electrodes, millifluidics 1. INTRODUCTION Many regions in the world suffer from brackish ground water and increasing salinity levels, a trend that is expected to continue, affecting the lives of billions of people. 1-8 Reverse osmosis (RO) and thermal processes are the currently used technologies for large-scale water desalination. 9 In these approaches, desalted water is produced from sea or brackish water either by passing water through water-permeable membranes under pressure, or by distillation, respectively. However, in those processes, where desalted water is removed from the feed water, energy consumption is inherently significant. For instance, for an energy consumption in RO of 4 kWh per m 3 of fresh water produced from sea water, operation is at about 4× the thermodynamic minimum (water recovery = 0.6, c salt,sea = 0.5 M). 5,10 It is not unreasonable to assume that for brackish water of relatively low ionic strength it is energetically favorable to remove the salt ions instead of the water. Technologies where ions are removed from water by electrical fields include electro-deionization and electrodialy- sis, 11,12 water desalination using microchannels 13 and bat- teries, 14 and capacitive deionization (CDI). 8,10,15-27 In the present work, we focus on CDI. Capacitive deionization (CDI) is a millifluidic water desalination technology in which the salt ions in brackish water are removed by flowing the water through a spacer channel with porous electrodes on each side. Upon applying a potential difference between the two electrodes, cations (Na + ) move to and are accumulated in the cathode, and anions (Cl - ) are absorbed by the anode. In this way, partially desalted water is obtained. After some time, the voltage is reduced, or even reversed, and ions are released from the electrodes, leading to a concentrated salt product stream which is discharged. Note that reversal of voltage is only Received: May 25, 2011 Accepted: February 13, 2012 Published: February 13, 2012 Research Article www.acsami.org © 2012 American Chemical Society 1194 dx.doi.org/10.1021/am201683j | ACS Appl. Mater. Interfaces 2012, 4, 1194-1199