Electrochimica Acta 56 (2010) 441–447 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Limitations of charge efﬁciency in capacitive deionization processes III: The behavior of surface oxidized activated carbon electrodes Eran Avraham ∗ , Malachi Noked, Yaniv Bouhadana, Abraham Soffer, Doron Aurbach Department of Chemistry, Bar Ilan University, Ramat Gan 52900, Israel article info Article history: Received 6 May 2010 Received in revised form 30 August 2010 Accepted 30 August 2010 Available online 22 September 2010 Keywords: Capacitive deionization (CDI) Water desalination Activated carbon electrodes Electro-adsorption Surface groups abstract In previous papers we reported on attempts to improve the performance of water desalination using capacitive de-ionization (CDI) processes by understanding the ions transport and adsorption/desorption behavior of activated carbon electrodes as a function of the applied potential. We also investigated the charge efﬁciency in CDI processes of brackish water in symmetrical cells containing identical highly porous activated carbon electrodes. In this work, we study the inﬂuence of oxygen-containing surface groups on activated carbon electrodes on the adsorption/desorption behavior of ions in brackish water. A special methodology was developed in order to estimate the charge efﬁciency of CDI processes which include the ability to prepare various kinds of activated carbon electrodes (ACEs) with controlled porosity and surface groups, measuring the PZC (potential of zero charge) of ACE in solutions and simultaneous adsorption and desorption of ions into/from them. The presence of polar, oxygen containing surface groups on ACE does not affect the electroadsorption behavior of Na + and Cl - ions into porous carbons whose average pore size is greater than 0.58 nm, apart of considerably changing the PZC. This results in a shift of the entire curves of ion adsorption vs. potential. The possible use of ACE with oxidized surfaces in CDI processes is discussed. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Capacitive deionization (CDI) is considered to be a promising energy-efﬁcient approach for the desalination of brackish water. Brieﬂy, in the CDI method, the solution is fed through or along pairs of activated carbon electrodes (ACEs) [1,2]. The removal of salt from the water is achieved by the electrostatic adsorption of the ions due to the application of a potential difference between the pairs of electrodes in the cell. In an ideal situation, the only interactions are electrostatic (no Faradaic processes are involved) so that CDI cells can undergo very extensive cycling without capacity fading. In previous publications on this topic, we explored the extent to which the capacitive current measured upon polarization of CDI cells is due to both adsorption of counter ions and desorption of co-ions. The contribution of the latter process is most pronounced with carbons that possess wide pores (compared to the ion size) and at potentials close to the PZC. The efﬁciency of the CDI process is deﬁned as  = Fdn s dq , (1) ∗ Corresponding author. E-mail address: eranchem@gmail.com (E. Avraham). where F is the Faraday number, dq is the differential of charge passed through the electrical circuit and dn s is the differential of the molar amount of actual ions removed from the solution. As mentioned above, the net charge passed upon polarization is due to the opposing adsorption and desorption processes dq F = d - 1 - d + 1 , (2) where d - 1 and d + 1 are the inﬁnitesimal changes in the surface excess of anions and cations, respectively. We have developed expressions and methodology for cal- culating the charge efﬁciency of the salt removal in the CDI cells containing activated carbon electrodes, based on experimental work in which data concerning ions and electro- adsorption–desorption processes related to single electrodes can be obtained. We have shown that the efﬁciency of CDI processes depends very strongly on the ranges of potential (charge and dis- charge) applied to the cell [3,4]. In the present work, we examine the role of oxygen surface groups on the ionic adsorption processes into ACEs. The inﬂuence of surface groups on electro-adsorption processes onto porous car- bon electrodes and possible pseudo-capacitive reactions of surface groups was studied in connection with super-capacitors, and their energy density and kinetics [5]. A good example is the surface interaction of quinone–hydroquinone surface groups, ﬁrst mentioned by Garten 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.056