Electrochem. Energy Technol. 2016; 2:1–5 Review Article Open Access Mahendra S. Gaikwad* and Chandrajit Balomajumder Polymer coated Capacitive Deionization Electrode for Desalination: A mini review DOI 10.1515/eetech-2016-0001 efits of being an eco-friendly desalination technology, hav- Received November 10, 2015; revised December 3, 2015; accepted ing lower energy consumption and working costs, sim- December 7, 2015 plicity in regeneration and maintenance compared with Abstract: This mini review deals with a recently develop- other conventional techniques of desalination [3, 4]. The ing water purification technology, i.e. capacitive deioniza- main aspects of capacitive deionization are salinity, scal- tion. It presents the current progress achieved with poly- ability, efficiencies of electrodes and cost effectiveness in mer coated electrodes in capacitive deionization for desali- comparison to other desalination techniques [5, 6]. Work- nation. The introduction covers capacitive deionization, ing mechanism of capacitive deionization operates in two application of polymer or polymer composite in capacitive steps, namely purification and regeneration. Purification deionization electrode, comparative study and discussion step and regeneration step are schematically presented on fabrication of electrode. This paper aims at indicating in Figure 1. In capacitive deionization porous electrodes novel research prospects in capacitive deionization tech- are used to adsorb the salt ions from the salt solution. nology for desalination. In purification step, when voltage is applied to electrodes cations and anions are attracted towards negatively and Keywords: Capacitive Deionization, polymer coating, ion positively charged electrodes respectively, thus pure wa- exchange, desalination, electrode ter will leave the capacitive deionization cell. When the electrode is saturated due to salt ion deposition on elec- trodes there is need of regeneration of electrode by apply- 1 Introduction ing a reverse voltage or short-circuit for desorption of ions from the electrode. A modified form of capacitive deion- This mini review presents the progress of the polymer ization is membrane capacitive deionization (MCDI). Only coated electrode capacitive deionization technology. The difference is cation and anion exchange membrane are objectives of this article are: (1) to present an introduc- used in front of the porous electrodes [7, 8]. Schematic tion about capacitive deionization; (2) to describe broadly of membrane capacitive deionization and polymer coated the application of polymer coated electrodes for capacitive capacitive deionization are mentioned in Figure 2. Under deionization (3) comparison of different polymer coated the application of a potential to a capacitive deionization capacitive deionization electrodes and (4) to discuss the cell, counter ions are attracted to the surface of the elec- polymer coating in capacitive deionization. Shortage of trode, concurrently co-ions expelled from the counter elec- fresh water is an increasing concern on the earth, and re- trode [27]. Such “co-ion” effect could successfully be pre- quires suitable solutions to increase water availability [1, vented by using ion exchange membranes in capacitive 2]. Most of the well-known desalination technologies such deionization. Membrane capacitive deionization shows as reverse osmosis, thermal processes and electrodialy- adsorption capacity much higher than capacitive deion- sis need specialized and expensive infrastructure. On the ization without membrane. Also energy consumption was other side capacitive deionization (CDI) signifies a novel reduced providing an economical advantage of membrane and energy-efficient desalination technique. CDI has ben- capacitive deionization [28]. Materials of electrodes should have a sufficient elec- tronic conductivity, high surface area, suitable pore size *Corresponding Author: Mahendra S. Gaikwad: Department arrangement for removal of ions [9, 10]. Carbon material of Chemical Engineering, Indian Institute of Technology Roorkee, is mostly used for fabrication of electrodes due to its prop- Roorkee-247667, Uttarakhand, India, E-mail: erties such as high surface area, better chemical stability [email protected] and eco-friendliness [11]. Mainly activated carbon, carbon Chandrajit Balomajumder: Department of Chemical Engineering, aerogel and mesoporous carbon are preferred for electrode Indian Institute of Technology Roorkee, Roorkee-247667, Uttarak- hand, India, E-mail: [email protected] production [11–13]. After improvement in research it was © 2016 Mahendra S. Gaikwad and Chandrajit Balomajumder, licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 2 Ë Mahendra S. Gaikwad and Chandrajit Balomajumder found that application of nanomaterials in electrode pro- The diffusion layer on the membrane surface will become vides better response in desalination [14–16]. thick at low feed concentration of contaminants [17] such phenomena creates problem like a high interfacial resis- tance of the membrane. Such weaknesses or problems are solved by casting thin layers of the ion exchanger on the surface of carbon electrodes, which supports a decreased contact resistance between the electrode of MCDI and ion exchanger [18]. Basically two types of fabrication method are used: coating method and compressing method. Com- pressing method requires high pressure to fabricate the electrode from composite materials, conductive agent and binder material [19]. Recently researchers focus on the (a) polymer coating electrode for capacitive deionization [20, 21, 25, 26]. 2 Progress of polymer coated capacitive deionization electrodes (b) The current progress of various polymer coated capaci- Figure 1: Schematic of capacitive deionization steps (a) Purification tive deionization electrodes is comparatively and system- Step (b) Regeneration Step. atically shown in Table 1. 3 Discussion of various polymer coated electrodes fabrication Kim and Choi [20] reported that for membrane capacitive deionization a cation-exchange polymer coated electrode was prepared by simple and inexpensive way. A mixture of poly(vinyl alcohol) and sulfosuccinic acid mixture was (a) used as coating solution to introduce negatively charged ion-exchange groups. Such electrode was prepared at vari- ous crosslinking temperatures and sulfosuccinic acid con- tents. Electrode performance shows that the specific ca- pacitance and the electric resistance of the coated car- bon electrodes were affected strongly by the crosslinking temperature. Despite this, when compared with commer- cial ion-exchange membranes, the area resistance of the coated layer was relatively low [20]. Multiplication of spe- (b) cific surface area carbon and double layer capacitance (C ) is known as specific capacitance (C /F cm−2) [29]. Figure 2: Schematic (a) membrane capacitive deionization and (b) dl dl polymer coated capacitive deionization. Coating on carbon cloths was achieved by spraying with bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide). Cation exchange and anion exchange layers are In MCDI, there is need of strong physical pressure to formed on electrode by sulfonation and amination respec- create smooth contact between electrode and membrane. tively. Such novel electrodes improve weakness of mem- Table 1: Comparison of various polymer coated capacitive deionization electrodes. Polymer coated Electrode Coating Surface Area Flow rate Voltage Feed Concentration Salt Adsorption Percent Publication year thickness (mm) (m2/g) (ml/min) (V) (mg/L) (mg/g) removal (%) and references Carbon electrode coated 0.01 – 20 1.5 200 – 85 2010 [30] with an ion-exchange polymer (a mixture of poly (vinyl alcohol) and sulfosuccinic acid) Bromomethylated poly (2, – – 4 1.8 100 – 83.4 2011 [18] 6-dimethyl-1, 4-phenylene oxide) ion exchanger lay- Polymer coated Capacitive Deionization Electrode for Desalination: A mini review ered electrodes Ion selective polymer coat- 0.0205 30 1.5 250 5.50 77.23 2013 [21] ing Anion exchange poly- – 50 1.2 50a – 93 2014 [22] mer (dimethyl diallyl ammonium chloride) and cation exchange (polyethyleneimine) poly- mer into carbon nanotubes (CNTs) electrodes Polyaniline-modified acti- – 618.4 – 1.2 250 3.15 – 2014 [23] vated carbon electrodes Polypyrrole/carbon nan- 185.21 – 1.4 1000a 43.99 – 2014 [24] otube composites Coating-type polypyr- 0.3 – – 1.4 1000a 93.68 – 2015 [25] role/carbon nanotube composite electrode Synthesized anion (am- 1-2 – 23 1.0 100 – 100 2015 [26] inated polysulfone) and cation (poly(phenylene oxide) exchange polymer coating on the electrodes Ë Note: all feed solutions are NaCl , a: Initial conductivity of feed solution (µS/cm). 3 4 Ë Mahendra S. Gaikwad and Chandrajit Balomajumder brane capacitive deionization by minimizing the interfa- by minimizing the interfacial resistance between the car- cial resistance between the carbon electrode and ion ex- bon electrode and ion exchanger layer. Evidence is re- changer layer [18]. ported that the coated electrode was significantly superior Salt removal efficiency was improved and contact to the traditional compressed electrode in terms of the spe- resistances decreases by combined used of ion selec- cific adsorption capacity and specific mass capacitance. tive membrane and polymer in CDI cell [21]. Applica- Contact adhesion between electrodes and ion exchange tion of polyethyleneimine (cation exchange polymer) and polymers was enhanced compared