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Capacitive deionization using a carbon electrode prepared with water-soluble poly(vinyl alcohol) binder
Journal of Industrial and Engineering Chemistry 17 (2011) 717–722
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Capacitive deionization using a carbon electrode prepared with water-soluble poly(vinyl alcohol) binder Byeong-Hee Park a, Yu-Jin Kim a, Jin-Soo Park b, Jaehwan Choi a,* a b
Institute for Rare Metals, Department of Chemical Engineering, Kongju National University, 275 Budae-dong, Seobuk-gu, Cheonan, Chungnam, 331-717, South Korea Department of Environmental Engineering, Sangmyung University, 300 Anseo-dong, Dongnam-gu, Cheonan, Chungnam 330-720, South Korea
A R T I C L E I N F O
Article history: Received 29 September 2010 Accepted 25 November 2010 Available online 13 May 2011 Keywords: Capacitive deionization Water-soluble polymer Wettability Specific capacitance Cross-linking
A B S T R A C T
To increase the wettability of carbon electrodes, we have fabricated a carbon electrode using a poly(vinyl alcohol) (PVA) binder (a water-soluble polymer) by cross-linking PVA with glutaric acid (GA). The PVAbonded carbon electrodes were prepared at various cross-linking temperatures and GA contents, and their electrochemical properties were analyzed using cyclic voltammetry (CV) and impedance spectroscopy. All carbon electrodes prepared in this study maintained their integrity within boiling water, which demonstrated the chemical stability of PVA cross-linking with GA. CV and impedance spectroscopy showed that the cross-linking temperature can significantly affect the resistance of PVAbonded carbon electrodes by influencing the degree of cross-linking. The electric resistance increased with decreasing cross-linking temperature, which was attributed to the swelling of PVA that was permitted by inadequate cross-linking of PVA with GA at low temperatures. As the mole ratio of GA to PVA increased from 5 to 20 mol/mol, specific capacitance increased also; this behavior appeared to result from the unreacted GA, which carried a negative charge. The desalting performance of the carbon electrodes prepared in this study indicates that they are suitable for capacitive deionization applications. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Capacitive deionization (CDI) is a desalination process in which an aqueous solution flows through the space between two porous carbon electrodes. When an electric potential is applied to an electrode, ionic compounds such as sodium, chloride, ammonium and nitrate are attracted to and electrostatically adsorbed onto the surface of the charged electrode. Once the electric potential is removed, the adsorbed ions are quickly released back into the bulk solution [1–3]. With growing concerns about the effect of energy consumption on the environment, the CDI process has increasingly become utilized as a choice desalination technology. The CDI process is regarded as an energy-efficient process because it operates at a relatively low voltage (approximately 1.2 V), one at which no electrolysis reactions occur [4–6]. During the CDI process, adsorption and desorption are achieved by shorting the circuit or reversing the polarity of the electrodes. Thus, it is possible to carry out the CDI process in an environmentally friendly manner because it does not require any additional chemicals to regenerate electrodes [7–10].
* Corresponding author. Tel.: +82 41 521 9362; fax: +82 41 554 2640. E-mail address: [email protected] (J. Choi).
Many carbon materials such as carbon fiber, carbon cloth, carbon nanotubes and recently carbon aerogels are used as electrodes for CDI applications [3,4,11–15]. These carbon materials are known to have good electrical conductivity and high specific surface area. In particular, recent research has been focused on carbon aerogels because they have an easy to control pore size with superior conductivity. The aerogel has a specific surface area of less than 400–1100 m2/g with an electrical resistance of less than 40 mV cm [4]. Even though these carbon materials show excellent physical and electrical properties for CDI applications, they require relatively complicated manufacturing processes, which result in high costs. Thus, activated carbon powders (ACPs) have been widely used as an electrode material for CDI processes. However, to manufacture an electrode consisting of ACPs, a polymer is required to bind the ACPs. There are two methods for preparing a polymer-bonded carbon electrode. One is a compression method where the mixture of ACPs, polymer powder, and water is pressed at a high temperature [7]. However, fabricating carbon electrodes in this manner can cause ACPs to break off of the electrodes because of the weak physical bonding power of the polymer binder. Polymer-bonded carbon electrodes can also be fabricated through a drying process after casting a slurry blended with ACPs in a polymer solution [16]. In this case, the carbon electrode has a higher mechanical strength
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.05.015
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because the polymer binder can effectively bond the ACPs. However, the use of a polymer binder will increase the internal resistance and may block some pores of the carbon powder, resulting in a reduced capacitance. It has been shown that poly(tetrafluoroethylene) (PTFE) and poly(vinylidenefluoride) (PVdF) exhibit good chemical and thermal stabilities [15]. Thus, these materials are often used as a binder for preparing carbon electrodes. To achieve high desalination performance during the CDI process, it is essential to enhance the adsorption capacity (capacitance) of the carbon electrodes. The capacitance strongly depends on the properties of the electrode materials, such as their surface area, pore structure, pore size distribution and functional group [5,17–19]. In addition, the wettability of a carbon electrode plays a critical role in adsorption capacity because the electrode surface that does not come into contact with the aqueous solution. Thus, it is very important to increase the wetted surface area of carbon electrodes when seeking good CDI performance. When a carbon electrode is prepared by using a hydrophobic polymer binder such as PTFE or PVdF, some portion of the surface area may not be wetted due to the hydrophobic nature of the binder, which may result in decreased adsorption capacity. Water-soluble polymers such as poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) can be good candidates for increasing the wettability of polymer-bonded carbon electrodes. It is known that PVA possesses good membrane-forming properties and good chemical and thermal stability [20,21]. Thus, in this study, we have prepared carbon electrodes using water-soluble PVA as a polymer binder. To prevent the re-dissolution of PVA in the PVA-bonded carbon electrodes, PVA was cross-linked with glutaric acid (GA) using an esterification reaction. PVA-bonded carbon electrodes were prepared with various combinations of cross-linking concentrations and cross-linking temperatures. The electrochemical properties of these electrodes were analyzed using cyclic voltammetry (CV) and electric impedance spectroscopy. Additionally, the desalination performance was evaluated using a CDI unit cell equipped with the PVA-bonded carbon electrodes prepared in this study. 2. Experiment 2.1. Materials Activated carbon powder P-60 (BET surface area = 1260 m2/g) was purchased from Daedong AC Co. (Korea). The ACPs were dried in an oven at 80 8C over 24 h and stored in a desicator before use. Fully hydrolyzed PVA (molecular weight 89,000–98,000) and GA (used as a cross-linking agent) were purchased from Aldrich Co. Ltd. and used without further purification. Aqueous PVA solution (10 wt%) was prepared by dissolving PVA in distilled water at 90 8C for 4 h; this solution was then used to prepare the carbon electrodes. Graphite sheet was supplied by Dongbang Carbon Co. (Korea) (Cat. No. 02511) and used as a current collector.
Table 1 Preparation conditions for carbon electrodes. Carbon electrode
GA/PVA ratioa (mol/mol)
Cross-linking temperature (8C)
GA10_T100 GA10_T110 GA10_T120 GA10_T130 GA05_T120 GA10_T120 GA15_T120 GA20_T120
0.10 0.10 0.10 0.10 0.05 0.10 0.15 0.20
100 110 120 130 120 120 120 120
a The number of moles of PVA was determined by dividing the mass of PVA by the formula weight of the monomer.
cross-link the PVA with GA by an esterification reaction. Detailed preparation conditions for the PVA-bonded carbon electrodes are summarized in Table 1. The weights of these electrodes were measured to determine the amount of carbon that was coated on the graphite sheet. The carbon electrodes were soaked in a 0.5 M KCl solution for 24 h to analyze their electrochemical properties. Prior to electrochemical characterization, the electrodes were vacuum impregnated in the electrolyte to remove any adsorbed gases from the inner pore volume. 2.3. Physical and electrochemical characterization of carbon electrodes The surface and cross-section of the electrodes were observed with scanning electron microscopy (SEM, MIRA LMH, TESCAN Ltd.) to determine the their physical structure. The electrode samples were covered with gold using an ion sputtering method, and SEM images were obtained at an acceleration voltage of 15.0 kV. To examine the electrochemical properties and electrochemical behavior of the electrodes, CV and electrical impedance spectroscopy (EIS) were measured using a 3-electrode system. The carbon electrode was inserted into the specimen holder with an exposed surface area of 1.77 cm2. A porous carbon rod and a saturated Ag/ AgCl electrode were used as a counter electrode and a reference electrode, respectively. Voltage on the test electrode was controlled by an AutoLab PGST30 potentiostat. The electrolyte solution consisted of 0.5 M KCl. All experiments were maintained in a water bath at 25 0.1 8C. CV measurements of the electrodes were made in the potential range of 0.5 to 0.5 V (vs. Ag/AgCl) at a potential sweep rate of 5 mV/s. Impedance measurements were performed with an AutoLab FRA impedance analyzer. The impedance spectra were obtained at a potential of 0.0 V in a frequency range of 100 Hz to 20 mHz. An alternating sinusoidal signal of 25 mV peak-to-peak was superimposed on the direct current (dc) potential. 2.4. Capacitive deionization experiments
2.2. Fabrication of carbon electrodes PVA-bonded carbon electrodes were fabricated by slurry coating a graphite sheet, drying and then thermally cross-linking PVA with GA. The carbon slurry was prepared by mixing ACPs with the PVA solution. GA was added to the mixture to serve as a crosslinking agent. The mole ratios of GA to PVA in the mixture were in the range of 5–20 mol/mol. The mixture was vigorously stirred at room temperature for 12 h to ensure homogeneity. The slurry was then casted onto a graphite sheet and allowed to dry at 50 8C in an oven for 1 h. Finally, the dried electrodes were heated for 1 h in a thermostat oven at temperatures of 100, 110, 120 and 130 8C to
To evaluate the desalting performance of the PVA-bonded carbon electrodes, desalination experiments were conducted by constructing a CDI unit cell with a pair of carbon electrodes prepared in this study. The CDI unit cell consisted of two parallel carbon electrodes separated by a non-conductive spacer. The size of each carbon electrode was 100 mm 100 mm. A solution with 200 mg/L NaCl was supplied to the cell at a flow rate of 20 mL/min. A variable potential was applied to the CDI cell using a potentiostat (WPG100, WonA Tech Corp., Korea). The conductivities of the effluent were automatically measured at intervals of 1.0 s by connecting a conductivity sensor (CON-BTA, Vernier Corp.) to the
B.-H. Park et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 717–722
interface (LabQuest, Vernier Corp.). The detailed description of the CDI cell has been reported in our previous manuscripts [22,23].
3. Results and discussion 3.1. Carbon electrode morphology Fig. 1 shows SEM images of the cross-section of the PVA-bonded carbon electrodes for which the mole ratio of GA to PVA are (a) 5 mol/mol and (b) 20 mol/mol. As shown in this figure, PVA bound to ACPs may exhibit various structural forms. For the carbon electrode containing a 5 mol/mol ratio of GA to PVA, PVA spread onto the carbon powder in the shape of fibrils or nets. On the contrary, for carbon electrodes containing a 20 mol/mol ratio of GA to PVA, PVA took the shape of discrete particles or rods. In other words, when the concentration of the cross-linking agent is low, PVA fibrils spread like a thin film or net; however, when the GA content increases, this shape takes the form of a rod. The cross-linking reaction of PVA with GA is illustrated in Fig. 2. The –OH groups in PVA react with –COOH groups in GA, forming an
ester and a water molecule at high temperature. The GA cross-links the PVAs so that the products cannot be dissolved in water. For the case of carbon electrodes prepared with 5 mol/mol of GA to PVA, the PVA spreads over the carbon particles due to the membraneforming properties of PVA. As shown in Fig. 1(b), increased crosslinking of PVA with GA may produce a more rigid and compact polymer structure. ACPs on all carbon electrodes prepared in this study remained attached to the electrode surface when the surface was rubbed by hand. To confirm the thermal stability of these bonds, the PVA-bonded carbon electrodes were immersed in boiling water for 1 h. All of the electrodes remained in their original state without deformation, indicating that the PVA was in fact cross-linked with GA. 3.2. Effect of cross-linking temperature on electrochemical properties Fig. 3 shows the cyclic voltammograms obtained for the PVAbonded carbon electrodes prepared at various cross-linking temperatures at a potential sweep rate of 5 mV/s. An ideal capacitor would produce a rectangular shape on a cyclic voltammogram. It is known that deviation from a rectangular shape on a cyclic voltammogram is due to charging resistance [24,25]. As shown in Fig. 3, a gradual increase of anodic (Ia) and cathodic (Ic) currents was observed. Moreover, the increase in current is more evident with decreasing reaction temperature. The slope of the charging curve between 0.5 and 0.4 V results from the charging resistance of the electrode. Fig. 3 shows that the resistance of PVA-bonded electrodes increases with decreasing cross-linking temperature. This can be attributed to the swelling of PVA in the carbon electrode. The esterification reaction of PVA with GA occurs at high temperature; yet, it is thought that even a temperature of 100 8C may not be high enough to produce an adequate reaction. The swelling of PVA in the carbon electrodes may be responsible for the increase in contact resistance of the carbon powders. To investigate the charging resistance and capacitance of the carbon electrodes, EIS was utilized because it is capable of distinguishing resistance from capacitance. Specific capacitance (C) can be derived from the imaginary component (Z00 ) of the impedance spectra using the following equation [25]: 1 C ¼ 00 vZ
Fig. 1. SEM images of PVA-bonded carbon electrodes with a mole ratio of GA to PVA (a) 5 mol/mol and (b) 20 mol/mol.
719
(1)
Fig. 2. Esterification reaction mechanism of PVA and GA.
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Fig. 3. Cyclic voltammograms for PVA-bonded carbon electrodes prepared at various reaction temperatures at a potential sweep rate of 5 mV/s.
Fig. 5. Charging resistance of carbon electrodes (derived from the impedance data) as a function of frequency.
where v denotes the angular frequency of the applied alternating current (ac) signal. The corresponding plots of capacitance as a function of frequency are shown in Fig. 4. Specific capacitance increased steeply with decreasing frequency; however, it remained fairly constant at lower frequencies. The ac signal reached and charged more inner surface sites of the carbon electrodes as frequency decreased, resulting in a higher capacitance. At frequencies below 50 mHz, the signal reached nearly the entire surface of the carbon, yielding a fairly constant capacitance for all electrodes. It is notable that the plateau frequency is lower when the reaction temperature decreases. For example, when PVA-bonded electrodes were prepared at a reaction temperature of 130 8C (GA10_T130), the capacitance plateau reached approximately 0.3 Hz; however, when the electrodes were prepared at a reaction temperature of 100 8C (GA10_T100), the specific capacitance increased sharply to approximately 0.05 Hz. Moreover, for frequencies less than 0.5 Hz, the capacitance increased slowly with decreasing the frequency, whereas the capacitance values remained fairly constant for the GA10_T130 electrode. This indicates that preparing carbon electrodes at a higher temperature improves penetration of the ac signal. Propagation of the ac signal within inner pore structures is characterized by longer electrolyte pathways, which results in an
increased real component (charging resistance) of the impedance [25,26]. The charging resistance at a particular frequency corresponds to the real component of the impedance minus the high frequency (100 Hz) resistance. Fig. 5 shows the charging resistance of the carbon electrodes as a function of frequency; the charging resistance showed definitive differences. The frequencydependant resistance of PVA-bonded electrodes increased with decreasing cross-linking temperature. The charging resistance of the electrodes was strongly influenced by the degree of crosslinking between PVA and GA within the carbon electrode. As mentioned previously, the degree of cross-linking may be controlled by modulating the cross-linking temperature. The high charging resistance of PVA-bonded carbon electrodes cross-linked at 100 8C resulted from the swelling of PVA due to a low degree of cross-linking. 3.3. Effect of cross-linking agent concentration on electrochemical properties To observe the effect of cross-linking agent concentration, PVAbonded carbon electrodes were prepared with various concentrations of GA at a cross-linking temperature of 120 8C. The cyclic voltammograms for these electrodes showed a shape similar to that shown in Fig. 3. The specific current increased with additional GA. The specific capacitance from the cyclic voltammograms can be calculated from the following equation [27]: C¼
Fig. 4. Specific capacitance of carbon electrodes prepared at various reaction temperatures as a function of frequency.
Ia Ic 2n m
¼
DI 2n m
(2)
where C is specific capacitance, v is potential sweep rate, m is mass of the carbon powder and Ia and Ic are anodic and cathodic currents, respectively. To determine the capacitance as a function of potential, DI values were obtained at various potentials. Fig. 6 shows the specific capacitances of PVA-bonded carbon electrodes prepared with various GA concentrations. As shown in Fig. 6, the specific capacitance increased with increasing GA concentration. The specific capacitances were in the range of 96.4–109.8 F/gcarbon at a potential of 0 V for a GA concentration that increased from 5 to 20 mol/mol. It is an interesting phenomena that specific capacitance at various potentials exhibits different tendencies according to GA concentration. For the case of the GA05_T120 electrode, the minimum specific capacitance is approximately 0.1 V, while the minimum
B.-H. Park et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 717–722
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Fig. 7. Changes in effluent NaCl concentration during adsorption and desorption. Fig. 6. The specific capacitances of carbon electrodes (derived from cyclic voltammograms) as a function of potential.
shifts toward more positive potentials with increasing GA concentration. It is known that the capacitance of an electrode reaches its minimum at a potential of zero charge [28]. This means that the surface potential of a carbon electrode increases negatively with increasing GA concentration. This appears to result from the cross-linking action of GA. GA carries two carboxylic acid groups on each molecule. If only a single carboxylic acid group reacts with an –OH group on PVA, then one carboxylic acid group remains on the GA molecule, which yields a negative surface potential. Meanwhile, the capacitance does not reach a minimum for the case of the GA20_T120 electrode. The specific capacitance increased for the negative potential ranges, while remaining nearly constant for the positive potential ranges. This is attributed to the cation selectivity of the unreacted carboxylic acid groups of GA on the carbon electrodes. When applying a negative potential, cations can easily be adsorbed onto the electrode. When a positive potential is applied, however, the negative charge of the carboxylic acid groups prevents the migration of anions, resulting in a reduced current. From these results, we conclude that PVA-bonded carbon electrodes may be used as a cation-selective electrode.
potential, which indicates that the desorption occurred very quickly during the CDI process. The rapid desorption rate increases the recovery ratio of the feed solution by decreasing the desorption time during the CDI process. Higher effluent concentration during desorption can be achieved by controlling the flow rate. Biesheuvel et al. reported that a highly concentrated solution can be produced by using the ‘‘stop-flow’’ mode of operation [29]. In this mode, the flow rate is temporarily set to zero, after which the solution flow rate is turned back on at the beginning of the ‘‘waste’’-step. This mode of operation yields a much smaller product stream of higher salt concentration than does the typical operation mode where the flow rate remains constant throughout the full cycle of adsorption and desorption. Recently, a membrane capacitive deionization (MCDI) system (one that includes an ion-exchange membrane) was incorporated into a CDI system to enhance desalination efficiency [29,30]. In this study, we found that carbon electrodes with high concentrations of GA showed cation selectivity due to unreacted carboxylic groups carrying a negative charge. Based on these results, we think that PVA-bonded carbon electrodes cross-linked with GA may be used as a cation-selective electrode in MCDI applications if cross-linking is adjusted properly.
3.4. Desalination performance of PVA-bonded electrodes 4. Conclusion To examine the desalting performance of the PVA-bonded carbon electrodes, desalination experiments were carried out using a CDI unit cell equipped with a pair of GA10_T120 electrodes. After applying a cell potential of 1.2 V for 3 min, the cell potential was immediately changed to 0.0 V for 2 min to desorb the ions attached to the electrode surface. Fig. 7 shows the changes in effluent NaCl concentration over time at a cell potential of 1.2 V. The CDI process was stable and reproducible for 5 cycles of adsorption and desorption. Upon applying the adsorption potential, the effluent concentration decreased rapidly from an initial 200 mg/L to a minimum of 14.8 mg/L. After reaching its minimum, the effluent concentration began to increase slowly as the adsorption capacity of the carbon electrode became saturated. The salt removal efficiency and the amount of adsorbed NaCl during the adsorption period were approximately 85% and 9.96 mg NaCl, respectively. Regarding the desorption performance, the effluent concentration increased sharply to its maximum of 750 mg/L (3.8-fold higher than the influent concentration) when a cell potential of 0.0 V was applied. Most of the adsorbed ions (approximately 72%) were desorbed within 60 s following the application of the desorption
For CDI applications, it is desirable for carbon electrodes to possess hydrophilic properties to increase the wetted surface area. To increase the wettability of carbon electrodes, we have fabricated carbon electrodes using a PVA binder (a water-soluble polymer) by cross-linking PVA with GA. PVA-bonded carbon electrodes were prepared at various cross-linking temperatures and GA concentrations, and the electrochemical properties and desalting performance were analyzed in this study. All carbon electrodes prepared in this study maintained their integrity within boiling water, which demonstrated the chemical stability of PVA cross-linking with GA. From CV and impedance spectroscopy, we found that the electric resistance of these carbon electrodes increased with decreasing cross-linking temperature. This was attributed to the swelling of PVA that was permitted by inadequate cross-linking of PVA with GA at low temperatures. In contrast, specific capacitance increased as GA concentration increased from 5 to 20 mol/mol. This behavior appears to have resulted from the cross-linking activity of the GA. The results of CDI experiments revealed that PVA-bonded carbon electrodes showed a good desalting performance. We conclude that carbon electrodes
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fabricated using a water-soluble polymer binder, such as PVA, can be utilized in CDI applications. Acknowledgement This work was supported by the research grant of the Kongju National University in 2009. References [1] J.B. Lee, K.K. Park, S.W. Yoon, P.Y. Park, K.I. Park, C.W. Lee, Desalination 237 (2009) 155. [2] C.J. Gabelich, T.D. Tran, I.H.M. Suffet, Environ. Sci. Technol. 36 (2002) 3010. [3] Y. Oren, Desalination 228 (2008) 10. [4] T.J. Welgemoed, C.F. Schutte, Desalination 183 (2005) 327. [5] L. Li, L. Zou, H. Song, G. Morris, Carbon 47 (2009) 775. [6] P. Xu, J.E. Drewes, D. Heil, G. Wang, Water Res. 42 (2008) 2605. [7] K.K. Park, J.B. Lee, P.Y. Park, S.W. Yoon, J.S. Moon, H.M. Eum, C.W. Lee, Desalination 206 (2007) 86. [8] B.H. Park, J.H. Choi, Electrochim. Acta 55 (2010) 2888. [9] J.A. Lim, N.S. Park, J.S. Park, J.H. Choi, Desalination 238 (2009) 37. [10] J.C. Farmer, D.V. Fix, G.V. Mack, R.W. Pekala, J.F. Poco, J. Electrochem. Soc. 143 (1996) 159.
[11] T.Y. Ying, K.L. Yang, S. Yiacoumi, C. Tsouris, J. Colloid Interf. Sci. 250 (2002) 18. [12] K.L. Yang, T.Y. Ying, S. Yiacoumi, C. Tsouris, E.S. Vittoratos, Langmuir 17 (2001) 1961. [13] M.W. Ryoo, G. Seo, Water Res. 37 (2003) 1527. [14] H.H. Jung, S.W. Hwang, S.H. Hyun, K.H. Lee, G.T. Kim, Desalination 216 (2007) 377. [15] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, John Wiley Sons, New York, 1988. [16] J.Y. Choi, J.H. Choi, J. Ind. Eng. Chem. 16 (2010) 401. [17] Y. Han, X. Quan, S. Chen, S. Wang, Y. Zhang, Electrochim. Acta 52 (2007) 3075. [18] L. Zou, L. Li, H. Song, G. Morris, Water Res. 42 (2008) 2340. [19] T. Momma, X. Liu, T. Osaka, Y. Ushio, Y. Sawada, J. Power Sources 60 (1996) 249. [20] L.Z. Zhang, Y.Y. Wang, C.L. Wang, H. Xiang, J. Membr. Sci. 308 (2008) 198. [21] J.Y. Rhim, H.B. Park, C.S. Lee, J.H. Jun, D.S. Kim, Y.M. Lee, J. Membr. Sci. 238 (2004) 143. [22] J.H. Lee, W.S. Bae, J.H. Choi, Desalination 258 (2010) 159. [23] Y.J. Kim, J.H. Choi, Water Res. 44 (2010) 990. [24] C.T. Hsieh, H. Teng, Carbon 40 (2002) 667. [25] H. Pro¨bstle, M. Wiener, J. Fricke, J. Porous Mater. 10 (2003) 213. [26] H. Pro¨bstle, C. Schmitt, J. Fricke, J. Power Sources 105 (2002) 189. [27] E. Gileadi, Electrode Kinetics for Chemists, Chemical Engineers and Materials Scientists, Wiley-VCH, NY, 1993. [28] K.I. Yang, S. Yiacoumi, C. Tsouris, J. Electroanal. Chem. 540 (2003) 159. [29] P.M. Biesheuvel, A. van der Wal, J. Membr. Sci. 346 (2010) 256. [30] Y.J. Kim, J.H. Choi, Sep. Purif. Technol. 71 (2010) 70.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Capacitive deionization using a carbon electrode prepared with water-soluble poly(vinyl alcohol) binder Byeong-Hee Park a, Yu-Jin Kim a, Jin-Soo Park b, Jaehwan Choi a,* a b
Institute for Rare Metals, Department of Chemical Engineering, Kongju National University, 275 Budae-dong, Seobuk-gu, Cheonan, Chungnam, 331-717, South Korea Department of Environmental Engineering, Sangmyung University, 300 Anseo-dong, Dongnam-gu, Cheonan, Chungnam 330-720, South Korea
A R T I C L E I N F O
Article history: Received 29 September 2010 Accepted 25 November 2010 Available online 13 May 2011 Keywords: Capacitive deionization Water-soluble polymer Wettability Specific capacitance Cross-linking
A B S T R A C T
To increase the wettability of carbon electrodes, we have fabricated a carbon electrode using a poly(vinyl alcohol) (PVA) binder (a water-soluble polymer) by cross-linking PVA with glutaric acid (GA). The PVAbonded carbon electrodes were prepared at various cross-linking temperatures and GA contents, and their electrochemical properties were analyzed using cyclic voltammetry (CV) and impedance spectroscopy. All carbon electrodes prepared in this study maintained their integrity within boiling water, which demonstrated the chemical stability of PVA cross-linking with GA. CV and impedance spectroscopy showed that the cross-linking temperature can significantly affect the resistance of PVAbonded carbon electrodes by influencing the degree of cross-linking. The electric resistance increased with decreasing cross-linking temperature, which was attributed to the swelling of PVA that was permitted by inadequate cross-linking of PVA with GA at low temperatures. As the mole ratio of GA to PVA increased from 5 to 20 mol/mol, specific capacitance increased also; this behavior appeared to result from the unreacted GA, which carried a negative charge. The desalting performance of the carbon electrodes prepared in this study indicates that they are suitable for capacitive deionization applications. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Capacitive deionization (CDI) is a desalination process in which an aqueous solution flows through the space between two porous carbon electrodes. When an electric potential is applied to an electrode, ionic compounds such as sodium, chloride, ammonium and nitrate are attracted to and electrostatically adsorbed onto the surface of the charged electrode. Once the electric potential is removed, the adsorbed ions are quickly released back into the bulk solution [1–3]. With growing concerns about the effect of energy consumption on the environment, the CDI process has increasingly become utilized as a choice desalination technology. The CDI process is regarded as an energy-efficient process because it operates at a relatively low voltage (approximately 1.2 V), one at which no electrolysis reactions occur [4–6]. During the CDI process, adsorption and desorption are achieved by shorting the circuit or reversing the polarity of the electrodes. Thus, it is possible to carry out the CDI process in an environmentally friendly manner because it does not require any additional chemicals to regenerate electrodes [7–10].
* Corresponding author. Tel.: +82 41 521 9362; fax: +82 41 554 2640. E-mail address: [email protected] (J. Choi).
Many carbon materials such as carbon fiber, carbon cloth, carbon nanotubes and recently carbon aerogels are used as electrodes for CDI applications [3,4,11–15]. These carbon materials are known to have good electrical conductivity and high specific surface area. In particular, recent research has been focused on carbon aerogels because they have an easy to control pore size with superior conductivity. The aerogel has a specific surface area of less than 400–1100 m2/g with an electrical resistance of less than 40 mV cm [4]. Even though these carbon materials show excellent physical and electrical properties for CDI applications, they require relatively complicated manufacturing processes, which result in high costs. Thus, activated carbon powders (ACPs) have been widely used as an electrode material for CDI processes. However, to manufacture an electrode consisting of ACPs, a polymer is required to bind the ACPs. There are two methods for preparing a polymer-bonded carbon electrode. One is a compression method where the mixture of ACPs, polymer powder, and water is pressed at a high temperature [7]. However, fabricating carbon electrodes in this manner can cause ACPs to break off of the electrodes because of the weak physical bonding power of the polymer binder. Polymer-bonded carbon electrodes can also be fabricated through a drying process after casting a slurry blended with ACPs in a polymer solution [16]. In this case, the carbon electrode has a higher mechanical strength
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.05.015
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because the polymer binder can effectively bond the ACPs. However, the use of a polymer binder will increase the internal resistance and may block some pores of the carbon powder, resulting in a reduced capacitance. It has been shown that poly(tetrafluoroethylene) (PTFE) and poly(vinylidenefluoride) (PVdF) exhibit good chemical and thermal stabilities [15]. Thus, these materials are often used as a binder for preparing carbon electrodes. To achieve high desalination performance during the CDI process, it is essential to enhance the adsorption capacity (capacitance) of the carbon electrodes. The capacitance strongly depends on the properties of the electrode materials, such as their surface area, pore structure, pore size distribution and functional group [5,17–19]. In addition, the wettability of a carbon electrode plays a critical role in adsorption capacity because the electrode surface that does not come into contact with the aqueous solution. Thus, it is very important to increase the wetted surface area of carbon electrodes when seeking good CDI performance. When a carbon electrode is prepared by using a hydrophobic polymer binder such as PTFE or PVdF, some portion of the surface area may not be wetted due to the hydrophobic nature of the binder, which may result in decreased adsorption capacity. Water-soluble polymers such as poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) can be good candidates for increasing the wettability of polymer-bonded carbon electrodes. It is known that PVA possesses good membrane-forming properties and good chemical and thermal stability [20,21]. Thus, in this study, we have prepared carbon electrodes using water-soluble PVA as a polymer binder. To prevent the re-dissolution of PVA in the PVA-bonded carbon electrodes, PVA was cross-linked with glutaric acid (GA) using an esterification reaction. PVA-bonded carbon electrodes were prepared with various combinations of cross-linking concentrations and cross-linking temperatures. The electrochemical properties of these electrodes were analyzed using cyclic voltammetry (CV) and electric impedance spectroscopy. Additionally, the desalination performance was evaluated using a CDI unit cell equipped with the PVA-bonded carbon electrodes prepared in this study. 2. Experiment 2.1. Materials Activated carbon powder P-60 (BET surface area = 1260 m2/g) was purchased from Daedong AC Co. (Korea). The ACPs were dried in an oven at 80 8C over 24 h and stored in a desicator before use. Fully hydrolyzed PVA (molecular weight 89,000–98,000) and GA (used as a cross-linking agent) were purchased from Aldrich Co. Ltd. and used without further purification. Aqueous PVA solution (10 wt%) was prepared by dissolving PVA in distilled water at 90 8C for 4 h; this solution was then used to prepare the carbon electrodes. Graphite sheet was supplied by Dongbang Carbon Co. (Korea) (Cat. No. 02511) and used as a current collector.
Table 1 Preparation conditions for carbon electrodes. Carbon electrode
GA/PVA ratioa (mol/mol)
Cross-linking temperature (8C)
GA10_T100 GA10_T110 GA10_T120 GA10_T130 GA05_T120 GA10_T120 GA15_T120 GA20_T120
0.10 0.10 0.10 0.10 0.05 0.10 0.15 0.20
100 110 120 130 120 120 120 120
a The number of moles of PVA was determined by dividing the mass of PVA by the formula weight of the monomer.
cross-link the PVA with GA by an esterification reaction. Detailed preparation conditions for the PVA-bonded carbon electrodes are summarized in Table 1. The weights of these electrodes were measured to determine the amount of carbon that was coated on the graphite sheet. The carbon electrodes were soaked in a 0.5 M KCl solution for 24 h to analyze their electrochemical properties. Prior to electrochemical characterization, the electrodes were vacuum impregnated in the electrolyte to remove any adsorbed gases from the inner pore volume. 2.3. Physical and electrochemical characterization of carbon electrodes The surface and cross-section of the electrodes were observed with scanning electron microscopy (SEM, MIRA LMH, TESCAN Ltd.) to determine the their physical structure. The electrode samples were covered with gold using an ion sputtering method, and SEM images were obtained at an acceleration voltage of 15.0 kV. To examine the electrochemical properties and electrochemical behavior of the electrodes, CV and electrical impedance spectroscopy (EIS) were measured using a 3-electrode system. The carbon electrode was inserted into the specimen holder with an exposed surface area of 1.77 cm2. A porous carbon rod and a saturated Ag/ AgCl electrode were used as a counter electrode and a reference electrode, respectively. Voltage on the test electrode was controlled by an AutoLab PGST30 potentiostat. The electrolyte solution consisted of 0.5 M KCl. All experiments were maintained in a water bath at 25 0.1 8C. CV measurements of the electrodes were made in the potential range of 0.5 to 0.5 V (vs. Ag/AgCl) at a potential sweep rate of 5 mV/s. Impedance measurements were performed with an AutoLab FRA impedance analyzer. The impedance spectra were obtained at a potential of 0.0 V in a frequency range of 100 Hz to 20 mHz. An alternating sinusoidal signal of 25 mV peak-to-peak was superimposed on the direct current (dc) potential. 2.4. Capacitive deionization experiments
2.2. Fabrication of carbon electrodes PVA-bonded carbon electrodes were fabricated by slurry coating a graphite sheet, drying and then thermally cross-linking PVA with GA. The carbon slurry was prepared by mixing ACPs with the PVA solution. GA was added to the mixture to serve as a crosslinking agent. The mole ratios of GA to PVA in the mixture were in the range of 5–20 mol/mol. The mixture was vigorously stirred at room temperature for 12 h to ensure homogeneity. The slurry was then casted onto a graphite sheet and allowed to dry at 50 8C in an oven for 1 h. Finally, the dried electrodes were heated for 1 h in a thermostat oven at temperatures of 100, 110, 120 and 130 8C to
To evaluate the desalting performance of the PVA-bonded carbon electrodes, desalination experiments were conducted by constructing a CDI unit cell with a pair of carbon electrodes prepared in this study. The CDI unit cell consisted of two parallel carbon electrodes separated by a non-conductive spacer. The size of each carbon electrode was 100 mm 100 mm. A solution with 200 mg/L NaCl was supplied to the cell at a flow rate of 20 mL/min. A variable potential was applied to the CDI cell using a potentiostat (WPG100, WonA Tech Corp., Korea). The conductivities of the effluent were automatically measured at intervals of 1.0 s by connecting a conductivity sensor (CON-BTA, Vernier Corp.) to the
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interface (LabQuest, Vernier Corp.). The detailed description of the CDI cell has been reported in our previous manuscripts [22,23].
3. Results and discussion 3.1. Carbon electrode morphology Fig. 1 shows SEM images of the cross-section of the PVA-bonded carbon electrodes for which the mole ratio of GA to PVA are (a) 5 mol/mol and (b) 20 mol/mol. As shown in this figure, PVA bound to ACPs may exhibit various structural forms. For the carbon electrode containing a 5 mol/mol ratio of GA to PVA, PVA spread onto the carbon powder in the shape of fibrils or nets. On the contrary, for carbon electrodes containing a 20 mol/mol ratio of GA to PVA, PVA took the shape of discrete particles or rods. In other words, when the concentration of the cross-linking agent is low, PVA fibrils spread like a thin film or net; however, when the GA content increases, this shape takes the form of a rod. The cross-linking reaction of PVA with GA is illustrated in Fig. 2. The –OH groups in PVA react with –COOH groups in GA, forming an
ester and a water molecule at high temperature. The GA cross-links the PVAs so that the products cannot be dissolved in water. For the case of carbon electrodes prepared with 5 mol/mol of GA to PVA, the PVA spreads over the carbon particles due to the membraneforming properties of PVA. As shown in Fig. 1(b), increased crosslinking of PVA with GA may produce a more rigid and compact polymer structure. ACPs on all carbon electrodes prepared in this study remained attached to the electrode surface when the surface was rubbed by hand. To confirm the thermal stability of these bonds, the PVA-bonded carbon electrodes were immersed in boiling water for 1 h. All of the electrodes remained in their original state without deformation, indicating that the PVA was in fact cross-linked with GA. 3.2. Effect of cross-linking temperature on electrochemical properties Fig. 3 shows the cyclic voltammograms obtained for the PVAbonded carbon electrodes prepared at various cross-linking temperatures at a potential sweep rate of 5 mV/s. An ideal capacitor would produce a rectangular shape on a cyclic voltammogram. It is known that deviation from a rectangular shape on a cyclic voltammogram is due to charging resistance [24,25]. As shown in Fig. 3, a gradual increase of anodic (Ia) and cathodic (Ic) currents was observed. Moreover, the increase in current is more evident with decreasing reaction temperature. The slope of the charging curve between 0.5 and 0.4 V results from the charging resistance of the electrode. Fig. 3 shows that the resistance of PVA-bonded electrodes increases with decreasing cross-linking temperature. This can be attributed to the swelling of PVA in the carbon electrode. The esterification reaction of PVA with GA occurs at high temperature; yet, it is thought that even a temperature of 100 8C may not be high enough to produce an adequate reaction. The swelling of PVA in the carbon electrodes may be responsible for the increase in contact resistance of the carbon powders. To investigate the charging resistance and capacitance of the carbon electrodes, EIS was utilized because it is capable of distinguishing resistance from capacitance. Specific capacitance (C) can be derived from the imaginary component (Z00 ) of the impedance spectra using the following equation [25]: 1 C ¼ 00 vZ
Fig. 1. SEM images of PVA-bonded carbon electrodes with a mole ratio of GA to PVA (a) 5 mol/mol and (b) 20 mol/mol.
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(1)
Fig. 2. Esterification reaction mechanism of PVA and GA.
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Fig. 3. Cyclic voltammograms for PVA-bonded carbon electrodes prepared at various reaction temperatures at a potential sweep rate of 5 mV/s.
Fig. 5. Charging resistance of carbon electrodes (derived from the impedance data) as a function of frequency.
where v denotes the angular frequency of the applied alternating current (ac) signal. The corresponding plots of capacitance as a function of frequency are shown in Fig. 4. Specific capacitance increased steeply with decreasing frequency; however, it remained fairly constant at lower frequencies. The ac signal reached and charged more inner surface sites of the carbon electrodes as frequency decreased, resulting in a higher capacitance. At frequencies below 50 mHz, the signal reached nearly the entire surface of the carbon, yielding a fairly constant capacitance for all electrodes. It is notable that the plateau frequency is lower when the reaction temperature decreases. For example, when PVA-bonded electrodes were prepared at a reaction temperature of 130 8C (GA10_T130), the capacitance plateau reached approximately 0.3 Hz; however, when the electrodes were prepared at a reaction temperature of 100 8C (GA10_T100), the specific capacitance increased sharply to approximately 0.05 Hz. Moreover, for frequencies less than 0.5 Hz, the capacitance increased slowly with decreasing the frequency, whereas the capacitance values remained fairly constant for the GA10_T130 electrode. This indicates that preparing carbon electrodes at a higher temperature improves penetration of the ac signal. Propagation of the ac signal within inner pore structures is characterized by longer electrolyte pathways, which results in an
increased real component (charging resistance) of the impedance [25,26]. The charging resistance at a particular frequency corresponds to the real component of the impedance minus the high frequency (100 Hz) resistance. Fig. 5 shows the charging resistance of the carbon electrodes as a function of frequency; the charging resistance showed definitive differences. The frequencydependant resistance of PVA-bonded electrodes increased with decreasing cross-linking temperature. The charging resistance of the electrodes was strongly influenced by the degree of crosslinking between PVA and GA within the carbon electrode. As mentioned previously, the degree of cross-linking may be controlled by modulating the cross-linking temperature. The high charging resistance of PVA-bonded carbon electrodes cross-linked at 100 8C resulted from the swelling of PVA due to a low degree of cross-linking. 3.3. Effect of cross-linking agent concentration on electrochemical properties To observe the effect of cross-linking agent concentration, PVAbonded carbon electrodes were prepared with various concentrations of GA at a cross-linking temperature of 120 8C. The cyclic voltammograms for these electrodes showed a shape similar to that shown in Fig. 3. The specific current increased with additional GA. The specific capacitance from the cyclic voltammograms can be calculated from the following equation [27]: C¼
Fig. 4. Specific capacitance of carbon electrodes prepared at various reaction temperatures as a function of frequency.
Ia Ic 2n m
¼
DI 2n m
(2)
where C is specific capacitance, v is potential sweep rate, m is mass of the carbon powder and Ia and Ic are anodic and cathodic currents, respectively. To determine the capacitance as a function of potential, DI values were obtained at various potentials. Fig. 6 shows the specific capacitances of PVA-bonded carbon electrodes prepared with various GA concentrations. As shown in Fig. 6, the specific capacitance increased with increasing GA concentration. The specific capacitances were in the range of 96.4–109.8 F/gcarbon at a potential of 0 V for a GA concentration that increased from 5 to 20 mol/mol. It is an interesting phenomena that specific capacitance at various potentials exhibits different tendencies according to GA concentration. For the case of the GA05_T120 electrode, the minimum specific capacitance is approximately 0.1 V, while the minimum
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Fig. 7. Changes in effluent NaCl concentration during adsorption and desorption. Fig. 6. The specific capacitances of carbon electrodes (derived from cyclic voltammograms) as a function of potential.
shifts toward more positive potentials with increasing GA concentration. It is known that the capacitance of an electrode reaches its minimum at a potential of zero charge [28]. This means that the surface potential of a carbon electrode increases negatively with increasing GA concentration. This appears to result from the cross-linking action of GA. GA carries two carboxylic acid groups on each molecule. If only a single carboxylic acid group reacts with an –OH group on PVA, then one carboxylic acid group remains on the GA molecule, which yields a negative surface potential. Meanwhile, the capacitance does not reach a minimum for the case of the GA20_T120 electrode. The specific capacitance increased for the negative potential ranges, while remaining nearly constant for the positive potential ranges. This is attributed to the cation selectivity of the unreacted carboxylic acid groups of GA on the carbon electrodes. When applying a negative potential, cations can easily be adsorbed onto the electrode. When a positive potential is applied, however, the negative charge of the carboxylic acid groups prevents the migration of anions, resulting in a reduced current. From these results, we conclude that PVA-bonded carbon electrodes may be used as a cation-selective electrode.
potential, which indicates that the desorption occurred very quickly during the CDI process. The rapid desorption rate increases the recovery ratio of the feed solution by decreasing the desorption time during the CDI process. Higher effluent concentration during desorption can be achieved by controlling the flow rate. Biesheuvel et al. reported that a highly concentrated solution can be produced by using the ‘‘stop-flow’’ mode of operation [29]. In this mode, the flow rate is temporarily set to zero, after which the solution flow rate is turned back on at the beginning of the ‘‘waste’’-step. This mode of operation yields a much smaller product stream of higher salt concentration than does the typical operation mode where the flow rate remains constant throughout the full cycle of adsorption and desorption. Recently, a membrane capacitive deionization (MCDI) system (one that includes an ion-exchange membrane) was incorporated into a CDI system to enhance desalination efficiency [29,30]. In this study, we found that carbon electrodes with high concentrations of GA showed cation selectivity due to unreacted carboxylic groups carrying a negative charge. Based on these results, we think that PVA-bonded carbon electrodes cross-linked with GA may be used as a cation-selective electrode in MCDI applications if cross-linking is adjusted properly.
3.4. Desalination performance of PVA-bonded electrodes 4. Conclusion To examine the desalting performance of the PVA-bonded carbon electrodes, desalination experiments were carried out using a CDI unit cell equipped with a pair of GA10_T120 electrodes. After applying a cell potential of 1.2 V for 3 min, the cell potential was immediately changed to 0.0 V for 2 min to desorb the ions attached to the electrode surface. Fig. 7 shows the changes in effluent NaCl concentration over time at a cell potential of 1.2 V. The CDI process was stable and reproducible for 5 cycles of adsorption and desorption. Upon applying the adsorption potential, the effluent concentration decreased rapidly from an initial 200 mg/L to a minimum of 14.8 mg/L. After reaching its minimum, the effluent concentration began to increase slowly as the adsorption capacity of the carbon electrode became saturated. The salt removal efficiency and the amount of adsorbed NaCl during the adsorption period were approximately 85% and 9.96 mg NaCl, respectively. Regarding the desorption performance, the effluent concentration increased sharply to its maximum of 750 mg/L (3.8-fold higher than the influent concentration) when a cell potential of 0.0 V was applied. Most of the adsorbed ions (approximately 72%) were desorbed within 60 s following the application of the desorption
For CDI applications, it is desirable for carbon electrodes to possess hydrophilic properties to increase the wetted surface area. To increase the wettability of carbon electrodes, we have fabricated carbon electrodes using a PVA binder (a water-soluble polymer) by cross-linking PVA with GA. PVA-bonded carbon electrodes were prepared at various cross-linking temperatures and GA concentrations, and the electrochemical properties and desalting performance were analyzed in this study. All carbon electrodes prepared in this study maintained their integrity within boiling water, which demonstrated the chemical stability of PVA cross-linking with GA. From CV and impedance spectroscopy, we found that the electric resistance of these carbon electrodes increased with decreasing cross-linking temperature. This was attributed to the swelling of PVA that was permitted by inadequate cross-linking of PVA with GA at low temperatures. In contrast, specific capacitance increased as GA concentration increased from 5 to 20 mol/mol. This behavior appears to have resulted from the cross-linking activity of the GA. The results of CDI experiments revealed that PVA-bonded carbon electrodes showed a good desalting performance. We conclude that carbon electrodes
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