- Email: [email protected]
Use of mesoporous conductive carbon black to enhance performance of activated carbon electrodes in capacitive deionization technology
Desalination 268 (2011) 182–188
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Use of mesoporous conductive carbon black to enhance performance of activated carbon electrodes in capacitive deionization technology Suresh Nadakatti ⁎, Mahesh Tendulkar, Manoj Kadam 1 Unilever R&D Bangalore, Hindustan Unilever Ltd, 64, Main Road, Whitefield, Bangalore-560066, India
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 6 October 2010 Accepted 7 October 2010 Available online 20 November 2010 Keywords: Capacitive deionization Conductive carbon black Desalination Electrosorption Activated carbon electrode
a b s t r a c t Capacitive deionization (CDI) is a technology for removal of dissolved salts from water by electrosorption of ions onto oppositely charged electrodes. The electrodes are usually made from activated carbon due to its high surface area. This paper reports the use of low levels of mesoporous conductive carbon blacks (MCCB) containing high fraction of mesopore surface area to enhance the capacitance of powdered activated carbon (PAC) electrodes. Different grades of MCCB with mesopore surface area varying from 65 m2/g to 455 m2/g were used. The capacitance of powdered activated carbon electrode could be enhanced from 13.8 F/g to 45.0 F/g at a scan rate of 5 mV/s by replacing 10% of powdered activated carbon with MCCB. The process for making these electrodes does not require use of organic solvent. The capacitance of improved electrode was comparable to that of commercially available carbon aerogel and activated carbon cloth electrodes. The removal of dissolved ions from water using a pair of electrodes (each weighing 11 g) at 10 ml/min was 75% and recovery was ~ 55% at applied potential of 1.2 V. Activated carbon electrodes containing low levels of MCCB offer high potential for use in capacitive deionization technology. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Capacitive deionization technology (CDI) is an exciting new method of removing dissolved salts from water [1–5]. The applications in drinking water include enhancing taste of water by controlling the dissolved salts level and converting brackish water into potable water. Some of the potential applications of the technology include: (i) water softening because of its preferential adsorption for divalent ions such as Mg and Ca [6], (ii) removal of nitrates, ammonium ions from brackish water [7], (iii) treating oil-containing brackish water [7,8], (iv) use in integrated plants along with reverse osmosis (RO) for reject water recovery to recycle the treated water to RO feed [9] and (v) removal of heavy metals such as ferric ions from water [10]. In CDI, a brackish water stream flows between pairs of high surface area carbon electrodes that are held at a potential difference of ~ 1.2 V. The treatment involves two steps—purification step and regeneration step. During the purification step, the ions and other charged particles are attracted to and held on the electrode of opposite charge. The negative electrode (cathode) attracts positively charged ions (cations) such as calcium (Ca2+), magnesium (Mg2+) and sodium (Na+), while the positively charged electrode (anode) attracts negative ions (anions) such as chloride (Cl−) and nitrate
⁎ Corresponding author. Tel.: +91 80 39831077; fax: +91 80 28453086. E-mail address: [email protected] (S. Nadakatti). 1 Present address: OTS-Advance Solution, Honeywell Automation India Limited, SP Infocity, Bldg No. II, 3rd Floor, Fursungi, Hadapsar, Pune 412 308, Maharashtra, India. 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.10.020
(NO− 3 ). Eventually the electrodes become saturated with ions and must be regenerated. During the regeneration step, the applied potential is removed. The adsorbed ions are released back into feed stream producing a more concentrated brine stream. Typically, for a gallon of brackish water fed to the CDI process, more than 50% emerges as fresh, deionized potable water, and the remainder is discharged as a concentrated brine solution. The key advantage of CDI lies in its potentially lower operating cost, which is about one third that of RO [5] and high recovery of N50% as compared to ~15% for RO [5,7]. One of the key factors that has limited the scale-up and commercialisation of CDI technology is availability of low cost, high efficiency electrodes. The initial electrodes for use in CDI were made from porous activated carbon and polymeric binder [1,4,11]. The key problems with conventional activated carbon electrodes were weak physical bonding of activated carbon, high electrical resistance and poor capacitance. Consequently the salt removal was low and recovery was poor. Much of the effort in recent years has been on development of electrodes with novel materials that have high electrical conductivity and high capacitance. Carbon aerogel is one such material which has high surface area and better ion-removal capacitance [12–16]. Researchers have experimented on various options for improvement of the existing electrode materials such as surface modification with titanium oxide [17,18], use of water-soluble polymer binder [19], use of mesoporous carbon electrode [3,20], etc. Other materials of current interest include carbon nanotubes and carbon nanofibers [21–24]. Recently, researchers have focussed on novel routes for enhancing the salt removal such as the charge barrier concept [7], membrane
S. Nadakatti et al. / Desalination 268 (2011) 182–188
capacitive deionization [25], use of ion-selective membranes [26] and coating electrode with ion-exchange polymer [27,28]. Kinetics of adsorption of salts and thermodynamic studies relating to this technology have been reported [24,29]. Mechanisms of adsorption, desorption, and electrode reactions in CDI have also been investigated by Lee and Choi [30]. The aforementioned carbon substances demonstrate good electrode characteristics in terms of electrical conductivity and specific surface area, but still suffer from their relatively complicated manufacturing process and high cost. The high cost of superior performance electrodes such as carbon aerogels has limited the use of these electrodes in commercial CDI applications. One of the key reasons for improved performance of carbon aerogels is the ability to control the pore size distribution (PSD). It has been shown [31] that carbon aerogels that contain higher proportion of mesopore (pore size N2 nm) surface area to total surface area have higher capacitance and faster kinetics of ion adsorption. Several researchers have investigated the role of micropores and mesopores on the performance of electrodes [3,12,13,15]. Micropores increase the surface area of activated carbon and hence contribute to high specific capacitance. Mesopores provide more favourable and quicker pathway for ions to penetrate the pores, thereby improving the adsorption and desorption kinetics. Thus, an electrode for use in CDI should have both micropores and mesopores. The conventional activated carbon-polymeric binder electrodes have large surface area but most of it is due to micropores. The capacitance at low scan rates (1 mV/s) is high but at high scan rates (5 to 100 mV/s) the capacitance is very low. Poor capacitance at high scan rates corresponds to poor salt removal and poor recovery in flow-through-capacitor applications. This drawback has limited the use of activated carbon-polymeric binder electrodes for commercial applications. The other drawback with activated carbon-binder electrodes is that the processes described in the literature require use of an organic solvent. The organic solvent has to be subsequently removed by air drying and/or vacuum drying thereby rendering complexity to the manufacturing process. Recently, Choi [32] has fabricated a relatively inexpensive carbon electrode comprising activated carbon powder and poly(vinylidenefluoride) (PVDF) binder. The process for making the electrode required use of di-methyl acetamide to ensure homogeneity. The electrode exhibited good electrosorptive characteristics and salt removal capability. There have been very limited studies on the use of conductive carbon blacks (CCB) in activated carbon electrodes for improving the electrical conductivity. It is reported [33] that the capacitance of activated carbon electrodes can be enhanced by incorporating CCBs. CCBs with high proportion of mesopore surface area are now available. Mesoporous CCBs (MCCB) can potentially offer advantages of both improved electrical conductivity and enhanced charge– discharge rates. Use of MCCBs in activated carbon electrodes has not been studied so far. The objectives of the present study are to improve the electrosorptive characteristics of the electrodes made from powdered activated carbon (PAC) and polymeric binder and to develop a simple process for making the electrodes. In this novel study, we report for what we believe to be the first time results on electrosorptive desalination on electrodes made from powdered activated carbon and polymeric binder incorporating low levels of MCCB. The specific capacitance and the kinetics of removal of dissolved salts from water are studied. A process has been proposed for making electrodes comprising PAC, MCCB, and polymeric binder that does not use organic solvent. 2. Materials and methods 2.1. Electrode The processes described in the literature for fabricating electrode from activated carbon are organic solvent based [2,32–35]. The
183
present study uses a process which is aqueous based. Powdered activated carbon was procured from Active Carbon India Pvt. Ltd, Hyderabad, India. The powder was sieved to get particles in the size range 75 to 250 microns. The surface area of activated carbon was measured using BET technique. The total surface area was 885 m2/g and the surface area of mesopores (N2 nm) was 89 m2/g. The thermoplastic polymeric binder used was Ultra-high molecular weight polyethylene from Ticona Gmbh, Germany, having a melt flow rate of b1 g/10 min., particle size between 40 and 50 micron and bulk density of 0.2–0.25 g/cm3. Various grades of CCBs with varying surface area and pore size distribution were used in the study. The suppliers and specifications of CCBs used in this study are shown in Table 1. The surface area data was given by the suppliers. The total surface area varied from 65 m2/g for Ensaco 250 G (Supplier: TIMCAL) to 706 m2/g for Ensaco 350G (TIMCAL), while the mesopore area varied from b65 m2/g to 455 m2/g respectively. CCB from TIMCAL Ensaco 350G with mesopore area of 455 m2/g is referred to as mesoporous CCB (MCCB) hereafter. Ultra pure water taken from Millipore unit (MilliQ apparatus) was used for all experiments. Graphite sheet for use as current collector was procured from Graphite India Pvt. Ltd. The thickness of the current collector was 0.3 mm and was cut into a circular disc of 15 cm diameter. The electrode was fabricated by mixing and curing process. The detailed process for making the electrode has been described elsewhere [36]. The sheet comprising PAC, binder, and CCB cast onto graphite current collector was used as the electrode for salt removal experiments. No organic solvent was used for making the electrode. 2.2. Measurement of capacitance The samples for measurement of capacitance of the electrode composition were cut from the electrodes made as described in Section 2.1. Cyclic Voltammetry (CV) is a technique used [2,28,32,34,35] to characterize the electrode material for salt removal capacity (capacitance). The voltammogram is a curve of change in current as a function of voltage. The integration over the entire set of data per unit weight of the material gives specific capacitance of the electrode material (F/g). Cyclic voltammetry experiments were performed using a Bioanalytical Systems (BAS, West Lafayette, IN) voltammetric analyzer connected to a BAS C2 cell stand. A small piece (of 9 mg) and large piece (of ~270 mg) were cut from the electrode material to be tested. The reference electrode used was Ag/AgCl electrode immersed in 3 M sodium chloride solution. The working electrode was a platinum wire with a small piece of electrode material (9 mg) attached to it. The auxiliary electrode was also platinum wire with the large piece (270 mg) of electrode material attached to it. 0.5 M sodium chloride solution was used as an electrolyte. Before any measurements were made, the electrode samples were placed in the aqueous solution and cycled between the potential window of interest (−0.4 to + 0.6 V) for at least three times to ensure that no bubbles were trapped in the Table 1 Suppliers and specifications of conductive carbon black (CCB). Specific capacitance values are for 70% (wt) powdered activated carbon, 20% binder, 10% CCB. CCB supplier
CCB grade
TIMCAL Graphite & Carbon Cabot Corporation Cabot Corporation TIMCAL Graphite & Carbon
ENSACO 250G REGAL VULCAN X72R ENSACO 350G
Total surface area (m2/g)
Mesopore surface area (m2/g)
Specific capacitance (F/g) (at scan rate 5 mV/s)
65
b 65
16.7
90 210 706
b 90 158 455
17.4 30.3 45.0
184
S. Nadakatti et al. / Desalination 268 (2011) 182–188
sample. Sample was subjected to scan rate (dV/dt) of 1, 5, 15, 25, 50 and 100 mV/s. Current–voltage data was recorded. Capacitance (C) was calculated from the data using the relationship: C = ∫i
dt dV
Capacitance for the activated carbon based electrodes was measured for various levels of binder and CCB in the electrode composition. The capacitance of the electrode compositions of the present study were compared with two types of commercial electrodes used in capacitive deionization applications. One of the commercial electrodes was a carbon aerogel procured from M/s MarkeTech International, Inc., WA, USA. The sample was reported (by the supplier) to have a total surface area of 443 m2/g and a mesopore area (N2 nm) of 423 m2/g. The other commercial electrode was activated carbon cloth (Spectracarb 2225) supplied by M/s. Engineered Fibers Technology LLC, CT, USA. 2.3. Experimental setup for salt removal Salt removal experiments were carried out using a pair of electrodes (cathode and anode) prepared using the process described in Section 2.1. The electrodes were separated by a 0.22 mm thick Whatman paper no. 41, average pore size of 20 microns. A hole measuring 5 mm diameter was made at the centre of one of the electrodes and flexible tubing was connected to it along with a flow regulator. The two-electrode assembly was held together using nonconductive clips. The electrodes were connected to wires by soldering on a silver-glue-coated area on the outer (non-coated) surface of graphite sheet (current collector). The entire electrode assembly was placed horizontally in a 20 cm diameter acrylic cylindrical chamber (the electrode chamber) with an opening at the bottom for outlet. The flexible tubing was guided out of the opening and connected to a pair of solenoid valves which directed the flow of purified and waste (concentrated) water to separate collection chambers. Refer to Fig. 1 for schematic. Water to be purified was filled in the electrode chamber. The water was thus designed to enter from the periphery into and between the electrodes and exit from the centre hole via the flexible tubing and flow regulator. The solenoid valves would then switch the flow into the purified or waste collection chambers as applicable. The purification step involved applying a DC voltage of 1.2 V to the electrodes. Feed water was allowed to flow for 12 min. Water during the first 4 min was rejected and the following 8 min was collected as purified water. The applied voltage was then removed and electrodes shorted for 30 s. During the regeneration step applied voltage was reversed (−1.2 V) for 30 s followed by voltage removal and electrode shorting for 3 min. The flow rate of purified water was controlled at 10 ml/min in salt removal experiments. + To Voltage Source Water In
Electrode
The real-time data-logging for the values of voltage and conductivity was accomplished by a data-logging machine and software (National Instruments, USB-6009, DAQ-MX driver software) connected in parallel with the voltage source and the data-recorder output of conductivity meter. The data was plotted to give the charge– discharge curves. The electrodes were tested for salt removal performance with 1000 ppm NaCl solution (conductivity 2.04 mS/cm) and tap water (~670 ppm total dissolved salts in water; conductivity 1.47 mS/cm). NaCl solution was prepared using distilled water of conductivity 0.01 mS/cm. The tap water had a mixture of chlorides, carbonates and bicarbonates and the source of the water was bore well. % salt removal and % recovery were noted for all the electrodes before and after MCCB incorporation in both tap water and NaCl solution systems.
3. Results and discussion 3.1. Optimization of binder and MCCB levels The binder used in this study is non-conductive and binds together the carbon particles essentially by melting and sticking to the carbon surfaces. The disadvantage is that the surface area covered by the binder particles is inaccessible to ions for electrosorption. At low levels of binder, the electrode is mechanically unstable and weak whereas, at higher levels, the active sites of carbon are blocked, thereby making it less efficient. The binder levels were optimised based on mechanical integrity of the electrode and electrode capacitance. Table 2 shows the specific capacitance of the electrode material with 10% MCCB (Ensaco 350G) for 20% and 30% (w/w) binder. Binder level was optimized at 20% (w/w) since capacitance was higher and mechanical integrity was acceptable. The parameters for consideration for optimizing the level of MCCB were capacitance and leach of MCCB particles. At low levels of MCCB (b2% w/w), there was no significant increase in capacitance values whereas, at higher levels, the MCCB particles leached out of the electrode when contacted with water as the particle size of MCCB is smaller as compared to the activated carbon and binder particles. The optimum level of MCCB was found to be 10% w/w. The best performance was given by electrodes with 10% MCCB and 20% Binder. Park et al. [33] have studied the effect of polytetrafluoroethylene (PTFE) binder level on the capacitance of the activated carbon electrodes. They also found that that use of high levels of polymeric binder resulted in decrease in the capacitance. In the same paper they reported that the optimum level of CCB was 12%. CCB used by Park et al. did not have high levels of mesopore area as is the case in present study. The electrode was prepared by using isopropyl alcohol as solvent. Lee and Choi [30] and Choi and Choi [34] have used PVDF polymeric binder to make an electrode of activated carbon powder. The process for making the electrode requires the use of di-methylacetamide (DMAc) as solvent to ensure homogeneity. The solvent is subsequently removed using drying oven and vacuum oven. The
Table 2 Effect of binder level on capacitance of electrode materiala. Scan rate (mV/s)
Flow Regulator
Current Collector Water Out
Fig. 1. Schematic diagram of experimental setup for salt removal studies.
100 50 25 15 5
Sp. Capacitance (F/g) 20% Binder
30% Binder
3.2 7.7 17.4 28.4 45.0
2.7 6.2 12.7 20.0 36.0
a X% (wt.) binder, 10% MCCB (Timcal Ensaco 350G), (90-X)% powdered activated carbon.
S. Nadakatti et al. / Desalination 268 (2011) 182–188
3.2. Effect of mesoporous conductive carbon black on the capacitance Different grades of CCB from various suppliers were tested at different levels. The difference in these grades was in the total surface area and most importantly, the mesopore area. The CV results for different grades of CCB at 10% (w/w) are tabulated in Table 1 for scan rate of 5 mV/s. Electrode made with high mesopore area CCB (MCCB; TIMCAL Ensaco 350G) has given the highest value of capacitance. Table 1 shows that capacitance of the electrode increases with mesopore area of CCB. There was nearly a three-fold increase in the capacitance of the electrode material after the incorporation of 10% MCCB. The values of capacitance for a PAC electrode before and after the incorporation of MCCB (TIMCAL Ensaco 350G) were 13.8 and 45.0 F/g respectively. MCCB levels below 2% did not show appreciable rise in capacitance. Mesopores provide area for better ion accessibility and also, serve as pathways to access the micropores located in the inner surface of carbon particles. Thus, they act as pore interconnectors providing mobility to the ions from the bulk of the electrolyte to the micropores. Ying et al. [14] have found that two types of carbon aerogel electrodes with different surface areas had similar salt adsorption capacities. They attributed this finding to the observation that the mesopore area fraction in the two types of carbon aerogel was comparable. Hou et al. [15] have reported that carbon aerogels that have higher fraction of mesopores showed higher capacitance than carbon aerogels that have lower fraction of mesopores. Similar observations have been reported for carbon aerogels by Yang et al. [12,13]. Transfer of ions in mesopores is faster and easier than transfer in micropores. When a pore has a width smaller than the specific value (cut-off pore width), it does not contribute to the total capacity because of electrical double layer overlapping effect. This effect greatly reduces the electrosorption capacity for electrodes with significant numbers of micropores, as is the case with the PAC electrode of the present study without incorporation of mesoporous CCB. The effect of the double-layer overlapping on the electrosorption capacitance can be reduced by increasing the pore size. This effect was seen in the present study wherein the capacitance of PAC electrode could be significantly enhanced by using CCB with high mesopore area. Zou et al. [3] have reported higher specific capacitance values for Ordered Mesoporous Carbon (OMC) electrodes compared to activated carbon (AC) suggesting that the former had a higher desalting capacity compared to the latter. OMC had a mesopore fraction of 82% whereas AC had a mesopore fraction of 41%. Under the same electrochemical condition the specific capacitance value of OMC electrode (at a sweep rate of 1 mV/s in 0.1 M NaCl solution) was 133 F/g and that for AC electrode was 107 F/g. Furthermore OMC showed a better rate capacity than the AC electrode. In addition, it was found that the adsorbed ion amounts in OMC were almost 3 times higher as compared to those on AC electrode. The excellent adsorptive desalination performance of OMC electrode was attributed to the presence of high level of mesopores in OMC compared to AC and also to the ordered mesoporous structure. In a second paper by the same group [20] it is reported that a modified sol–gel process involving nickel salts resulted in OMCs with higher BET surface area and smaller mesopores. The OMCs synthesized using the modified sol–gel process exhibited higher capacitance compared to their earlier study. The specific capacitance values reported for OMCs cannot be directly compared with those of the present study since the experimental conditions are different.
3.3. Dependence of capacitance on scan rate Capacitance was measured as a function of scan rate for the electrode material. The data is shown in Table 2 and Fig. 2. The capacitance decreases with an increase in scan rate. The capacitance at 1 mV/s and 100 mV/s was 54.7 F/g and 3.2 F/g respectively. This can be explained by considering the pore size distribution of the electrode material. At high scan rates, the ions can penetrate only the larger mesopores and thus, only a smaller proportion of the total area is effectively utilized for electrosorption. However, at low scan rates, the smaller micropores are accessible to the ions, thereby increasing the electrosorption capacity. A similar observation was made by Seo and Park [18] for activated carbon cloth (AC cloth) and activated carbon composite (AC composite). Seo et al. reported capacitance values of 41.5 F/g for AC cloth and 12.1 F/g for AC composite at a scan rate of 1 mV/s. In contrast, the capacitance values at scan rate of 100 mV/s were 0.02 mV/s and 0.12 mV/s respectively. The higher capacitance of AC composite at 100 mV/s was attributed to higher fraction of mesopores in AC composite compared to AC cloth. Yang et al. [13] have shown that the dependence of capacitance on scan rate depends on several factors such as electrolyte concentration, applied voltage, and pore size distribution (PSD). Typically, at high electrolyte concentrations the capacitances obtained by slow and fast rates show little difference. At low electrolyte concentrations the capacitance of the electrode is high at low scan rate and low at higher scan rate. 3.4. CV results Fig. 3 shows cyclic voltammograms for PAC + Binder electrode and also for PAC + MCCB + binder electrode at scan rate of 5 mV/s. Use of low levels of MCCB resulted in a rectangular shaped CV curve indicative of an ideal electrosorption capacitive behavior. The voltammogram of PAC-binder electrode is distorted which may be due to the slow ion transport in micropores. It is therefore proposed that PACbinder electrode is not suitable for quick charge/discharge of ions. In contrast, the PAC-binder-MCCB electrode shows excellent rate capacity due to the presence of mesopores. It has been reported by many investigators that the cyclic voltammogram of an ideal doublelayer capacitor has a rectangular shape. Rectangular shaped cyclic voltammograms have been reported for electrodes such as carbon aerogels [15], carbon electrode with Poly (vinylidene fluoride) (PVDF) binder [32,34], titanium dioxide modified activated carbon electrode [18], polyvinyl alcohol (PVA)-bonded activated carbon electrode [19], and many other high performance electrodes. 60 50
Sp.Capacitance, F/g
optimum level of PVDF binder was found to be 10–13% (by wt.). Li et al. [10] have used PTFE as binder to fabricate an electrode comprising grapheme nano flakes. They used 8% PTFE binder and 20% graphite conductive material. Low levels of ethanol were used to wet the mixture.
185
40 30 20 10 0 0
20
40
60
80
100
Scanrate, mV/s Fig. 2. Effect of scan rate on capacitance of electrode material. Electrode composition: 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB (TIMCAL Ensaco 350G) electrode (♦).
186
S. Nadakatti et al. / Desalination 268 (2011) 182–188 Table 4 Salt removal and recovery for 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB electrode. Electrode % Salt Removal % Recovery
Fig. 3. Cyclic voltammetry curve for powdered activated carbon with MCCB (—) and without MCCB (—).
Zou et al. [3] have shown that the CV curve for ordered mesoporous carbon (OMC) electrode maintains the rectangular shape in the voltage sweep rate 1 to 10 mV/s. In OMC, 82% of the total surface area comprised of mesopores. By contrast, the shape of the CV curve, for the activated carbon (AC) electrode comprising largely micropores, was distorted. The findings of the present study relating to shape of the CV curve are therefore consistent with that reported in the literature. 3.5. Comparison with commercial electrodes The capacitance of the electrode of the present study at scan rate of 5 mV/s is 45.0 F/g. The capacitance of commercially available carbon aerogel and activated carbon cloth under comparable experimental conditions were 30.0 and 31.6, respectively (cf. Table 3). High capacitance of the electrode of present study can be attributed to very high levels of mesopores that are present in the conductive carbon black that was incorporated at 10% (w/w). The MCCB used in present study has a mesopore area of 455 m2/g. The capacitance of carbon aerogel has been studied by many investigators such as [12,13,15], etc. The activated carbon cloth from Spectracarb Corp. has been studied by Conway et al. [37], Ayranci et al. [38] and Niu and Conway [39,40]. The capacitance values obtained in the present study cannot be directly compared with that reported in literature since the experimental conditions were different. However, both carbon aerogel and Spectracarb 2225 are reported to have high capacitance, rectangular shaped cyclic voltammograms, and good electrode performance characteristics. 3.6. Salt removal and recovery The salt removal and recovery data for the electrodes, before and after the inclusion of MCCB is tabulated in Table 4. Both removal of dissolved salts from water and recovery improved significantly by incorporation of 10% of MCCB in the electrode composition. Removal of dissolved salts from tap water and from NaCl feed solution was about 30–35% for electrode without incorporation of MCCB at an
Table 3 Comparison of electrode capacitance. Electrode
Capacitance at scan rate 5 mV/s
Carbon aerogel Carbon cloth Present studya
30.0 31.6 45.0
a 70% (wt) powdered activated carbon, 20% binder, 10% MCCB (TIMCAL Ensaco 350G).
Without MCCB With MCCB Without MCCB With MCCB
Tap Water (~670 ppm)
NaCl Solution (1000 ppm)
35 75 25 55
30 70 25 55
applied voltage of 1.2 V and input TDS of ~ 1000 ppm. The flow rate was 10 ml/min. Incorporation of 10% MCCB in the electrode resulted in % salt removal going up to 70%. MCCB incorporation also helped in increasing the recovery from 25% to 55% in both NaCl feed solution and tap water. Choi [32] has reported salt removal for an electrode made from activated carbon powder and PVDF binder. The average salt removal efficiency at a flow rate of 20 ml/min was 77.8%. However, the input salt levels in Choi's study were significantly lower at 200 ppm compared to input levels in present study at 1000 ppm and therefore salt removal efficiency is not strictly comparable. Seo et al. [6] have investigated removal of hardness ions using activated carbon cloth and activated carbon composite. They reported 80% of Ca and Mg ions removal after 3 min of operation at a flow rate of 16 ml/min for one pair of electrodes. The electrode cell was operated with 4 min of purification cycle and 1 min regeneration cycle thus yielding a recovery of 80%. The salt removal and recovery of present work cannot be directly compared with that reported by Seo et al. since the feed water composition and electrode surface area used in two studies are very different. One of the key advantages of capacitive deionization technology over conventional reverse osmosis (RO) technology for removal of dissolved salts from water is higher recovery of feed water. In conventional RO, recovery is typically 15% i.e., for 10 l of feed water, only ~ 1.5 l of purified water is obtained. In the present study the recovery using CDI technology is significantly higher at 55%. Thus there is less wastage of water in CDI technology. The other key advantage of CDI over RO is that CDI is a low pressure process. In the present study gravity pressure was sufficient to carry out deionization of feed water. In RO high pressures (~100 psi) are required which is achieved using a pump. Anderson et al. [5] have indicated that CDI due to its low pressure operation can potentially result in 1/3 less energy consumption compared to RO for brackish water treatment. 3.7. Charge–discharge curves Cyclic tests of purification and regeneration were performed for the evaluation of CDI applicability. Fig. 4 shows charge–discharge curves for 9 cycles. The feed water was 670 ppm tap water. During the purification stage, low conductivity of treated water was obtained. During regeneration process, on the other hand, higher conductivity than that of initial feed solution was obtained due to the extracted ions from the electrical double layers in both electrodes. The conductivity-time plot shows the following features: (i) charge– discharge response is very fast and (ii) high reversibility of adsorption-desorption behaviour. These two are key parameters in selection of electrodes for use in CDI technology. The % salt removal (as determined by % reduction in conductivity) is ~ 70% and recovery is ~ 55%. Fig. 5 shows the performance of the electrode for an extended period of usage of more than 7 h. It was found that the performance of the electrode does not deteriorate with repeat cycles. Seo et al. [6] in their study of selective removal of divalent ions from water have reported similar charge–discharge curves for activated carbon cloth wherein cyclic tests were performed with the
S. Nadakatti et al. / Desalination 268 (2011) 182–188
187
Voltage Conductivity 1.4
3.0
1.0
0.6
Volts (V)
2.0 0.2 1.5 -0.2 1.0 -0.6 0.5
-1.0
-1.4 0:00
Conductivity (mS/cm)
2.5
0.0 0:14
0:28
0:43
0:57
1:12
1:26
1:40
1:55
2:09
Time (h:mm) Fig. 4. Charge–discharge, conductivity (—), voltage (—) graph for 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB electrode. Feed water is 670 ppm tap water.
five-cell stack. Cyclic test data has been reported for 4 cycles and electrode performance was found to be consistent. 4. Conclusions The optimum composition of the electrode for use in CDI technology is 70% (w/w) PAC, 10% MCCB, and 20% polymeric binder. Capacitance of an electrode comprising 80% PAC and 20% polyethylene binder at scan rate of 5 mV/s was 13.8 F/g. The capacitance could be enhanced to 45.0 F/g by replacing 10% of the PAC with MCCB having a total surface area of 706 m2/g and mesopore area of 455 m2/ g. The capacitance of commercially available carbon aerogel and
activated carbon cloth under comparable conditions were 30.0 F/g and 31.6 F/g respectively. Thus, the capacitance of electrodes made from PAC, polymeric binder, and low levels of MCCB is high and comparable to commercially available carbon aerogel and activated carbon cloth. Incorporation of low levels of MCCB in electrodes made from PAC and polymeric binder resulted in a rectangular shaped (mirror-imaged) CV curve indicative of an ideal electrosorption capacitive behavior. The charge/discharge response is fast. These characteristics have enabled high removal of dissolved salts from feed water (~75% removal) at recovery of ~ 55%. The performance of the electrodes was tested for an extended period of time (7 h of operation and 30 cycles) and found to be consistent. The process for preparing Voltage Conductivity
1.4
1.0
3.0
0.6
Volts (V)
2.0 0.2 1.5 -0.2 1.0 -0.6 0.5
-1.0
-1.4 0:00
Conductivity (mS/cm)
2.5
0.0 1:00
2:00
3:00
4:00
5:00
6:00
7:00
Time (h:mm) Fig. 5. Extended run data of charge–discharge, conductivity (—), voltage (—) for 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB electrode. Feed water is 670 ppm tap water.
188
S. Nadakatti et al. / Desalination 268 (2011) 182–188
electrodes does not require any organic solvent. PAC electrodes containing low levels of MCCB can potentially be ideal materials for use in capacitive deionization technology. Acknowledgements The authors thank Raghunandan S. for his help in experiments and Venkataraghavan R. for his guidance in preparing the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14]
Y. Oren, Desalination 228 (2008) 10–29. L. Zou, G. Morris, D. Qi, Desalination 225 (2008) 329–340. L. Zou, L. Li, H. Song, G. Morris, Water Research 42 (2008) 2340–2348. K.Y. Foo, B.H. Hameed, Journal of Hazardous Materials 170 (2009) 552–559. M.A. Anderson, A.L. Cudero, J. Palma, Electrochimica Acta 55 (2010) 3845–3856. S.J. Seo, H. Jeon, J.K. Lee, G.Y. Kim, D. Park, H. Nojima, J. Lee, S.H. Moon, Water Research 44 (2010) 2267–2275. R. Broseus, J. Cigana, B. Barbeau, C.D. Martinez, H. Suty, Desalination 249 (2009) 217–223. Y.J. Kim, J. Hur, W. Bae, J.H. Choi, Desalination 253 (2010) 119–123. L.Y. Lee, H.Y. Ng, S.L. Ong, G. Tao, K. Kekre, V. Balakrishnan, W. Lay, H. Seah, Water Research 43 (2009) 4769–4777. H. Li, L. Zou, L. Pan, Z. Sun, Separation and Purification Technology 75 (2010) 8–14. B.E. Conway, Electrochemical Supercapacitors, Scientific fundamentals and technological applications, Kluver Academic / Plenum Publishers, New York, 1999, pp. 615–616. K. Yang, T. Ying, S. Yiacoumi, C. Tsouris, E.S. Vittoratos, Langmuir 17 (2001) 1961–1969. K. Yang, S. Yiacoumi, C. Tsouris, Journal of Electroanalytical Chemistry 540 (2003) 159–167. T. Ying, K. Yang, S. Yiacoumi, C. Tsouris, Journal of Colloid and Interface Science 250 (2002) 18–27.
[15] C.H. Hou, C. Liang, S. Yiacoumi, S. Dai, C. Tsouris, Journal of Colloid and Interface Science 302 (2006) 54–61. [16] P. Xu, J.E. Drewes, D. Heil, G. Wang, Water Research 42 (2008) 2605–2617. [17] M.W. Ryoo, G. Seo, Water Research 37 (2003) 1527–1534. [18] M.K. Seo, S.J. Park, Current Applied Physics 10 (2010) 391–394. [19] B.H. Park, J.H. Choi, Electrochimica Acta 55 (2010) 2888–2893. [20] L. Li, L. Zou, H. Song, G. Morris, Carbon 47 (2009) 775–781. [21] H. Li, Y. Gao, L. Pan, Y. Zhang, Y. Chen, Z. Sun, Water Research 42 (2008) 4923–4928. [22] Y. Gao, L. Pan, H. Li, Y. Zhang, Z. Zhang, Y. Chen, Z. Sun, Thin Solid Films 517 (2009) 1616–1619. [23] L. Pan, X. Wang, Y. Gao, Y. Zhang, Y. Chen, Z. Sun, Desalination 244 (2009) 139–143. [24] H. Li, L. Pan, Y. Zhang, L. Zou, C. Sun, Y. Zhan, Z. Sun, Chemical Physics Letters 485 (2010) 161–166. [25] P.M. Biesheuvel, A. Wal, Journal of Membrane Science 346 (2010) 256–262. [26] Y.J. Kim, J.H. Choi, Separation and Purification Technology 71 (2010) 70–75. [27] Y.J. Kim, J.H. Choi, Water Research 44 (2010) 990–996. [28] J. Kim, J. Choi, Journal of Membrane Science 355 (2010) 85–90. [29] P.M. Biesheuvel, Journal of Colloid and Interface Science 332 (2009) 258–264. [30] J. Lee, W. Bae, J. Choi, Desalination 258 (2010) 159–163. [31] R.W. Pekala, J.C. Farmer, C.T. Alviso, T.D. Tran, S.T. Mayer, J.M. Miller, B. Dunn, Journal of Non-Crystalline Solids 225 (1998) 74–80. [32] J.H. Choi, Separation and Purification Technology 70 (2010) 362–366. [33] K. Park, J. Lee, P. Park, S. Yoon, J. Moon, H. Eum, C. Lee, Desalination 206 (2007) 86–91. [34] J.Y. Choi, J.H. Choi, Journal of Industrial and Engineering Chemistry 16 (2010) 401–405. [35] J.A. Lim, N.S. Park, J.S. Park, J.H. Choi, Desalination 238 (2009) 37–42. [36] M.K. Kadam, S.M. Nadakatti, M.S. Tendulkar, International patent WO 2009/077276 A1. [37] B.E. Conway, J. Niu, W.G. Pell, Kemical Industry 54 (4) (2005) 187–198. [38] E. Ayranci, B.E. Conway, Journal of Applied Electrochemistry 31 (2001) 257–266. [39] J. Niu, B.E. Conway, Journal of Electroanalytical Chemistry 546 (2003) 59–72. [40] J. Niu, B.E. Conway, Journal of Electroanalytical Chemistry 529 (2002) 84–96.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Use of mesoporous conductive carbon black to enhance performance of activated carbon electrodes in capacitive deionization technology Suresh Nadakatti ⁎, Mahesh Tendulkar, Manoj Kadam 1 Unilever R&D Bangalore, Hindustan Unilever Ltd, 64, Main Road, Whitefield, Bangalore-560066, India
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 6 October 2010 Accepted 7 October 2010 Available online 20 November 2010 Keywords: Capacitive deionization Conductive carbon black Desalination Electrosorption Activated carbon electrode
a b s t r a c t Capacitive deionization (CDI) is a technology for removal of dissolved salts from water by electrosorption of ions onto oppositely charged electrodes. The electrodes are usually made from activated carbon due to its high surface area. This paper reports the use of low levels of mesoporous conductive carbon blacks (MCCB) containing high fraction of mesopore surface area to enhance the capacitance of powdered activated carbon (PAC) electrodes. Different grades of MCCB with mesopore surface area varying from 65 m2/g to 455 m2/g were used. The capacitance of powdered activated carbon electrode could be enhanced from 13.8 F/g to 45.0 F/g at a scan rate of 5 mV/s by replacing 10% of powdered activated carbon with MCCB. The process for making these electrodes does not require use of organic solvent. The capacitance of improved electrode was comparable to that of commercially available carbon aerogel and activated carbon cloth electrodes. The removal of dissolved ions from water using a pair of electrodes (each weighing 11 g) at 10 ml/min was 75% and recovery was ~ 55% at applied potential of 1.2 V. Activated carbon electrodes containing low levels of MCCB offer high potential for use in capacitive deionization technology. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Capacitive deionization technology (CDI) is an exciting new method of removing dissolved salts from water [1–5]. The applications in drinking water include enhancing taste of water by controlling the dissolved salts level and converting brackish water into potable water. Some of the potential applications of the technology include: (i) water softening because of its preferential adsorption for divalent ions such as Mg and Ca [6], (ii) removal of nitrates, ammonium ions from brackish water [7], (iii) treating oil-containing brackish water [7,8], (iv) use in integrated plants along with reverse osmosis (RO) for reject water recovery to recycle the treated water to RO feed [9] and (v) removal of heavy metals such as ferric ions from water [10]. In CDI, a brackish water stream flows between pairs of high surface area carbon electrodes that are held at a potential difference of ~ 1.2 V. The treatment involves two steps—purification step and regeneration step. During the purification step, the ions and other charged particles are attracted to and held on the electrode of opposite charge. The negative electrode (cathode) attracts positively charged ions (cations) such as calcium (Ca2+), magnesium (Mg2+) and sodium (Na+), while the positively charged electrode (anode) attracts negative ions (anions) such as chloride (Cl−) and nitrate
⁎ Corresponding author. Tel.: +91 80 39831077; fax: +91 80 28453086. E-mail address: [email protected] (S. Nadakatti). 1 Present address: OTS-Advance Solution, Honeywell Automation India Limited, SP Infocity, Bldg No. II, 3rd Floor, Fursungi, Hadapsar, Pune 412 308, Maharashtra, India. 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.10.020
(NO− 3 ). Eventually the electrodes become saturated with ions and must be regenerated. During the regeneration step, the applied potential is removed. The adsorbed ions are released back into feed stream producing a more concentrated brine stream. Typically, for a gallon of brackish water fed to the CDI process, more than 50% emerges as fresh, deionized potable water, and the remainder is discharged as a concentrated brine solution. The key advantage of CDI lies in its potentially lower operating cost, which is about one third that of RO [5] and high recovery of N50% as compared to ~15% for RO [5,7]. One of the key factors that has limited the scale-up and commercialisation of CDI technology is availability of low cost, high efficiency electrodes. The initial electrodes for use in CDI were made from porous activated carbon and polymeric binder [1,4,11]. The key problems with conventional activated carbon electrodes were weak physical bonding of activated carbon, high electrical resistance and poor capacitance. Consequently the salt removal was low and recovery was poor. Much of the effort in recent years has been on development of electrodes with novel materials that have high electrical conductivity and high capacitance. Carbon aerogel is one such material which has high surface area and better ion-removal capacitance [12–16]. Researchers have experimented on various options for improvement of the existing electrode materials such as surface modification with titanium oxide [17,18], use of water-soluble polymer binder [19], use of mesoporous carbon electrode [3,20], etc. Other materials of current interest include carbon nanotubes and carbon nanofibers [21–24]. Recently, researchers have focussed on novel routes for enhancing the salt removal such as the charge barrier concept [7], membrane
S. Nadakatti et al. / Desalination 268 (2011) 182–188
capacitive deionization [25], use of ion-selective membranes [26] and coating electrode with ion-exchange polymer [27,28]. Kinetics of adsorption of salts and thermodynamic studies relating to this technology have been reported [24,29]. Mechanisms of adsorption, desorption, and electrode reactions in CDI have also been investigated by Lee and Choi [30]. The aforementioned carbon substances demonstrate good electrode characteristics in terms of electrical conductivity and specific surface area, but still suffer from their relatively complicated manufacturing process and high cost. The high cost of superior performance electrodes such as carbon aerogels has limited the use of these electrodes in commercial CDI applications. One of the key reasons for improved performance of carbon aerogels is the ability to control the pore size distribution (PSD). It has been shown [31] that carbon aerogels that contain higher proportion of mesopore (pore size N2 nm) surface area to total surface area have higher capacitance and faster kinetics of ion adsorption. Several researchers have investigated the role of micropores and mesopores on the performance of electrodes [3,12,13,15]. Micropores increase the surface area of activated carbon and hence contribute to high specific capacitance. Mesopores provide more favourable and quicker pathway for ions to penetrate the pores, thereby improving the adsorption and desorption kinetics. Thus, an electrode for use in CDI should have both micropores and mesopores. The conventional activated carbon-polymeric binder electrodes have large surface area but most of it is due to micropores. The capacitance at low scan rates (1 mV/s) is high but at high scan rates (5 to 100 mV/s) the capacitance is very low. Poor capacitance at high scan rates corresponds to poor salt removal and poor recovery in flow-through-capacitor applications. This drawback has limited the use of activated carbon-polymeric binder electrodes for commercial applications. The other drawback with activated carbon-binder electrodes is that the processes described in the literature require use of an organic solvent. The organic solvent has to be subsequently removed by air drying and/or vacuum drying thereby rendering complexity to the manufacturing process. Recently, Choi [32] has fabricated a relatively inexpensive carbon electrode comprising activated carbon powder and poly(vinylidenefluoride) (PVDF) binder. The process for making the electrode required use of di-methyl acetamide to ensure homogeneity. The electrode exhibited good electrosorptive characteristics and salt removal capability. There have been very limited studies on the use of conductive carbon blacks (CCB) in activated carbon electrodes for improving the electrical conductivity. It is reported [33] that the capacitance of activated carbon electrodes can be enhanced by incorporating CCBs. CCBs with high proportion of mesopore surface area are now available. Mesoporous CCBs (MCCB) can potentially offer advantages of both improved electrical conductivity and enhanced charge– discharge rates. Use of MCCBs in activated carbon electrodes has not been studied so far. The objectives of the present study are to improve the electrosorptive characteristics of the electrodes made from powdered activated carbon (PAC) and polymeric binder and to develop a simple process for making the electrodes. In this novel study, we report for what we believe to be the first time results on electrosorptive desalination on electrodes made from powdered activated carbon and polymeric binder incorporating low levels of MCCB. The specific capacitance and the kinetics of removal of dissolved salts from water are studied. A process has been proposed for making electrodes comprising PAC, MCCB, and polymeric binder that does not use organic solvent. 2. Materials and methods 2.1. Electrode The processes described in the literature for fabricating electrode from activated carbon are organic solvent based [2,32–35]. The
183
present study uses a process which is aqueous based. Powdered activated carbon was procured from Active Carbon India Pvt. Ltd, Hyderabad, India. The powder was sieved to get particles in the size range 75 to 250 microns. The surface area of activated carbon was measured using BET technique. The total surface area was 885 m2/g and the surface area of mesopores (N2 nm) was 89 m2/g. The thermoplastic polymeric binder used was Ultra-high molecular weight polyethylene from Ticona Gmbh, Germany, having a melt flow rate of b1 g/10 min., particle size between 40 and 50 micron and bulk density of 0.2–0.25 g/cm3. Various grades of CCBs with varying surface area and pore size distribution were used in the study. The suppliers and specifications of CCBs used in this study are shown in Table 1. The surface area data was given by the suppliers. The total surface area varied from 65 m2/g for Ensaco 250 G (Supplier: TIMCAL) to 706 m2/g for Ensaco 350G (TIMCAL), while the mesopore area varied from b65 m2/g to 455 m2/g respectively. CCB from TIMCAL Ensaco 350G with mesopore area of 455 m2/g is referred to as mesoporous CCB (MCCB) hereafter. Ultra pure water taken from Millipore unit (MilliQ apparatus) was used for all experiments. Graphite sheet for use as current collector was procured from Graphite India Pvt. Ltd. The thickness of the current collector was 0.3 mm and was cut into a circular disc of 15 cm diameter. The electrode was fabricated by mixing and curing process. The detailed process for making the electrode has been described elsewhere [36]. The sheet comprising PAC, binder, and CCB cast onto graphite current collector was used as the electrode for salt removal experiments. No organic solvent was used for making the electrode. 2.2. Measurement of capacitance The samples for measurement of capacitance of the electrode composition were cut from the electrodes made as described in Section 2.1. Cyclic Voltammetry (CV) is a technique used [2,28,32,34,35] to characterize the electrode material for salt removal capacity (capacitance). The voltammogram is a curve of change in current as a function of voltage. The integration over the entire set of data per unit weight of the material gives specific capacitance of the electrode material (F/g). Cyclic voltammetry experiments were performed using a Bioanalytical Systems (BAS, West Lafayette, IN) voltammetric analyzer connected to a BAS C2 cell stand. A small piece (of 9 mg) and large piece (of ~270 mg) were cut from the electrode material to be tested. The reference electrode used was Ag/AgCl electrode immersed in 3 M sodium chloride solution. The working electrode was a platinum wire with a small piece of electrode material (9 mg) attached to it. The auxiliary electrode was also platinum wire with the large piece (270 mg) of electrode material attached to it. 0.5 M sodium chloride solution was used as an electrolyte. Before any measurements were made, the electrode samples were placed in the aqueous solution and cycled between the potential window of interest (−0.4 to + 0.6 V) for at least three times to ensure that no bubbles were trapped in the Table 1 Suppliers and specifications of conductive carbon black (CCB). Specific capacitance values are for 70% (wt) powdered activated carbon, 20% binder, 10% CCB. CCB supplier
CCB grade
TIMCAL Graphite & Carbon Cabot Corporation Cabot Corporation TIMCAL Graphite & Carbon
ENSACO 250G REGAL VULCAN X72R ENSACO 350G
Total surface area (m2/g)
Mesopore surface area (m2/g)
Specific capacitance (F/g) (at scan rate 5 mV/s)
65
b 65
16.7
90 210 706
b 90 158 455
17.4 30.3 45.0
184
S. Nadakatti et al. / Desalination 268 (2011) 182–188
sample. Sample was subjected to scan rate (dV/dt) of 1, 5, 15, 25, 50 and 100 mV/s. Current–voltage data was recorded. Capacitance (C) was calculated from the data using the relationship: C = ∫i
dt dV
Capacitance for the activated carbon based electrodes was measured for various levels of binder and CCB in the electrode composition. The capacitance of the electrode compositions of the present study were compared with two types of commercial electrodes used in capacitive deionization applications. One of the commercial electrodes was a carbon aerogel procured from M/s MarkeTech International, Inc., WA, USA. The sample was reported (by the supplier) to have a total surface area of 443 m2/g and a mesopore area (N2 nm) of 423 m2/g. The other commercial electrode was activated carbon cloth (Spectracarb 2225) supplied by M/s. Engineered Fibers Technology LLC, CT, USA. 2.3. Experimental setup for salt removal Salt removal experiments were carried out using a pair of electrodes (cathode and anode) prepared using the process described in Section 2.1. The electrodes were separated by a 0.22 mm thick Whatman paper no. 41, average pore size of 20 microns. A hole measuring 5 mm diameter was made at the centre of one of the electrodes and flexible tubing was connected to it along with a flow regulator. The two-electrode assembly was held together using nonconductive clips. The electrodes were connected to wires by soldering on a silver-glue-coated area on the outer (non-coated) surface of graphite sheet (current collector). The entire electrode assembly was placed horizontally in a 20 cm diameter acrylic cylindrical chamber (the electrode chamber) with an opening at the bottom for outlet. The flexible tubing was guided out of the opening and connected to a pair of solenoid valves which directed the flow of purified and waste (concentrated) water to separate collection chambers. Refer to Fig. 1 for schematic. Water to be purified was filled in the electrode chamber. The water was thus designed to enter from the periphery into and between the electrodes and exit from the centre hole via the flexible tubing and flow regulator. The solenoid valves would then switch the flow into the purified or waste collection chambers as applicable. The purification step involved applying a DC voltage of 1.2 V to the electrodes. Feed water was allowed to flow for 12 min. Water during the first 4 min was rejected and the following 8 min was collected as purified water. The applied voltage was then removed and electrodes shorted for 30 s. During the regeneration step applied voltage was reversed (−1.2 V) for 30 s followed by voltage removal and electrode shorting for 3 min. The flow rate of purified water was controlled at 10 ml/min in salt removal experiments. + To Voltage Source Water In
Electrode
The real-time data-logging for the values of voltage and conductivity was accomplished by a data-logging machine and software (National Instruments, USB-6009, DAQ-MX driver software) connected in parallel with the voltage source and the data-recorder output of conductivity meter. The data was plotted to give the charge– discharge curves. The electrodes were tested for salt removal performance with 1000 ppm NaCl solution (conductivity 2.04 mS/cm) and tap water (~670 ppm total dissolved salts in water; conductivity 1.47 mS/cm). NaCl solution was prepared using distilled water of conductivity 0.01 mS/cm. The tap water had a mixture of chlorides, carbonates and bicarbonates and the source of the water was bore well. % salt removal and % recovery were noted for all the electrodes before and after MCCB incorporation in both tap water and NaCl solution systems.
3. Results and discussion 3.1. Optimization of binder and MCCB levels The binder used in this study is non-conductive and binds together the carbon particles essentially by melting and sticking to the carbon surfaces. The disadvantage is that the surface area covered by the binder particles is inaccessible to ions for electrosorption. At low levels of binder, the electrode is mechanically unstable and weak whereas, at higher levels, the active sites of carbon are blocked, thereby making it less efficient. The binder levels were optimised based on mechanical integrity of the electrode and electrode capacitance. Table 2 shows the specific capacitance of the electrode material with 10% MCCB (Ensaco 350G) for 20% and 30% (w/w) binder. Binder level was optimized at 20% (w/w) since capacitance was higher and mechanical integrity was acceptable. The parameters for consideration for optimizing the level of MCCB were capacitance and leach of MCCB particles. At low levels of MCCB (b2% w/w), there was no significant increase in capacitance values whereas, at higher levels, the MCCB particles leached out of the electrode when contacted with water as the particle size of MCCB is smaller as compared to the activated carbon and binder particles. The optimum level of MCCB was found to be 10% w/w. The best performance was given by electrodes with 10% MCCB and 20% Binder. Park et al. [33] have studied the effect of polytetrafluoroethylene (PTFE) binder level on the capacitance of the activated carbon electrodes. They also found that that use of high levels of polymeric binder resulted in decrease in the capacitance. In the same paper they reported that the optimum level of CCB was 12%. CCB used by Park et al. did not have high levels of mesopore area as is the case in present study. The electrode was prepared by using isopropyl alcohol as solvent. Lee and Choi [30] and Choi and Choi [34] have used PVDF polymeric binder to make an electrode of activated carbon powder. The process for making the electrode requires the use of di-methylacetamide (DMAc) as solvent to ensure homogeneity. The solvent is subsequently removed using drying oven and vacuum oven. The
Table 2 Effect of binder level on capacitance of electrode materiala. Scan rate (mV/s)
Flow Regulator
Current Collector Water Out
Fig. 1. Schematic diagram of experimental setup for salt removal studies.
100 50 25 15 5
Sp. Capacitance (F/g) 20% Binder
30% Binder
3.2 7.7 17.4 28.4 45.0
2.7 6.2 12.7 20.0 36.0
a X% (wt.) binder, 10% MCCB (Timcal Ensaco 350G), (90-X)% powdered activated carbon.
S. Nadakatti et al. / Desalination 268 (2011) 182–188
3.2. Effect of mesoporous conductive carbon black on the capacitance Different grades of CCB from various suppliers were tested at different levels. The difference in these grades was in the total surface area and most importantly, the mesopore area. The CV results for different grades of CCB at 10% (w/w) are tabulated in Table 1 for scan rate of 5 mV/s. Electrode made with high mesopore area CCB (MCCB; TIMCAL Ensaco 350G) has given the highest value of capacitance. Table 1 shows that capacitance of the electrode increases with mesopore area of CCB. There was nearly a three-fold increase in the capacitance of the electrode material after the incorporation of 10% MCCB. The values of capacitance for a PAC electrode before and after the incorporation of MCCB (TIMCAL Ensaco 350G) were 13.8 and 45.0 F/g respectively. MCCB levels below 2% did not show appreciable rise in capacitance. Mesopores provide area for better ion accessibility and also, serve as pathways to access the micropores located in the inner surface of carbon particles. Thus, they act as pore interconnectors providing mobility to the ions from the bulk of the electrolyte to the micropores. Ying et al. [14] have found that two types of carbon aerogel electrodes with different surface areas had similar salt adsorption capacities. They attributed this finding to the observation that the mesopore area fraction in the two types of carbon aerogel was comparable. Hou et al. [15] have reported that carbon aerogels that have higher fraction of mesopores showed higher capacitance than carbon aerogels that have lower fraction of mesopores. Similar observations have been reported for carbon aerogels by Yang et al. [12,13]. Transfer of ions in mesopores is faster and easier than transfer in micropores. When a pore has a width smaller than the specific value (cut-off pore width), it does not contribute to the total capacity because of electrical double layer overlapping effect. This effect greatly reduces the electrosorption capacity for electrodes with significant numbers of micropores, as is the case with the PAC electrode of the present study without incorporation of mesoporous CCB. The effect of the double-layer overlapping on the electrosorption capacitance can be reduced by increasing the pore size. This effect was seen in the present study wherein the capacitance of PAC electrode could be significantly enhanced by using CCB with high mesopore area. Zou et al. [3] have reported higher specific capacitance values for Ordered Mesoporous Carbon (OMC) electrodes compared to activated carbon (AC) suggesting that the former had a higher desalting capacity compared to the latter. OMC had a mesopore fraction of 82% whereas AC had a mesopore fraction of 41%. Under the same electrochemical condition the specific capacitance value of OMC electrode (at a sweep rate of 1 mV/s in 0.1 M NaCl solution) was 133 F/g and that for AC electrode was 107 F/g. Furthermore OMC showed a better rate capacity than the AC electrode. In addition, it was found that the adsorbed ion amounts in OMC were almost 3 times higher as compared to those on AC electrode. The excellent adsorptive desalination performance of OMC electrode was attributed to the presence of high level of mesopores in OMC compared to AC and also to the ordered mesoporous structure. In a second paper by the same group [20] it is reported that a modified sol–gel process involving nickel salts resulted in OMCs with higher BET surface area and smaller mesopores. The OMCs synthesized using the modified sol–gel process exhibited higher capacitance compared to their earlier study. The specific capacitance values reported for OMCs cannot be directly compared with those of the present study since the experimental conditions are different.
3.3. Dependence of capacitance on scan rate Capacitance was measured as a function of scan rate for the electrode material. The data is shown in Table 2 and Fig. 2. The capacitance decreases with an increase in scan rate. The capacitance at 1 mV/s and 100 mV/s was 54.7 F/g and 3.2 F/g respectively. This can be explained by considering the pore size distribution of the electrode material. At high scan rates, the ions can penetrate only the larger mesopores and thus, only a smaller proportion of the total area is effectively utilized for electrosorption. However, at low scan rates, the smaller micropores are accessible to the ions, thereby increasing the electrosorption capacity. A similar observation was made by Seo and Park [18] for activated carbon cloth (AC cloth) and activated carbon composite (AC composite). Seo et al. reported capacitance values of 41.5 F/g for AC cloth and 12.1 F/g for AC composite at a scan rate of 1 mV/s. In contrast, the capacitance values at scan rate of 100 mV/s were 0.02 mV/s and 0.12 mV/s respectively. The higher capacitance of AC composite at 100 mV/s was attributed to higher fraction of mesopores in AC composite compared to AC cloth. Yang et al. [13] have shown that the dependence of capacitance on scan rate depends on several factors such as electrolyte concentration, applied voltage, and pore size distribution (PSD). Typically, at high electrolyte concentrations the capacitances obtained by slow and fast rates show little difference. At low electrolyte concentrations the capacitance of the electrode is high at low scan rate and low at higher scan rate. 3.4. CV results Fig. 3 shows cyclic voltammograms for PAC + Binder electrode and also for PAC + MCCB + binder electrode at scan rate of 5 mV/s. Use of low levels of MCCB resulted in a rectangular shaped CV curve indicative of an ideal electrosorption capacitive behavior. The voltammogram of PAC-binder electrode is distorted which may be due to the slow ion transport in micropores. It is therefore proposed that PACbinder electrode is not suitable for quick charge/discharge of ions. In contrast, the PAC-binder-MCCB electrode shows excellent rate capacity due to the presence of mesopores. It has been reported by many investigators that the cyclic voltammogram of an ideal doublelayer capacitor has a rectangular shape. Rectangular shaped cyclic voltammograms have been reported for electrodes such as carbon aerogels [15], carbon electrode with Poly (vinylidene fluoride) (PVDF) binder [32,34], titanium dioxide modified activated carbon electrode [18], polyvinyl alcohol (PVA)-bonded activated carbon electrode [19], and many other high performance electrodes. 60 50
Sp.Capacitance, F/g
optimum level of PVDF binder was found to be 10–13% (by wt.). Li et al. [10] have used PTFE as binder to fabricate an electrode comprising grapheme nano flakes. They used 8% PTFE binder and 20% graphite conductive material. Low levels of ethanol were used to wet the mixture.
185
40 30 20 10 0 0
20
40
60
80
100
Scanrate, mV/s Fig. 2. Effect of scan rate on capacitance of electrode material. Electrode composition: 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB (TIMCAL Ensaco 350G) electrode (♦).
186
S. Nadakatti et al. / Desalination 268 (2011) 182–188 Table 4 Salt removal and recovery for 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB electrode. Electrode % Salt Removal % Recovery
Fig. 3. Cyclic voltammetry curve for powdered activated carbon with MCCB (—) and without MCCB (—).
Zou et al. [3] have shown that the CV curve for ordered mesoporous carbon (OMC) electrode maintains the rectangular shape in the voltage sweep rate 1 to 10 mV/s. In OMC, 82% of the total surface area comprised of mesopores. By contrast, the shape of the CV curve, for the activated carbon (AC) electrode comprising largely micropores, was distorted. The findings of the present study relating to shape of the CV curve are therefore consistent with that reported in the literature. 3.5. Comparison with commercial electrodes The capacitance of the electrode of the present study at scan rate of 5 mV/s is 45.0 F/g. The capacitance of commercially available carbon aerogel and activated carbon cloth under comparable experimental conditions were 30.0 and 31.6, respectively (cf. Table 3). High capacitance of the electrode of present study can be attributed to very high levels of mesopores that are present in the conductive carbon black that was incorporated at 10% (w/w). The MCCB used in present study has a mesopore area of 455 m2/g. The capacitance of carbon aerogel has been studied by many investigators such as [12,13,15], etc. The activated carbon cloth from Spectracarb Corp. has been studied by Conway et al. [37], Ayranci et al. [38] and Niu and Conway [39,40]. The capacitance values obtained in the present study cannot be directly compared with that reported in literature since the experimental conditions were different. However, both carbon aerogel and Spectracarb 2225 are reported to have high capacitance, rectangular shaped cyclic voltammograms, and good electrode performance characteristics. 3.6. Salt removal and recovery The salt removal and recovery data for the electrodes, before and after the inclusion of MCCB is tabulated in Table 4. Both removal of dissolved salts from water and recovery improved significantly by incorporation of 10% of MCCB in the electrode composition. Removal of dissolved salts from tap water and from NaCl feed solution was about 30–35% for electrode without incorporation of MCCB at an
Table 3 Comparison of electrode capacitance. Electrode
Capacitance at scan rate 5 mV/s
Carbon aerogel Carbon cloth Present studya
30.0 31.6 45.0
a 70% (wt) powdered activated carbon, 20% binder, 10% MCCB (TIMCAL Ensaco 350G).
Without MCCB With MCCB Without MCCB With MCCB
Tap Water (~670 ppm)
NaCl Solution (1000 ppm)
35 75 25 55
30 70 25 55
applied voltage of 1.2 V and input TDS of ~ 1000 ppm. The flow rate was 10 ml/min. Incorporation of 10% MCCB in the electrode resulted in % salt removal going up to 70%. MCCB incorporation also helped in increasing the recovery from 25% to 55% in both NaCl feed solution and tap water. Choi [32] has reported salt removal for an electrode made from activated carbon powder and PVDF binder. The average salt removal efficiency at a flow rate of 20 ml/min was 77.8%. However, the input salt levels in Choi's study were significantly lower at 200 ppm compared to input levels in present study at 1000 ppm and therefore salt removal efficiency is not strictly comparable. Seo et al. [6] have investigated removal of hardness ions using activated carbon cloth and activated carbon composite. They reported 80% of Ca and Mg ions removal after 3 min of operation at a flow rate of 16 ml/min for one pair of electrodes. The electrode cell was operated with 4 min of purification cycle and 1 min regeneration cycle thus yielding a recovery of 80%. The salt removal and recovery of present work cannot be directly compared with that reported by Seo et al. since the feed water composition and electrode surface area used in two studies are very different. One of the key advantages of capacitive deionization technology over conventional reverse osmosis (RO) technology for removal of dissolved salts from water is higher recovery of feed water. In conventional RO, recovery is typically 15% i.e., for 10 l of feed water, only ~ 1.5 l of purified water is obtained. In the present study the recovery using CDI technology is significantly higher at 55%. Thus there is less wastage of water in CDI technology. The other key advantage of CDI over RO is that CDI is a low pressure process. In the present study gravity pressure was sufficient to carry out deionization of feed water. In RO high pressures (~100 psi) are required which is achieved using a pump. Anderson et al. [5] have indicated that CDI due to its low pressure operation can potentially result in 1/3 less energy consumption compared to RO for brackish water treatment. 3.7. Charge–discharge curves Cyclic tests of purification and regeneration were performed for the evaluation of CDI applicability. Fig. 4 shows charge–discharge curves for 9 cycles. The feed water was 670 ppm tap water. During the purification stage, low conductivity of treated water was obtained. During regeneration process, on the other hand, higher conductivity than that of initial feed solution was obtained due to the extracted ions from the electrical double layers in both electrodes. The conductivity-time plot shows the following features: (i) charge– discharge response is very fast and (ii) high reversibility of adsorption-desorption behaviour. These two are key parameters in selection of electrodes for use in CDI technology. The % salt removal (as determined by % reduction in conductivity) is ~ 70% and recovery is ~ 55%. Fig. 5 shows the performance of the electrode for an extended period of usage of more than 7 h. It was found that the performance of the electrode does not deteriorate with repeat cycles. Seo et al. [6] in their study of selective removal of divalent ions from water have reported similar charge–discharge curves for activated carbon cloth wherein cyclic tests were performed with the
S. Nadakatti et al. / Desalination 268 (2011) 182–188
187
Voltage Conductivity 1.4
3.0
1.0
0.6
Volts (V)
2.0 0.2 1.5 -0.2 1.0 -0.6 0.5
-1.0
-1.4 0:00
Conductivity (mS/cm)
2.5
0.0 0:14
0:28
0:43
0:57
1:12
1:26
1:40
1:55
2:09
Time (h:mm) Fig. 4. Charge–discharge, conductivity (—), voltage (—) graph for 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB electrode. Feed water is 670 ppm tap water.
five-cell stack. Cyclic test data has been reported for 4 cycles and electrode performance was found to be consistent. 4. Conclusions The optimum composition of the electrode for use in CDI technology is 70% (w/w) PAC, 10% MCCB, and 20% polymeric binder. Capacitance of an electrode comprising 80% PAC and 20% polyethylene binder at scan rate of 5 mV/s was 13.8 F/g. The capacitance could be enhanced to 45.0 F/g by replacing 10% of the PAC with MCCB having a total surface area of 706 m2/g and mesopore area of 455 m2/ g. The capacitance of commercially available carbon aerogel and
activated carbon cloth under comparable conditions were 30.0 F/g and 31.6 F/g respectively. Thus, the capacitance of electrodes made from PAC, polymeric binder, and low levels of MCCB is high and comparable to commercially available carbon aerogel and activated carbon cloth. Incorporation of low levels of MCCB in electrodes made from PAC and polymeric binder resulted in a rectangular shaped (mirror-imaged) CV curve indicative of an ideal electrosorption capacitive behavior. The charge/discharge response is fast. These characteristics have enabled high removal of dissolved salts from feed water (~75% removal) at recovery of ~ 55%. The performance of the electrodes was tested for an extended period of time (7 h of operation and 30 cycles) and found to be consistent. The process for preparing Voltage Conductivity
1.4
1.0
3.0
0.6
Volts (V)
2.0 0.2 1.5 -0.2 1.0 -0.6 0.5
-1.0
-1.4 0:00
Conductivity (mS/cm)
2.5
0.0 1:00
2:00
3:00
4:00
5:00
6:00
7:00
Time (h:mm) Fig. 5. Extended run data of charge–discharge, conductivity (—), voltage (—) for 70% (wt.) powdered activated carbon, 20% binder, 10% MCCB electrode. Feed water is 670 ppm tap water.
188
S. Nadakatti et al. / Desalination 268 (2011) 182–188
electrodes does not require any organic solvent. PAC electrodes containing low levels of MCCB can potentially be ideal materials for use in capacitive deionization technology. Acknowledgements The authors thank Raghunandan S. for his help in experiments and Venkataraghavan R. for his guidance in preparing the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14]
Y. Oren, Desalination 228 (2008) 10–29. L. Zou, G. Morris, D. Qi, Desalination 225 (2008) 329–340. L. Zou, L. Li, H. Song, G. Morris, Water Research 42 (2008) 2340–2348. K.Y. Foo, B.H. Hameed, Journal of Hazardous Materials 170 (2009) 552–559. M.A. Anderson, A.L. Cudero, J. Palma, Electrochimica Acta 55 (2010) 3845–3856. S.J. Seo, H. Jeon, J.K. Lee, G.Y. Kim, D. Park, H. Nojima, J. Lee, S.H. Moon, Water Research 44 (2010) 2267–2275. R. Broseus, J. Cigana, B. Barbeau, C.D. Martinez, H. Suty, Desalination 249 (2009) 217–223. Y.J. Kim, J. Hur, W. Bae, J.H. Choi, Desalination 253 (2010) 119–123. L.Y. Lee, H.Y. Ng, S.L. Ong, G. Tao, K. Kekre, V. Balakrishnan, W. Lay, H. Seah, Water Research 43 (2009) 4769–4777. H. Li, L. Zou, L. Pan, Z. Sun, Separation and Purification Technology 75 (2010) 8–14. B.E. Conway, Electrochemical Supercapacitors, Scientific fundamentals and technological applications, Kluver Academic / Plenum Publishers, New York, 1999, pp. 615–616. K. Yang, T. Ying, S. Yiacoumi, C. Tsouris, E.S. Vittoratos, Langmuir 17 (2001) 1961–1969. K. Yang, S. Yiacoumi, C. Tsouris, Journal of Electroanalytical Chemistry 540 (2003) 159–167. T. Ying, K. Yang, S. Yiacoumi, C. Tsouris, Journal of Colloid and Interface Science 250 (2002) 18–27.
[15] C.H. Hou, C. Liang, S. Yiacoumi, S. Dai, C. Tsouris, Journal of Colloid and Interface Science 302 (2006) 54–61. [16] P. Xu, J.E. Drewes, D. Heil, G. Wang, Water Research 42 (2008) 2605–2617. [17] M.W. Ryoo, G. Seo, Water Research 37 (2003) 1527–1534. [18] M.K. Seo, S.J. Park, Current Applied Physics 10 (2010) 391–394. [19] B.H. Park, J.H. Choi, Electrochimica Acta 55 (2010) 2888–2893. [20] L. Li, L. Zou, H. Song, G. Morris, Carbon 47 (2009) 775–781. [21] H. Li, Y. Gao, L. Pan, Y. Zhang, Y. Chen, Z. Sun, Water Research 42 (2008) 4923–4928. [22] Y. Gao, L. Pan, H. Li, Y. Zhang, Z. Zhang, Y. Chen, Z. Sun, Thin Solid Films 517 (2009) 1616–1619. [23] L. Pan, X. Wang, Y. Gao, Y. Zhang, Y. Chen, Z. Sun, Desalination 244 (2009) 139–143. [24] H. Li, L. Pan, Y. Zhang, L. Zou, C. Sun, Y. Zhan, Z. Sun, Chemical Physics Letters 485 (2010) 161–166. [25] P.M. Biesheuvel, A. Wal, Journal of Membrane Science 346 (2010) 256–262. [26] Y.J. Kim, J.H. Choi, Separation and Purification Technology 71 (2010) 70–75. [27] Y.J. Kim, J.H. Choi, Water Research 44 (2010) 990–996. [28] J. Kim, J. Choi, Journal of Membrane Science 355 (2010) 85–90. [29] P.M. Biesheuvel, Journal of Colloid and Interface Science 332 (2009) 258–264. [30] J. Lee, W. Bae, J. Choi, Desalination 258 (2010) 159–163. [31] R.W. Pekala, J.C. Farmer, C.T. Alviso, T.D. Tran, S.T. Mayer, J.M. Miller, B. Dunn, Journal of Non-Crystalline Solids 225 (1998) 74–80. [32] J.H. Choi, Separation and Purification Technology 70 (2010) 362–366. [33] K. Park, J. Lee, P. Park, S. Yoon, J. Moon, H. Eum, C. Lee, Desalination 206 (2007) 86–91. [34] J.Y. Choi, J.H. Choi, Journal of Industrial and Engineering Chemistry 16 (2010) 401–405. [35] J.A. Lim, N.S. Park, J.S. Park, J.H. Choi, Desalination 238 (2009) 37–42. [36] M.K. Kadam, S.M. Nadakatti, M.S. Tendulkar, International patent WO 2009/077276 A1. [37] B.E. Conway, J. Niu, W.G. Pell, Kemical Industry 54 (4) (2005) 187–198. [38] E. Ayranci, B.E. Conway, Journal of Applied Electrochemistry 31 (2001) 257–266. [39] J. Niu, B.E. Conway, Journal of Electroanalytical Chemistry 546 (2003) 59–72. [40] J. Niu, B.E. Conway, Journal of Electroanalytical Chemistry 529 (2002) 84–96.