CHAPTER 4 CHEMICAL MODIFICATION OF ACTIVATED CARBON CLOTH FOR POTENTIAL USE AS ELECTRODES IN CAPACITIVE DEIONIZATION PROCESS

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1 CHAPTER 4 CHEMICAL MODIFICATION OF ACTIVATED CARBON CLOTH FOR POTENTIAL USE AS ELECTRODES IN CAPACITIVE DEIONIZATION PROCESS 4.1 INTRODUCTION Capacitive deionization (CDI) is one of the promising energy efficient technologies for desalination of water. 48,32 Adsorption of ions on to electrode surface during the introduction of electrical potential is a basic concept of CDI. 145 The electrosorption is a process in which a known potential is applied to an electrode of an electrochemical cell. As a result the inorganic ions present in the electrolyte moved to the electrical double layer (EDL) of the electrode surface and if the potential is removed, the ions are released back to the solution. This process operates at low cell voltages and hence it is more energy efficient than electrodialysis and even reverse osmosis when the low salt content feed is treated. 146,41,119 The expensive high surface area carbon aerogel electrodes are used at present in electrochemical treatment procedures and which is a major challenge to be tackled in order to make this process commercially viable. 38,58 Many other alternatives for carbon electrodes such as carbon nanotube composites 99 are being investigated for potential application in CDI. Activated carbon cloth (ACC) is one of the cost effective high surface area electrode material which finds commercial application in wastewater treatment

2 Preparation and activation methods for improving the surface activity and porosity of ACC materials 122,148 including electrochemical activation 149,150 are currently under investigation. These materials are also optimized for potential use as electrodes in supercapacitors in energy storage devices Highly porous nanostructured ACC carbon, 50,126 titania modified material, 59 ordered mesoporous carbon (OMC), 73 carbon naotubes (CNT) and its carbon nanofiber composite, 152 zinc oxide 128 and modified ACC electrodes have been found to be applicable in CDI processes. The current initiatives thus indicate that there is a considerable scope for further studies in this area. In the present work, the effect of chemical activation of commercially available carbon cloth in 1M and 8M HNO 3 was studied and reported. The activated carbon cloth (ACC) materials were systematically characterized by X-ray diffraction meter (XRD) (XRD-6000 SHIMADZU), scanning electron microscopy (SEM) (JSM-6390, JOEL) and BET-surface analyzer to understand the physical characteristics of carbon cloth materials. In addition to the above, the untreated and treated carbon cloth materials were subjected to electrochemical studies such as cyclic voltammetry (CV) and Chronocoulometry (CC) with an electrochemical workstation (CHI6038D, CH Instruments, USA) and the results obtained were reported and discussed EXPERIMENTAL Materials The carbon cloth with a surface area 1000 m 2 /g and pore volume ( cc/g) [HEG Limited, India], graphite sheet [Chemapol Industries, India], graphite conductive adhesive [Electron Microscopy Sciences, India], sodium sulphate anhydrous (Na 2 SO 4 99%) and concentrated nitric acid (HNO 3 70%) [Himedia, India] were used in this study. 49

3 4.2.2 Activation of carbon cloth The carbon cloth was chemically activated in nitric acid by varying the nitric acid concentration (1 and 8 M) for about 9 h. The activated carbon cloth (ACC) was taken out and thoroughly washed with de-ionized water until the wash solution attained neutral ph (7.0 ± 0.2). The ACC samples were then dried at 60 o C for 6 h before fixing it on the graphite paper for electrochemical measurements. 4.3 RESULTS AND DISCUSSION BET Surface analysis The BET surface area analysis was characterized by N 2 - adsorption/desorption isotherm at 77 K. All the samples were gassed at 150 C for 12 h before carrying out the analysis. The obtained N 2 adsorption/desorption profile of untreated carbon cloth, activated carbon cloth in 1 M HNO 3 and 8M HNO 3 are illustrated in Fig. 4.1a, 4.1b and 4.1c respectively. As shown in Fig. 4.1a, the observation of extended hysteresis loop indicates the existence of the macropores in the untreated carbon cloth. But, the hysteresis loop becomes discrete in Fig. 4.1b, in which the hysteresis loop appears at lower relative pressure range ( ) due to the presence of micropores and loop appears at high relative pressure range ( ) due to the occurrence of the macropores in the sample. The macropores found in untreated sample was converted into micropores in the relative pressure range ( ) because of the chemical treatment in 1 M HNO 3 for 9 hours. Further, increasing the HNO 3 concentration (from 1 M to 8 M), the high relative pressure hysteresis loop is completely disappeared and get a clear low pressure hysteresis loop ( ) which clearly indicates the complete formation of micropores in the sample. The above study confirms that macropores found in 50

4 carbon cloth completely converted into micropores by the treatment of 8 M HNO 3 for 9 h. The surface characteristics obtained by BET method on carbon cloth and chemically activated carbon cloths are indicated in Table 4.1. From the table, it was found that the pore size and the pore width of the treated carbon cloth with HNO 3 for 9 h are found to be less than the other samples. This indicates the influence of HNO 3 in decreasing the surface area of the samples effectively. 51

5 Figure 4.1 Adsorption-desorption isotherms of N 2 at 77 K for (a) untreated carbon cloth (b) carbon cloth treated in 1 M HNO 3 and (c) carbon cloth treated in 8 M HNO 3 for 9 h Table 4.1 Surface characteristics obtained by BET method on carbon cloth and chemically activated cloths Samples Untreated carbon cloth Carbon cloth treated with 1M HNO 3 for 9 h Carbon cloth treated with 8M HNO 3 for 9h BET surface area (m 2 /g) Micro pore area (m 2 /g) Micro pore Volume (cm 3 /g) Pore size (nm) Pore width (nm)

6 4.3.2 SEM analysis The SEM picture obtained for untreated carbon cloth and carbon cloth treated in HNO 3 presented in Fig Fig. 4.2a (i & ii) exhibits the SEM images of the untreated carbon cloth material. It shows that each bundles comprised of a number of fibers having 3 µm in diameter. The SEM images of untreated carbon cloth material fibers are quite similar to those already in the literature with diameter having 20 µm. 19 The SEM pictures of treated carbon cloth sample with HNO 3 (1 M and 8 M for 9 h) are indicated in Fig. 4.2b (i & ii) and 4.2c (i & ii) respectively. From the studies, it was observed that the fiber bundles are splited by HNO 3 treatment. Their diameter tends to decrease after the treatment. It was reported that the chemical treatment may enhance the electro-adsorption and electro-desorption characteristics of carbon cloth materials. 58 Hence, the analysis confirmed that chemical modification of carbon cloth can contribute for the better enhancement of physico-chemical characteristics in them. 53

7 Figure 4.2 SEM images of (a) untreated carbon cloth, (b) carbon cloth treated with 1 M HNO 3 and (c) carbon cloth treated with 8 M HNO 3 for 9 h at low resolution (1) and higher resolution of SEM images at selected area are shown in (2) XRD analysis The XRD patterns obtained on untreated carbon cloth, treated carbon cloth in 1 M HNO 3 for 9 h and treated carbon cloth in 8 M HNO 3 for 9 h are indicated in Fig. 4.3 (a), (b) and (c) respectively. These figures exhibit the crystalline behavior of the sample. The intense peaks appeared at 2θ = 24 and 44 are corresponding to the (002) and (100) crystal planes at graphite as reported. 153 These graphitic sites may be involved in the electronic conduction as well as electrosorption of ionic species. 54

8 Figure 4.3 XRD patterns obtained on untreated carbon cloth (a), treated carbon cloth in 1 M HNO 3 for 9 h (b) and carbon cloth treated in 8 M HNO 3 for 9 h (c) Cyclic voltammetry analysis Typical cyclic voltammograms (CV) obtained for untreated carbon cloth and chemically treated carbon cloth in 1 M and 8 M HNO 3 for 9 h at different sweep rates (1, 2 and 4 mv/s) in 0.1 M Na 2 SO 4 are shown in Fig. 4.4(a), (b) and (c) respectively. In the potential range between -0.5 V and +0.5 V, the CV curves of the untreated sample show a linear resistance behavior along with the capacitance loop (Fig. 4.4a). From the results, it was found that the current values are also less sensitive to sweep rate. The specific capacitance of untreated and chemically treated electrode values were calculated and listed in Table The CV curves obtained for HNO 3 treated samples show much higher capacitance values (Fig. 4.4b). The increasing specific capacitance values for chemically modified carbon cloth electrodes may be due to the enhancement of electrosorption capacitance in the materials 55

9 due to the chemical treatment with HNO 3. The CV curves obtained on the treated electrodes in 1 M HNO 3 and 8 M HNO 3 for 9 h Fig. (4.4b and 4.4c) at low scan rates show rectangular shape, which indicates the good capacitive properties of the electrodes. The electrosorption of the electrodes is higher at lower scan rates. It is because of the diffusion of ions from the solution could gain more access to the electrode surface leading to more surface adsorption/desorption of ions. As the scan rate increases, the rectangles of the CV curves for the samples become more slanting as shown in (Fig. 4.4b and 4.4c). However, at higher scan rate, the effective inner surface adsorption of ions would be reduced. Table 4.2 Specific capacitance values obtained for untreated carbon cloth (1 M for 9 h) and treated carbon cloth (8 M for 9 h) by cyclic voltammetry and Chronocoulometry. Samples Specific Capacitance (F/g) CV CC Untreated carbon cloth Carbon cloth treated with 1M HNO 3 for 9 h Carbon cloth treated with 8M HNO 3 for 9 h The specific capacitance for untreated carbon cloth electrodes samples at three different sweep rates are presented in Fig For example, the specific capacity increases from 49 F/g to 154 F/g for the ACC electrode treated in 8 M HNO 3 for 9 h. The specific capacitance values obtained by CV 56

10 experiments are found to depend on the sweep rate due to the time dependence of the ionic charging process. Figure 4.4 Cyclic voltammetry obtained on (a) un treated carbon cloth (b) carbon cloth 1 M HNO 3 for 9h and (c) carbon cloth treated in 8 M HNO 3 for 9 h in 0.1 M Na 2 SO 4 solution at 1, 2 and 4 mv/s. 57

11 Figure 4.5 Comparison of activated carbon cloth specific capacitance for nontreated and 1 M, 4M and 8 M HNO 3 treated 9 h for 0.1M Na 2 SO 4 solution sweep rate 1mV/s (obtained by cyclic voltammetry) Chronocoulometry analysis Chronocoulometry experiments carried out with a charging voltage of 0.25 V and discharging voltage of 0.0 V for 1000 seconds in each step to get the charging (C charge ) and discharging (C discharge ) capacitance values at these two potentials respectively. The ratio between these two values provides the charge recovery achieved during each charge/discharge cycle. Typical chronocoulometric curves obtained for untreated carbon cloth, treated carbon cloth (1 M HNO 3 for 9 h) and treated carbon cloth (8 M HNO 3 for 9 h) in 0.1 M Na 2 SO 4 solution are shown in (Fig. 4.6a, b and c). The capacitance values calculated for all the samples are indicated in Table The results indicate that specific capacity remains fairly high at around 154 F/g for the sample 58

12 treated with HNO 3 (1M and 8M) for 9 h in 0.1 M Na 2 SO 4 solution. The charge recovery ratio also remains high at around 85%. These results are also comparable to the specific capacitance values reported in the literature for nano particle doped ACC electrodes

13 (c) Figure 4.6 Chronocoulometry curves obtained on carbon cloth electrodes with applied potential of 250 mv/s. All the cycles, were obtained in 0.1 M Na 2 SO 4 solution for (a) untreated carbon cloth (b) carbon cloth treated in 1 M HNO 3 and (c) carbon cloth treated in 8 M HNO 3 for 9 h Electrosorption measurements by CDI process A series of electrosorption experiments were conducted by lab scale CDI flow cell. In these experiments, the concentration of TDS feed solution was gradually increased from 100 to 500 mg/l at room temperature at a flow rate of 2 ml/min. The results are shown in Fig 4.7. In addition, electrosorption experiments were conducted using carbon based electrodes such as activated carbon cloth and carbon aerogel (grade I & II) electrodes. The results clearly show that the TDS removal efficiency decreases gradually with increase in the feed of TDS. Table 4.3 shows TDS removal efficiency of carbon electrodes at various concentrations of TDS at constant applied voltage of 1.2 V and flow rate of 2 ml/min. CDI processes showed higher removal efficiency for the first 16 min. From the results (Fig. 4.7 and Table 4.3), it was found that the this may be due to the quick adsorption of ions on the adsorption sites of the 60

14 electrode surfaces as repoted. 68 With increase in treatment time, the adsorption capacity of carbon electrodes decreased, resulting in no adsorption of ions after 80 minutes. 61

15 Table 4.3 The removal efficiency of TDS at various initial concentration ( ppm) at constant applied voltage 1.2 V and constant flow rate 2 ml/min Time Removal efficiency (%) (min) Carbon aerogel-i (ppm) Carbon aerogel-ii (ppm) Activated carbon cloth (ppm)

16 a b c Figure 4.7 The electrosorption removal efficiency obtained in CDI flow cell (a) carbon aerogel-i, (b) carbon aerogel-ii and (c) activated carbon cloth 63

17 (treated in 8 M HNO 3 for 9 h) electrodes at various initial feed TDS at flow rate of 2 ml/min and room temperature Comparison of three carbon electrodes Effect of TDS concentration The electrosorption removal efficiency was compared for three carbon electrodes as shown in Fig 4.8a. From the results, it was noticed that the electrosorption removal efficiency was found to be 39-24% for carbon aerogel-i, 60-41% for carbon aerogel-ii and 74-49% for activated carbon cloth electrodes for different concentration of feed solution after 16 minutes. These results clearly indicate that activated carbon cloth electrode achieved higher removal efficiency. This may be due to higher surface area and higher specific capacitance of ACC as reported. 50 a Concentration of TDS 64

18 b Concentration of TDS Figure 4.8 Comparison of three carbon electrode materials based on (a) TDS removal efficiency and (b) charge efficiency at various concentration of the feed solution after 16 minutes in a CDI flow cell The charge efficiency of the electrical double layer (EDL) is an important property for the porous carbon electrodes, which refers to the ratio between the amount of salt ions adsorbed from the bulk solution and the amount of electronic charge transferred between the electrodes. The dependence of charge efficiency on the salt concentration of the feed solution ( ) ppm at a constant flow rate of 2 ml/min and applied voltage of 1.2 V is shown Fig 4.8b. It is clearly shown that the charge efficiency decreased gradually from 38 to 25% for carbon aerogel-i, 54 to 42.3% for carbon aerogel-ii and 76.6 to 62% for activated carbon cloth electrodes for different concentration of feed solution after 16 minutes. These results are consistent with the results reported by Biesheuvel. 154 In addition; these results confirm the behavior of charge efficiency predicted by the Gouy-Chapman-Stern double layer model, which indicated that the charge efficiency was higher at lower salt concentrations. When the feed salt concentration is low, lower 65

19 charge will be tranferred between the CDI electrodes. At the same time, the electrodes are still able to adsorb the salt ions as equivalent to the transferred charge. This confirms that CDI is more suitable for desalting process relatively low salt concentration water Effect of applied voltage Fig 4.9a shows the TDS removal efficiencies and charge efficiencies for a concentration of feed solution at 100 ppm and flow rate of 2 ml/min at various applied voltage 0.8, 1.0 and 1.2 V respectively in a CDI flow cell. Once the electrical voltage was applied during the electrosorption process, the salt concentration in the feed solution decreased dramatically as ions were quickly adsorbed onto the surface of the electrode. The obtained results show that electrosorption removal efficiency increased with the increase of applied potential. The higher voltage seems to enhance the electrostatic force between the electrode and adsorbed ions. Fig. 4.9b depicts the variation of the charge efficiency related to various applied voltages. The charge efficiency increased from % when the applied voltage was increased from 0.8 to 1.2 V for different carbon electrodes at flow rate 2 ml/min for an initial feed concentration of 100 ppm. It can be concluded that the charge efficiency was directly related to the applied voltage. a 66

20 b Figure 4.9 Comparison of three electrodes in effect of applied voltage of (a) removal efficiency and (b) charge efficiency at different applied voltage 0.8, 1.0 and 1.2 V and flow rate of 2mL/min at concentration of TDS 100 ppm and room temperature 4.4 CONCLUSION A simple study to evaluate the electrochemical characteristics of the carbon cloth material (untreated and treated with HNO 3 ) in the capacitive deionization (CDI) process for the waste water treatment is reported. BET studies obtained on the treated carbon cloth using 8 M HNO 3 resulted as a component with micropores which is required for the enhancement of ionadsorption characteristics in CDI process. The XRD studies revealed the crystalline behavior at both untreated and treated samples. The electrochemical studies based on cyclic voltammetry and chronocoulometry exhibited a better specific capacitance ( F/g) values for the treated carbon material (with 8M HNO 3 for 9 hours). The desalination experiment was performed for different carbon electrodes based on the effect of TDS concentration and the effect of various applied potential in a CDI flow cell. It was found that the electrosorption removal efficiency and charge efficiency were directly related 67

21 to the applied voltages. This was mainly due to the increase of the electrostatic force between the ions and the CDI electrodes. Moreover, treated carbon cloth materials with HNO 3 can be used effectively in CDI process for the treatment of waste water. 68