The carbon cloth with a surface area 1000 m²/g and pore volume (0.3-0.4 cc/g) [HEG Limited, India], graphite sheet [Chemapol Industries, India], graphite conductive adhesive [Electron Microscopy Sciences, India], sodium sulphate anhydrous (Na₂SO₄ 99%) [Himedia, India] and concentrated nitric acid (HNO₃ 70%) [Himedia, India] were used in this study.

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℃ for 6 h before fixing it on the graphite paper for electrochemical measurements.

The carbon cloth (untreated and activated carbon cloth) was fixed on a graphite paper through graphite conducting adhesive to ensure good electrical contact. Two identical electrodes facing each other (1 cm²) served as working and counter electrodes in a 25 mL capacity single compartment electrochemical cell. A freshly prepared Ag/Ag₂SO₄ electrode stored in saturated Na₂SO₄ solution served as the reference electrode since aqueous Na₂SO₄ solution was mainly used as the electrolyte in CDI studies. The schematic representation of the cell setup is given in Fig. 1.

The BET surface area analysis was carried out by N₂-adsorption/desorption isotherm at 77 K. All the samples were gassed at 150℃ for 12 h before carrying out the analysis. The obtained N₂ adsorption/desorption profile of untreated carbon cloth and activated carbon cloth in 1 M HNO₃ and 8 M HNO₃ are illustrated in Fig. 2a, 2b and 2c respectively. As shown in Fig. 2a, the observation of extended hysteresis loop appears at high relative pressure range (0.4-0.8) indicates the existence of the macropores size in the untreated carbon cloth. But, the hysteresis loop becomes discrete in Fig. 2b, in which the hysteresis loop appears at lower relative pressure range (0.0-0.4) due to the presence of micropores in the sample. The macropores found in untreated sample was converted into micropores in the relative pressure range (0.0-0.4) because of the chemical treatment in 1 M HNO₃ for 9 h. Further increasing the HNO₃ concentration (from 1 M to 8 M), the high relative pressure hysteresis loop is completely disappeared and resulted with a clear low pressure hysteresis loop (0.01-0.2) which clearly specifies that the complete formation of micropores the sample. The above study confirms that macropores found in untreated carbon cloth completely has been converted into micropores by the treatment of 8 M HNO₃ for 9 h.

The surface characteristics obtained by BET method on carbon cloth and chemically activated carbon cloths are indicated in Table 1. From the table, it was found that the pore size and the pore width of the treated carbon cloth with HNO₃ for 9 h are found to be less than the other samples. This indicates the influence of HNO₃ in decreasing the surface area of the samples in an effective manner.

The SEM pictures obtained an untreated carbon cloth and carbon cloth treated in HNO₃ are presented in in Fig. 3a, 3b and 3c respectively. SEM images of the untreated carbon cloth material are shown in Fig. 3a (i & ii). Carbon cloth having bundles with fibers having 3 µm in diameter is shown Fig. 3a. 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 HNO₃ (1 M and 8 M for 9 h) are indicated in Fig. 3b (i & ii) and 3c (i & ii) respectively. Fig. 3 (a) and Fig. 3 (b) have shown the reduction in the diameter of the carbon fibers due to the chemical treatment of HNO₃ acid. It was reported that the chemical treatment may enhance the electro-adsorption and electro-desorption characteristics of carbon cloth materials [8]. Therefore, the chemical treatment analysis has contributed significantly in the chemical modification of carbon cloth materials which may enhance their performance in the electrochemical studies.

The XRD patterns obtained on untreated carbon cloth, treated carbon cloth in (1 M HNO₃ for 9 h) and treated carbon cloth in (8 M HNO₃ for 9 h) are indicated in Fig. 4 (a), (b) and (c) respectively. From the figures, it was found that the samples exhibited the crystalline behavior. The intense peak appeared at 2θ = 24° and 44° may be attributed to the (002) and (100) crystal planes for graphite as reported in the literature [23]. These graphitic sites may be involved in the electronic conduction as well as electrosorption of ionic species during the electrochemical treatment of water.

Typical cyclic voltammograms (CV) obtained with untreated carbon cloth and chemically treated carbon cloths (in 1 M and 8 M HNO₃ for 9 h) at different sweep rates (1, 2 and 4 mV/s) in 0.1 M Na₂SO₄ are shown in Fig. 5(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. 5a). From the results, it was found that the resulted current values may be less sensitive to the sweep rate. The specific capacitance values of the untreated and chemically treated electrodes were calculated and indicated in Table. 2. The CV curves obtained for HNO₃ treated samples have shown higher capacitance values (Fig. 5b). The increase in specific capacitance values for the chemically modified carbon cloth electrodes may due to the enhancement of electrosorption capacitance in these materials due to the chemical treatment with HNO₃ [18]. The CV curves obtained on the treated electrodes [in 1 M HNO₃ and 8 M HNO₃ for 9 h] at low scan rates show rectangular shaped curve (Fig. 5b and 5c), 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 to the electrode surface due to adsorption/desorption mechanism [9]. As the scan rate increases, the rectangular shape of the CV curves for the chemically treated samples became more slanting as shown in Fig. 5b and 5c. This may be due to the higher electrosorption behavior of the samples because of the chemical treatment with nitric acid. However, the effective inner surface adsorption of ions was reduced at higher scan rates in all the samples as reported [9]. The difference in anodic (i_a) and cathodic (i_c) current was used to calculate the approximate specific capacitance in F_{(C Charge)} value as indicated in equation (1).

where ‘C’ is the specific capacitance (F/g), ‘dv/dt’ is the sweep rate in V/s, ‘m’ is mass of activated carbon cloth and ‘i_a’ and ‘i_c’ are the anodic and cathodic current in A respectively. The weight ‘m’ of 1 cm² electrode material is found to be 0.016 g. From Table 2, it was found that the specific capacitance value of the untreated sample increased from 49 F/g to 154 F/g due to the chemical treatment with 8 M HNO₃ for 9 h. The specific capacitance values obtained by CV experiments for the samples are found to depend on the sweep rates due to the time dependence of the ionic charging process [9,18].

Chronocoulometry experiments were 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. The charging capacitance is calculated based on the following equation (2).

Where ‘C_R’ is charge recovery, ‘Q_f’ forwarded charge in coulombs, ‘Q_r’ reverse discharge in coulombs respectively. Typical chronocoulometric curves obtained on untreated carbon cloth, treated carbon cloth (1 M HNO₃ for 9 h) and treated carbon cloth (8 M HNO₃ for 9 h) in 0.1 M Na₂SO₄ solution are shown in (Fig .6a, b and c). The capacitance values calculated for all the samples are indicated in Table 2. The results indicated that the specific capacity remains fairly high at around 104 and 154 F/g respectively for the samples treated with HNO₃ (1 M and 8 M) for 9 h in 0.1 M Na₂SO₄ solution. The charge recovery ratio was also remained high at around 85% for both the treated samples. These results are also comparable to the specific capacitance values (120 F/g) reported in the literature for the TiO₂ nanoparticle coated ACC electrodes obtained using chronocoulometry [19]. However, the specific capacitance obtained for the sample treated with 8 M HNO₃ is found to be good when compared with the reported result.