Capacitive desalination of WO₃/carbon cloth supercapacitor and morphology analysis

Capacitive deionization (CDI) is a technology of deionizing water by use electrochemical method. Through applying an electrical potential difference over two capacitive electrodes, these ions will be removed from the water due to being absorbed on the polarized electrode with opposite charges.

Therefore, in order to improve the ion removing efficiency for a CDI system, the capacitive property of the electrodes is an important factor. According to recent reports, the combination of metal oxide with carbon materials has great potential to improve further the specific capacitance and thus the efficiency of capacitance deionization (CDI) systems.

To date, several different kinds of metal oxides have been frequently investigated for CDI applications (Yang et al. 2012; Liu et al. 2013; Yin et al. 2013; Kim et al. 2014; Myint et al. 2014). Although the involvement of these metal oxides improves the electrosorption capacity, they have poor stability when exposed to acidic or basic solutions. In contrast, tungsten oxide exhibits better stability and it is already the preferable candidate as the cation storage container in the electrochromic applications (Gui & Blackwood 2013, 2014). However, there is a lack of reports on the use of WO₃ decorated carbon-based materials for CDI applications. Hence, in this work the carbon cloth (CC) coated with tungsten oxide was employed as cathode for CDI applications, which was firstly synthesized by self-anodization method.

The carbon cloth was immersed into 250 ml mixture solutions of concentrated H2SO4 and HNO₃ with a volume ratio of 1:1 under 80 °C for 24 h, then washing by deionized water (DI water) followed by rinsing at 100 °C for 1 h and waiting for use.

Placing carbon cloth into the chamber of a DC Magnetron Sputter. The distance between pure tungsten target and the stage was 8 cm. The sputtering of W was conducted in an inert argon atmosphere (2.0 × 10⁻⁶ torr) with a sputtering power of 100 watt.

Four pieces of W/CC, each with dimensions of 2.5 cm × 5 cmm were immersed into 50 ml mixture solutions of 3 M HCl and 3 M HNO₃ with different volume ratio of 1:0, 1:1, 5:1 and 0:1 respectively at room temperature for 24 h. Next the cloths were washed by deionized water (DI water) followed by rinsing at 100 °C for 1 h and waiting for characterization. These anodized products are named as WO₃/CC-a, WO₃/CC-b, WO₃/CC-c and WO₃/CC-d correspondingly.

The self-anodized WO_X was uniformly distributed on the entire carbon cloth and mainly composed of a flaky morphology. These as-prepared WO_X/CC are interconnected to form pore structures as seen from their top view image and vertically adhered on the carbon cloth from the corresponding cross section as displayed in Figure 1. Accordingly, from their morphology differences, it can be deduced that the addition of HNO₃ has assisted in accelerating the oxidation of tungsten. This is indicated by the finding of tungsten residues in Figure 1, in which the obvious particles under the formed flakes are the sputtered tungsten atoms. In addition, the atomic ratio of W/O is 1:1 for WO₃/CC-a and 1:3 for , WO₃/CC-b, WO₃/CC-c and WO₃/CC-d, as confirmed through the SEM-EDS, which proves the fully oxidized tungsten in the appearance of HNO₃ in the solution. BET surface analysis revealed that the specific surface area of WO₃ deposited carbon cloth is 20 m²·g⁻¹ on average, 25 times of 0.8 m²·g⁻¹ of the bare carbon cloth. This enhancement will give contributions to the ion adsorption needed in CDI applications.

The preliminary desalination experiments were conducted in the static 1 M NaCl aqueous solution over a potential range from −1 V to 0.2 V vs SCE (Figure 2(a)). By integrating the current densities in the cyclic voltammograms in Figure 2(a), it was found that the absorbed amount of ions on the modified carbon cloth is 21.4, 36.6, 41.5 and 24.4 times of bare carbon cloth for WO₃/CC-a, WO₃/CC-b, WO₃/CC-c and WO₃/CC-d respectively (Figure 2(b)). The enhancement exhibits their advantages in ion removing abilities for the CDI applications.

The chronoamperometry or charge/discharge curve (Figure 3) provides an assessment of the salt removal ability. The result reveals that the amount of ions electrosorbed by the WO₃/CC is an order of magnitude higher than the bare carbon cloth on average. Besides, through comparing the specific capacitance among all these four WO₃/CC materials, the one anodized under the mixture acidic solution with 17 vol. % HNO₃ shows the best performance as a capacitor for CDI applications.

These preliminary results show a promising enhancement of WO₃/CC in the aspect of surface area, capacitance and ion electrosorption abilities as compared to the bare carbon cloth, which, therefore, exhibit great advantages on CDI applications.

This research grant is supported by the Singapore National Research Foundation under its Environmental & Water Research Programme (Project Ref No: 1301-IRIS-33) and administered by PUB, Singapore's national water agency.