Elsevier

Water Research

Volume 37, Issue 7, April 2003, Pages 1527-1534
Water Research

Improvement in capacitive deionization function of activated carbon cloth by titania modification

https://doi.org/10.1016/S0043-1354(02)00531-6Get rights and content

Abstract

Activated carbon cloth (ACC) was modified by the reaction between polar groups on its surface and metal alkoxides of titanium, silicon, aluminum and zirconium to enhance its capacitive deionization (CDI) performance. Incorporated state of metals and surface property of modified ACC were deduced from surface analysis results obtained using FE-SEM, XRD, XPS and zeta-potential meter. Titania was highly dispersed on the ACC surface with tetrahedral coordination, and the incorporated titania was effective to decrease physical adsorption of NaCl and to increase electric field adsorption, resulting in a significant enhancement of CDI performance. The negligible contribution of silica, alumina and zirconia modifications suggested that the small oxidation–reduction potential of titania was responsible for the enhancement of the electric field adsorption. Reversibility of adsorption and desorption operation on titania-modified ACC were also discussed relating to its CDI function.

Introduction

Water is not only essential for living organisms, but also for fundamental resources in industries with various purposes: cleaners, solvents, diluents, coolants, and reactants [1]. Such a variety of water in use is attributed to its high dielectric property, and thus various electrolytes are highly dissolved in water [2], [3]. However, the amounts of dissolved ions in water are important for its applications. For instance, the dissolved amount of ions in ultra-pure water used in nuclear power plants [4], [5] and semiconductor manufacturing processes [6], [7] is strictly regulated to obtain high stability in operation and high production yields. Therefore, many kinds of deionization processes are operated to produce pure water by removing ions.

Water deionization processes can be classified by the use of electric power. Classical processes such as adsorption, ion exchange and reverse osmosis are based on the difference in physico-chemical potentials of ions. Although these processes have been widely used for deionization, they require expensive regeneration steps of adsorbent, ion-exchange resin and membrane. Recently, the electrodialysis system (EDI) operated by electric power has been commercialized as a new method applicable to demineralization of brine water without using hazardous chemicals [8], [9]. A continuous purification of water by only supplying electricity provides a high convenience, but a significant consumption of electricity—for electrolysis of water, producing a vast amount of gas—cannot be avoidable because the EDI is operated with high DC voltage [10].

Capacitive deionization (CDI) means the removal process of ions using capacitive adsorption. Ions are adsorbed onto the surface of porous electrodes by applying electric field, producing deionized water. Adsorbed ions are desorbed from the surface of the electrodes by eliminating electric field, resulting in the regeneration of the electrodes [11].

Since CDI is operated with low voltage electricity, the loss of electricity due to electrolysis of water can be excluded [12]. However, the efficiency of CDI strongly depends upon the surface property of electrodes such as their surface area and adsorption properties [13], [14]. Electrodes with a large number of adsorption sites capable of concentrating ions under an electric field exhibit high CDI functions. High electric conductivity, large surface area and high chemical stability of carbon materials bring about their application as CDI electrodes. Easy processibility and variety in shapes also enhance the feasibility for their application. In spite of these advantages of carbon materials as CDI electrodes the adsorbed amount of ions on carbon surface is not large compared to the exchanged amount of ions in ion-exchange resin, so high compact CDI systems composed of a large number of electrodes were suggested. Stack disk flow-through-capacitor demonstration system with a sufficient surface area for the deionization of metric ton per hour was reported to be under design [15]. Activated carbon [16], carbon aerogel sheet [17] and carbon honeycomb [18] are suggested as effective CDI electrode materials achieving large contact area with water as well as high compactness.

Activated carbon cloth (ACC), a woven product of activated carbon fiber, is suitable for the electrodes of CDI in the aspects of high surface area, high electric conductance and variety in shaping. The large number of micropores developed along the surface causes fast adsorption and desorption of ions along with charging and discharging [19]. Moreover, many functional groups located on the surface are useful to immobilize transition metal atoms [20], which can work as adsorption sites of ions when an electric field is charged.

In this study, ACC modified by reacting with alkoxides of metals such as titanium, zirconium, silicon, and aluminum were used as CDI electrodes. The incorporated state of metal atoms on ACC was examined, and the enhancement of CDI performance with the modification was discussed.

Section snippets

ACC and its modification

ACC (CH900-20, Kuraray Chem., Japan) woven activated carbon fiber of 10 μm in diameter, was used as an electrode material. Its BET surface area was about 2000 m2/g and average pore diameter was 18 Å.

Dried ACC in an oven at 110°C for 24 h was immersed in anhydrous ethanol (>99.5%, Dae Jung Chem. Co., Ltd.) solution containing metal alkoxides: titanium(IV)-butoxide (>99%, Johnson Matthey Co.), zirconium(IV)-n-propoxide (70%, Johnson Matthey Co.), 3-trimethoxypropylsilane (>98%, Aldrich) and aluminum

Characterization

The incorporated amounts of modifying metals on ACC were determined using a thermogravimetric analyzer (TG/DTA 200, Seico). Temperature increased from ambient temperature to 800°C in air flow with a ramping rate of 5°C/min. The remained weights of modified ACC after carbon burning provided the incorporating amounts of metal on ACC, followed by compensating ash content.

Specific surface areas of modified ACC were calculated using the BET equation from adsorption isotherms of nitrogen obtained at

CDI performance

Electrodes with a dimension of 5×5 cm2 were prepared using modified ACC. Current collectors of sus 316 with 32 mesh had the same dimension. Electrodes were placed face to face at both sides of an insulating polyethylene film (Cellgard 3501) and contacted with current collectors as shown in Fig. 1. The assembled electrodes were packed by electrode holders with an open square of 4.5×4.5 cm2.

Adsorption of ions on electrodes was measured using a batch testing apparatus shown in Fig. 2. An assembled

Characterization of modified ACC

Since carbon is burned out in air, TG results in airflow provide the incorporated amounts of metal as metal oxides. Fig. 3 shows typical runs of TG experiments. Ash content of the ACC was 1.9%. By compensating weight increases due to oxide formation and ash content, incorporated amounts of metal could be obtained from the remaining weights. The incorporated amounts of metal on ACC were listed in Table 1. The numbers in the parentheses of electrode names denote the incorporated amounts of metal

Capacitive deionization of modified ACC

Since ACC composed of activated carbon fiber has an extremely large surface area and many charging sites, ions are adsorbed on it. If an electric potential is applied to ACC electrodes assembled face to face, more ions can be adsorbed. Adsorption of ions—either physically or electrically—brought about the decrease in the concentration of the electrolyte as shown in Fig. 7. Profile (a) showed physical adsorption of ions without any electric field. A charging electric field of DC 1.0 V caused

Conclusions

Titania could be highly dispersed on ACC surface by reacting titanium butoxide molecules with ACC. Tetrahedrally coordinated titania reduced polar groups of ACC surface, resulting in a considerable reduction of physical adsorption of NaCl. However, electric field adsorption of NaCl on the ACC was significantly enhanced by titania modification due to the increase in the number of adsorption sites for ions under electric field by the participation of titanium atoms. The reduction of physical

Acknowledgments

This work was supported by the BK21 Project (EI 238) and Hankook Jungsoo Industries Co., Ltd.

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