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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.31 No.5 pp.552-559
DOI : https://doi.org/10.14478/ace.2020.1064

Activated Carbon-Nickel (II) Oxide Electrodes for Capacitive Deionization Process

Karl Adrian Gandionco*,**, Jin Won Kim***, Joey D. Ocon*,***, Jaeyoung Lee**,***,†
*Laboratory of Electrochemical Engineering, Department of Chemical Engineering, College of Engineering, University of the Philippines Diliman, Quezon City 1101, Philippines
**Electrochemical Reaction and Technology Laboratory, School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, South Korea
***Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, South Korea
†Corresponding Author: School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Electrochemical Reaction and Technology Laboratory, Gwangju 61005, South Korea Tel: +82-62-970-2440 e-mail: jaeyoung@gist.ac.kr
August 18, 2020 ; September 13, 2020 ; September 14, 2020

Abstract


Activated carbon-nickel (II) oxide (AC-NiO) electrodes were studied as materials for the capacitive deionization (CDI) of aqueous sodium chloride solution. AC-NiO electrodes were fabricated through physical mixing and low-temperature heating of precursor materials. The amount of NiO in the electrodes was varied and its effect on the deionization performance was investigated using a single-pass mode CDI setup. The pure activated carbon electrode showed the highest specific surface area among the electrodes. However, the AC-NiO electrode with approximately 10 and 20% of NiO displayed better deionization performance. The addition of a dielectric material like NiO to the carbon material resulted in the enhancement of the electric field, which eventually led to an improved deionization performance. Among all as-prepared electrodes, the AC-NiO electrode with approximately 10% of NiO gave the highest salt adsorption capacity and charge efficiency, which are equal to 7.46 mg/g and 90.1%, respectively. This finding can be attributed to the optimum enhancement of the physical and chemical characteristics of the electrode brought by the addition of the appropriate amount of NiO.


Capacitive deionization , Nickel (II) oxide , Activated carbon , Salt adsorption capacity , Charge efficiency

초록

    1. Introduction

    The scarcity of clean water has remained a global issue for the past several decades. According to the United Nations Water (UN-Water), two-thirds of the world’s population currently live in areas that experience water scarcity for at least one month a year, and about 500 million people live in areas where water consumption exceeds the locally renewable water resources by a factor of two[1]. Water scarcity is becoming more severe by the year as the demand for clean water has been increasing brought by the unhindered growth of the global population. The current world population is 7.6 billion, and it is projected to reach up to 9.8 billion in 2050[2]. Moreover, the irresponsible exploitation of water resources has resulted in a shortage of fresh water supply. It was reported that only 0.5% of the 1.4 billion cubic kilometers of water in the world is accessible fresh water[3,4]. The current state of the world’s fresh water supply and the vastness of sea water have paved the way for the development of desalination technologies as alternative sources of clean water[4,5]. Membrane separation systems, specifically reverse osmosis and electrodialysis, and thermal sep-aration methods, particularly multistage flash distillation, multi-effect distillation, and mechanical vapor compression, are the most widely used processes for carrying out desalination[6]. These technologies are relatively well developed, but most of them are energy-intensive and are not cost-effective in terms of infrastructure cost. At present, the common goal of most researchers around the world is to develop alternative technologies that require less energy and are cost-effective. One of the most promising technologies that is currently studied is capacitive deionization (CDI).

    Capacitive deionization is an emerging and attractive technology for the removal of ions from aqueous solutions[7,8]. In CDI, ions are removed by applying an electrical voltage difference between two porous carbon electrodes. Upon the application of voltage difference, the ions are electrosorbed in the electrodes. Many electrochemical processes can play a role in CDI, but the formation of an electrical double layer is considered as the main mechanism by which ions are immobilized and removed from aqueous solutions[7].

    Carbons have been the electrode material of choice for CDI because of their availability in a variety of forms, cost-effectiveness, stability, porosity, and high specific surface area[9]. Several carbon materials such as activated carbon (AC), activated carbon cloth (ACC), carbide- derived carbon, ordered mesoporous carbon, carbon nanofiber, carbon nanotube, carbon aerogel, carbon black, and graphene have been intensively studied as electrodes for the deionization of aqueous salt solutions[10]. Among all the carbon materials, AC stands out be-cause it is the most cost-efficient[7]. Activated carbon has been used as CDI electrode material since the early years of the technology. It was employed as the electrode material in one of the groundbreaking studies in CDI[11]. More recent CDI studies that used activated carbon as electrode performed some modifications in the pristine carbon material to improve its deionization efficiency[12-17].

    One of the drawbacks of carbon materials is that they have low polarization resistance, which leads to a reduction in the electric field and the deionization performance[18]. To address this problem, researchers have investigated the use of dielectric materials (e.g. TiO2, ZrO2, and ZnO) as additives to carbon materials. Dielectric materials can strengthen the electric field through self-polarization, which eventually increases the capacitance and salt adsorption capacity[18-20]. Moreover, dielectric materials specifically transition metal oxides can improve CDI efficiency by providing additional adsorption sites for the ions, enhancing electrode wettability, improving the oxygen reduction reaction, and reducing physical adsorption[16,21-23]. Several transition metal oxides were investigated as additives to carbon-based CDI electrodes, such as manganese oxide, titanium dioxide, and zinc oxide[24].

    Aside from manganese oxide, titanium dioxide, and zinc oxide, nickel (II) oxide (NiO) is also a promising additive to carbon materials for CDI electrodes. Nickel (II) oxide-carbon materials have been studied as electrodes for supercapacitor applications because of their relatively high specific capacitance (theoretical specific capacitance of 3750 F/g), inexpensiveness, non-toxicity, and hydrophilicity[25-30]. Even though NiO has attractive characteristics, there has been no study that employs it as an electrode material for CDI.

    2. Experimental

    2.1. Electrode fabrication

    Activated carbon-nickel (II) oxide electrodes were prepared by stirring a mixture of PVDF binder, activated carbon (YP50-F from Kuraray), and NiO (< 50 nm particle size, Sigma-Aldrich) in NMP solvent at 60 degree Celsius for a total of 3 hours. After the thermal treatment, the resulting slurry was applied on a graphite sheet (thickness: 0.23 mm) using a doctor blade film applicator. The electrodes were vacuum dried in an oven at 60 ℃ for 12 h. The amount of NiO in the as-prepared electrodes was set at 10, 20, and 30% of the amount of the activated carbon. The corresponding representations for the as-prepared electrodes are AN-10, AN-20, and AN-30 and that of the pure carbon electrode is AC. The dimensions and thickness of all the electrodes were 10 cm × 10 cm and 260 μm, respectively.

    2.2. Structural and chemical characterization

    Field emission-scanning electron microscope (FE-SEM, Hitachi S-4700) was used to investigate the morphology of the as-prepared electrodes. Energy-dispersive X-ray spectroscopy (EDX, Horiba 7200-H) and X-ray diffraction (XRD, Rigaku Miniflex II) were employed to determine the elemental compositions and the bulk crystal structures, respectively. Nitrogen gas sorption experiments were performed at 77 K using a BET analyzer (Bel Japan Inc. Belsorp-Max) to determine the Brunauer-Emmet-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size distribution. The sessile drop method of a standard goniometer with DROPimage was used to examine the electrode wettability. Thermogravimetric analysis (SDT Q600) was performed on hand-milled samples at 10 ℃/min from 20 ℃ to 693 ℃ using a synthetic air which flows at 100 mL/min to determine the NiO content of the as-prepared electrodes.

    2.3. Electrochemical measurements

    The electrochemical performance of the electrodes was investigated by performing cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a 1M aqueous NaCl solution. Electrochemical impedance spectroscopy was performed using a modular electrochemical workstation (Zahner Zennium) in a two-electrode setup. The as-prepared electrode was used as the working electrode and platinum mesh as the counter electrode. The amplitude of the alternating voltage was 5 mV, and the data were collected in the frequency range from 100 mHz to 100 kHz. Cyclic voltammetry was performed using a potentiostat/ galvanostat (Autolab PGSTAT204) in a three-electrode system, which consisted of the as-prepared electrode as the working electrode, Ag/AgCl (3 M KCl, Metrohm) as the reference electrode, and platinum mesh as the counter electrode. The specific capacitance was calculated using the following equation[31]:

    C S = ∫ V 1 V 2 I d V 2 m υ ( V 2 − V 1 )
    (1)

    where CS is the specific capacitance, I is current, V is potential, m is the total weight of the electrodes, and (V2 - V1) is the potential window.

    2.4. Capacitive deionization experiments

    The deionization of the aqueous NaCl solution was conducted using a stack, which comprised of Plexiglas plates, polymer sheet, nylon cloth (thickness: 0.105 mm), and the as-prepared electrodes (see Figure S1 in the Supplemental Information). The adsorption and desorption were performed by applying a constant voltage difference values of 1.2 and 0 V, respectively, using a source measure unit (Keithley 2400). Both the adsorption and desorption steps had a 30-minute duration. Peristaltic pump (MasterFlex L/S) was employed to supply the aqueous 10 mM NaCl solution to the CDI stack at a flow rate of 5 mL/min, and a conductivity meter (YSI 3200) was used to measure the variation of the ionic conductivity of the deionized solution. Please refer to Figure 1 for the CDI experimental setup that was used in this study.

    The effectiveness of the as-prepared electrodes as CDI materials was determined by calculating their salt adsorption capacity and charge efficiency. Salt adsorption capacity is the ratio of the weight of the adsorbed ions and the total weight of the two electrodes whereas charge efficiency is the ratio of the equilibrium salt adsorption over electrode charge. The two quantities can be calculated using the following equations[ 32,33]:

    Salt adsorption capacity,   Γ s a l t ( m g   s a l t g   e l e c t r o d e ) =                     V M ˙ m e l e c t r o d e ∫ 0 t ( C e f f − C i n ) d t
    (2)

    Charge efficiency, % = F M s a l t ⋅ Γ s a l t ( Σ m e l e c t r o d e ) × 100 %
    (3)

    3. Results and Discussion

    3.1. Structural and chemical characterization

    Scanning electron microscopy was used to investigate the morphology of the as-prepared electrodes. As shown in Figure 2, all of the as-prepared electrodes reveal a rougher surface relative to the pristine activated carbon (see Figure S2 in the Supplemental Information), indicating the addition of PVDF binder and NiO to the carbon. Energy dispersive X-ray (EDX) spectroscopy was performed to verify the presence of NiO in the as-prepared electrodes and to determine the bulk concentration of NiO in the activated carbon-metal oxide electrodes. As shown in Table 1 and Figure 3, the composition of Ni increased correspondingly to the amount of NiO that was used in the fabrication of the electrodes. The result of the EDX analysis provides evidence that NiO was incorporated to the as-prepared activated carbon-metal oxide electrodes.

    The XRD patterns of the as-prepared electrodes were obtained to further verify the incorporation of NiO with the activated carbon. Figure 4 shows the X-ray diffractograms of the samples. The peaks at 2θ values of 37.28°, 43.37°, 62.90°, 75.41°, and 79.41° correspond to the (111), (200), (220), (311), and (222) crystal planes of cubic NiO (JCPDS no. 47-1049), respectively. The broad peaks at 2θ values of 24° and 43.8° indicate the (002) reflection which arises from the crystal plane of graphite from the activated carbon and the (101) reflection which signifies the high degree of interlayer condensation of carbon, respectively. These findings confirm the successful preparation of the as-prepared electrodes.

    The results of the nitrogen gas sorption experiments are presented in Table 2 and Figures S3 and S4. All of the N2 adsorption-desorption isotherms show a type IV-shaped isotherms which means that the electrode materials contain mesopores. The steep portion of the adsorption isotherms at p/p0 values from 0 to 0.05 signifies the presence of micropores in the as-prepared electrodes. Furthermore, the adsorption isotherms of the as-prepared electrodes resemble type II-shaped isotherm, which indicates that there are also macropores in the electrode materials[ 34]. These findings are well supported by the BJH pore size distribution of the electrode materials shown in Figure S4. The hierarchical porosity of the as-prepared electrodes promotes the facile diffusion of ions through their pore network. The easy transport of ions ensures the maximization of the available specific surface area. Moreover, the specific surface area of the electrode materials decreased as the amount of NiO in the activated carbon-metal oxide electrodes was increased. This is logical since some of the pores of the activated carbon might be blocked or occupied by the NiO. As shown in Figure S5 and Table S1, the pristine activated carbon material consists mainly of micropores and mesopores, which can be easily occupied or blocked by the NiO which has a particle diameter of less than 50 nm. It is very important to evaluate the pore characteristics of the electrode materials because the specific surface area and the pore characteristics greatly affect the deionization performance. As mentioned earlier, the main process that governs CDI is the formation of an electrical double layer, which is strongly dependent on the available surface area. This implies that more ions will be adsorbed if the material has a higher specific surface area. As presented in Table 2, the AC shows the highest specific surface area among all the electrodes. If all other properties and characteristics of the electrodes are the same except for the specific surface area, most certainly the AC will have the best deionization performance.

    The wettability of the electrode is another important factor that affects deionization performance. Figure 5 shows the images of the contact angle measurements of the as-prepared electrodes. As can be observed, the AN-30 electrode has the lowest contact angle which means that it is the most hydrophilic among all the electrodes. The AN-30 is followed by the AN-20, AN-10, and the AC electrodes. This observation is consistent with the fact that metal oxides can improve hydrophilicity by providing additional oxygen group. Desalination performance depends on the hydrophilicity of the electrode because good wetting behaviour ensures the maximization of the available specific surface area[35].

    The actual quantity of NiO in the as-prepared electrodes was de-termined by Thermogravimetric analysis (TGA). As presented in Figure 6, the TGA curves of the as-prepared electrodes show three sections of weight loss, namely between 20 to 250 ℃, 250 to 550 ℃, and 550 to 700 ℃. The weight loss from 20 to 250 ℃ is attributed to the removal of adsorbed water and unstable functional groups. At the temperatures of 250 to 550 ℃, the weight loss is due to the evaporation of surface water and volatile components. The final section of the weight loss which is from 550 to 700 ℃ is ascribed to the complete removal of the carbon material[23]. The burn off residue for AC, AN-10, AN-20, and AN-30 electrodes are 2.51, 13.3, 20.67, and 25.37%, respectively. These values are close to the expected NiO content for AC, AN-10, AN-20, and AN-30 electrodes which are 0, 8.33, 15.38, and 21.43%, respectively.

    3.2. Electrochemical performance

    Cyclic voltammetry and electrochemical impedance spectroscopy were carried out in 1M aqueous NaCl solution to investigate the electrochemical performance of the as-prepared electrodes. As shown in Figure 7, the voltammograms are almost in quasi-rectangular shapes, and there is no observable redox peak. The current responses in both regions are mirror images, indicating a close to ideal capacitive behavior. The specific capacitance of AC, AN-10, AN-20, and AN-30 electrodes are 32.53, 46.53, 35.03, and 26.22 F/g, respectively. Both the AN-10 and AN-20 electrodes have a lower specific surface area compared to the AC electrode. However, the two AC-NiO electrodes have a higher capacitance relative to the pure carbon electrode. The specific surface area of the AC electrode is almost thrice that of the AN-30 electrode, but the difference of the specific capacitance of the two electrodes is not more than 10 F/g. These findings can be attributed to the addition of NiO to the activated carbon. The increased hydrophilicity of AC-NiO electrodes promotes the maximization of the available specific surface area. The addition of NiO can also strengthen the electric field, which leads to the attraction of more ions and eventually results in increased charge storage. It is interesting to note that the AN-10 electrode shows the highest specific capacitance. This suggests that it has the best combination of structural and chemical properties among all the electrodes.

    As presented in Figure 8, the Nyquist plots in 1M aqueous NaCl solution of the as-prepared electrodes exhibit two regions, namely the semi-circle and linear regions. This observation is consistent with published studies on metal oxide-carbon CDI electrodes[24,36-44], except that the semi-circle part in the higher frequency region is not well-defined. The low-frequency region of the Nyquist plot is indicative of the diffusion dynamics. Infinite diffusion occurs if the linear part of the Nyquist plot resembles the vertical line whereas semi-infinite diffusion happens if the linear part is characterized by a slope of 45 °[45]. Among all the electrodes, the AC has the linear part that resembles the vertical line. It is followed by AN-10, AN-20, and AN-30 electrodes. This implies that the diffusion of ions in the pure activated carbon electrode is least restricted in the pure activated carbon electrode. The presence of the added NiO has somehow affected the movement of ions through the porous electrode. The facile transport of ions through the electrode materials entails a more accessible specific surface area for ion adsorption[46].

    3.3. Capacitive deionization performance

    The deionization performance of the as-prepared electrodes was studied using a 10 mM aqueous NaCl solution in a single-pass mode CDI setup. The adsorption and desorption were performed at 1.2 V and 0 V, respectively, and each potential was applied for 30 minutes. Three samples per electrode were employed for the deionization of aqueous NaCl solution. Figures S6 and S7 show the CDI profiles for the as-prepared electrodes. As presented in the representative CDI profiles in Figure 9, the AN-10 electrode shows the biggest areas for adsorption and desorption compared to all the electrodes. The enclosed areas of the adsorption and desorption curves with respect to the corresponding equilibrium values of the ionic conductivity signify the amount of salt that was adsorbed on the electrodes. Tables 3 and 4 show the results of the calculations for salt adsorption capacity (SAC) and charge efficiency based on the obtained CDI profiles. Capacitive deionization is primarily governed by the electrical double layer formation, hence the value of the specific capacitance obtained from the cyclic voltammetry measurement is indicative of the salt adsorption capacity of electrode. As can be observed, the order of the values of the specific capacitance of the as-prepared electrodes is consistent with the order of the values of salt adsorption capacity. Moreover, other factors could also contribute to the deionization performance of CDI electrodes. In this work, the electrode wettability, specific surface area, and diffusion resistance of the electrodes are the most significant factors that lead to the different deionization performance of the as-prepared electrodes.

    Note that the AC electrode has the highest specific area and has the least restricted diffusion dynamics among all the electrodes, but it is also the most hydrophobic. The AN-30 electrode is found to be the most hydrophilic, however, it has the lowest specific surface area and the most restricted diffusion dynamics. The AN-10 electrode has the highest salt adsorption capacity, indicating that it has the optimum characteristics among all the electrodes. The AN-10 electrode has a relatively high specific surface area, good wettability, and has diffusion dynamics that is quite comparable to the AC electrode. Furthermore, it very interesting to note that the AN-10 and AN-20 electrodes have higher salt adsorption capacity compared to the AC electrode even if the latter electrode has the highest specific area among all the electrodes. The addition of a dielectric material like NiO to the carbon material resulted in the enhancement of the electric field which attracted more ions in the vicinity of the electrode and eventually led to an improved deionization performance of the AC-NiO electrodes. Dielectric materials can enhance the electric field through self-polarization. This process enables charge storage as dipoles distributed within the material core and on its surface. Each stored charge generates an associated electric field around it, and a combination of such charges leads to the magnification of the electric field and results in the attraction of more ions in the vicinity[18,20,47].

    Charge efficiency is another important performance metric in capacitive deionization. It is the ratio of the total adsorbed salt in terms of Coulomb over the total invested charge. Charge efficiency indicates how energy efficient a CDI system in carrying out deionization of a solution. As presented in Table 4, the AN-10 has the highest charge efficiency among all the electrodes. It is noteworthy that all the activated carbon-metal oxide electrodes have higher charge efficiency compared to the pure activated carbon. The enhanced charge efficiency can be attributed to the presence of the NiO in the electrode material. Only the current that was used in the adsorption of counter-ions contributes to the overall deionization. Co-ion expulsion consumes current without any contribution to the total deionization[23]. The presence of the NiO in the activated carbon-metal oxide electrodes strengthens the electric field which results in the efficient adsorption of positive and negative ions on their corresponding counter electrodes. This finding is similar to a study wherein charge efficiency was observed to have bigger values when the applied potential was increased[48]. Based on electrostatics, the magnitude of the applied potential is directly proportional to the associated electric field[20]. That is, bigger values of charge efficiency will be obtained if the electric field will be enhanced.

    4. Conclusions

    In this work, AC-NiO electrodes were fabricated by the physical mixing of activated carbon, NiO, and PVDF binder in NMP solvent at 60 ℃ for 3 h, and by casting the resulting mixture on a graphite sheet. The addition of NiO to the carbon material enhances the deionization performance. The specific surface area of the AC-NiO electrodes is observed to be much smaller than that of the AC electrode, but the salt adsorption capacity of AN-10 and AN-20 is slightly higher, and the charge efficiency of the AC-NiO electrodes are all higher. Dielectric materials like NiO can increase the electric field by self-polarization, which results in the attraction of more ions in the vicinity of the electrode and eventually leads to an improved deionization performance. The AC-NiO electrodes in this study have comparable deionization performance (7.46 mg/g and 90.1%) to carbon-metal oxide CDI electrodes in the literature. Furthermore, to the best of the authors’ knowledge, this is the first study in CDI literature that shows the feasibility of AC-NiO as electrodes for the deionization of aqueous sodium chloride solution.

    Acknowledgements

    This work was supported by GIST Research Institute (GRI) grant funded by the GIST in 2020.

    Figures

    ACE-31-5-552_F1.gif
    CDI experimental setup.
    ACE-31-5-552_F2.gif
    SEM images of as-prepared a) AC, b) AN-10, c) AN-20, and d) AN-30 electrodes.
    ACE-31-5-552_F3.gif
    EDX spectra of the as-prepared a) AC, b) AN-10, c) AN-20, and d) AN-30 electrodes.
    ACE-31-5-552_F4.gif
    XRD diffractograms of the as-prepared electrodes and the pristine AC and NiO.
    ACE-31-5-552_F5.gif
    Contact angle measurements of the as-prepared a) AC, b) AN-10, c) AN-20, and d) AN-30 electrodes.
    ACE-31-5-552_F6.gif
    Thermogravimetric curves of the as-prepared electrodes.
    ACE-31-5-552_F7.gif
    Cyclic voltammograms of the as-prepared electrodes at a scan rate of 5 mV/s in 1 M NaCl solution.
    ACE-31-5-552_F8.gif
    Nyquist plots of the as-prepared electrodes in 1M aqueous NaCl solution collected in the frequency range from 100 mHz to 100 kHz.
    ACE-31-5-552_F9.gif
    Representative CDI profiles of the as-prepared electrodes. Variations of a) Ionic conductivity and b) Current with time in the deionization of 10 mM NaCl solution.

    Tables

    EDX Elemental Composition of the as-prepared Electrodes (Weight %)
    Pore Characteristics of the as-prepared Electrodes
    Salt Adsorption Capacities (mg NaCl/g Electrode) of as-prepared Electrodes
    Charge Efficiencies (%) of as-prepared Electrodes

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