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activated carbon composite electrode materials prepared by a microwave-assisted ionothermal synthesis method
Journal of Colloid and Interface Science xxx (2015) xxx–xxx
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Effects of activated carbon characteristics on the electrosorption capacity of titanium dioxide/activated carbon composite electrode materials prepared by a microwave-assisted ionothermal synthesis method Po-I Liu a,b, Li-Ching Chung a,⇑, Chia-Hua Ho a, Hsin Shao a, Teh-Ming Liang a, Ren-Yang Horng a,⇑, Min-Chao Chang a, Chen-Chi M. Ma b,⇑ a b
Material and Chemical Research Laboratories, Industrial Technology Research Institute (ITRI), Bldg. 17, 321 Sec. 2, Kuang Fu Road, Hsinchu 30011, Taiwan Department of Chemical Engineering, National Tsing Hua University, Rm. 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 9 October 2014 Accepted 2 December 2014 Available online xxxx Keywords: Activated carbon Anatase titanium dioxide Capacitive deionization Microwave Ionothermal synthesis Electrosorption capacity
a b s t r a c t Titanium dioxide (TiO2)/ activated carbon (AC) composite materials, as capacitive deionization electrodes, were prepared by a two-step microwave-assisted ionothermal synthesis method. The electrosorption capacity of the composite electrodes was studied and the effects of AC characteristics were explored. These effects were investigated by multiple analytical techniques, including X-ray photoelectron spectroscopy, thermogravimetry analysis and electrochemical impedance spectroscopy, etc. The experimental results indicated that the electrosorption capacity of the TiO2/AC composite electrode is dependent on the characteristics of AC including the pore structure and the surface property. An enhancement in electrosorption capacity was observed for the TiO2/AC composite electrode prepared from the AC with higher mesopore content and less hydrophilic surface. This enhancement is due to the deposition of anatase TiO2 with suitable amount of Ti–OH. On the other hand, a decline in electrosorption capacity was observed for the TiO2/AC composite electrode prepared from the AC with higher micropore content and highly hydrophilic surface. High content of hydrogen bond complex formed between the functional group on hydrophilic surface with H2O, which will slow down the TiO2 precursor-H2O reaction. In such situation, the effect of TiO2 becomes unfavorable as the loading amount of TiO2 is less and the micropore can also be blocked. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction The capacitive deionization (CDI) has been considered as an emerging technology for the low strength brackish water desalina⇑ Corresponding authors. Fax: +886 357 32349 (L.-C. Chung), +886 357 32633 (R.-Y Horng), +886 357 15408 (C.-C.M. Ma). E-mail addresses: [email protected] (L.-C. Chung), [email protected] (R.-Y. Horng), [email protected] (C.-C.M. Ma).
tion due to its low energy consumption and no requirement of chemical cleaning process [1–3]. In CDI process, the charged ions are removed by electrosorption at the surface of polarized electrodes. This process is based on the formation of electrical double layers (EDLs) inside the pores of electrode materials. Many carbon materials such as carbon aerogel, activated carbon (AC) and graphene have been used as the CDI electrode materials. Among these materials, AC is widely used due to its reasonable cost, scalable manufacturing and favorable stability [4,5].
http://dx.doi.org/10.1016/j.jcis.2014.12.007 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
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In CDI process, the electrosorption capacity and cycle reversibility of the AC electrode are influenced by the physiochemical characteristics of the AC material in addition to the electrolyte concentration [6,7]. Many studies demonstrated that loading certain amount of titanium dioxide (TiO2) onto the AC matrix can enhance the electrosorption capacity and cycle reversibility of the AC electrode [8,9]. In a recent paper, Kim et al. also reported that the enhancement of electrosorption capacity is owing to the increasing surface wettability in TiO2/AC composite electrode [10]. On the other hand, the cycle reversibility of AC electrode can be affected by the polarization of the electrode. Several researchers considered that the polarization of AC can be reduced by the deposition of TiO2 onto the AC matrix and the cycle reversibility of the electrode can be improved [11,12]. The TiO2/AC composite material can be prepared by sol–gel method. In conventional sol–gel method, it takes longer reaction time at high temperature above 400 °C [13]. In our previous study, a novel two-step microwave-assisted ionothermal synthesis (MAIS) method was developed to prepare the nanostructured anatase TiO2/AC composite [14]. In this method, the anatase TiO2 nanoparticles were in situ synthesized and evenly deposited in the AC matrix within less than 2 h under ambient conditions. In comparison to the pristine AC (mesoporous type) electrode, the electrosorption capability and cycle reversibility of the anatase TiO2/AC composite electrode can be enhanced. The intrinsic properties of AC material can affect the structure and property of the TiO2/AC composite material. However, the systematic investigation about the effect of the different AC characteristics on the electrosorption behavior of TiO2/AC composite electrode has not been found in literature. Hence, in this study, various types of AC materials with different surface property and pore structure were used to prepare the TiO2/AC composites by MAIS method. Then, the electrosorption capacity of the pristine AC electrode and TiO2/AC composite electrode was investigated. Their structure–property characteristics were also analyzed by several instrumental techniques. In addition, the electrochemical impedance spectroscopy (EIS) was also used to study the electrochemical characteristics of the pristine AC electrode and TiO2/AC composite electrode. The correlation between the deposited TiO2 content in TiO2/AC composite and electrosorption capacity was also discussed. 2. Materials and methods 2.1. AC materials The petroleum coke based activated carbon (AC-p) was used as an electrode material. Three kinds of AC-p were prepared by different activation temperature (i.e., 700 °C, 800 °C and 900 °C). The raw material of AC-p, supplied from the Formosa plastics corporation (Taiwan), was further chemically activated. In the chemical activation process [15], petroleum coke was carefully grounded with potassium hydroxide (KOH, SHOWA). The KOH/carbon weight ratio was 4:1. During activation, nitrogen gas at a flow rate 0.06 m3/h was used as purge gas. The sample was heated to setting temperature with a rate of 10 °C/min, and maintained for 1 h then followed by cooling to room temperature. The remaining KOH and salts formed during the activation were removed by washing thoroughly with deionized water until the pH of solution was neutral. All carbon powders were dried overnight at 120 °C prior to use. 2.2. Two-step microwave-assisted ionothermal synthesis of TiO2/AC composite Two-step microwave-assisted ionothermal synthesis method includes a controlled sol–gel reaction step and a subsequent IL-
inducing crystallization step. The starting materials included titanium tetraisopropoxide (TIP) (Merck), water, isopropyl alcohol (IPA) (Merck), and 1-butyl-3- methylimidazolium tetrafluoroborate ([Bmim]+[BF4]) ionic liquid (IL) (Merck). In the reaction step, 2.0 g of AC was dispersed in 200 ml of IPA and then mixed with 0.5 g of TIP and 0.4 g of IL. A high amount of IPA was required for effective AC dispersion. Then, 0.5 g of deionized water was added to the reaction mixture for sol–gel reaction. The molar ratio of TIP and H2O was 1:15 at a constant TIP/IL molar ratio of 1:1. The experimental device used for sample preparation was equipped with a Milestone START D MR microwave system (Milestone Srl, Italy). It was fitted to a water reflux condenser (the cooling water temperature was 5 °C) for operation at atmospheric pressure. In conducting the experiment, the entire mixture was stirred under microwave irradiation for 30 min. The microwave was set at a power of 800 W and a frequency of 2.45 GHz. The temperature of the reaction system was monitored online using an infrared sensor connected to a computer. In the crystallization step, 2.0 g of the product from the reaction step was dispersed in 80 ml of IPA and then mixed with 7.2 g of IL and 9.6 g of H2O. The mixture was stirred under microwave irradiation for 60 min. In this case, the molar ratio of IL and H2O was set at 1:18. Finally, the resulting TiO2/AC composite was washed with ethanol three times and dried in an oven at 70 °C overnight. 2.3. Fabrication of electrode To fabricate an electrode, a carbon slurry was prepared by mixing AC powder (80 wt%), polyvinylidene fluoride (PVDF, M.W. 514,000, Aldrich) (10 wt%) dissolved in N-methyl-2-pyrrolidone (NMP, Aldrich), and graphite (10 wt%). The mixing time was 2 h. The homogenous carbon slurry was then casted onto a titanium foil used as a current collector by applying a doctor-blade technique (wet thickness 300 lm). The coated electrode was dried at 140 °C in an oven for 2 h to remove all organic solvents. 2.4. Characterization The specific surface area (SBET) of the electrode material was measured and calculated using a NOVA4000e and Autosorp MP1 instrument (Quanta-chrome) analyzer based on the Brunauer– Emmett–Teller (BET) method. Using the Barrett–Joyner–Halenda (BJH) model, the pore size distribution was derived from N2 desorption branch of the isotherm. The total pore volume (Vt) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.99, and the mesopore volume (Vmeso) was estimated by the BJH method using a desorption branch of isotherm. Micropore volume (Vmicro) was derived from the intercept of the t-plot. The microstructure of the TiO2/AC composite was determined by transmission electron microscopy (TEM) (PHILIPS M100) coupled with energy dispersive spectroscopy (EDS) at an accelerating voltage of 200 kV. The chemical structure bonding of the AC and TiO2/AC composite was examined by X-ray photoelectron spectroscopy (XPS) measurement on the PHI 1600/3057 spectrometer equipped with a Mg Ka source (Physical Electronics). The crystalline phase was investigated by Raman spectroscopy. For Raman analysis, a Horiba Jobin–Yvon Lab Ram HR800 spectrometer was used with the laser source of 632.8 nm HeNe laser at a power of 17 mW. The percent loading of TiO2 in TiO2/AC composite was determined by conducting thermogravimetric analysis (TGA) (Erkin ElmerTAC7/DX, Thermal Analysis Controller). While measuring the sample mass, the sample was heated from 30 to 800 °C to burn off any carbon-based material. The heating rate was 20 °C/min. The weight difference between the coated and uncoated sample was used to determine the percent loading of the TiO2.
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2.5. Electrochemical measurement
2.6. The electrosorption capacity of the electrode
The EIS electrochemical analysis was conducted by CHI 614D (CH Instruments, Austin, TX) working station. The measurement was performed using a three-electrode cell at room temperature. The three-electrode includes a working electrode (porous electrode), a counter electrode and a reference electrode for measurement of the porous characteristics of the electrode. The counter and reference electrodes were composed of a platinum wire and an Ag/AgCl electrode, respectively. The EIS measurements were carried out using ac perturbation amplitude of 5 mV around the equilibrium potential (0 V). The data were collected in the frequency range from 100 kHz to 1 mHz. The NaCl solution with a concentration of 100 mg/l was also used as electrolyte. The capacitance was calculated from the imaginary part of the impedance. C = |1/2pf Z00 |/w where C is the capacitance (F/g), f is the frequency (Hz), Z00 is the imaginary part of the impedance, and w is the weight of active material in carbon electrode.
The electrosorption capacity of the electrode was evaluated with a semi-batch type apparatus, which has been specially described in our previous work [14]. The assembled electrodes were packed in electrode holders with an opening area of 8 5 cm2. The mass of each electrode was ca. 125 mg. The influent rate is 12 ml/min. The concentration of NaCl was adjusted to 100 mg/l. As a potential difference of 1.2 V was applied, the change in the concentration of NaCl solution was monitored by an ion conductivity meter (EC-410, Suntex). A computerized data acquisition system logged the measured concentrations every 10 s.
3. Results and discussion 3.1. The electrosorption capacity of the pristine AC-p electrodes and TiO2/AC-p composite electrodes Three kinds of AC-p with different characteristics were used to prepare TiO2/AC composites. They were prepared by different activation temperature (i.e., 700 °C, 800 °C and 900 °C). Before further investigation on the electrosorption capacity of each electrode, the phase of TiO2 in TiO2/AC-p composite was first characterized. The Raman spectra of the representative pristine AC-p and TiO2/AC-p composite material are shown in Fig. 1. The phase of TiO2 in TiO2/AC-p composite showed the anatase phase and no Table 1 Electrosorption capacity of electrodes prepared from different pristine AC-p and TiO2/ AC-p composite electrodes.
Fig. 1. Raman spectra of the pristine AC-p (900 °C) and TiO2/AC-p (900 °C) composite.
Electrode materials
Electrosorption capacity (mg/g)
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C) TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
9.2 ± 0.3 9.1 ± 0.2 8.0 ± 0.3 6.9 ± 0.7 9.3 ± 0.4 11.5 ± 0.6
Fig. 2. TEM micrograph of the nanostructured anatase TiO2/AC-p (900 °C) composite. The inset shows the crystalline size and the Ti element on the composite surface.
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Table 2 Characteristics of different pristine AC-p and TiO2/AC-p composite materials. Electrode materials
SBET (m2/g)
Vt (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Vmeso/Vmicro
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C) TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
2844 2767 2715 1999 2019 1685
1.38 1.43 1.55 1.17 1.19 0.95
0.29 0.16 0.06 0.24 0.17 0.17
0.64 0.99 1.10 0.83 0.96 0.70
2.19 6.14 17.40 3.51 5.59 4.11
corresponding peak was found in pristine AC-p [16]. The TEM picture of the anatase TiO2/AC-p composite is shown in Fig. 2. The magnified image indicated that the crystalline size of the anatase TiO2 particle was around 7–12 nm. Table 1 lists the electrosorption capacity of the pristine AC-p and TiO2/AC-p composite electrode. For the pristine AC materials, AC-p (700 °C) electrode has the highest electrosorption capacity. However, in the TiO2/AC-p composite electrodes, TiO2/AC-p (900 °C) composite electrode showed the highest electrosorption capacity. It’s worth noting that the decline in electrosorption capacity was found for the TiO2/AC-p (700 °C) composite electrode and an enhancement in electrosorption capacity was observed for the other TiO2/AC-p composite electrodes, especially for the TiO2/AC-p (900 °C) composite electrode. It is well known that the electrosorption capacity of AC electrode is related to the characteristics of AC such as the total surface area, pore size and the surface functional group [3]. The pore characteristics of these AC-p materials were first investigated. The measured pore characteristics data are listed in Table 2. In the pristine AC material, it can be seen that the surface area of pristine AC-p decreases when the activation temperature increases from 700 °C to 900 °C. The decrease of micropores (<2 nm) and increase of mesopores (2–50 nm) were observed as well. This is due to the fact that the carbon wall separating each microporous pore was destroyed and the larger pores were formed at higher temperature [15]. In TiO2/AC-p composite, the reduction of surface area in the TiO2/AC-p (700 °C) composite may be resulted from the pore blocking of micropores by TiO2. Similar phenomenon was found in the study of Chang et al. [17]. In Table 2, TiO2/AC-p (800 °C) and TiO2/AC-p (900 °C) both have increase in micropores and decrease in mesopores. It might be resulting from the partial deposition of TiO2 in some small size of mesopores and the newly formed micropores between TiO2 interparticles [18]. In general, the micropore has often been considered to achieve high electrosorption ability whereas mesopore is beneficial for fast ion transfer [19]. For the other condition being equal, the electrode fabricated from the AC material with high surface area and suitable composition of mesopore and micropore can get high electrosorption capacity [20]. However, it can be seen that the AC-p (700 °C) material with the lowest meso/microporosity ratio has the highest electrosorption capacity. How the surface property of AC-p (700 °C) material affects the ion electrosorption in TiO2/AC-p
Table 3 XPS O1s fitting results for three AC-p materials. Electrode materials
C@O (%)
OAC@O (%)
CAOH (%)
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C)
18.0 18.6 25.1
0.7 5.1 5.5
81.3 76.3 69.4
composite electrode is needed to consider. In addition, the TiO2/ AC-p (900 °C) composite has the lowest surface area and its electrode showed the highest electrosorption capacity. It seems that the loading of TiO2 has also an overwhelming effect on the electrosorption capacity. Hence, the detailed investigation about how the characteristics of pristine AC-p affects the electrosorption capacity of the TiO2/AC-p composite electrode as well as the pristine AC-p electrode is meaningful. 3.2. The effect of surface property of the pristine AC-p on the characteristics of TiO2/AC-p composite The XPS data for AC-p are shown in Fig. 3 and Table 3. It can be seen that AC-p (700 °C) has higher content of C–OH group. It indicates that the surface of AC-p (700 °C) has more hydrophilic feature. For the TiO2/AC-p composite materials, the data of TGA analysis and XPS analysis results are shown in Fig. 4 and Table 4. The XPS curves of TiO2/AC-p show that the O1s regions are fitted into three peaks. The main peak located at 530.2 eV is attributed to the Ti–O of TiO2 and the peak located at 531.3 eV is assigned to the hydroxyl group of the titanium compound [14]. As shown in Table 4, the TiO2 content of TiO2/AC-p composite increases with increasing activation temperature and mesopore volume of pristine AC-p. In Table 4, the C–OH group content becomes less for all of the TiO2/AC-p composite samples. However, the higher content of TiO2 and Ti–OH were observed for the TiO2/AC-p (800 °C) composite and TiO2/AC-p (900 °C) composite. The two-step MAIS method includes a controlled sol–gel reaction step and a subsequent IL-inducing crystallization step. The proposed mechanism of TiO2 modification of AC material is described in Fig. 5. In the sol–gel reaction step, the amorphous TiO2/AC-p composite is first formed. At the subsequent crystallization step, the water and IL
Fig. 3. XPS curves of the O1s spectrum for the three AC-p materials and the O1s fitting results for AC-p (700 °C) (a) AC-p (800 °C) (b) and AC-p (900 °C) (c).
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Fig. 4. XPS curves of O1s spectrum for the three nanostructured anatase AC-p/TiO2 composite materials and O1s fitting results for TiO2/AC-p (700 °C) (a) TiO2/AC-p (800 °C) (b) and TiO2/AC-p (900 °C) (c).
Table 4 Characteristics of TiO2/AC-p composite prepared from different AC-p materials. Electrode materials
TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
TGA analysis
XPS analysis (O1s)
TiO2 content (%)
CAOH (%)
TiAOH (%)
TiAO (%)
5.2 8.1 10.2
28.5 24.0 22.8
67.3 71.4 71.6
4.3 4.7 5.6
around the TiO2 are the critical components for the surface anatase crystallization. In sol–gel reaction step, the C–OH group of AC-p may form hydrogen bonding complex with H2O and kinetically slow down the hydrolysis reaction of TiO2-precursor [21]. Therefore the lower TiO2 content formed in TiO2/AC-p (700 °C) composite is due to the high C–OH content of AC-p (700 °C) at the same reaction time and TIP/H2O ratio. The ion accessibility to electrode surface is also an important factor affecting the electrosorption capacity of the electrode. The ion accessibility is related to the surface hydrophilicity or wettability of electrode. The electrode with high hydrophilic surface can provide high ion accessibility and electrosorption capacity [22]. Therefore, the pristine AC-p (700 °C) electrode has the highest electrosorption capacity. In TiO2/AC composite electrode, the electrical conductivity of TiO2 is another important factor affecting the electrosorption capacity of the TiO2/AC composite electrode. In our previous study, the nanostructured TiO2/AC composite with higher anatase and Ti– OH content contributes to higher electronic conductivity [14]. High electronic conductivity of electrode can lead to high electrosorption capacity [23]. In Table 4, The TiO2/AC-p (800 °C) composite and TiO2/AC-p (900 °C) composite showed the higher content of
TiO2 and Ti–OH. Consequently, the electrical conductivity effect of TiO2 in these two composite electrodes could be overwhelming and an improvement in electrosorption capacity was observed. On the contrary, the electrical conductivity effect of TiO2 is insignificant in TiO2/AC-p (700 °C) composite because of the lower TiO2 content. It was considered that the decline of electrosorption capacity in TiO2/AC-p (700 °C) composite electrode was mainly due to the higher proportion of micropore blockage by TiO2 nanoparticles. 3.3. EIS investigation for the pristine AC electrodes and TiO2/AC composite electrodes The electrochemical characteristics of the electrode can be investigated by cyclic voltammetry (CV) and EIS method. The CV measurements are generally conducted at constant and high electrolyte concentration (at 0.5 M). On the other hand, the EIS can measure for whole electrolyte concentration and provide more detailed electrochemical information [24]. In this study, the initial concentration of NaCl for electrosorption capacity measurement was set at 100 mg/l. Hence, the EIS was used to study the electrochemical characteristics of the pristine AC electrode and TiO2/AC composite electrode. The EIS analysis is conducted by applying a small voltage signal with sinusoidally changing over time in a large range of frequencies. The responding current with a time-lag is analyzed to construct Nyquist plots and other representations that allow one to know the magnitude of resistances and capacitance in the electrode [25,26]. In the Nyquist plot of EIS, each point on the plot represents the impedance at a certain frequency. The imaginary impedance (Z00 ) indicates the capacitive character of
Fig. 5. Proposed mechanism of TiO2 modification of carbon material by MAIS method.
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Po-I Liu et al. / Journal of Colloid and Interface Science xxx (2015) xxx–xxx Table 5 The calculated capacitance obtained from EIS data for each AC-p electrodes. Electrode materials
EIS capacitance (F/g)
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C) TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
51.3 38.0 24.7 40.4 42.6 64.5
Fig. 6. Nyquist plots for AC-p (a) and TiO2/AC-p (b) composite electrodes.
Fig. 8. Adsorption–desorption cycles of AC-p (900 °C) (a) electrode and TiO2/AC-p (900 °C) (b) electrode at initial NaCl concentration of 100 mg/l and applied potential difference of 1.2 V.
Fig. 7. Calculated capacitance for AC-p (a) and TiO2/AC-p (b) composite electrodes.
the electrode while the real impedance (Z0 ) characterizes the solution resistance, electrode charge transfer and ionic diffusion resistance.
The Nyquist plots for the three pristine AC-p electrodes and TiO2/AC-p composite electrodes are shown in Fig. 6. The semicircle at the high frequency region indicates the impedance at the interface between the current collector and the AC, the interface between the ACs as well as the interface between AC electrode and the solution. The semicircle represents the existence of the interfacial capacitance along with the interface resistance of the electrode. From Fig. 6(a), it can be seen that the pristine AC-p (700 °C) has the lowest real impedance and the smallest interfacial resistance because of its more hydrophilic surface. In the case of TiO2/AC-p electrodes, the approaching vertical lines were found at the low frequency region. It implies that these electrodes approach capacitive behavior and allow the solution ions to be more efficiently electrosorped by the electrode, especially for TiO2/AC-p (900 °C) composite electrode. It is emphasized that higher content of anatase TiO2 and Ti–OH for the TiO2/AC-p (900 °C) electrode material was contributed to low electron transport resistance and ion transport resistance [14,27]. From Fig. 6(b) and TGA analysis listed in Table 4, lower electron transport and ionic transport resistance are found for the TiO2/AC-p (900 °C) composite electrode and then a high capacitive behavior was observed.
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The capacitance of each electrode can be calculated from EIS data, shown in Fig. 7 and Table 5. The calculated capacitance for the TiO2/AC-p (700 °C) composite electrode was decline while that for TiO2/AC-p (800 °C) composite electrode and TiO2/AC-p (900 °C) composite electrode was both enhanced. The calculated EIS capacitance has the same trend as the electrosorption capacity results. 3.4. The adsorption–desorption cycle test for the pristine AC-p and TiO2/AC-p composite electrode The ion adsorption–desorption test was conducted for the pristine AC-p and TiO2/AC-p composite electrode. Fig. 8 demonstrates the adsorption–desorption cycles of the pristine AC-p (900 °C) electrode and the TiO2/AC-p (900 °C) composite electrode at initial NaCl concentration of 100 mg/l and applied potential difference of 1.2 V. It can be seen that the TiO2/AC-p (900 °C) composite electrode showed the improved electrosorption capacity and reversibility. In our experiment, the electrosorption capacity was calculated according to the difference between the influent and the effluent NaCl concentration divided by the weight of active material. The electrosorption capacity increased from 8.0 to 11.5 mg/g with an improved reversibility during the 120 cycles. 4. Conclusion The TiO2/AC-p composite materials for capacitive deionization electrode prepared by a two-step MAIS method were investigated. This method includes a controlled sol–gel reaction step and a subsequent ionic liquid-inducing crystallization step. It was found that the AC-p characteristics can influence the TiO2 precursor-H2O reaction in the reaction step. Therefore the electrosorption capacity of the TiO2/AC-p composite electrode can be affected. An enhancement in electrosorption capacity and cycle reversibility was observed for the TiO2/AC-p composite electrode prepared from AC-p with higher content of mesopores and less hydrophilic surface. The electrosorption capacity can be increased from 8.0 to 11.5 mg/g with improved reversibility during the 120 cycles for the TiO2/AC-p (900 °C) composite electrode. In addition, a decline in electrosorption capacity was found for the TiO2/AC-p composite
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electrode prepared from the AC-p with higher micropore content and highly hydrophilic surface. The high content of hydrogen bond complex formed between the functional group on hydrophilic surface with H2O, which slows down the TiO2 precursor-H2O reaction. In such situation, the effect of TiO2 becomes unfavorable as the loading amount of TiO2 is less and the micropore can also be blocked. References [1] Y. Oren, Desalination 228 (2008) 10. [2] H.B. Li, L.K. Pan, C.Y. Nie, Z. Sun, J. Nanosci. Lett. 2 (2012) 9. [3] S. Porada, R. Zhao, A. van der Wal, V. Presser, P.M. Biesheuvel, Prog. Mater Sci. 58 (2013) 1388. [4] C.H. Hou, J.F. Huang, H.R. Lin, B.Y. Wang, J. Taiwan Inst. Chem. Eng. 43 (2012) 473. [5] J.H. Choi, Sep. Purif. Technol. 70 (2010) 362. [6] E. Pollak, I. Genish, G. Salitra, A. Soffer, L. Klein, D. Aurbach, J. Phys. Chem. B 110 (2006) 7443. [7] L. Han, K.G. Karthikeyan, M.A. Anderson, K.B. Gregory, J. Colloid Interface Sci. 430 (2014) 93. [8] M.W. Ryoo, J.H. Kim, G. Seo, J. Colloid Interface Sci. 264 (2003) 414. [9] M.W. Ryoo, G. Seo, Water Res. 37 (2003) 1527. [10] C. Kim, J. Lee, S. Kim, J. Yoon, Desalination 342 (2014) 70. [11] H. Liang, F. Chen, R. Li, L. Wang, Z. Deng, Electrochim. Acta 49 (2004) 3463. [12] M.K. Seo, S.J. Park, Curr. Appl. Phys. 10 (2010) 391. [13] Y. Huai, X. Hu, Z. Lin, Z. Deng, J. Suo, Mater. Chem. Phys. 113 (2009) 962. [14] P.I. Liu, L.C. Chung, H. Shao, T.M. Liang, R.Y. Horng, C.C.M. Ma, M.C. Chang, Electrochim. Acta 96 (2013) 173. [15] T. Otowa, R. Tanibata, M. ltoh, Gas Sep. Purif. 7 (1993) 241. [16] W. Ma, Z. Lu, M. Zhang, Appl. Phys. A 66 (1998) 621. [17] L.M. Chang, X.Y. Duan, W. Liu, Desalination 270 (2011) 285. [18] J.H. Lee, J.H. Choi, Desalin. Water Treat. 51 (2013) 503. [19] M. Noked, E. Avraham, A. Soffer, D. Aurbach, J. Phys. Chem. C 113 (2009) 21319. [20] Z. Peng, D. Zhang, T. Yan, J. Zhang, L. Shi, Appl. Surf. Sci. 282 (2013) 965. [21] N. Venkatachalam, M. Palanichamy, V. Murugesan, Mater. Chem. Phys. 104 (2007) 454. [22] H.J. Oh, J.H. Lee, H.J. Ahn, Y. Jeong, Y.J. Kim, C.S. Chi, Thin Solid Films 515 (2006) 220. [23] Z. Chen, X. Sun, H. Guo, C. Song, Adv. Mater. Res. 113–114 (2010) 2134. [24] M.E. Suss, T.F. Baumann, M.A. Worsley, K.A. Rose, T.F. Jaramillo, M. Stadermann, J.G. Santiago, J. Power Sources 241 (2013) 266. [25] Z. Peng, D.S. Zhang, L.Y. Shi, T.T. Yan, J. Mater. Chem. 22 (2012) 6603. [26] Z. Peng, D.S. Zhang, L.Y. Shi, T.T. Yan, S.A. Yuan, H.R. Li, R.H. Gao, J.H. Fang, J. Phys. Chem. C 115 (2011) 17068. [27] A.S. Wochnik, M. Handloser, D. Durach, A. Hartschuh, C. Scheu, ACS Appl. Mater. Interfaces 5 (2013) 5696.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Effects of activated carbon characteristics on the electrosorption capacity of titanium dioxide/activated carbon composite electrode materials prepared by a microwave-assisted ionothermal synthesis method Po-I Liu a,b, Li-Ching Chung a,⇑, Chia-Hua Ho a, Hsin Shao a, Teh-Ming Liang a, Ren-Yang Horng a,⇑, Min-Chao Chang a, Chen-Chi M. Ma b,⇑ a b
Material and Chemical Research Laboratories, Industrial Technology Research Institute (ITRI), Bldg. 17, 321 Sec. 2, Kuang Fu Road, Hsinchu 30011, Taiwan Department of Chemical Engineering, National Tsing Hua University, Rm. 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 9 October 2014 Accepted 2 December 2014 Available online xxxx Keywords: Activated carbon Anatase titanium dioxide Capacitive deionization Microwave Ionothermal synthesis Electrosorption capacity
a b s t r a c t Titanium dioxide (TiO2)/ activated carbon (AC) composite materials, as capacitive deionization electrodes, were prepared by a two-step microwave-assisted ionothermal synthesis method. The electrosorption capacity of the composite electrodes was studied and the effects of AC characteristics were explored. These effects were investigated by multiple analytical techniques, including X-ray photoelectron spectroscopy, thermogravimetry analysis and electrochemical impedance spectroscopy, etc. The experimental results indicated that the electrosorption capacity of the TiO2/AC composite electrode is dependent on the characteristics of AC including the pore structure and the surface property. An enhancement in electrosorption capacity was observed for the TiO2/AC composite electrode prepared from the AC with higher mesopore content and less hydrophilic surface. This enhancement is due to the deposition of anatase TiO2 with suitable amount of Ti–OH. On the other hand, a decline in electrosorption capacity was observed for the TiO2/AC composite electrode prepared from the AC with higher micropore content and highly hydrophilic surface. High content of hydrogen bond complex formed between the functional group on hydrophilic surface with H2O, which will slow down the TiO2 precursor-H2O reaction. In such situation, the effect of TiO2 becomes unfavorable as the loading amount of TiO2 is less and the micropore can also be blocked. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction The capacitive deionization (CDI) has been considered as an emerging technology for the low strength brackish water desalina⇑ Corresponding authors. Fax: +886 357 32349 (L.-C. Chung), +886 357 32633 (R.-Y Horng), +886 357 15408 (C.-C.M. Ma). E-mail addresses: [email protected] (L.-C. Chung), [email protected] (R.-Y. Horng), [email protected] (C.-C.M. Ma).
tion due to its low energy consumption and no requirement of chemical cleaning process [1–3]. In CDI process, the charged ions are removed by electrosorption at the surface of polarized electrodes. This process is based on the formation of electrical double layers (EDLs) inside the pores of electrode materials. Many carbon materials such as carbon aerogel, activated carbon (AC) and graphene have been used as the CDI electrode materials. Among these materials, AC is widely used due to its reasonable cost, scalable manufacturing and favorable stability [4,5].
http://dx.doi.org/10.1016/j.jcis.2014.12.007 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
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In CDI process, the electrosorption capacity and cycle reversibility of the AC electrode are influenced by the physiochemical characteristics of the AC material in addition to the electrolyte concentration [6,7]. Many studies demonstrated that loading certain amount of titanium dioxide (TiO2) onto the AC matrix can enhance the electrosorption capacity and cycle reversibility of the AC electrode [8,9]. In a recent paper, Kim et al. also reported that the enhancement of electrosorption capacity is owing to the increasing surface wettability in TiO2/AC composite electrode [10]. On the other hand, the cycle reversibility of AC electrode can be affected by the polarization of the electrode. Several researchers considered that the polarization of AC can be reduced by the deposition of TiO2 onto the AC matrix and the cycle reversibility of the electrode can be improved [11,12]. The TiO2/AC composite material can be prepared by sol–gel method. In conventional sol–gel method, it takes longer reaction time at high temperature above 400 °C [13]. In our previous study, a novel two-step microwave-assisted ionothermal synthesis (MAIS) method was developed to prepare the nanostructured anatase TiO2/AC composite [14]. In this method, the anatase TiO2 nanoparticles were in situ synthesized and evenly deposited in the AC matrix within less than 2 h under ambient conditions. In comparison to the pristine AC (mesoporous type) electrode, the electrosorption capability and cycle reversibility of the anatase TiO2/AC composite electrode can be enhanced. The intrinsic properties of AC material can affect the structure and property of the TiO2/AC composite material. However, the systematic investigation about the effect of the different AC characteristics on the electrosorption behavior of TiO2/AC composite electrode has not been found in literature. Hence, in this study, various types of AC materials with different surface property and pore structure were used to prepare the TiO2/AC composites by MAIS method. Then, the electrosorption capacity of the pristine AC electrode and TiO2/AC composite electrode was investigated. Their structure–property characteristics were also analyzed by several instrumental techniques. In addition, the electrochemical impedance spectroscopy (EIS) was also used to study the electrochemical characteristics of the pristine AC electrode and TiO2/AC composite electrode. The correlation between the deposited TiO2 content in TiO2/AC composite and electrosorption capacity was also discussed. 2. Materials and methods 2.1. AC materials The petroleum coke based activated carbon (AC-p) was used as an electrode material. Three kinds of AC-p were prepared by different activation temperature (i.e., 700 °C, 800 °C and 900 °C). The raw material of AC-p, supplied from the Formosa plastics corporation (Taiwan), was further chemically activated. In the chemical activation process [15], petroleum coke was carefully grounded with potassium hydroxide (KOH, SHOWA). The KOH/carbon weight ratio was 4:1. During activation, nitrogen gas at a flow rate 0.06 m3/h was used as purge gas. The sample was heated to setting temperature with a rate of 10 °C/min, and maintained for 1 h then followed by cooling to room temperature. The remaining KOH and salts formed during the activation were removed by washing thoroughly with deionized water until the pH of solution was neutral. All carbon powders were dried overnight at 120 °C prior to use. 2.2. Two-step microwave-assisted ionothermal synthesis of TiO2/AC composite Two-step microwave-assisted ionothermal synthesis method includes a controlled sol–gel reaction step and a subsequent IL-
inducing crystallization step. The starting materials included titanium tetraisopropoxide (TIP) (Merck), water, isopropyl alcohol (IPA) (Merck), and 1-butyl-3- methylimidazolium tetrafluoroborate ([Bmim]+[BF4]) ionic liquid (IL) (Merck). In the reaction step, 2.0 g of AC was dispersed in 200 ml of IPA and then mixed with 0.5 g of TIP and 0.4 g of IL. A high amount of IPA was required for effective AC dispersion. Then, 0.5 g of deionized water was added to the reaction mixture for sol–gel reaction. The molar ratio of TIP and H2O was 1:15 at a constant TIP/IL molar ratio of 1:1. The experimental device used for sample preparation was equipped with a Milestone START D MR microwave system (Milestone Srl, Italy). It was fitted to a water reflux condenser (the cooling water temperature was 5 °C) for operation at atmospheric pressure. In conducting the experiment, the entire mixture was stirred under microwave irradiation for 30 min. The microwave was set at a power of 800 W and a frequency of 2.45 GHz. The temperature of the reaction system was monitored online using an infrared sensor connected to a computer. In the crystallization step, 2.0 g of the product from the reaction step was dispersed in 80 ml of IPA and then mixed with 7.2 g of IL and 9.6 g of H2O. The mixture was stirred under microwave irradiation for 60 min. In this case, the molar ratio of IL and H2O was set at 1:18. Finally, the resulting TiO2/AC composite was washed with ethanol three times and dried in an oven at 70 °C overnight. 2.3. Fabrication of electrode To fabricate an electrode, a carbon slurry was prepared by mixing AC powder (80 wt%), polyvinylidene fluoride (PVDF, M.W. 514,000, Aldrich) (10 wt%) dissolved in N-methyl-2-pyrrolidone (NMP, Aldrich), and graphite (10 wt%). The mixing time was 2 h. The homogenous carbon slurry was then casted onto a titanium foil used as a current collector by applying a doctor-blade technique (wet thickness 300 lm). The coated electrode was dried at 140 °C in an oven for 2 h to remove all organic solvents. 2.4. Characterization The specific surface area (SBET) of the electrode material was measured and calculated using a NOVA4000e and Autosorp MP1 instrument (Quanta-chrome) analyzer based on the Brunauer– Emmett–Teller (BET) method. Using the Barrett–Joyner–Halenda (BJH) model, the pore size distribution was derived from N2 desorption branch of the isotherm. The total pore volume (Vt) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.99, and the mesopore volume (Vmeso) was estimated by the BJH method using a desorption branch of isotherm. Micropore volume (Vmicro) was derived from the intercept of the t-plot. The microstructure of the TiO2/AC composite was determined by transmission electron microscopy (TEM) (PHILIPS M100) coupled with energy dispersive spectroscopy (EDS) at an accelerating voltage of 200 kV. The chemical structure bonding of the AC and TiO2/AC composite was examined by X-ray photoelectron spectroscopy (XPS) measurement on the PHI 1600/3057 spectrometer equipped with a Mg Ka source (Physical Electronics). The crystalline phase was investigated by Raman spectroscopy. For Raman analysis, a Horiba Jobin–Yvon Lab Ram HR800 spectrometer was used with the laser source of 632.8 nm HeNe laser at a power of 17 mW. The percent loading of TiO2 in TiO2/AC composite was determined by conducting thermogravimetric analysis (TGA) (Erkin ElmerTAC7/DX, Thermal Analysis Controller). While measuring the sample mass, the sample was heated from 30 to 800 °C to burn off any carbon-based material. The heating rate was 20 °C/min. The weight difference between the coated and uncoated sample was used to determine the percent loading of the TiO2.
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2.5. Electrochemical measurement
2.6. The electrosorption capacity of the electrode
The EIS electrochemical analysis was conducted by CHI 614D (CH Instruments, Austin, TX) working station. The measurement was performed using a three-electrode cell at room temperature. The three-electrode includes a working electrode (porous electrode), a counter electrode and a reference electrode for measurement of the porous characteristics of the electrode. The counter and reference electrodes were composed of a platinum wire and an Ag/AgCl electrode, respectively. The EIS measurements were carried out using ac perturbation amplitude of 5 mV around the equilibrium potential (0 V). The data were collected in the frequency range from 100 kHz to 1 mHz. The NaCl solution with a concentration of 100 mg/l was also used as electrolyte. The capacitance was calculated from the imaginary part of the impedance. C = |1/2pf Z00 |/w where C is the capacitance (F/g), f is the frequency (Hz), Z00 is the imaginary part of the impedance, and w is the weight of active material in carbon electrode.
The electrosorption capacity of the electrode was evaluated with a semi-batch type apparatus, which has been specially described in our previous work [14]. The assembled electrodes were packed in electrode holders with an opening area of 8 5 cm2. The mass of each electrode was ca. 125 mg. The influent rate is 12 ml/min. The concentration of NaCl was adjusted to 100 mg/l. As a potential difference of 1.2 V was applied, the change in the concentration of NaCl solution was monitored by an ion conductivity meter (EC-410, Suntex). A computerized data acquisition system logged the measured concentrations every 10 s.
3. Results and discussion 3.1. The electrosorption capacity of the pristine AC-p electrodes and TiO2/AC-p composite electrodes Three kinds of AC-p with different characteristics were used to prepare TiO2/AC composites. They were prepared by different activation temperature (i.e., 700 °C, 800 °C and 900 °C). Before further investigation on the electrosorption capacity of each electrode, the phase of TiO2 in TiO2/AC-p composite was first characterized. The Raman spectra of the representative pristine AC-p and TiO2/AC-p composite material are shown in Fig. 1. The phase of TiO2 in TiO2/AC-p composite showed the anatase phase and no Table 1 Electrosorption capacity of electrodes prepared from different pristine AC-p and TiO2/ AC-p composite electrodes.
Fig. 1. Raman spectra of the pristine AC-p (900 °C) and TiO2/AC-p (900 °C) composite.
Electrode materials
Electrosorption capacity (mg/g)
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C) TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
9.2 ± 0.3 9.1 ± 0.2 8.0 ± 0.3 6.9 ± 0.7 9.3 ± 0.4 11.5 ± 0.6
Fig. 2. TEM micrograph of the nanostructured anatase TiO2/AC-p (900 °C) composite. The inset shows the crystalline size and the Ti element on the composite surface.
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Table 2 Characteristics of different pristine AC-p and TiO2/AC-p composite materials. Electrode materials
SBET (m2/g)
Vt (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Vmeso/Vmicro
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C) TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
2844 2767 2715 1999 2019 1685
1.38 1.43 1.55 1.17 1.19 0.95
0.29 0.16 0.06 0.24 0.17 0.17
0.64 0.99 1.10 0.83 0.96 0.70
2.19 6.14 17.40 3.51 5.59 4.11
corresponding peak was found in pristine AC-p [16]. The TEM picture of the anatase TiO2/AC-p composite is shown in Fig. 2. The magnified image indicated that the crystalline size of the anatase TiO2 particle was around 7–12 nm. Table 1 lists the electrosorption capacity of the pristine AC-p and TiO2/AC-p composite electrode. For the pristine AC materials, AC-p (700 °C) electrode has the highest electrosorption capacity. However, in the TiO2/AC-p composite electrodes, TiO2/AC-p (900 °C) composite electrode showed the highest electrosorption capacity. It’s worth noting that the decline in electrosorption capacity was found for the TiO2/AC-p (700 °C) composite electrode and an enhancement in electrosorption capacity was observed for the other TiO2/AC-p composite electrodes, especially for the TiO2/AC-p (900 °C) composite electrode. It is well known that the electrosorption capacity of AC electrode is related to the characteristics of AC such as the total surface area, pore size and the surface functional group [3]. The pore characteristics of these AC-p materials were first investigated. The measured pore characteristics data are listed in Table 2. In the pristine AC material, it can be seen that the surface area of pristine AC-p decreases when the activation temperature increases from 700 °C to 900 °C. The decrease of micropores (<2 nm) and increase of mesopores (2–50 nm) were observed as well. This is due to the fact that the carbon wall separating each microporous pore was destroyed and the larger pores were formed at higher temperature [15]. In TiO2/AC-p composite, the reduction of surface area in the TiO2/AC-p (700 °C) composite may be resulted from the pore blocking of micropores by TiO2. Similar phenomenon was found in the study of Chang et al. [17]. In Table 2, TiO2/AC-p (800 °C) and TiO2/AC-p (900 °C) both have increase in micropores and decrease in mesopores. It might be resulting from the partial deposition of TiO2 in some small size of mesopores and the newly formed micropores between TiO2 interparticles [18]. In general, the micropore has often been considered to achieve high electrosorption ability whereas mesopore is beneficial for fast ion transfer [19]. For the other condition being equal, the electrode fabricated from the AC material with high surface area and suitable composition of mesopore and micropore can get high electrosorption capacity [20]. However, it can be seen that the AC-p (700 °C) material with the lowest meso/microporosity ratio has the highest electrosorption capacity. How the surface property of AC-p (700 °C) material affects the ion electrosorption in TiO2/AC-p
Table 3 XPS O1s fitting results for three AC-p materials. Electrode materials
C@O (%)
OAC@O (%)
CAOH (%)
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C)
18.0 18.6 25.1
0.7 5.1 5.5
81.3 76.3 69.4
composite electrode is needed to consider. In addition, the TiO2/ AC-p (900 °C) composite has the lowest surface area and its electrode showed the highest electrosorption capacity. It seems that the loading of TiO2 has also an overwhelming effect on the electrosorption capacity. Hence, the detailed investigation about how the characteristics of pristine AC-p affects the electrosorption capacity of the TiO2/AC-p composite electrode as well as the pristine AC-p electrode is meaningful. 3.2. The effect of surface property of the pristine AC-p on the characteristics of TiO2/AC-p composite The XPS data for AC-p are shown in Fig. 3 and Table 3. It can be seen that AC-p (700 °C) has higher content of C–OH group. It indicates that the surface of AC-p (700 °C) has more hydrophilic feature. For the TiO2/AC-p composite materials, the data of TGA analysis and XPS analysis results are shown in Fig. 4 and Table 4. The XPS curves of TiO2/AC-p show that the O1s regions are fitted into three peaks. The main peak located at 530.2 eV is attributed to the Ti–O of TiO2 and the peak located at 531.3 eV is assigned to the hydroxyl group of the titanium compound [14]. As shown in Table 4, the TiO2 content of TiO2/AC-p composite increases with increasing activation temperature and mesopore volume of pristine AC-p. In Table 4, the C–OH group content becomes less for all of the TiO2/AC-p composite samples. However, the higher content of TiO2 and Ti–OH were observed for the TiO2/AC-p (800 °C) composite and TiO2/AC-p (900 °C) composite. The two-step MAIS method includes a controlled sol–gel reaction step and a subsequent IL-inducing crystallization step. The proposed mechanism of TiO2 modification of AC material is described in Fig. 5. In the sol–gel reaction step, the amorphous TiO2/AC-p composite is first formed. At the subsequent crystallization step, the water and IL
Fig. 3. XPS curves of the O1s spectrum for the three AC-p materials and the O1s fitting results for AC-p (700 °C) (a) AC-p (800 °C) (b) and AC-p (900 °C) (c).
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Fig. 4. XPS curves of O1s spectrum for the three nanostructured anatase AC-p/TiO2 composite materials and O1s fitting results for TiO2/AC-p (700 °C) (a) TiO2/AC-p (800 °C) (b) and TiO2/AC-p (900 °C) (c).
Table 4 Characteristics of TiO2/AC-p composite prepared from different AC-p materials. Electrode materials
TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
TGA analysis
XPS analysis (O1s)
TiO2 content (%)
CAOH (%)
TiAOH (%)
TiAO (%)
5.2 8.1 10.2
28.5 24.0 22.8
67.3 71.4 71.6
4.3 4.7 5.6
around the TiO2 are the critical components for the surface anatase crystallization. In sol–gel reaction step, the C–OH group of AC-p may form hydrogen bonding complex with H2O and kinetically slow down the hydrolysis reaction of TiO2-precursor [21]. Therefore the lower TiO2 content formed in TiO2/AC-p (700 °C) composite is due to the high C–OH content of AC-p (700 °C) at the same reaction time and TIP/H2O ratio. The ion accessibility to electrode surface is also an important factor affecting the electrosorption capacity of the electrode. The ion accessibility is related to the surface hydrophilicity or wettability of electrode. The electrode with high hydrophilic surface can provide high ion accessibility and electrosorption capacity [22]. Therefore, the pristine AC-p (700 °C) electrode has the highest electrosorption capacity. In TiO2/AC composite electrode, the electrical conductivity of TiO2 is another important factor affecting the electrosorption capacity of the TiO2/AC composite electrode. In our previous study, the nanostructured TiO2/AC composite with higher anatase and Ti– OH content contributes to higher electronic conductivity [14]. High electronic conductivity of electrode can lead to high electrosorption capacity [23]. In Table 4, The TiO2/AC-p (800 °C) composite and TiO2/AC-p (900 °C) composite showed the higher content of
TiO2 and Ti–OH. Consequently, the electrical conductivity effect of TiO2 in these two composite electrodes could be overwhelming and an improvement in electrosorption capacity was observed. On the contrary, the electrical conductivity effect of TiO2 is insignificant in TiO2/AC-p (700 °C) composite because of the lower TiO2 content. It was considered that the decline of electrosorption capacity in TiO2/AC-p (700 °C) composite electrode was mainly due to the higher proportion of micropore blockage by TiO2 nanoparticles. 3.3. EIS investigation for the pristine AC electrodes and TiO2/AC composite electrodes The electrochemical characteristics of the electrode can be investigated by cyclic voltammetry (CV) and EIS method. The CV measurements are generally conducted at constant and high electrolyte concentration (at 0.5 M). On the other hand, the EIS can measure for whole electrolyte concentration and provide more detailed electrochemical information [24]. In this study, the initial concentration of NaCl for electrosorption capacity measurement was set at 100 mg/l. Hence, the EIS was used to study the electrochemical characteristics of the pristine AC electrode and TiO2/AC composite electrode. The EIS analysis is conducted by applying a small voltage signal with sinusoidally changing over time in a large range of frequencies. The responding current with a time-lag is analyzed to construct Nyquist plots and other representations that allow one to know the magnitude of resistances and capacitance in the electrode [25,26]. In the Nyquist plot of EIS, each point on the plot represents the impedance at a certain frequency. The imaginary impedance (Z00 ) indicates the capacitive character of
Fig. 5. Proposed mechanism of TiO2 modification of carbon material by MAIS method.
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Po-I Liu et al. / Journal of Colloid and Interface Science xxx (2015) xxx–xxx Table 5 The calculated capacitance obtained from EIS data for each AC-p electrodes. Electrode materials
EIS capacitance (F/g)
AC-p (700 °C) AC-p (800 °C) AC-p (900 °C) TiO2/AC-p (700 °C) TiO2/AC-p (800 °C) TiO2/AC-p (900 °C)
51.3 38.0 24.7 40.4 42.6 64.5
Fig. 6. Nyquist plots for AC-p (a) and TiO2/AC-p (b) composite electrodes.
Fig. 8. Adsorption–desorption cycles of AC-p (900 °C) (a) electrode and TiO2/AC-p (900 °C) (b) electrode at initial NaCl concentration of 100 mg/l and applied potential difference of 1.2 V.
Fig. 7. Calculated capacitance for AC-p (a) and TiO2/AC-p (b) composite electrodes.
the electrode while the real impedance (Z0 ) characterizes the solution resistance, electrode charge transfer and ionic diffusion resistance.
The Nyquist plots for the three pristine AC-p electrodes and TiO2/AC-p composite electrodes are shown in Fig. 6. The semicircle at the high frequency region indicates the impedance at the interface between the current collector and the AC, the interface between the ACs as well as the interface between AC electrode and the solution. The semicircle represents the existence of the interfacial capacitance along with the interface resistance of the electrode. From Fig. 6(a), it can be seen that the pristine AC-p (700 °C) has the lowest real impedance and the smallest interfacial resistance because of its more hydrophilic surface. In the case of TiO2/AC-p electrodes, the approaching vertical lines were found at the low frequency region. It implies that these electrodes approach capacitive behavior and allow the solution ions to be more efficiently electrosorped by the electrode, especially for TiO2/AC-p (900 °C) composite electrode. It is emphasized that higher content of anatase TiO2 and Ti–OH for the TiO2/AC-p (900 °C) electrode material was contributed to low electron transport resistance and ion transport resistance [14,27]. From Fig. 6(b) and TGA analysis listed in Table 4, lower electron transport and ionic transport resistance are found for the TiO2/AC-p (900 °C) composite electrode and then a high capacitive behavior was observed.
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The capacitance of each electrode can be calculated from EIS data, shown in Fig. 7 and Table 5. The calculated capacitance for the TiO2/AC-p (700 °C) composite electrode was decline while that for TiO2/AC-p (800 °C) composite electrode and TiO2/AC-p (900 °C) composite electrode was both enhanced. The calculated EIS capacitance has the same trend as the electrosorption capacity results. 3.4. The adsorption–desorption cycle test for the pristine AC-p and TiO2/AC-p composite electrode The ion adsorption–desorption test was conducted for the pristine AC-p and TiO2/AC-p composite electrode. Fig. 8 demonstrates the adsorption–desorption cycles of the pristine AC-p (900 °C) electrode and the TiO2/AC-p (900 °C) composite electrode at initial NaCl concentration of 100 mg/l and applied potential difference of 1.2 V. It can be seen that the TiO2/AC-p (900 °C) composite electrode showed the improved electrosorption capacity and reversibility. In our experiment, the electrosorption capacity was calculated according to the difference between the influent and the effluent NaCl concentration divided by the weight of active material. The electrosorption capacity increased from 8.0 to 11.5 mg/g with an improved reversibility during the 120 cycles. 4. Conclusion The TiO2/AC-p composite materials for capacitive deionization electrode prepared by a two-step MAIS method were investigated. This method includes a controlled sol–gel reaction step and a subsequent ionic liquid-inducing crystallization step. It was found that the AC-p characteristics can influence the TiO2 precursor-H2O reaction in the reaction step. Therefore the electrosorption capacity of the TiO2/AC-p composite electrode can be affected. An enhancement in electrosorption capacity and cycle reversibility was observed for the TiO2/AC-p composite electrode prepared from AC-p with higher content of mesopores and less hydrophilic surface. The electrosorption capacity can be increased from 8.0 to 11.5 mg/g with improved reversibility during the 120 cycles for the TiO2/AC-p (900 °C) composite electrode. In addition, a decline in electrosorption capacity was found for the TiO2/AC-p composite
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Please cite this article in press as: P.-I. Liu et al., J. Colloid Interface Sci. (2015), http://dx.doi.org/10.1016/j.jcis.2014.12.007