Activation of hierarchically ordered mesoporous carbons for enhanced capacitive deionization application

Synthetic Metals 205 (2015) 48–57

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Activation of hierarchically ordered mesoporous carbons for enhanced capacitive deionization application Yuan-Cheng Tsai, Ruey-an Doong * Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 December 2014 Received in revised form 20 March 2015 Accepted 25 March 2015 Available online xxx

In this study, an environmentally benign strategy was used to fabricate hierarchically ordered mesoporous carbons (HOMCs) using sugarcane bagasse as the scaffold and then activated with nitric acid or carbon dioxide for capacitive deionization (CDI) application. Calcium ions were selected as the model species because of the importance to hard water. The electronic microscopic images show that the HOMCs contain large domains of highly ordered hexagonal arrays of mesopores with 1-D channels and the ordered structures are retained after activation with nitric acid (HOMC-H). In addition, the oxygen and nitrogen content increase in HOMC-H. Different from HOMC-H, a partial distortion of mesostructures with the increase in micropore surface area is observed after physical activation with CO2. The electrochemical performance of HOMCs shows ideal rectangular voltammograms with non-faradic reaction at scan rates of 1–10 mV s
1 and the specific capacitance of HOMC-H is 1.4–20 times higher than those of as-prepared and CO2-activated HOMCs. The impedance measurement shows good transport of the bulk electrolyte to the electrolyte/electrode interface of HOMC-H with excellent reversibility and ideal capacitive properties. In addition, the specific electrosorption capacity of HOMC-H electrode materials for Ca2+ removal can be up to 115.4 mmol g
1 at 1.2 V. The excellent electrochemical performance of HOMC-H is mainly attributed to the increased mesoporous structures and hydrophilic functional groups after chemical activation. Results clearly indicate that the HOMC-H is a promising electrode which could facilitate good charge propagation and fast ion adsorption to treat grey and brown waters. ã 2015 Published by Elsevier B.V.

Keywords: Hierarchically ordered mesoporous carbons (HOMCs) Sugarcane bagasse Activation Capacitive deionization (CDI) Electrosorption capacity

1. Introduction Ordered mesoporous carbons (OMCs) have recently been regarded as the novel nanomaterials for a wide application in the fields of electrochemical adsorption, supercapacitor, lithium ion batteries, catalyst support and water purification [1–6]. Several technologies including soft and hard template methods have been used to fabricate various morphologies of OMCs [7,8], and the solvent evaporation-induced self-assembly (EISA) has been demonstrated to be a promising method for fabrication of OMCs [9]. However, the produced amounts of OMCs by EISA are usually limited and cannot provide the sufficient quantity for application. More recently, the use of natural and artificial templates including sugarcane bagasse [10], polyurethane foam [11] and crab shell [12] as the scaffolds for mass production of OMCs has been developed.

* Corresponding author. Tel.: +886 3 5726785; fax: +886 3 5718649. E-mail address: [email protected] (R.-a. Doong). http://dx.doi.org/10.1016/j.synthmet.2015.03.026 0379-6779/ ã 2015 Published by Elsevier B.V.

Huang and Doong [13] used natural sugarcane bagasse as the sacrificial scaffold to prepare hierarchically porous carbon materials (HPCMs) and found that the bagasse-based HPCMs had a good electrochemical performance with the specific capacitances of 190–234 F g
1 at the scan rates of 5–50 mV s
1 in acidic solutions. Although OMCs show good electrochemical performance on energy storage, the hydrophobic characteristics on the surface of OMCs could lower the wettability and electrolyte accessibility of OMCs, resulting in the decrease in transfer rates of electrons and ions. Several physicochemical methods have been developed to modify the surface characteristics of OMCs and chemical oxidation by oxidants such as HNO3, H2O2 and KMnO4 has been proven to be an effective method to introduce oxygencontaining groups including hydroxyl, carboxyl, carbonyl, lactone, and quinone groups to the surface of carbon materials [13–15]. Among the reagents used for modification, nitric acid is one of the most often used chemical because of the good ability to remove impurities on carbon surface. Tripathi et al. [16] used

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nitric acid to functionalize OMC prepared by using SBA-15 silica as the template and found that both N and O contents were increased after oxidation while ordered structures were maintained. Chou et al. [17] found that purification of singlewall carbon nanotubes by nitric acid not only increased the internal surface area but also introduced oxygen-containing surface groups. The supply of sufficient clean water for daily demand has recently received consideration attention because of the need of industrial and agricultural activities and, therefore, the development of an efficient technology for separation and purification of water is required. Capacitive deionization (CDI) has been regarded as an energy-efficient process to remove a wide variety of ions because of the low energy consumption, low pressure demand and environmental friendliness [14,15]. Similar to the energy storage in supercapacitors, the mechanism for salt removal by CDI involves the application of an electric field to force ionic species toward opposite electrodes, and results in the accumulation of ions within the electrical double layers near the electrode surfaces [16]. The electrosorption plays an important role in determination of efficiency of CDI, and, therefore, the search of electrode materials with good pore textures and high specific surface areas is highly needed. Activated carbons (ACs) have been widely applied as the electrode materials for supercapacitors and CDI [21]. However, the wide pore distribution may cause severe reduction in capacitance at a short current drain time [17,18]. Several graphene-based composite materials including graphene/mesoporous carbon, graphene-coated hollow mesoporous carbon and three-dimensional graphene-based hierarchically porous carbon have been developed for CDI application [19–26]. The graphene-based materials with 3-D interconnected structures and high specific surface areas can provide more adsorption sites for enhanced desalination efficiency, resulting in the improvement of electrochemical properties and the reduction of inner resistance for ion transfer. More recently, the 3-dimensional hierarchically OMC (HOMC) has been considered as a promising material for energy storage and CDI application because of its ability to reduce the resistance of electrolyte diffusions as well as to provide suitable ion transport pathway for electrolytes in hierarchical structures [27–29]. However, the application of HOMCs for CDI process to remove ions has received less attention. In addition, the activation of carbon materials with chemical reagents or CO2 may alter the surface hydrophilicity of HOMCs, resulting in the enhancement of CDI efficiency. Therefore, it is desirable to develop a fabrication strategy for mass production and activation of HOMCs so that the carbon materials can have high specific surface area and well inter-connective pore channels to enhance the electrosorption capacity toward ion separation. In this study, an environmentally benign strategy for the fabrication of HOMCs using sugarcane bagasse as the scaffold for mass production was developed for CDI application. Calcium ions were selected as the model species because of its importance to hard waters. The surface functionality of HOMCs was then activated with CO2 or HNO3 for enhanced electrocapacitive performance of CDI processes. The morphology, specific surface area, surface property and change in elements of HOMCs before and after the activation were investigated. In addition, the electroechemical performance of as-prepared and activated HOMCs was examined by using cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS). The electrosorption capacity of as-prepared and activated HOMCs for removal of calcium ions was also evaluated.

49

2. Experimental 2.1. Chemicals Phenol (99%) and formaldehyde (24 wt%) were obtained from Acros Organics (Pittsburg, PA). Absolute ethanol (99.8%) was purchased from Riedel-deHaën (Seelze, Germany). Pluronic F127 (EO106PO70EO106), the amphiphilic triblock copolymer for mesostructural directing agent, was purchased from Sigma–Aldrich Co. (Milwaukee, WI). Commercial activated carbons were obtained from China Activated Carbon Industries Ltd. (New Taipei City, Taiwan). All other chemicals were of analytical grade and were used as received without further purification. In addition, aqueous solutions were prepared using bi-distilled deionized water (Millipore Co., 18.3 MV cm) unless otherwise mentioned. 2.2. Preparation of phenol-formaldehyde (PF) resins The basic polymerization method was used to synthesize lowmolecular-weight PF resins (MW 500–5000) in the presence of phenol and formaldehyde. Briefly, 32 g of phenol was melted at 45  C followed by the addition of 6.8 g of 20 wt% NaOH solution under stirring conditions. After 10 min of mixing, 87.3 g of formaldehyde were added into the solution dropwise, and the mixture was stirred at 65  C for 60 min. The pH was adjusted to 6.0–7.0 using 2 N HCl solutions after cooling down to room temperature, and then the water content was removed in vacuum at temperature lower than 50  C. The PF resins were re-dissolved in 50 wt% of ethanol solution. 2.3. Mass production of HOMCs Fig. 1 illustrates the mass production of HOMCs by EISA in the presence of sugarcane bagasse. The sugarcane bagasse was first sliced into the size of 20  10 mm2, and the mixture containing 5 g of F127 and 10 g of 50 wt% PF resins was prepared under homogeneous stirring for 1 h at room temperature. The sliced sugarcane bagasse was submerged in the mixture under stirring for 0.5 h. After infiltration, the sliced carbon materials were stood for 3 h at 25  C and then at 50  C for another 24 h to evaporate the

Fig. 1. Schematic illustration of hierarchically ordered mesoporous carbons (HOMCs) using sugarcane bagasse as the scaffold.

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solvent. After drying, the monoliths were heated to 100  C for 24 h for further thermo-polymerization and then to 150  C for another 24 h for the production of high-quality HOMCs with well interconnection channels [11]. Subsequently, the polymeric monoliths were calcined at 900  C for 3 h at a rate of 1  C min
1 under N2 atmosphere in a tubular furnace (denoted as as-prepared HOMCs). In addition, the carbon materials derived from the carbonization of pure sugarcane bagasse at 900  C (denoted as SB) were prepared under the same conditions for further comparison. 2.4. Activation of HOMCs The as-prepared HOMCs were physically activated by CO2 at high temperature of 850  C or chemically modified by HNO3 to remove impurities on carbon surfaces as well as to alter the surface properties. For chemical activation, 0.2 g of HOMCs were homogeneously stirred in 30 mL of concentrated HNO3 at room temperature for 2 h, filtered and washed with deionized water until the pH of filtrates was at neutral, and then dried at 108  C for 24 h (denoted as HOMC-H). In addition, a modification with carbon dioxide at high temperature was used to remove impurities as well as to increase the specific surface area of carbon materials. For physical activation, HOMCs were heated at 850  C for 2.5 h at a rate of 2  C min
1 under CO2 atmosphere in a tubular furnace (denoted as HOMC-C). 2.5. Characterization and measurement The crystalline structures of HOMCs were examined using an X-ray diffractometer (XRD, Bruker NEW D8 ADVANCE, Germany) with Ni-filtered Cu Ka radiation (l = 1.5406 Å) at 40 kV and 40 mA. The small-angle XRD patterns were obtained from 0.5 to 6 2u , while the wide-angle XRD patterns were acquired from 10 to 90 2u. The inter-planar spacing (d) was estimated using Bragg’s

formula (d = nl/(2 sinuB)), where l is the wavelength of Cu Karadiation source and uB is the Bragg’s angle of 2u /2. The grain size of HOMCs was determined by transmission electron microscopy (TEM) images by using a JEOL JEM 2100 microscope operated at 200 kV on a holey carbon film supported on a copper grid. The surface morphology and porosity of HOCMs were examined by field-emission scanning electron microscope (FE-SEM, JEOL, JSM-6330F) at an accelerating voltage of 15 kV. A thin layer of Au was coating on the samples to improve the resolution. The specific surface areas of HOMCs were determined by nitrogen adsorption–desorption isotherms at 77 K using N2 adsorption analyzer (Micromeritics, ASAP 2020). Samples were degassed at 180  C for 12 h in vacuum and then the specific surface areas were determined by the Brunauer–Emmett–Teller method using adsorption data in a relative pressure (P/P0) range of 0.02–0.2. The total pore volumes were estimated from the adsorbed amount at the P/P0 value of 0.995. The micropore surface areas as well as pore volumes were calculated from the t-plot method where the t values were calculated using the de Bore equation, t (Å) = [13.99/(log(P0/P) + 0.0340)]1/2 in the P/P0 range of 0.08–0.30. Micro Raman spectroscopy (DU420A) was used to investigate the band characterization of carbon-based materials. The change in weight ratio of major elements in the HOMCs was determined by Elementar VarioEL-III (GmbH) for C, H, N, S and by Thermo Flash 2000 (Italy) for oxygen. Fourier-transform infrared spectrometer (FTIR, Bomem DA8.3, Canada) was used to characterize the change in functional groups of HOMCs before and after the activation processes. Pure KBr was prepared as the baseline. The FTIR was recorded in the wavenumber range 400–4000 cm
1 with 100 scans collected at 4 cm
1. The surface chemical compositions of HOMCs were analyzed by an ESCA PHI 1600 photoelectron spectrometer (Physical Electronics, Eden Prairie, MN) using an Al Ka radiation of 1486.6 eV as the excitation source.

Fig. 2. SEM images of (A) low (400) and (B) high (2000) magnitudes of sugarcane bagasse and TEM images of (C) as-prepared HOMCs, (D) HOMC-H and (E) HOMC-C.

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galvanostatic charge/discharge measurements were investigated at different current densities ranging from 0.1 to 1.0 A g
1.

2.6. Electrochemical measurement The electrochemical properties of carbon materials were evaluated by Autolab PGSTAT 128N electrochemical test system (Metrohm Autolab B.V., Netherlands) in a three-electrode system at 25  C. The Ag/AgCl electrode and platinum wire were used as the reference and counter electrodes, respectively. The working electrode material was a mixture consisting of carbon materials (as-prepared HOMCs, HOMC-C, HOMC-H), polyvinylidene fluoride (PVDF) and carbon black (CB) at a weight ratio of 8:1:1. The mixture was homogeneously suspended in N-methyl-2-pyrrolidone and spread on the current collector (titanium plate) and dried at 60  C for 24 h. The cyclic voltammetry (CV) of as-prepared and activated HOMCs was investigated at the scan rates of 1–10 mV s
1 in a voltage window from
0.6 to 0.6 V. The specific capacity (SC) was evaluated by adding 1.0 M calcium chloride solutions under acidic conditions. In addition, the temperature effect on the SC of carbon materials at 10–40  C was investigated. The specific capacitance can be calculated from CV curves using the following equation: qa þ jqc j 2m  DV

(1)

where Cs is the specific capacitance, qa and qc represent the anodic and cathodic voltammetric charges, respectively, m is the mass of HOMCs on the electrode, and DV is the potential windows of cyclic voltammogram. EIS measurements were recorded from 10 kHz to 0.1 Hz with an alternate current amplitude of 10 mV, and the base potential was 0.2 V vs. Ag/AgCl in 1.0 M CaCl2 electrolyte to minimize the pseudo-capacitive reaction. In addition,

(A)

3. Results and discussion 3.1. Characterization of hierarchically ordered mesoporous carbon Fig. 2 shows the SEM images of sugarcane bagasse and TEM images of HOMCs before and after the activation. The sugarcane bagasse shows large and replicated void volumes (Fig. 2(A) and (B)), which can provide interconnected channels for impregnation, self-assembly and thermopolymerization for the fabrication of

100

110 HOMC

100

002

(B)

002

Intensity (a.u.)

Intensity (a.u.) Adsorbed amount /(cm3(STP)g)-1

The electrosorption efficiency of calcium ions on the HOMCbased electrodes was evaluated using a flow-through system including a CDI unit cell, a peristaltic pump and a thermostat controller (Fig. S1, see Supplementary data). A piece of working electrode at around 24 mg was stuck on a size of 40  40  0.2 mm3 of titanium plate, and the calcium ion concentrations of 10, 20 and 40 mg-Ca2+ L
1 were used as the target ions to evaluate the CDI performance on water softening. In addition, commercial ACs were used as the carbon electrode for comparison. The solution was pumped and recirculated to the CDI unit cell by 2.5 mL min
1 at 25  C and at the applied potential of 1.2 V. The deionization efficiency was calculated by the change in calcium ion concentration determined by inductively couple plasma optical emission spectrometry (ICP-OES) at 393.4 nm.

100

1

(C)

2.7. Electrosorption of calcium ion by capacitive deionization

110 200

100 HOMC,R=1.19 100 HOMC-H, R=1.71

HOMC-H HOMC-C 2

3

4

5

HOMC-C 10

6

20

30

40

800 700 600

(D)

HOMC HOMC-H HOMC-C

500 400 300 200 100 0.0

0.2

0.4

0.6

Relative Pressure (P/P0)

50

60

70

80

90

2 Theta / degree

2 Theta / degree

0.8

1.0

Differential pore volume (cm3g-1)

Cs ¼

51

0.5

HOMC HOMC-H HOMC-C

0.4 0.3 0.2 0.1 0.0

2

4

6

8

10

12

14

16

Pore Size (nm)

Fig. 3. (A) Small-angle and (B) wide-angle XRD patterns, (C) nitrogen adsorption–desorption isotherms and (D) pore size distribution of HOMC-based materials including asprepared HOMC, HOMC-H and HOMC-C.

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HOMCs. As illustrated in Fig. 2(C), the TEM image of as-prepared HOMCs after carbonization at 900  C under N2 atmosphere shows the highly ordered 1-D strip-like hexagonal arrays of mesopores. The size of channels and pore walls in as-prepared HOMCs are 3.7  0.1 and 5.84 nm, respectively. After activation with nitric acid, the HOMC-H show large domains of highly ordered 1-D strip-like channels (Fig. 2(D)) and the 2-D hexagonal mesostructures are still retained. The diameter of 1-D channels and pore wall are 3.7  0.1 nm and 5.78 nm, respectively, which mean that the activation with HNO3 has little effect on the alternation of channel structures of HOMCs. However, a partial distortion of mesostructure is observed when HOMCs are activated by CO2 at 850  C for 2.5 h. The size of channels and pore wall in HOMC-C are 3.9  0.1 and 5.48 nm, respectively, clearly showing that the physical activation with CO2 slightly changes the ordered mesostructures of HOMCs, and results in the structural shrinkage inside the pores (Fig. 2(E)). The small- and wide-angle XRD patterns of HOMCs confirm the generation of highly ordered mesostructures in the framework. As shown in Fig. 3(A), the as-prepared HOMCs and HOMC-H have major small diffraction peaks at around 0.92 , 1.6 and 1.92 2u , which can be assigned as the 10, 11 and 20 reflections of 2-D hexagonal mesostructure with symmetry (p6mm space group) [30]. Different from the chemical activation processes, poorly resolved small-angle XRD peaks are observed when HOMCs are activated with high temperature of CO2. In addition, the low intensity of scattering peaks indicates the reduction and distortion of the mesostructured regularity, which can be clearly observed in TEM images (Fig. 2(E)). The wide-angle XRD pattern of as-prepared HOMCs (Fig. 3(B)) shows two distinct broad peaks centered at 22.3 and 43.2 2u , which can be indexed as the 0 0 2 and 1 0 0 planes of the carbon materials [31]. After activation with nitric acid, the XRD peaks of HOMC-H slightly shift to 22.8 and 43.7 2u, indicating the decrease in d spacing of the lattice. The poorly resolved XRD peaks of HOMC-C are mainly attributed to the distortion of the mesostructured rigidity. In addition, the single-layered carbon sheets in disordered carbon can be estimated by the ratio of heights between the Bragg peaks at around 22 2u and background [13,32]. The ratio increases from 1.19 for as-prepared HOMCs to 1.71 for HOMC-H, which indicates the formation of single-layered carbon sheets and the increase in degree of graphitization after chemical activation with HNO3. Fig. 3(C) shows that nitrogen adsorption–desorption curves of as-prepared HOMCs, HOMC-H, and HOMC-C. All the as-prepared and activated HOMCs show type-IV isotherms with a steep increase in the P/P0 region of <0.1, depicting the generation of micropores in the framework after pyrolysis and carbonization under N2 atmosphere. In addition, the H4 hysteresis loop shown in the P/P0 range of 0.4–0.78 indicates the co-existence of slit-shaped micropores and mesopores. As illustrated in Table 1, the specific surface area of as-prepared HOMCs is 508 m2 g
1 and the average

Table 1 Specific surface area and pore volumes of various hierarchically ordered mesoporous carbons (HOMCs) before and after activation. Carbons

SBET (m2 g
1)

Smicro (m2 g
1)

Vtot (cm3 g
1)

Vmicro (cm3 g
1)

SB HOMC HOMC-H HOMC-C

2.7 508 487 1517

– 247 (49%)a 179 (37%) 710 (47%)

– 0.395 0.404 1.154

– 0.112 (28%) 0.08 (20%) 0.319 (27%)

SBET: specific surface area, Vtot: total pore volume at P/P0 = 0.995; Smicro: specific surface area of micropores calculated by t-plot analysis; and Vmicro: pore volume of micropores. a Values shown in parentheses are the percentages of micropore specific surface areas and micropore volume in SBET and Vtot, respectively.

pore diameters, derived from density function theory, is 3.86 nm (Fig. 3(D)) with the total pore volume of 0.395 cm3 g
1, which is comparable with the results obtained in previous study [13]. Activation of HOMCs with nitric acid slightly decreases the specific surface area to 487 m2 g
1. However, the average pore diameter increases to 3.92 nm. In addition, the pore volume of HOMC-H increases to 0.404 cm3 g
1 while the micropore volume decreases from 0.112 to 0.08 cm3 g
1, indicating the increase in mesopores after the chemical activation with nitric acid. Different from the HOMC-H nanomaterials, the specific surface area as well as pore volume of HOMC-C increases to 1517 and 1.154 cm3 g
1, respectively. In addition, the specific surface area of micropore and average pore size diameter of HOMC-C increase to 710 m2 g
1 and 3.92 nm, respectively, presumably attributed to the removal of residual carbons and impurities onto the surface of HOMCs. A previous study [33] used CO2 to activate the ordered porous carbons obtained from the colloidal silica-F127 templating method, and found that the porosity of carbon increased with the increase in activation time, which is in good agreement with the result obtained in this study. To further understand the change in surface properties before and after the activation, the change in elemental compositions of HOMCs was examined. Table S1 (see Supplementary data) shows the elemental compositions of HOMCs. The carbonized sugarcane bagasse contains 70.6 wt% of C, 0.36 wt% of N and 4.77 wt% of O. After impregnation with F127 and PF resins to form as-prepared HOMCs, the elemental composition of carbon increases to 85.38 wt %. However, the carbon contents decrease to 81.23–79.6 wt% with the concomitant increase in hydrogen contents (1.33–1.48 wt%) after activation, clearly depicting the removal of residual carbons and impurities of as-prepared HOMC. The nitrogen and sulfur contents in HOMC-C increase slightly after the physical activation with CO2, presumably attributed to the fact that the sugarcane bagasse contains trace amounts of N and S and the removal of impurities increases the weight ratios of these two intrinsic elements in HOMCs. In addition, the increase in oxygen and nitrogen contents in HOMC-H indicates the increase in functional groups of HOMCs after chemical activation with nitric acid. Fig. 4 shows the C1s XPS spectra of as-prepared HOMC, HOMC-H and HOMC-C after peak deconvolution. The C1s XPS spectra of asprepared HOMC and HOMC-C show three different peaks centered at 284.5, 285.1 and 286.2 eV, which can be assigned as C¼C, C

C and C

O bonds, respectively. It is clear that five peaks which correspond to C¼C (284.5 eV), C

C (285.1 eV), C

H (285.5 eV), C

O (286.2 eV) and

O

C¼O (289.1 eV) bonds are observed in the XP spectra of HOMC-H and the ratios are 51.1% for C¼C bonds, 20.7% for C

C bonds, 10.1% for C

H bonds, 10.8% for C

O bonds, and 7.3% for

O

C¼O bonds. These results indicate that the activation with nitric acid not only increases the O and N contents, but also modifies the surface functionality of the carbon materials, which may have specific effects on electrochemical behaviors and ion transfer [34]. Fig. S2 (see Supplementary data) shows the FTIR spectra of various ordered mesoporous carbons. A broad band at 1100 cm
1, which belongs to the stretch vibration of C

O bonds, is observed in all HOMC materials. After activation with nitric acid, another two bands at 1530–1570 cm
1 for C

O stretching and 1385 cm
1 for NO3
stretching are observed in HOMC-H. A weak band appeared at 820 cm
1 due to out-of-plane deformation of C

H and another weak band near 1220 cm
1 contributing from

OH bending in carboxylic group (

COOH) also appears in HOMC-H spectra. The broad peak at 3410–3480 cm
1 is the OH group. In addition, the HOMC-C spectrum shows weak C

O and N

O stretching peaks at 1570 and 1439 cm
1, respectively. These results are in good agreement with the elemental analysis shown in Table S1, and the increase in functional groups of HOMCs after

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high value of IG/ID means the high degree of graphitization. The IG/ ID values of HOMC, HOMC-H and HOMC-C are calculated to be 1.11, 1.06 and 1.05, respectively, indicating the existence of sp2 structures with two-dimensional hexagonal lattice in pore structures. The relatively low IG/ID values of HOMC-C is mainly attributed to the partial distortion of mesostructures resulting from the oxidation of the pore walls, which has been evidenced by XRD patterns and TEM images.

(A) Sum Fit Background C=C C-C C-O

Intensity (a.u.)

53

3.2. Electrochemical properties of HOMCs

298

296

294

292

290

288

286

284

282

280

Binding energy (eV) (B)

Intensity (a.u.)

Sum Fit Background C=C C-C C-O

298

296

294

292

290

288

286

284

282

280

Binding energy (eV)

Intensity (a.u.)

(C)

Sum Fit Background C=C C-C C-H -O-C=O C-O

In this study, the electrochemical property of various ordered porous carbon materials on capacitive rate performance was investigated. Fig. 6(A) shows the CV profile by HOMC, HOMC-H and HOMC-C in the 1.0 M CaCl2 solution at a scan rate of 5 mV s
1 and at 25  C. All the CV curves of HOMC materials show ideal rectangular and symmetry voltammograms with non-Faradic reaction, indicating the typical capacitive behaviors with good charge propagation and easy ion transport in the electrode materials. It is noteworthy that the specific surface area of HOMC-C (1517 m2 g
1) is higher than those of as-prepared HOMC and HOMC-H (487–508 m2 g
1). However, the specific capacitance for HOMC-C (8.7 F g
1) is far lower than those of as-prepared and HNO3activated HOMC materials (51.8–88.4 F g
1). The excellent capacitive behaviors of as-prepared HOMC and HOMC-H are mainly attributed to the contribution of the interconnected channels in the mesoporous carbon materials to form double-layer capacitance for ion transport. Several studies have depicted that mesopores play a crucial role in electrochemical performance of mesoporous materials [18,36]. It is noteworthy that the specific surface areas of as-prepared HOMCs and HOMC-H are similar, but the average pore diameter of HOMC-H is slightly larger than that of as-prepared HOMC. The large mesopores can facilitate the fast ion transport in the interconnected meso- and micropores, resulting in maintenance of the excellent capacitive behavior of HOMC-H. In addition, the surface oxygen-containing functional groups make the HOMC-H more hydrophilic than that of as-prepared HOMCs, and subsequently accelerate the ion transport from bulks solution to the surface of ordered mesoporous carbons. Fig. 6(B) shows the CV curves of HOMC-H electrode at various scan rates ranging from 1 to 10 mV s
1. All the CV curves of HOMCs show ideal rectangular voltammograms, which indicate the good charge propagation and easy ion transport in the electrode materials. The specific capacitances are in the range 84–93 F g
1 at scan rates of 1–10 mV s
1. Several studies have depicted that the specific capacitance was in the range 45–122 F g
1 at a scan rate of

300 298 296 294 292 290 288 286 284 282

Fig. 4. The C1s XPS spectra of (A) as-prepared HOMC; (B) HOMC-C and (C) HOMC-H.

activation could result in the enhancement of electrochemical property of HOMCs [35]. Fig. 5 shows the Raman spectra of as-prepared and activated HOMCs. The Raman spectra of HOMCs show two characteristic peaks at 1330 and 1590 cm
1. The D-band of disordered carbon at 1330 cm
1 is mainly attributed to the existence of imperfections and defects within the graphitic structure, while the peak at 1590 cm
1 belongs to Raman-active E2g mode, which indicates the presence of G band of graphitic sp2 carbon structures within a twodimensional hexagonal lattice [31]. In addition, the intensity ratio of G and D bands (IG/ID) represents the graphitized degree, and the

G

Intensity (a.u.)

Binding energy (eV)

D

HOMC

HOMC-H HOMC-C 750

1000

1250

1500

1750

2000

2250

-1

Raman shift (cm ) Fig. 5. Raman spectra of as-prepared and activated HOMCs.

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(A)

6.0

(B) 100

Current (mA)

4.0

HOMC-H

2.0

80

0.0 60

-2.0

As-prepared HOMC

-4.0

40

-6.0

HOMC

-8.0 -10.0 -0.8

20

HOMC-C

HOMC-C 0

-0.6

-0.4

-0.2

0.0

0.2

0.4

0

0.6

2

E(V) vs Ag/AgCl

Current (mA)

5.0 0.0 -5.0

10 mV/s 5 mV/s 2 mV/s 1 mV/s

-15.0 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E(V) vs Ag/AgCl

Specific capacitance (F/g)

(D)

(C) 10.0

-10.0

4

6

8

10

Scan rate (mV/s)

140 120 100 80 60 40 20 0

10 degC

25 degC

40 degC o

Temperature ( C)

Fig. 6. (A) Cyclic voltammetric curves of various HOMC-based electrodes at a scan rate of 5 mV s
1; (B) specific capacitances of HOMC-based electrode at various scan rates ranging from 1 to 10 mV s
1; (C) cyclic voltammetric curves of HOMC-H electrode at various scan rates, and (D) temperature effect on the specific capacitance of HOMC-H electrode at 5.0 mV s
1.

1 mV s
1 [13,27], which is in good agreement with the results obtained in this study. Fig. 6(C) shows the specific capacitance of porous carbon materials as a function of scan rate ranging from 1 to 10 mV s
1. The specific capacitances of as-prepared HOMC decreased from 64.3 F g
1 at 1 mV s
1 to 43.6 F g
1 at 10 mV s
1. The physical activation of HOMC decreases the specific capacitance and only 4.2–27.7 F g
1 are obtained for HOMC-C electrodes, presumably due to the distortion of mesostructures and the lack of interconnected mesopores for formation of double-layered capacitance. However, the calculated specific capacitance for HOMC-H electrode at 1 mV s
1 can reach 93.2 F g
1, which is 1.45 and 3.45 times higher than those of as-prepared HOMCs and HOMC-C, respectively. The high specific capacitance of HOMC-H is mainly attributed to the interconnected mesoporous channels and the oxygen-containing functional groups such as hydroxyl (

OH) and nitro (

NO2) groups in carbon materials. Previous studies have depicted that the oxygen-containing functional groups improved the surface hydrophilicity and provided reversible electrochemical active sites for electrolyte accessibility in porous structures at low scan rate, resulting in the acceleration of ion transport and increase in specific capacitance [37–42]. The effect of temperature on the specific capacitance of HOMCH electrode was further investigated. Fig. 6(D) shows the specific capacitance of HOMC-H electrode at various reaction temperatures using 1.0 M CaCl2 as the electrolyte. The specific capacitance for HOMC-H electrode increases from 88 F g
1 at 10  C to 117.7 F g
1 at 40  C, showing that high temperature can increase the specific capacitance of HOMC-H electrodes.

The electrochemical performance of HOMC-based materials was further evaluated by galvanostatic charge/discharge cycling and EIS spectra. Fig. 7 shows the charge/discharge cycling curves of HOMC-based electrode materials under fixed and various current densities conditions ranging from 0.1 to 1.0 A g
1. The charge– discharge cycling curves are highly symmetric at a fixed current density of 0.1 A g
1 and still maintain the typical triangular shape at 0.1–10 A g
1, which indicate the good reversibility with ideal capacitive properties and less Faradic reaction. Fig. 8 shows the Nyquist plot of HOMC-based electrode materials in 1.0 M CaCl2. In the low frequency regions, the linear lines of HOMC and HOMC-H electrodes parallel to the imaginary axis are clearly observed, which indicate the ideal capacitive behavior because of the ordered pore structure and interconnected channels. The equivalent series resistances (Res) of HOMC, HOMC-H and HOMC-C electrodes shown in the inset of Fig. 8 are 0.58, 0.48 and 0.51 V, respectively, depicting that the resistance of the electrolyte between working and reference electrodes is very low. In addition, The semicircle of HOMC-C electrode in high frequency region was observed, which means that HOMC-C electrode is an electrochemically reactioncontrol process [6,34]. 3.3. Enhancement of capacitive deionization The capacitive deionization of HOMCs for removal of calcium ions was evaluated using various HOMC-based electrodes. Fig. 9 shows the SEC of HOMC-based electrodes toward the calcium ion removal as a function of applied voltage. The SEC for Ca2+ removal

Y.-C. Tsai, R.- Doong / Synthetic Metals 205 (2015) 48–57

12

(A) 1.0

HOMC HOMC-H HOMC-C

10

0.6 0.4 0.2

8 1.0

Impedance (Z", Ohm)

0.8 Impedance (Z", Ohm)

Potential (V, vs Ag/AgCl)

55

6

4

2

0.0

0.8 0.6 0.4 0.2 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Impedance (Z', Ohm)

0

1000

2000

3000

4000

5000

6000

0 0

2

4

6

Time (sec)

8

10

12

14

16

18

20

Impedance (Z', Ohm) Fig. 8. The Nyquist plot of as-prepared and activated HOMCs. -1

0.1 Ag 0.2 Ag-1 0.4 Ag-1 0.6 Ag-1 0.8 Ag-1 1.0 Ag-1

1.0 0.8 0.6 0.4 0.2 0.0 0

200

400

600

800

1000

1200

Time (sec) Fig. 7. (A) The charge–discharge curves of HOMC-H at (A) constant current density of 0.1 Ag
1 in electrolyte and (B) different current densities (0.1–1.0 Ag
1) in 1.0 M CaCl2 electrolyte.

by HOMC-based electrodes increases upon increasing applied voltage and a linear relationship between SEC and applied voltage is observed. The SEC of HOMC-based electrodes for Ca2+ removal is 3.3 mg g
1 for as-prepared HOMC and HOMC-C, and 4.6 mg g
1 for HOMC-H at 1.2 V. In this study, the effect of various initial Ca2+ concentrations on SEC was also investigated. As shown in Fig. 10, the SEC increases with the increase in initial concentration. The high calcium ion concentration in bulk solution can produce high concentration gradient, and then increases the ion transfer rate from bulk solution to the surface of carbon electrode. It is noteworthy that the SEC of AC-based electrode for Ca2+ was higher than those of as-prepared HOMC- and HOMC-C-based electrodes, but is lower than that of HOMC-H-based electrode. The high SEC of HOMC-H is mainly attributed to the large domains of highly ordered 1-D strip-like channels and 2-D hexagonal mesostructures. In addition, the functional groups on the electrode surface after activation also enhanced the electrochemical property and deionization performance of HOMC-H. The regeneration of carbon electrode by discharging is an important process. Ions can be transferred from bulk solutions to the surface of electrode and then adsorbed onto the carbon electrode during the charging process, leading to the decrease in conductivity in bulk solutions. However, the adsorbed ions could be released to the bulk solutions after discharging. Fig. S3 shows

the electrosorption–desorption profiles of electrolyte using different carbon-based electrodes. During the charging processes at 1.2 V, no obvious difference in electrosorption efficiency between HOMC-H- and AC-based electrodes is observed. However, the HOMC-H-based electrode exhibits a better regeneration performance than that of AC-based electrode, presumably due to the regular mesostructure and uniform pore size distribution of HOMC-H. Several studies have used various carbon materials as the CDI electrodes to remove ions from solutions and found that the CDI electrodes have different electrosorption behaviors for ions, depending on the atomic mass, valence, initial concentration, and hydrated ionic radius [43–49]. A previous study [23] has fabricate the a three-dimensional (3-D) graphene-based hierarchically porous carbon (GHPC) for CDI and found that the electrosorptive capacity of GHPC was 6.18 mg g
1, which was higher than that of the 3-D graphene alone. Mossad and Zou [44] have used ACs as the electrode materials to separate ions and found that the SECs for Na+, Ca2+, Mg2+ and Fe3+ were 2.97, 1.04, 0.74 and 0.026 mg g
1, respectively. Peng et al. [46] have fabricated threedimensional micro/mesoporous carbon composites with carbon nanotube networks for CDI application and found that the desalination efficiency of NaCl by OMC/CNT electrode was in the range 10.67–11.83 mmol g
1. Yang and Zou [48] used calcium waste

Specific electrosorption capacity (mg/g)

Potential (V, vs Ag/AgCl)

(B)

5 HOMC HOMC-H HOMC-C

4

3

2

1

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Applied potential (V) Fig. 9. Specific electrosorption capacity (SEC) of as-prepared and activated HOMCs at various applied potentials ranging from 0 to 1.2 V.

56

Specific electrosorptive capacity (mg/g)

Y.-C. Tsai, R.- Doong / Synthetic Metals 205 (2015) 48–57

4. Conclusions

6.0 5.5 5.0 4.5 4.0 3.5

HOMC HOMC-H HOMC-C AC

3.0 2.5 10

15

20

25

30

35

40

Initial Ca2+ concentration (mg/L) Fig. 10. Specific electrosorptive capacity (SEC) of as-prepared HOMC, HOMC-H, HOMC-C and commercial AC at various initial Ca2+ concentrations.

Table 2 The specific electrosorptive capacity (SEC) of ions by using various carbon materials as the CDI electrodes. Ions 2+

Carbon materialsa Applied volt SEC, mg g
1 (mmol g
1) References

In this study, we have successfully used the activated HOMCs as the novel electrode materials for enhanced CDI application. A well 2-D interconnected mesostructure is obtained for as-prepared HOMCs and the 2-D hexagonal mesostructures with large domains of highly ordered 1-D strip-like channels are retained after activation. However, a partial distortion of mesostructure is observed when HOMCs are physically activated with CO2 at 850  C for 2.5 h. The XRD patterns and Raman spectra confirm the generation of large domains of single-layered carbon sheets, resulting in the increase in degree of graphitization after chemical activation with HNO3. Although the activation with nitric acid slightly decreases the specific surface of HOMCs, the mesoporous volume as well as functional groups of HOMC-H increases. All the CV curves of ordered mesoporus carbon materials show ideal rectangular and symmetry voltammograms with non-faradic reaction at scan rates of 1–10 mV s
1, indicating the typical capacitive behaviors with good charge propagation and easy ion transport in the electrode materials. The good electrochemical reversibility with ideal capacitive properties is mainly attributed to the well-obtained 2-D hexagonal mesostructures. The specific capacitance of HOMC-H electrode materials is 1.45 and 3.45 times higher than those of as-prepared HOMC and HOMC-C materials, respectively, and the SEC for Ca2+ is 115.4 mmol g
1 at 1.2 V. Results obtained in this study clearly demonstrate that the activated HOMCs can provide large interconnected mesostructures to facilitate fast ion adsorption and open an avenue for development of higher efficiency of capacitive deionization technology for treatment of grey and brown waters.

HOMC HOMC-H HOMC-C

1.2

3.26 (81.5) 4.62 (115.4) 3.28 (81.9)

This study

Activated carbon Activated carbon Activated carbon MC-on-graphite 3DG 3DGHPC OMC–CNT Porous carbon MC

1.0–1.4 1.5 1.2 1.2 1.2

[15] [44] [45] [15] [23]

1.2 1.4 2.0

1.3–5.5 2.97 2.26 (38.68) 6.1–14.6 4.41 (75.4) 6.18 (105.6) 0.62–0.69 (10.7–11.8) 11.68 1.36 (23.33)

[46] [47] [48]

The authors thank the Ministry of Science and Technology (MOST), Taiwan for financial support under grant No. 101-2221-E007-084-MY3.

Ca2+

Activated carbon Activated carbon

1.5 1.2

1.04 2.28 (57.22)

[44] [45]

Appendix A. Supplementary data

Mg2+

Activated carbon

1.5

0.74

[44]

Fe3+

Activated carbon

1.5

0.026

[44]

Ca

Na+

a HOMC: hierarchically ordered mesoporous carbon; HOMC-H: HNO3-actived HOMC; HOMC-C: CO2-activated HOMC; MC: mesoporous carbon; OMC: ordered mesoporous carbon; 3DG: three-dimensional graphene; 3DGHPC: three-dimensional graphene-based hierarchically porous carbon.

to fabricate mesoporous carbon for CDI, and the SEC for Na+ could reach 23.33 mmol g
1. In addition, Hou and Huang [45] used activated carbon-based electrodes to compare the electrosorption efficiency of Na+ and Ca2+ and found that the electrosorption capacity for Ca2+ (57.22 mmol g
1) was 19% higher than that of Na+ (38.68 mmol g
1), presumably due to the fact that divalent calcium ions have strong Columbic interactions between the charged surfaces. In this study, the SECs of HOMC electrodes for Ca2+ are in the range 81.5–115.4 mmol g
1, which are superior to those reported data shown in Table 2 [16,19,35–40]. This result clearly indicates the excellent electrosorption ability of HOMC toward hardness ion removal. The enhanced CDI efficiency after modification is attributed to the fact that a large fraction of mesopores in HOMCs provides large surface area to generate double layers for ions. In addition, the functional groups such as

O,

OH and

NO can increase the hydrophilicity of electrode, resulting in the acceleration of the electrosorption of calcium ions onto the surface of HOMCs.

Acknowledgements

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