Cellulose Framework Directed Construction of Hierarchically Porous

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Letter

Cellulose Framework Directed Construction of Hierarchically Porous Carbons Offering High-Performance Capacitive Deionization of Brackish Water Saikat Dutta, Shu-Yun Huang, Cephas Chen, Jeffrey E. Chen, Zeid Abdullah Alothman, Yusuke Yamauchi, Chia-Hung Hou, and Kevin C.-W. Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01587 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Cellulose Framework Directed Construction of Hierarchically Porous Carbons Offering High-Performance Capacitive Deionization of Brackish Water Saikat Dutta,a Shu-Yun Huang,b Cephas Chen,a Jeffrey E. Chen,a Zeid A. ALOthman,*,c Yusuke Yamauchi,d Chia-Hung Hou*,b and Kevin C.-W. Wu*,a aDepartment

of Chemical Engineering, National Taiwan University, No 1, Sec 4, Roosevelt Rd., Taipei 10617, Tai-

wan. bGraduate

Institute of Environmental Engineering, National Taiwan University, No 1, Sec 4, Roosevelt Rd., Taipei

10617, Taiwan. cChemistry

d

Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia.

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Insti-

tute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail addresses: [email protected] (ZA ALOthman), [email protected] (CH Hou), [email protected] (KCW Wu). KEYWORDS. Cellulose template, N-doped carbon, electrosorption, hierarchical electrodes, water desalination

ABSTRACT: We demonstrate a cellulose-templating method for synthesizing a hierarchically porous carbon electrode that is capable of high-performance capacitive deionization (CDI). Hierarchically porous carbons (denoted as HPC-X, X = 500-900 °C) of an exceptionally high surface area up to 2535 m2g-1 and wide-range pore size distribution (macro-, meso-, and micropores) were obtained via the pyrolysis of macroporous cellulose fibrous-templated resorcinolformaldehyde-triaminopyrimidine (RF-TPF) polymers. The improved electrosorption performance of HPC-800 electrode can be ascribed to the enhanced specific surface area, favorable hierarchical structure, and excellent capacitive electric double layer behaviors.

INTRODUCTION Increasing groundwater salinity levels have continued to affect lives in the form of clean freshwater shortages in numerous regions in the world.1,2 Thermal distillation, electro-dialysis, and reverse osmosis are currently the most effective technologies employed for large-scale freshwater supply by desalination of saline water. However, all of these technologies require

ACS Paragon Plus Environment


Page 2 of 23

special and expensive infrastructures and high energy consumption, which limit their practical worldwide applications.3,4 Con-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sequently, developing innovative desalination technology with simple materials and equipment has been highly demanded.5 Capacitive deionization (CDI) is an emerging technique that is effective for the electrosorption of salt ions from aqueous solutions by forming an electrical double layer at the electrode/electrolyte interface within porous electrodes.6 As a low-pressure and non-membrane based desalination process, CDI can be operated at a low applied voltage (0.8-1.2 V) and thereby, can drastically reduce energy usage and operational costs. Noteworthy, the biggest advantage of CDI technique lies in its high energy efficiency for desalination of brackish water (≈1 kWh m-3) that is superior to that of reverse osmosis (≈2.9-3.7 kWh m-3 depending on the salt concentration).7 Despite the great potential of CDI in desalination, the lack of desired porous materials featuring high electrosorption capacity and rapid rate of transportation of ions has inhibited formidable success of CDI-based applications. In principle, a high-performance CDI electrode requires: (i) large capacitance enforced by high specific surface area; (ii) high electrical conductivity; (iii) rapid response to ion adsorption-desorption (charge-discharge kinetics); (iv) robustness to chemical and electrochemical process. A full understanding of the electrode’s desired electrochemical features and the relationship between structure and charge/mass transfer during electrosorption process is essential for constructing an ideal CDI electrode material. Generally, carbon materials such as activated carbons (AC), carbon aerogel,8 nanoporous/mesoporous carbon,9 carbon nanotubes,10,11 and graphene12,13 have been used as the electrode materials for CDI to exhibit NaCl electrosorption capacities in the range of 0.1-10 mg/g.14 For further improvement, recent efforts were witnessed in developing different graphene-based15 materials or other hybrid materials.16 Hybrid materials such as graphene/mesoporous carbon,17 graphene/metal oxide,16 graphene-coated hollow mesoporous carbon,18 AC-coated carbon nanotubes,19 and 3D hierarchical AC/CNT hybrids20 have been reported to enhance CDI performance. However, the fabrication of such hybrids with desired pore interconnectivity is still a challenging task especially with a prerequisite of a high dispersion of graphene nanosheets. In the case of graphene hybrids, impeding the aggregation of individual graphene sheets while maintaining the graphene-coated structure add additional challenges to this graphene-based strategy. As realized from previous electrochemical applications using hierarchical porous carbon21 or graphene22 composites, specific surface area and pore volume along with pore size are the key physical parameters that determine the performance of a carbon-based electro-adsorbent. For this stimulating task of capacitive charge storage, hierarchically porous carbons (HPCs) exhibiting micro-, meso-, and macropores are believed to adsorb more charged species on their electrical double layer under an applied potential.23-27 Thus, we envisage that a template containing large macroporous framework and subsequent strategies to install meso- and micropores will be appropriate for constructing a new type of HPCs with desired and tailored porous properties. As a natural polysaccharide with three-dimensional (3D) porous structure, cellulose fibrous have been used by several researchers as the template for making metallic and metal oxide nanostructures.28-30 However, such a construction of cellulos three-dimensional (3D) porous framework for forming HPCs has not been attempted, possibly due to the limitations of


Page 3 of 23


mass diffusion31 and the optimization of the 3D structure in carbon matrix. Herein, we utilize cellulose fibrous as the template

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and coat a nitrogen-containing polymer on the surface. After subsequent activation and pyrolysis processes, a HPC electrode with micro-, meso-, macroporosity, super-high specific surface areas, and interconnected pores could be obtained. We further applied the synthesized HPC for CDI application, and one of the samples (i.e., HPC-800) achieved an excellent electrical double-layer capacitance (EDLC) ensuring high capacity of electrosorption of salt ions (3.75mg/g of electrode) from brackish water. This is of great significance for the further design and development of advanced porous carbon electrodes by exploring abundant and structurally diverse range of cellulosic templates for imparting desired porous features.

EXPERIMENTAL SECTION 2,4,6-triaminopyrimidine, formaldehyde, resorcinol, potassium hydroxide (pellets) were purchased from Sigma-Aldrich. Cellulose filter template (No. 5C) was purchased from Advantec, Japan. Synthesis of cellulose filer-templated HPC: 2,4,6-triaminopyrimidine (0.6975 g, 4.37 mmol) was dissolved in DI water (30 mL) and to this was added two Advantec cellulose filter templates (LCFTs) (4 µm average pore size, 55 mm diameter) and stirred for overnight for crushing the LCFTs to obtain a thick mixture containing aqueous solution of 2,4,6triaminopyrimidine. To this was added formaldehyde (1.07 g, 13.25 mmol) and the mixture (TPF) stirred at 80 °C for 10 minutes. In another vial, resorcinol (0.485 g, 4.37 mmol) and formaldehyde (0.7125 g, 8.75 mmol) was dissolved in distilled water (30 mL) and the mixture (RF) was stirred for 1h at 40 °C. To the RF at 40 °C, TPF was added and the resulting mixture was stirred for 30 min at 60 °C to allow the formation of light brown precipitate during the mixing. The resulting mixture was then transferred to a Teflon autoclave and heated at 120 °C for 24 h under static condition. The reddish-brown solid product RF-TPF@LCFT like spongy material was recovered by filtration and air-dried at 100 °C for 24 h. As-made composite RFTPF@LCFT (1 g) was immersed in a 20 mL KOH solution (5 M) in water. The mixture was left for 3h at room temperature and then heated at 50 °C for another 3 h. After cooling down to ambient condition, the extra KOH solution was removed by briefly filtering. Residual volume of KOH solution in the RF-TPF@LCFT composites was about 1 ml. Then, the mixture was washed by ethanol in a fixed volume ratio (residual KOH solution:ethanol ~ 1:20) and with the aid of ultra-sonication for 5 min. After pouring away the ethanol, the as-activated RF-TPF@LCFT were collected and dried in oven at 80 °C for 20 min. The resulting KOH mixed RF-TPF@LCFT was calcined at 150 °C under N2 flow for 1 h by raising the temperature at a ramp of 5 °C min-1 from room temperature. The sample was then pyrolyzed at different maximum temperatures (500, 600, 800, and 900 °C) for 3h under N2 by rising temperature with a ramp rate 5 °C min-1. After cooling down to room temperature, the products were treated with aqueous HCl solution (2 M) to remove the residual alkali (e.g. K2CO3), and then repeatedly rinsed with ultrapure water until a pH value of 7 was obtained. The resulting samples (HPC) were dried in an air-oven at 80 °C, 12 h for further characterization.

RESULTS AND DISCUSSION



Page 4 of 23

Synthesis of HPC. The synthesis process of the HPCs is shown in Scheme 1 with details described in ESI. Cellulose

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

filters with macroporous (around 4 µm in pore diameter) and fibrous framework (denoted as LCFT) were used as a 3D porous template. We further coated 2,4,6-triaminopyrimidine (TPF) on the template (denoted as TPF-LCFT), and performed the polymerization with the presence of resorcinol (RF) in a steel-jacketed Teflon reactor under hydrothermal condition (120 °C for 24 h in aqueous medium). The product was denoted as RF-TPF@LCFT. The pyrolysis process was then carried out under N2 at different temperatures (500 to 900 °C) after the KOH-assisted activation process, denoted as HPC-X (X=500 to 900 °C). The yields of HPC-X after the KOH treatment are summarized in Table S1. Micropores and mesopores in the synthesized HPCs were created by the KOH-assisted activation, while the macropores of the HPCs were created from the cellulose filters. Altered pyrolysis temperature would result in samples with different specific surface area and nitrogen content, which affected their electrochemical properties. A control experiment without using the cellulose template was also conducted, and the resulting sample exhibited low specific surface area (110.6 m2g-1). This result indicates the importance of the cellulose template.

Scheme. 1

Structural Characterization of Cellulose Template and HPC. Surface morphology of samples after and before subject to chemical treatments was investigated with field-emission scanning electron microscopy (FE-SEM). As shown in Fig. 1, a fibrous network with a porous structure could be observed for the LCFT template (Fig. 1(a)). The average pore size was determined to be around 4.2 µm from mercury intrusion porosimetry measurement (Table S2). After polymerization process, spherical polymers of RF-TPF formed on the surface of LCFT and the LCFT template color changed from white to dark orange, as shown in Fig. 1(b). We found that the RF-TPF polymer tend to form spherical particles with size around 2 µm after polymerization (Supporting information, Fig. S1(a)). The RF-TPF@LCFT samples were then treated with KOH prior to pyrolysis at various temperatures to form HPCs. The color of the sample changed from dark orange to black after pyrolysis at 800 °C, which indicates a successful conversion to carbon materials. In addition, the HPC-800 exhibited po.rous surface as evidenced from its SEM image (Fig. 1(c)). In contrast, the control sample without KOH treatment exhibited smooth surface (Supporting information, Fig. S1(b)), which suggested that KOH treatment would help create pores on the surface of the RF-TPF polymers.

Fig. 1

Thermogravemetric (TG) analysis results justified the reason for KOH treatment. As shown in Fig. 1d, the LCFT exhibited a significant loss of weight at 350 °C and near complete depletion after 450 °C, which indicates a low thermal stability (blue line in Fig. 1d). After the RF-TPF polymer coating, about 50% of the sample remained with temperatures of over 350 °C to suggest that the RF-TPF polymer exhibited higher thermal stability and was indeed the carbon source for the final HPC


Page 5 of 23


samples. However, the RF-TPF polymer was completely burned out above 600 °C (black line in Fig. 1(d)). Surprisingly, the

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

thermal stability of the RF-TPF polymer was greatly improved after KOH treatment. Several instances of weight loss at temperatures of 100, 350, and 600 °C could be observed due to the loss of ethanol, LCFT template, and certain amounts of RFTPF polymer, respectively. However, more than 50% of the sample remained when the calcination temperature exceeded 600 °C (up to 900 °C). This result clearly evidenced the enhancement of the RF-TPF polymer structure after KOH treatment. A high-resolution TEM image in Fig. 1(e-1) again shows the porous surface of the synthesized HPC-800 sample. In addition, elemental mapping images of carbon, nitrogen and oxygen for the HPC-800 demonstrate the uniform distribution of nitrogen generated from the RF-TPF polymer on the carbon matrix (Fig. 1(e-2 to e-4)). The effect of pyrolysis temperature on the porous properties of the fabricated HPCs was studied. In addition, the porous features of the HPCs made without using the LCFT template and without KOH treatment were also compared. As shown in Table 1 and Fig. 2a, the resulting N2 adsorption isotherms for HPC-500 and HPC-600 were typical type-I according to the IUPAC classification32 which indicates that large volumes of N2 molecules are adsorbed at relatively low-pressure range (P/P0 < 0.1) and that the electrode exhibit microporous features. This is also evidenced from the micropoe size distribution result of HPC-600, as shown in Supporting information Fig. S2. However, the specific surface area of the HPCs was increased from 380 to over 900 m2/g as the pyrolysis temperature increased from 500 to 600 °C. We suggest that this was due to the complete removal of RF-TPF polymer at 600 °C, according to the TG result. With a pyrolysis temperature of 800 °C, the curves for HPC-800 were closed to type-IV isotherm owing to the appearance of a capillary condensation step in the P/P0 range 0.4-0.6 to indicate the existence of mesopores. We suggested such a great increase in surface area for HPC-800 resulted from the increase of meso- and microporosity33 that was created from the removal of nitrogen atoms in the HPCs framework (vide infra). At 900 °C, the surface area of HPC-900 slightly decreased even though the microporosity still increased, due to the collapse of the macro- and mesoporous structure of HPCs. This can be explained by the isotherm of HPC-900 returning to type-I. The corresponding pore size distribution results in Fig. 2b and Supporting information Fig. S2 demonstrate the appearance of mesopores (around 2 nm) and micropore (around 1.1 nm), respectively, for all HPC-X samples, which is best seen in the HPC-800 sample. These results indicate that an optimized pyrolysis temperature at 800 °C would generate HPC samples with the maximum specific surface area and uniform mesopores.

Table 1 Fig. 2 To demonstrate the significance of the LCFT template and KOH treatment, HPC samples made without using the two treatments (i.e., HPC-600NT and HPC-600UA, respectively) were also examined. The HPC-600NT exhibited a spherical morphology with low specific surface area around 110 m2/g. By comparing HPC-600NT to the corresponding HPC-600 sample, we could conclude that the LCFT template is essential for the 3D scaffold of polymer coating. Similarly, HPC-600UA also exhibited low specific surface area with a severe lack of pores. By comparing HPC-600UA to the corresponding HPC-600 with KOH



Page 6 of 23

treatment, we concluded that the KOH treatment would create more micropores (or even mesopores when adjacent mi-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cropores merge together). Similar KOH-assisted pore-generating treatments have also been reported.34,35 From these results, we can conclude that the following three factors are all significant for synthesizing HPCs with high surface area: (1) a suitable structure template as the 3D scaffold for polymer coating, (2) an optimized pyrolysis temperature for generating carbon framework, and (3) a KOH treatment for creating more pores and enhancing porous structure. To evaluate the amount of carbon, nitrogen, and oxygen in all HPC samples, we conducted elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), and SEM energy dispersive X-ray (EDX) mapping (Supporting information, Table S3 and Fig. S3). Figure S3 shows a homogeneous distribution of carbon, nitrogen and oxygen for a typical HPC-600 sample. It is assumed that the HPCs are derived from single carbon precursor; therefore, C and N atoms are evenly distributed in the carbon matrix of HPC-500 and HPC-600 with a similar C/N ratio (~80 wt% for C and ~8 wt% for N) (Table S3). These results indicate that nitrogen atoms doped in the HPCs were preserved. At higher pyrolysis temperatures over 800 °C, the amount of nitrogen decreased to around 2 wt%. After combining this result with N2 sorption isotherm, we suggest that the enhanced surface area of HPC-800 was caused by the removal of nitrogen atoms. In addition to high-temperature pyrolysis, KOH treatment was also found to be useful for removing nitrogen atoms to thus create more pores. The amount of nitrogen for HPC-600UA was ~73 wt%, in contrast to that for HPC-600 (~8 wt%). The successful incorporation of nitrogen in HPCs was further confirmed by X-ray photoelectron spectroscopy (XPS) (Supporting information, Fig. S4). Typical XPS spectra of the synthesized HPCs revealed the atomic percentage of nitrogen, which decreased with the increase of the RF-TPF@LCFTs pyrolysis temperature (Supporting information, Table S3). From the de-convoluted plot of high-resolution N1s spectrum, the contribution of pyridinic, pyrrolic and other types of nitrogen was estimated, which is listed in Table S4. The signals in the range of 397-399 eV correspond to pyridinic N species, and the signal at 400 eV can be ascribed to pyrrolic N species. These results confirm the appearance of nitrogen doped in the HPC framework, and the amount of nitrogen greatly decreased at higher pyrolysis temperatures. This observation is consistent with that of HPCs obtained from other synthetic strategies.36-38 Interestingly, N-doping in carbon framework would alter the electronic and crystalline structure of carbon, thus enhancing the capacity of carbon materials towards surface wettability,39 adsorption capacity,40 charge storage capacity,41 and electrical conductivity.42 Electrochemical Characterization of HPCs. Electrochemical impedance spectra (EIS) analysis was conducted to evaluate the electrical conductivity of HPC electrodes. Fig. 3(a) shows the experimental data presented in the Nyquist plot with two regions between Z' (real axis) and Z'' (imaginary axis). It is revealed that all of EIS for HPCs exhibited a quasisemicircle in high-frequency region, followed by a linear part in the low-frequency region. Noteworthy, the Nyquist plots of HPC-800 and HPC-900 share a similar shape and their quasi-semicircle diameters are much shorter than that of HPC-500 and HPC-600. In addition, the Warburg tail results from the frequency-dependence of ion diffusion/transport in aqueous solution to the electrode structure. This entails the straight line with 45 o inclination at the middle frequency region, which is


Page 7 of 23


observed for HPC-500. This is not evident for HPC-800 and HPC-900 electrodes, which suggest that the ion diffusion exhib-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

its no obvious difference and thus, ions are able to easily access the electrode surface.40

Fig. 3

More importantly, the intersection of the quasi-semicircles at the real axis in the range of high frequency represents the equivalent series resistance (ESR) of the electrodes, which includes electronic resistance of electrode, the interfacial resistance between the electrode and the current collector, and diffusion resistance of ions in the small pores of electrode materials and through the separator.43 In other words, the diameter of the quasi-semicircle corresponds to the charge-transfer resistance of the electrodes and solution interface. Herein, a smaller quasi-semicircle at high frequency is pursed and the corresponding diameter means a low polarization resistance, which is closer towards pure capacitive behaviour of the electrode. A vertical straight line represents an ideal electrical double-layer capacitance at low frequencies. From the plot in the Supporting information Fig. S5, it can be seen that the ESR of HPC-800 is significantly smaller than that of HPC-600. This indicates that the HPC-800 electrode has a decreased inner resistance and an improved conductivity property due to the mesopores created at the higher pyrolysis temperature via KOH activation. This also means a faster charge/discharge rate with lesser internal loss.44 Additionally, the inclined line of the HPC-800 is more vertical than that of the HPC-600 to thus reflect the superior capacitive behaviour of HCP-800. Similar trends for the HPC materials can be observed by the EIS measurements in 1 M H2SO4 electrolyte solution as seen from their Nyquist plots (Supporting information, Fig. S6). This further demonstrates that the HPC-800 electrode exhibits excellent capacitive charge storage capability with electrical double layer mechanism to ensure an efficient electrosorption behavior. The reversibility of electrodes for the electrosorption process was observed by the galvanostatic charge/discharge (GC) test in 1 M NaCl solution at a current density of 0.1 A/g. As shown in Figs. 3b and Supporting information S7, the GC measurements exhibited continuous GC profiles for all HPC samples. It is evident that all GC curves contain a well-retained triangular shape, which indicates the good reversibility of HPC electrodes. The observed symmetric charge/discharge characteristics further demonstrate a good capacitive behaviour resulting from electrostatic attraction rather than faradaic reaction.45 Noteworthy, the discharge time of the HPC-800 is considerably longer than that of the other HPC electrodes, which indicates the increased ion storage capacity at the corresponding current density (Fig. 3b). The enhanced capacity of HPC-800 results from the increase in specific surface area and suitable pore size distribution, in terms of meso- and microporosity. Especially, the higher ratio of mesopores to total pore volume (46%) of HPC-800 than that of the HPC-900 (24%), better facilitates the transport of ions and electrons into the inner region of the electrode.46 In addition, potential drop measurements (iR drop) is an effective parameter to examine interconnected pore channels, which can decrease diffusion distance and reduce inner resistance for ion transport into the pore network. The iR drop (Ohmic drop) of HPC-800 (0.12 V) was lower than that of the



Page 8 of 23

HPC-500 (0.18 V), which indicates that the electrode HPC-800 contains a low internal resistance. This low potential drop also

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

confirms the superior conductivity of HPC-800 electrodes as a candidate for electrosorption process. Cyclic voltammetry (CV) measurement was implemented to evaluate the fundamental behavior of electric double layer capacitance. The CDI performance of porous carbon electrodes were noted, and from wherein the specific capacitances were derived accordingly.47 Herein, the electrosorption capacity for desalination has a strong dependence on the capacitance of electrodes. Fig. 3c depicts the CV profiles of the HPC electrodes in a 0.01 M NaCl solution (salt ion conc. of brackish water) with a potential range from -0.4 to +0.6 V at a scan rate of 5 mV/s. It is found that all the CV curves of HPC electrodes display near box-shaped profile without any humps to suggest the absence of Faradic reaction in the chosen potential range. This reveals that the response of specific capacitance is a result of electrical double layer formation where ions are captured by the Coulombic interactions at the electrode surface upon polarization rather than the electrochemical oxidation/reduction reaction.8 Generally, CV profile that has a larger enclosed area possesses a higher specific capacitance and ion storage capacity. As shown in Fig. 3d, the specific capacitances calculated from the CV profiles at 5 mV/s for HPC-500, HPC-600, HPC-800, and HPC-900 are 11.0, 14.4, 50.8, and 31.0 F/g, respectively. Herein, the HPC-800 electrode has the highest specific capacitance, which is approximately 64% higher than that of HPC-900 electrode. The evidently improved specific capacitance of HPC-800 can be ascribed to the following reasons: i) the enhancement of total pore volume by association of larger pores at higher pyrolysis temperature to significantly increase specific surface area and ensure more effective sites for the salt ion (e. g. NaCl) adsorption; ii) the effective combination of micropores, mesopores and macropores of HPCs provide more pore openings within the porous channels for ion penetration and transportation into the interior of bulk phase with a shorter ion diffusion. CV profiles of HPC-800 in 1, 0.1, and 0.01 M NaCl solutions at various scan rates (5 to 500 m V/s) are shown in the Supporting information Fig. S8. Clearly, a typical rectangular shape of CV profile can be observed at higher NaCl concentrations or slower scan rates, indicating an ideal capacitive behaviour of the HPC-800 electrode. As the scan rate increases from 5 to 500 mV/s, the CV profile is distorted into a leaf-like shape associated with a decrease in specific capacitance. Moreover, Fig. 3d and Supporting information Fig. S9 show the scan-rate dependency of HPC electrodes in 0.01 M NaCl at a scan rate ranging from 5 to 500 mV/s. The specific capacitance decreasing with an increase in scan rate can be attributed to the following reasons: i) ions barely have time to penetrate into the deeper pores with increasing scan rates, which results in a reduced accessible surface area for ion electrosorption ii) the Ohmic resistance for ion transport at high scan rates has influenced the formation of the electric double layer. Herein, ions may not fully develop an electrical double layer at the electrode/solution interface in the pores. It is noteworthy that the HPC-800 exhibits substantially higher specific capacitance than that of the other HPC electrodes at any sweep rate in the chosen range to indicate better electrosorption behavior in the CDI process. It is believed that the mesoporous channels can shorten the diffusion distance and possess a smaller resistance for the ion transport pathway through the porous carbon. Therefore, the HPC-800 electrode with enormous mesopores allows fast ion penetration into the deeper region, which ensures an efficient utilization of the pore surface area and thus more absorbed salt ions onto the inner surface sites.


Page 9 of 23

ACS Sustainable Chemistry & Engineering Performance of HPC Electrodes in Electrosorption of Salt Ions. According to our results, the HPC-800

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrode can be a good candidate as a CDI electrode due to its high specific surface area, desirable porous structure, and excellent capacitive double layer characteristics. The desalination performance of HPC-800 was further evaluated by batch mode CDI experiments with an initial conductivity of NaCl solution at 68.6 µs/cm (30 mg/L). The working voltage of CDI is usually limited by the potential of water electrolysis (1.23 V). As seen in Fig. 4, increasing the applied voltage from 0.8 to 1.2 V enhances the electrosorption capacity. The electrosorption capacities of HPC-800 at 0.8, 1.0 and 1.2 V are determined to be 1.74, 2.73, and 3.24 mg/g, respectively. In addition, the electrosorption capacity increases with increasing salt solution concentration due to the compression of electric double layer thickness. For example, with an initial NaCl solution of 580 mg/L, a electrosorption capacity of 7.75 mg/g can be achieved at 1.2 V. Noteworthy, at low salt concentration, the HPC-800 electrode has the highest electrosorption capacity (3.24 mg/g) as compared to the performance of other common carbon electrodes reported in literature (Table S5), including AC (0.25 mg/g), OMC (0.68 mg/g), graphene (1.30 mg/g), and as well as graphene hybridized mesoporous carbon electrodes.48,49 One can conclude that the desalination performance of HPC-800 outperforms other porous carbon materials for high performance CDI electrodes.

Fig. 4

Good reproducibility is essential for the utilization of electrode materials in CDI. Fig. 5(a)-(c) shows the electrosorption and regeneration cycles of HPC-800 in 30 mg/L NaCl solution at an applied voltage of 1.2 V. Once the voltage is imposed, ions are electroadsorbed onto the oppositely charged electrode surface and the conductivity decreases correspondingly, while the current arises dramatically due to the transport of ions. Ions are released to the bulk solution and the conductivity returns to the initial value by the discharging process (0.0 V) to demonstrate the good reversibility ability of HPC-800 electrode.

Fig. 5

It is worth noting that the large surface area and the hierarchical pore structure are postulated to be crucial to high desalination capability. More specifically, the porous carbon electrodes may experience a very limited mass transfer of salt ions at low NaCl solution concentrations (e.g., 0.01 M) where large amounts of disordered micropores are inaccessible for ion electrosorption. In order to avoid such barrier, nitrogen-enriched ordered mesoporous carbons obtained through direct pyrolysis in ammonia has been for capacitive performance,50 however hierarchical ordered mesoporous carbon was found to be more suitable for capacitive performance of brackish water.51 At the verge of exciting development of cellulose based materials, we demonstrate a cellulose fiber-templated electrode synthesis technique with potential application to CDI process, as well as the enhanced scope for the optimization of po-



Page 10 of 23

rous structures for cellulose-directed electrodes. A significant discrimination of the N2 isotherms between HPC-800 and HPC-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

900 (Fig. 2a) indicates a difference mesopores and micropores composition. As seen, the N2 isotherm of HPC-900 can be classified as type I. Herein, the ratio of micropores to the total pore volume of HPC-900 (76 %) is much higher than that of HPC-800 (54 %). This observation is justified as per the higher microporous nature of HPC-900, which may cause limited mass transport and possible inaccessibility of some micropores to salt ions. In contrast, N2 isotherm of HPC-800 exhibits a typical type II isotherm with higher specific surface area, but lower micropore surface area, in which the presence of mesopores can facilitate the ion transport to guarantee high capacitive double layer capacity. This enables HPC-800 to act better as a salt-ion reservoir with contribution from larger pore volume and the coexistence of macro-, meso- and micropores. This feature is in accordance with the results obtained by the electrochemical measurements. As pointed out previously, the HPC800 offers more preferential capacitive characteristics, such as enhanced electrical conductivity, longer charge/discharge time, less scan-rate dependence, and higher specific capacitance. Therefore, the HPC-800 with high specific surface area, large pore volume, suitable pore size distribution, and hierarchical structure can be considered as a promising electrode material for CDI application. Noteworthy, CDI applications are usually operated at low electrolyte concentrations (e. g. brackish water). The desalination capacity of the resultant HPC-800 electrode is quite promising in similar experimental conditions as compared to other novel three-dimensional (3D) porous carbon materials (Table S5), such as 3D macroporous graphene architecture (3DMGA)52 that can be fabricated with sacrificial polystyrene microspheres. In the case of our HPC-800, a novel macroporous cellulose-fiber templating was successful to offer similar performance. Additionally, the electrosorption capacity of HPC-800 is higher than that of a three-dimensional hierarchical porous carbon (3DHPC)53, which contains a bimodal pore arrangement of meso- and macropores prepared via a double-templating strategy.53 The improved desalination performance could be ascribed to the increased pore volume and enhanced specific surface area.54 However, the graphene-based hierarchically porous carbon (3DGHPC)55 prepared by dual templating, via incorporation of porous carbon into the 3D graphene (3DG) for constructing hierarchical pore networks with a bimodal pore distribution, still displays superiority in desalination performance. To the best of our knowledge, the cellulose-framework directed HPC created in this work is an advanced and unique model for the preparation of high-performance CDI electrode materials, especially without installing a second component (graphene) into the macroporous framework for the creation of graphene-based HPCs.

CONCLUSION In conclusion, we disclose the potential of cellulose filter paper with macroporous framework to direct the construction of a series of HPC-X electrodes consisting of hierarchical pores and exceptionally high surface areas (maximum 2535 m2 g-1 for HPC-800) that can be tuned by varying pyrolysis temperatures. HPC-X exhibit promising capacitive electric double layer performance with enhanced electrical conductivity, high specific capacitance, and low inner resistance to ion diffusion at the interface of electrode-electrolyte. Noteworthy, the HPC-800 electrode exhibits superior performance to electroadsorb salt ions from brackish water (0.01 M NaCl) with an electrosorption capacity of 7.75 mg/g at an applied voltage of 1.2 V. This ensures


Page 11 of 23


wide scope cellulose framework directed hierarchical porous electrode fabrication for large-scale capacitive charge storage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

devices, electroactive sorbent in Li-batteries etc. with interesting ion dynamics and transport.

ASSOCIATED CONTENT The experimental details, CDI measurement, characterization of the synthesized samples are provided in Supporting Information. Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng…….

AUTHOR INFORMATION Corresponding Author *Tel: 886-2-3366-3064. E-mail: [email protected] (ZA ALOthman), [email protected] (KCW Wu), [email protected] (CH Hou)

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS We would like to thank the Ministry of Science and Technology (MOST), Taiwan and National Taiwan University (104R7706) for the funding support (103-2113-M-008-001; 104-2119-M-008-010). The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this Prolific Research Group (PRG-1436-04).

REFERENCES (1)

Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and Technology for Water Purification in

the Coming Decades. Nature 2008, 452, 301-310. (2)

Barnett, T. P.; Adam, J. C.; Lettenmaier, D. P. Potential Impacts of a Warming Climate on Water Availability in Snow-Dominated Regions.

Nature 2005, 438, 303-309. (3)

Elimelech, M.; Phillip, W. A. The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333, 712-717.

(4)

Service, R. F. Desalination Freshens Up. Science 2006, 313, 1088-1090.

(5)

Kim, S. J.; Ko, S. H.; Kang, K. H.; Han, J. Direct Seawater Desalination by ion Concentration Polarization. Nat. Nano 2010, 5, 297-301.

(6)

Biesheuvel, P. M.; Bazant, M. Z. Nonlinear Dynamics of Capacitive Charging and Desalination by Porous Electrodes. Phys. Rev. E 2010, 81,

031502. (7)

Anderson, M. A.; Cudero, A. L.; Palma, J. Capacitive Deionization as an Electrochemical Means of Saving Energy and Delivering Clean Water.

Comparison to Present Desalination Practices: Will it Compete?. Electrochim. Acta 2010, 55, 3845-3856.


ACS Sustainable Chemistry & Engineering (8)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

Haro, M.; Rasines, G.; Macias, C.; Ania, C. O. Stability of a Carbon gel Electrode When Used for the Electro-Assisted Removal of Ions from

Brackish Water. Carbon 2011, 49, 3723-3730. (9)

Malgras, V.; Ji, Q.; Kamachi, Y.; Mori, T.; Shieh, F.-K.; Wu, K.C.-W.; Ariga, K.; Yamauchi, Y. Templated Synthesis for Nanoarchitectured

Porous Materials. Bull. Chem. Soc. Jpn. 2015, 88, 1171-1200. (10)

Li, H.; Pan, L.; Zhang, Y.; Zou, L.; Sun, C.; Zhan, Y.; Sun, Z. Kinetics and Thermodynamics Study for Electrosorption of NaCl onto Carbon

Nanotubes and Carbon Nanofibers Electrodes. Chem. Phys. Lett. 2010, 485, 161-166. (11)

Wang, L.; Wang, M..; Huang, Z.-H.; Cui, T.; Gui, X.; Kang, F.; Wang, K.; Wu, D. Capacitive Deionization of NaCl Solutions Using carbon

Nanotube Sponge Electrodes. J. Mater. Chem., 2011, 21, 18295-18299. (12)

Li, H.; Lu, T.; Pan, L.; Zhang, Y.; Sun, Z. Electrosorption Behavior of Graphene in NaCl Solutions. J. Mater. Chem. 2009, 19, 6773-6779.

(13)

Zhang, D.; Yan, T.; Shi, L.; Peng, Z.; Wen, X.; Zhang, J. Enhanced Capacitive Deionization Performance of Graphene/Carbon nanotube

Composites. J. Mater. Chem. 2012, 22, 14696-14704. (14)

Biesheuvel, P. M. Thermodynamic Cycle Analysis for Capacitive Deionization. J. Colloid Int. Sci. 2009, 332, 258-264.

(15)

Yang, Z.-Y.; Jin, L.-J.; Lu, G.-Q.; Xiao, Q.-Q.; Zhang, Y.-X.; Jing, L.; Zhang, X.-X.; Yan, Y.-M.; Sun, K.-N. Sponge-Templated Preparation of

High Surface Area Graphene with Ultrahigh Capacitive Deionization Performance. Adv. Funct. Mater. 2014, 24, 3917-3925. (16)

Yin, H.; Zhao, S.; Wan, J.; Tang, H.; Chang, L.; He, L.; Zhao, H.; Gao, Y.; Tang, Z. Three-Dimensional Graphene/Metal Oxide Nanoparticle

Hybrids for High-Performance Capacitive Deionization of Saline Water. Adv. Mater. 2013, 25, 6270-6276. (17)

Zhang, D.; Wen, X.; Shi, L.; Yan, T.; Zhang, J. Enhanced Capacitive Deionization of Graphene/Mesoporous Carbon Composites. Nanoscale

2012, 4, 5440-5446. (18)

Wang, H.; Shi, L.; Yan, T.; Zhang, J.; Zhong, Q.; Zhang, D. Design of Graphene-Coated Hollow Mesoporous Carbon Spheres as high

Performance Electrodes for Capacitive Deionization. J. Mater. Chem. A 2014, 2, 4739-4750. (19)

Shi, K.; Ren, M.; Zhitomirsky, I. Activated Carbon-Coated Carbon Nanotubes for Energy Storage in Supercapacitors and Capacitive Water

Purification. ACS Sustain. Chem. & Eng. 2014, 2, 1289-1298. (20)

Zhou, F.; Liu, Q.; Kang, D.; Gu, J.; Zhang, W.; Zhang, D. A 3D Hierarchical Hybrid Nanostructure of Carbon Nanotubes and Activated carbon

for High-Performance Supercapacitors. J. Mater. Chem. A 2014, 2, 3505-3512. (21)

Huang, X.; Kim, S.; Heo, M. S.; Kim, J. E.; Suh, H.; Kim, I. Easy Synthesis of Hierarchical Carbon Spheres with Superior Capacitive

Performance in Supercapacitors. Langmuir, 2013, 29, 12266-12274. (22)

Wen, X.; Zhang, D.; Yan, T.; Zhang, J.; Shi, L. Three-Dimensional Graphene-Based Hierarchically Porous Carbon Composites Prepared by a

Dual-Template Strategy for Capacitive Deionization. J. Mater. Chem. A 2013, 1, 12334-12344. (23)

Xia, Y.; Mokaya, R.; Walker, G. S.; Zhu, Y. Superior CO2 Adsorption Capacity on N-doped, High-Surface-Area, Microporous Carbons

Templated from Zeolite. Adv. Energ. Mater. 2011, 1, 678-683. (24)

Zhao, Y.; Liu, X.; Yao, K. X.; Zhao, L.; Han, Y. Superior Capture of CO2 Achieved by Introducing Extra-framework Cations into N-doped

Microporous Carbon. Chem. Mater. 2012, 24, 4725-4734. (25)

Qie, L.; Chen, W.; Xu, H.; Xiong, X.; Jiang, Y.; Zou, F.; Hu, X.; Xin, Y.; Zhang, Z.; Huang, Y. Synthesis of Functionalized 3D Hierarchical

Porous Carbon for High-Performance Supercapacitors. Energy Environ. Sci. 2013, 6, 2497-2504. [26)

Estevez, L.; Dua, R.; Bhandari, N.; Ramanujapuram, A.; Wang, P.; Giannelis, E. P. A Facile Approach for the Synthesis of Monolithic

Hierarchical Porous Carbons – High Performance Materials for Amine Based CO2 Capture and Supercapacitor Electrode. Energy Environ. Sci. 2013, 6, 1785-1790.


Page 13 of 23 (27)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering Srinivas, G.; Krungleviciute, V.; Guo, Z.-X.; Yildirim, T. Exceptional CO2 Capture in a Hierarchically Porous Carbon with Simultaneous High

Surface Area and Pore Volume. Energy Environ. Sci. 2014, 7, 335-342. (28)

Zhang, L.; Liu, Z.; Cui, G.; Chen, L. Biomass-Derived Materials for Electrochemical Energy Storages. Progress Polym. Sci. 2015, 43, 136-164.

(29)

He, J.; Kunitake, T.; Watanabe, T. Porous and Nonporous Ag Nanostructures Fabricated Using Cellulose Fiber as a Template. Chem. Commun.

2005, 795-796. (30)

Aoki, Y.; Huang, J.; Kunitake, T. Electro-Conductive Nanotubular Sheet of Indium Tin Oxide as Fabricated from the Cellulose Template. J.

Mater. Chem. 2006, 16, 292-297. (31)

Wang, D.-W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H.-M. 3D Aperiodic Hierarchical Porous Graphitic Carbon Material for High-Rate

Electrochemical Capacitive Energy Storage. Angew. Chem. Int. Ed. 2008, 47, 373-376. (32)

Sing, K. S. W. Reporting Physisorption Data for Gas/Solid Systems. Pure Appl.Chem. 1982, 54, 2201-2218.

(33)

Wang, J.; Senkovska, I.; Oschatz, M.; Lohe, M. R.; Borchardt, L.; Heerwig, A.; Liu, Q.; Kaskel, S. Highly Porous Nitrogen-Doped Polyimine-

Based Carbons with Adjustable Microstructures for CO2 Capture. J. Mater. Chem. A 2013, 1, 10951-10961. (34)

Qie, L.; Chen, W.; Xu, H.; Xiong, X.; Jiang, Y.; Zou, F.; Hu, X.; Xin, Y.; Zhang, Z.; Huang, Y. Synthesis of Functionalized 3D Hierarchical

Porous Carbon for High-Performance Supercapacitors. Energy Environmen. Sci. 2013, 6, 2497-2504. (35)

Armandi, M.; Bonelli, B.; Geobaldo, F.; Garrone, E. Nanoporous Carbon Materials Obtained by Sucrose Carbonization in the Presence of KOH.

Microporous Mesoporous Mater. 2010, 132, 414-420. (36)

Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Bandosz, T. J. Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of

Microporous Activated Carbon on its Electrochemical Performance in Supercapacitors. Adv. Funct.Mater. 2009, 19, 438-447. (37)

Su, F.; Poh, C. K.; Chen, J. S.; Xu, G.; Wang, D.; Li, Q.; Lin, J.; Lou, X. W. Nitrogen-Containing Microporous Carbon Nanospheres with

Improved Capacitive Properties. Energy & Environmen. Sci. 2011, 4, 717-724. (38)

Li, M.; Xue, J. Integrated Synthesis of Nitrogen-Doped Mesoporous Carbon from Melamine Resins with Superior Performance in

Supercapacitors. J. Phys. Chem. C 2014, 118 (5), 2507-2517. (39)

Guo, H.; Gao, Q. Boron and Nitrogen Co-Doped Porous Carbon and its Enhanced Properties as Supercapacitor. J. Power Sources 2009, 186,

551-556. (40)

Hao, G.-P.; Li, W.-C.; Qian, D.; Lu, A.-H. Rapid Synthesis of Nitrogen-Doped Porous Carbon Monolith for CO2 Capture. Adv. Mater. 2010, 22,

853-857. (41)

Jeong, H. M.; Lee, J. W.; Shin, W. H.; Choi, Y. J.; Shin, H. J.; Kang, J. K.; Choi, J. W. Nitrogen-Doped Graphene for High-Performance

Ultracapacitors and the Importance of Nitrogen-Doped Sites at Basal Planes. Nano Lett. 2011, 11, 2472-2477. (42)

Lee, Y.-H.; Lee, Y.-F.; Chang, K.-H.; Hu, C.-C. Synthesis of N-doped Carbon Nanosheets from Collagen for Electrochemical energy

Storage/Conversion Systems. Electrochem. Commun. 2011, 13, 50-53. (43)

Lei, Z.; Christov, N.; Zhao, X. S. Intercalation of Mesoporous Carbon Spheres Between Reduced Graphene Oxide Sheets for Preparing High-

Rate Supercapacitor Electrodes. Energy Environ. Sci. 2011, 4, 1866-1873. (44)

Li, Y.; Van Zijll, M.; Chiang, S.; Pan, N. KOH Modified Graphene Nanosheets for Supercapacitor Electrodes. J. Power Sources, 2011, 196,

6003-6006. (45)

Kim, K.-S.; Park, S.-J. Influence of Multi-Walled Carbon Nanotubes on the Electrochemical Performance of Graphene Nanocomposites for

Supercapacitor Electrodes. Electrochim. Acta 2011, 56, 1629-1635. (46)

Lu, X.; Dou, H.; Gao, B.; Yuan, C.; Yang, S.; Hao, L.; Shen, L.; Zhang, X. A Flexible Graphene/Multiwalled Carbon Nanotube Film as a High

Performance Electrode Material for Supercapacitors. Electrochem. Acta 2011, 56, 5115-5121.


ACS Sustainable Chemistry & Engineering (47)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

Wang, K.; Wang, Y.; Wang, Y.; Hosono, E.; Zhou, H. Mesoporous Carbon Nanofibers for Supercapacitor Application. J. Phys. Chem. C 2008,

113, 1093-1097. (48)

Zhang, D.; Wen, X.; Shi, L.; Yan, T.; Zhang, J. Enhanced Capacitive Deionization of Graphene/Mesoporous Carbon Composites. Nanoscale

2012, 4, 5440-5446. (49)

Wang, H.; Shi, L.; Yan, T.; Zhang, J.; Zhong, Q.; Zhang, D. Design of Graphene-Coated Hollow Mesoporous Carbon Spheres as High

Performance Electrodes for Capacitive Deionization. J. Mater. Chem. A, 2014, 2, 4739-4750. (50)

Wang, X.; Liu, C.-G.; Neff, D.; Fulvio, P. F.; Mayes, R. T.; Zhamu, A.; Fang, Q.; Chen, G.; Meyer, H. M.; Jang, B. Z.; Dai, S. Nitrogen-

Enriched Ordered Mesoporous Carbons Through Direct Pyrolysis in Ammonia with Enhanced Capacitive Performance, J. Mater. Chem. A 2013, 1, 79207926. (51)

Mayes, R. T.; Tsouris, C.; Kiggans Jr. J. O.; Mahurin S. M.; DePaoli D. W.; Dai, S. Hierarchical Ordered Mesopo-

rous Carbon from Phloroglucinol-Glyoxal and its Application in Capacitive Deionization of Brackish Water J. Mater. Chem. 2010, 20, 8674-8678. (52)

Wang, H.; Zhang, D.; Yan, T.; Wen, X.; Zhang, J.; Shi, L.; Zhong, Q. Three-Dimensional Macroporous Graphene Architectures as High

Performance Electrodes for Capacitive Deionization. J. Mater. Chem. A 2013, 1, 11778-11789. (53)

Wen, X.; Zhang, D.; Shi, L.; Yan, T.; Wang, H.; Zhang, J. Three-Dimensional Hierarchical Porous Carbon with a Bimodal Pore Arrangement for

Capacitive Deionization. J. Mater. Chem. 2012, 22, 23835-23844. (54)

Dutta, S.; Bhaumik, A.; Wu, K. C. W. Hierarchically Porous Carbon Derived from Polymers and Biomass: Effect of Interconnected Pores on

Energy Applications. Energy Environ. Sci. 2014, 7, 3574-3592. (55)

Wen, X.; Zhang, D.; Yan, T.; Zhang, J.; Shi, L. Three-Dimensional Graphene-Based hierarchically Porous Carbon Composites Prepared by a

Dual-Template Strategy for Capacitive Deionization. J. Mater. Chem. A, 2013, 1, 12334-12344.


Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Fabrication of HPC-X (X = 500–900 °C) from RF-TPF polymer via macroporous cellulosetemplating strategy.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

Figure 1. FE-SEM images (a-c) of LCFT (a), RF-TPF@LCFT before pyrolysis (b), and HPC obtained by pyrolysis of RF-TPF@LCFT at 800 °C. The inset in each image displays the digital image of the corresponding sample. (d) Thermogravemetric (TG) analysis of LCFT, RT-TPF@LCFT, and KOH-activated RT-TPF@LCFT samples. (e1) A TEM image and (e-2 to e-4) the corresponding EDX mapping images for the HPC-800 sample.

16


Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cellulose Framework Directed Construction of Hierarchically Porous Carbons Offering High-Performance Capacitive Deionization of Brackish Water Saikat Dutta, Shu-Yun Huang, Cephas Chen, Jeffrey E. Chen, Zeid A. ALOthman, Yusuke Yamauchi, Chia-Hung Hou and Kevin C.-W. Wu Cellulose-framework templated hierarchically porous carbon materials were synthesized and used for capacitive deionization of brackish water (i.e., in 0.01 M NaCl, the electrosorption capacity is 7.75 mg/g at an applied voltage of 1.2 V).

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

18


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Summary of porous properties of HPC samples.

Sample

HPC900 HPC800 HPC600 HPC500 HPC600UA[b] HPC600NT[c]

Specific surface area (m2/g)

Average pore diameter (nm)

Total Micropore Total mipore Surface cropore volume (m2/g)[a] volume 3 (cm /g) (cm3/g)[a]

2378

2.01

1.19

1637.80

0.902

2535

2.37

1.50

1409.85

0.805

939

2.20

0.95

902.60

0.468

380

2.31

0.22

165.82

0.136

208

2.42

0.13

-

-

110

2.36

0.07

-

-

[a] t-Plot method, [b] UA = un-activated, [c] NT = non-templated.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

Figure 2. (a) N2 sorption isotherms and (b) pore size distribution of HPCs after different treatments. HPC-X (X = 500, 600, 800, and 900) indicates different pyrolysis temperatures. HPC-600NT and HPC-600UA represent the HPC-600 sample synthesized without using the LCFT template and KOH treatment, respectively.

20


Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. EIS presented as Nyquist plots in 1 M NaCl (a); typical GC curves with current density at 100 mA/g (b); CV cure at 5 mV/s (c) and specific capacitance vs. scan rate (d) in a 0.01 M NaCl.

21



3.5 Electrosorption capacity (mg/g electrode HPC-800)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

3.0

0.8 V

1.0 V

1.2 V

2.5 2.0 1.5 1.0 0.5 0.0 Voltage (V)

Figure 4. EIS presented as Nyquist plots in 1 M NaCl (a); typical GC curves with current density at 100 mA/g (b); CV cure at 5 mV/s (c) and specific capacitance vs. scan rate (d) in a 0.01 M NaCl.

22


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Multiple electrosorption and regeneration cycles at an applied voltage of 1.2 V in ~30 mg/L NaCl solution for the conductivity (a), working voltage (b), and current (c) measurements.

23


Cellulose Framework Directed Construction of Hierarchically Porous

Recommend Documents