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Creating 3D hierarchical carbon architectures with micro-#meso- and macropores via a simple selfblowing strategy for flow-through deionization capacitor Shanshan Zhao, Tingting Yan, Hui Wang, Jianping Zhang, Liyi Shi, and Dengsong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03704 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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ACS Applied Materials & Interfaces

Creating 3D Hierarchical Carbon Architectures with Micro-，，Meso- and Macropores via a Simple Self-Blowing Strategy for Flow-Through Deionization Capacitor Shanshan Zhao, Tingting Yan, Hui Wang, Jianping Zhang, Liyi Shi, Dengsong Zhang* Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China. Fax: 86 21 66136079; E-mail: [email protected]

Abstract In this work, 3D hierarchical carbon architectures (3DHCA) with micro-，meso- and macropores was well prapared via a simple self-blowing strategy as highly efficient electrodes for flow-through deionization capacitor (FTDC). The obtained 3DHCA has hierarchically porous structure, large accessible specific surface area (2061 m2 g-1) and good wettability. The electrochemical tests show that the 3DHCA electrode has a high specific capacitance and good electric conductivity. The deionization experiments demonstrate that the 3DHCA electrodes possess a high deionization capacity of 17.83 mg g-1 in a 500 mg L-1 NaCl solution at 1.2 V. Moreover, the 3DHCA electrodes present fast deionization rate in 100-500 mg L-1 NaCl solutions at 0.8-1.4 V. The 3DHCA electrodes also present a good regeneration behavior in the reiterative regeneration test. These above factors render the 3DHCA a promising FTDC electrode material.

Key words Carbon; Resin; 3D hierarchical carbon architectures; Capacitor; Electrodes.

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INTRODUCTION Nowadays the freshwater shortage has become a worldwide problem because of the industrial development and population increase1, 2. Therefore, it is very urgent to find effective ways to produce freshwater. The desalination of seawater is a significant and promising way to solve the freshwater shortage problem3-5. Conventional desalination methods include membrane techniques, reverse osmosis, evaporation and electrodialysis. However, most of these methods have some inevitable drawbacks including great energy consumption, low yield and environmental pollution4, 6. Thus, finding low-cost and effective strategies for seawater desalination is very important. The flow-through deionization capacitor (FTDC) can be used in seawater and brackish water desalination, which is designed from the principle of electric double layer capacitor (EDLC). When a cell voltage is applied, the electrodes would be charged and the salt ions would move towards the opposite-charged electrodes and then be adsorbed into the electrodes. After the cell voltage is removed, the ions can be desorbed immediately7-9. Therefore, the deionization technology of the FTDC is a low-cost, energy-efficient and reversible method for seawater desalination with no secondary pollution10-12. In consideration of the above EDLC mechanism, the deionization performance largely depends on the properties of the FTDC electrode materials6,

13

. Recently, highly porous

materials have attracted much attention in the area of ion adsorption and storage14-17 A good FTDC electrode material also should have rational porous structure, appropriate pore size distribution, high accessible surface area, good wettability and good conductivity13. The electrosorption performance is also influenced by applied voltage, spacer thickness, retention 2


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time and chemical surface charge18, 19. Carbon materials are ideal candidates because of their unique properties. Up to now, activated carbon20,

21

, graphene22-24, carbon nanotubes25-27,

carbon nanofibers28, 29, carbon aerogels30, 31, mesporous carbon32, 33 and their composites34-36 have been widely used as FTDC electrode material by researchers. Activated carbon is the most used electrodes due to its high specific surface area. However, the activated carbon is microporous dominated and the micropores restrict the salt ions diffusion and not beneficial to the mass transferring. Mesoporous carbon materials have narrow mesopore size distribution and a large number of mesoporous channels, which would be in favor of the transporting and penetrating of the ions. Therefore, mesoporous carbon materials show a preferable salt adsorption performance compared with the activated carbon37. Unfortunately, mesoporous carbon has shortcomings of low effective surface area and long diffusion distance, which restrict its improvement of the salt adsorption capacity. In order to solve these problems, many researchers are working on preparing novel carbon materials with different pore size. Currently, hierarchically porous carbon has been regarded to be a promising electrode material and has exhibited an excellent performance in the area of electrochemistry17, 38, 39. Hierarchically porous carbon materials combine the advantages of different pore structures. The macropores can form ion-buffering reservoirs and minimize the ion diffusion distance and thus accelerate the transportation of the salt ions. The mesopores guarantee a minimized resistance for the salt ions in the porous carbon materials, and the micropores can provide a large number of adsorption sites and the resulting enhanced electrical double-layer capacitance17,

40

. Moreover, hierarchically porous carbon materials

have been used as FTDC electrodes by us and other groups and show good salt adsorption 3



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performance. We synthesized three-dimensional hierarchically porous carbon by using SiO2 as templates, triblock copolymer as soft templates and phenolic resin as precursor. The salt adsorption experiments show that the obtained three-dimensional hierarchically porous carbon have a deionization capacity of 2.16 mg g-1 in a 30 mg L-1 NaCl solution at 2 V, which is higher than ordered mesoporous carbon (1.94 mg g-1)41. Wang and coworkers prepared activated carbon nanofiber with well-developed hierarchically with micro-，meso- and macroporous structure through electrospinning process followed by CO2 activation, and they found that the obtained hierarchical activated carbon nanofiber possess a deionization capacity of ~4.5 mg g-1 in a 90 mg L-1 NaCl solution at 1.6 V, which is better than activated carbon (~3 mg g-1)42. However, these methods are multi-step, fussy and time-cosuming, which cannot meet the large-scale production. Most recently, we prepared graphene-like carbon nanosheets using a Fe-catalyzed glucose-blowing strategy, in which NH4Cl was used as a blowing agent43. The obtained graphene-like carbon nanosheets possess a salt adsorption capacity of ~12.86 mg g-1 in a 500 mg L-1 NaCl solution at 1.6 V. Here, we propose a new self-blowing strategy by using KHCO3 as self-blowing agent to prepare 3D hierarchical carbon architectures (3DHCA) with micro-, meso- and macropores. The strategy is very simple, time-saving and cost-effective and thus has a good prospect for commercial application. The schematic illustration for the preparation of the 3DHCA is shown in Figure 1. The pretreated cation exchange resin was mixed with KHCO3 and then calcined. KHCO3 decompose and generate a lot of gas, which react with the as-formed carbon. Throughout the entire process, gas are generated constantly and blow out macropores with three dimensional structure. The reactions between 4


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self-blowing agent and as-formed carbon contribute to the formation of mesopores and micropores. As a result, the 3DHCA with hierarchical pores consisting of macro-, meso- and micropores and high specific surface area was obtained. Importantly, the preparation process is very simple, environmently friendly and effective. The 3DHCA electrode is demonstrated to be a promising candidate for the FTDC application.

EXPERIMENTAL SECTION Preparation The cation exchange resin was purchased from Shanghai Hualing Resin Co., Ltd (China). The graphite papers were purchased from Zhongbang Hardware Store (Beijing East Road, Shanghai), and the thickness of the graphite paper is 1 mm. Other chemicals were provided by Sinopharm Chemical Reagent Co., Ltd. Typically, the cation exchange resin was immersed in deionized water for a few days and then dried at 60 o C. Then the pretreated cation exchange resin was mixed with KHCO3 (in 20 mL deionized water, the ratio of the resin to KHCO3 was 1:2, 1:4, 1:5, 1:6 and 1:8). The mixture was evaporate to dryness at 60 o C and then put into the tube furnace and calcined at 600-800 o C for 3 h in N2 atmosphere (5 o C min-1). The resulting sample was then washed by using 2 mol/L HCl solution so as to remove the inorganic impurity. The obtained black powers were named as 3DHCA/x-y, in which x is the mass ratio (KHCO3:Resin) and y is the calcination temperature. For comprasion, direct calcined carbon (DC) was prepared with the absence of KHCO3 by direct heating the resin at 800 o C for 3 h in N2 atmosphere (5 o C min-1). 5



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Characterization The materials were investigated TG, N2 adsorption-desorption mearsurments, Hg porosimetry analysis, SEM, TEM, XRD, Raman spectroscopy, dynamic contact angle analysis, CV, EIS and GC measurements. The specific details are presented in Supporting Information.

Deionization experiments of the FTDC The FTDC electrodes consist of 80 wt% of 3DHCA or DC, 10 wt% of Super P and 10 wt% of PTFE. The mixtures were mixed in ethanol homogeneously and the slurries were pressed on the graphite paper. Afterwards, the electrodes were dried at 120 o C for 12 h. The mass, thickness and size of the electrodes were 0.2 g, ~ 0.2 mm and 6 cm × 6 cm. Finally the electrodes were equipmented into the deionization device to test the deionization performance. The FTDC device consists of two electrodes separated by a piece of insulating grid spacer, and the NaCl solution can flow through the device circularly driven by a peristaltic pump. The device was connected with a conductivity meter, which can measure the conductivity value of NaCl solution in real-time. The volume of NaCl solution was 30 mL. The flow rate of NaCl solution was 40 mL min-1. The concentrations of NaCl were 100-500 mg L-1. The cell voltages were 0.8-1.4 V. The salt adsorption capacity (SAC) of the FTDC electrodes were obtained from the formula (1):

SAC = (C0 − C)V/m

(1)

in which C0 and C are the initial and final NaCl concentrations, V is the total volume of the solution, and m is the total mass of the active materials. The charge efficiency (Λ) were calculated from the the formula (2): 6


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Λ = (Γ × F)/Σ

(2)

in which Γ is the SAC (mol g-1), F is the Faraday constant (96485 C mol-1), and ∑ (charge, C g-1) is calculated through integrating the current.

RESULTS AND DISCUSSION Characteristics The calcination temperature and the ratio of KHCO3 to the resin could affect the specific surface area and pore structure, so different calcination temperature and ratio were used to prepare different 3DHCA/x-y samples. The N2 sorption isotherms of all the samples are presented in Figure S2 and the surface texture properties of all the samples are listed in Table 1. At 800 o C and the ratio of 4, the obtained 3DHCA/4-800 possesses the highest specific surface area. Therefore, the 3DHCA/4-800 sample was selected to study the electrochemical and deionization performance. For convenience, the 3DHCA/4-800 is named as 3DHCA in the following text. Figure 2a depicts the N2 adsorption-desorption isotherm of the 3DHCA sample. The isotherm is mixed type I (b) and type IV (a), according to the IUPAC Technical Report44. The curve is steep at low pressure, indicating the microporous characteristic of the 3DHCA45-47. Furthermore, a hysteresis loop (0.4＜P/P0＜0.8) can be observed, indicating the exisitence of mesopores in the 3DHCA48. As calculated, the specific surface area of the 3DHCA is 2061 m2 g-1, which is significantly higher than that of the DC (19 m2 g-1). The pore size distributions obtained from Quenched-solid Density Functional Theory (QSDFT) model and Hg penetration analysis are presented in Figure 2b. Sharp peaks centered in the micropore 7



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(0.55 nm and 1.14 nm), mesopore (3.10 nm) and macropore (1053 nm and 30241 nm) ranges can be seen, indicating the existence of micropores, mesopores and macropores. The pore volume of the 3DHCA is 1.01 cm3 g-1 and the average pore width is 0.55 nm. In contrast, the DC is 0.02 cm3 g-1 and 1.92 nm. From the Hg penetration tests, the 3DHCA have a high pososity of 89.30 % and ultra-low bulk density of 0.06 g mL-1, suggesting the 3DHCA is quite lightweight and have rich macropores49. The average pore diameter of macropore size is 3920 nm. As for the formation mechanism of the 3DHCA, the macropores are generated during the decomposition of KHCO3 by means of blowing effect (equation S2, Supporting Information) while mesopores and micropores are generated by the reaction between the activator and carbon material (equation S3-S6, Supporting Information). In conclusion, the 3DHCA have hierarchically porous structure consisting of micro-, meso- and macropores. The hierarchically porous structure could minimize the ion diffusion distance and beneficial to the ion transporting into the inner channels of the 3DHCA. The micropores guarantee adequate adsorption sites for ions in the deionization process. Figure 2c exhibits the XRD patterns of the 3DHCA and DC in the wide angle region. For both the 3DHCA and DC, two broad peaks appear at ~22o and ~43o are corresponding to the (002) and (100) diffraction peaks of amorphous graphitic carbon50. No any other peak is observed, indicating the purity of two samples. The intensity of (002) peak of the 3DHCA is much lower than the DC, indicating the 3DHCA have higher amorphous degree51, 52. For the Raman spectra in Figure 2d, the D band and G band peaks appear around 1335 cm-1 and 1590 cm-1, respectively. The ID/IG of the 3DHCA is 1.11, which is higher than that of the DC (1.09), suggesting the 3DHCA have more defects53, 54, which is according with the XRD patterns. 8


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These results suggest that the 3DHCA have more defective structure because of the 3D hierarchical porous structure. The morphology of the 3DHCA was tested by SEM and TEM and the images are presented in Figure 3. From Figure 3a we can see that the 3DHCA has a 3D framework with interconnected channels and a large number of macropores. The diameters of the macropores are ranging from several hundreds nanometers to several micrometers, which is in accord with the Hg penetration result. The well-interconnected macropores form ion-buffering reservoirs and minimize the ion diffusion distance in the porous carbon materials for salt ions17. In contrast, the DC presents bulk morphology and it is difficult to find any pore structure (Figure S4a and S4b, Supporting Information). From the TEM image in Figure 3b, it is obviously that the 3DHCA shows a macroporous architecture, which is consistant with the SEM image. From Figure 3c, the 3DHCA have ultrathin characteristic, which can be attributed to the blowing effect of the gases during the calcination process. Furthermore, numerous of pores can be seen. From the HRTEM image at high magnification in Figure 3d, mesopores and numerous micropores can be well observed. The mesopores can reduce the resistance and the micropores can provide high accessible surface area, more adsorption sites and the resulting enhanced electrical double-layer capacitance41. The hydrophilicity is also an important factor affects the salt adsorption performance of the FTDC electrodes. Good hydrophilicity leads to a good wettability. An increased wettability leads to sufficient contact between the electrodes and solution, which is beneficial to salt solution infiltration, and thus enhance the salt adsorption performance55. Figure 4 presents the experimental results of water contact angle of the 3DHCA and DC electrodes. For the 9



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3DHCA electrodes, the contact angle was 60o at the beginning of the test, which is much smaller than the DC electrode (108o). Afterwards, the contact angle of the 3DHCA electrode was decreased fast with the time increased. At 25 s, the water droplet immersed into the pore structure of the 3DHCA electrode completely. In contrast, the DC still remains a wide angle of 102o. The result suggests that the 3DHCA electrode have good hydrophilicity and wettability due to its hierarchically porous structure.

Electrochemical performance CV tests were applied to evaluate potential applications of the 3DHCA and DC electrodes for FTDC. Figure 5a presents the CV curves of the 3DHCA and DC electrodes at 5 mV s-1 between -0.5 to 0.5 V. The CV curves are relatively rectangular and have no reduction or oxidation peak, indicating the capacitance behavior mostly results from the ideal EDLC behavior instead of Faradic reaction56, 57. The shape of CV curves are relatively rectangular and have a symmetry characteristic, which results from the formation and deformation of the EDL during the electro-adsorption and desorption process. The specific capacitance of the 3DHCA is calculated to be 142 F g-1, which is dramatically larger than that of the DC (64 F g-1), indicating an enhanced specific capacitance and a better electrochemical performance. The enhanced capacitive performance is mainly because of the following three factors. First, the 3DHCA has hierarchical pores, which provides more convenient transport for salt ions, shortens the ion transport pathway and reduces the inner resistance. Second, the specific surface area of the 3DHCA is much higher and it can provide more adsorptive sites for salt ions. Last, the 3DHCA have good wettability, which leads to sufficient contact between the electrodes and solution, and thus beneficial to salt solution infiltration. 10


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The CV behaviors at different scan rates of the 3DHCA and DC eletrodes were also tested (Figure S5a and S5b, Supporting Information). Figure 5b shows the specific capacitance of the electrodes at 1-40 mV s-1. With the increase of the scan rate, the specific capacitance decreased gradually. At 1 mV s-1, the specific capacitance of the 3DHCA electrode is 206 F g-1. When the scan rate increased to 40 mV s-1, the specific capacitance decreased to 40 F g-1 correspondingly. This result is chiefly caused by the following factors. At low scan rate, the time is sufficient for the ions to diffuse into the pore structure of the electrode materials and it is in favor of the formation of the EDLC. In this situation, more salt ions can be adsorbed. At high scan rate, the time is not enough for the salt ions to get into the pores of the electrode materials. Therefore, the EDLC cannot form completely. In addition, when the scan rate increases, the ohmic resistance is also increased and it is not beneficial to the formation of EDLC58. At all scan rates, the specific capacitance of the 3DHCA electrode is much higher than that of the DC electrode, and the major contributors are the 3D hierarchically porous structure, large surface area and good wettability. It is noteworthy that at 1 mV s-1, the specific capacitance of the 3DHCA was 206 F g-1, which is 92 F g-1 higher than that of the DC electrodes. However, at 40 mV s-1, this gap decreased to only 17 F g-1. This is chiefly because that the 3DHCA has a large number of pores, and the slower the scan rates, the higher the specific capacitance. This effect is so significant that the specific capacitance of the 3DHCA electrode at slow scan rates and high scan rates are quite different. However, the DC has almost no pore structure, and thus the scan rates have little effect on the specific capacitance of the DC electrodes. The CV curves in different NaCl solutions were also tested, and when the NaCl concentration was increased, the specific capacitance increases 11



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correspondingly (Figure S5c, Supporting Information). Moreover, with a potential windows of 0-1.2 V and in 100-500 mg L-1 NaCl solutions (the same conditions as the deionization experiments), the CV curves also have no reduction or oxidation peak (Figure S6, Supporting Information), suggesting that the 3DHCA electrode have ideal capacitive behavior between 0-1.2 V. The EIS measurement was employed to test the dynamic behavior of salt ions in the electrosorption process. The Nyquist profiles of the 3DHCA and DC electrodes are presented in Figure 5c. Clearly, two profiles have similar shape and both have a semicircle in the high-frequency and a slop line in the low-frequency. Generally speaking, the diameter of the small semicircle always reveals the polarization resistance (Rs) ， which reflects the charge-transfer resistance between the solution and the electrodes interface23, 59. It is clearly that the diameters of the semicircles are very small, indicating that the charge-transfer resistance is very small. The x-intercept of the plots reflect the equivalent series resistance (ESR), which is relevant to the intrinsic resistance of the electrodes, salt solution resistance and contact resistance between the carbon materials and current collectors60, 61. A smaller value of ESR indicates a smaller internal loss62. From the inset chart in Figure 5c, the intercept at the x-axis of the 3DHCA electrode is smaller than that of the DC electrode, indicating that the 3DHCA electrode possesses a smaller ESR, and further showing that the 3DHCA electrode has a smaller resistance and less energy loss. The slop line in the low-frequency reveals the capacitive behaviors. The profiles of the 3DHCA and the DC electrodes have similar line shape, indicating they have similar capacitive behaviors. In conclusion, the 3DHCA electrode possesses low internal resistance and good conductivity. 12


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This is because that the 3DHCA have 3D hierarchically porous structure, which provide more channels, fast transfer pathways and good wettability for salt ions and render the salt ions transfer more quickly and conveniently. Figure 5d exhibits the GC curves of the 3DHCA and DC electrodes at 200 mA g-1 in a 0.5 M NaCl solution. Both the 3DHCA and DC electrodes present a triangular shape, indicating an ideal EDLC behavior63, 64. At the same time, the profiles also show liner potential-time plots, illustrating the rapid I-V response in the charge-discharge process65. Furthermore, the discharge time of the 3DHCA electrode is more permanent than that of the DC electrode, showing that the 3DHCA electrode has higher specific capacitance, which corresponds to the CV results. Figure 5e shows the GC curves of the 3DHCA electrodes at 200-1000 mA g-1 in a 0.5 M NaCl solution. All the profiles exhibit triangular shapes, indicating that the 3DHCA electrode has an ideal EDLC behavior at every current density. From the GC curves, the iR drop can be seen at the start of the discharge curves, which reflects the resistance of the solution, electrodes and ion diffusion41. The analysis of the iR drops change with the current densities of two electrodes are presented in Figure 5f. Obviously, from 200 mA g-1 to 1000 mA g-1, the iR drops of two electrodes both increases correspondingly, suggesting a low current density could lower the inner resistance. At all current densities, the 3DHCA electrode exhibits reduced iR drop than that of the DC electrode. Moreover, the ESR of the electrodes can be reflected from the slop of iR drop lines and a higher slop represents larger resistance41. Obviously, the slop of the 3DHCA electrode is much smaller than that of the DC electrode, further demonstrating that the 3DHCA electrode has a lower resistance, which is consistent with the EIS analysis, and both confirm that the 3D hierarchically structure renders 13



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the 3DHCA electrode a reduced inner resistance, which would be beneficial to the salt adsorption process.

Deionization performance of the FTDC In consideration of the 3D hierarchically porous structure, high specific surface area and good wettability, the deionization performance of the 3DHCA electrodes were tested under different experimental conditions by batch mode experiments. Figure 6a depicts the SAC curves and transient current of the 3DHCA and DC electrodes in a 500 mg L-1 NaCl solution at 1.2 V. Obviously, with the time increased, the SAC of the 3DHCA electrodes increase dramatically, indicating the salt adsorption process is very fast. The SAC of the 3DHCA electrodes is 17.83 mg g-1, while the DC electrodes possess only 6.88 mg g-1, which is much lower. Moreover, the SAC of the 3DHCA electrodes is much higher than most FTDC electrode materials reported up to date (Table S1, Supporting Information). The charge efficiency has been an important index to evaluate the charge utilization9, 66. The charge efficiencies of the 3DHCA and DC electrodes obtained from the SAC value and current transient are to be 0.46 and 0.36, indicating the 3DHCA electrodes have higher charge utilization. In addition, the charge efficiency of the 3DHCA is higher than some FTDC electrodes (Table S2, Supporting Information), indicating the 3DHCA electrodes have a relatively higher charge utilization. However, the charge efficiency is always lower than the theoretical value of 1, which is because of these three factors: (i) we always use binder such as PTFE in the fabrication process of the FTDC electrode and the conductivity of the binder is always not good, so the inner resistance of the FTDC electrode is increased; (ii) the adhesive force between the electrode material and graphite paper is not closely inevitably, 14


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which increase the other portion of the charge consumption; (iii) in the deionization process, the repulsion effect of the co-ions also decreases the charge utilization9, 67. The salt adsorption rate (SAR) is also very important in deionization experiments and the Ragone plot has been an functional tool to evaluate the deionization performance of the FTDC9, 68. Figure 6b presents Ragone plots of SAR vs. SAC of two electrodes in a 500 mg L-1 NaCl solution at 1.2 V. It is obviously that the plot of 3DHCA electrodes is in the righter and upper rigion, indicating the 3DHCA electrodes have higher SAC and faster SAR. In addition, along with the deionization process, the SAC remains increasing while the SAR keeps decreasing. It can be explained that with the process of the deionization, more and more salt ions were adsorbed into the pores of the FTDC electrodes and thus the adsorption sites for the ions become less and less. In such a situation, the electrostatic repulsion becomes stronger, and this lead to the increasing of the SAC and decreasing of the SAR. The high SAC and fast SAR of the 3DHCA electrodes can be explained by the following reasons. The 3DHCA have 3D framework and hierarchical pores including macro-, mesoand micropores. The macropores povide ion-buffering reservoirs and accelerate the transportation of the ions, the mesopores reduce the resistance of the salt ions through the pores and channels, and the micropores provide a high specific surface area (2061 m2 g-1) and the resulting more adsorption sites. Furthermore, the 3DHCA have a good wettability resulting from the hierarchical porous structure, which lead to sufficient contact between the electrodes and solution and render the salt solution infiltrate more easily. The initial NaCl concentration is an important factor affects the deionization performance, so the deionization experiments of the 3DHCA electrodes in different concentrations of 100, 15



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300 and 500 mg L-1 at 1.2 V were carried out and the curves are presented in Figure 6c. When the concentration of NaCl solution increased, the SAC increase correspondingly from 9.96 mg g-1 to 17.83 mg g-1. When the concentration of NaCl increases, the slop of the curves also increases, indicating the increase of the SAR. Figure 6d displays the Ragone plots of SAR vs. SAC of the 3DHCA electrodes in different NaCl concentrations. It can be seen that from low concentration to high concentration, the plot turns to the upper and righter region, suggesting the SAC is beome higher and the SAR is become faster. The increase of the SAC can be explained that when the NaCl concentration increased, the EDLC would form more easily. The faster SAR is due to the following two reasons. First, the conductivity of the solution would increases with the increase of concentration. Second, with the increase of NaCl concentration, the ions would move more rapidly from the solution to the electrodes correspondingly68, 69. The deionization behavior of the 3DHCA electrodes in a 500 mg L-1 NaCl solution at 0.8-1.4 V were also investigated. It is noteworthy that choosing an appropriate cell voltage is very important. If the cell voltage is too low, the EDLC would form insufficiently and the SAC would be much lower than that at normal situation. If the cell voltage is too high, the electrolytic reaction would happen. In this work, cell voltages between 0.8 to 1.4 V were carefully choose. Figure 6e displays the adsorption curves from 0.8 V to 1.4 V in a 100 mg L-1 NaCl solution. From 0.8 V to 1.4 V, the SAC increased from 7.32 mg g-1 to 11.18 mg g-1, indicating a higher cell voltage leads to a higher SAC. In addition, the slope of the curves increase with the increase of the cell voltage, indicating the SAR increases. These results are also can be seen from Figure 6f, which depicts the Ragone plots at 0.8-1.4 V. From 0.8 V to 16


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1.4 V, the plot gradually turns to the upper and righter region, revealing that the SAC and SAR both increased. It is chiefly because that when the higher cell voltage is increased, the electrostatic forces would become stronger and the EDLC would become thicker. Under the influence of this factor, the SAC becomes higher and the SAR becomes faster when the cell voltages increase22, 68. To

study

the

regeneration

behavior

of

the

3DHCA

electrodes,

reiterative

deionization-regeneration experiment was carried out, in which the charge volage was 1.2 V and the discharge voltage was 0 V. Figure 7 depicts the regeneration curves of the 3DHCA electrodes in a 80 mg L-1 NaCl solution. It can be seen that when the cell voltage of 1.2 V was added, the conductivity decreased rapidly and once the voltage was removed, the conductivity can increase back to the origin value quickly. In addition, in all the process, the SAC of the 3DHCA electrodes almost has no obvious declination, showing that the 3DHCA electrodes have a good regeneration behavior.

CONCLUSIONS In this work, the 3DHCA was prepared via a simple self-blowing strategy. The obtained 3DHCA material has 3D framework with a large number of hierarchical pores and high specific surface area of 2061 m2 g-1. Because of the unique structure, the 3DHCA have good wettability. The 3DHCA electrode also has good electrochemistry performance. The deionization experiments show that the 3DHCA electrodes exhibit a high deionization capacity of 17.83 mg g-1 in a 500 mg L-1 NaCl solution at 1.2 V. Besides, the 3DHCA electrodes show a good regeneration behavior. Based on the above advantages, the 3DHCA 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

might be applied as electrode material for high performance FTDC and possesses good prospect for commercial application.

ASSOCIATED CONTENT Supporting Information The detailed information of characterization, TG and DTG curves, electrochemical measurements, N2 sorption isotherms and pore size distribution of all the samples, SEM images of the 3DHCA and DC, CV curves of the electrodes at 1-40 mV/s, CV curves of the 3DHCA electrode at 0.1-1 M NaCl solutions, comparison of SAC and charge effiency of various FTDC electrodes. This material is available free of charge via the Internet at

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgments We sincerely acknowledge the funding support of the National Basic Research Program of China (973 Program, 2014CB660803).

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Table 1 Surface texture properties of the 3DHCA and DC samples Samples

SBET

Smicro

Sext 27


Vtotal

Daverage


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

(m2 g-1)

(m2 g-1)

(m2 g-1)

Page 28 of 35

(cm3 g-1)

(nm)

3DHCA/4-600

985

919

66

0.39

0.55

3DHCA /4-700

1418

1310

108

0.58

0.52

3DHCA /4-800

2061

1354

707

0.99

0.55

3DHCA /2-800

1933

1317

616

0.93

0.52

3DHCA /5-800

1944

1330

614

0.86

0.60

3DHCA /6-800

1823

1325

498

0.82

0.62

3DHCA /8-800

1670

1309

361

0.74

0.63

DC

19

-

19

0.02

2.77

28


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Figure 1. Preparation process of the 3DHCA.

Figure 2. (a) N2 sorption isotherms and (b) pore size distribution of the 3DHCA; (c) XRD patterns and (d) Raman spectra of the 3DHCA and DC.

29



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Figure 3. (a) SEM, (b) TEM and (c, d) HRTEM images of the 3DHCA.

30


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Figure 4. Water contact angle measurement of the 3DHCA and DC electrodes.

31



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. (a) CV curves of the electrodes at 5 mV s-1; (b) the specific capacitance of the electrodes at 1-40 mV s-1; (c) Nyquist plots of the electrodes; (d) GC curves of the electrodes at 200 mA g -1; (e) GC curves of the 3DHCA electrode at 200-1000 mA g-1; (f) iR drops of the electrodes vs. current densities.

32


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Figure 6. (a) SAC and transient current and (b) Ragone plots of SAR vs. SAC of the 3DHCA and DC electrodes in a 500 mg L-1 NaCl solution at 1.2V; (c) the SAC curves and (d) Ragone plots of SAR vs. SAC of the 3DHCA electrodes in 100-500 mg L-1 NaCl solution at 1.2 V; (e) the SAC curves and (f) Ragone plots of SAR vs. SAC of the 3DHCA electrodes in a 500 mg L-1 NaCl solution at 0.8-1.4 V.

33



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Figure 7. Regeneration curve of the 3DHCA electrodes in a 80 mg L-1 NaCl solution at 1.2 V.

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Table of Contents Graphic

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Creating 3D Hierarchical Carbon Architectures ... - ACS Publications

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