Nitrogen-Doped Hollow Mesoporous Carbon Spheres for Efficient

Research Article pubs.acs.org/journal/ascecg

Nitrogen-Doped Hollow Mesoporous Carbon Spheres for Efficient Water Desalination by Capacitive Deionization Yang Li, Junwen Qi, Jiansheng Li,* Jiaming Shen, Yuxin Liu, Xiuyun Sun, Jinyou Shen, Weiqing Han, and Lianjun Wang* Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, No. 200 Xiaolingwei Street, XuanWu District, Nanjing 210094, People’s Republic of China S Supporting Information *

ABSTRACT: Water desalination performance of capacitive deionization (CDI) largely depends on electrode materials properties. Rational design and regulation of the structure and composition of electrode materials to acquire high CDI performance is of great significance. Herein, nitrogen-doped hollow mesoporous carbon spheres (NHMCSs) were investigated as electrode material for CDI application. To understand the effect of structure and composition on CDI performance, another two CDI electrode materials, i.e., hollow mesoporous carbon spheres (HMCSs) and solid mesoporous carbon spheres (SMCSs) were prepared for comparison. The obtained NHMCSs possessed unique hollow cavity and excellent nitrogen doping property, resulting in fast ion diffusion, good charge transfers ability and fine wettability. Compared with HMCSs and SMCSs electrodes, N-HMCSs electrode exhibited an improved electrosorption capacity and rate, demonstrating the dependence of CDI performance on the synergistic effect of hollow structure and nitrogen doping property. N-HMCSs electrode also present excellent cycle stability over 20 adsorption−desorption cycles. These results indicate the promising prospect of N-HMCSs for CDI application. KEYWORDS: Carbon spheres, Nitrogen-doped, Hollow mesoporous, Capacitive deionization

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INTRODUCTION In recent years, capacitive deionization (CDI) has drawn extensive attention as a promising desalination approach due to high desalination efficiency, energy saving and environmental friendliness.1,2 During the CDI process, the removal of the ions is achieved by their adsorption on the electrical double layer (EDL) at the electrodes surface.3,4 Similar to the EDL-based supercapacitors, porous carbon materials, including activated carbon,5 carbon nanotube,6 graphene,7,8 carbon nanofibers,9 carbon nanosheets,10 carbon aerogel,11 mesoporous carbon,12 and their composites,13−18 are preferential candidates as CDI electrodes. Fundamentally, the CDI performance essentially relies on the properties of the porous carbon electrode (specific surface area, porous structure, conductivity and wettability, etc.).19 Although many efforts devoted to improve CDI performance by tailoring these properties, exploring the novel carbon-based CDI electrode materials with excellent desalination performance is still highly desired, meanwhile challengeable. Hollow mesoporous carbon spheres (HMCSs), as one kind of porous carbon materials, have widely received attention in many fields such as adsorption,20 catalysis,21 drug delivery,22 and energy storage23,24 by virtue of their large surface-tovolume ratio, high specific surface area, low specific density, and unique architecture. Particularly, the hollow cavity offers a high specific surface area along with reduced mass and charge © 2017 American Chemical Society

diffusion lengths and the mesopores on the shell facilitate the ion diffusion kinetics by shorting the diffusion distance. These remarkable properties endow HMCSs as a promising candidate for EDLCs with the outstanding high-rate capacitive performances.25−27 Inspired by this, the HMCSs have been extended to the CDI fields. In some pioneering works, the HMCSs as the CDI electrode materials were prepared by the modified Stöber method with a SiO2 sphere as a hard template.28,29 However, the improvement of desalination performance for the obtained HMCSs is still needed due to the high inner resistance and poor wettability of amorphous carbon materials. In addition, the graphene-coated hollow mesoporous carbon spheres were also designed by the template method.30 Although the hierarchical structure and high conductivity of the resultant materials can facilitate the salt ions transport and charge transfer ability in the CDI application, the low specific surface area restricted the electrosorption capacity to only 2.3 mg g−1. Based on these results, some undesirable properties still exist in the HMCSs, which hinder their CDI performance. To address the above-mentioned issues, an efficient solution should be to simultaneously improve the charge transfer ability and wettability of HMCSs via a simple strategy. Recently, Received: March 23, 2017 Revised: June 26, 2017 Published: July 3, 2017 6635

DOI: 10.1021/acssuschemeng.7b00884 ACS Sustainable Chem. Eng. 2017, 5, 6635−6644

Research Article

ACS Sustainable Chemistry & Engineering

same way without melamine. And SMCSs were prepared in the same way by adding 1 mL of TEOS without melamine. Characterization. N2 adsorption/desorption isotherms were measured using Micromeritics ASAP-2020 at liquid nitrogen temperature (−196 °C). Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area and pore volume. The 2D nonlocal density functional theory (2D-NLDFT) method was used to analyze the pore size distribution. Scanning electron microscopy (SEM) analysis was performed on a FEI Quanta 250F electron microscope. Transmission electron microscopy (TEM) analysis was measured on a TECNAI G2 20 LaB6 system operated at 200 kV. The X-ray diffraction (XRD) analysis was performed by using a Bruker AXS D 8 advance powder diffraction system using Cu Kα (λ = 1.5418 Å) radiation. Raman spectra were recorded on an Aramis spectrometer at 532 nm. The X-ray photoelectron spectroscopy (XPS) was made by using a PHI Quantera II ESCA System with Al Kα radiation at 1486.8 V. The surface wettability was characterized by using Krüss DSA30 shape analysis system. Electrochemical Measurements. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge−discharge (GC) were tested using a CHI 660D electrochemical workstation in 1 M NaCl solution. A three-electrode system was employed where the sample was used as the working electrode, platinum foil as the counter electrode, and Ag/AgCl (3 M NaCl) electrode as the reference electrode. The working electrodes were fabricated by coating a mixture of 80 wt % sample, 10 wt % acetylene black, and 10 wt % polytetrafluoroethene (PTFE) onto a titanium plate, and dried at 120 °C overnight. The specific capacitances (C, F g−1) are obtained from the CV curves as follows:

nitrogen-doping has been considered as a facile and efficient strategy to the promotion of CDI performance.31−34 After incorporating nitrogen atoms into the carbon frameworks, the obtained nitrogen-doped (N-doped) porous carbon possesses good charge transfer ability and fine wettability so that a high salt adsorption capacity can be achieved. Unfortunately, to date, only limited studies reported the construction of nitrogendoped hollow mesoporous carbon spheres (N-HMCSs) as CDI electrodes. Zhang’s group first applied N-HMCSs that were prepared via a hard template strategy as CDI electrode, and the electrosorption capacity can be achieved about 12.95 mg g−1 in 500 mg L−1 NaCl solution.35 However, the understanding of the synergistic effect of the hollow structure and nitrogen doping property on CDI performance is unclear. The structure and composition are two key factors that influence the final CDI performance of electrode materials.36 Correspondingly, rationally designing the structure and tuning the composition of electrode materials provide an effective approach to enhancing CDI performance. Although the positive effect of hollow structure or nitrogen doping on the electrochemical performance of carbon electrodes have been observed in supercapacitor,37 Zn−air batteries,24 oxygen reduction reaction,38 and sodium-ion batteries,39 there is no direct proof to evidence the superiority by simultaneously controlling of structure and the composition in the CDI process. Therefore, deeply exploring the dependence of CDI performance on the structure and composition of N-HMCSs is of great significance, but has been rarely reported. Herein, we prepared N-HMCSs in one pot synthesis via a “silica-assisted” sol−gel strategy. To elucidate the synergistic effect of the hollow structure and nitrogen doping property on the CDI performance, another two CDI electrode materials, i.e., HMCSs and solid mesoporous carbon spheres (SMCSs) were also prepared. The advantage of our research strategy is rationally designing the control samples, subsequently exploring the dependence of CDI performance on the structure and composition of N-HMCSs. As expected, the N-HMCSs electrode shows a high electrosorption capacity, fast electrosorption rate, and good cycle stability. Because of the unique hollow structure and nitrogen doping property, N-HMCSs demonstrate the promising prospect in CDI technology.

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C=

∫ 2vIΔdVVm

(1)

where I (A) is the response current density, v (V s−1) is the scan rate, ΔV (V) is the voltage change, and m (g) is the active material mass. CDI Experiments. The CDI electrodes were fabricated following the same procedure mentioned above in electrochemical measurements. Titanium plate (Baoji Geely Ti Metal Material Co. Ltd., 10 cm × 10 cm, 1 mm) was used as current collector. The total mass and size of the electrodes were 50 mg and 5 cm × 5 cm × 100 μm, respectively. The CDI performance was measured by installing the electrodes into a self-made CDI apparatus. Details of the apparatus can be found in our previous work.36 All electrosorption experiments were performed by pumping 30 mL NaCl solution into the CDI apparatus at a flow rate of 25 mL min−1 under 1.6 V. The NaCl concentration was measured by monitoring the effluent conductivity with a DDS-308 conductivity meter. The electrosorption capacity (mg g−1) can be calculated according to the following equation:

EXPERIMENTAL SECTION Γ=

Chemicals. Ethanol, HF (40%), and NH4OH (25−28%) were purchased from Nanjing Chemical Reagent Co., Ltd. Resorcinol, formaldehyde (37 wt % solution), TEOS, and melamine were purchased from Sinopharm Chemical Reagent Co., Ltd. CTAC was purchased from Aladdin Corp. All reagents were used as received. Deionized water (Millipore) was used throughout the process. Preparation of N-HMCSs. N-HMCSs were prepared in one pot synthesis via a “silica-assisted” sol−gel strategy according the reported references.40,41 Briefly, 2 g of CATC was dissolved in a mixed solution containing 100 mL of H2O, 40 mL of ethanol, and 0.5 mL of NH4OH and stirred at 70 °C for 10 min. Subsequently, 0.55 g of resorcinol was added in the above solution. After 30 min, the “silica-assisted” sol−gel reaction was started by adding 3 mL of TEOS and 0.74 mL of formaldehyde solution. After stirring for another 30 min, 0.5 g of melamine and 0.55 mL of formaldehyde were added and continually stirred for 24 h. Finally, after centrifugation, washing with ethanol and drying at 80 °C overnight, a brownish red product was obtained. The as-prepared product was directly calcined at 700 °C for 3 h under N2 atmosphere at a heating rate of 3 °C min−1. After etching of the silica in 10% HF solution for 24 h, washing and drying, the N-HMCSs were finally obtained. For comparison, the HMCSs were prepared in the

(C0 − Ce)V m −1

(2) −1

where C0 (mg L ) is the initial NaCl concentrations, Ce (mg L ) is the equilibrated NaCl concentrations, V (L) is the NaCl solution volume, and m (g) is the active materials mass. The charge efficiency (Λ) can be defined as follows: Λ=

Γ×F ∑

(3)

where Γ (mol g−1) is the electrosorption capacity, F (96 485 C mol−1) is the Faraday constant, and Σ (C g−1) is the total charge according to the integrated corresponding current. The CDI performance in terms of the electrosorption capacity and electrosorption rate can be evaluated by the Ragone plot.3,42,43 The electrosorption capacity at a certain time (Γt, mg g−1) and the corresponding electrosorption rate (vt, mg g−1 min−1) were calculated as follows:

Γt = 6636

(C0 − Ct )V m

(4) DOI: 10.1021/acssuschemeng.7b00884 ACS Sustainable Chem. Eng. 2017, 5, 6635−6644

Research Article


Figure 1. SEM images of (a) HMCSs, (c) N-HMCSs, and (e) SMCSs. TEM images of (b) HMCSs, (d) N-HMCSs, and (f) SMCSs.

Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of HMCSs, N-HMCSs, and SMCSs.

vt =

Γt t

further confirmed by TEM images (Figure 1b,d,f). It can be clearly seen that both of HMCSs and N-HMCSs exhibit the uniform hollow nanostructures with a cavity diameter of about 220 nm and shell thickness of about 90 nm. The hollow structure can also be visually observed in the magnification image in Figure S1. Differently, SMCSs show well-defined nanospherical morphology with a solid structure. However, the nanospherical morphology of SMCSs cannot be maintained after pyrolysis without TEOS, suggesting the structure strengthening function of TEOS (Figure S2). Moreover, a large number of mesopores radially arranged can be seen in all samples due to the CTAC template. Based on the observations from SEM and TEM images, the hollow and solid structures of carbon spheres can be successfully tailored. Additionally, N-

(5)

where Ct (mg L−1) is the NaCl concentration at t min.

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RESULTS AND DISCUSSION Characteristics. The structure and morphology of HMCSs, N-HMCSs, and SMCSs were studied by the SEM and TEM analysis. As shown in Figure 1a,c,e, all samples exhibit uniform and well-defined nanospherical morphology with a diameter about 350−400 nm. The similar diameters can be ascribed to the same ammonia concentration and the ethanol/water volume ratio, which can govern the emulsion droplets size and final carbon spheres size.40,41 The solid and hollow are 6637


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weak peaks at 2θ = 25° and 43.7°, indicative of the amorphous carbon frameworks.49 The structures of HMCSs, N-HMCSs, and SMCSs were further investigated by the Raman spectra. As shown in Figure S3b, two peaks at 1350 and 1580 cm−1 can be observed in all samples, which are the D band and G band, respectively. The ID/IG value of HMCSs and SMCSs are 0.94 and 0.96 respectively, whereas ID/IG value of N-HMCSs is 1.01. The higher ID/IG value of N-HMCSs implies that much more structural disorders and defects in the carbon framework are generated by the nitrogen doping, which are beneficial for charge accumulation during the CDI process.50 Figure 3 shows the XPS analysis of HMCSs, N-HMCSs, and SMCSs. As shown in Figure 3a, only the peaks at about 285 and 534 eV are shown in the XPS spectrum of HMCSs and SMCSs, which corresponds to the C and O species, respectively. As expected, the N species located 398 eV at can be observed in the XPS spectrum of N-HMCSs, suggesting the successful nitrogen doping. Table 1 lists the elemental compositions of HMCSs, N-HMCSs, and SMCSs. The C, O, and N contents of N-HMCSs are 90.2, 6.1, and 3.7%, respectively. Compared with HMCSs and HMCSs, the decreased O content may be ascribed to that the highly active oxygen atoms are easily replaced by the N atoms during the nitrogen doping process.51 The highresolution N 1s XPS spectrum of N-HMCSs was performed to study the bonding configurations of N species (Figure 3b). As shown, four N species located at 398.2, 399.5, 401.1, and 402.6 eV can be observed, which are referred to pyridinic-N, pyrrolicN, graphitic-N, and oxided-N, respectively.52 According to the relative areas of four N species, the percentages of pyridinic-N, pyrrolic-N, graphitic-N, and oxided-N are 12.14, 15.93, 60.69, and 11.24%, respectively. The highest percentage of graphitic-N is favorable for the conductivity enhancement of carbon materials.53,54 Therefore, the improved CDI performance of N-HMCSs would be expected due to the enhanced conductivity. The wettability of the as-prepared electrodes was evaluated by the dynamic water contact angle measurements, as shown in Figure 4. For SMCSs and HMCSs electrodes, the initial contact angles are 129.3 and 116.5°, respectively. As the contact time increases, the final contact angles decrease to 125.6 and 111.5°, respectively. It is suggested that SMCSs and HMCSs are very hydrophobic due to the pure carbon environment. Different from SMCSs and HMCSs electrodes, the N-HMCSs electrode shows a low initial contact angle of 83.1° and final contact angle of 59.4° within 60 s. Obviously, the N-HMCSs electrode has a

HMCSs can still retain the similar hollow structure as that of HMCSs, indicating the introduction of melamine cannot affect the final morphology during the sol−gel process. The porous structures of HMCSs, N-HMCSs, and SMCSs were investigated by the N2 adsorption/desorption isotherms measurements. As shown in Figure 2a, a typical type IV isotherm with a H2 hysteresis loop can be found in all samples, indicating the presence of mesopores.44 The uptake adsorption at low relative pressure can be observed in the isotherms, demonstrating the microporous structure in the carbon shells. Figure 2b shows the pore size distribution calculated with 2DNLDFT method. Clearly, all samples show the strong mesopores peaks around 3 nm, which should be ascribed to removal of the template. The presence of mesoporous channels is desired for the CDI electrodes because it can provide a more accessible ion adsorption site and lower the ion transport resistance through the electrodes.45,46 Table 1 lists the Table 1. Structural Parameters and Elemental Compositions of HMCSs, N-HMNCSs and SMCSs Elemental composition (at. %) Sample HMCSs NHMNCSs SMCSs

Specific surface area (m2 g−1)

Pore volume (cm3 g−1)

C

1230 1099

1.35 1.13

92.6 90.2

1166

0.78

92.4

N

O

3.7

7.4 6.1 7.6

calculated structural parameters for HMCSs, N-HMCSs, and SMCSs. The specific surface area of HMCSs, N-HMCSs, and SMCSs are 1230, 1099, and 1166 m2 g−1, respectively. Specially, N-HMCSs have a slight decrease in specific surface area as compared to HMCSs, which is probably ascribed to the structural shrinkage and deposition of extra carbon induced by the decomposition of melamine.47 As reported previously, developed specific surface area is always desired for CDI desalination because more sites are provided for ion adsorption.48 However, in our work, to highlight the important effect of the hollow structure and nitrogen doping property on CDI performance, maintaining the comparable specific surface area of all samples would be a proper experimental design. Figure S3a shows the wide-angle XRD patterns of HMCSs, N-HMCSs, and SMCSs. All samples exhibit two broad and

Figure 3. (a) XPS spectra of HMCSs, N-HMCSs, and SMCSs and (b) high-resolution N 1s XPS spectra of N-HMCSs. 6638


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Figure 4. Dynamic water contact angles of HMCSs, N-HMCSs, and SMCSs electrodes.

Figure 5. CV properties of HMCSs, N-HMCSs, and SMCSs electrodes. (a) CV curves at a scan rate of 1 mV s−1 and (b) specific capacitances at different scan rates.

more inclined with a distortion from the original shape. Differently, the CV curves of HMCSs and N-HMCSs electrodes can still remain the original shape (Figure S4), indicating a good capacitive property due to the quickly ions diffusion in the unique hollow mesoporous structures.59 In addition, the CV enclosed area of the N-HMCSs electrode is larger than those of HMCSs and SMCSs electrodes, suggesting a higher specific capacitance. Based on eq 1, the calculated specific capacitances of HMCSs, N-HMCSs, and SMCSs electrodes are 102.3, 179.0, and 80 F g−1, respectively. Figure 5b further shows the specific capacitances of HMCSs, NHMCSs, and SMCSs electrodes at different scan rates. Clearly, N-HMCSs exhibits the highest specific capacitances among three electrodes. That is to say N-HMCSs electrode has improved electrochemical performance toward ion accumulation. The increased specific capacitance of N-HMCSs electrode can be explained as follows: (1) improved wettability; (2) the Faradaic effects arising from nitrogen doping; (3) decreased electrical resistance and enhanced charge transfer ability due to the hollow structure. The electrical resistance and capacitive behavior were evaluated by the EIS analysis. The Nyquist plots of HMCSs, N-HMCSs, and SMCSs electrodes at a frequency ranging from 105 to 0.1 Hz are shown in Figure 6. An equivalent electric circuit was used to fit and interpret the Nyquist plots (inset of Figure 6). At the low-frequency region, all three electrodes show the nearly vertical line, demonstrating the dominant EDL capacitive behavior.60 At the high-frequency, the x-intercept of the plots can be regarded as the bulk resistance (Rs), which corresponds to the intrinsic electrical properties of the electrode and electrolyte and contact resistance between the

significant improvement of the wettability. Compared to SMCSs and HMCSs electrodes, the improved wettability of N-HMCSs electrode should contribute to the variation caused by the nitrogen doping. On one hand, the introduction of N atoms can change the surface chemistry to improve the pure carbon environment of the original SMCSs and HMCSs electrodes. On the other hand, as proved by Raman spectra (Figure S3b), more structural defects are generated upon nitrogen doping. Thus, these two aspects facilitate the access of aqueous solution into the porous channels. In other words, the whole pores can be entirely utilized to the maximum electrosorption of ions. Electrochemical Performance. The electrochemical properties of HMCSs, N-HMCSs, and SMCSs electrodes were investigated by the electrochemical measurements in 1 M NaCl solution. Figure 5a shows the CV curves of HMCSs, NHMCSs, and SMCSs electrodes at a scan rate of 1 mV s−1. Overall, all electrodes exhibit the CV curves with relatively quasi-rectangular shape, indicative of the dominant EDL behavior. A slight deviation from the quasi-rectangular shape can be observed in N-HMCSs electrode. It has been suggested that this deviation is ascribed to the doped nitrogen species, especially the pyridinic-N, which can generate the pseudocapacitive effect between the electrolyte protons and the carbon materials by changing the electron donor−acceptor characteristic.55 It is noteworthy that this pseudocapacitive effect caused by nitrogen doping can exist in any electrolyte, no matter acidic (H2SO4), basic (KOH) or neutral solution (Na2SO4, K2SO4, NaCl).56−58 The presence of pseudocapacitive effect contributes positively to the final capacitance. When the scan rate increases from 1 to 50 mV s−1, the CV curve of SMCSs become 6639


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CDI Performance. The CDI performance of the fabricated electrodes was tested by a series of electrosorption experiments in NaCl solution. Figure 8a depicts the CDI profiles of HMCSs, N-HMCSs, and SMCSs electrodes in 100 mg L−1 NaCl solution. Clearly, the conductivity decrease quickly once the voltage is applied and then plateaued within 40 min, indicative of the electrosorption equilibrium. Clearly, the N-HMCSs electrode reaches the lowest conductivity of 159.7 μS cm−1, whereas 169.1 and 176.4 μS cm−1 for HMCSs and SMCSs electrodes. Next, the conductivity recovers to the initial value after being short-circuited, suggesting the complete generation of the adsorbed ions. The complete conductivity recovery in the first cycle can be observed. This may be ascribed to a negligible conductivity change induced by the weak faradaic side reaction due to the less oxygen in the solution under normal temperature and pressure without oxygen bubbling. Notably, the SMCSs electrode exhibits a slow conductivity increase compared with HMCSs and N-HMCSs electrodes, indicative of a longer generation time. The Ragone plots of the as-prepared electrodes are shown in Figure 8b. Generally, it is favorable to see an upper and right side shift in the plot, which means a higher electrosorption capacity and faster electrosorption rate. Obviously, the N-HMCSs electrode exhibits the most upper and right Ragone plot, indicative of the highest electrosorption capacity and fastest electrosorption rate among three electrodes. In other words, the CDI electrode can adsorb the largest amount of ions at the fastest adsorption rate. The charge utilization is an important index to evaluate the CDI performance, which can be reflected by the charge efficiency.3,19 Figure 9 shows the current response of HMCSs, N-HMCSs, and SMCSs electrodes in 100 mg L−1 NaCl solution. Based on eq 3, the calculated Λ values of HMCSs, NHMCSs, and SMCSs electrodes are about 0.41, 0.45, and 0.40, respectively. Obviously, the N-HMCSs electrode shows the highest charge efficiency. Based on the above description, the N-HMCSs electrode exhibits the best CDI performance. Unfortunately, the CDI performance of SMCSs electrode is the worst. This can be explained from the following two aspects: structure and composition. In the case of the SMCSs electrode, the solid structure increases the ion diffusion resistance within the porous channels of carbon spheres whereas the poor wettability makes all pores not easily accessible for the electrolyte.

Figure 6. Nyquist plots of EIS for HMCSs, N-HMCSs, and SMCSs electrodes.

electrode material and current collector.61 Clearly, the smallest x-intercept can be found in the N-HMCSs electrode indicative of the lowest Rs. The small semicircle in the plots reflects the charge transfer resistance (Rct) at the electrolyte/electrode interface.62 The fitted Rct values of HMCSs, N-HMCSs, and SMCSs electrodes are 3.2, 1.1, and 7.1 Ω, respectively. Thus, NHMCSs has the best charge transfer ability in aqueous solution. Such a low electrical resistance of N-HMCSs electrode maybe ascribed to the unique hollow mesoporous structures and nitrogen doping property. The GC curves of HMCSs, N-HMCSs, and SMCSs electrodes at the current density of 0.2 A g−1 are shown in Figure 7a. Clearly, all samples exhibit the nearly symmetric GC curves with a typical triangular shape, indicative of good EDL behavior. Moreover, the discharging time of N-HMCSs is the longest among three electrodes, suggesting the highest specific capacitance, which is in accordance with CV analysis. Moreover, at the initial stage of the discharge process, the GC curves show the iR drops, which correspond to the ion diffusion resistance and the internal resistance of the electrodes and electrolyte.63 Figure 7b shows the iR drops of HMCSs, NHMCSs, and SMCSs electrodes at different current densities. Clearly, N-HMCSs exhibits the lowest iR drops at all current densities, which agrees well with the EIS results. Such the lowest resistance is thought to be useful in the CDI application.

Figure 7. (a) GC curves of HMCSs, N-HMCSs, and SMCSs electrodes at the current density of 0.2 A g−1 and (b) iR drops of HMCSs, N-HMCSs, and SMCSs electrodes at different current densities. 6640


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Figure 8. (a) CDI profiles and (b) Ragone plots of HMCSs, N-HMCSs, and SMCSs electrodes in 100 mg L−1 NaCl solution.

SMCSs electrodes were further evaluated at different initial concentrations of NaCl solution and their electrosorption capacities are shown in Figure 10. The N-HMCSs electrode

Figure 9. Current response of HMCSs, N-HMCSs, and SMCSs electrodes in 100 mg L−1 NaCl solution.

Moreover, the increased the ion diffusion resistance hinders the ion desorption from the inner porous channel, resulting in a longer generation time for reaching an equilibrium. Thus, the SMCSs electrode has the lowest electrosorption capacity and slowest electrosorption rate. Compared to the SMCSs electrode, the CDI performance of the HMCSs electrode is improved. Considering the similar specific surface area, pore size, and surface chemistry between them, the improved CDI performance is mainly attributed to the superiority of hollow structure: (i) short the diffusion distance, (ii) reduce the diffusion resistance, and (iii) provide both outer and inner surface for ion adsorption due to the increased surface-tovolume ratio.41,64 As for the N-HMCSs electrode, the best CDI performance is ascribed not only to the hollow structure but also to the nitrogen doping property: (1) improved charge transfer ability and (2) enhanced wettability. Notably, NHMCSs has a slightly lower the specific surface area as compared to HMCSs. However, N-HMCSs has better CDI performance than HMCSs. Given the similar hollow mesoporous structure, it can be evolved that nitrogen doping actually play an important role in the electrosorption process and does greatly improve the CDI performance. Therefore, the final CDI performance of the N-HMCSs electrode is determined by the synergistic effect of the hollow structure and nitrogen doping property. To clarify to effect of hollow structure and nitrogen doping property, the CDI performance of HMCSs, N-HMCSs, and

Figure 10. Electrosorption capacities of HMCSs, N-HMCSs, and SMCSs electrodes at different initial concentrations of NaCl solution.

exhibits the highest electrosorption capacities in 100, 250, and 500 mg L−1 NaCl solutions, which are 11.5, 13.1, and 16.6 mg g−1, respectively. Table S1 summarizes the CDI performance of the reported carbon-based CDI electrodes. Clearly, the CDI performance of N-HMCSs stands a high level among various carbon electrodes. Notably, the electrosorption capacity of NHMCSs is relative high as compared with other carbon spheres materials (4.8−15.8 mg g−1).28,30,35,65,66 The cycle stability of CDI electrode is essential for the practical application. Figure 11 shows the cycle stability of N-HMCSs electrode in 100 mg L−1 NaCl solution. Clearly, the electrosorption capacity can still achieve 10.94 mg g−1 (95.1% of the initial value) after 20 adsorption−desorption cycles, indicating its excellent cycle stability in the CDI process. Therefore, it is looking forward to the promising prospects of N-HMCSs for CDI application.

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CONCLUSION In summary, N-HMCSs were prepared in one pot synthesis via a “silica-assisted” sol−gel strategy and applied as CDI electrode. Through this strategy, the rational regulation of the hollow structure and nitrogen doping property can be successfully achieved. The obtained N-HMCSs possessed a unique hollow cavity and excellent nitrogen doping property, resulting in fast 6641


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Figure 11. Cycle stability of N-HMCSs electrode in 100 mg L−1 NaCl solution.

ion diffusion, good charge transfer ability, and fine wettability. Compared with HMCSs and SMCSs electrodes, the N-HMCSs electrode showed an improved electrosorption capacity and rate, demonstrating the dependence of CDI performance on the synergistic effect of hollow structure and nitrogen doping property. Moreover, the N-HMCSs electrode also displayed good cycle stability over 20 adsorption−desorption cycles. Such promising results of N-HMCSs represent the promising prospect of this type of material for CDI application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00884. Magnification SEM image of N-HMCSs; SEM images of SMCSs prepared without TEOS; wide-angle XRD patterns and Raman spectra of HMCSs, N-HMCSs, and SMCSs; CV curves of HMCSs, N-HMCSs, and SMCSs electrodes at different scan rates; comparison of electrosorption capacities of various carbon electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*J. Li. Tel.: +86-025-84315351. E-mail: [email protected]. *L. Wang. E-mail: [email protected]. ORCID

Jiansheng Li: 0000-0002-3708-3677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51478224) and the priority academic program development of Jiangsu higher education institutions.

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