Creating Nitrogen-Doped Hollow Multiyolk@Shell Carbon as High

Research Article pubs.acs.org/journal/ascecg

Creating Nitrogen-Doped Hollow Multiyolk@Shell Carbon as High Performance Electrodes for Flow-Through Deionization Capacitors Hui Wang,†,‡ Tingting Yan,† Liyi Shi,† Guorong Chen,† Jianping Zhang,† and Dengsong Zhang*,† †

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Research Center of Nano Science and Technology, Shanghai University, No. 99 Shangda Road, BaoShan District, Shanghai 200444, People’s Republic of China ‡ School of Environmental Science and Engineering, Yancheng Institute of Technology, No. 1 of Hope Avenue Road, Yancheng, Jiangsu 224051, People’s Republic of China S Supporting Information *

ABSTRACT: A novel electrode material for flow-through deionization capacitors consisting of the hollow multiyolk@shell carbon (HMYSC) with effective nitrogen doping has been rationally designed and originally prepared by a template-directed coating method. The HMYSC can be divided into several hollow carbon spheres cores and the nitrogen-doped shell. The as-obtained HMYSC shows many favorable features for flowthrough deionization capacitors, such as large specific surface area (910 m2 g−1), hierarchical pores, high conductivity and good wettability. With the multiple synergistic effects of the above features, the as-prepared HMYSC electrode has higher specific capacitance, lower inner resistance and good stability. In the deionization test, the HMYSC electrode exhibits a high salt adsorption capacity of 16.1 mg g−1 under the applied voltages of 1.4 V in a 500 mg L−1 NaCl solution. Furthermore, it has been demonstrated that the HMYSC electrodes presented faster salt adsorption rate under the applied voltages of 0.8−1.4 V and in the NaCl solution with the concentration of 100−500 mg L−1. The HMYSC electrodes also exhibits an excellent regeneration performance in the repeated adsorption−desorption experiments. The HMYSC developed in this work is promising to be an effective electrode material for the flow-through deionization capacitors and other electrochemistry applications. KEYWORDS: Nitrogen doping, Hollow carbon, Capacitor, Water desalination

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INTRODUCTION Because of economic development and the continuous population rise, the rarity of freshwater resources poses significant threats to the survival and development of mankind.1−3 The impending demand of fresh water has promoted fast-growing research on desalination technologies. Such desalination technologies as reverse osmosis, thermal separation and multiple-effect distillation have been widely applied. However, such drawbacks of traditional methods as being cost-intensive, having excessive energy consumption and using hazardous chemicals during the purification stages are inevitable.4,5 Hence, the search for an eco-friendly and economical desalination technology is urgent. As an alternative, the capacitive deionization (CDI) has recently drawn extensive attention during the past decade. Depending on the electric double-layer capacitors (EDLCs) mechanism, the cations and anions in the flow-through capacitors are absorbed on the electrical double layer (EDL) regions of the oppositely charged electrode and subsequently stored inside them once the external voltage is applied during the CDI process.6−8 The absorbed ion could be released by reducing or reversing the external voltage when the adsorption equilibrium reached after a certain time.7 The CDI thereby potentially serves as an energy © 2017 American Chemical Society

saving, reversible, green and cost-efficient desalination technology.8−10 According to the above EDLCs mechanism, the deionization performance is mainly determined by the physical and chemical properties of electrode materials. The recent research on the CDI electrode materials has mainly focused on carbon materials because carbon materials have high surface area, excellent conductivity and highly chemical stability.4,11,12 Various carbon materials such as activated carbon (AC),13,14 carbon aerogel,15 carbon nanotubes,16,17 porous carbon,18,19 graphene20,21 and their composites22−24 have been extensively investigated by us and other groups. To acquire high deionization performance, a possible approach is to design a novel carbon material integrating high accessible surface area, suitable pore structure, good conductivity and wettability. The rational design of material structure has been one of the most effective strategies to resolve the above-mentioned issues. Various advanced nanostructures of electrodes have been developed, among which the hierarchically porous structures showed the most Received: December 28, 2016 Revised: February 21, 2017 Published: March 5, 2017 3329

DOI: 10.1021/acssuschemeng.6b03183 ACS Sustainable Chem. Eng. 2017, 5, 3329−3338

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the HMYSC.

enhanced deionization capacity.25,26 For example, the micropores (50 nm) serving as ion-buffering reservoirs can shorten the ion diffusion distance. The nanoporous carbonaceous spheres with solid, core−shell and/or hollow structures have been successfully synthesized by us and other research groups.18,27 In our previous works, we demonstrated that the hollow carbon spheres with mesoporous shell and macroporous void can be efficiently used as the CDI electrodes materials.27 The carbon spheres with core−shell or yolk−shell (core@ void@shell) structure combining micro/meso/macropores in a single particle could present good electrochemical performance because the hierarchically porous structure shows less limited mass transfer.28,25 However, to the best of our knowledge, there is no report on the study of yolk−shell carbon spheres as electrodes for flow-through deionization capacitors. Continuous scientific endeavors have been directed toward the surface modification of carbon materials, and the heteroatom doping on carbon frameworks has been demonstrated to be an attractive approach to improve the electrochemical performance of carbon materials.29 For example, the nitrogen-doped mesoporous carbon obtained from thermal annealing in ammonia showed improved conductivity and lower ion diffusion resistance.30 The wettability of nitrogendoped graphene hydrogels obtained from the hydrothermal reaction with urea as reducing-doped agents was also greatly enhanced, resulting in a larger specific capacitance.31 In addition, some defects produced in the nitrogen-doping process resulted in the increased porosity of carbon materials, which also contributed to a higher electrochemical performance.32 Therefore, the rational nitrogen doping should be beneficial to the CDI electrodes. Combining the advantages from rational structure design and heteroatom doping for carbon architectures, we originally developed a new type of nitrogen-doped hollow multiyolk@ shell carbon (HMYSC), and the general strategy for fabricating process is illustrated in Figure 1. The first step is a Stöber sol− gel coating process, where monodisperse SiO2 nanospheres were first coated with a polymeric layer of phenol formaldehyde resin (RF) to produce RF@SiO2 nanospheres after the introduction of resorcinol and formaldehyde, and subsequently coated with mesoporous silica (mSiO2) through the classical Stöber method using tetraethyl orthosilicate (TEOS) as a precursor and cetyltrimethylammonium bromide (CTAB) as the structure-directing agent, giving rise to the mSiO2@RF@ SiO2 nanospheres. The second coating step is a polymerization process, the dopamine was polymerized on the surface of mSiO2@RF@SiO2 in the Tris solution, and the polydopamine (PDA) layer was formed. The resulted sample was denoted as PDA@mSiO2@RF@SiO2. The third step is the carbonization

process under a N2 atmosphere, where the polymeric layer of RF can be converted into inner carbon layer and the PDA was converted into the nitrogen-doped carbon layer outside, to produce the C@mSiO2@C@ SiO2 core−shell nanospheres. The last step is the silica removal to yield the final HMYSC with a multiyolk@shell structure. The obtained HMYSC possesses remarkable features of high specific surface (910 m2 g−1) with abundant adsorption sites dispersing over the hierarchical pore structure, which favors rapid mass and charge transfer, and easy solution access. Moreover, the effective nitrogen doping and additional defects can improve the electronic conductivity and increase adsorption sites, leading to the exceptional performance as well. Ascribed to the unique features, the HMYSC is a promising candidate for flow-through deionization capacitors (FTDCs) due to the multiple synergistic effects from the unique structure and heteroatom doping.

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EXPERIMENTAL SECTION

Preparation. All the chemicals were purchased from Sinopharm Chemical Reagent Company and used without further purification. Deionized (DI) water was used throughout the process. In a typical synthesis, monodisperse SiO2 microspheres with a diameter of ∼100 nm were prepared according to the previously reported method.33 The following RF and mSiO2 coating was realized through the Stöber sol−gel method.34,35 For RF coating, 0.8 g of asobtained SiO2 spheres was first dispersed in 70.4 mL of DI water by sonication, and then 2.3 g of CTAB, 0.35 g of resorcinol, 28.2 mL of ethanol and 0.1 mL of ammonia were added into the described SiO2 dispersion, followed with stirring at 35 °C for 30 min to form a uniform dispersion. Then, 0.5 mL of a formalin solution was added to the dispersion under stirring. After 6 h of continuous stirring, the mixture was cooled to room temperature, and then aged at room temperature overnight. The product RF@SiO2 was collected by centrifugation and then washed with the DI water and ethanol several times. For the following use, the RF@SiO2 was dispersed in the ethanol through sonication. The next step was mSiO2 coating: 0.1 g of RF@SiO2, 0.16 g of CTAB and 1.5 mL of ammonia were added into the mixture solution of ethanol and DI water (v/v = 4:1). After sonication for 1 h, 1.54 g of TEOS diluted by ethanol were added drop by drop, and then stirred 12 h. The resulted mSiO2@RF@SiO2 was collected by vacuum filtration and then washed with ethanol several times. The mSiO2@RF@SiO2 was dried at 60 °C overnight. For the next PDA coating, 0.24 g of mSiO2@RF@SiO2 was dispersed in 75 mL of 10 mM Tris solution (pH = 8.6), and then 0.24 g of dopamine was added under stirring. After 24 h of reaction, the product PDA@ mSiO2@RF@SiO2 was washed by ethanol and DI water several times. The carbonization of the PDA@mSiO2@RF@SiO2 was completed at 400 °C for 2 h and 800 °C for 2 h under a N2 atmosphere with a heating rate of 1 °C min−1. The obtained silica−carbon composite was further treated with a 10 wt % HF aqueous solution to remove the silica template completely, followed by washing with plenty of DI water, and drying at 60 °C for 24 h. The resultant sample was denoted as HMYSC. For comparison, the HC was prepared by direct carbonization of RF@SiO2, and then etched with 10 wt % HF aqueous solution. For comparison, the yolk−shell carbon (YSC-1 and 3330


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Figure 2. (a) XRD pattern, (b) Raman spectra and (c) XPS spectra of the HMYSC. YSC-2) was also synthesized, and the only difference is the amount of TEOS. For YSC-1 and YSC-2, the ratio of TEOS/RF@SiO2 is 9.4 and 12.4. The nitrogen-doped carbon (N-doped C) was also obtained after carbonizing and etching PDA@SiO2. More details are provided in the Supporting Information. Characterization. The morphologies were examined by emission scanning electron microscopy (SEM, JEOL JSM-700F) and transmission electron microscopy (TEM, JEOL JEM-200CX). The graphitization degree of each sample was tested by X-ray diffraction (XRD, Rigaku) and Raman (JY H800UV) with a 633 nm laser. The specific surface area and pore volume were measured by nitrogen sorption (Autosorb-IQ2) after degassed overnight at 593 K in a vacuum line. The Brunauer−Emmett−Teller (BET) method was utilized to calculate the specific surface areas and the pore volumes, and the pore size distributions were derived from the desorption branches of the isotherms using nonlocal density functional theory (NLDFT). The nitrogen species on the surfaces of the HMYSC were characterized by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C). The contact angle of water on the surface of electrode was measured by a drop shape analysis system (Krüss, DSA100). The cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were measured with the CHI 660D. The galvanostatic charge−discharge (GC) was conducted with LAND battery test instrument. All the electrochemical properties were measured in a NaCl solution using a three-electrode system. A piece of graphite and a saturated calomel electrode were used as the counter and reference electrodes. FTDC Experiments. For preparation of the FTDC electrodes, the active component (90 wt %) were mixed with polytetrafluoroethylene (10 wt %) in ethanol. The obtained slurry was pressed onto graphite sheets and dried at above 110 °C overnight. The size of the electrode is 50 mm × 40 mm × 0.3 mm. The FTDC performance of electrodes was measured in a batch mode experiment, conducting in a continuous recycling system.36 The FTDC cell herein consists of two sided electrodes separated by an insulated grid spacer with a thickness of 0.27 mm. The NaCl aqueous solution was continually supplied to the cell using a pump with a flow rate of 30 mL min−1. The concentration change of the NaCl solution was measured by a conductivity meter at the outlet of the cell. The salt adsorption capacity (SAC) and salt adsorption rate (SAR) of electrodes was calculated according to the following equation:

SAC = (C0 − C)V /m

(1)

SAR = SAC/t

(2)

S (μS/cm) = 2.169C (mg L−1) + 2.55

(3)

observed at 2θ = 24.0° and 42.8°, corresponding to the (002) and (100) planes of HMYSC, respectively. Without the nitrogen-doped shell, the HC shows a similar peak position and intensity (Figure S1), indicating that the crystalline structure can be well maintained after the multicoating process and the silica templates can be removed thoroughly. The (002) peak of HMYSC has shifted to the lower value as compared to the standard graphite (26.6°), which indicates that the inter lamellar spacing between the (002) planes of HMYSC have become broader. Moreover, the (002) peak of HMYSC is broad and not like the sharp graphite (002) peak, ascribing to more pores and defects in the HMYSC structure.37 The (100) peak is relatively weak, which indicates that the as-prepared HMYSC has a low crystalline structure.38 The Raman spectroscopy using an excitation wavelength of 633 nm further reveals the structure characteristics of HMYSC. As shown in Figure 2b, the broad and overlapping D (1357 cm−1) and G (1578 cm−1) bands are observed, giving rise to (i) disordered graphitic lattice or sp3-rich phase and (ii) amorphous sp2bonded forms of carbon.39 In addition, the intensity ratio of G to D band is about 1.01 and the defects can be ascribed to the nitrogen doping and multiyolk@shell structure, which is consistent with the XRD results. The IG/ID value of HMYSC is relatively higher compared to that of the HC (Figure S2), indicating that the HMYSC has the more abundant graphitic layers even though the presence of disorder in the structure. The XPS measurement is performed to identify further the heteroatoms doping of the HMYSC. As calculated from the XPS analysis, the nitrogen content is about 3.1 at. % (Figure S3). As shown in Figure 2c, the high resolution N 1s spectra can be deconvoluted into three different types of nitrogen species located at 398.5 ± 0.2 eV, 400.4 ± 0.2 eV and 401.4 ± 0.2 eV corresponding to pyridinic N (N-6), pyrrolic N (N-5) and quaternary N (N-Q).39 The N-6 atoms are bonded with two C atoms in a C6 ring, leading to a pair of lone electrons and hence induce electrondonor properties of the graphitic layer. The N-5 is in five-membered rings or pyridine-N associated with the adjacent phenolic or carbonyl group in the neighbor ring. The N-Q atoms are in the center of graphitic plane, bonding with three carbon atoms.38 It has been demonstrated that the presence of N-Q can improve the electronic conductivity of carbon networks, which is highly needed for electrodes with high conductivity.39 These N 1s results indicate that most of the N atoms are embedded in the graphitic layer, and the structural improvement of carbon network could drastically enhance the electrochemical properties of HMYSC.38,40 The N-5 and N-6 components on the surface of HMYSC provide hydrophilic polar sites due to the proton donor property, which are beneficial for improving the wettability of carbon networks, and thus the accessibility of

where C0 and C are the initial and final concentrations of NaCl solution, V is the total volume, m is the total mass of the electrodes, t is the deionization time and S is the solution conductivity.

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RESULT AND DISCUSSION The structure of the synthesized material was characterized by various analytical methods. The XRD results for the HMYSC are shown in Figure 2a. Two broad diffraction peaks can be 3331


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Figure 3. TEM images of (a) YSC-1, (b) YSC-2 and (c) HMYSC; N2 sorption isotherms of (d) YSC-1, (e) YSC-2 and (f) HMYSC. The inset is the pore size distribution.

Figure 4. (a) SEM image, (b,c) TEM images and (d−f) energy filter TEM mapping images of the HMYSC.

salty solution can be improved correspondingly.40−42 Additionally, some expletive defects are produced in the carbon architectures due to the presence of N atoms, and thus additional adsorption sites are created as well.43 It is interesting that the HMYSC shows much lower electric resistivity (0.03 Ω cm−1) than that of the HC (0.37 Ω cm−1), which is obtained from the four-point probe methods. Therefore, it is confirmed that the conductivity of HMYSC is improved due to the nitrogen doping. The formation of multiyolk@shell structure is realized by adjusting the amount of TEOS. The simple yolk−shell structure is formed with a low amount of TEOS. Figure 3a−c compares the TEM images of the products during the modulation process, which clearly demonstrates the structure transformation from simple yolk−shell to multiyolk shell. The subtle change is attributed to the thickness of mSiO2 coating on the surface of RF@SiO2. It should be noted that the thickness of mSiO2 on RF@SiO2 can be tailored by adjusting the ratio between silicon source and RF@SiO2. With a low ratio

(TEOS/RF@SiO2 = 9.4−12.4), mSiO2 just coated on surface of RF@SiO2 one by one, and then dopamine polymerized on the separating surface of mSiO2@RF@SiO2. After carbonization and etching, a simple yolk−shell carbon was produced (Figure 3a,b). Continuing to increase the ratio (TEOS/RF@ SiO2 = 15.4), excessive TEOS would further hydrolyze on the surface of separating mSiO2@RF@SiO2 with a thinner layer, and the separating mSiO2@RF@SiO2 would be enveloping into a whole. Subsequently, the dopamine polymerization would occur on the whole piece of mSiO2@RF@SiO2, rather than one by one. Hence, the multiyolk shell structure is produced finally (Figure 3c). Eventually, after the systematical analysis on the multiyolk shell carbon, we find that this multiyolk structure has a hierarchically porous structure with more paths in the inner core for ion transport and storage. Figure 3d−f shows the nitrogen adsorption−desorption isotherm curves of all the samples. Obviously, all the samples exhibit the type IV isothermal curve with a distinct hysteresis loop at a relative pressure P/P0 ranging from 0.4 to 1.0, 3332


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Figure 5. Optical micrographs of the water contact angles on the surface of carbon electrodes as a function of contact time.

analyses demonstrate that the dopamine polymerization coating-directed synthetic pathway is a powerful route for fabrication of a homologous nitrogen-doped carbon networks. To study the wettability of the two kinds of carbon electrodes, the hydrophilic characteristics were provided by contact angle measurements. The optical micrographs of the water contact angles on the surface of carbon electrodes as a function of contact time are clearly shown in Figure 5. Obviously, the entire contact angles were decreased with the time increasing. At the beginning of the test, the contact angle of the HMYSC electrode is about 88.7°. When the contact time goes to 1.8 s, the contact angle was decreased to 9.3° and then disappeared on the surface of the HMYSC electrode. In contrast, the contact angle of HC is about 98° at first, and larger than that of the HMYSC. When the contact time increased to 1.8 s, the contact angle is about 90°, and much larger than that of the HMYSC at the same contact time. The contact angle of HMYSC decreased quite a lot with the increase of contact time, whereas the contact angle curve of HC is flat (Figure S4). The contact angle of the HMYSC electrode quickly reached to 0°, indicating that aqueous solution is easier to infiltrate into the HMYSC electrode. That is to say the HMYSC shows a much better hydrophilic property than the HC due to the effective nitrogen doping of the thin shell. The excellent wettability of the HMYSC means easier accessible surface and channels for salty solution permeating, which contributes much to its FTDC performance.44,45 To evaluate the effectiveness and advantages of HMYSC, the capacitive properties of all samples were first measured by using a three-electrode system in a 0.5 M NaCl solution. For comparison, the HC with a diameter of 100 nm was also prepared by a template-directed method. To show the advantages of HMYSC as FTDC electrode, the CV curves of the two electrodes at a scan rate of 1 mV s−1 are presented in Figure 6a. The CV curves have a rectangular-like shape and no obvious redox process is observed, indicating that the capacitive response mostly results from the EDL formation.46 It is worth noting that the CV curve of the HMYSC electrode exhibits a much larger area as compared with HC, suggesting a higher specific capacitance of HMYSC electrodes. The specific capacitance of HMYSC is 204.0 F g−1 at 1 mV s−1, much larger than that of HC (150.0 F g−1). The CV with a very low solution concentration is also tested, and the specific capacitance of HMYSC electrode is still higher than that of the HC (Figure S5). The CV curve deviates from ideal rectangular-like shape when the scan rate was increased to 10 mV s−1, because these is no abundant time for the ions transporting into the inner pores of electrode and thus the EDL formation is slightly impeded (Figure S6).47 The capacitance of HMYSC is also higher than those of N-doped C, YSC-1 and YSC-2 (Figure S6). The higher capacitance of HMYSC can be

suggesting a hierarchically porous structure, especially the existence of mesopores and macropores.33 As calculated by the BET method, the HMYSC has a higher specific surface area (910 m2 g−1) than that of theYSC-1 and YSC-2 (751 and 857 m2 g−1), indicating the multiyolk@shell structure can greatly increase the specific surface area. It is worth noting that the external area of HMYSC is as large as 641 m2 g−1, suggesting that mesopores are predominant in the HMYSC. This can be further confirmed by the pore size distribution profiles in the inset of Figure 3f. The mSiO2 coating on RF@SiO2 can serve as mesoporous templates for dopamine polymerization and the nitrogen-doped mesoporous carbon shell can be well formed after carbonization and etching, which favors higher external surface area of HMYSC. With a higher external specific surface area, the HMYSC could offer an ample interface for ion or charge accumulation and increase the accessible surface area. As seen from the corresponding NLDFT pore size distribution, the HMYSC exhibits dual distribution of micropore at 0.5 and 1.5 nm, and mesopore at 4.6 nm. Hence, combining the N2 adsorption curve and pore size distribution analysis, the HMYSC exhibits a hierarchical pore structure consisting of macro-, meso and micropores. In addition, the HMYSC has a total pore volume of 3.49 cm3 g−1, much larger than those of YSC-1 and YSC-2 (1.21 and 1.81 cm3 g−1). The larger volume of HMYSC can favor rapid ion diffusion and lead to an exceptionally enhanced rate performance of electrode material. In a word, the higher surface area and larger pore volume of HMYSC can be successfully obtained, which should accelerate the ion transport and adsorption in the FTDC application.38 As shown in the SEM image in Figure 4a, the HMYSC shows a spherical structure. The cores can be clearly observed in the broken area, indicating the yolk−shell structure of HMYSC is well formed. The TEM image in Figure 4b shows that the HMYSC exhibits a perfect multiyolk@shell structure with several HC cores well dispersed within the nitrogen-doped shell. This multiyolk@shell structure can provide more transport pathways and lowers the transport barriers. Both the shell and cores of HMYSC have ultrathin walls, which not only provide minimum diffusive resistance to mass transport on the interface but also offer easy ion transport by shortening the diffusion pathways. The energy filter TEM mapping images (Figure 4c−f) reveal that nitrogen atoms are effectively incorporated and homogeneously distributed throughout the shell at the nanoscale, which will induce some defects on the carbon architecture, increase available adsorption sites, effectively modulate surface property and improve the electronic conductivity. The N doping is mainly distributed on the outermost layer of HMYSC, and the inner carbon spheres are seldomly doped because the polydopamine with a larger molecule structure can hardly penetrate into the inner carbon surface. The particular morphology and structure 3333


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resistance of electrode.36,51 The iR drop at the low current density is inconspicuous, whereas with the increase of current density the value of iR drop increase. The detailed values of iR drop are present in Figure 6c. At any current density, the HMYSC electrode always shows much lower iR drop under the same testing conditions, indicating the much smaller inner resistance, corresponding to the EIS results.51 This result further confirms that the conductivity is improved after nitrogen doping and the inner resistance is reduced correspondingly. To compare further the ion-transport behavior and electrical resistance of HMYSC and HC electrodes, the EIS was used and analyzed, as shown in Figure 6d. In the low-frequency region, both of the electrodes exhibit a straight and nearly vertical line, indicating the capacitive behavior. In the high frequency region, the real axis intercept is the equivalent series resistance (ESR), and the width of semicircle plotted is indicative of the chargetransfer resistance caused by Faradaic reactions in the interface.52,53 Obviously, the ESR value of HMYSC is much lower than that of the HC (inset). Thereby, the synergistic effect of the multicomponent structures is validated: (i) The hierarchically porous structure of HMYSC is facilitate the contact between the salt ion and electrode material.51 (ii) The nitrogen doping of porous shell improves the conductivity of whole electrode and serves as multidimensional pathways, facilitating the transport of electrons in the bulk electrode. As shown from the curves of HMYSC and HC, the semicircle impedance loop is ignorable, indicating that the charge transfer resistance is quite small. The more vertical nature of the curves in the low frequency region indicates a near-ideal double layer capacitor behavior. The EIS results further demonstrate that the hollow multiyolk@shell structure with nitrogen doping can effectively decrease the inner resistance of HMYSC due to the enhanced conductivity as well as the rapid electron and ion transportation. To determine the FTDC performance of HMYSC and HC electrodes, the batch mode FTDC experiments were carried out in a NaCl solution with an initial conductivity of 1042 μS cm−1 at an applied potential of 1.4 V. A pair of electrodes was assembled to a sandwich structure and separated by an insulting spacer. In Figure 7a, the plot of SAC vs time under the external voltage is well presented. The SAC increases quickly at the initial stage of the deionization process and then slows down until reaching a plateau because of the reduced accessible surface area and enhanced electrostatic repulsion with the electrosorption time going by. Remarkably, the HMYSC electrodes always present much higher SAC than that of the HC electrodes, suggesting the better deionization rate performance of HMYSC electrodes, which indicates that more ions are adsorbed by the HMYSC electrodes. It should be noted that the HMYSC shows an SAC of 16.1 mg g−1, which is larger than that of HC (12.7 mg g−1) under the applied voltage of 1.4 V in the NaCl solution with the concentration of 500 mg L−1. Also, the HMYSC electrodes show higher SAC than other carbon materials reported in recent literature under the similar testing conditions (Figure S7 and Table S2). The charge efficiency of HMYSC, HC is 0.50 and 0.43, less than 1.0 due to the weak adhesion with current collector, co-ion effect and the voltage consumption from the binder (Figure S8).54−57 It is quite necessary to evaluate the overall desalination performance in consideration of SAC and SAR together. The Ragone plots of SAR vs SAC for FTDC electrodes provide an intuitive representation of the FTDC performance, in which the

Figure 6. (a) CV curves of the HMYSC and HC electrodes at a scan rate of 1 mV s−1; (b) GC curves of the HMYSC and HC electrodes with a current density of 0.2 A g−1; (c) iR drops of the electrodes vs current densities and (d) EIS presented as Nyquist plots of the HMYSC and HC electrodes. All the curves were obtained in a 0.5 M NaCl aqueous solution.

ascribed to the following aspects. First, the nitrogen doping can generate extrinsic defects on the carbon structure, which favors the ion diffusion and hence improve the capacitance.48 Second, the nitrogen as a dopant modifies the carbon structure and then improves the wettability and electronic conductivity of the HMYSC, which is beneficial to the interfacial contact with salty solution and charge transfer.49,50 Third, the unique multiyolk@ shell structure provides even larger specific surface area, increasing the amount of adsorption sites of HMYSC. The multiyolk@shell carbon possesses a hierarchically porous structure. Micropores are mainly from the cores derived from the RF decomposition, and mesopores are from the shell due to mSiO2 templates for dopamine polymerization. The hollow structure of HMYSC creats abundant macropores. As previously reported, the macropores can serve as ion-buffering reservoirs, which guarantees a shorter ion diffusion distance.33 In the multiyolk@shell carbon structure, various voids are effective ion-buffering reservoirs and hence the rapid transportation of ions into the interior of the bulk can be easily realized. The mesopores on the shell provides abundant ion transport pathways with a low resistance, so the ions in the bulk solution can easily penetrate and transport into the cores. The hollow multiyolk with a lot of micropores increases the specific surface area to adsorb and storage ions. Hence, combining macro-, meso- and micropores, the HMYSC shows a remarkable advantage in ion transportation and adsorption. Figure 6b shows the discharge curves of the as-prepared samples at the current density of 0.2 A g−1. It is obvious that the discharge time of HMYSC is much longer, as compared to that of the HC. It means that the HMYSC electrode has quite larger specific capacitance, which is consistent with the CV results. The higher capacitance determined from the GC further demonstrated that the moderate nitrogen doping on the carbon structure and the multiyolk@shell structure can greatly enhance the specific capacitance due to the improved conductivity, wettability and sufficient ion transport channel and short diffusion distance. The rate capability is another crucial criterion for estimating the practical application of HMYSCbased FTDC electrode. The sudden voltage drop (iR drop) at the beginning of discharge process can reflect the electronic 3334


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structure are favorable for buffering ions to shorten the diffusion distances from the bulk solution to the interior surfaces. Moreover, the appropriate amount of micropores in the meso- and macroporous skeleton guarantee effective EDL formation and promote ion adsorption. (iii) The wettability is another critical factor for electrosorption, and the aqueous solutions can be rapidly immersed into the HMYSC as shown in the contact angle testing, which accelerates the ions adsorption during the deionization process. (iv) The nitrogen doping assists the formation of some defects on the carbon architecture and modifies the electronic structure of carbon plane. On one hand, the defects increase the adsorption sites for ion adsorption. On the other hand, the effective nitrogen doping enhances the conductivity of HMYSC, accelerating the charge transfer and reducing the internal resistance of the electrode. In conclusion, combining the higher specific surface area, suitable pore structure, excellent wettability and conductivity, the HMYSC shows highly enhanced performance for the FTDC application. As is well-known, the applied voltage plays a key role on the FTDC efficiency. The low voltage may lead to insufficient EDL formation, and weakens the adsorbing ability of FTDC electrodes. Although the excessively high voltage could lead to Faradaic current, and bubbles would be produced on the surface of electrodes due to the electrolytic reaction.59 Herein, the electrosorption of NaCl was carefully conducted at different external voltages ranging from 0.8 to 1.4 V. Figure 7c shows the change of solution conductivity at different voltages using HMYSC electrodes. As shown, the solution conductivity declined with the time going by, but the falling trend is more obvious at higher applied voltage. That is to say the ions are easier to be adsorbed by the electrodes at higher voltage, which results in higher SAC due to the stronger Coulombic interaction. During the FTDC process, the weak adhesion between the current collector, electrodes materials and binder used in the fabrication of electrodes may lead to extra consumption of the applied voltage, so no oxygen evolution was observed even at 1.4 V.27,60 When the voltage increased from 0.8 to 1.4 V, the SAC of the HMYSC electrodes increased from 3.4 to 8.5 mg g−1, whereas the HC electrodes is from 2.7 to 6.9 mg g−1 in the NaCl solution with the concentration of 100 mg L−1. The HMYSC always exhibits much higher SAC than that of the HC at any applied voltage owing to its unique structure and special surface properties (Figure S9). As shown in Figure 7d, the higher voltage shifted the range plot toward upper and right region, suggesting both of the deionization capacity and rate are increased with the increase of applied voltage, which is ascribed to stronger Coulomic interactions between charged ions and electrode and a thicker electrical double layer. To evaluate further the FTDC performance of HMYSC, the experiments using the HMYSC in the NaCl solution with different initial concentrations (100−500 mg L−1) are also conducted, and the results are shown in Figure 7e,f. For initial NaCl solution of 100, 300 and 500 mg L−1, the SAC is 8.5, 14.2 and 16.1 mg g−1, respectively. The Ragone plot is shifted to the upper and right region when the initial NaCl concentration increased. The compact EDL formation in higher NaCl concentration favors the SAC increase. The enhanced ionic conductivity of NaCl solution with higher concentration accelerates faster ion transport into the electrodes, so the SAR is enhanced with the NaCl concentration increase.58

Figure 7. (a) Plots of SAC vs deionization time; (b) Ragone plots of SAR vs SAC for the HC and HMYSC electrodes in a NaCl solution with a concentration of 500 mg L−1; (c) plots of SAC vs deionization time and (d) Ragone plots under different voltage; (e) plots of SAC vs deionization time; and (f) Ragone plots under different concentration of the HMYSC electrodes. All the tests were conducted with the flow rate of 30 mL min−1.

salt adsorption rate (SAR) is plotted against the salt adsorption capacity (SAC).58 Figure 7b shows the comprehensive comparison of the HMYSC and HC electrodes in terms of the deionization performance. With the deionization time increase, the higher capacity can be obtained until reach adsorption equilibrium, whereas the rate is decreased with the deionization time increase. With the time going by, the adsorption sites for salt ion will decrease, and the electrostatic repulsion will be stronger, resulting in a slight increase of SAC and sharp decrease of SAR. The plot of HMYSC is located in the much upper and right region, indicating increased SAC and faster SAR. The larger specific surface area, high conductivity and good wettability of HMYSC accelerate ion transport and adsorption, so the capacity and rate are enhanced accordingly. That is to say the HMYSC is a better option for rapid deionization with high capacity as compared to the HC. The higher deionization capacity and rate of the HMYSC electrodes can be ascribed to the following reasons: (i) The multiyolk@shell structure of HMYSC provides much larger surface area and pore volume, which increases the accessible surface area for ions adsorption during the deionization process. The walls of the shell and cores are ultrathin, which is beneficial to the ion diffusion and transportation by offering minimum diffusive resistance and short diffusion distance. (ii) It is wellknown that pore structure is a very important factor for the electrosorption performance. The HMYSC with an appropriate amount of micropores, and mesopores and macropores shows a suitable pore structure for electrosorption, as compared to the HC. The mesopores in thin walls provide smooth pathways for ion transportation, the macropores within muti-yolk@shell 3335


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electrode material notably for the FTDC, but also for other electrochemistry application.

As known, the regeneration performance of the FTDC electrode is very important to the practical application. Figure 8

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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.6b03183. XRD patterns and Raman spectrum of HC; XPS spectrum of HMYSC; plots of water contact angles on different carbon electrodes versus contact time; CV profiles of the HMYSC and HC electrodes; CV profiles of the HMYSC, HC, YSC-1, YSC-2 and N-doped C electrodes; FTDC profiles of HMYSC electrode; current transient for HC and HMYSC electrodes; electrosorption comparison of HC and HMYSC electrodes; detailed specific surface and pore volume data of the HMYSC and HC; comparison of the FTDC performance among different carbon-based electrodes (PDF)

Figure 8. Regeneration curves of the HMYSC electrodes in a NaCl solution with a concentration of 100 mg L−1 with the flow rate of 30 mL min−1.

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presents the several adsorption−desorption cycles of the HMYSC and HC electrode in a NaCl solution with an initial conductivity of 220 μS cm−1. A direct voltage was applied on the electrodes in the electrosorption charge process whereas the potential was removed for the desorption process. The profiles clearly show that the conductivity can come back to the initial value in desorption, suggesting the ions can be released and back to the bulk solution after the voltage removal. This confirms that the electrode could be regenerated very well without any driving energy and secondary pollution. Moreover, no obvious declination of the electrosorption performance is observed in the repeated adsorption−desorption experiments, which is ascribed to faster ion transfer and shorter diffusion distance in the hierarchical porous structure of HMYSC.33 The easier regeneration performance of the electrodes further demonstrated that the HMYSC is a promising candidate for FTDC application.

AUTHOR INFORMATION

Corresponding Author

*Dengsong Zhang. E-mail: [email protected]. ORCID

Dengsong Zhang: 0000-0003-4280-0068 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of the National Natural Science Foundation of China (U1462110) and the Science and Technology Commission of Shanghai Municipality (16JC1401700). The authors thank Mr. P. F. Hu from the Analysis and Test Center of SHU for help with the TEM measurements.

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CONCLUSION In summary, the nitrogen-doped hollow multiyolk@shell carbon with an excellent FTDC performance is rationally designed and originally prepared by a template-directed coating method. The HMYSC consists of nitrogen-doped carbon shell and multiple nanosphere cores. The multiyolk@shell structure could be controlled by adjusting the amount of silicon source using the coating process. The as-obtained HMYSC with a multiyolk@shell structure exhibits large specific surface area (910 m2 g−1), hierarchical pores, good wettability and high conductivity. These characteristics present favorable multiple synergistic effects, which is beneficial for high performance FTDCs. The electrochemical characterization reveals that the as-prepared HMYSC electrode has higher specific capacitance, lower inner resistance and good stability. In the following FTDC test, the HMYSC electrode exhibits much higher SAC than HC electrodes at any chosen voltage, especially the SAC can reach 16.1 mg g−1 in a NaCl solution with a concentration of 500 mg L−1 under the applied voltages of 1.4 V. Moreover, the HMYSC electrodes shows enhanced salt removal rate as compared to HC. The HMYSC electrodes exhibit an excellent regeneration performance in the repeated adsorption− desorption experiments. Impressively, with the aspect to large surface area, hierarchically porous structure and other good surface properties, we expect the HMYSC offers a desirable

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Creating Nitrogen-Doped Hollow Multiyolk@Shell Carbon as High

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