Synthesis of Dispersible Mesoporous Nitrogen-Doped Hollow Carbon

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Synthesis of Dispersible Mesoporous Nitrogen-doped Hollow Carbon Nanoplates with Uniform Hexagonal Morphologies for Supercapacitors Jie Cao, Charl J. Jafta, Jiang Gong, Qidi Ran, Xianzhong Lin, Roberto Félix, Regan G. Wilks, Marcus Bär, Jiayin Yuan, Matthias Ballauff, and Yan Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08946 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 21, 2016

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ACS Applied Materials & Interfaces

Synthesis of Dispersible Mesoporous Nitrogendoped Hollow Carbon Nanoplates with Uniform Hexagonal Morphologies for Supercapacitors Jie Cao,a Charl J. Jafta,a Jiang Gong,b Qidi Ran,a Xianzhong Lin,c Roberto Félix,d Regan G. Wilks,de Marcus Bär,def Jiayin Yuan,b Matthias Ballauffa and Yan Lu*a a

Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie,

Hahn-Meitner-Platz 1, 14109 Berlin, Germany. b

Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Am

Mühleberg 1, 14476 Potsdam, Germany. c

Institute for Heterogeneous Material Systems, Helmholtz-Zentrum Berlin für Materialien und

Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany. d

Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Lise-

Meitner-Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany. e

Energy Materials In-Situ Laboratory Berlin (EMIL), Helmholtz-Zentrum Berlin für Materialien

und Energie GmbH, Albert-Einstein-Straße 15, 12489 Berlin, Germany. f

Institut für Physik und Chemie, Brandenburgische Technische Universität Cottbus-Senftenberg,

Platz der Deutschen Einheit 1, 03046 Cottbus, Germany. Keywords: polydopamine, silica nanocasting, hollow carbon nanoplates, carbon nanostructure, supercapacitors.

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ABSTRACT: In this study, dispersible mesoporous nitrogen-doped hollow carbon nanoplates have been synthesized as a new anisotropic carbon nanostructure using gibbsite nanoplates as templates. The gibbsite-silica core-shell nanoplates were first prepared before the gibbsite core was etched away. Dopamine as carbon precursor was self-polymerized on the hollow silica nanoplates surface assisted by sonification, which not only favors a homogeneous polymer coating on the nanoplates but also prevents their aggregation during the polymerization. Individual silicapolydopamine core-shell nanoplates were immobilized in a silica gel in an insulated state via a silica nanocasting technique. After pyrolysis in a nanoconfine environment and elimination of silica, discrete and dispersible hollow carbon nanoplates are obtained. The resulting hollow carbon nanoplates bear uniform hexagonal morphology with specific surface area of 460 m2·g−1 and fairly accessible small mesopores (~3.8 nm). They show excellent colloidal stability in aqueous media and are applied as electrode materials for symmetric supercapacitors. When using polyvinylimidazolium-based nanoparticles as a binder in electrodes, the hollow carbon nanoplates present superior performance in parallel to polyvinylidene fluoride (PVDF) binder.


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Introduction Along the rapid development of materials science, substantial research interest has concentrated on the controllable synthesis of nanomaterials featured with a hollow interior, which show a wide range of promising applications based on their properties such as light weight, low density, high surface-to-volume ratio, and possible shellcontrolled permeability.1–5 Among them, hollow carbon nanomaterials are unique examples due to their excellent physicochemical properties such as rich abundance, chemical inertness and tunable electrical conductivity, making them appealing candidates for energy storage, water remediation, and catalyst support.6–10 For example, carbon nanocapsules with enhanced adsorption capability towards toluene and methanol vapours have been reported as a result of their high surface area.11 Hollow carbon nanotubes (CNTs) have been applied as a carrier system to support catalytic components. Different types of catalytically active metal nanoparticles (such as Pd, Au, and Au-Pd alloy) and metal oxide nanoparticles (for example ZnO, Co3O4, and TiO2) have been immobilized onto the CNTs, which can be applied as catalyst for selective oxidation reaction of benzyl alcohol to benzaldehyde.12 Generally, novel properties may arise from morphological manipulation of carbon-based materials due to the variation of the interfacial/mutual interaction. The development of hollow, mesoporous carbon nanomaterials with regular morphologies would provide great opportunities to explore the full property spectrum of carbons. Previously, spherical or tube-like morphology of hollow carbon (nano)materials have been reported.13–16 To date, two-dimensional (2D) carbon materials with high aspect ratios, finite lateral sizes and porous structures have attracted increasing interest and have shown potential applications


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in adsorption, and energy storage.17–21 Carbon nanoplates are exotic carbon 2D nanostructures that have been studied in only limited examples due to their restricted accessability.22,23 For instance, 2D microporous carbon nanoplates and porous carbon nanosheets were synthesized and tailored for use in supercapacitors. Compared with spherical carbon materials, 2D nanostructured carbon materials could reduce the ion transport distance in the nanoscaled dimension, and could exhibit significantly improved electrochemical performance.22,23 In comparison, to our best knowledge hollow carbon nanoplates are unknown carbon nanostructures to be explored. In parallel, most carbon nanostructures when produced at high temperature will inevitably conglutinate in aqueous media due to their hydrophobic nature, which leads to nondispersible state in the absence of stabilization agent. One of the methods to overcome this limitation was reported by Lu et al.24 by using phenol and formaldehyde as carbon precursors via confined pyrolysis of polymer-silica particles. Later, Soll et al. synthesized highly crystalline, water dispersible carbon nanobubbles by using poly(ionic liquid) as carbon precursor via a silica nanocasting technique.25 These carbon nanostructures are structurally isotropic and self-dispersing in water without dispersants. Inspired by our recent work,26 gibbsite-polymer core-shell nanoplates can be synthesized by a dopamine-based approach. In the present study, we report the fabrication of water dispersible nitrogen-doped mesoporous hollow carbon nanoplates via a silica nanocasting technique using dopamine as carbon precursor and hexagonal-shaped gibbsite as template as shown in Figure 1. These hollow carbon nanoplates can be further applied as electrode materials for supercapacitors, i.e. electrochemical double layer (ECDL) capacitors.


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Figure 1. Schematic synthesis route to prepare hollow carbon nanoplates (HCPs).

Experimental Synthesis of gibbsite nanoparticles Platelike Gibbsite nanoparticles were prepared as our previous work26 following the approach developed by Wierenga et al.27 Details for the synthesis can be found in the Supporting Information. Synthesis of hollow silica nanoplates Gibbsite nanoparticles with silica coating were prepared according to the modified Stöber method.28 In a typical synthesis, polyvinylpyrrolidone (PVP)-stabilized gibbsite nanoplates were first prepared by adding 25 ml gibbsite nanoplates dispersion (40g/L) into 500 mL PVP solution (100g/L). The mixture was kept under stirring for 24 h and was then centrifuged for 20 h at a speed of 500 g. The sediment was dispersed in 794 mL of ethanol via ultrasonification. 46 mL of ammonia was added to adjust ammonia concentrations to 5.5% (v/v). While vigorous stirring, 0.8 mL tetraethyl orthosilicate


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(TEOS) was injected slowly into the mixture. The reaction lasted for 6 h. The products were cleaned by washing twice with ethanol and three times with water, and then redispersed in concentrated HCl (37 %). The gibbsite cores were etched away eventually by concentrated HCl for 5 days. The as-obtained hollow silica nanoplates were washed several times with deionized water until neutral pH was reached. Coating of hollow silica nanoplates with a polydopamine shell The deposition of polydopamine shell onto the hollow silica nanoplates was conducted by the self-oxidation polymerization of dopamine. In a typical run, 0.5 mg/mL hollow silica nanoplates were exposed to a dopamine monomer solution (0.5 mg/mL) in air at pH 8.5 in 10 mM tris(hydroxymethyl) aminomethane (TRIS) buffer. Meanwhile, ultrasonification is applied to avoid aggregation of polydopamine. After 3 h’s reaction, the polydopamine coated hollow silica nanoplates were cleaned by centrifugation with deionized water. This process was repeated for several times until the secondary polydopamine particles were totally removed from the dispersion. Synthesis of hollow carbon nanoplates The silica nanocasting process was realized as follows. 2 mL of HCl (0.1 M) was added to 20 mL of polydopamine coated hollow silica nanoplates dispersion (40 g/L) under stirring. After 10 min’s stirring followed with 10 min of ultrasonification, 15 mL TEOS were added in portions under the liquid surface of mixture while being vigorously stirred. The reaction was run at room temperature overnight under stirring. After removing ethanol by rotary evaporation, the mixture was freeze-dried. Carbonization was then carried out at 800 °C for 2 h under argon (heating rate of 2 °C/min). The pyrolyzed product was treated by the ammonium hydrogen difluoride (NH4HF2) aqueous solution to


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remove the silica. The desired products hollow carbon nanoplates were obtained after careful washing. Synthesis of polyvinylimidazolium (PIL) nanoparticles PIL binders were prepared previously.29 Typically, 1,3,5-tribromomethyl benzene was treated with 1-vinylimidazole, which is in excess, to produce Br- containing monomer which consists of three vinylimidazolium bromide units. The polymerization was then conducted to form Br- containing PIL polymer, which are nanoparticles of three dimensionally interconnected networks. The final product PIL nanoparticle binder was obtained by anion exchange to replace Br- by bis(trifiuoromethane)sulfonimide (TFSI). Material characterization Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) measurements were conducted by using JEOL JEM-2100 instrument. Raman spectrum was taken by a confocal Raman microscope (α300; WITec, Ulm, Germany) equipped with a 532 nm laser. Energy-dispersive X-ray Analysis (EDX) were conducted using LEO GEMINI 1530 scanning electron microscope equipped with a Thermo Fisher Scientic Xray silicon drift detector. Combustion elemental analysis was performed on a Vario Micro setup to determine the content of C, H, and N in the final product. Hard X-ray photoelectron spectroscopy (HAXPES) measurements were conducted at the High Kinetic Energy Spectrometer (HiKE) endstation located at the KMC-1 beamline of the BESSY II light source.30,31 A 2 keV excitation energy and a VG SCIENTA R4000 electron analyzer were used for these measurements. For energy calibration, a clean Au foil was measured and the binding energy (BE) of the Au 4f7/2 core level was set to 84.00


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eV.33 N2 adsorption/desorption measurements were performed with a Quantachrome Autosorb-1 instrument at 77 K.16 X-ray diffraction (XRD),32 Thermo Gravimetric analysis (TGA), and zeta potential measurements were performed as in our previous study.26 Electrochemical Tests A symmetrical two electrode cell was applied for the electrochemical tests. To prepare the electrodes, 80 wt% of hollow carbon nanoplates powder as active material was mixed with 10 wt% of acetylene black as conductive additive and 10 wt% of PIL or polyvinylidene fluoride (PVDF) as binder in 1-methyl-2-pyrrolidinone (NMP) to form a slurry. Afterwards, the homogeneous slurry was coated onto stainless steel foil (1.44 cm2) and dried at 60 °C in a vacuum oven for 24h to remove the solvent. The mass loading was around 1 mg/cm2. The capacitor was realized by sandwiching a porous glass microfiber membrane GF/A between two electrodes, using 1 M Li2SO4 aqueous solution as electrolytes. The electrochemical measurements were performed using a Biologic MPG-2 potentiostat/galvanostat. The specific capacitance (Csp) is determined from the slope of the charge-discharge curves using the equations:34

=

∆⁄∆

· =

4

where ∆/∆ is the slope obtained from the discharge curve in seconds (s), is the applied current in ampere (A), is the weight of two electrodes in gram (g), and is the capacitance calculated in Farad (F). In addition, the specific energy density as well as the power density can be obtained according to the equations:22


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ℎ · =

1 1 2 3.6

W · =

where is the cell voltage after ohmic drop in Volt (V), and is the time for discharge in hour (h).

Results and discussion The synthetic strategy is illustrated in Figure 1. Gibbsite nanoplates possessing an intrinsically hexagonal shape (Figure S1) can be produced in a large scale according to literature, thus serving as the anisotropic nanotemplate. Silica coated gibbsite particles are then synthesized using a modified Stöber method.28 As shown in Figure 2b, the silica shell of around 8 nm in thickness can be seen clearly. After acid leaching using a concentrated HCl solution, hollow silica nanoplates (HSP) are easily recognizable in the TEM image in Figure 2d by a light homogeneous core with a dark edge, indicating that gibbsite cores have been effectively removed without destroying the integrity of the particles. The absence of Al signal from EDX results in Figure S2 further proves the complete removal of gibbsite cores. It must be mentioned that the silica layer is necessarily required to offer colloidal stability of nanoplates during the growth of polydopamine (PDA) layer, discussed later.


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Figure 2. (a, b) TEM images of the silica coated gibbsite nanoparticles, average size 215 ± 27 nm. The blue arrows point to the silica layer (~8 nm thick). (c, d) TEM images of hollow silica nanoplates after acid leaching, average size 214 ± 24 nm. (e, f) TEM images of the HSP@PDA core-shell nanoparticles, average size 232 ± 27 nm. The size distribution curves of these particles have been presented in Figure S3 in the supporting information. After etching off the flat gibbsite core, HSP@PDA core-shell nanoparticles are then prepared by the self-polymerization of dopamine onto the hollow silica nanoplates surface with controllable PDA thickness according to the method of our previous work.26 As shown in Figure 2e, no aggregates can be observed for the HSP@PDA core-shell nanoparticles because of constant ultrasonification applied during the polymerization, which prevents the aggregation of hollow silica nanoplates. The hollow structure appears as a homogeneous contrast (Figure 2f) due to


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PDA coating. The successful PDA coating has been further confirmed by zeta-potential measurements. As shown in Figure S4, these HSP@PDA nanoparticles exhibit zwitterionic properties. When the pH is higher than 7 or lower than 3, these particles are stable in the aqueous solution. As a next step, in order to obtain discrete and dispersible hollow carbon nanoplates, a dense silica matrix is formed to confine individual HSP@PDA. Considering that HSP@PDA nanoparticles are positively charged and stable due to electrostatic repulsion at pH 2, silica coating is realized by dropwise addition of the silica precursor TEOS into the aqueous dispersion of the HSP@PDA nanoparticles at pH 2 under vigorous stirring. The mixture slowly solidifies and forms a viscous gel-like network. By using this silica-nanocasting technique, individual nanoparticles are initially covered with a thin silica shell on the surface, which are eventually immobilized in a compact silica gel. TEM measurements confirm that HSP@PDA nanoparticles are well embedded in the silica gel and kept isolated. The dashed white lines in Figure 3a indicate the isolated HSP@PDA nanoparticles in the silica matrix. A higher magnification in Figure 3b shows that the trapped core-shell nanostructure can be seen clearly due to the electron contrast difference between silica and PDA. Hollow core, silica shell and PDA outer layer can be observed and are indicated with arrows.


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Figure 3. TEM images of (a) HSP@PDA nanoparticles/silica hybrids before carbonization. The dashed white lines indicate the isolated HSP@PDA nanoparticles in silica gel. (b) HSP@PDA nanoparticles/silica hybrids with high magnification. The black arrow points to the hollow core. The red one points to the SiO2 shell. And the blue one points to the PDA layer. Subsequently, HSP@PDA nanoparticles trapped in the silica gel are converted into isolated carbon by pyrolysis at 800 °C under Ar2 flow. Discrete and monodispersed hollow carbon nanoplates are obtained after elimination of the silica portion in the particle core as well as outside of the shell. The TEM images in Figure 4a and b demonstrate the structural integrity of the hollow carbon nanoplates. The obtained particles have a hollow core of ~25 nm in height with a carbon shell of ~9 nm in thickness, and no other amorphous clustered materials can be observed. To further investigate the details of the local structure order, hollow carbon nanoplates are characterized by HR-TEM. As shown in Figure 4c and d, onion-like graphitic phase can be observed clearly. The d spacing is around 0.35 nm (see panels e and f in Figure 4), typical for graphitic layers, which agrees well with previous report.35 The formation of graphitic


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nanostructure may result from a layered-stacking supramolecular structure of PDA, which was converted to graphitic phase at 800 °C.36

Figure 4. (a) TEM image of the hollow carbon nanoplates, average size 231 ± 20 nm. Inset: a standing hollow carbon nanoplate, which indicates clearly the hollow structure. (b) TEM image of one single hollow carbon nanoplate. The blue arrows point to the carbon layer (~9 nm thick). (c, d) HR-TEM images of the enlarged graphite-like domain in the hollow carbon nanoplates, which contains over 10 stacking layers. (e, f) HR-TEM images of the parts of the hollow carbon nanoplate indicated in (c) and (d). The inlayer distance (i.e., the d spacing) is 0.35 nm. The as-synthesized hollow carbon nanoplates, when etching off the silica matrix, are welldispersed in water without precipitation for days. Since no soft surfactant has been used to


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stabilize the hollow carbon nanoplates, the superior dispersibility mainly results from the strong electrostatic repulsion between charged hollow carbon nanoplates.25 This is proven by the zeta potential value of the hollow carbon nanoplates (–25 mV) under neutral pH conditions. However, hollow carbon nanoplates precipitate within 1 day in acidic or basic conditions. Thus, the stability of the hollow carbon nanoplates in aqueous media cannot be caused by the ion adsorption (Figure 5a).

Figure 5. (a) Photographs of the dispersed hollow carbon nanoplates aqueous dispersion at different pH conditions. (b) Nitrogen adsorption/desorption isotherms and (c) EDX pattern of the hollow carbon nanoplates. (d) HAXPES spectrum of the N 1s core level of the hollow carbon nanoplates. (e) XRD pattern of the hollow carbon nanoplates. A Brunauer-Emmett-Teller specific surface area of 460 m2·g−1 has been determined for the hollow carbon nanoplates from nitrogen sorption isotherm in Figure 5b with a pore volume of 0.45 cm3·g−1. From the BJH pore size distribution curve shown in Figure S5 in the supporting


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information, a peak with pore size of ~3.8 nm can be found. The EDX spectrum (Figure 5c) shows that C, O and N are the major elements in the hollow carbon nanoplates, indicating the complete removal of the inorganic parts (i.e., Si and Al), which is further demonstrated by TGA analysis in Figure S6. The corresponding content of each element can be obtained by combustion elemental analysis. As listed in Table S1, the N/C ratio is 0.09, and the nitrogen content is as high as 7.2 wt%, which is quite beneficial to improve the polarity and wettability of the carbon surface as well as to enhance the electric conductivity.37,38 To clarify the structure of N atoms in the hollow carbon nanoplates, HAXPES characterization was further performed. The highresolution N 1s spectrum of the hollow carbon nanoplates, shown in Figure 5d, displays two prominent peaks at binding energy values i) 398.18 and ii) 400.75 eV, attributed to pyridinic-N and graphitic-N, respectively.39 The graphitic-N peak is more prominent, which indicates that the incorporation of nitrogen into the graphitic carbon is dominant, which is similar to the report by Soll et al. for the carbon nanobubble system.25 XRD and Raman spectroscopy were further applied to investigate the graphitic structure of the hollow carbon nanoplates. The XRD pattern of the hollow carbon nanoplates is shown in Figure 5e. Two diffraction reflections at 2θ = 25° and 43° are observed. The reflection at 2θ = 25° is assigned to the (002) of the graphitic carbon. The broad (10) reflection at 2θ = 43° originates from the in-plane reflection of the graphitic carbon.40 From the Raman spectrum shown in Figure S7 in the supporting information, two bands at ~1345 and ~1575 cm−1 can be observed clearly, which correspond to the D band and G band, respectively. It is known that the D band is recognized to be a disordered band, and the G band corresponds to the in plane stretching motion between sp2 carbon atoms.41 The D/G intensity ratio is ~1.5, which indicates that the hollow carbon nanoplates are partially


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graphitized. This means they contain some graphitic domains in the amorphous phase. This agrees well with the results from the HR-TEM measurements. Considering a combination of the unique plate-like hollow structure, the nano-sized dimension as well as high nitrogen doping, the as-synthesized hollow carbon nanoplates are expected to be a potential electrode material for electrochemical devices. In our study, the as-synthesized hollow carbon nanoplates have been applied for symmetric carbon/carbon supercapacitors as electrode materials. Moreover, a recent study shows that the binder materials might influence the performance of supercapacitors.42 As one of the most commonly used binders in supercapacitors, PVDF is the benchmark product. But some drawbacks of PVDF as binders have been already suggested, for instance less of sufficient surface activity for binding all the electrode components eventually, and reduction of pore volume of the active materials.42–44 Recently, poly(ionic liquid) (PIL) based polymers as a new generation of binder materials have been employed in lithium ion batteries, which showed better performance than the commercial standard PVDF.29,45 In the present study, PIL nanoparticle binder is used in parallel to PVDF as binder to reach the optimal performance.


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Figure 6. (a) CV curves for the hollow carbon nanoplates based symmetric supercapacitor (50 mV/s, 1 mol/L Li2SO4, PIL binder), (b) CV curves of the hollow carbon nanoplates at different scan rates ranging from 10 to 300 mV/s by using PIL as binder, (c) galvanostatic charge/discharge curves of the hollow carbon nanoplates under different current densities in the range of 0.25 to 10 A/g by using PIL as binder, and (d) the specific capacitance versus current densities of the hollow carbon nanoplates by using PIL and PVDF as binder, respectively. In order to establish the maximum stable potential for this system, CV was run under constant scan rate of 50 mV/s in different potential windows. From Figure 6a, it can be found that the maximum potential for this system is 1.6 V, at which there is a noticeable positive current tail. This current tail can be attributed to anodic oxidation of water46 and therefore the potential window between 0.0 V and 1.4 V is used. It is observed in Figure


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6b that the CV curves take on a square shape which is typical of double-layer capacitive behaviour. Increasing the scan rate from 10 to 300 mV/s, we can see that there is tiny deviation in the shape of the CV curves. The rectangular shape of the cyclic voltammogram indicates low internal resistance of the supercapacitor and high power.47,48 At scan rates higher than 20 mV/s, commonly used materials for supercapacitor electrodes, such as activated carbon, become less capacitive and more resistive49. Given the individual onion-like graphitic structure of the hollow carbon nanoplates, the electrodes facilitate ion accessibility via the external surface area ensuring the higher rate handling.22 The specific capacitance as well as the power density and energy density were determined from the galvanostatic charge-discharge experiments (Figure 6c). These curves show a linear dependence between voltage and time, which is characteristic for electrical double layer capacitor50. The specific capacitance values obtained at 0.5, 1, 2.5, 4 and 5 A/g are 88, 84, 76, 70 and 67 F/g, respectively. Figure 6d shows the specific capacitance of the hollow carbon nanoplates at different current densities, which was compared to the previous works

published

on

symmetric

carbon/carbon

supercapacitors

with

neutral

electrolytes46,50,51,54,56. It should be noted that hollow carbon nanoplates have an impressive rate handling, which is comparable to that obtained by onion-like carbons.52,53 A specific capacitance of 56 F/g is retained at high current density (10 A/g) as shown in Figure 6d, demonstrating the excellent high power capability. For comparison, electrode was also prepared using the commercially available PVDF as binder and tested (Figure 6d). As observed, the specific capacitance of the hollow carbon nanoplates with PVDF as binder is only 103 F/g compared to 124 F/g at 0.25 A/g for hollow carbon nanoplates with PIL binder, suggesting that PIL is a better binder for supercapacitors. This observation is


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related to a recent finding that the PIL network is expected to form a homogeneous, comparably better conductive matrix than PVDF binder, enhancing the charge flow and transfer29. Table 1. Summary of the present symmetric carbon/carbon supercapacitor results and the reported data on the carbon-based symmetric supercapacitors.*

Sample Activated carbon xerogel Commercial carbon Seaweed carbon 2D porous carbon nanosheets Ball-milled graphite PE-RGO HCPs (PIL)

SBET Voltage Electrolyte 2 (m /g) (V)

Current density (A/g)

Capacitance (F/g)

Ref.

2876

Na2SO4

1.0

0.2

140

54

2250

Na2SO4

1.6

0.2

135

51

1082

Na2SO4

1.6

0.2

123

46

719

KOH

1.0

0.2

110

55

422

Na2SO4

1.4

0.2

79

56

395

Na2SO4

1.6

0.2

~82

50

460

Li2SO4

1.4

0.25

124

This work

HCPs (PVDF) 103 * The data in all cases for comparison is extracted from a symmetric cell with the two electrodes of the carbon materials using aqueous electrolytes. Table 1 lists the capacitances of other reported carbon materials as symmetric supercapacitors electrodes in aqueous electrolytes. In general, hollow carbon nanoplates show high capacitance, which is comparable or superior to carbon materials with very high surface areas, such as commercial carbon,51 activated carbon xerogel,54 seaweed carbon,46 and 2D porous carbon nanosheets55. Comparing with carbon materials containing similar surface areas, for example ball-milled graphite,56 partially exfoliated and reduced graphene oxide (PE-RGO),50 the present


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hollow carbon nanoplates show much better rate handling. The excellent performance of the hollow carbon nanoplates in the symmetric supercapacitors can be ascribed to the following features. (a) The 2D and onion-like graphitic structure of nano-sized hollow carbon nanoplates allows not only the electrons and ions diffuse fast, but also the soaking of the electrolyte with a high speed.22 (b) The hollow interiors could act as ion-buffering reservoirs that the volume expansion caused from the charge/discharge cycles can be diminished efficiently, resulting in an enhanced stability.16 (c) The nitrogen doping increases electrical conductivity with improved wettability of the hollow carbon nanoplates-based electrode in the electrolyte, which will then enhance the efficiency of ion transfer.57,58 In addition, the specific energy density versus specific power density of the hollow carbon nanoplates based symmetric cell was plotted in a Ragone plot (Figure S8). The maximum energy and power densities of this device are 8.4 Wh·kg-1 and 7000 W·kg-1, respectively. The cyclability of hollow carbon nanoplates was measured by the galvanostatic charge-discharge under constant current density (2.5 A/g). The hollow carbon nanoplates maintain about 95% of its initial specific capacitance after 3000 cycles (see in Figure 7), indicating good electrochemical stability. Even after 10,000 cycles, the specific capacitance of the hollow carbon nanoplates based supercapacitor still retains about 90% of the initial value (Figure S9).


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120

Capacitance retention (%)

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100 80 60 40 20 0 0

500

1000

1500

2000

2500

3000

Cycle number

Figure 7. The cyclic performance of hollow carbon nanoplates based symmetric supercapacitor by using PIL as binder under current density of 2.5 A/g in the voltage ranging from 0 to 1.4 V.

Conclusions A facile approach has been successfully developed to synthesize highly dispersible, monodispersed and mesoporous nitrogen-doped hollow carbon nanoplates with uniform hexagonal nanomorphologies by using hexagonal gibbsite nanoplates as template and dopamine as a carbon precursor via a silica nanocasting technique. The as-synthesized structurally anisotropic hollow carbon nanoplates contain a moderate specific surface area (460 m2·g−1) but highly accessible mesopores of uniform size (~3.8 nm). The obtained hollow carbon nanoplates have been successfully applied as electrode materials for symmetric supercapacitors using poly(ionic liquid) nanoparticles as binder, offering high capacitance and excellent electrochemical stability. The present study lights on a design route of a new carbon 2D


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nanomaterial suitable for various potential applications, beside electrochemical energy storage, from biomedicines and drug delivery to catalyst supports. ASSOCIATED CONTENT Supporting Information. TEM image of the colloidal gibbsite platelets and its XRD pattern, EDX patterns of GS and HSP, Size distribution curves of the as-obtained nanoparticles, Zeta potential of the obtained HSP and HSP@PDA particles in aqueous solution at different pH values, BJH pore-size distribution curve of the HCPs, TGA curve of the HCPs in air, Raman spectrum of the HCPs, Elemental composition information of the HCPs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT J. C. gratefully acknowledges financial support of CSC scholarship. R. F., R. G. W., and M. B. acknowledges support by the Impuls- und Vernetzungsfonds of the Helmholtz Association (VHNG-423).


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Synthesis of Dispersible Mesoporous Nitrogen-Doped Hollow Carbon

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