Thin and Small N‑Doped Carbon Boxes Obtained from MicroporousOrganic Networks and Their Excellent Energy Storage Performanceat High Current Densities in Coin Cell SupercapacitorsJunpyo Lee,†,‡ Jaewon Choi,†,‡ Daye Kang,† Yoon Myung,∥ Sang Moon Lee,§ Hae Jin Kim,§

Yoon-Joo Ko,∞ Sung-Kon Kim,*,ϕ and Seung Uk Son*,†

†Department of Chemistry, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Korea∥Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul05006, Korea§Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Korea∞Laboratory of Nuclear Magnetic Resonance, National Center for Inter-University Research Facilities (NCIRF), Seoul NationalUniversity, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, KoreaϕSchool of Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 54896, Korea

*S Supporting Information

ABSTRACT: This work reports the thinnest and smallest hollow N-dopedcarbon boxes among recently reported hollow N-doped carbon materials. Hollowand N-rich microporous organic networks (H-NMONs) were prepared by theazide−alkyne Huisgen cycloaddition of tetra(4-ethynylphenyl)methane and 1,4-diazidobenzene on the surface of Cu2O nanocubes and the successive acidetching of inner Cu2O. The Cu2O nanocubes played roles of templates andnetworking catalysts. The networking reaction generated N-rich triazole rings inthe MON. Heat treatment of H-NMONs under argon resulted in the formationof hollow N-doped carbon boxes (H-NCBs). The diameter and shell thickness ofH-NCBs were 130 and 12 nm, respectively. The H-NCBs showed superiorelectrochemical performance in H2SO4 electrolyte as energy storage materials forsupercapacitors, compared with that in KOH electrolyte. Among the H-NCBs,H-NCB-900 which was obtained by the heat treatment of H-NMON at 900 °Cshowed the best performance with capacitances of 286 and 251 F/g at currentdensities of 1 and 10 A/g in two electrode coin cell type supercapacitors and maintained the capacitances of 228 and 206 F/g athigher current densities of 50 and 100 A/g. Moreover, the H-NCB-900 showed excellent cycling stabilities with ∼95% retentionof the first capacitance after 10 000 cycles. The excellent electrochemical performance of H-NCBs can be attributed to theirefficient N-doping, hollow structure, and thin thickness of shells.

KEYWORDS: Microporous organic network, Polymer, Hollow material, N-Doped carbon, Supercapacitor

■ INTRODUCTION

Due to their high power densities and fast charge/dischargeprocesses, various supercapacitors have been engineered usingcarbon-based energy storage materials.1 In continuing efforts toimprove storage performance, the morphology engineering ofcarbon materials has attracted the attention of scientists.2 Forexample, hollow morphologies of carbon materials can bebeneficial because thin shells induce facile mass diffusion ofelectrolytes into materials, maximizing the utilization of storagematerials.3 Moreover, the doping of heteroatoms to carbonmaterials is also an attractive strategy to improve the storageperformance because heteroatoms such as nitrogen can notonly tune the electronic properties of carbon materials but alsoact as redox active sites to result in pseudocapacitiveperformance.4,5 Thus, hollow N-doped carbon materials areattractive materials and, thus, have been engineered.6−18

However, further synthetic exploration for hollow N-dopedcarbon materials is required not only for methodologicaldiversity19,20 but also for more delicate control of materialparameters such as thin shell fabrication.Recently, there have been efforts to find new carbon

precursors and new synthetic chemistry for the developmentof energy storage materials.21−31 For a recent example, Cooperand co-workers reported the application of conjugatedmicroporous polymers (CMPs) as precursors of carbonmaterials for supercapacitors.32 These CMPs were preparedby the Sonogashira coupling of aryl alkynes and arylhalides.33−36 Because chemical components of CMPs can be

Received: October 22, 2017Revised: January 18, 2018Published: January 27, 2018

Research Article

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© 2018 American Chemical Society 3525 DOI: 10.1021/acssuschemeng.7b03836ACS Sustainable Chem. Eng. 2018, 6, 3525−3532

easily controlled by changing the building blocks, nitrogencould be introduced into CMPs via a predesigned buildingblock approach. Moreover, post-treatment of CMPs withammonia gas can introduce N contents into materials. Thepyrolysis of N-containing CMPs resulted in N-doped carbonmaterials that showed promising performance as energy storagematerials for supercapacitors.32

Our research group has shown that microporous organicnetworks (MONs) can be engineered by template meth-ods.37,38 The functions of MONs can be improved throughengineering of shapes.39 For example, hollow MON catalystsshowed the enhanced activities due to the facile diffusion ofsubstrates into MON networks. The same morphologicalbenefits can be applied to energy storage materials insupercapacitors.40

The MONs can be prepared by various coupling reaction ofbuilding blocks.41−46 Nitrogen can be incorporated into MONsbased on the azide−alkyne Huisgen cycloaddition reaction(click reaction) of organic building blocks.47−51 Because theconnection of aryl azides and aryl alkynes results in triazolerings with three Ns in each ring, the resultant MON materialsare N-rich. Moreover, hollow N-rich MON materials can beprepared using Cu2O nanomaterials as networking catalysts andtemplates.52 The pyrolysis of hollow N-rich MON materialsmay result in N-doped carbon materials.In this work, we report the synthesis of very thin and small

hollow N-doped carbon nanoboxes (H-NCBs) by pyrolysis ofhollow MON boxes rich in triazole rings (H-NMONs) andtheir enhanced electrochemical performances as energy storagematerials for supercapacitors.

■ EXPERIMENTAL SECTIONGeneral Information. Scanning electron microscopy (SEM) was

conducted using a JSM6700F instrument. Powdery samples wereloaded on a carbon tape for SEM analysis. Transmission electronmicroscopy (TEM) was conducted using a JEOL 2100F. For TEManalysis, the powdery samples were dispersed in methanol and drop-casted on a carbon film on a copper grid. Powder X-ray diffraction(PXRD) studies were conducted using a Rigaku MAX-2200 (Cu−Kαradiation) instrument. N2 adsorption−desorption isotherm curveswere obtained at 77K using a BELSORP II-mini. The isotherm curveswere analyzed based on the Brunauer−Emmett−Teller (BET) theory.The pore size distribution was analyzed based on the densityfunctional theory (DFT). Thermogravimetric analysis (TGA) wasconducted using a Seiko Exstar 7300. Solid state 13C nuclear magneticresonance (NMR) spectroscopy was conducted with a mode of cross-polarization/total sideband suppression (CP-TOSS) using a 500 MHzBruker ADVANCE II NMR spectrometer. The spin rate was 5 kHz.Infrared absorption (IR) spectroscopy was conducted using a BrukerVERTEX 70 FT-IR instrument. Raman spectroscopy was conductedusing a Renishaw inVia Raman Instrument. The peaks were analyzedby Gaussian function using a PeakFIT 4.12 program. X-rayphotoelectron (XPS) spectroscopy was conducted using a ThermoVG and Al−Kα radiation. Elemental analysis was conducted using aCE EA110 analyzer. Electrochemical studies were conducted using aWonAtech ZIVE SP1 Electrochemical Workstation.Synthetic Procedures for H-NMON. To be used as templates

and networking catalysts, Cu2O nanocubes were prepared by thesynthetic procedures reported in the literature.53 In our work, thefollowing procedures were applied. Distilled water (800 mL) wasadded to a 1 L round bottomed flask. Aqueous solution of trisodiumcitrate (0.90 M, 4.0 mL, 3.6 mmol trisodium citrate) was added to theflask. After stirring for 20 min, aqueous solution of CuSO4 (1.20 M,4.0 mL, 4.8 mmol CuSO4) was added to the solution in the flask. Afterstirring for 5 min, aqueous solution of NaOH (4.80 M, 4.0 mL, 19mmol NaOH) was added to the solution in the flask. After stirring for

5 min, aqueous solution of ascorbic acid (1.20 M, 4.0 mL, 4.8 mmol,ascorbic acid) was added to the solution in the flask. After stirring for30 min, Cu2O powder was separated by centrifugation, washed threetimes with methanol (40 mL each), and dried at room temperatureunder vacuum.

For the synthesis of Cu2O@NMON, Cu2O nanocubes (0.20 g)were dispersed in a mixture of dimethyl sulfoxide (DMSO, 31 mL)and H2O (4 mL) in a 50 mL Schlenk flask. The mixture was sonicatedfor 1 h. Tetra(4-ethynylphenyl)methane54 (20 mg, 48 μmol) and 1,4-diazidobenzene55 (15.5 mg, 96 μmol) were dissolved in DMSO (5mL). This solution was added to the mixture containing Cu2Onanocubes and heated at 80 °C for 20 h. After cooling the mixture toroom temperature, the powder was separated by centrifugation,washed three times with acetone (40 mL each), three times with a 1:1mixture of methylene chloride and hexane (40 mL each), and dried atroom temperature under vacuum. The obtained Cu2O@NMON wasadded to HCl aqueous solution (2 M, 40 mL) in an 80 mL Falcontube. After stirring for 30 min, the H-NMON was separated bycentrifugation, washed four times with a 1:1 mixture of methanol andwater (40 mL each), and transferred to a 20 mL vial. The H-NMONwas further washed two times with a 1:1 mixture of methanol andmethylene chloride (10 mL each) and dried at room temperatureunder vacuum.

Synthetic Procedures for H-NCBs. H-NMON (45 mg) wasloaded on a crucible and added to a furnace filled with argon. Afterargon flowed for 30 min, the temperature in the furnace was graduallyincreased to 900 °C (temperature increase rate 5 °C/min). Then, thesample was treated at 900 °C for an additional 3 h and cooled to roomtemperature. The resultant product was denoted at H-NCB-900. Forthe synthesis of H-NCB-600, H-NCB-700, and H-NCB-800, thetreating temperatures were adjusted to 600, 700, and 800 °C,respectively. Other procedures were the same as those applied for H-NCB-900.

Procedures for Electrochemical Studies. For the fabrication ofworking electrodes, H-NCB (20 mg), Super P carbon black (2.5 mg),and polyvinylidene fluoride (PVDF 2.5 mg, 19 mg of 13% PVDF inNMP) were dispersed in N-methylpyrrolidinone (NMP, 25 mg) bygrinding using a mortar and pestle. Using a spatula, the slurry wascoated on the surface of a Ti foil (0.77 cm × 0.64 cm, 0.127 mm thick,annealed, Alfa Aesar Co.), dried at 80 °C in an oven for 3 h, and thendried overnight at 100 °C in a vacuum oven. The loading amount ofH-NCB was calculated as 0.97 mg/cm2. The thicknesses of electrodematerials were measured as 58.1, 57.5, 57.4, and 59.7 μm for H-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900, respectively, using athickness gauge (Mitutoyo Co.).

Two working electrodes were prepared for the fabrication of coin-cell type supercapacitors. One working electrode was loaded on onecap of CR2032 cell. An electrolyte solution (1 M H2SO4 or 6 M KOHaqueous solution, 80 mg) was loaded on the working electrode. Aseparator (No. 20 filter paper, Hyundai Micro Co., HD20 MN020 5−8 μm) was loaded on the working electrode. The second workingelectrode was loaded on the separator. The active material on thesecond working electrode was located in the direction to the separator.Electrolyte solution (1 M H2SO4 or 6 M KOH aqueous solution, 80mg) was loaded on the working electrode. A space disc, a spring, andthe other cap of CR2032 cell were loaded on the second workingelectrode. Using a crimper, coin-cells were assembled. Electrochemicaltests were conducted after the cells standing for 24 h. Cell voltageswere scanned in a range of 0−1 V. Cell capacitance (Ccell) wascalculated by the following eq 1:

= Δ ΔC V t mI/[( / ) ]cell (1)

,where I is the applied current (A), ΔV/Δt is the slope of dischargecurves after an IR drop at the beginning of the discharge curve, and mis the total mass of electrode materials (g). The specific capacitance ofthe single electrode was calculated by eq 256

=C C4s cell (2)

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■ RESULTS AND DISCUSSIONA synthetic scheme for H-NCBs is shown in Figure 1. Thecopper-catalyzed click reaction is a facile coupling reaction of

aryl azide and aryl alkyne.57−59 MON materials have beenprepared by this reaction.49−51 Moreover, using silica templates,hollow MON spheres were engineered.60 Various copperspecies are known to catalyze this reaction. Among Cu(+1)species, Cu2O powder has been used to catalyze thisreaction.61−64 In addition, Cu2O nanomaterials have beenengineered to show enhanced catalytic activities.53,61−64 Thus,the Cu2O nanomaterials can be used not only as catalysts forthe synthesis of MONs based on the click reaction but also astemplates.52

For using nanotemplates, Cu2O nanocubes were prepared bythe synthetic procedures reported in the literature.53 MONlayers were formed on the surface of Cu2O nanocubes by theclick reaction of 1 equiv tetra(4-ethynylphenyl)methane54 with2 equiv 1,4-diazidobenzene. Because the surface of Cu2Onanocubes is catalytically active for the click reaction of buildingblocks, the MON layers were formed exclusively on the surfaceof Cu2O nanocubes. The inner Cu2O could be etched by thetreatment of hydrochloric acid to form hollow MON boxesbearing triazole rings (H-NMONs). The heat treatment of H-NMONs at 600, 700, 800, and 900 °C resulted in the H-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900, respectively.The Cu2O nanotemplates and the synthesized intermediate/

final materials were investigated by scanning (SEM) andtransmission electron microscopies (TEM). As shown in Figure2a, the Cu2O nanomaterials had a cube shape with a diameterof 105 nm. The SEM image of Cu2O@NMON showed a turbidcontrast due to the formation of organic layers on the Cu2Omaterials, compared with that of Cu2O nanocubes (Figure 2b).The magnified SEM images of Cu2O@NMON showeduniform coating of MON layers on the Cu2O nanocubes(Figure S1 in the Supporting Information (SI)). Thethicknesses of the MON layers were measured in the range

of 12−17 nm. The SEM and TEM images of H-NMONsshowed the hollow nature of materials with a shell thickness of∼15 nm and a diameter of ∼135 nm, matching well with theoriginal thickness of NMON in Cu2O@NMON (Figure S2 inthe SI). The carbon materials of H-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900 showed the retention of theoriginal box shape and shell thicknesses of H-NMON materials(Figures 2c−h and S3 in the SI). The shell thicknesses anddiameters of H-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900 were nearly same and were observed as ∼12 and∼130 nm, respectively.The physical and chemical properties of materials were

investigated by various analysis methods. According to analysisof N2 adsorption−desorption isotherm curves based on theBrunauer−Emmett−Teller (BET) theory, the Cu2O nanocubesshowed a surface area of 16 m2/g. The Cu2O@NMON showedan increase of a surface area to 132 m2/g through the formationof MON layers with high surface areas, compared to thenonporous Cu2O nanocubes (Figure 3a). The H-NMONmaterials showed a surface area of 886 m2/g, supporting thehigh porosity of materials. The analysis of pore size distributionof H-NMON based on the density functional theory (DFT)showed microporosity (main pore sizes <2 nm) with amicropore volume (Vmic) of 0.25 cm3/g. The surface areas ofH-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900 wereobserved as 643 (Vmic 0.23 cm

3/g), 496 (Vmic 0.16 cm3/g), 455

(Vmic 0.15 cm3/g), and 443 m2/g (Vmic 0.14 cm3/g),respectively, indicating the decomposition of MON materialsand the generation of new micropores in carbon materials(Figure 3b).The powder X-ray diffraction (PXRD) studies on the Cu2O

and Cu2O@MON showed (110), (111), (200), (220), (310),and (222) diffraction peaks at 29.7, 36.6, 42.5, 61.5, 73.7, and

Figure 1. Synthetic scheme for hollow N-rich microporous organicnetworks (H-NMONs) and hollow N-doped carbon nanoboxes (H-NCBs).

Figure 2. SEM images of (a) Cu2O nanocubes, (b) Cu2O@NMON,and (g) H-NCB-900. TEM images of (c) H-NMON, (d) H-NCB-600,(e) H-NCB-700, (f) H-NCB-800, and (h) H-NCB-900.

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77.5° (2θ value), respectively, indicating that crystallinematerials are cubic phase Cu2O (JCPDS no. 78-2073) (Figure3c). The H-NMON showed amorphous characteristic, which isa conventional property of MON materials prepared by theclick coupling reaction of organic building blocks.49−51 All H-NCBs also showed conventional amorphous characteristic(Figure 3d).Solid state 13C nuclear magnetic resonance (NMR) spec-

troscopy of H-NMON showed the 13C peak of quaternaryaromatic carbon of triazole rings and benzyl carbon originatingfrom the tetrahedral building blocks were observed at 147 and64 ppm, respectively (Figure 3e). The 13C peaks from otheraromatic rings were observed at 121, 126, and 135 ppm. The13C spectrum of H-NMON matches well with the expectedchemical structure and those of the MONs prepared by theclick reaction in the literature.49−51 Infrared (IR) spectroscopyon the H-NMON showed the NN vibration peak oftriazole rings at 1607 cm−1, matching with that of MONsprepared by the click reaction in the literature (Figure S4 in theSI).49−51,65 Elemental analysis of H-NMON by combustionshowed N contents of 18.20 wt %. The N contents in H-NCB-600, H-NCB-700, H-NCB-800, and H-NCB-900 were observed

as 5.51, 3.77, 3.23, and 2.89 wt %, respectively. Thermogravi-metric analysis (TGA) showed that thermal stability of H-NCBincreases with an increase of carbonization temperature (FigureS5 in the SI).The chemical surroundings of nitrogen in N-doped carbon

materials are critical in their storage performance for super-capacitors. Thus, H-NCBs were further investigated by X-rayphotoelectron (XPS) and Raman spectroscopy. In XPS spectra,the N 1s orbital peaks of pyridinic, pyrrolic, graphitic, and oxidenitrogen in H-NCBs appeared at 398.3, 399.7, 400.8, and 402.1eV, respectively (Figure 4a).5 As carbonization temperature

increased from 600 to 700, 800, 900 °C, the graphitic N portionin N contents of H-NCBs increased from 39% to 41%, 47%,and 51%, respectively. In contrast, the pyridinic N portion in Ncontents of H-NCBs gradually decreased from 36% to 30%,25%, and 23%, respectively. Also, the pyrrolic N portions in Ncontents of H-NCBs gradually decreased from 21%, 20%, 18%,and 14%, respectively. The location of N 1s orbital XPS peaksof H-NMON was significantly different from those of H-NCBsand analyzed by a 1:1:1 combination of three peaks observed at399.4, 400.2, and 401.5 eV, matching well with the fact that thetriazole ring contains three chemically different Ns (Figure 4a).The Raman spectra showed that when carbonization temper-ature increased from 600 to 700 °C, the ID/IG ratio of D(disordered carbon) and G (graphitic carbon) band intensitiesat 1349 and 1585 cm−1 increased from 0.67 to 0.81, indicatingthat N-doping induced disorder in carbon materials could besignificantly formed at around 700 °C. The ID/IG ratios weremaintained at 0.83 and 0.84 in H-NCB-800 and H-NCB-900,respectively (Figure 4b). The ID/IG ratios of most N-dopedhollow carbon materials in the literature were reported in therange of 0.92−1.5.6−18 The lower ID/IG ratios of H-NCB-700,H-NCB-800, and H-NCB-900, compared to those of N-dopedhollow carbon materials in the literature,6−18 imply greatercontents of sp2 graphitic carbon and higher conductivities.Thus, the H-NCBs were expected to show good conductivitiesand promising rate performance at high current densities asenergy materials for supercapacitors.Considering the N-doping and the thin shell thicknesses of

H-NCBs, we studied their energy storage performance forsupercapacitors. Figures 5, 6, and S6−8 in the SI summarize theresults.

Figure 3. (a and b) N2 adsorption−desorption isotherm curves at 77K, pore size distribution diagrams based on the DFT method, (c andd) PXRD patterns of Cu2O, Cu2O@NMON, H-NMON, and H-NCBs. (e) Solid state 13C NMR spectrum of H-NMON.

Figure 4. (a) XPS spectra of N 1s orbital in H-NMON and H-NCBs.(b) Raman spectra of H-NCBs.

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We fabricated coin cell type (CR2032) two electrodesupercapacitors using H-NCBs (loading amount ∼1 mg/cm2). Overall, as the carbonization temperature increasedfrom 600 to 700, 800, and 900 °C, the capacitances of H-NCBsgradually increased from 40 to 189, 333, and 365 F/g (currentdensity 0.1 A/g), respectively (Figure 5a, b, and e). Among theH-NCBs, the H-NCB-900 showed the best performance incapacitance. When we increased the loading amount of H-

NCB-900 from 0.97 to 2, 3, and 4 mg/cm2, capacitancesdecreased from 286 to 202, 165, and 125 F/g (current density 1A/g) (Figure S8 in the SI).We found that H-NCBs showed very different performance

depending on the electrolytes: basic 6 M KOH and acidic 1 MH2SO4 (Figure 5). The cyclic voltammograms (CVs) of H-NCB-900 were relatively more rectangular in the KOHelectrolyte, compared with those with additional redox peaksin the H2SO4 electrolyte (Figures 5a and b and S5−6 in theSI).4−18 The charge/discharge profiles of H-NCBs were moresymmetrical in the KOH electrolyte than those in the H2SO4electrolyte, indicating that while the storage mechanisms of H-NCBs in the KOH are closer to conventional electrostaticdouble layer capacitive storage, those in the H2SO4 indicateadditional pseudocapacitive behaviors (Figure 5a−d). Wesuggest that the enhanced performance of H-NCBs in theH2SO4, compared with that in the KOH, is attributable to moreefficient chemical attraction (acid−base reaction) of the acidelectrolyte with basic N moieties in H-NCBs. In addition, thethin shell thickness of H-NCBs can enhance the difference.The H-NCBs showed clearly better rate performance in the

H2SO4 electrolyte than those in the KOH electrolyte (Figure5e). Interestingly, the capacitances of H-NCB-900 at a currentdensity of 0.3 A/g are the same as 294 F/g in the cases of theH2SO4 and the KOH electrolytes, respectively. The capaci-tances of H-NCB-900 in H2SO4 maintained the 286 (97%),251 (85%), 244 (83%), 228 (78%), and 206 F/g (70%) at thecurrent densities of 1, 10, 20, 50, and 100 A/g, respectively,compared with that at current density of 0.3 A/g. In contrast,the capacitances of H-NCB-900 in KOH significantly decreasedto the 270 (92%), 225 (77%), 206 (70%), 159 (54%), and 116F/g (39%) at the current densities of 1, 10, 20, 50, and 100 A/g, respectively, compared to that at current density of 0.3 A/g.These results imply that the H-NCBs have higher conductiv-ities in H2SO4, compared with those in KOH.The Nyquist plots were obtained through the electro-

chemical impedance spectroscopy (EIS), showing that as thecarbonization temperature increased from 600 to 700, 800, and900 °C, charge transfer resistances (Rct) of H-NCBs in KOHelectrolyte gradually decreased from >100 to 62, 8.7, and 6.0 Ω,respectively (Figure 5f). In comparison, as the carbonizationtemperature increased from 600 to 700, 800, and 900 °C, thecharge transfer resistances of H-NCBs in H2SO4 electrolytegradually decreased from >100 to 4.5, 1.6, and 0.98 Ω,respectively (Figure 5g). The higher conductivities of H-NCBsin H2SO4 electrolyte compared with those in KOH electrolyte

Figure 5. Electrochemical performance of symmetrical coin cell typesupercapacitors. Cyclic voltammograms (scan rate 100 mV/s) of H-NCBs in (a) 6 M KOH and (b) 1 M H2SO4. Charge/dischargeprofiles (current density 1 A/g) of H-NCBs in (c) 6 M KOH and (d)1 M H2SO4. (e) Rate performance of H-NCBs in 6 M KOH (blankcircle) and 1 M H2SO4 (solid circle). Nyquist plots of H-NCBs in (f) 6M KOH and (g) 1 M H2SO4.

Figure 6. Cycling tests of coin-cell type supercapacitors of H-NCB-900 at the current densities of 20, 50, and 80 A/g (electrolyte 1 MH2SO4).

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are attributable to the efficient interaction of acidic electrolytewith basic N-doped carbon materials, possible to formammonium salt moieties in the H-NCBs.Next, cycling tests were conducted to study the electro-

chemical stability of H-NCB-900 as a representative material.As shown in Figure 6, the coin cell type supercapacitors of H-NCB-900 showed good cycling performance at various currentdensities. At the current densities of 20, 50, and 80 A/g, thecells of H-NCB-900 in H2SO4 electrolyte showed capacitancesof 244, 233, and 215 F/g at the first cycle. After 10 000 cycles,the cells maintained 231 (94.7% of the first cycle), 221 (94.8%of the first cycle), and 205 F/g (95.3% of the first cycle), atcurrent densities of 20, 50, and 80 A/g, respectively. Thesupercapacitor cells recovered after cycling tests maintained thehigh conductivity with a charge transfer resistance of 1.6 Ω(Figure S9 in the SI).We surveyed the electrochemical performances of the related

N-doped carbon materials for supercapacitors reported in thetop journals and compared those with our results (Figure 7 and

Table S1 in the SI).6−18,32,66,67 In the case of the synthesis ofnonhollow N-doped carbon materials using microporousorganic polymer materials as precursors, very recently, Cooperand co-workers reported the synthesis of N-doped carbonmaterials by the thermolysis of CMPs which were prepared bythe Sonogashira coupling of organic building blocks.32 Thematerials showed a capacitance of 149 F/g at a current densityof 10 A/g in a three electrode system using 1 M H2SO4 as anelectrolyte. In another example, Zhi and co-workers reportedthe synthesis of N-doped carbon powders by the thermolysis ofN-rich porous organic polymers which were prepared by thecross-linking of terephthalonitrile monomers.66 The resultantN-doped carbon materials showed specific capacitance up to173 F/g (current density 10 A/g) in two electrode cells using 1M H2SO4 as electrolyte. In comparison, H-NCB-900 in thepresent work showed a specific capacitance of 251 F/g (currentdensity 10 A/g) in two electrode systems with 1 M H2SO4electrolyte.In the very recent synthetic case, Lou and co-workers

reported the synthesis of hollow N-doped carbon materials bythe thermolysis of the porous zeolitic imidazolate framework(ZIF-8) and showed a specific capacitance of 252 F/g at thecurrent density of 10 A/g in a two electrode system using 2 MH2SO4 as electrolyte.

67 While this performance is nearly samewith ours (251 F/g at 10 A/g), the materials in the literature

showed capacitances of 235 and 193 F/g at the currentdensities of 20 and 50 A/g, respectively. In comparison, H-NCB-900 in the present work showed capacitances of 244, 228,and 206 F/g at the current densities of 20, 50, and 100 A/g.As far as we are aware, our work may be the first example of

the engineering of hollow N-doped carbon materials using themicroporous polymer materials as precursors. The enhancedelectrochemical performance of H-NCBs is attributable toseveral factors, i.e., the efficient N-doping as a result of buildingblock approach, the good distribution of N moieties, the goodconductivity (Rct up to 0.98 Ω), and the hollow structure.Especially, the thickness (12 nm) and diameter (130 nm) of H-NCB-900 are thinnest and smallest, respectively, among therecent hollow N-doped carbon materials6−18,32,66,67 (excepting2D materials14) (Figure 7 and Table S1 in the SI), which arebeneficial in the efficient contact of materials with an acidicelectrolyte and in the ultimate utilization of electrochemicallyactive sites.3

■ CONCLUSION

This work shows that the Cu2O template synthesis of hollowN-rich MONs can be applied for the engineering of energystorage materials for supercapacitors. Azide−alkyne Huisgencycloaddition of building blocks resulted in N-rich triazolemoieties on the surface of templates. The homogeneousdistribution of triazole rings over MONs can be applied for thesynthesis of N-doped carbon materials. Moreover, the templatesynthesis resulted in the very thin shell N-doped carbonmaterials with hollow box shapes. The resultant H-NBC-900showed excellent electrochemical performance with capacitan-ces of 286, 251, 228, and 206 F/g at current densities of 1, 10,50, and 100 A/g, respectively, in two electrode coin cellsupercapacitors. H-NBC-900 showed excellent cycling stabil-ities with ∼95% retention of the capacitance of the first cycleafter 10 000 cycles. Interestingly, H-NBCs showed clearlysuperior electrochemical performance in an acidic electrolyte,compared with a basic one. We suggest that these featuresoriginate from the efficient N-doping and the thin shellthickness of H-NCBs. We believe that the chemical/physicalparameters of H-NCBs can be further optimized by screeningof the organic building blocks.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.7b03836.

SEM images of Cu2O@NMON and H-NMON, IRspectrum of H-NMON, cyclic voltammograms of H-NCBs, and comparison of electrochemical performanceof H-NCBs with the recent N-doped hollow carbonmaterials (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail address: sson@skku.edu (S.U.S.).*E-mail address: skkim@jbnu.ac.kr (S.K.K.).

ORCIDYoon Myung: 0000-0002-5774-6183Seung Uk Son: 0000-0002-4779-9302

Figure 7. Comparison of (a) electrochemical performance and (b) sizeparameters of H-NCB-900 and recent N-doped hollow carbonmaterials6−8,32,66,67 used for supercapacitors (refer to Table S1 inthe SI for detailed values).

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Author Contributions‡These authors contributed equally to this work.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by Basic Science Research Program(2016R1E1A1A01941074) through the National ResearchFoundation of Korea (NRF) funded by the Ministry ofScience, ICT and Future Planning, and the grants CAP-15-02-KBSI (R&D Convergence Program) of the National ResearchCouncil of Science & Technology (NST) of Korea.

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