Graphene-directed Formation of Nitrogen-Doped Porous Carbon

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Graphene-directed Formation of Nitrogen-Doped Porous Carbon Sheet with High Catalytic Performance for Oxygen Reduction Reaction Lei Qin, Yifei Yuan, Wei Wei, Wei Lv, Shuzhang Niu, Yan-Bing He, Dengyun Zhai, Feiyu Kang, Jang-Kyo Kim, Quan-Hong Yang, and Jun Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12327 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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The Journal of Physical Chemistry

Graphene-Directed Formation of Nitrogen-Doped Porous Carbon Sheet with High Catalytic Performance for Oxygen Reduction Reaction

Lei Qin,a, c Yifei Yuan,e Wei Wei,b, d Wei Lv,*a Shuzhang Niu,a Yan-Bing He,a Dengyun Zhai,a Feiyu Kang,a Jang-Kyo Kim,c Quan-Hong Yang,b and Jun Lu*e

a

Engineering Laboratory for Functionalized Carbon Materials, Baotou Graphene Innovation

Center (Shenzhen), Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China. b

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR

China. c

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Clear Water Bay, Kowloon, Hong Kong, PR China. d

Division of Fuel Cell & Battery, Dalian National Laboratory for Clean Energy, Dalian Institute

of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China. e

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700S Cass

Avenue, Argonne, Illinois 60439, USA. E-mail: [email protected], [email protected]

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ACS Paragon Plus Environment

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ABSTRACT: Nitrogen (N)-doped porous carbon sheet is prepared by in-situ polymerization of pyrrole on both sides of graphene oxide, following which the polypyrrole layers are then transformed to the Ndoped porous carbon layers during the following carbonization and a sandwich structure is formed. Such a sheet-like structure possesses a high specific surface area and more importantly, guarantees the sufficient utilization of the N-doping active porous sites. The internal graphene layer acts as an excellent electron pathway, and meanwhile, the external thin and porous carbon layer helps to decrease the ion diffusion resistance during electrochemical reactions. As a result, this sandwich structure exhibits prominent catalytic activity toward oxygen reduction reaction in alkaline media, as evidenced by a more positive onset potential, a larger diffusion-limited current, better durability and poison-tolerance than commercial Pt/C. This study shows a novel method of using graphene to template the traditional porous carbon into two-dimensional, thin and porous carbon sheet, which greatly increases the specific surface area and boosts the utilization of inner active sites with suppressed mass diffusion resistance.

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1. INTRODUCTION Oxygen reduction reaction (ORR) is the essential reaction in fuel cells and metal-air batteries.1-3 Due to the originally sluggish kinetics of ORR, the use of catalysts such as noble metals and their alloys are demanded in the pursuit of high performance.4-13 However, the quick degradation of catalytic activity of these noble metals as well as their low poison-tolerance and high cost limit the practical applications. Therefore, developing low-cost catalysts with competitive catalytic activity and long-term stability attracts considerable attention.14-17 Newly developed carbonbased catalysts can deliver high ORR activity by compositing different carbon allotropes together or heteroatom-doping.18-21 Specially, the incorporation of heteroatoms (e.g. N, B, S) into the carbon skeleton significantly improved the electron conductivity and the surface polarity, which greatly enhanced the resulted catalytic performance.14 As a typical case, nitrogen-doped porous carbon showed super ORR activity.22-24 The porous structure and composition of these carbons normally depend on the carbon precursors and carbonization process, which always result in the complicated porous structure.24 In addition, the conventional nitrogen-doping methods utilizing ammonia gas or direct carbonization of the nitrogen-containing precursors usually suffered from the poor electrical conductivity and the unsatisfactory nitrogen doping extent. Both of them lead to the low utilization of active sites, poor catalytic activity, and slow reaction kinetics.25-27 Besides, the dopant types and doping configurations also greatly affect the catalytic performance.22, 23 To improve the catalytic performance, it is highly desirable to obtain carbon materials possessing a porous structure with shortened pore length and increased specific surface area that is electrochemically accessible.28, 29 As a typical two-dimensional (2D) carbon, graphene has been widely investigated in electrochemical applications due to its high specific surface area and excellent conductivity.30 Moreover, graphene and its derivatives (e.g. graphene

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oxide) have been applied as versatile templates to prepare 2D porous carbons, which benefit from the structural advantages of both porous carbon and graphene.31 Such 2D porous carbons have good chemical stability and provide much higher electron conductivity and shortened pore length, as a result of which the lower ion transport/diffusion resistance than conventional porous carbons is achieved.29 Besides, the inner active sites in these carbons are easily accessed and utilized, guaranteeing the great potential of these carbon materials as ORR catalysts.32 Herein, we used graphene oxide (GO) as the template to prepare a 2D nitrogen-doped porous carbon sheet (NPCS) showing high ORR performance owing to the sheet-like structure, which guarantees fast mass transfer and provides sufficient accessible active sites. The NPCS is prepared by homogeneous polymerization and deposition of pyrrole on both sides of GO and the following carbonization reduces GO to graphene sandwiched by porous carbons (derived from the polypyrrole), through which a 2D porous carbon sheet forms. The carbonization of polypyrrole generates abundant nitrogen-doped active sites in the carbon framework (4.25 at% for N), and meanwhile the graphene template suppresses the aggregation of the carbonized products. Therefore, the highly porous structure with a large specific surface area is successfully synthesized. In alkaline media, the NPCS shows remarkable ORR performance as evidenced by a high onset potential (0.989 V vs. reversible hydrogen electrode), a large diffusion-limited current and a comparable electron transfer number (n = 3.92 at 0.67 V) to that of commercial Pt/C catalysts. Moreover, it exhibits much better long-term stability and poisoning tolerance against methanol than that of commercial Pt/C catalysts.

2. METHODS 2.1. Materials. Pt/C catalyst (20 wt% of Pt) was purchased from Johnson Matthey company.

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Nafion solution (5 wt%) from Sigma-Aldrich company was used as received. Pyrrole, potassium hydroxide (KOH) and ferric chloride (FeCl3·6H2O) were purchased from Sinopharm Chemical Reagent company. 2.2. Preparation of NPCS-(T). Graphite oxide was first synthesized by the modified Hummers’ method33 and the graphene oxide (GO) suspension with a concentration of 2 mg mL-1 was obtained by ultrasonication of graphite oxide in aqueous solution for 2 h. 4 mL Pyrrole was then mixed thoroughly with the GO suspension containing 160 mg GO with the aid of high-shear dispersion homogenizer, and after adding ferric chloride aqueous solution containing 9.5 g FeCl3·6H2O in it dropwise, the formed mixture was stirred for another 24 h in ice bath. After that, the formed precipitate was washed several times with deionized water and ethanol and then dried at 70 oC. The product was thermally treated at different temperatures (750, 850 or 950 oC) for 1 h under Ar atmosphere. After washing with 8 M HCl solution, the final product was obtained and denoted as NPCS-T, in which T represents the carbonization temperatures. The intermediate sample before acid washing was denoted as Fe/NPCS. For comparison, nitrogen-doped porous carbon (NPC) was synthesized with the same process but without the addition of GO. In addition, sample GO-T was obtained by annealing treatment of graphite oxide. 2.3. Fabrication of a catalyst-modified glassy-carbon electrode. The glassy carbon electrode (GCE) was polished with 1, 0.2 and 0.05 µm polishing powders and then sonicated in the mixture of water and ethanol. To prepare the working electrode, 1.0 mg catalyst was ultrasonically dispersed in the mixture of isopropanol (500 µL), deionized water (500 µL) and 5 wt% Nafion (20 µL). Then about 32 µL above suspension was pipetted out and transferred onto the GCE with 4 mm in diameter and naturally dried at ambient condition. The typical mass

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loading of the active material was around 250 µg cm-2. 2.4. Material characterization and electrochemical test. X-ray diffraction (XRD) measurements were performed on a Rigaku D/max 2500/PC X-ray (40 kV, 100 mA) using Cu Kα radiation (k = 0.154 nm). The morphology of samples was recorded on scanning (Hitachi S4800, SEM) and transmission (JEOL 2100F, TEM) electron microscopy. X-ray photoelectron spectroscopy (XPS) data were collected on an X-ray photoelectron spectrometer (PHI5802). Nitrogen adsorption experiments were conducted on a BELsorp mini-instrument (BEL). All the samples were degassed in vacuum at 200 oC for 10 h before the adsorption measurements and the specific surface areas were estimated from the adsorption isotherms with the Brunauer-EmmettTeller (BET) method. Thermo-gravimetric (TG) and differential scanning calorimetry (DSC) measurements were carried out on a NETZSCH STA 449 F3 Jupiter analyzer starting from room temperature to 800 oC at a heating rate of 5 oC min-1 under a N2/O2 (v/v = 4:1) atmosphere or from room temperature to 950 oC under a N2 atmosphere. Atomic force microscopy (AFM) was done on Dimension Icon (ScanAsyst, Bruker). Electrochemical measurements were conducted in a standard three-electrode glass cell using a catalyst-modified GCE as the working electrode, Pt wire as a counter electrode and Ag/AgCl (in 3 M KCl) as the reference electrode, respectively. The rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) experiments using a glassy carbon rotating disk or ring-disk electrode of 4 mm in diameter were performed on the Autolab electrochemical workstation. The electrolyte was 0.1 M KOH solution prepared using Milli-Q water (18 MΩ cm-1). All data were obtained under the oxygen-saturated condition to maintain the O2/H2O equilibrium potential at 1.23 V vs. reversible hydrogen electrode (RHE) potential. The polarization curves for ORR were swept from -0.8 to 0.2 V vs. Ag/AgCl with a scan rate of 5 mV s-1 and various rotating speeds

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from 400 to 2025 rpm were implemented in the experiment. For the RRDE measurements, the working electrode was swept from -0.8 to 0.2 V vs. Ag/AgCl at 5 mV s-1 with the rotating speed of 1600 rpm. The ring potential was set at 0.6 V vs. Ag/AgCl. Cyclic voltammetry (CV) experiments were recorded at 50 mV s-1. The current density was normalized to the geometrical area and the measured potential (vs. Ag/AgCl) was converted to the RHE potential according to the corresponding Nernst equation (ERHE = EAg/AgCl + 0.059*PH + 0.205). The number of electrons transferred (n) in the ORR process was calculated based on the Koutecky-Levich (K-L) relationship in Equation 1 which was analyzed at -0.3, -0.35 and -0.4 V vs. Ag/AgCl: 1 1 1 1 1 = + = + (1) / where J is the measured current density, JL and JK are the diffusion-limited current and kinetic current, w represents the electrode rotation angular velocity. Theoretically, the value of the Levich slope (B) that is related to the Koutecky-Levich plots (J-1 vs. w-1/2) can be expressed using the following relationship in Equation 2: = 0.62 (/) ( /) (2) where n is the transferred electron number, F is the Faraday constant (96485 C mol-1), C0 is the bulk concentration (solubility) of O2 in 0.1 M KOH (1.2×10-6 mol cm-3), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm2 s-1) and υ represents the kinematic viscosity of the electrolyte (1.13×10-2 cm2 s-1). The Tafel curve reflects the relationship between the overpotential (η) and the kinetic current JK. Here, η refers to the difference between the standard reversible potential and applied potential in the ORR process and JK at a given potential can be calculated by Equation 3:

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∗ (3)

The percentage of H2O2 and the electron transfer number (n) calculated from RRDE can be determined by the following expressions in Equation 4 and 5: %(! " ) = 200 ∗

=4∗

#$ ⁄% (4) #' + #$ ⁄%

#' (5) #' + #$ ⁄%

where Ir is the ring current, Id is the disk current and N is RRDE current collection efficiency of the Pt ring with a value of 0.37.

3. RESULTS AND DISCUSSION

Scheme 1 Preparation process for nitrogen-doped porous carbon sheet (NPCS). The preparation process of NPCS is illustrated in Scheme 1. The GO dispersion and pyrrole were first uniformly mixed together by stirring. The polymerization of pyrrole was then initiated by adding ferric chloride (FeCl3) into the mixture placed in ice-bath, forming polypyrrole (PPy) that were attached on GO surface. After that, the formed GO-PPy was carbonized at 850 oC in an Ar atmosphere, and the Fe-containing compounds were then removed by acid washing. Finally, the NPCS was obtained, and the reference sample prepared under the same condition but without 8


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the addition of GO was denoted as NPC. The samples without removing the Fe compounds were labeled as Fe/NPCS and Fe/NPC correspondingly. In XRD pattern of Fe/NPCS, the Fecontaining compounds can be defined as Fe3O4 (JCPDS card No. 65-3107), Fe2O3 (JCPDS card No. 33-0664) and Fe3C (JCPDS card No. 65-2412) (Fig. S1a). These Fe-containing compounds can promote the crystallization of amorphous carbon in the annealing treatment due to the catalytic graphitization effect,34 which is confirmed by the lessening peak width at 26o with the increase of carbonization temperature (Fig. S1b).

Fig. 1 (a) SEM image of NPCS-850. (b-c) TEM image and elemental mappings of NPCS-850. (d) XPS spectrum of NPCS-850 recorded from 0 to 1000 eV (inset: the contents of carbon, nitrogen and oxygen according to XPS data). High-resolution (e) N 1s and (f) Fe 2p XPS spectra of NPCS-850. The morphology and composition of the GO-PPy before and after carbonization were characterized. The GO-PPy exhibits a sheet-like structure much thicker than GO, suggesting that PPy is successfully deposited onto the GO sheets due to the strong π-π interaction between them (Fig. S2).35 After thermal treatment at high temperature, PPy was transformed into the porous 9



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carbon while the GO was reduced into graphene, forming NPCS with corrugated platelet morphology (Fig. 1a). The TEM image in Fig. 1b shows the crumpled NPCS, which is similar to that of thermally treated GO (Fig. S3), suggesting that the NPCS also has similar thin sheet-like structure. Elemental mappings of NPCS show that the element of C, N and O are evenly distributed in the sheet-like structure (Fig. 1c). The signals of Fe and Cl are also detected mainly because some initially added FeCl3 is embedded in the carbon framework and hardly to be removed. The X-ray photoelectron spectroscopy (XPS) confirms the existence of a trace amount of Fe species (0.33 at%) (Fig. 1d). Note that the NPCS has a high N content (4.25 at%), and the N 1s spectrum can be deconvoluted into four peaks at 398.5, 399.3, 400.0 and 401.1 eV, which can be assigned to pyridinic N, Fe-N, pyrrolic N and graphitic N, respectively (Fig. 1e).14, 28, 34, 36 The graphitic N has the highest charge mobility that can facilitate O2 adsorption on the adjacent carbon atoms, while the carbon atoms with Lewis basicity next to pyridinic N is regarded as the ORR active sites in the conventional N-doped carbon materials.37-39 Furthermore, the Fe 2p peaks in Fig. 1f can be identified at 711.0 eV (Fe 2p3/2) and 724.9 eV (Fe 2p1/2),36 and it has been reported that pyridinic N may coordinate with Fe to form Fe-N species, serving as potential active sites for ORR.40-45

Fig. 2 (a) AFM image of NPCS-850 with the inset showing the height profile corresponding to its thickness. (b-c) Nitrogen adsorption/desorption isotherm of NPCS-850 and the corresponding pore size distribution. 10


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The atomic force microscopy (AFM) image of NPCS and the corresponding height profile are shown in Fig. 2a. The sheet thickness of ~20 nm was deduced and this could be explained by the addition of porous carbon covering the thin graphene in the middle.32,

33, 35

The adsorption

isotherm of NPCS combines the type-Ⅰ and type-Ⅳ character, suggesting the existence of micropores and mesopores.33 Its specific surface area is 594 m2/g with a pore volume of 0.77 cm3 g-1, and the corresponding pore size distribution further confirms the formation of micropores (12 nm) and small mesopores (3-5 nm) (Fig. 2b-c). Considering the sheet-like structure, the pore length is greatly shortened, which can thus facilitate the diffusion of active species (e.g. O2, H2O, OH-, etc.) for enhanced ORR kinetics.46 The influences of pyrolysis temperature on the microstructure of NPCS was investigated. As depicted in Fig. S4, the thermogravimetric analysis (TGA) curve of GO-PPy shows an obvious exothermic signal around 675 oC, indicating that pyrolysis of PPy occurs. In addition, the TGA curves of NPCS depict two main weight-loss regions of carbon in a mixture of N2/O2 atmosphere, which can be caused by the presence of both graphene and amorphous carbon (Fig. S5). According to the TG profile in Figure S4, the content of graphene is estimated to be 38.8 wt% in NPCS-850. The related calculation process can be found in the supporting information. As shown in Fig. S6, the specific surface areas of NPCS-750, NPCS-850 and NPCS-950 are 468, 594 and 628 m2/g. Note that the content of N dopants decreases from 5.10 at% in NPCS-750 to 4.25 at% in NPCS-850 and to 2.12 at% in NPCS-950 (Table S1). Moreover, based on the integrated N 1s peak profiles, it can still be deconvoluted into four peaks for NPCS-750, but there are no obvious signals of pyrrolic N, pyridinic N or Fe-N in NPCS-950 (Fig. S7). The NPC prepared without GO shows rough spherical morphology (Fig. S8a-b), and it is shown that nitrogen content is 2.12 at% in the XPS spectra of NPC (Fig. S8c-e), lower than that

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of NPCS (4.25 at%). In addition, NPC is characterized by a lower specific surface area of 452 m2 g-1 and a large H4 type hysteresis loop, suggesting that there are many narrowly slit-shaped mesopores (Fig. S8f). In comparison, NPCS has a higher specific surface area and hierarchical porous structure, which improves the utilization of active sites in the pores.

Fig. 3 (a) Cyclic voltammetry and (b) RDE response for NPCS-T (T=750, 850 and 950) and Pt/C catalysts in O2-saturated 0.1 M KOH aqueous solution. Scan rate of LSVs: 5 mV s-1. Rotation rate: 1600 rpm. Scan rate of CVs: 50 mV s-1. The ORR performance was evaluated in O2-saturated 0.1 M KOH aqueous solution using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The NPCSs prepared under different temperatures deliver obvious cathodic peaks and the peak potential is 0.78 V (vs. RHE) for NPCS-750, 0.79 V for NPCS-850 and 0.71 V for NPCS-950 (Fig. 3a). As depicted in Fig. 3b, the onset potential of these three samples is 0.962, 0.989 and 0.908 V, respectively. Note that the onset potential of NPCS-850 (0.989 V) is even more positive compared to that of commercial Pt/C (0.986 V). Moreover, it also delivers the highest limiting current of 5.76 mA cm-2 at 0.45 V, much higher than that of NPCS-750 (3.87 mA cm-2), NPCS-950 (3.79 mA cm-2) and Pt/C (5.23 mA cm-2). The commercial Pt/C performance is comparable to that reported in the previous literatures.12, 28 Though NPCS-950 has a larger specific surface area, it shows inferior 12


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ORR performance to NPCS-850, which may be ascribed to its lower nitrogen content (Table S1).28 At the same time, NPC-850 and GO-850 show the cathodic peak potentials of 0.70 V and 0.61 V with the onset potentials of 0.923 V and 0.719 V (Fig. S9), indicating their much worse ORR activity. Compared with NPC, the highly conductive graphene in the middle of NPCS can provide high electron conductive path and sheet-like hierarchical structure that shorten the ion diffusion path and increase the accessibility of nitrogen active sites.47 The inferior performance of GO-850 should be ascribed to the limited number of active sites on its surface without nitrogen doping.48 It is reported that the nitrogen can bond with Fe and the formed N-binding Fe species are the active moiety for ORR.49 Thus, the Fe-N in NPCS should act as the additional active sites for ORR.

Fig. 4 (a) LSV curves of NPCS-850 at different rotation rates (in rpm) and (b) the corresponding Koutecky-Levich plots derived from the LSV curves from 0.57 to 0.67 V, (c) Electron transfer numbers of NPCS-T (T=750, 850 and 950) and commercial Pt/C catalyst, (d) RRDE voltammograms in O2-saturated 0.1 M KOH for commercial Pt/C and NPCS-850 at 1600 rpm and 5 mV s-1 and the ring electrode was polarized at 1.57 V, (e) Percentage of H2O2 and electron

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transfer number of Pt/C and NPCS-850 catalyst at various potentials on the basis of the related RRDE data in d, (f) Tafel plots of Pt/C and NPCS-850 derived by the mass-transport correction of RDE data. The corresponding linear sweep voltammetry (LSV) curves and Koutecky-Levich (K-L) plots of these samples are shown in Fig. 4a and Fig. S10. The K-L analysis (J-1 vs. w-1/2) is conducted at 0.57, 0.62 and 0.67 V and the plot of NPCS-850 exhibits good linear relationship with a constant slope, implying the first-order reaction kinetics for ORR process (Fig. 4b). The electron transfer number is 3.92 for NPCS-850, approaching an ideal 4 electron reaction of direct reduction of O2 to OH- ions in alkaline solution.50 Also, the electron transfer number is estimated to be 3.78 for NPCS-750 and 3.1 for NPCS-950 (Fig. 4c). As shown in Fig. 4d-e, the H2O2 yield is lower than 5% over the potential range of 0.20 to 0.80 V for NPCS-850 and the electron number is larger than 3.9, which is coincident with the K-L calculation results, showing its high ORR activity and fast reaction kinetics. This is further confirmed by its small Tafel slope of 68 mV/decade at the low overpotential range, approaching that of commercial Pt/C (62 mV/decade) (Fig. 4f). The ORR performance of NPC, commercial Pt/C and NPCSs prepared under different temperatures is listed in Table S2 and Table S3. The ideal ORR catalyst for fuel cells is also expected to have satisfactory tolerance ability towards fuel molecules (e.g. methanol). The LSV responses of NPCS-850 show neglectable changes after injecting 3 M methanol into the system, suggesting its excellent resistance towards methanol crossover (Fig. S11a). In contrast, the commercial Pt/C undergoes an abrupt decrease in the current response (Fig. S11b-c). The current-time (i-t) chronoamperometric response was recorded to assess the long-term stability of ORR catalysts (Fig. S11d). The commercial Pt/C suffers a 27.6% decrease in current density after 30,000 s while NPCS-850 exhibits a modest

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11.8% loss, which further demonstrates the superior stability of NPCS-850. Moreover, NPCS850 is among the best noble metal-free ORR catalysts reported so far in terms of their ORR activity (Table S4). 4. CONCLUSIONS A nitrogen-doped porous carbon sheet with a sandwich-like structure is prepared using graphene oxide as the substrate and template for the polymerization and carbonization of PPy. For this structure, the aggregation of the carbonization products was restrained and thin hierarchical porous carbon layers on graphene surface were thus formed. Consequently, the pore length is shortened while the high specific surface area is achieved, which shorten the ion diffusion path and increase the accessibility of the nitrogen-based catalytic active sites in the pores. The highly conductive graphene as the middle layer ensures fast electron transfer for the catalytic reactions. The NPCS-850 prepared with the carbonization temperature of 850 oC shows a high specific surface area while with a high N content, leading to the high activity towards ORR in alkaline media that is comparable to that of commercial Pt/C catalyst. Moreover, NPCS-850 also shows a larger diffusion-limited current and higher tolerance towards methanol poisoning than commercial Pt/C catalyst. Overall, this study provides a low-cost and high-performance metalfree ORR catalyst of great potential to be commercialized in fuel cells, metal-oxygen batteries, and related energy conversion devices. Furthermore, it also demonstrates a facile and novel strategy to improve the utilization of inner pores and active sites of traditional porous carbon materials by simply incorporating graphene as the template.

ASSOCIATED CONTENT Supporting Information

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Structure and composition characterization results, including the SEM and TEM images, XRD, N2 adsorption isotherms, TG and XPS spectra; The electrochemical characterization results, including CV profiles, ORR polarization curves, the Koutecky-Levich plots, and the comparison of catalytic performance. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Guangdong Natural Science Funds for Distinguished Young Scholar (2017B030306006), National Natural Science Foundation of China (Nos. 21506212, 51772164 and U1601206), National Science Fund for Distinguished Young Scholars, China (No.51525204), National Basic Research Program of China (2014CB932400) and Shenzhen Basic Research Project (No. JCYJ20150529164918734). We also thank the Youth Research Funds of Graduate School at Shenzhen, Tsinghua University (QN20160001).

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Graphene-directed Formation of Nitrogen-Doped Porous Carbon

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