A Thermally Decomposable Template Route to ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

A Thermally Decomposable Template Route to Synthesize NitrogenDoped Wrinkled Carbon Nanosheets as Highly Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction Daguo Gu,† Fangfang Wang,‡ Kang Yan,*,§ Ruguang Ma,*,‡ and Jiacheng Wang*,‡ †

School of Materials Engineering, Yancheng Institute of Technology, 1 Xiwang Avenue, Yancheng 224051, Jiangsu Province, P. R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China § State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, P. R. China S Supporting Information *

ABSTRACT: We successfully developed a thermally decomposable template route to prepare wrinkled carbon nanosheets with a high level of nitrogen functional moieties by direct carbonization of biomass glucose and dicyandiamide as the renewable feedstocks. Confined pyrolysis of glucose within the interlayers of dicyandiamide-derived g-C3N4 as a thermally removable template results in the formation of two-dimensional (2D) wrinkled carbon nanosheets as well as simultaneous high-level nitrogen doping. The textural properties and nitrogen contents could be controlled by adjusting the mass ratio of glucose/ dicyandiamide. Among various samples, the sample prepared with the dicyandiamide/glucose mass ratio of 7/1 has optimal activity for the electrocatalytic oxygen reduction (onset potential −0.12 V vs saturated calomel electrode (SCE); limiting current density 4.73 mA/cm2) in 0.1 M KOH solution, the half-wave potential of which is only 67 mV larger than that for 20 wt % Pt/C. Moreover, it demonstrates a highly efficient four-electron reaction process, as well as superior durability and tolerance to MeOH crossover to Pt/C. The excellent activity is mainly attributed to the high content of pyridinic and graphitic-N groups, highly graphitized structures, and wrinkled 2D nanostructures, efficiently promoting the increased exposure of actives sites and fast mass/electron transfer. KEYWORDS: Thermal decomposition, Biomass, Graphene, Nitrogen doping, Electrocatalysts, Oxygen reduction reaction

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INTRODUCTION Due to its slow reaction kinetics, the oxygen reduction reaction (ORR) process is the rate-determining step, which is critical to improve the overall performance of clean energy devices including fuel cells and metal−air batteries in the near future.1−4 Therefore, great effort has been devoted to develop high-performance ORR electrocatalysts, which could reduce the reaction overpotential and speed the reaction.5,6 It has been reported that Pt and Pt-based alloys show superior ORR performance,7−10 but the rarity, high-cost, and nondurability of Pt-based ORR electrocatalysts are unavoidable to limit the commercialization of these energy devices. Thus, much © XXXX American Chemical Society

attention has been paid to develop and explore nonprecious-metal ORR electrocatalysts containing Co, Fe, Ni, Mn, etc.5,11−19 However, these metal-based catalysts still have various disadvantages, such as low activity and durability, detrimental environmental effects derived from catalyst wastes, and unwanted side-products. Therefore, it is still necessary to search for more environmentally friendly, stable, and highly active ORR electrocatalysts. Received: September 20, 2017 Revised: November 28, 2017

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DOI: 10.1021/acssuschemeng.7b03370 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

durability and MeOH crossover resistance to commercial Pt/ C.

Compared to metal-based materials, carbon-based materials possess more desirable advantages including low-price, lightweight, variable porosity, adjustable morphologies, controllable heteroatom doping, and high stability.20−22 Thus, carbon materials can be utilized in a wide range of fields including gas adsorption and storage, electrochemical energy conversion and storage, catalysis, adsorption for waste oil and heavy metal ions, etc.23−26 Moreover, the doping of various heteroatoms (e.g., N, B, S, P) within the carbon framework could improve the ORR reactivity of carbon because the introduced heteroatoms could evidently modify their electronic structures and chemical activities due to different electronegativity.27,28 Thus, heteroatom-doped carbons have been considered as potential metal-free ORR electrocatalysts to replace expensive Pt-based electrocatalysts. Among various carbon materials, graphene has become one of the most promising candidates as metal-free electrocatalysts because of its remarkable mechanical flexibility, good electrical conductivity, and large surface area.27,29,30 Particularly, Ndoped graphene nanosheets have demonstrated high activity for ORR ascribed to the uniform charge distribution on the carbon atoms adjacent to the nitrogen atoms. The traditional synthesis strategy for the N-doped graphene needs two steps: (1) the oxidation of graphite resulting in the formation of graphene oxide; (2) postreaction of graphene oxide with Ncontaining precursors. However, this strategy starting from natural graphite is very complicated and requires the consumption of a large amount of dangerous and hazardous reagents. In addition, the transition metal impurities (Mn, Fe, Co, Ni, etc.) derived from graphite and chemical reagents could contaminate the final N-doped graphene materials, and thus the reported “metal-free” catalysis based on these catalysts is not strictly “metal-free”.31 Recently, a series of wrinkled graphene-based materials have been prepared via simpler, more controllable, and lower cost approaches. For example, a sugarblowing process has been designed to obtain N-doped graphene networks.32 Moreover, the direct carbonization of organic precursors and high-nitrogen-content molecules (e.g., melamine, urea, NH4Cl, etc.) could lead to the formation of nitrogen-doped wrinkled graphene materials.33 However, these N-doped graphene wrinkles have not been studied as electrocatalysts toward the ORR. In this work, we develop a thermally decomposable template route to synthesize N-doped wrinkled graphene materials by one-pot thermal treatment of glucose and dicyandiamide as the renewable feedstocks at 950 °C. During the pyrolysis, dicyandiamide could condense into g-C3N4, which could act as the in situ template for confined growth of 2D graphene from glucose. At the higher temperature, the g-C3N4 template could completely decompose, and thus no further posttreatment removal of template was needed. The advantages of this strategy are not only the confined growth of graphene within the interlayers of dicyandiamide-derived g-C3N4, but also the simultaneous nitrogen doping into the framework of graphene. Compared to those strategies of preparing N-doped graphene starting from graphite, the present process is very safe and practical, and it does not consume any organic solvents or aggressive chemical reagents. The effect of the glucose/dicyandiamide mass ratios on the textural properties of the final samples was also studied in detail. Due to the unique nanostructures, the resulting N-doped graphene materials possess not only excellent ORR activity with a close four-electron ORR process but also superior long-term

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EXPERIMENTAL SECTION

Reagents. Potassium hydroxide, glucose, and dicyandiamide were purchased from Sinopharm Chemical Reagent Co., Ltd. Nafion solution (5%) was bought from Sigma-Aldrich. The commercial 20 wt % Pt/C catalyst was obtained from Johnson Matthey (United Kingdom). All chemicals were used as received without further purification. Materials Synthesis. Synthesis of N-Doped Wrinkled Carbon Nanosheets (NWCNs). A series of NWCNs were prepared via onestep pyrolysis of a homogeneous mixture of glucose and dicyandiamide, which were used as the carbon source and nitrogen source, respectively. In a typical process, glucose (1 g) was first dissolved in distilled water under magnetic stirring at room temperature, and a calculated amount of dicyandiamide was also dissolved in distilled water by heating at 60 °C. Then, the dicyandiamide solution was poured into the glucose aqueous solution under stirring at 110 °C until a white solid was formed after the complete evaporation of water. Then, after being ground into a fine powder using a mortar with a pestle, this white mixture was further carbonized at 450 °C for 1 h and then at 950 °C for another 1 h in a quartz tube furnace under Ar atmosphere. After cooling down, the resultant black material was ground into a fine powder for further analyses. Three samples were synthesized by adjusting the amount (1, 4, and 7 g) of dicyandiamide. The final samples were designated as NWCN-X, where NWCN means N-doped wrinkled carbon nanosheet and X is the mass ratio of dicyandiamide to glucose. For comparison, pure N-free porous carbon material (PCM) was synthesized by direct carbonization of glucose at 950 °C for 1 h in Ar. Structural Characterization. The morphology of various carbons was observed by scanning electron microscopy (SEM) on an FEI field emission Magellan 400 SEM equipped with an Oxford Instruments XEDS system. The sample was dispersed in ultrahigh purity ethanol, and then a drop of the suspension was dropped onto a Al coil. Samples were prepared for transition electron microscopy (TEM) examination by dispersing the catalyst powder in ultrahigh purity ethanol. A drop of the suspension was then allowed to evaporate on a carbon microgrid supported by a 300 mesh copper grid. Samples were examined in a JEOL JEM 2100F transmission electron microscope operating at an accelerating voltage of 200 kV. Nitrogen sorption isotherms were measured using a Micromeritics ASAP 2010 surface area and pore size analyzer at liquid nitrogen temperature (−196 °C). Prior to measurement, the hierarchically porous materials were dehydrated under vacuum at 200 °C overnight. The specific surface areas were calculated by the Brunauer−Emmett− Teller (BET) method. The total pore volume was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distribution curves were calculated from the analysis of the desorption branch of the isotherm using the Barrett−Joyner−Halenda (BJH) model. The specific surface areas were calculated by the multipoint BET method at the relative pressure range of P/P0 = 0.05− 0.20. X-ray photoelectron spectroscopy (XPS) was recorded with an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα (hν = 1486.6 eV) radiation. The carbon monoliths were ground to powder and then glued onto indium (In) metal particles by pressing for measurements. All spectra were calibrated using 284.5 eV as the line position of adventitious carbon. For each sample, the XPS measurements were performed three times, and the average data of the N contents were reported. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific Co., USA) with 532 nm excitation length. Electrochemical Experiments. The electrocatalytic measurements were performed in a standard three-electrode electrochemical cell, which was connected to an electrochemical workstation (WaveDriver20 Bipotentionstat/Galvanostat, Pine Research InstruB


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Figure 1. Schematic drawing of confined growth of nitrogen-doped wrinkled carbon nanosheets (NWCNs) as an efficient metal-free ORR electrocatalyst via a thermally removable template route by one-pot carbonization of glucose and dicyandiamide as the renewable feedstocks. where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring (N = 0.37).

mentation, USA) coupled with a rotating disk electrode (RDE) system (AFMSRCE3529, Pine Research Instrumentation, USA).34,35 Electrochemical activity of the working electrode was studied by cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ringdisc electrode (RRDE) measurements using a Pt plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. An aqueous solution of 0.1 M KOH was used as the electrolyte for the electrochemical studies. The catalyst ink was prepared by ultrasonic dispersion of the active material (5 mg), deionized water (0.25 mL), ethanol (0.25 mL), and Nafion solution (25 μL) for 1 h. For the RDE measurement, the ink (10 μL) containing catalyst (95.2 μg) was transferred onto the glassy RDE with 5 mm diameter (geometric surface area 0.1963 cm2), yielding a catalyst loading level of 0.49 mg/cm2. The electrode with the catalyst was dried at 60 °C for 20 min, which was used as the working electrode for further electrochemical measurements. Prior to the measurements, the electrolyte (0.1 M KOH) was saturated by bubbling O2 for 15 min unless stated specifically. The CV measurements were carried out at a scan rate of 50 mV/s over the potential range of 0.1 to −1 V (vs SCE). Before collection of the data, the working electrode was activated by performing CV cycles at a scan rate of 200 mV/s at least 100 times. The LSV tests were performed from 0.1 V to −1 V (vs SCE) at a sweep rate of 10 mV/s with the electrode rotating from 400 to 2025 rpm. The current densities were normalized by the geometric surface area. The Koutechy−Levich (K-L) plots were obtained by analyzing the RDE data at −0.6 V (vs SCE).7 The slopes of their linear fitting lines are used to calculate the electron transfer number (n) based on the following K-L eqs 1 and 2:

1 1 1 = + J Jk Bω1/2

(1)

B = 0.2nFCO2DO2 2/3v−1/6

(2)

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RESULTS AND DISCUSSION In the present research, a thermally removable template route was adopted to prepared N-functionalized graphene materials via one-pot pyrolysis of glucose and dicyandiamide at high temperature. During this process, dicyandiamide acts as not only the precursor for forming 2D g-C3N4 as the thermally decomposable template but also the nitrogen source for Ndoped graphene.33 As presented in Figure 1, glucose was carbonized in the presence of dicyandiamide in a staged pyrolysis process. When heated at 450 °C, dicyandiamide condensed into g-C3N4 nanosheets,36 which can further act as the template for the confined carbonization of glucose into graphene nanosheets. Moreover, the N-rich dicyandiamide could act as the nitrogen source to in situ functionalize graphene materials by doping N atoms during the polymerization. When the pyrolysis temperature increased to 950 °C, the as-formed g-C3N4 template was readily removed off by thermal degradation. Due to the usage of dicyandiamide with high N content, this one-pot process was accompanied by simultaneous doping of N atoms into the graphene nanostructure. The morphologies of pure PCM and NWCN-X samples were investigated by scanning electron microscopy (SEM). As presented in Figure 2a, pure PCM prepared by direct pyrolysis of glucose at 950 °C in Ar has a very smooth surface. No sheetlike nanostructures or pores could be observed on the surface. When the mixture of glucose and dicyanamide with 1:1 mass

where J and Jk are the measured current density and kinetic-limiting current density, respectively, n is the electron transfer number, F is the Faraday constant (96485 C/mol), v is the viscosity of the electrolyte (0.01 cm2/S), CO2 is the concentration of O2 (1.2 × 10−6 mol/cm3), and DO2 is the diffusion coefficient (1.9 × 10−5 cm2/s). The coefficient 0.2 is adopted when the rotating speed is expressed in rpm. For the RRDE test, a GC disk (0.2475 cm2 geometric surface area) surrounded by a Pt ring (0.1866 cm2 geometric surface area) was used as the working electrode to load the catalysts. The catalyst ink (20 μL) containing catalyst (190.4 μg) was transferred onto the RRDE, yielding a catalyst level of 0.77 mg/cm2. During the measurement, the ring potential was held at 0.2 V (vs SCE), and the rotating speed was set as 1600 rpm. The LSV data were collected at a scan rate of 10 mV/s. The transferred electrons number (n) during the ORR process can be calculated via eq 3: n=4

Id Id + Ir /N

Figure 2. SEM images of (a) PCM, (b) NWCN-1, (c) NWCN-4, and (d) NWCN-7.

(3) C


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Figure 3. (a) TEM, (b) HR-TEM images, and (c) XRD pattern of NWCN-7.

ratio is pyrolyzed, the surface of the resulting NWCN-1 becomes a little uneven compared to that for pure PCM and also some pores could be formed due to the decomposition of dicyandiamide or dicyandiamide-derived g-C3N4 at high temperatures. For the sample NWCN-4 prepared with dicyandiamide/glucose of 4, its surface is composed of many irregular particles with wrinkled structures. It implies great effect of dicyandiamide on the final morphology of porous carbons. When the glucose/dicyandiamide ratio decreased to 1/7, it is clearly observed that the resultant NWCN-7 material has a more wrinkled surface with some macropores, very similar to that of reduced graphene oxide (rGO) prepared by the Hummers method.37 It could imply the successful preparation of 2D carbon nanosheets with improved graphitization degree via this simple one-pot pyrolysis process. It can be believed that the as-formed g-C3N4 nanosheets derived from dicyandiamide could act as the confined template for the growth of 2D carbon nanosheets.33 The highly graphitized graphene nanostructures are beneficial for fast electron transfer, thus improving the activity. Moreover, the aggregating macropores from these carbon nanosheets are advantageous for fast mass diffusion and transfer of the electrolytes, thus greatly improving the reaction kinetics.38 The formation of 2D carbon nanosheets was further confirmed by TEM observation. As shown in Figure 3a, it is clear that NWCN-7 is composed of a large perfect 2D sheet with some wrinkles, similar to the classical 2D sp2-hybridized graphene-like carbon nanosheets.39 The high-resolution TEM image reveals the lattice fringes of the few layers with wrinkle morphology for NWCN-7, implying its highly graphitic microstructure.12,40 The interlayer distances for these nanosheets vary in the range from 3.49 to 3.66 Å in different regions (Figure 3b), evidently larger than the value of graphite (3.35 Å), which should be caused by the existence of microstructural defects (e.g., carbon atom arrangement) and the substitution of N atoms.41,42 Indeed, the XRD pattern of NWCN-7 shows a typical (002) peak at 25.1° (Figure 3c), implying the development of graphitic structure to some extent. The lattice spacing derived from the diffraction degree is calculated to be 3.54 Å, matching well with the results of HRTEM. These structural defects as well as N dopants are potential active sites for activating oxygen molecules, thus improving the ORR activity. Due to the use of dicyandiamide-derived g-C3N4 with high N content as a thermally decomposable template, the resultant NWCNs were successfully doped with N functional groups, determined by the XPS analysis (Figure 4). The XPS spectra of NWCNs demonstrate three peaks at ca. 284.5, 401.0, and 533.0 eV, which are attributed to C 1s, N 1s, and O 1s, respectively (Figure 4a),43−45 while no N 1s peak was observed in the XPS spectrum of PCM, implying the successful doping

Figure 4. (a) XPS survey spectra of pure PCM and NWCN-X prepared with different glucose/dicyandiamide mass ratio, highresolution N 1s XPS spectra of (b) NWCN-1, (c) NWCN-4, and (d) NWCN-7, and (e) relative atomic percentage (%) of nitrogen functionalities obtained from the deconvoluted N 1s peaks for NWCNs samples, and (f) schematic drawing of N functional moieties in the NWCN-X samples.

of N atoms within the carbon framework by one-pot thermal treatment of glucose and dicyandiamide.46 With the glucose/ dicyandiamide mass ratio of 1/1, the resulting NWCN-1 has nitrogen doping of 4.85 atom % (Table 1). The decrease of the glucose/dicyandiamide mass ratio resulted in the formation of NWCNs with higher N contents. For example, NWCN-4 and NWCN-7 show the N doping of 5.68 and 6.08 atom %, respectively. In sharp contrast, the oxygen content significantly decreases to 3.42 atom % when the mass ratio decreased to 1/ 7. This result implies that some oxygen-containing groups could react with the nitrogen-based volatiles, thus causing nitrogen doping into the carbon framework accompanying the removal of oxygen atoms in the forms of NOx and COx.47 The doping of heteroatoms will evidently affect the electrocatalytic activity of carbon nanosheets because the doping will modify the electronic structures, chemical activities, and Fermi level of the adjacent carbon atoms, favorably adsorbing and activating O2 molecules.42,48−50 D


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Table 1. Contents of C, O, and N and Relative Nitrogen Species Analyzed by N 1s XPS Spectra of PCM and NWCN-X Samples samples

C (atom %)

O (atom %)

N (atom %)

pyridinic-N (atom %)

graphitic-N (atom %)

pyrrolic-N (atom %)

oxidized-N (atom %)

PCM NWCN-1 NWCN-4 NWCN-7

94.05 90.52 90.71 90.50

5.95 4.63 3.61 3.42

0 4.85 5.68 6.08

15.06 13.90 15.56

52.57 47.91 54.60

13.39 13.63 11.46

18.98 24.56 18.38

The chemical surroundings of doped nitrogen atoms were further evaluated by high-resolution XPS, revealing four peaks at pyridinic (398.0 eV), pyrrolic (399.3 eV), and graphitic N (400.7 eV) and N-oxide (401.8 eV) (Figure 4b−d).43,44,48 Figure 4e displayed the evolution of the percentage of four types of N-bonding configurations calculated based on the areas of the deconvoluted peaks, and the relative contents of various nitrogen groups are listed in Table 1. It is clearly found that the graphitic nitrogen groups account for the majority of the N and the contents of the graphitic N for all samples are evidently larger than those of the pyridinic N. It is believed that the high-temperature pyrolysis (e.g., 950 °C) is beneficial for the transformation of the pyrrolic and pyridinic to graphitic nitrogen groups. It has been reported that nitrogen-doping could improve ORR activity, but the total atomic content of nitrogen is not an important factor affecting the activity. Both quaternary and pyridinic N play important roles in the electrocatalytic ORR.51 Thus, the NWCN-7, showing the highest contents of pyridinic and graphitic N groups, could possess superior ORR activity, as discussed later. The corresponding atomic structure is shown in Figure 4f on the basis of the XPS analysis. The presence of dicyandiamide not only changed the morphology of the final carbons but also evidently influenced their textural properties. The N2 adsorption−desorption isotherms, pore size distribution curves, and cumulative pore size distribution plots are presented in Figure 5, and the textural properties are shown in Table 2. Pure PCM prepared by direct carbonization of glucose possesses a high N2 uptake at low relative pressure and thus has a typical type I isotherm,44 indicating its microporous texture.52,53 Indeed, the surface area (422.9 m2/g) in the micropore range dominates ∼80% of total specific surface area (531 m2 g−1). Notably, the pyrolysis of glucose and dicyandiamide with 1:1 mass ratio resulted in the significant decrease in the N2 uptake for NWCN-1 (Figure 5a), implying that NWCN-1 has very low specific surface area (10.4 m2/g), possibly ascribed to the dense multilayered graphitized structures. When the dicyandiamide/glucose ratio is increased to 4/1 and 7/1, the adsorption isotherms changed from type I to IV with a distinct hysteresis loop at the relative pressure from 0.47 to 1.0,38,43,54,55 suggesting that mesopores are dominant in the NWCNs.56 The position of the P/P0

Table 2. Textural Properties for Pure PCM Prepared by Direct Carbonization of Glucose and Various NWCNs Prepared by One-Pot Carbonization of Glucose and Dicyandiamide samples

SBETa (m2 g−1)

Vtotalb (cm3 g−1)

PCM NWCN-1 NWCN-4 NWCN-7

531.0 10.4 124.9 121.8

0.258 0.019 0.397 0.478

Dporec (nm)

Smicrod (m2 g−1)

Vmicrod (cm3 g−1)

422.9

0.193

2.2 2.2

a

The surface areas (SBET) were calculated by the multipoint BET method. bThe total pore volumes (Vtotal) were estimated at P/P0 = 0.993. cThe Dpore values were obtained from the pore size distribution curves. dMicropore surface area (Smicro) and micropore volume (Vmicro) were calculated using the t-plot method.

inflection point is related to the diameter in the mesopore range. Indeed, no micropore surface area was obtained in NWCN-4 and NWCN-7. And both NWCN-4 and NWCN-7 had similar specific areas of 124.9 and 121.8 m2/g, respectively. The surface areas for these NWCNs are evidently smaller than theoretical surface area (∼2600 m2/g) of a graphene monolayer due to the stacking textures of multilayered nanosheets with smaller interlayer distances (3.49−3.66 Å determined by the HRTEM observation) of NWCNs. Furthermore, it is evident that the pore sizes of NWCN-4 and NWCN-7 increase to the range of mesopores (Figure 5b), indicating the great effects of dicyandiamide-derived g-C3N4 on the texture of final NWCN-X samples. The NWCNs prepared with dicyandiamide/glucose mass ratios of 4 and 7 show an evident mesopore cantered at 2.2 nm. The layered structure of g-C3N4 could act as the template to promote the growth of stacked carbon nanosheets.57 It is notable that the total pore volumes of NWCNs evidently increase as the dicyandiamide/glucose mass ratios increase (Figure 5c and Table 2), and NWCN-7 has the largest pore volume of 0.478 cm3 g−1. The large surface areas and pore volumes and aggregating pores are beneficial for not only the exposure of the catalytic sites but also mass transport of reactants and products and thus could greatly improve the catalytic activity.49 Raman spectra are further used to get microstructure information on carbon materials through the determination

Figure 5. (a) N2 adsorption−desorption isotherms, (b) pore size distribution curves calculated from the adsorption branches, and (c) cumulative pore size distribution plots. E


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ACS Sustainable Chemistry & Engineering of the G bands at ∼1580 cm−1 and D bands at ∼1340 cm−1.58−60 The intensity ratio of the D to G band (ID/IG) is used to provide qualitative information on the graphitic degree in carbon materials. As shown in Figure 6, the peak intensities

ID/IG values for NWCN-1, NWCN-4, and NWCN-7 are 1.01, 1.05, and 1.06, respectively, all of which are evidently lower than 1.12 for PCM. It implies a higher degree of carbonization and carbon ordering for NWCN-X samples than that for PCM.61 The N-doping in the amorphous carbon matrix could provide large amounts of defective and N-functional active sites for adsorbing and activating oxygen molecules, thus efficiently improving the electrocatalytic ORR activity. The ORR activity of the resulting catalysts was studied by cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques in a conventional three-electrode system. Figure 7a and S1 shows the CV curves of NWCN-7 deposited on a glassy carbon electrode (GCE) in N2- and O2-saturated 0.1 M KOH aqueous electrolyte at 50 mV/s. The CVs collected in both N2 and O2-saturated 0.1 M KOH solution show nearly rectangular shapes, implying the high conductivity with superior capacitive current. Notably, there is a well-defined oxygen peak at −0.25 V (vs SCE) in the CV curve obtained in O2-saturated solution, while no such a peak in N2-saturated one was found, showing that oxygen is reduced on the surface of the NWCN-7modified electrode. This result demonstrates efficient ORR activity of the metal-free NWCN-7 in alkaline electrolyte. The electrocatalytic activity of all samples as well as commercial Pt/C catalyst was further studied by the rotating disk electrode (RDE) technique in O2-saturated 0.1 M KOH

Figure 6. Raman spectra of pure PCM and NWCN-X samples.

of NWCN-X samples are much lower than that for PCM, showing that NWCN-X samples are more amorphous in the carbon microstructure than PCM. Moreover, the calculated

Figure 7. (a) CV curves of NWCN-7 in O2- or N2-saturated 0.1 M KOH solution (sweep speed 50 mV/s), (b) LSV curves of NWCN-7 in O2saturated 0.1 M KOH solution at different rotating speeds (sweep speed 10 mV/s), (c) LSV curves of different samples and commercial Pt/C in O2-saturated 0.1 M KOH solution at 1600 rpm (sweep speed 10 mV/s), (d) the comparison of the limiting current density for samples, (e) the K-L plots based on the ORR curves of NWCN-7 at different potentials (vs SCE), and (f) the K-L plots of different samples and Pt/C at −0.6 V (vs. SCE). The catalyst loading level is 0.49 mg/cm2. F


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of high importance in fuel cells, because the peroxides can be poisonous to the cell.49 Moreover, the possibility of a trace amount of transition metal impurities (e.g., Ni, Co) responsible for high ORR activity could be ruled out,11,12,66 because no transition metal precursors were utilized for preparing the NWCNs. Anyway, the NWCN-7 shows the best comprehensive performance toward the ORR in terms of E1/2, Eonset, limiting current density, and n value among the resulting PCM and NWCN materials. To further investigate the effects of the N-doping and pore structures on the reaction mechanism and catalytic activity of the resulting carbons, the LSV curves of PCM and NWCN-7 were collected by a RRDE technique in O2-saturated 0.1 M KOH medium at 1600 rpm.67 During the measurements, the ring potential was set at 0.2 V (vs SCE). Figure 8a indicates the

aqueous solution at different rotating speeds (scan rate 10 mV/ s).62 The linear sweep voltammetry (LSV) curves for PCM, NWCN-X, and Pt/C are presented in Figure 7b and S2. It is clearly observed that the increase of the electrode rotating speed results in larger current density, resulting from the improved mass diffusion on the electrode surface. The ORR activity of PCM, NWCN-X, and Pt/C was compared by the LSV measurements in O2-saturated 0.1 M KOH solution at 1600 rpm (Figure 7c). Pure PCM shows poor ORR activity with a large onset potential (Eonset) of −0.22 V (vs. SCE) and small limiting current density of 2.56 mA/cm2 owing to the lack of N-relative active sites, because the lone-pair electrons of the N atoms can enrich the electron density of the adjacent carbon atoms and thus form active ORR sites.63 Although NWCN-1 has a high N doping of 4.85%, it is still less active for the ORR possibly due to its low specific surface area and pore volume. When the mass ratio of glucose/dicyandiamide is reduced to 1/4 and 1/7, the resulting NWCN-4 and NWCN-7 have significantly better ORR activity than PCM and NWCN1 in terms of Eonset. Both NWCN-4 and NWCN-7 have an Eonset value of −0.12 V (vs SCE), superior to those of PCM (−0.22 V) and NWCN-1 (−0.28 V). Moreover, the limiting current densities of NWCN-4 (4.08 mA/cm2) and NWCN-7 (4.73 mA/cm2) are also much improved compared to those for PCM and NWCN-1 (Figure 7d). Although the NWCN-X samples have similar N contents, NWCN-7 has the largest limiting current density possibly due to the increased masstransfer kinetics linked with its largest pore volume. These results show that the mass ratio of glucose/dicyandiamide has a great effect on the ORR activity, reflected by obvious positive shift in the Eonset from −0.28 to −0.12 V and half-wave potential (E1/2) from −0.38 to −0.22 V and the increase of the limiting current density from 2.16 to 4.73 mA/cm2 with decreasing the mass ratio from 1/1 to 1/7 (Figure 7d). Among these NWCN-X samples, NWCN-7 shows the best ORR activity and demonstrates comparable ORR activity to commercial Pt/C catalyst in terms of close limiting current densities and negatively shifted E1/2 just by 67 mV. This E1/2 gap (67 mV) between Pt/C and NWCN-7 is comparable to or smaller than those of other carbon-based ORR catalysts, such as N,B-co-doped graphene (144 mV),64 nitrogen-doped graphene (62 mV),34 and N−P-doped graphene (70 mV).65 The electron transfer numbers (n) were calculated on the basis of the Koutecky−Levich (K-L) equation. The K-L plots of NWCN-7 within the potential range of −0.4 to −0.8 V (vs SCE) exhibit good linearity (Figure 7e), suggesting first-order reaction kinetics toward the dissolved oxygen on the surface of the electrode. The exact electron transfer numbers (n) were calculated according to the slopes of the linear fitted K-L plots. The n values of NWCN-7 varied from 3.92 to 4.07 at a wide potential range of −0.4 to −0.8 V (vs SCE), suggesting a stable four electron transfer pathway for the ORR. The K-L plots of PCM, NWCN-X, and Pt/C with the calculated n values at the potential of −0.6 V (vs SCE) are shown in Figure 7f. The n values for PCM and NWCN-1 are 1.64 and 1.94, respectively, implying a 2e−-dominant reaction pathway with peroxide as the main product. The NWCN-4 and NWCN-7 have similar n values of 3.98 and 3.96, very close to that for Pt/C (∼4.0), showing that the ORR reaction catalyzed by NWCN-4 and NWCN-7 is a 4e−-dominant pathway with water as the main product. The great improvement mainly results from their large surface areas and pore volumes and high contents of graphitic and pyridinic-N groups. The four-electron process is

Figure 8. (a) The RRDE voltammograms for PCM, NWCN-7, and Pt/C in O2-saturated 0.1 M KOH medium (rotating speed 1600 rpm; scanning rate 10 mV/s; ring potential 0.2 V (vs SCE)), and (b) the calculated transferred electrons number (n) over the potential range from −0.8 to −0.4 V (vs SCE) based on the RRDE curves. The catalyst loading level is 0.77 mg/cm2.

ring and disc current density for NWCN-7, as well as other reference samples (PCM and commercial Pt/C). The calculated n values for three samples are displayed in Figure 8b. The disc current density and Eonset of these samples demonstrate the same trend as those in Figure 7c. Furthermore, the E1/2 for NWCN-7 is only 69 mV more negative than of Pt/C (Figure 8a), which is similar as the result obtained by the RDE measurements. NWCN-7 shows n values varying from 3.75 to 3.90, very close to 3.79−3.95 for commercial Pt/C over the potential of −0.4 to −0.8 V (vs SCE), implying a 4e−-dominant pathway in the NWCN-7catalyzed ORR to greatly reduce peroxide yields and thus obtain maximum energy capacity.13,48,68 Indeed, the peroxide yields calculated based on eq 4 are in the range of 5−12.5% for NWCN-7, a little larger than 2.5−10.5% for commercial Pt/C. It is very important for fuel cells because peroxides produced via a 2e− process can poison the cells by corroding the membrane and catalyst layer.67,69 In contrast, pure PCM prepared by direct pyrolysis of glucose showed much lower n values of 2.14−2.39, implying a 2e− reaction process on the PCM. Thus, the calculated peroxide yields are as high as 80.5− 90.5% over the potential of −0.4 to −0.8 V. This comparison indicates the key roles of N-relative active sites including pyridinic and graphitic N groups in catalyzing the 4e− ORR process.70 Thus, the resulting NWCN-7 is a potential low-cost, metal-free ORR electrocatalyst to replace Pt/C in alkaline fuel cells. %(H2O2) = 200 × G

Ir / N Id + Ir /N

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dicyandiamide/glucose mass ratios of 4/1 and 7/1 show more than ten times larger specific surface areas. The pore volume of the NWCNs evidently increases as the content of dicyandiamide in the precursor, and the NWCN prepared with the dicyandiamide/glucose mass ratio of 7/1 has the largest pore volume of 0.478 cm3/g, possibly due to the utilization of excess amount of dicyandiamide and thus the formation of larger amount of macropores within the graphene interlayers. Among various samples, NWCN-7 shows the optimal ORR activity in 0.1 M KOH solution with much more positive onset potential (−0.12 V vs SCE), and larger current density of 4.78 mA/cm2 than PCM and other NWCNs. The half-wave potential of NWCN-7 is only 67 mV larger than that for commercial Pt/C. Furthermore, NWCN-7 demonstrates a highly efficient four-electron reaction process, indicating that water is the main product for the ORR catalyzed by NWCN-7. It is notable that NWCN-7 shows superior durability and tolerance to MeOH crossover to commercial Pt/C ascribed to its metal-free textures. It can be suggested that the large pore volume and high relative contents of pyridinic and graphitic N groups are responsible for the excellent ORR activity for NWCN-7. Moreover, the highly graphitized microstructures and wrinkled nanosheets of the NWCNs are also beneficial for increased exposure of N-relative active sites and fast mass/ electron transfer, thus improving the ORR activity. The resulting NWCNs with increased graphitization degree, high level of N dopants, nanosheet morphology, and large pore volumes can also be further studied in various applications including batteries, supercapacitors, adsorbents, etc.

Both the long-period stability and tolerance to methanol crossover effects are vital considerations for practical applications of non-precious-metal electrocatalysts in future fuel cell devices.67 The stability of NWCN-7 and Pt/C was compared using the I−t chronoamperometric response at −0.6 V (vs SCE) in the O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. As shown in Figure 9a, the

Figure 9. The I−t chronoamperometric responses of NWCN-7 and commercial Pt/C electrodes at −0.6 V (vs SCE) in (a) O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm for 5 h and (b) O2-saturated 0.1 M KOH solution with 3.0 M methanol added at ∼200 s.

optimized NWCN-7 still retained 97% of the kinetic current density after 5 h long-term operation, while the ORR current density of commercial Pt/C catalyst evidently decreases to 76% under the same test conditions. This result indicates that NWCN-7 has a much better long-term durability than commercial Pt/C catalyst toward the ORR in basic medium. Moreover, the resistance to methanol crossover effects of NWCN-7 and Pt/C were also studied by using the chronoamperometric measurements in O2-saturated 0.1 M KOH solution with 3 M methanol added at ∼200 s. It is clearly observed that Pt/C has a great decrease of 42% in the relative current density after the addition of MeOH into the cell while the value for NWCN-7 only decreases by 5% (Figure 9b). This comparison reveals that NWCN-7 has much better resistance for MeOH crossover than Pt/C. It can be believed that the metal-free features and high chemical durability of the Nrelated active sites for NWCN-7 could enhance long-term stability and the resistance to methanol crossover effects for the ORR.48

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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.7b03370. CV and LSV curves of PCM and NWCN-X samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K. Yan). *E-mail: [email protected] (R. Ma). *E-mail: [email protected] (J. Wang).

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CONCLUSIONS In summary, we have explored a facile approach for confined growth of nitrogen-doped wrinkled carbon nanosheets via onepot carbonization of glucose and dicyandiamide blend. The dicyandiamide-derived g-C3N4 could act as a thermally decomposable template for confined synthesis of 2D carbon nanosheets. This strategy does not consume any organic solvents or involve template etching using aggressive chemicals. Contrary to the microporous textures for the glucose-derived carbons, the N-doped carbons derived from co-pyrolysis of glucose and dicyandiamide are composed of 2D wrinkled stacked nanosheets observed by TEM analysis, which are typical for graphene materials. It indicates the successful confined growth of 2D graphene materials from glucose within the interlayers of dicyandiamide-derived g-C 3 N 4. The increased dicyandiamide/glucose mass ratio resulted in the increased total N contents for final samples. The graphitic N groups occupied the majority of total nitrogen groups ascribed to the high pyrolysis temperature. Compared to NWCN-1, the samples (NWCN-4 and NWCN-7) prepared with the

ORCID

Jiacheng Wang: 0000-0003-4327-1508 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the National Key Research and Development Program of China (2016YFB0700204), Natural Science Foundation of Jiangsu Province (No. BK20140472), NSFC (51602332), Science and Technology Commission of Shanghai Municipality (15520720400, 15YF1413800, 14DZ2261203, 16DZ2260603), and One Hundred Talent Plan of Chinese Academy of Sciences.

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