N, S Codoped Hierarchical Porous Graphene Nanosheets Derived

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Materials and Interfaces

N, S co-doped hierarchical porous graphene nanosheets derived from petroleum asphalt via in-situ texturing strategy for high-performance supercapacitors Wang Yang, Bijian Deng, Liqiang Hou, Jingbo Tian, Yushu Tang, Shuo Wang, Fan Yang, and Yongfeng Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00222 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Industrial & Engineering Chemistry Research

N, S co-doped hierarchical porous graphene nanosheets derived from petroleum asphalt via in-situ texturing strategy for high-performance supercapacitors

Wang Yang#, Bijian Deng#, Liqiang Hou, Jingbo Tian, Yushu Tang, Shuo Wang, Fan Yang and Yongfeng Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Changping 102249, China *Corresponding

author: E-mail: [email protected]; Tel: +86-10-89739028.

ABSTRACT Facile yet rational strategy is highly desired for the synthesis of high value-added carbon materials from cheap petroleum asphalt. Here, we develop an in situ texturing strategy for synthesis of nitrogen and sulfur co-doped hierarchical porous graphene nanosheets (N, S-hPGNs) by using dual-functional graphitic carbon nitride templates. The synthetic strategy avoids using additional sulfur sources and cumbersome acid washing process. The well-developed pores and abundant heteroatoms endow the N, S-hPGNs with a high capacity of 302 F g–1 at 1 A g–1 and an excellent cycling stability. Moreover, the as-fabricated supercapacitors display a high energy density of 10.42 Wh kg–1 at the power density of 250 W kg–1, outperforming other previous carbon materials from petroleum asphalt. This work not only demonstrates the great 1 ACS Paragon Plus Environment

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potential of N, S-hPGNs for supercapacitors, but also provides a new avenue for the high value-added utilization of petroleum asphalt.

Keywords: petroleum asphalt; hierarchical porous graphene; heteroatoms; high value-added utilization; supercapacitor

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1. INTRODUCTION Petroleum asphalt, as a byproduct of vacuum distillation for crude oil, has been well known to everyone because of its low cost and abundant source. At present, the annual output of petroleum asphalt in China is over 25 million tons. Note that, in most instances, the petroleum asphalt has been directly used as paving materials for road construction or burned as fuel.1 In a manner of speaking, the petroleum asphalt is still faced to the low levels of utilization, which is a great waste of resources. Actually, petroleum asphalt contains plentiful polycyclic aromatic hydrocarbons, high carbon content and low ash content.2–3 Herein, it is reasonable to expect petroleum asphalt to be a promising raw carbon resource for manufacture of high value-added carbon materials through aromatization and carbonization processes under high temperature. Up to now, substantial efforts have been devoted to utilizing petroleum asphalt to synthesis a variety of carbon materials, such as carbon nanotube,4 porous carbon,5–8 graphene9–11 and so forth. Nevertheless, petroleum asphalt usually tends to confront with severe aggregation when subjected to direct pyrolysis. So, appropriate templates are essential during the synthesis process, which also exert crucial roles in determining the final structures of carbon materials. For instance, He et al. proposed a strategy for production of corrugated graphene nanosheets from petroleum asphalt with the assistance of sheet-like MgO templates.12 Lately, hierarchical porous carbon materials derived from petroleum asphalt have been synthesized by using SiO2 nanospheres as hard templates.13 These intensively employed template approaches have demonstrated the feasible of transformation petroleum asphalt into high-quality 3 ACS Paragon Plus Environment


carbon materials. However, it is striking to note that these deliberately introduced rigid templates have to be removed through the time-consuming acid washing process (e.g., HCl or even highly corrosive HF solution).7–9,11–14 This incurs inevitable environmental cost to the final product and also hinders the scale-up potential of these protocols. Therefore, it is still highly challenging but imperative to fulfill the facile fabrication of high value-added carbon materials from petroleum asphalt. In the meantime, to satisfy the demand for high-power systems, supercapacitors have attracted widespread attention due to their very high power density and long cycle lifetime.15–17 Ideal electrode materials for supercapacitors should possess high electrical conductivity, large specific surface area and long-term stability.18–21 Currently, carbon materials are well-recognized as the leading electrode materials for supercapacitors.22–27 Considering the fact that high-performance carbon materials can be yielded from petroleum asphalt, it is logical to anticipate that these obtained petroleum-asphalt-based carbon materials will greatly boost the development of supercapacitors. More importantly, if the original heteroatoms (such as S or N) of petroleum asphalt can be in-situ incorporated into the resultant carbon matrix, the related performance will be further enhanced to a great extent. In turn, this promising application can realize the high value-added utilization of petroleum asphalt as well. To date, there are several reports about using petroleum-asphalt-based carbon materials as electrodes for supercapacitors.12,13,28–30 For example, Wu et al. lately synthesized petroleum-asphalt-derived N-doped porous carbons and exhibited a capacitance of 277 F g–1 at 0.05 A g–1 in KOH aqueous electrolyte.31 Despite these 4 ACS Paragon Plus Environment

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tremendous achievements, a number of issues still need to be addressed, ranging from complexity of synthesis to controllable preparation of desirable structures. Herein, we have proposed a facile dual-functional g-C3N4 templating route for producing N, S co-doped hierarchical porous graphene nanosheets (N, S-hPGNs) from low-cost petroleum asphalt. Particularly, apart from as C sources, the petroleum asphalt also serves as S sources, which avoids using additional S sources. The accompanying evolved gases generated from the pyrolysis of g-C3N4 will induce a well-developed porous structure with ultrahigh pore volume, which facilitates the ion transport and electron storage. Besides, the in situ doping of N and S atoms into the carbon skeleton will improve the wettability and conductivity to a great extent, which effectively increases the ion-accessible surface area and reduces the charge transfer resistance. Consequently, the as-synthesized N, S-hPGNs electrodes exhibit a large specific capacitance (302 F g–1 at 1 A g–1 and even 192 F g–1 at 50 A g–1) and excellent cycling stability. Furthermore, the assembled supercapacitors deliver a high energy density (10.42 Wh kg–1) and power density (12.5 kW kg–1), demonstrating their great potentials for practical application. More profoundly, this strategy may unfold an exciting opportunity in high value-added utilizing of petroleum asphalt.



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2. EXPERIMENTAL SECTION 2.1 Synthesis of N, S co-doped hierarchical porous graphene nanosheets (N, S-hPGNs) The templates of g-C3N4 are firstly prepared according to the previously reported method.32,33 The petroleum asphalt is provided by PetroChina Co Ltd. (C: 82.1 wt.%, S: 5.0 wt.%, N: 0.8 wt.%, O: 5.5 wt.% and H: 6.6 wt.%). Typically, petroleum asphalt (0.25 g) and different amount of g-C3N4 templates (0.5, 1, 1.5 and 2 g) are mixed in the toluene solution. A homogenous mixture solution is obtained through ultrasound and stirring treatments at room temperature. Then the toluene solvent is fully removed through a rotary evaporating apparatus. It should be pointed out that the toluene can be

completely

collected

and

recycled.

Subsequently,

the

as-prepared

g-C3N4/petroleum asphalt is placed in a tube furnace and pyrolyzed at 850 °C for 1.5 h with a heating rate of 5 oC min–1 under flowing N2 atmosphere, and cooled naturally to room temperature. Finally, the fluffy black products are obtained and directly employed in subsequent characterization and electrochemical testing. 2.2 Material characterization The morphologies and microstructures of the as-prepared N, S-hPGNs samples are characterized by using field-emission scanning electron microscopy (SEM; FEI Quanta 200F) and transmission electron microscopy (TEM; FEI, F20) equipped with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) analyses are performed on a Thermo Fisher K-Alpha spectrometer. Raman spectra are collected via the confocal Raman microscope system with a green LED laser (λ = 532 6 ACS Paragon Plus Environment

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nm). The Brunauer-Emmett-Teller (BET) specific surface area is recorded through N2 adsorption/desorption isotherms at 77 K with a Micromeritics ASAP 2020 instrument. 2.3 Electrochemical characterization All electrochemical tests are performed by using the CHI760E electrochemical workstation (Chenhua Instrument, Shanghai, China) at room temperature. Moreover, a Pt foil is used as the counter electrode and a Hg/HgO electrode serves the reference electrode. To evaluate the supercapacitive performance of the N, S-hPGNs samples, a symmetric supercapacitor of coin-cell type is assembled. For preparation of working electrodes, 70 wt.% as-obtained products, 20 wt.% acetylene black and 10 wt.% PVDF binder are homogeneously mixed into a paste. Then the paste is casted into the nickel foam current collector with a diameter of 1.3 cm and dried in a vacuum oven at 80 oC for overnight. The average mass loading of the active materials is 1.0 mg cm–2. Finally, the dried electrodes/collectors are assembled in CR2032 coin-typed cell with the 6 M KOH aqueous solution and porous cellulose membrane as electrolyte and separator, respectively. The electrochemical performances of the samples are examined by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). Among them, EIS is performed from 10–2 to 105 Hz with amplitude of 10 mV. The gravimetric specific capacitance (Cs, F g–1) is calculated from galvanostatic charge/discharge curves using the following equations: Cs =

2 × Icons × ∆t m × ∆V



Where Icons (A) is the constant current in discharging process, ∆t (s) presents the discharge time, m (g) is the mass of active materials on a single electrode and ∆V (V) is the potential window during the discharge process (excluding the IR drop). The energy density E (Wh kg–1) is obtained by: E =

Cs × ΔV2 2 × 4 × 3.6

The power density P (W kg-1) is calculated by: P =

E × 3600 ∆t


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3. RESULTS & DISCUSSION The overall synthesis strategy for N, S-hPGNs is schematically illustrated in Figure 1. Specifically, the g-C3N4 is elaborately selected as a template due to its high nitrogen content and easily decomposed feature.34–36 Firstly, petroleum asphalt and g-C3N4 templates are well-mixed together in toluene. After removal of the solvent, the dry petroleum pitch/g-C3N4 powder is subjected to heating under N2 atmosphere. It is worth to mention that in this stage, the g-C3N4 templates will be completely pyrolyzed into various N-containing gases, which induces the in-situ pore engineering and N doping. Besides, the S atoms derived from the petroleum asphalt are synchronously incorporated into the carbon matrix. In this study, different N, S-hPGNs-x materials have been prepared to reveal the roles of g-C3N4 templates (x represents the g-C3N4/petroleum pitch mass ratio in the starting mixture). Representative scanning electron microscopy (SEM) images show that the resultant N, S-hPGNs-6 presents a honeycomb-like morphology feature, which is composed of interconnected wrinkled nanosheets (Figures 2a–b). Clearly, abundant pores can also be seen on the surface of overall frameworks. It is interesting to find that with the increase in g-C3N4/petroleum asphalt ratio, the obtained N, S-hPGNs samples exhibit a more fluffy and crumpled morphologies (Figure S1). Besides, the samples prepared without using g-C3N4 present a distinct morphology with large carbon blocks (Figure S2). These results verify that the g-C3N4 templates exert a critical effect on the final morphologies of N, S-hPGNs. The nanostructures are further characterized via transmission electron microscopy (TEM) images. Indeed, the N, S-hPGNs-6 exhibits a thin 2D geometry 9 ACS Paragon Plus Environment


with distinct crumpled or rippled structures (Figures 2c–d), which is well consistent with the SEM observations. This is maybe that large amounts of released gases and instantaneous high internal pressure will blow the carbon matrix to generate pore-rich and ultrathin graphene nanosheets. Note that such unique wrinkles will favor the infiltration of electrode materials and improve utilization of surfaces, which is beneficial for enhancing the electrochemical performance.37,38 Furthermore, a high-resolution TEM picture depicts well-graphitized graphene few layers with twisted morphologies and some broken fringes in the shells (Figure 2e). Meanwhile, the energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 2f) suggests the presence of C, N and S elements, which are homogenously distributed throughout the skeleton of the N, S-hPGNs-6. The Raman spectrum exhibits a D band at 1350 cm–1 and G band at 1585 cm–1, corresponding to the defective sp3 carbon and graphitized sp2 carbon, respectively (Figure 3a).39 It is found that the ID/IG ratio gradually increases from N, S-hPGNs-2 to N, S-hPGNs-8, indicates that the g-C3N4 template exerts a critical role in regulating the defects. Besides, the broad 2D peak to some extent confirms a relatively high degree of graphitization and the presence of multi-layer graphene layers.40,41 To further analyze the chemical states and compositions of the elements in N, S-hPGNs samples, X-ray photoelectron spectroscopy (XPS) has been employed. Indeed, due to the reaction with the N-containing gaseous species generated from the pyrolysis of g-C3N4, the N elements are distinctly observed in the survey spectra for all samples (Figure 3b). Surprisingly, apart from the C, O and N, the signal of S element has also 10 ACS Paragon Plus Environment

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been detected. Note that we have not used any additional S sources during the synthesis process. In other words, this approach can take full advantage of the characteristics of high S contents in petroleum asphalt to in situ generate S-doped carbon matrix. The detailed surface atomic contents are summarized in Table S1. It is found that the elemental content of N and S in N, S-hPGNs-6 is 6.07 at.% and 1.34 at.%, respectively. But further increasing the amount of g-C3N4 leads to an significant decrease in the contents of N (5.18 at.%) and S (0.51 at.%) for N, S-hPGNs-8. For all N, S-hPGNs samples, the N 1s spectrum can be fitted into three major peaks (Figure 3c): pyridinic-N (398.2 eV), pyrrolic-N (399.8 eV) and quaternary-N (401.2 eV).42,43 And the high-resolution S 2p spectrum (Figure 3d) shows two main peaks at 163.9 and 164.9 eV, being attributed to the S 2p3/2 and S 2p1/2 peaks for the covalent bond, while the minor peak located at 168.3 eV can be ascribed to the existence of C–SOx– C state.44,45 Taken together, these results suggest that the N and S atoms have been firmly incorporated into the carbon backbones. On the other hand, the N and S dual-doped strategy endows the N, S-hPGNs with outstanding wettability as demonstrated in Figure 4a. This will increase the carbon surface affinity to the electrolyte and ion accessible surface area.46,47 For EDLCs, the pore structures exert pivotal roles as well in determining the capacitance performance.38,48,49 Thus, N2 adsorption/desorption measurements have been conducted to analyze the surface area and porosity structure of the as-obtained N, S-hPGNs. As shown in Figure 4b, all the samples exhibit a typical IV isotherm, and the detailed parameters are summarized in Table S2. The crescent-like hysteresis 11 ACS Paragon Plus Environment


loop in the pressure range of 0.5–1 with no clear boundary between the sorption regions and no limiting adsorption at high P/P0 indicates a loose network and coexisting meso- and macropores. With the increasing of C3N4 mass, the specific surface areas increase from 415 (N, S-hPGNs-2) to 1093 m2 g–1 (N, S-hPGNs-6), and then decrease to 876 m2 g–1 (N, S-hPGNs-8). Likewise, the total pore volumes change from 1.87 (N, S-hPGNs-2) to 5.97 cm3 g–1 (N, S-hPGNs-6), and then reduce to 4.16 cm3 g–1 (N, S-hPGNs-8). This very high surface area and pore volume should be attributed to the amounts of gases released from the decomposition of g-C3N4. Furthermore, according to the Barrett-Joyner-Halenda (BJH) calculation results, the pore size distribution of N, S-hPGNs indeed ranges from mesoporous to macropores (Figure 4c). It is worth noting that such hierarchical porous channels especially ultrahigh pore volumes are significant for providing abundant sites for adsorbing ions and accelerating electron transfer, which can significantly enhance the performance of supercapacitors.19,25,50–51 Considering the aforementioned features of N, S-hPGNs, it is highly anticipated to employ them as high-performance electrodes materials for supercapacitors. Thus, the capacitive performances of N, S-hPGNs-x have been evaluated by two-electrode coin-type supercapacitors in 6 M KOH aqueous electrolyte for practical application. Figure 5a depicts the typical cyclic voltammetry (CV) curves of different N, S-hPGNs-x samples at a scan rate of 50 mV s-1. Apparently, all the four N, S-hPGNs-x samples manifest nearly symmetrical rectangular shapes, revealing ideal capacitive behavior with efficient separation of charges at the electrode/electrolyte 12 ACS Paragon Plus Environment

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interface. Although high N content can bring about pseudocapacitance, no distinct peak has been detected in CV curves for all samples, consistent with previous literatures.19,52 Moreover, the CV profile of N, S-hPGNs-6 presents the largest enclosed integral area, much better than the case for other counterparts, clarifying its excellent capacitive performance. Even at a high scan rate of 1000 mV s–1, the CV curve of N, S-hPGNs-6 still remains its basic quasi-rectangular shape without obvious distortion (Figure 5b), suggesting its low electrochemical internal series resistance and high rate capability. This should be attributed to the fast electrons transfer and ion diffusion channels within the interconnected porous graphene matrix. Galvanostatic charge/discharge (GCD) measurement has been further conducted to determine the specific capacitance. As displayed in Figure 5c and Figure S3, all the GCD curves at various current densities display symmetric and linear triangular slopes, confirming electrical double layer type storage mechanism. It is found that the voltage (IR) drop of N, S-hPGNs-6 is considerably small (0.003V) at 10 A g–1, which is much lower than those of other N, S-hPGNs samples (Figure S4). This signifies that the N, S-hPGNs-6 possesses a high power capability and low equivalent series resistance. Furthermore, Figure 5d describes the relationships between the specific capacitance and current density of different N, S-hPGNs-x samples. Obviously, among the four samples, N, S-hPGNs-6 exhibits the highest specific capacitance, which is well consistent with the CV analysis results. Specifically, the capacitance of N, S-hPGNs-6 is as high as 302 F g–1 at 1 A g–1 and slowly decays to 192 F g–1 at 50 A g–1, which is comparable to most of previously reported carbon materials (Table 13 ACS Paragon Plus Environment


S3). Meanwhile, as calculated, the capacitance retention reaches 62%, demonstrating its good rate capability. To get more insights into the electrochemical behavior for these N, S-hPGNs-x electrodes, electrochemical impedance spectroscopy (EIS) has been performed, as shown in Figure 5e. Similar shapes of Nyquist plots are obtained for all samples. Generally, the intercept at the real axis reflects the equivalent series resistance (RESR).53 It is seen that N, S-hPGNs-6 exhibits the smallest RESR of 0.48 Ω, in accordance with the minimum IR drop in GCD curve, indicating its high conductivity. Besides, the nearly vertical line at low-frequency range and miniature semicircle at high-frequency region of N, S-hPGNs-6 further demonstrate its fast ion diffusion and smaller charge-transfer resistance. Figure 5f elaborates the Ragone plots of symmetric supercapacitors assembled by N, S-hPGNs-6. It reveals that the energy density of N, S-hPGNs-6 electrode can reach 10.42 and 6.26 Wh kg–1 with the corresponding power density of 0.25 and 12.5 kW kg–1, respectively. Importantly, this overall performance in KOH aqueous electrolyte is comparable or even superior to those of recently reported carbon materials derived from petroleum asphalt (Figure 5f and Table S4). As another crucial factor for practical application of supercapacitors, electrochemical stability has also been evaluated using continuous GCD process at a current density of 10 A g–1. The N, S-hPGNs-6 electrodes deliver a high capacitance retention (97%) with nearly 100% coulombic efficiency even after 20000 cycles (Figure 6), highlighting its excellent cycle stability. Therefore, on one hand, these preceding results manifest that the N, S-hPGNs materials derived from petroleum asphalt indeed show great potential for 14 ACS Paragon Plus Environment

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supercapacitors. Such satisfying performances should be mainly ascribed to the following factors. First, the resultant N, S-hPGNs possess a hierarchical porous structure which combines the merits of large surface area, ultrahigh pore volume and abundant wrinkles. The synergism of these features provides convenient paths for ion transport and ensures sufficient space for charges storage. Besides, the high content of N and S heteroatoms endows the N, S-hPGNs with good wettability and high conductivity, which results in a large ion-accessible surface area and fast electrons movement. On the other hand, these results confirm that this adopted strategy is feasible and productive to realize the high value-added utilization of petroleum asphalt.



4. CONCLUSION In summary, we have developed a facile synthetic approach to in situ fabricate N, S co-doped hierarchical porous graphene nanosheets by using g-C3N4 as a template and N source, as well as petroleum asphalt as both C and S sources. The as-prepared N, S-hPGNs integrate the superiorities of large surface area, ultrahigh pore volume, abundant heteroatoms, excellent wettability, and conductivity. Owing to these desirable merits, the assembled supercapacitors based on N, S-hPGNs electrodes achieve a high capacitance of 302 F g–1 at 1 A g–1 and a high cyclic performance of 97% capacitance retention even after 20000 cycles. Meanwhile, the highest energy density of assembled supercapacitors can reach up to 10.42 Wh kg–1 and still maintains 6.26 Wh kg–1 at a high operating power density of 12.5 kW kg–1, which outperforms those of recently reported petroleum-asphalt-based carbon materials. Importantly, this strategy has completely avoided the polluting and time-consuming processes for post-synthetic template-removal. More significantly, this also may open up a new opportunity for scalable synthesis of high value-added carbon materials from abundant and cheap petroleum asphalt.


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Supporting information The Supporting Information is available free of charge on the ACS Publications website. Additional data and figures including BET surface area, pore volume, surface element concentration, electrochemical performance, comparisons, SEM images, CV, and GCD curves.

Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements W. Yang and B. J. Deng contributed equally to this work. We gratefully acknowledge the financial support from the China Postdoctoral Science Foundation (Nos. 2017M620084 and 2018T110187), Science Foundation of China University of Petroleum, Beijing (Nos. 2462018YJRC009, 2462017YJRC051, and C201603) and National Natural Science Foundation of China (Nos. 21776308 and 21576289).



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Figure captions Figure 1 Schematic illustration of N, S-hPGNs synthesis strategy. Stage I: preparation of a heterogeneous mixture of petroleum asphalt/g-C3N4. Stage II: synthesis of N, S-hPGNs via a heating process. Figure 2 Morphological and structural characterizations of the N, S-hPGNs-6 sample. The SEM (a,b) and TEM images (c,d) of N, S-hPGNs-6. e) A high-resolution TEM image of N, S-hPGNs-6 demonstrates its few-layer feature, the red arrows present the twisted and broken layers. (f) EDX element mapping images of C, N, and S respectively. Figure 3 (a) Raman spectra, (b) XPS survey scan, the high-resolution N 1s (c) and S 2p (d) XPS spectra of different N, S-hPGNs-x samples. Figure 4 (a) Dynamic water contact angle measurements for N, S-hPGNs-x samples. The photograph at 0 s is taken immediately after resting the water droplet on the N, S-hPGNs-x surface. (b) N2 adsorption-desorption isotherm and (c) pore-size distribution curves of different N, S-hPGNs-x samples. Figure 5 Electrochemical performances of N, S-hPGNs-x samples measured with a two-electrode system in 6 M KOH electrolyte. (a) CV curves of N, S-hPGNs-x at a scan rate 50 mV s–1. (b) CV curves of N, S-hPGNs-6 at different scan rates. (c) GCD curves of N, S-hPGNs-6 at various current densities from 1 to 50 A g–1. (d) Gravimetric capacitances versus current density. (e) Nyquist plots, Inset in (e) is the magnified data of the high-frequency region. (f) Ragone plots of the N, S-hPGNs-6. 26 ACS Paragon Plus Environment

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Figure 6 Cycling stability and Coulombic efficiency of the N, S-hPGNs-6 at 10 A g–1.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



For Table of Contents Use Only

High value-added N, S co-doped hierarchical porous graphene nanosheets are derived from low-cost petroleum asphalt through a facile dual-functional templating strategy, and exhibit great potentials as electrodes for supercapacitors.


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N, S Codoped Hierarchical Porous Graphene Nanosheets Derived

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