Sulfur-Codoped

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Article Cite This: ACS Omega 2019, 4, 5904−5914

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Ammonium Nitrate-Assisted Synthesis of Nitrogen/Sulfur-Codoped Hierarchically Porous Carbons Derived from Ginkgo Leaf for Supercapacitors Leizhi Zheng,†,§ Shuang Wang,‡,§ Yanfang Yang,† Xiaoqi Fu,*,† Tingshun Jiang,† and Juan Yang‡ †

School of Chemistry and Chemical Engineering and ‡School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China

ACS Omega 2019.4:5904-5914. Downloaded from pubs.acs.org by 193.9.158.151 on 03/28/19. For personal use only.

S Supporting Information *

ABSTRACT: Biomass-derived carbons for supercapacitors provide a promising and sustainable strategy to address the worldwide energy and climate change challenges. Here, we have designed and constructed three-dimensional nitrogen/ sulfur-codoped hierarchically porous carbons for highperformance supercapacitor electrode materials in a one-step process, in which ginkgo leaf is used as a carbon source and S source and ammonium nitrate (AN) is used as an activating agent and a N source. During the synchronous carbonization and activation process, AN could be decomposed completely into gaseous byproducts and be removed easily without leaving residues after product formation. The as-synthesized ginkgo leaf-derived carbons exhibited a high specific capacitance of 330.5 F g−1 at a current density of 0.5 A g−1 and a capacitance of 252 F g−1 even at a high current density of 10 A g−1 with an excellent capacitance retention of 85.8% after 10 000 cycles in 6 mol L−1 KOH electrolyte. The present study provides an efficient, sustainable, and facile approach to prepare renewable hierarchically porous carbons as advanced electrode materials for energy storage and conversion.

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INTRODUCTION Biomass-derived carbons, referred to as highly aromatic and carbon-rich porous materials, can be synthesized through carbonization and activation of natural biomass1−3 and have been greatly concerned in energy storage and conversion area because of their large surface area, high porosity, excellent thermal and chemical stability, low cost, and high energy and power density in electric devices.4−6 During the carbonization process, biomass undergoes a series of complex chemical processes, including chemical bond cleavage, isomerization, polymerization, and heteroatom removal, resulting in threedimensional (3D) skeletal networks of porous carbons.7 The porous structures and large surface areas of biomass-derived carbons make them promising for energy storage, especially for supercapacitors. Supercapacitors can be generally classified into two types based on the charge-storage mechanism.8−10 One is the pseudocapacitor whose capacitances come from the reversible faradic reactions occurring at the electrode surface. The other is the electrical double-layer capacitor (EDLC), whose capacitances arise from the rapid accumulation of ionic charges at electrode/electrolyte interface. Typically, carbon materials mainly exhibit EDLC characteristics because of their inertness, high electrical conductivity, large surface area, and high porosity.2,7 The large surface areas and adapted porous structures facilitate the rapid diffusion of electrolyte ions to the inner suface.3,11 It is evidenced that micropores can © 2019 American Chemical Society

increase the specific surface area and provide large specific capacitances but hinder the diffusion of electrolyte ions, increasing the diffusion resistance, whereas mesopores and macropores can provide broad ion transport pathways, reducing the diffusion resistance, promoting the electrical double layer formation, and improving the rate capacitance performance.12−15 Therefore, hierarchically porous carbons (HPCs) with interconnected micropores, mesopores, and macropores could provide both large surface areas and fast channels for electrolyte ion transport, preferred for highperformance EDLCs. In addition to carbonization, activation also facilitates generating large amounts of nanopores, especially micropores and mesopores.3,16 Activation refers to a process of converting inactive or less active carbonaceous materials or carbon skeletons into activated carbons, which was often conducted simultaneously or after the carbonization. To activate biomassderived carbons, two main methods, physical and chemical activation, can be used.3 The physical activation, using activating agents such as steam,17,18 CO2,19,20 and air,21,22 is a simple and green synthetic strategy, which does not require postsynthesis washing to remove any impurities and byproducts generated during the activation process, but often Received: December 21, 2018 Accepted: March 18, 2019 Published: March 27, 2019 5904

DOI: 10.1021/acsomega.8b03586 ACS Omega 2019, 4, 5904−5914

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Figure 1. Scheme illustrating the synthesis of ginkgo-leaf-derived hierarchically porous carbon.

Figure 2. (A−C) Scanning electron microscopy (SEM) images, at different magnifications, of the as-synthesized NSHPC1.5. (D, E) TEM and (F) HRTEM images showing the structures of the NSHPC1.5. (G) PXRD patterns of the NSHPC0 and NSHPC1.5 at a lower angle. For clarity, the intensity of PXRD patterns of NSHPC1.5 was divided by 10. (H) EDS spectrum of the NSHPC1.5. The Au signals in the EDS originated from the 2 nm thick Au film sputter-coated on the samples for SEM imaging. The inset shows the weight percentages of C, N, O, and S in the NSHPC1.5. (I) SEM image and spatial distributions of C, N, O, and S elements mapped by EDS.

specific surface area of 2,494 m2 g−1 and a pore volume of 2.28 cm3 g−1, exhibiting a capacitance of 242 F g−1 and energy density of 102 Wh kg−1. Compared with physical activation, chemical activation tends to achieve higher specific surface and pore volume but brings burden in postsynthesis, which often requires postactivation washing or acid washing to remove the excess activating reagent or byproducts yielded during the activation process. Some other techniques have also been employed to prepare HPCs such as the hard and soft template method,28,29 still requiring removal of the templating agents in the postsynthesis process. Herein, ammonium nitrate (AN) activation is performed combined with pyrolysis of ginkgo leaf (GL) to prepare a 3D hierarchically porous carbon in a one-step process. As we know, AN is a strong oxidizer, could decompose into gaseous

needs high temperature. In comparison, the chemical activation of porous carbons is more commonly used, due to its lower activation temperature and well-developed and interconnected hierarchical pores.1 KOH,1,16,23,24 ZnCl2,12,25 and H3PO426 are the most commonly used chemical activating agents. Zhang et al.27 prepared a 3D hierarchically porous carbon through pyrolysis of garlic skin followed by KOH activation. The obtained garlic skin-derived carbon has a high specific surface area of 2,818 m2 g−1 and a pore volume of 1.327 cm3 g−1. As an electrode for a supercapacitor, the obtained carbon displayed excellent specific capacitances of 427 and 315 F g−1 at current densities of 0.5 and 50 A g−1, respectively. Hou et al.12 used natural silk to prepare hierarchically porous nitrogen-doped carbon nanosheets via ZnCl2 activation. The obtained carbon nanosheets have a 5905


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Figure 3. Weight percentages of heteroatoms of N and S doped in the NSHPCs synthesized at (A) various AN/GL mass ratios at a constant carbonization temperature of 700 °C and at (B) various carbonization temperatures at a constant mass ratio of 1.5, quantified by EDS. Intensity ratios of the G to D bands (IG/ID) were evaluated from Raman spectra of the NSHPCs synthesized at (C) various AN/GL mass ratios and (D) various carbonization temperatures.

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RESULTS AND DISCUSSION The key steps involved in the preparation of NSHPCs are schematically illustrated in Figure 1. GLs were chosen as a carbon source and thermally carbonized to produce HPCs because of their wide range of biomass sources, low cost, easy processability, and high carbon yield and because they are rich in N, S heteroatoms and suitable water conveyance channels. The GLs underwent pulverization, activation, and carbonization processes, resulting in a 3D hierarchically porous architecture. AN was used as the activating agent to generate large amounts of pores on carbons during the carbonization process. The representative synthesized NSHPC1.5 prepared at a carbonization temperature of 700 °C with AN/GL mass ratio of 1.5 underwent three major weight losses: 45.8% starts from 160 to 203 °C, 22.3% from 203 to 234 °C, and 13.2% from 234 to 800 °C (Figure S1, Supporting Information). An exothermic peak centered at 200 °C corresponds to partial oxidation of the biomass carbon by AN and its decomposition product of HNO3(g). Another exothermic peak centered at 222 °C could be ascribed to deep decomposition of AN and further reactions of the biomass carbons with gaseous products such as NO2 and NH3.30,38 During these reactions, massive gaseous byproducts like NO2, N2O, N2, CO2, and H2O gases were generated. The sudden massive released gases converted the as-synthesized NSHPC bush to a graphene-like layered carbon material (Figure 2A), which has well-constructed interconnected open macropores (Figure 2B) and a few water conveyance holes (Figure 2C), while majority of pores are water conveyance holes in the GL-derived carbons carbonized without AN (Figure S2). Furthermore, large amounts of nanopores were observed on the NSHPC1.5 carbon sheets (Figure 2D,E). It was found that some inorganic salts contained in plant leaves, mainly CaCO3, MgCO3, CaSO4, and MgSO4, could generate large amounts of small pores

phase of HNO3(g) and NH3(g) as long as it was heated up to 110 °C,30 and is expected to etch the GL-derived carbons through a redox reaction to generate large amounts of pores. The massive gaseous byproducts such as CO2, H2O, NO2, N2O, and N2,30,31 from the redox reactions, could further promote the formation of porous structures. The NH3(g) is an efficient N dopant for carbon materials,32 which could enhance conductivity of the carbon electrode, increase specific capacitance through pseudocapacitance, and improve wettability of the electrode.33 Moreover, AN decomposes completely into gaseous products at temperatures higher than 220 °C and can be removed easily without leaving residues after product formation. In autumn, GLs, the carbon source, wither and fall off the ginkgo trees, which are largely cultivated in China, France, and South Carolina, being waste on the ground. The dry GL is composed of about 45.1% carbon, 34.0% oxygen, 1.2% nitrogen, 0.7% sulfur, 5.5% hydrogen, and 13.4% ash.34,35 The relatively higher content of N and S compared to that in other biomass such as coconut sheath36 and corn stover37 indicates that the GL-derived porous carbons can be self-doped with N and S heteroatoms. It is found that the incorporation of N and S increases electrical conductivity, electrocatalytic activity, and wettability of the electrode and contributes to pseudocapacitances.38−41 In this work, we demonstrated a one-step process for the synthesis of N/S-codoped hierarchically porous carbons (NSHPCs) with GL and AN as a carbon source and an activating agent, respectively. The as-synthesized NSHPCs with 3D interconnected hierarchically porous structures displayed a large specific surface area and excellent capacitance performance with high energy and power density. 5906


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Figure 4. (A) XPS survey and high-resolution spectra of (B) C 1s, (C) N 1s, and (D) S 2p of the NSHPC1.5.

during the acid washing process.27 We examined whether these hierarchical pores were induced by AN, acid washing, or both. Considerable numbers of nanopores were still observed in the non-acid-washed NSHPC1.5 sample (Figure S3), which means AN can not only create macropores but also create small nanopores. The AN used here as the activating agent may perform the following main reactions NH4NO3(S) → HNO3(g) + NH3(g) (adsorbed state)

(1)

NH4NO3 → N2O + 2H 2O(g)

(2)

2NH4NO3 + C → CO2 + 2N2 + 4H 2O(g)

(3)

4HNO3(g) + C → 4NO2 + CO2 + 2H 2O(g)

(4)

2NO2 + C → 2NO + CO2

(5)

(Figure 2F), indexed to the amorphous graphitic carbon layers with a spacing of 3.6 Å, were clearly resolved. Powder X-ray diffraction (PXRD) patterns of the NSHPC1.5 showed a broad graphitic stacking peak at 24.9° and a weak peak at 43.6° (Figure S4). In addition, the as-obtained NSHPC1.5 showed a much sharper diffraction peak at a lower angle compared with the NSHPC0 (Figure 2G), indicating that more regular and fluffy layer structures were formed when GL was carbonized with AN. As quantified by energy dispersive spectroscopy (EDS) in Figure 2H, the weight % values of C, N, O, and S in the NSHPC1.5 were 72.64, 11.28, 10.31, and 5.77%, respectively. Correlated EDS mapping showed that C, N, O, and S elements were clearly distributed on the NSHPC1.5 (Figure 2I), confirming that the as-synthesized carbon materials were successfully doped with N and S. Because GL is rich in N and S elements, the GL-derived HPCs can self-dope with N and S elements, as can be seen from Figure 3A that 4.56% weight of N and 6.43% weight of S were doped in the NSHPC0 carbonized without AN. In Figure 3A, the weight % of the N element in the NSHPCs carbonized at 700 °C gradually increased from 4.56 to 13.06% with the mass ratio of AN/GL raised from 0 to 2, indicating that AN could be used as a N-doping agent to produce N-doped carbons. It should be noted that the higher the amount of the AN added, the lower the yields of the NSHPCs (Table S1 in the Supporting Information). The loss of NSHPCs could be ascribed to the consistent redox reaction of the GL-derived carbons with AN. The weight % of the S element displayed a slight drop as the content of AN increased, mainly due to the increasing content of N. Furthermore, the weight % of the N element of NSHPC1.5 decreased from 16.38 to 8.69% as the carbonization temperature increased from 500 to 800 °C (Figure 3B), due to removal of foreign atoms at high temperature. It should be noted that a higher carbonization temperature would cause lower yields of the NSHPCs (Table

As the carbonization temperature increased, AN gradually melted and partially decomposed into HNO3(g), NH3(g), and N2O (1, 2) and the oxidation (3, 4) of the biomass carbons by AN and HNO3(g) occurred simultaneously. The redox reactions between biomass carbons and AN/HNO3(g) etched the carbon skeletons and generated large amounts of nanopores. The massive gaseous byproducts, such as NO2, N2O, N2, CO2, and H2O gases, swelled the biomass carbons to graphene sheetlike carbon structures and further promoted the formation of porous structures. As we know, the micropores can increase the specific surface area and provide active sites to enhance the specific capacitance, while mesopores and macropores can provide pathways for high-rate transport of electrolyte ions, facilitate a decrease in diffusion resistance, and subsequently improve the electrochemical performance.12−14,42 More detailed microstructure information on the representative NSHPC1.5 was obtained from high-resolution transmission electron microscopy (HRTEM) images. The massive nanopores (Figure 2E) and (002) characteristic lattice fringes 5907


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Figure 5. (A) Nitrogen adsorption/desorption isotherms and (B) pore size distribution curves of the NSHPCs. The inset shows magnified pore size distribution. Comparison of the corresponding specific surface area and average pore size of the NSHPCs synthesized at (C) various AN/GL mass ratios and (D) various carbonization temperatures.

S1), perhaps due to the digestion of carbon skeleton at high temperature. Raman spectroscopy is a powerful technique to provide detailed structural information of the carbon skeleton, especially considering the fact that conjugated and double carbon−carbon bonds lead to high Raman intensities.43−45 The Raman spectra of all NSHPC samples showed two wellknown bands: The G-band located at around 1580 cm−1 is a result of sp2-graphitized carbon, and the D-band at around 1350 cm−1 is associated with the disordered carbon induced by structural defects and impurities.45 The intensity ratio of the G-band to D-band (IG/ID) represents the degree of structural order, as well as the graphitization level.43,44 As seen from Figure 3C,D, the IG/ID values of NPHPC0, NSHPC1, NSHPC 1.5 , NSHPC 2 , NSHPC-500, NSHPC-600, and NSHPC-800 were 1.31, 1.03, 0.91, 0.77, 0.76, 0.84, and 0.92, respectively. The IG/ID value gradually decreased with the increasing amount of AN added at constant temperature, due to the heteroatom N doping. Furthermore, the higher carbonization temperature led to a higher IG/ID value, which means higher structural alignment. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the surface feature and chemical composition of the NSHPCs. As shown in Figure 4A, XPS survey spectra of the NSHPC1.5 displayed four peaks comprising C 1s, O 1s, N 1s, and S 2p, indicating the presence of N and S dopants with the content of 9.91 and 2.66%, consistent with the EDS results. The high-resolution C 1s spectrum showed a broadened peak with a small tail that can be deconvoluted into five individual subpeaks of carbon components (Figure 4B), with the binding energies centered at 283.8, 284.4, 285.5, 286.7, and 289.0 eV corresponding to C−S−C, sp2-hybridized CC, sp3-hybridized C−C, C−N, and CO, respectively.46 In the N 1s spectra, four subpeaks of N species were clearly resolved (Figure 4C), corresponding to pyridinic-N (398.0 eV), pyrrolic-N (399.6 eV), graphitic-N (400.2 eV), and oxidizedN (405.9 eV).29,47,48 Among them, the pyridinic-N and pyrrolic-N possess a desirable electron-donating feature and high charge transfer, which are propitious to improve electrochemical performance, while the graphitic-N has a significant effect on accelerating electron transfer and enhancing electrical conductivity of carbon materials. The oxidized-N addressed at the defect of the graphite lattice center improves specific conductance as well. The S 2p spectrum could be deconvoluted into three individual subpeaks

associated with C−S−C 2p3/2 at 163.9 eV, C−S−C 2p1/2 at 165.1 eV, and C−SOx−C at 168.3 eV (Figure 4D). The S species could contribute to causing changes in the electronic density of state and arousing pseudocapacitances.29 The doping of heteroatoms (N, S, and O) into the NSHPCs could improve their wettability and, more importantly, could tailor electrical properties for enabling high-performance supercapacitors. Besides the electrical conductivity and wettability, for EDLCs, the specific surface area (SBET) and corresponding pore size distribution (PSD) also play important roles in electrical capacitance. We performed N2 sorption isotherms on all NSHPCs to characterize their surface properties. As seen from Figure 5A, all NSHPCs displayed a hybrid type I/IV isotherm profile,49,50 a sharp rise at low pressure (P/P0 < 0.01) contributed by the microporous structure, and an H3 hysteresis loop at a high relative pressure (P/P0 > 0.4) arisen from mesoporous and macroporous structures, indicating hierarchically porous structures in the as-synthesized NSHPCs. The pore volume (Vpore) of NSHPCs increased obviously after AN activation, exhibiting more considerable porosity and higher SBET. As the AN/GL mass ratio increased from 0 to 2 at a constant temperature of 700 °C, SBET increased up to 15.66 times from 47.9 to 672.2 m2 g−1, while Vpore increased up to 13 times from 0.04 to 0.52 cm3 g−1. The detailed porous properties of the NSHPCs are listed in Table S1. As the activation temperature increased from 500 to 800 °C at the AN/GL mass ratio of 1.5, the SBET and Vpore of NSHPCs increased from 127.5 and 0.14 to 503.6 m2 g−1 and 0.41 cm3 g−1, respectively. However, both the SBET and Vpore reduced dramatically when the activation temperature became 800 °C, probably due to the digestion of carbon skeleton and the collapse of pores at high carbonization temperature. The PSD is presented in Figure 5B. The cumulated volume is separated by a vertical dashed line at 2 nm, left of which denotes micropores (50 nm). The dV/dD numerical values of NSHPCs are greater than zero, ranging from 1 to 100 nm, indicating that the coexistence of micropores, mesopores, and macropores formed hierarchical porous structures in the NSHPCs. Notably, the mesopores accounted for more percentage than micropores and macropores in the ANactivated NSHPCs. The AN activation created significantly large amount of mesopores, which increased with the AN/GL mass ratio (Figure 5C) and the activation temperature, except 5908


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Figure 6. Electrochemical performances measured in a three-electrode system in the 6 mol L−1 KOH electrolyte. (A) Cyclic voltammograms (CV) of the as-synthesized NSHPCs at 10 mV s−1. Comparison of the corresponding specific capacitance calculated from CV curves at (B) various AN/ GL mass ratios and (C) various carbonization temperatures. (D) Galvanostatic charge/discharge (GCD) curves of the NSHPC2 at various current densities. (E) Comparison of the corresponding discharge capacitance calculated from GCD curves. (F) Cycling performance of the NSHPC2 at a current density of 10 A g−1 for 10 000 cycles. The inset shows the GCD cycles. (G) Nyquist plots of the NSHPCs prepared at various AN/GL mass ratios.

tended to slow down as the AN/GL mass ratio increased from 0 to 2, whereas the NSHPC yield acceleratingly decreased (Table S1 in the Supporting Information). We tried the reaction with the AN/GL mass ratio of 3 and found a big drop in NSHPC yield to 12.1%. As seen from the thermogravimetric/differential scanning calorimetry (TG/DSC) curves in Figure S1, the decomposition of AN released large amounts of heat in the carbonization and activation process, probably causing the digestion of carbon skeleton and the low NSHPC yield. Considering the input−output ratio, the reactions were conducted with the AN/GL mass ratio no more than 2. In addition, the increasing calcination temperature is beneficial for improving the specific capacitance, as can be seen in Figure 6C, which is due to the enhanced graphitization and conductivity of the NSHPCs. However, the capacitance reduced significantly to 191 F g−1 when the activation temperature increased to 800 °C, due to the reduction of surface area and graphitization level arisen from the digestion of carbon skeleton and the collapse of pores at high carbonization temperature. CV profiles of the AN-activated NSHPCs (Figure S5) at various scan rates maintained the rectangular shape without obvious transformation even at a very high scan rate of 500 mV s−1, indicating rapid ion mobility and excellent frequency response, which can be ascribed to the large amount of mesopores that promoted electrolyte ion diffusion. Galvanostatic charge/discharge (GCD) measurements were performed at various current densities to further reveal the capacitance performance of the NSHPCs. All GCD curves of the obtained NSHPCs in Figures 6D and S6 displayed typical symmetrical triangular shapes at current densities from 0.5 to 10 A g−1, demonstrating good reversibility for charge/

for NSHPC-800 due to the digestion of carbon skeleton and the collapse of pores at high carbonization temperature (Figure 5D). The large SBET and hierarchically porous structure are in favor of promoting capacitor performance.51 Based on the high specific surface area and hierarchically porous characteristics, it is expected that NSHPCs would be a promising electrode material for high-performance supercapacitors. Their capacitive performances were characterized by cyclic voltammetry (CV) in a three-electrode system in 6 mol L−1 KOH solution at room temperature. Figure 6A shows typical CV curves of NSHPCs treated at various AN/GL mass ratios and various activation temperatures measured at a scan rate of 10 mV s−1 with a potential window between −1 and 0 V (versus saturated calomel electrode (SCE)). The CV profiles of all samples show approximately rectangular peaks, suggesting that EDLC capacitance is mainly involved in the as-synthesized NSHPCs. Small redox humps were also observed in these CV curves during the charge/discharge process, indicating partial contribution from pseudocapacitance, due to the redox reaction of the doped heteroatoms (N, S, and O).51,52 As seen from Figure 6A,B, the NSHPC2 exhibited the largest area of the CV curve and the highest specific capacitance (277.5 F g−1 calculated from the CV curve). The specific capacitance increased with the AN/GL mass ratio, revealing that AN activation significantly improved the capacitance. This improvement is mainly contributed by the gradually increased surface area and the number of mesopores and micropores, which agrees well with the Brunauer−Emmett−Teller (BET) results in Figure 5, partly contributed by the pseudocapacitance arisen from the redox reaction of heteroatoms, especially N. It should be noted that the increased rate of specific capacitance and surface area 5909


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Figure 7. Electrochemical performances measured in a two-electrode system in the poly(vinyl alcohol)−KOH gel electrolyte. (A) CV of the NSHPC2-based symmetrical device at various scan rates. (B) GCD curves of the NSHPC2 device at various current densities. (C) Ragone plot of the NSHPC2 device. (D) Cycling performance of the NSHPC2 device at a current density of 2 A g−1 for 5000 cycles.

the NSHPC2 as a function of charge/discharge cycle number, n, from continuous GCD curves. The specific capacitance decreased gradually from 252 to 218 F g−1 in the initial 1250 cycles and then showed better stability, and about 85.8% of the capacitance was preserved even after 10 000 cycles. The ANactivated NSHPC2 exhibited excellent specific capacitance performance with a long cycle life. To further understand the electrochemical behaviors of the NSHPCs, electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency range from 100 kHz to 10 mHz. All Nyquist plots of the NSHPCs prepared at various AN/GL mass ratios (Figure 6G) and various calcination temperatures (Figure S7) exhibit a small semicircle in the high-frequency region and a straight line in the low-frequency region. As we know, the intersection of the Nyquist plot with a real axis in the high-frequency region represents the internal resistance of the system (RS), while the radius of the semicircle corresponds to the charge-transfer resistance (RCT) and the slope of the straight line represents the Warburg impedance (Zw). The small RS values of NSHPC1, NSHPC1.5(700), NSHPC2, NSHPC-500, NSHPC-600, and NSHPC-800 are 0.719, 0.712, 0.673, 0.690, 0.735, and 0.692 Ω, respectively, indicating good conductivities of the NSHPCs in the electrolyte. Their RCT values are 0.245, 0.141, 0.111, 0.435, 0.153, and 0.239 Ω, respectively, revealing that they have low charge-transfer resistances between the NSHPCs and the electrolyte interface. Almost vertical lines are observed in the low-frequency region, indicating the typical double-layer capacitance behavior with low diffusive resistances. A shortlength line with a slope of 45° in the medium-frequency region is also observed for all of the AN-activated NSHPCs, named Warburg resistance, which is the typical diffusive resistance of electrolyte ions to porous electrode materials. The short length of the Warburg slope indicates the low resistance of the ions transport to the NSHPCs. The NSHPC2 displayed the smallest RCT, indicating that the charge/ion is efficiently transported,

discharge processes as typical EDLCs. It should be noted that some imperfect symmetries were observed from 0 to −0.3 V, mainly due to the existence of Faraday redox electrochemical reactions of the N and S groups in the NSHPCs,29,53 in agreement with the CV curves. Severe deformations were observed in the GCD curves of NSHPC0 (Figure S6A), due to small SBET with a few mesopores, promoting ion diffusion during the charge/discharge processes. The specific capacitances of the NSHPCs were calculated by GCD curves at various current densities ranging from 0.5 to 10 A g−1 (Figure 6E). At a current density of 0.5 A g−1, the NSHPC2 exhibited the largest specific capacitance of 330.5 F g−1, much higher than that of the non-AN-activated NSHPC0 with a capacitance of 48.8 F g−1, indicating that AN activation manifestly promoted the capacitance performance, which not only increased the SBET but also introduced large amounts of hierarchical pores. The high content of AN and high activation temperature are propitious to improve the specific capacitance, in good agreement with the results of CV curves. NSHPC-800 is an exception, which showed a decline in the specific capacitance due to the reduction of SBET and number of hierarchical pores arisen from the digestion of carbon skeleton and the collapse of pores at high carbonization temperature. Figure 6E shows that the specific capacitance decreased gradually with the increase of current density, probably due to the fact that electrolyte ions did not have sufficient time to diffuse to the inner pore interface at high current density in the rapid charge/discharge process.54 The decrease gradually slowed down since the current density was greater than 2 A g−1. The capacitance of NSHPC2 still maintained at 252 F g−1 even at 10 A g−1, demonstrating the excellent capacitance. The NSHPC2 displayed a high specific capacitance compared with the reported carbon-based supercapacitors (Table S2). Longterm cycling stability of the NSHPC2 was further investigated using a continuous GCD measurement at a current density of 10 A g−1. Figure 6F shows the special capacitance retention of 5910


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(KOH, 95%), potassium chloride (KCl, 99.8%), and methanol were purchased from Sinopharm Chemical Reagent Corporation (Shanghai, China). All reagents were used as received without further purification. Ultrapure water used for all experiments was distilled through a Millipore Synergy water purification system with a resistivity of 18.2 MΩ. Synthesis of N/S-Codoped Hierarchically Porous Carbons (NSHPCs). NSHPCs were synthesized by pyrolysis of GLs with AN as the activator. Briefly, fresh GLs were rinsed with distilled water, dried at 80 °C for 36 h, ground to powder, and then sieved through 200 μm mesh. The as-obtained (1 g) leaf powder was impregnated into 10 mL of aqueous solution of AN (1.5 g) under sonication for 6 h, followed by drying at 70 °C until a paste texture was obtained. The as-obtained paste was put in a tube furnace for carbonization at 700 °C for 1 h with a heating ramp rate of 3 °C min−1 under nitrogen flow. After cooling to room temperature, the black NSHPC powder was washed with 1 mol L−1 HCl to remove foreign ions and then washed with water until the filtrate reached a neutral pH and subsequently dried at 80 °C. For clarity, the NSHPCs activated at various temperatures of 500, 600, 700, and 800 °C are denoted NSHPC-500, NSHPC-600, NSHPC-700, and NSHPC-800, respectively. The NSHPC 0 , NSHPC 1 , NSHPC1.5, and NSHPC2 represent the NSHPCs synthesized by the reactants AN and GL at mass ratios of 0, 1, 1.5, and 2, respectively, at a constant carbonization temperature of 700 °C. Material Characterizations. Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) elemental distributions were obtained using a JEOL JSM7001F field emission scanning electron microscope. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Powder X-ray diffraction (PXRD) patterns were recorded using a Bruker D8 advanced X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm). X-ray photoelectron spectra (XPS) were acquired using a Thermo ESCALAB250 X-ray photoelectron spectrometer with a monochromatized Al Kα line source (hν = 1486.6 eV). Thermogravimetric/differential scanning calorimetry (TG/DSC) analysis was performed using a Netzsch STA 449F3 TGA instrument at a heating rate of 10 °C min−1 in a N2 atmosphere. Raman spectra were collected using a Renishaw inVia confocal Raman microscope equipped with a diode laser beam (532 nm, 1 mW) with an acquisition time of 10 s. The specific surface area and pore volume were analyzed using a Micromeritics ASAP 2020 surface area and porosity analyzer and calculated via the Brunauer−Emmett−Teller (BET) method and the Barrett−Joyner−Halenda model, respectively. Electrochemical Measurement. All electrochemical measurements were carried out using a CHI 660E workstation (CH Instruments, China) at room temperature. For the threeelectrode system, a saturated calomel electrode (SCE) served as the reference electrode, a platinum foil served as the counter electrode, a nickel foam compressed with a homogeneous slurry of the as-obtained carbon materials (80 wt %), poly(tetrafluoroethylene) (PTFE, 10 wt %), and acetylene black (10 wt %) served as the working electrode, and a 6 mol L−1 KOH solution was used as the electrolyte. The homogeneous slurry was painted onto a piece of 1 cm × 4 cm nickel foam and vacuum-dried at 80 °C for 24 h. Subsequently, the nickel foam with slurry was pressed under a

exhibiting excellent electrochemical performance, benefiting from the hierarchical pores and large heteroatom-doped surface area. To evaluate the electrochemical capacitive practical performances, a symmetrical two-electrode system was assembled with a polypropylene membrane separating the two same nickel foam electrodes painted with the NSHPC2 and poly(vinyl alcohol) (PVA)−KOH gel55 was used as a semisolid polymer electrolyte. The electrochemical performance of the assembled symmetrical NSHPC2-based device was further examined by CV and GCD tests at various scan rates from 0.5 to 200 mV s−1 and current densities from 0.5 to 10 A g−1 within an operation voltage window of 0−1.6 V. The relatively rectangular CV profiles at various scan rates (Figure 7A) and the symmetrical triangular GCD profile at various current densities (Figure 7B) without obvious transformation indicate an ideal electric double-layer capacitor behavior. The Ragone plot for the NSHPC2 device in Figure 7C shows the relation between energy density and power density, which was calculated on the basis of eqs 8 and 9, showing a specific energy density of 43.6 Wh kg−1 at a power density of 400 W kg−1 and remaining 14.2 Wh kg−1 at a high-power density of 8000 W kg−1, demonstrating the desired electrode material for a supercapacitor. The specific capacitance retention of the NSHPC2 device at a current density of 2 A g−1 is shown to be 79% after 5000 circulations in Figure 7D, exhibiting excellent capacitance stability in practical applications.

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CONCLUSIONS In summary, we have demonstrated a facile and sustainable strategy to fabricate 3D N/S-codoped hierarchically porous carbons for high-performance supercapacitors in a one-step synthesis using ginkgo leaf as a carbon source and S source and ammonium nitrate (AN) as an activating agent and a N source. The use of AN generated large amounts of mesopores and micropores through chemical etching of the carbon skeletons by redox reactions and released massive gaseous byproducts, which further promoted the formation of hierarchical pores. The hierarchically porous structure greatly increased the specific surface area and promoted charge transport and rate performance. Consequently, the as-synthesized NSHPC2 possessed a high specific surface area of 672.2 m2 g−1, exhibited a high specific capacitance of 330.5 F g−1 at a current density of 0.5 A g−1, and a capacitance of 252 F g−1 even at a high current density of 10 A g−1 with an excellent capacitance retention of 85.8% after 10 000 cycles. Moreover, the assembled NSHPC2-based symmetric supercapacitor displayed a high energy density of 43.6 Wh kg−1 at a power density of 400 W kg−1. This work provides a novel method to activate biomass to hierarchically porous carbons for high-performance supercapacitors, expected to be applied to most biomass, e.g., pine needles and orange leaves (Figure S8).

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EXPERIMENTAL SECTION

Materials. Fresh GLs were collected from Jiangsu University. Ammonium nitrate (AN, 99%) was obtained from Xilong Scientific Corporation (Guangdong, China). Poly(tetrafluoroethylene) emulsion (PTFE, 60 wt %) and acetylene black were purchased from MTI Corporation (Shanghai, China). Nafion perfluorinated resin solution (5 wt %) was purchased from Alfa Aesar. Hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4, 98%), potassium hydroxide 5911


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Author Contributions

pressure of 12 MPa. The mass of the as-obtained active carbon materials used for each electrode was measured to be 4 mg. The potentials for cyclic voltammetry (CV) scans were within the window from −1 to 0 V at various potential scanning rates in the range of 5−500 mV s−1. The average specific capacitances of the electrodes can be calculated from the CV curves using the following equations

Cs =

Sarea 2mνΔV

§

L.Z. and S.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Natural Science Foundation of China (51302113, 51572114) and the Natural Science Foundation of Jiangsu Province (No. BK20130512). L.Z. was partially supported by a Graduate Research Grant (17A129) from the Jiangsu University. S.W. was partially supported by the Jiangsu Postdoctoral Research Foundation (2018K058C).

(6)

where Cs is the specific capacitance (F g−1), Sarea is the integral area of the entire CV loop, m is the mass of active materials (g), ν is the scan rate (V s−1), and ΔV is the operation voltage window (V). Electrochemical impedance spectroscopy (EIS) was carried out under a frequency range from 0.01 Hz to 100 kHz. The galvanostatic charge/discharge (GCD) tests were performed at different current densities from 0.5 to 10 A g−1. The gravimetric specific capacitances of the electrodes can be calculated depending on the galvanostatic charge/discharge curves according to the following equation24 Cs =

C I Δt = m mΔV

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(7)

where Cs is the specific capacitance (F g−1), I is the discharge current (A), Δt is the discharge time (s), m is the mass of active materials (g), and ΔV is the operation voltage window (V). A test symmetrical supercapacitor was assembled with a polypropylene membrane separating the two same nickel foam electrodes painted with the NSHPC2, and a poly(vinyl alcohol)−KOH (PVA−KOH) gel55 was used as a semisolid polymer electrolyte. The PVA−KOH gel was prepared by adding 3.36 g of KOH to a viscous solution containing 6 g of poly(vinyl alcohol) and 60 mL of water. The energy density (E, Wh kg−1) and power density (P, W kg−1) were evaluated by the following equations

■

E=

CsΔV 2 2 × 3.6

(8)

P=

3600E Δt

(9)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03586. TG/DSC analysis, SEM and TEM images, PXRD patterns, CV and GCD profiles, Nyquist plots, yields and pore characteristics, and comparison of capacitance performance of recent biomass-derived carbon supercapacitors (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: 86-511-88791800. ORCID

Xiaoqi Fu: 0000-0003-1029-4017 Tingshun Jiang: 0000-0002-8859-1890 5912


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