Large-Scale and Low-Cost Motivation of Nitrogen-Doped Commercial

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Cite This: ACS Appl. Energy Mater. 2019, 2, 4234−4243

Large-Scale and Low-Cost Motivation of Nitrogen-Doped Commercial Activated Carbon for High-Energy-Density Supercapacitor Shuai Zhang,† Xiaoze Shi,† Xuecheng Chen,*,†,‡ Dengsong Zhang,§ Xianjie Liu,∥ Zhibin Zhang,⊥ Paul K. Chu,*,# Tao Tang,*,‡ and Ewa Mijowska†

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†

Nanomaterials Physicochemistry Department, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § Research Center of Nano Science and Technology, Shanghai University, No. 99 Shangda Road, Baoshan District, Shanghai 200444, China ∥ Department of Physics, Chemistry and Biology, Linkoping University, SE-58183 Linkoping, Sweden ⊥ Division of Solid State Electronics, Department of Engineering Sciences, Uppsala University, Uppsala 75237, Sweden # Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: The growing requirement for high-performance energy-storage devices has spurred the development of supercapacitors, but the low energy density remains a technical hurdle. In this work, porous nitrogen-doped activated carbon (NAC) is prepared on a large scale from commercial activated carbon (AC) and inexpensive chemicals by a one-step method. The NAC material with 3.1 wt % nitrogen has a high specific surface area of 1186 m2 g−1 and shows a specific capacitance of 427 F g−1 in a symmetric cell with an aqueous electrolyte. 98.2% of the capacity is reserved after 20 000 cycles at 20 A g−1. The energy densities of the NAC are 17.2 and 87.8 Wh kg−1 in acidic and organic electrolytes, respectively. Moreover, this simple process is readily scalable to address commercial demand and can be extended to the motivation of a variety of carbonbased materials with poor capacitances. KEYWORDS: supercapacitor, heteroatom doping, activated carbon, CVD, low cost, energy storage metal−organic frameworks (CMOFs),11 carbon spheres,12 carbon nanocages,13,14 and carbon aerogel,15 as electrode materials in supercapacitors. Despite recent advances, the lowcost mass production of carbon-based materials is still challenging,16 and low-cost porous carbon materials such as activated carbon (AC) have attracted attention.17 AC is used in commercial capacitors on account of the large SSA, narrow PSD, low cost, and chemical stability.18 Despite recent development, the specific capacitance, energy density, and power density of AC-based materials are still not satisfactory because of the large pore size and low conductivity.19 Therefore, it is challenging to control the porosity (especially micromesopore size distributions) as well

1. INTRODUCTION Supercapacitors have drawn considerable attention based on advantages such as fast charge−discharge rates, high power density, and long-term cyclability;1,2 however, the energy densities of present supercapacitors are much lower than those of the commercial lithium ion batteries, consequently hampering wider application.3 The ideal supercapacitor electrode should have high conductivity, a large ion-accessible surface area, a balanced pore-size distribution (PSD), a large ion transport rate, as well as high electrochemical stability.4,5 Much effort has been made to address the low energy density of supercapacitors, and porous carbon-based materials are considered suitable electrode materials for supercapacitor electrodes due to the large specific surface area (SSA), balanced PSD, and high electric conductivity.6,7 Various types of carbon-based materials have been investigated, for example, graphene,8,9 carbon nanotubes (CNTs),10 carbonized © 2019 American Chemical Society

Received: March 5, 2019 Accepted: May 29, 2019 Published: May 29, 2019 4234

DOI: 10.1021/acsaem.9b00481 ACS Appl. Energy Mater. 2019, 2, 4234−4243

Article

ACS Applied Energy Materials

Scheme 1. Preparation of NAC from Ethanol and Ammonia by Chemical Vapor Deposition and Implementation in a Symmetrical Supercapacitora

a

Weight of NAC in the beaker is 14.7 g.

Figure 1. Structural characterization of NAC and AC materials. (a) TEM images of commercial AC, NAC-0N/200C, NAC-200N/0C, and NAC120N/80C (inset: HR-TEM image of NAC-120N/80C with the arrows pointing to the formed micropores). (b) STEM image and EDS maps of NAC-120N/80C. (c) XRD spectra of the AC and NAC samples. (d) N2 adsorption/desorption isotherms of the NAC-0N/200C, NAC-120N/ 80C, NAC-200N/0C, and AC samples, with the red rectangle highlighting the variation of the micropores. (e) PSD of NAC-120N/80C determined by the N2 and CO2 techniques (inset).

nitrogen-doped AC with excellent electrochemical performance and high energy density have seldom been reported, and it is necessary to develop commercially viable processes to improve the supercapacitive properties of AC. Herein the large-scale preparation of nitrogen-doped activated carbon (NAC) with well-defined PSD and improved conductivity using commercial AC as the precursor and ammonia and ethanol as the nitrogen and carbon sources, respectively, is described. For demonstration, as high as ∼15 g of the NAC product can be produced in one pot (Scheme 1). Owing to the enhanced conductivity and optimal PSD, the NAC delivers excellent supercapacitive performance. In the aqueous electrolyte, a high specific capacitance value of 427 F

as the conductivity of AC, which are crucial to the supercapacitor performance.20 Several techniques including polymer coating,21 heteroatom doping,22,23 chemical activation via alkali24,25 and acid treatment,26 and hybridization with metal oxides27,28 have been proposed to functionalize ACbased materials to enhance the electrochemical performance. In particular, the heteroatom doping, especially nitrogen doping, can increase the specific capacitance of carbon-based materials by contributing to the pseudocapacitive redox reactions.29−31 Compared with the pristine carbon materials, the wettability of nitrogen-doped carbon in an aqueous electrolyte is improved significantly, leading to a high specific capacitance;32 however, low-cost supercapacitors comprising 4235


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Figure 2. Surface chemistry and hydrophilic analysis. X-ray photoelectron spectroscopy (XPS) spectra of NAC-120N/80C: (a) Survey spectrum. High-resolution XPS spectra: (b) N 1s and (c) C 1s. (d) Contact angles on the NAC samples with different doping concentrations. The liquid is 1 M H2SO4, which is the same as the electrolyte.

g−1 at a scanning rate of 1 mV s−1 is observed from the symmetrical cells. In the organic electrolyte, an energy density of 87.81 Wh kg−1 is achieved at a power density of 621 W kg−1. Furthermore, excellent cyclability with 98.2% of the initial capacitance maintained after 20 000 cycles at a current density of 20 A g−1 is observed. To the best of our knowledge, this is the most cost-effective method to prepare AC-based electrode materials with the highest capacitance so far and can be readily extended to other types of carbon materials such as carbon black, CNTs, and graphene.

functionalize AC, NAC-200N/0C has a similar morphology as the pristine AC. Figure 1a displays the TEM and highresolution transmission electron microscopy (HR-TEM) images of NAC-120N/80C prepared with both ethanol and ammonia. Many micropores can be observed from the edge of NAC-120N/80C. On the basis of the experimental results, NAC samples with different PSDs and ordered open pores are prepared (Figure S6). The porous carbon layers are grown on the walls of open-pore channels (Scheme 1), leading to a reduced channel size and a bimodal distribution of the pore size. The narrow channels in the original AC substrate can be used for electrolyte storage and transportation and shorten the ion transfer distance and lower the current resistance to improve the rate performance.33,34 Energy-dispersive X-ray spectroscopy (EDS) is carried out to determine the nitrogen distribution, and, as shown in Figure 1b, homogeneous N and O distributions are revealed on the carbon substrate, verifying the functionalization of the AC materials. In contrast, there is no N signal is observed from the original AC (Figure S3). Thermogravimetric analysis (TGA) is conducted to evaluate the thermal stability (Figure S4), and all of the NAC samples have higher thermal stability than the original AC. The original AC and NAC samples are analyzed by powder X-ray diffraction (XRD). The spectra in Figure 1c and Figure S5a exhibited the broad peaks at 23.5°. With the increase in the nitrogen concentration, the intensity of the diffraction peaks at 23.5 and 43.2° decreases, indicating the formation of more disordered carbon. The Raman scattering spectra of the AC and NAC samples are depicted in Figure S5b. The peaks at 1320 and 1590 cm−1 are the D band and G band of carbon, representing disordered and graphitic carbon structures, respectively,35 and the ID/IG ratios of NAC-0N/200C, NAC120N/80C, and NAC-200N/0C are 1.57, 1.64, and 1.73, respectively. Nitrogen increases the ID/IG ratios, and the Raman results correspond to the XRD data. N2 adsorption−desorption experiments are performed to investigate the porosity of the AC and NAC samples (Figure 1d), and the isotherms indicate that micropores and

2. RESULTS AND DISCUSSION As shown in Scheme 1, the highly porous NAC materials are prepared from inexpensive chemicals by chemical vapor deposition (CVD). Ethanol and ammonia are used to functionalize the commercial AC, which can be implemented directly as electrode material in supercapacitors. In our experiments, 14.7 g of NAC can be prepared at one time at a low cost. The NAC samples are denoted by the volume ratio of ammonia solution (N)/ethanol (C) as NAC-0N/200C to NAC-200N/0C. After CVD, carbon and nitrogen atoms are selectively introduced into the AC using ethanol as the carbon source and ammonium as the nitrogen source. The nitrogen and carbon atoms codeposited on the surface of the AC substrate. With the addition of ethanol, the NH3 molecules react with ethanol vapor at high temperature; then, the intermediate product deposits on the AC surface to form a nitrogen-rich CVD carbon layer. To optimize the synthesis efficiency, a series of NAC samples with different ratios of ethanol/ammonia (Figures S1 and S2) are prepared. Transmission electron microscopy (TEM) is performed, and Figure 1a shows the morphology of AC. The original AC with ordered open-pore channels not only serves as the substrate but also is used for electrolyte storage and transportation. As shown in Figure 1a, when only ethanol is used (NAC-0N/200C), the AC is fully covered by a thick carbon layer, demonstrating that CVD is very effective. Figure 1a shows that when only ammonia is used to 4236


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Figure 3. Electrochemical properties of NACs with different N and C ratios in 1 M H2SO4 electrolyte. (a) CV curves at a scanning rate of 200 mV s−1. (b) Specific capacitance for different scanning rates. (c) GCD curves. (d) Specific capacitance for different current densities. (e) Nyquist plots (inset: high-resolution Nyquist plot). (f) Stability of NAC-120N/80C at a current density of 20 A g−1 (inset: comparison of the 1st and 20 000th GCD curves).

spectrum (Figure S8) reveal the presence of oxygen atoms in the carbonyl groups shown in the C 1s spectrum, and other oxygen groups participate in the faradaic reactions to enhance the wettability and pseudocapacitance. Table S1 lists the corresponding XPS elemental results of the NAC samples. With increasing amounts of ammonia, both the nitrogen and oxygen concentrations increase, as expected, and NAC-200N/ 0C has the largest dopant concentration. Doping of carbon has been shown to improve the wettability between the electrolyte and electrode (Figure 2d).42 We further investigate the XPS spectra of the other NAC samples (Figure S9). A high ratio (∼403.1 eV) of the pyridinic-N-oxide-type (N-X) nitrogen atoms exists in the NAC materials (NAC-160N/40C and NAC-200N/0C) with a higher amount of ammonia treatment, indicating the direct reaction between the ammonia and oxygen-containing groups on the AC surface.43 The different graphitic and pyridinic-N-oxide-type nitrogen atoms on the NAC samples with different ethanol ratio treatments suggest that the nitrogen and carbon atom codeposited on the surface of the AC substrate. With the addition of ethanol, the NH3 molecules can react with ethanol vapor at a high temperature; then, the intermediate product deposits on the AC surface to form a nitrogen-rich CVD carbon layer. Contact-angle measurements are also used to evaluate the wettability of NAC samples, and 1 M H2SO4 aqueous is used as the test liquid, which consists of the electrolyte in the electrochemical measurement. As illustrated in Figure 2d, with the increase in the heteroatom doping ratio, the wettability of the material is improved, as expected, which suggests that the electrode surface can be efficiently saturated by the electrolyte ions during the charge−discharge process. The electrochemical properties of the NAC samples are determined using a two-electrode system in 1 M H2SO4 electrolyte. Figure 3a compares the results of the original AC and NAC samples, and a dramatically enhanced electrochemical performance at a scan rate of 200 mV s−1 is observed from the latter. With an increasing volume ratio of ammonia/

mesopores coexist in the NAC samples. The enhanced adsorption capacities at low pressure (0 to 0.05) indicate the presence of micropores. The SSAs of the pristine AC, NAC0N/200C, NAC-120N/80C, and NAC-200N/0C are 1338, 934, 1186, and 1633 m2 g−1. NAC-200N/0C has the largest SSA because AC is further activated by H2O at a high temperature (Figure S6),36 and it is consistent with XRD and Raman scattering. As shown in Figure 1e, the PSD of NAC120N/80C is calculated by the density functional theory (DFT). The micropore distribution is also measured by means of CO2 adsorption. Micropores with the size of 0.6 to 1 nm (inset in Figure 1e) and mesopores of 2.5 nm are observed. Figure S6 shows the PSDs of the NAC and AC samples. With the increase in carbon content, the amount of micropores increases while the number of mesopores decreases, indicating narrower channels consistent with TEM. Previous theoretical and experimental investigations have shown that micropores mainly contribute to the capacitance of supercapacitors.37,38 The increase in micropores and balanced PSD in NAC suggests improved capacitive characteristics. To determine the chemical composition of the NAC samples, X-ray photoelectron spectroscopy (XPS) is performed (Figure 2a−c and Figure S7). As shown in Figure 2a, C, N, and O are detected from NAC-120N/80C, and the high-resolution N 1s and C 1s spectra are presented in Figure 2b,c. The XPS C 1s spectrum of the NAC sample is decomposed, and three distinct peaks can be recognized. The peak at 284.6 eV refers to the sp2-hybridized carbon from the deposition of ethanolderived carbon layer at high temperature, and the other one at 286.3 eV is ascribed to hydroxyl groups complementing the molecules. The peak at 288.7 eV indicates a small amount of the carboxylic groups on the material surface.39 In the N 1s spectrum, the peak at 398.0 eV belongs to pyridinic nitrogen species, which are assumed to contribute to the pseudocapacitance.40 The peak at 400.8 eV is assigned to graphitic (quaternary) nitrogen, which enhances the electrical conductivity.41 The peaks at 531.4 and 533.6 eV in the O 1s 4237


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Figure 4. Electrochemical performances of NAC sample in different inorganic electrolytes and with different mass loadings. (a,b) Specific capacitances calculated from (a) CV and (b) GCD curves of NAC-120N/80C in different inorganic electrolytes. (c) Capacitances of NAC-120N/ 80C electrodes versus scanning rates for different areal mass loadings.

20 000 energy-storage cycles at a high current density of 20 A g−1. Very little difference is found in the initial and last cycles in the GCD curve, indicating excellent electrochemical stability. The increase in the capacitance in the NAC samples after the treatment can be attributed to the synergistic effects of the heteroatom doping and the modification of the porosity compared with the raw AC material. According to the energystorage mechanism of the electric double-layer capacitor, the electrolyte ions are accumulated, adsorbed, and stored on the micropore structure of the electrode materials, so the lack of the micropore structure in the raw AC material decreases the electrochemical performance and the capacitive behavior in the following measurements. The NAC-120N/80C material is treated with ethanol to tune the PSD via the CVD method at high temperature. The additional carbon layer deposited on the porous structure of AC and partly blocked the macropore structure to form the micropores, which is beneficial to the capacitance behavior. On the contrary, the heteroatom doping of NAC-120N/80C material introduces nitrogen-containing functional groups, which can undergo a Faraday reaction and improve the wettability of the pore walls, and nitrogen doping could increase the conductivity of the carbon material. The synergistic effects of the heteroatom doping and the modification of the porosity lead to a capacitance increase in NAC samples. To evaluate the electrochemical behavior of NAC-120N/ 80C in different electrolytes, the capacitive performance in 6 M KOH and 1 M Li2SO4 in the voltage window between 1.2 and 1.6 V is determined (Figure 4a,b). The capacitances derived from CV and GCD are less than those in H2SO4 due to the N−H mechanism.44 The electrochemical properties of the NAC-120N/80C-based supercapacitors with different areal mass loadings are determined (Figure 4c). When the areal mass loading is increased from 2 to 8 mg cm−2, the capacitance value decreases from 427 to 392 F g−1 at 1 mV s−1, and a ∼20% decrease from 264 to 207 F g−1 is observed at 200 mV s−1. At a large areal mass loading, the capacitance is higher than that of carbon-based supercapacitor materials.48 Carbon materials are the most commonly used supercapacitor electrode materials in industrial preparation. However, the unsatisfied capacitance values of commercial carbon materials limited their application in the energy-storage field. Thus a low-cost and universal motivation method for increasing the capacitance values of carbon materials is longawaited. To further evaluate the versatility of this functionalization scheme, three types of carbon materials, namely, acetylene black (ACB), CNTs, and reduced graphene oxide (RGO), are subjected to the CVD process using the optimal

ethanol, the capacitance initially increases and then slightly decreases. The largest specific capacitance of 427 F g−1 is obtained from NAC-120N/80C at 1 mV s−1 (Figure 3b, Figures S10 and S11). Furthermore, NAC-120N/80C does not exhibit significant diffusion limitation when the scanning rate is increased from 20 to 200 mV s−1. The capacitance of NAC120N/80C is maintained at 264 F g−1 despite a high scan rate of 200 mV s−1, demonstrating excellent rate performance. The enhanced specific capacitance of NAC-120N/80C sample can be attributed to the nitrogen and oxygen doping on the material surface, which would lead to an increase in the pseudocapacitance. On the contrary, the suitable PSD as well as the ordered porous structure of NAC enhance the contact between the electrolyte/electrode surface and benefit the electrolyte ion storage and transfer during the energy-storage process in the supercapacitor. Because of the interaction between the electrolyte ions and the electron pair in the graphitic (quaternary) nitrogen (N-Q) hybrid orbital, the adsorption of the proton in the acidic electrolyte increases on NAC.44 Galvanostatic charge−discharge (GCD) measurements are conducted on the NAC samples and original AC at current densities between 1 and 20 A g−1, and the symmetrical features with linear slopes indicate excellent capacitive properties. The largest value of 356 F g−1 is observed from NAC-120N/80C, and it is nearly six times better than the untreated commercial AC (60 F g−1) at 1 A g−1 (Figure 3c,d and Figure S13). This value of 356 F g−1 at 1 A g−1 is the highest among recently published results (Table S2). The energy density of NAC-120N/80C is calculated to be 17.8 Wh kg−1 at 1 A g−1 in an aqueous electrolyte, and the power density is 298 W kg−1. On the basis of a packing density of 0.614 g cm−3, the volumetric capacitance as well as the energy density are calculated to be 219 F cm−3 and 11 Wh L−1, respectively, which are also the highest values so far. The Nyquist plots are obtained to evaluate the conductivity (Figure 3e and Figure S14). The conductivities of all of the NAC samples increase, and the results are in agreement with the cyclic voltammetry (CV) and TGA data. The series resistances of AC, NAC-0N/200C, NAC-120N/80C, and NAC-200N/0C are 26.4, 1.8, 4.5, and 15.1Ω, respectively.45 From Table S1, the conductivities of AC and NAC samples corresponded to the trend of the carbon content from XPS data. Interestingly, although the NAC-80N/120C and the NAC-120N/80C have the same carbon content, a higher conductivity of NAC-120N/ 80C can be attributed to the high ratio of graphitic nitrogen (N-Q) atoms, which will have a lower Warburg resistance and will be beneficial to the specific capacitance.46,47 Figure 3f shows that 98.2% of the initial capacitance is retained after 4238


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Figure 5. Comparison of capacitive behaviors of acetylene black (ACB), carbon nanotubes (CNTs), and reduced graphene oxide (RGO) before and after treatment. Scheme: The ammonia solution/ethanol vapor treatment process and the TEM images of ACB, CNT, and RGO before and after treatment. (a−c) Capacitance values of various carbon materials from CV measurements from 1 to 200 mV s−1. (d−f) Capacitance values of various carbon materials from GCD measurements from 1 to 20 A g−1.

Figure 6. Electrochemical behaviors of NAC-120N/80C in BMIM BF4/PC electrolyte: (a) CV curves, (b) GCD curves, (c) Nyquist plot, (d) Bode plot, (e) cycling stability of NAC-120N/80C (inset: Nyquist plot before and after 10 000 charging/discharging cycles at a current density of 20 A g−1), and (f) Ragone plot of NAC-120N/80C compared with results in the literature39,54−56 (inset: NAC-120N/80C-based symmetrical supercapacitor powering four LEDs).

capacitances of the treated carbon materials increase, and on the basis of the GCD curves at 1 A g−1, the capacitance ratios are 120 times, 94 times, and 892 times those of the untreated ACB, CNTs, and graphene oxide, respectively, which suggests the great potential application of this modification method in the energy-storage field.

ammonia/ethanol ratio. As shown in Figure 5, carbon layers are formed, and the homogeneous distribution of N on graphene confirms that the technique is suitable for other types of carbon-based materials (Figure S18). Figure 5a−f and Figures S15−S17 exhibit the electrochemical performances of the carbon materials before and after motivation. The capacitances of all three materials increase significantly. The 4239


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3. CONCLUSIONS Using commercial AC and inexpensive chemicals, a one-step CVD process is designed to produce porous N-doped NAC with a large accessible surface area, balanced PSD, and improved conductivity on a large scale. In the H2SO4 electrolyte, NAC-120/80C has a capacity of 427 F g−1, rate capability, and cyclability with 98.2% of the capacitance maintenance after 20 000 cycles. Moreover, it shows a high energy density of 87.8 Wh kg−1 in an organic electrolyte. The strategy can be extended to other types of carbon materials to increase the capacitive behaviors of energy-storage devices.

Because the energy density of the capacitor is related to the square of the voltage, organic electrolytes are usually used based on the extended operating voltage window. Hence, the electrochemical properties of NAC-120N/80C and AC are assessed in the 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4)/propylene carbonate (PC) electrolyte.49 As shown in Figure 6a, the CV curves are nearly rectangular from 0 to 3 V at 200 mV s−1. As shown in Figure 6b, the GCD curves show a nearly symmetrical triangular shape with a small IR drop (0.03 V at a current density of 1 A g−1), and NAC120N/80C shows a high capacitance of 318 F g−1 at 1 A g−1. Even for the current density in the GCD measurement up to 20 A g−1, the capacitance is maintained at 194 F g−1 (Figure 6b). In comparison, commercial AC is evaluated under the same conditions (Figure S19), and the capacitance (0.53 F g−1 at 1 A g−1 and 0.13 F g−1 at 20 A g−1) is far below those of NAC-120N/80C. Because of the low conductivity of the organic electrolyte, the serial resistance (6.7 Ω) in BMIM BF4/ PC electrolyte is higher than that in the aqueous electrolyte (4.2 Ω) (Figure 6c). However, beneficial to the modification of NAC material via the nitrogen-doping method, the polarized solvent of PC can have good contact with the material surface, which will enhance the electrochemical performance.50,51 The dependence of the phase angle on the frequency of NAC120N/80C is −79° in the low-frequency region, and the characteristic frequency f 0 at a phase angle of −45° is 1.03 Hz, corresponding to a time constant τ0 (= 1/f 0) of 0.97 s. It is less than that of AC-based supercapacitors (∼10 s)52 (Figure 6d). NAC-120N/80C with the BMIM BF4/PC electrolyte exhibits excellent cyclability with 81.4% of the capacitance maintenance after 10 000 charging/discharging cycles at a high current density of 20 A g−1. The Nyquist plots show small differences before and after 10 000 cycles, which suggests a good stability of this material in different electrolytes (Figure 6e). The Ragone plot of NAC-120N/80C is shown in Figure 6f, and the high specific capacitance results in a superior energy density of 87.81 Wh kg−1, which is comparable to that of a lead−acid battery.53 At this energy density, the power density is 621 W kg−1 and the energy density is 46.56 Wh kg−1, even when the power density is elevated to 14 966 W kg−1. On the basis of the extraordinary electrochemical performance of NAC-120N/ 80C in the organic electrolyte, a coin-cell-type supercapacitor is constructed and is demonstrated to power four yellow LEDs, thus verifying the large potential in energy storage. The results demonstrate that by controlling the PSD, wettability, and conductivity of commercial AC-derived NAC, excellent electrochemical properties can be obtained. The convenient and cost-effective process can be readily scaled up to address commercial demand. The superior energy-storage performance of NAC-120N/80C can be attributed to the unique ordered pore structure and nitrogen doping. The Ndoped nanoporous carbon layer on the AC pores provides the abundant electrode−electrolyte interface to form electric double layers for high performance in an aqueous electrolyte. Furthermore, the NAC structure has a bimodal PSD. The AC not only supports the NAC layers but also shortens the diffusion pathway in the mesoporous structure for enhanced electrolyte storage. Last but not least, nitrogen increases the wettability and the conductivity of the NAC samples to improve the ion transfer efficiency.

4. EXPERIMENTAL SECTION 4.1. Materials. The AC powder, 60% PTFE water dispersion, BMIM BF4, PC, and CNTs were purchased from Sigma-Aldrich, and ethanol (99.5%) and ammonia (40%) were obtained from ChemLand. Polyvinylidenedifluoride (PVDF) was from Solvay. All chemicals were used without further purification. The TF4530 separator was purchased from NKK, Japan. 4.2. Preparation of N-Doped Activated Carbon. 120 mL of ethanol and 80 mL of the ammonia solution were mixed in a 250 mL bottle at a temperature of 23.4−26.6 °C, and the bottle was connected to a tube furnace. 14.7 g of AC was transferred into the furnace on a corundum boat. N2 gas was introduced at 200 sccm through the ethanol and ammonia solution in the furnace to replace the air inside. Another bottle filled with DI water was connected to the other end of the tube furnace to absorb the exhaust gas. The furnace was heated to 900 °C at a rate of 15 °C min−1 and then cooled to room temperature naturally under gas protection after heating for 3 h to obtain the NAC samples. Different ammonia/ethanol ratios (Table 1) were used to prepare the NAC samples under the same condition.

Table 1. Ethanol and Ammonia Solutions Used for Preparation (v/v ratios, mL) ammonia solution ethanol

0

40

80

100

120

140

160

200

200

160

120

100

80

60

40

0

4.3. Electrochemical Measurements. The electrodes for supercapacitor application were produced by mixing the active material (NAC, 80 wt %), conductive agent (CNTs, 10 wt %), and PVDF (10 wt %) and pressed by10 MPa pressure to form the electrode tablet with a diameter of 1 cm. The mass of the active materials in each tablet was 2 mg, and 1 M H2SO4 (0.1% w/w SDS) was used as the aqueous electrolyte. The 1 M Li2SO4 and 6 M KOH aqueous solutions were also used as the electrolytes.57 The electrochemical properties were determined by CV, GCD, and electrochemical impedance spectroscopy (EIS) using the EC-LAB electrochemical workstation. The CV measurements were executed in the potential window between 0 and 1.2 V at the scan rates from 1 to 200 mVs−1. The GCD tests were performed in the potential window from 0 and 1.2 V at different current densities between 1 and 20 A g−1. With 1 M Li2SO4 aqueous electrolyte, the voltage window was extended to 1.6 V. EIS was conducted in the frequency range between 100 kHz and 1 mHz with an amplitude of 10 mV. The electrochemical performance of the NAC and AC samples was also evaluated in 1 M BMIM BF4 in the PC electrolyte. Two mg of the NAC-120N/80C was blended with 25 μL of the 5% PTFE suspension; then, the slurry was spread on a 15 mm circular aluminum foil. The foil with the electrode materials was pressed to 35 μm thickness by a roller. Then, the electrodes were transferred to an argon-filled glovebox after drying at 100 °C overnight under vacuum. A CR2032 coin-type cell was used for symmetric capacitor assembly, and the TF4530 membrane was used as a separator. With the BMIM BF4/PC electrolyte, the voltage window was extended to 3 V. The capacitance value is calculated from the CV curves with the equation 4240


ACS Applied Energy Materials C=

ij 1 jj m × Δv × s jk

∫v

v

i dv +

0

∫v

v0

Article

y i dvzzzz {

where C is the specific capacitance value (F g−1), m is the active material content in the single electrode (mg), Δv is the potential range (V) in the CV measurement, and s is the scanning rate (mV s−1). The integral is the calculated area from the CV curve at each scanning rate. The specific capacitance is calculated from the GCD curves with the equation C=

■

*E-mail: [email protected] (X.C.). *E-mail: [email protected] (P.K.C.). *E-mail: [email protected] (T.T.).

ΔV Δt

( )

where C is the specific capacitance (F g−1), I is the discharging current (mA), m is the mass (mg) of the active materials in one electrode, and ΔV/Δt is the slope of the discharge measurement (V s−1). The energy density is evaluated by the following equation E=

ORCID

Xuecheng Chen: 0000-0002-8874-5642 Dengsong Zhang: 0000-0003-4280-0068 Zhibin Zhang: 0000-0003-0244-8565

(CV 2) (8 × 3.6)

Notes

The authors declare no competing financial interest.

where E is the energy density (Wh kg−1), C is the capacitance of the materials (F g−1), and V is the potential window in the electrochemical test. The power density is evaluated by the following equation P=

■

ACKNOWLEDGMENTS We acknowledge financial support from the National Science Centre, Poland (OPUS 2017/252/B/ST8/02702) and City University of Hong Kong Strategic Research Grant (SRG) (no. 7005105).

3600E Δt

■

where P is the power density (W kg−1), E is the energy density, and t is the discharge time.

■

AUTHOR INFORMATION

Corresponding Authors

2I m×

after ammonia/ethanol treatment at 900 °C. Figure S18: EDS maps of RGO and ammonia/ethanol-treated graphene. Figure S19. Electrochemical performance of commercial AC in BMIM BF4/PC electrolyte. Table S2: Electrochemical performance of various active carbonbased electrochemical double layer capacitors (EDLCs) (PDF)

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00481. Figure S1. SEM images of the AC and NAC samples. Figure S2: TEM images of NAC-40N/160C, NAC80N/120C, and NAC-160N/80C. Figure S3: STEM image and EDS maps of the original AC. Figure S4. TGA profiles of the AC and NAC samples. Figure S5: XRD spectra of NAC-160N/40C, NAC-80N/120C, and NAC-40N/160C. Raman scattering spectra of all NAC and AC samples. Figure S6. DFT PSDs of the commercial AC and NAC samples. Figure S7: XPS spectra and high-resolution XPS spectra of the NAC and AC samples. Table S1: Relative concentrations of carbon, oxygen, and nitrogen in AC and NACs obtained by deconvolution analysis of the high-resolution XPS spectra. Figure S8. O 1s survey spectrum of NAC-120N/ 80C. Figure S9: High-resolution XPS spectrum of NAC200N/0C sample. Figure S10. CV curves of the NAC samples in the acidic electrolyte. Figure S11. Specific capacitances of NACs and commercial AC calculated from the CV curves in 1 M H2SO4 electrolyte. Figure S12. GCD curves of the NAC samples in the acidic electrolyte. Figure S13. Specific capacitances of NACs and commercial AC calculated from the GCD curves in 1 M H2SO4 electrolyte. Figure S14. Nyquist plot of the NAC samples and active carbon in the acidic electrolyte. Figure S15. TEM images and electrochemical properties of ACB before and after ammonia/ethanol treatment at 900 °C. Figure S16. TEM images and electrochemical properties of CNTs before and after ammonia/ethanol treatment at 900 °C. Figure S17: TEM images and electrochemical properties of graphene oxide before and 4241


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