Facile hydrothermal synthesis of N-doped commercial activated carbon for zinc ion hybrid supercapacitor

A one-step hydrothermal method was employed to treat commercial activated carbon (AC) with sodium azide as the nitrogen source. The characterizations confirmed the successful incorporation of N dopant into the surface of commercial activated carbon (NAC) and the specific surface area increased to 1739 m2·g−1, which indicates that nitrogen-doped activated carbon has excellent properties in ion adsorption and electron transfer. An NAC-based aqueous zinc ion hybrid supercapacitor (ZHS) is fabricated and shows a specific capacity of 232.4 mA·h·g−1 and an energy density of 232.4 Wh·kg−1 at 0.1 A·g−1 with a voltage window of 2.0 V and long-term GCD stability up to 10,000 cycles. Considering the low-cost raw materials and surface modification at a relatively low temperature, the proposed technical route can be viewed as a promising method for the modification of carbon-based materials and application in next-generation energy storage devices.


Introduction
Ion batteries and supercapacitors have been extensively researched as energy storage devices and have practical applications in a wide range of industrial fields and consumer electronics [1,2]. Because of their different energy storage mechanisms, they have the main advantages of batteries in energy density and supercapacitors in power density, as well as long cycle life [3]. Apparently, the power density and energy density shortcomings of batteries and supercapacitors limit the various demands for energy storage devices in all fields [4,5].
In recent years, zinc ion hybrid supercapacitors with metal zinc as the negative electrode and porous materials as the positive anode have been developed [6]. The energy storage mechanism of the negative anode is based on the electrochemical reaction at the electrode, and the capacitance of the positive electrode is derived from ion adsorption/desorption (EDLC mechanism). It is certain that the EDLC-based charge storage mechanism has a higher power density than the electrochemical reaction mechanism, implying that the zinc ion hybrid supercapacitor has a higher power density than the battery [7].
A series of carbon-based materials with unique properties, such as high chemical stability to withstand long-term charge/discharge cycles, good electrical conductivity for highly efficient electronic transmission, and tunable pore structure and high specific surface area for ionic adsorption, have been widely investigated for the preparation of ZHSs [8]. Wu et al. used chemically activated graphene to build a porous carbon as a positive electrode of a ZHS device with a 3 M Zn(CF 3 SO 3 ) 2 electrolyte, resulting in a high energy density of 106.3 Wh·kg −1 and a cycling life of 80 000 GCD cycles [9]. To assemble the ZHSs, Li et al. used a porous fibrous carbon containing O/N groups as a positive electrode and a zinc foil as a negative electrode, yielding an energy density of 127 Wh·kg −1 and a cycle life of 6000 GCD cycles with a 7% capacity loss at a current density of 20 A·g −1 [10]. Yang et al. employed electrochemical graphene oxide coated with polypyrrole as a positive electrode, and their ZHS device demonstrated a maximum energy density of 117.7 Wh kg −1 at a power density of 0.34 kW·kg −1 , owing to the high electrical conductivity and porous structure of the PPy coated composite [11]. Wang et al. introduced N and P dopants into onion-like carbon applied to a ZHS device, and the synergistic effect of N and P dopants for ionic adsorption is highly significant, resulting in a high energy density of 149.5 Wh·kg −1 and a long-term cycling lifespan [12]. Using ZnO as a sacrificial template, Yuksel et al. synthesized nitrogen-doped tubular carbon with necklace feathers derived from ZIF-8 particles and ZIF-8 layer [13]. The assembled ZHS has an energy density of up to 189.6 Wh·kg −1 .
Activated carbon with a rich porous structure and a high specific surface area has been identified as a promising candidate for energy storage. According to previous research [14,15], the pores of conventional activated carbon are primarily less than 0.5 nm in size, making them "useless" for energy storage. Meanwhile, the structure of activated carbon is primarily amorphous, which results in poor electrical conductivity and is not conducive to electron transfer. As a result, conventional activated carbon cannot be used directly in energy storage devices. Recently, commercial activated carbons with an average pore size of approximately 1 nm have been developed for use in supercapacitors that can load with different ions in an aqueous electrolyte [16]. Furthermore, doping technology is thought to be an efficient method for increasing the electrical conductivity of electrode materials, including carbon-based materials [17,18]. However, the majority of N-doped carbon material preparation is done at high temperatures, such as the carbonization process at 800 °C to prepare porous N-doped carbon using C 3 N 4 @ PDA hybrid as starting material [19], atmospheric pressure CVD at 1000 °C growing N-doped graphene on copper foil [20] and calcination route at 800 °C to synthesize N-doped CNTs microspheres by pyrolyzing the melamine [21], which is both energy-intensive and environmentally unfriendly.
This work proposes a facile method for doping commercial activated carbon at low temperatures. The commercial AC was produced directly through a one-step hydrothermal treatment to form nitrogen-doped products with sodium azide as the nitrogen source, which is an industrially viable technology. The prepared N-doped AC-based ZHS has a maximum energy density of 232.4 Wh·kg −1 and excellent long-term GCD stability up to 10,000 cycles, implying a high potential for next-generation energy storage devices.

Material synthesis
A facile hydrothermal process was employed to carry out the preparation of N-doped AC (NCC) from hydrophilic commercial AC (YEC ® -8 A, Fuzhou Yihuan Carbon LTD, China). Typically, approximately 500 mg of commercial AC was ultrasonically dispersed in 60 mL of deionized water and a 20 ml solution containing 100 mg of NaN 3 and 60 mg of NaOH was added to the above solution at room temperature. The precursor was then placed in a vacuum system under a pressure of 2 × 10 3 Pa to allow the gas to escape from the AC. Subsequently, the precursor was transferred to a 100 ml Teflon-lined stainless steel autoclave, and the reactor was placed in a high-temperature oven and treated at 180 °C for 12 h. After the reactor cooled to room temperature, the solid particles were obtained by centrifugalization and rigorously rinsed with deionized water to remove impurities. To complete the synthesis of NAC, the sample was dried in a vacuum system at 80 °C for 10 h.

Material characterization
Scanning electron microscopy (SEM, Inspect F, FEI Co., USA) was used to observe the morphologies of the samples. The micromorphology was further observed by transmission electron microscope (HRTEM, Libra 200FE, Germany) and selective area electron diffraction (SAED) was employed to identify the components of the samples. The surface chemical states of NAC were ascertained by X-ray photoelectron spectroscopy (XPS, XSAM800, KRATOS, UK). The crystal structure was detected by X-ray diffraction (XRD, D-Max-γ type A, Rigaku Co., Japan). The N 2 adsorption-desorption data was derived from A JW-BK112 Surface Characterization Analyzer (Beijing JWGB Sci & Tech Co., China) and Brunauer-Emmett-Teller (BET) model was used to analyze the surface area. Raman spectroscopy was employed to characterize the structural features on an XploRA PLUS Raman spectrometer.

Electrochemical measurements
A slurry containing 80 wt% of NCC, 10 wt% of acetylene black, 10 wt% of PVDF, and a small amount of isopropanol was ground repeatedly in an agate mortar, and then hotpressed onto carbon cloth current collector with a mass loading of 5 mg·cm − 1 . All the electrodes dried in a vacuum oven at 80 °C overnight. Electrochemical data were investigated using assembled standard 2032 coin-type cells, while zinc foil and as-prepared NAC, glass fiber cloth, and 2 M ZnSO 4 solution were used as a negative anode, positive anode, separator, and aqueous electrolyte. The electrochemical properties include cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), electrochemical impedance spectroscopy (EIS), and long-term cycle stability were evaluated on an electrochemical workstation (CHI760E, Chenhua, China) and a CT2001A rapid sampling battery testing system (LAND, China).

3
The calculation formulas of capacity (C) and energy density (E) based on the discharge curves of devices are as follow: where I, t, ΔV, and m are discharge current, discharge time, voltage, and mass of NAC, respectively.

Structural characteristics
SEM was used to examine the AC samples before and after hydrothermal treatment in NaN 3 solution, and the SEM images are shown in Fig. 1. It is discovered from Fig. 1a, b that the AC and NAC contain the majority of micron-sized particles and a few macro-sized pores. Notably, almost no change in particle surface features is observed after hydrothermal treatment. In theory, the N-doping reactions between AC and NaN 3 occur only on the exposed solid surface, which has little effect on the surface feature. Given that SEM can only observe structure morphology on a micron scale, additional characterization methods including SEM, XPS, Raman spectra, and N 2 adsorption-desorption experiment will be further used to distinguish between their structures. The TEM shows that the carbon material has an amorphous structure in most areas and lattice stripes in others (Fig. 1c), indicating that it contains graphitic carbon microparticles. Meanwhile, abundant dark regions can be seen, implying that NAC has a large number of nanopores that facilitate energy storage via ion adsorption-desorption. Furthermore, a weak diffraction ring corresponds to the (002) crystal plane of graphite, while the other vaguely visible diffraction ring corresponds to the (100) crystal plane, indicating that the NAC structure is primarily amorphous with low crystallinity of the graphite phase. The crystalline structure of NAC was investigated using powder XRD analysis. Figure 1d shows two broad peaks at 2θ degrees of 23° and 43°, which are ascribed to the (002) and (100) crystal planes of graphite, respectively [22]. The characteristic peaks indicate that NAC has a low degree of graphitization, which is consistent with the result of the TEM analysis.
The surface chemical states of NAC were analyzed by an XPS measurement and the results are shown in Fig. 2. Distinctly, the survey spectrum clearly shows C1s, N1s, and O1s peaks (Fig. 2a), implying the presence of C, N,  (Fig. 2b), corresponding to the C[O] OH, C=O, C-N, and C=C groups, respectively. The C-OOH (534.0 eV), C-O/C-OH (533.1 eV), and C=O (531.5 eV) peaks are fitted into the O1s spectrum (Fig. 2c). Furthermore, two nitrogen-based groups at 401.7 eV and 398.6 eV are detected, indicating that the nitrogen element has been successfully doped into the activated carbon (Fig. 2d). The former is defined as N-Q (quaternary N) and the latter as N-5 (pyrrolic N), which can facilitate the electron transfer to improve the conductivity of activated carbon and induce the pseudocapacitive interaction to increase the electrochemical capacity of electrode materials [23].
The results of N 2 adsorption-desorption experiments on the specific surface areas and pore size distributions of AC and NAC materials are shown in Fig. 3. Based on Fig. 3a, the specific surface area of activated carbon is 1210 m 2 ·g −1 ; however, when the sample is treated in NaN 3 solution in a hydrothermal environment, the specific surface area  Fig. 3b. However, the number of pores in NAC is greater than that in AC, indicating that the N doping process can not only introduce the N element to improve conductivity but also increase the area available for ion adsorption to improve electrode material capacity [24]. Figure 4 depicts the Raman spectra of commercial AC and NAC materials. The first peak at 1335 cm −1 is the D-band of disordered carbon with a defective structure, and the second peak at 1581 cm −1 is the G-band of sp2 graphitic carbon, as shown in Fig. 4a. The D-band and G-band of the AC doped with N are located at 1334 cm −1 and 1576 cm − 1 (Fig. 4b), respectively. Obviously, nitrogen doping shifts the G-band, altering the electronic structure [25]. More importantly, the NAC has a higher intensity ratio of D and G peaks (ID/IG) than the AC, implying that the N bonding configurations in NAC cause abundant defects, which promotes the growth of electrochemically active sites [26].

Electrochemical performance
The comparative electrochemical performance of commercial AC and NAC was investigated by assembling ZHS devices using AC and NAC as positive electrodes. The CV curves of the AC-based device are shown in Fig. 5a (low scan rate range of 1-5 mV·s −1 ) and 5b (high scan rate range of 10-100 mV·s −1 ). The CV curves in Fig. 5a display similar shapes and the polarization phenomenon becomes very noticeable when the voltage exceeds 1.75 V, which is due to AC's low electrical conductivity. The polarization becomes inconspicuous as the scan rate increases to a high range, which is attributed to fast kinetic processes [27]. Furthermore, redox peaks can be found in all CV curves, indicating that Faradaic reactions take place during energy storage caused by oxygen-containing groups on the surface of AC. Figure 5c depicts the GCD curves of ACbased ZHS at various current densities. All of the charge and discharge curves are nonlinear, indicating that there is additional electrochemical energy storage in addition to ion adsorption energy storage, which is consistent with the CV measurement results. At current rates of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 A·g −1 , the AC-based ZHS has specific capacities of 39.1, 31.2, 22.7, 16.7, 12.5, 9.5, and 8.5 mA h g −1 . Obviously, the specific capacity is insufficient, and the pores' adsorption capacity in activated carbon is limited by the low electrical conductivity.
The CV curves of the NAC-based device at low and high scan rates are shown in Fig. 5d and e, respectively. Remarkably, the CV area of a NAC-based device is much larger than that of an AC-based device. The polarization of the NACbased device is significantly weaker than in Fig. 5c. Similarly, the redox peaks are still visible on the CV curves in Fig. 5a and b, indicating that the nitrogen group is involved in energy storage [28]. It is obvious that the hydrothermal nitrogen doping process improves the electrical conductivity of commercial AC, thereby stimulating more electrochemically active sites for ion adsorption. Simultaneously, nitrogen-containing groups formed on the surface of the AC participate in the redox reaction, providing additional capacity for the device. Figure 5f depicts the GCD curves of the NAC-based device at various current densities. The existence of redox reactions for energy storage is further confirmed by the nonlinear discharge curves. All discharge curves lack a discharge plateau, indicating that the main working mechanisms are zinc ion deposition/stripping on the negative electrode and ion adsorption/desorption on the positive electrode [29]. The NAC-based device has specific capacities of 232.4, 147.1,  [35] and Na + preintercalated δ-MnO 2 nanoflakes (74.3 Wh·kg −1 at 0.2 A·g −1 ) [36]. The technological route proposed in this work greatly increases the application value of commercial CC for energy storage due to its simplicity and industrial capability.
Dunn's method led to capacitive-and diffusion-controlled contributions [37]. The relationship between peak current i(V) and scan rate (v) is given by i(V) = av b and the fitted value of the b parameter is in the 0.5-1 range. Our previous work contains a detailed treatment for extracting the capacitive-and diffusion-controlled contributions [38]. Figure 6a shows the fitted capacity contribution based on the CV curve at 5 mV·s −1 , with the capacitive contribution accounting for 75% of the capacity contribution. As the scan rate increases from 3 mV·s −1 to 5, 10, 20, and 50 mV·s −1 , the capacitive contribution gradually increases in sequence to 81%, 87%, 91%, and 95% (Fig. 6b), indicating a surface capacitivedominated process with fast electrochemical kinetics at high scan rate.
The GCD test was used to evaluate the practicability of the assembled ZHS based on the NAC electrode for longterm cycling stability at a current density of 1 A·g −1 and the results are shown in Fig. 7. This device exhibits excellent GCD stability up to 10,000 cycles with a capacity loss of less than 1% of the initial value and a high coulombic efficiency close to 100%, indicating the viability of NAC-based ZHS. In fact, carbon-based materials are well known for their inherent electrochemical stability [39]. The N-doping modification has no effect on the main structure of AC and only introduces some surface defects, improving electrical conductivity and increasing active sites.
EIS measurements were used to demonstrate the benefits of N-doping in AC. In the frequency range of 10 − 2 -10 5 Hz, Nyquist plots of two types of devices were tested, and the results are shown in Fig. 8 (The equivalent circuit model is shown in the inset.). Two samples' Nyquist plots show a semicircle in the high-frequency region and a straight line in the low-frequency region. It is discovered that two ZHSs have a similar slope in the low-frequency region. In the highfrequency region, however, two samples deliver the differentiated semicircle. In Fig. 7, the solution/electrolyte resistance (Rs) and charge-transfer resistance (Rct) are calculated directly from the real axis. AC-and NAC-based ZHSs have Rs values of 15.1 and 26.9 Ω, respectively. Based on XPS analysis, commercial AC is rich in oxygen-containing groups, resulting in excellent infiltration between the electrode material and the electrolyte. As some of the oxygencontaining groups are substituted by nitrogen-containing groups, the infiltration of NAC is weakened, resulting in a higher Rs. It is important to note that the Rct value of the NAC-based ZHS is 15.9 Ω, which is significantly lower than the Rct value of the NAC-based ZHS (26.1 Ω). The decreased Rct is attributed to the N-doping process, which results in NAC's excellent electrical conductivity.

Conclusion
In summary, we proposed a simple method for synthesizing NAC using sodium azide as the nitrogen source in a hydrothermal environment using low-cost commercial AC. The prepared NAC has a higher specific surface area of 1739 m 2 ·g −1 than commercial AC (1210 m 2 ·g −1 ) and contains Oand N-containing groups. The XPS confirms the presence of N-Q (quaternary N) and N-5 (pyrrolic N), which improve activated carbon conductivity and electrochemical active site. In an aqueous ZHS, a zinc foil, NAC, and 2 M ZnSO 4 serve as the negative electrode, positive electrode, and electrolyte solution, respectively. The device has a maximum energy density of 232.4 Wh·kg −1 at 0.1 A·g −1 and an energy density of 59.6 Wh kg −1 at 10 A·g −1 , and long-term cycling stability with a capacity loss of less than 1% of the initial value when the voltage window is controlled in the range of 0-2.0 V. Given the significantly increased capacity, the proposed route can be viewed as a promising modification Fig. 6 a CV curve of the ZHS at the scan rate of 5 mV·s −1 with shadowed area representing the capacitive contribution, b Charge storage contribution from capacitive mechanism at various scan rates