Plasma-induced, nitrogen-doped graphene-based aerogels for high-performance supercapacitors

Commonly used energy storage devices include stacked layers of active materials on two-dimensional sheets, and the limited specific surface area restricts the further development of energy storage. Three-dimensional (3D) structures with high specific surface areas would improve device performance. Herein, we present a novel procedure to fabricate macroscopic, high-quality, nitrogen-doped, 3D graphene/nanoparticle aerogels. The procedure includes vacuum filtration, freeze-drying, and plasma treatment, which can be further expanded for large-scale production of nitrogen-doped, graphene-based aerogels. The behavior of the supercapacitor is investigated using a typical nitrogen-doped graphene/Fe3O4 nanoparticle 3D structure (NG/Fe3O4). Compared with 3D graphene/Fe3O4 structures prepared by the traditional hydrothermal method, the NG/Fe3O4 supercapacitor prepared by the present method has a 153% improvement in specific capacitance, and there is no obvious decrease in specific capacitance after 1000 cycles. The present work provides a new and facile method to produce large-scale, 3D, graphene-based materials with high specific capacitance for energy storage.

In addition, the capacitance of intrinsic graphene is not sufficient for commercial applications but can be improved by N-doping [35][36][37][38][39] . The preparation of N-doped graphene sheets by arc discharge/plasma treatment and chemical vapor deposition (CVD) thermal annealing of graphene oxide (GO) with NH 3 have been reported 35,40,41 . Plasma treatment is an eco-friendly and efficient way to produce N-doped graphene sheets, and several reports have demonstrated N-doping of graphene by plasma 42,43 . However, these reports address 2D structures and are not aimed at energy storage. A method for fabricating highquality, N-doped, graphene-based, hybrid, 3D structures does not exist.
In this work, we report a novel method for preparing N-doped, 3D, graphene/Fe 3 O 4 , nanoparticle aerogel (NG/Fe 3 O 4 ), which can be expanded for large-scale production of nitrogen-doped, graphenebased aerogel and various active nanomaterials can be incorporated into the 3D hybrid structures. The high-quality NG/Fe 3 O 4 aerogels are acquired by controllable physical treatment of GO. Compared with the 3D reduced-graphene/Fe 3 O 4 (RGO/Fe 3 O 4 ) prepared by the commonly used hydrothermal method, the present method produces greatly improved porous networks and exhibits significantly enhanced supercapacitor performance. The present work provides a new and facile method to produce high-quality, 3D, graphene-based materials for application in energy storage.  (Figure 1). The as-prepared sample was denoted as NG/Fe 3 O 4 . A schematic view of the HCD system used for the plasma treatment and the plasma experiment parameters are shown in Supplementary Fig. S1 and Table SI Characterization and electrochemical measurements X-ray diffraction (XRD) measurements were performed with CuKa radiation (D-MAX II A, λ = 0.15406 nm). A VG ESCALAB MKII (Thermo Scientific, Waltham, MA, USA) was used for the X-ray photoelectron spectroscopy (XPS) investigation. Transmission electron microscopy (TEM) images were acquired by a JEOL2010 (JEOL, Tokyo, Japan). Fourier transform infrared spectroscopy (FTIR) curves were obtained on a VERTEX 70 (Bruker, Ettlingen, Germany). The electrical conductivity of NG/Fe 3 O 4 and RGO/Fe 3 O 4 aerogel samples was determined via the four-probe method at room temperature. An IVIUMSTAT (Ivium, Eindhoven, Netherlands) electrochemical workstation was used for the electrochemical investigations, and the electrolyte was 6 M KOH. The galvanostatic charge − discharge was measured under different current densities between − 1.0 and 0 V. The cyclic voltammetry (CV) was measured at different scan rates (5, 20, 100 and 200 V s − 1 ) between − 1.0 and 0 V. The electrochemical impedance spectroscopy was acquired from 100 kHz to 0.01 Hz by applying a signal of 14.14 mV. Figure 1 shows the experimental procedures of the hydrothermal synthesis of NG/Fe 3 O 4 . The GO/Fe 3 O 4 mixed suspension was deposited onto Ni-foam by vacuum filtration, followed by freeze-drying. Finally, the GO/Fe 3 O 4 composites were reduced and nitrogen-doped simultaneously by plasma treatment. Using the hydrothermal method, a gel-like cylinder of RGO/Fe 3 O 4 was constructed, as shown in Figure 2a. The formation of a 3D porous network with micrometer-sized pores was confirmed by scanning electron microscopy (SEM), as shown in Figure 2b and 2c. However, as in the commonly used hydrothermal methods, the aggregation of graphene sheets during hydrogel formation was inevitable due to the reduction-induced strong p-stacking interaction between graphene sheets, which is originally prohibited by the oxygen-containing surface groups of GO. The network walls of RGO/Fe 3 O 4 show a tendency of layered aggregation, even though the decoration of Fe 3 O 4 NPs as spacers on graphene nanosheets partially prevents the aggregation. , and the aggregation of graphene sheets during hydrogel formation was observed. This result indicates that the preparation method of the aerogel plays a key role in avoiding the aggregation tendency in the reduction process. The larger pore sizes and thinner pore walls increased the specific surface area of NG/Fe 3 O 4 (92 m 2 g − 1 ) compared to that of RGO/Fe 3 O 4 (55 m 2 g − 1 ) based on the BET results ( Supplementary Fig. S3). These properties of NG/Fe 3 O 4 are directly related to the potential applications from adsorbents to supercapacitors.

RESULTS AND DISCUSSION
Both samples had similar TEM images, and the nanosized Fe 3 O 4 particles were anchored on graphene uniformly, suggesting efficient assembly between the graphene sheets and the NPs (Supplementary  Supplementary Fig. S5, was replaced by a broad peak between 20°and 30°, which results from the (002) reflection of the graphene of NG/Fe 3 O 4 and RGO/Fe 3 O 4 , indicating that GO was reduced by the hydrothermal and plasma treatment.  Figure 3a. For all three samples, peaks corresponding to the C 1s and O 1s were observed. Compared with GO/Fe 3 O 4 , the O 1s peak intensities of RGO/Fe 3 O 4 and NG/Fe 3 O 4 decreased, suggesting an increased C/O ratio after reduction by the hydrothermal process and plasma treatment, and the oxygen-related functional groups were efficiently removed. This hypothesis was confirmed by the deconvoluted C 1s spectra (Figure 3b). The weak signals of C-O and C = O in RGO/Fe 3 O 4 compared with that of GO/Fe 3 O 4 suggest that most of the GO was reduced, and the residual oxygen-related functional groups resulted from the incomplete reduction during the hydrothermal process. However, the oxygen-related peaks in the NG/Fe 3 O 4 were nearly invisible, which indicates that the plasma treatment was more efficient for the reduction of 3D GO-based hybrids than the hydrothermal method. The formation of Fe 3 O 4 in RGO/Fe 3 O 4 and NG/Fe 3 O 4 was further confirmed by the Fe 2p spectra (Figure 3c). Two characteristic peaks corresponding to Fe 2p 1/2 and 2p 3/2 at approximately 724.8 and 711.3 eV were observed, which is consistent with the XRD results. The survey spectra in Figure 3a indicate the presence of nitrogen in both RGO/Fe 3 O 4 and NG/Fe 3 O 4 . For RGO/Fe 3 O 4 , the introduction of nitrogen is attributed to the reduction agents used in the hydrothermal process, and the nitrogen in NG/Fe 3 O 4 results from N 2 plasma treatment. The analysis of the N chemical bonding is shown in Figure 3d, and the N 1s peak can be deconvoluted into three components. The pyridinic and pyrrolic N at 398.2 and 400.1 eV correspond to the N atoms of the π-conjugated system 35  Nitrogen-doped graphene for supercapacitors XY Zhang et al replacing the C atoms inside graphene sheets, which could be observed clearly for NG/Fe 3 O 4 but was nearly invisible for RGO/Fe 3 O 4 , as shown in Figure 3d. The first two types of N atoms located in the πconjugated system account for most of the N in graphene and contribute one or two p-electrons. The graphitic N atoms can be considered to be threefold coordinated sp 2 N in the hexagonal rings of graphene, which plays an important role in regulating the electronic properties of graphene in electrochemical systems 48,49 . According to our results, the graphitic N doping is difficult to achieve by the hydrothermal method, and plasma treatment is crucial to achieve a high-quality, N-doping, graphene-based aerogel. A typical three-electrode method was used in this work to investigate the electrochemical behavior. The working electrodes were prepared from RGO/Fe 3 O 4 and NG/Fe 3 O 4 . The CV curves of RGO/ Fe 3 O 4 and NG/Fe 3 O 4 are shown in Figure 4a and 4b. The specific capacitances C (F g À1 ) can be calculated from the CV curves using the following equation 44 : where V, I, m and v are the potential window (V), the current (A), the mass of the active materials (g) and the scan rate (mV s À1 ), respectively. Figure 4c summarizes the specific capacitance of the two samples as a function of the scan rate. The NG/Fe 3 O 4 electrode reached a maximum of 386 F g − 1 at 5 mV s − 1 , which was much higher than that of the RGO/Fe 3 O 4 electrode (253.3 F g −1 ). Due to the in situ preparation of the RGO/Fe 3 O 4 @ Ni-foam electrode, the specific capacitance at 5 mV s − 1 was 267 F g − 1 , which was slightly improved compared with that of the RGO/Fe 3 O 4 electrode but was still far behind the NG/Fe 3 O 4 electrode. The galvanostatic charge − discharge lines of the NG/Fe 3 O 4 electrode exhibit an almost symmetric triangular shape (Figure 4d), indicating a high reversibility in the charge and discharge cycle [50][51][52] . Figure 5a shows the Nyquist plots of the NG/Fe 3 O 4 and RGO/Fe 3 O 4 electrodes. For both samples, the Nyquist plots consist of two distinct parts: a linear part at low frequency and a semicircle part at high frequency. The two samples exhibit similar plots. In the high-frequency part (inset of Figure 5a), the charge transfer resistance (Rct) was calculated as 0.9 and 0.85 Ω for the NG/Fe 3 O 4 and RGO/Fe 3 O 4 electrodes, respectively. The bulk electrical conductivity of the NG/Fe 3 O 4 aerogel sample was 174 S m − 1 , three times greater than that of RGO/Fe 3 O 4 (55 S m − 1 ). Although the conductive agent (conductive carbon black in this work) was absent in the preparation progress of the NG/Fe 3 O 4 electrode, the Rct of the NG/Fe 3 O 4 electrode has a similar value to that of the RGO/Fe 3 O 4 electrode, which is also lower than in some previous studies 44,53 , indicating the excellent conductivity of NG/Fe 3 O 4 .The NG/Fe 3 O 4 electrode shows excellent cycling stability, as shown in Figure 5b, and there is no obvious decrease in capacitance after 1000 cycles, which is crucial for commercial applications of supercapacitors [54][55][56] . The addition of pseudocapacitor materials is an efficient way to improve the performance of graphene-based supercapacitors. Two very important pseudocapacitor materials are transition metal compounds and conducting polymers. Generally, supercapacitors based on conducting polymers have higher specific capacitance than transition metal compounds; however, their cyclic stability is often poor 57 . Transition metal compounds have improved cyclic stability, but the weaknesses of the transition metal compounds are poor mechanical strength and low electrical conductivity. An efficient strategy to improve supercapacitor performance would be a combination of transition metal compounds in highly conductive 3D graphene frameworks. In the present work, we further developed the commonly used hydrothermal method and have shown that plasma-treated NG/Fe 3 O 4 would greatly enhance supercapacitor performance. Due to the non-toxicity, easy redox reactions and low cost of Fe 3 O 4 , it has become a good candidate as a pseudocapacitor material, although its theoretical specific capacitance is lower than that of some other transition metal compounds, such as MnO 2 , RuO 2 and V 2 O 5 20,58-60 . Table 1 summarizes the performances of supercapacitors prepared with similar 3D systems 20,58,59,[61][62][63][64][65][66][67] . For 3D graphene aerogels prepared by CVD with Ni-foam and integrated with oxides 58,59,67 , they exhibit large specific surface area and low defects, and these aerogels can be used directly without further reduction. However, the CVD methods generally require rigorous conditions, such as high temperature, templates and dangerous gas. Furthermore, the limited output prevents its expansion for industrialization. In the present work, 3D structures were prepared by in situ plasma reduction, which is a simple and feasible strategy that can be expanded for large-scale production of nitrogen-doped, graphene-based aerogel, and various active nanomaterials can be incorporated into the 3D hybrid structures. In addition, the present method demonstrates competitive specific capacitance compared with CVD methods.
Compared with other supercapacitors based on Fe 3 O 4 /RGO structures [61][62][63][64] , the present NG/Fe 3 O 4 shows excellent specific capacitance and charge transfer ability. Because of the intrinsic properties of the materials, the specific capacitance of the NG/Fe 3 O 4 electrode is  still lower than that of MnO 2 /RGO and RuO 2 /RGO electrodes 20,58,59 . However, the present method is a simple and feasible one compared with the traditional hydrothermal process and CVD method. The active nanomaterials can be further expanded for other compounds of transition metals, such as Co, Ni, Mn, Mo and V. The dip-coating and plasma treatment strategy works well for the 3D NG/Fe 3 O 4 system and would also be effective for other compounds of transition metals. The properties of some other materials, such as cobalt oxide and Ni(OH) 2 , which have 'battery' electrochemical behavior, are not compared 68 .

CONCLUSIONS
In conclusion, we have developed a plasma treatment approach to fabricate 3D NG/Fe 3 O 4 nanostructures as high-performance supercapacitor electrode materials. During the plasma process, the GO of the GO/Fe 3 O 4 materials was reduced and N-doped. The as-prepared NG/Fe 3 O 4 electrode exhibited good electrochemical performance, especially high specific capacitance, excellent stability and low charge transfer resistance. As a mature, simple, efficient, low-cost and environmentally friendly method, plasma treatment is a promising process for the preparation and modification of energy storage materials. Abbreviations: C, retention rate of C s after the cycle life test; C s , specific capacitance; CNT, carbon nanotube; GE, graphene; GS, graphene sheet; R, internal resistance obtained from the electrochemical impedance spectra measurements; T, cycles of the cycle life test.