Facile synthesis of functionalized graphene hydrogel for high performance supercapacitor with high volumetric capacitance and ultralong cycling stability

Accepted Manuscript Full Length Article Facile synthesis of functionalized graphene hydrogel for high performance supercapacitor with high volumetric capacitance and ultralong cycling stability Yangyang Tan, Dongling Wu, Tao Wang, Penggao Liu, Jia Guo, Dianzeng Jia PII: DOI: Reference:

S0169-4332(18)31475-2 https://doi.org/10.1016/j.apsusc.2018.05.161 APSUSC 39437

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

8 February 2018 19 May 2018 21 May 2018

Please cite this article as: Y. Tan, D. Wu, T. Wang, P. Liu, J. Guo, D. Jia, Facile synthesis of functionalized graphene hydrogel for high performance supercapacitor with high volumetric capacitance and ultralong cycling stability, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.05.161

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Facile synthesis of functionalized graphene hydrogel for high performance supercapacitor with high volumetric capacitance and ultralong cycling stability Yangyang Tan, Dongling Wu,* Tao Wang, Penggao Liu, Jia Guo and Dianzeng Jia* Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Xinjiang 830046, P. R. China * Corresponding authors. E-mail: [email protected] E-mail: [email protected]

Tel: +86-0991-8581183. Tel: +86-0991-8583083.

Abstract：Graphene gels have attracted intense research due to their excellent gravimetric performances in supercapacitors. However, their low volumetric capacitance and cycling stability limit their practical application. In this work, three-dimensional (3D) reduced graphene aerogel (RGAs) with hierarchical porous and high electrical conductivity were synthesized via simple low temperature chemical reduction method using graphene oxide as precursor and L-cysteine as reducing and functional agent. The sample RGAs-8 that prepared after reduction for 8 hour displayed the highest mass density, higher electrical conductivity and cross-linked porous structure. Electrochemical measurements showed that the gravimetric capacitance and the volumetric capacitance of RGAs-8 reached as high as 203.9 F g-1 and 293.6 F cm-3 at 0.5 A g-1 in 6 M KOH aqueous electrolyte, respectively. In particular, the capacitance of RGAs-8 showed no capacitance loss even after 300000 charge/discharge cycles, clearly demonstrating a robust long-term stability. Keywords: L-Cysteine, graphene, supercapacitor, long cycle life, high volumetric capacitance

1. Introduction Graphene has been an attractive material in the past few years due to its high specific surface area, excellent electrical conductivity, extraordinary chemical stability and mechanical flexibility[1-3]. However, because of the strong π-π interaction between graphene sheet, it is easy make the obtained two-dimensional (2D) graphene flakes tend to form irreversible aggregates or overlapping to graphitic structure, and thus the inherent structures and 1

properties of graphene sheets cannot be translated into the architecture built up with them, which limits their practical applications[4,5]. In order to retain the intrinsic surface and advantaged properties of individual graphene sheets as much as possible, much effort has been devoted to self assemble 2D graphene sheets into three-dimensional (3D) framework structures, such as graphene hydrogels[6-8]. The 3D framework of graphene hydrogel can effectively prevent the aggregation of individual graphene sheets. In addition, the 3D graphene hydrogel with continuous crosslinked porous network structure not only has high specific surface area, but also provides efficient ion transport channels to facilitate further infiltration of liquid into the interior of the material. Furthermore, the self supporting structure of the monolithic graphene hydrogel has good mechanical strength and electronic conductivity, which is favorable for practical applications[5-9]. In a word, 3D graphene hydrogel not only have the inherent properties of individual graphene sheets, but also exhibit outstanding performance due to their unique framework structures, and thus it has a great potential for practical applications in various fields such as energy storage, adsorption, catalysis, sensing and so on[10,11]. Many methods have been reported to prepare 3D graphene hydrogels, including electrochemical reduction[13,14], hydrothermal/solvothermal reduction[15,16], chemical reduction self-assembly and so on. Compared with other methods, chemical reduction self-assembly has the characteristics of simplicity, mild reaction conditions and rapid assembly[17-27]. Most of the chemical reduction process can be achieved within a couple of hours below 100 °C, while the porous structures and mechanical properties of 3D graphene hydrogels obtained by chemical reduction can be comparable to that of other methods[5]. In addition, the chemical reduction reaction is not limited to autoclave used in hydrothermal reactions, and the shape of the graphene hydrogel can be changed easily with the container[18]. As early as 2011, Yan et al. have synthesized 3D graphene hydrogels by using different reducing reagents such as Vitamin C, NaHSO3, Na2S, and HI at 95 °C[18]. Since then, a lot of reducing agent have been used, such as Fe2+[19], phenolic acid[20], thioglycolic acid[21],

ammonium

sulfide[22],

dopamine[23],

hydroquinone[26], phenylalanine[27] and so on. 2

thiourea[24],

carbohydrazide[25],

Due to its rich pore structure, good electrical conductivity and strong self-supporting structure, 3D graphene hydrogel has been investigated as electrode materials for application in supercapacitors, and exhibited excellent gravimetric capacitance[5]. Although the rich pore structure of graphene-based material is an important factor in achieving excellent gravimetric capacitance, simultaneously it also leads to low packing density and thus poor volumetric capacitance, which is a great limitation for its development in practical applications[28-30]. In recent years, the volumetric capacitance has become the focus of attention, and much effort has been made to improve the volumetric capacitance of 3D graphene hydrogels[31-33]. Wang and coworkers synthesized dense graphene-based hydrogel composite by introduced activated carbon. The packing density of the aerogel is increased to 0.67 g cm-3, and the gravimetric capacitance and volumetric capacitance is 128.2 F g-1 and 85.9 F cm-3 at 0.5A g-1, respectively[31]. Yang et al. obtained high density porous graphene by using the vacuum evaporation method to remove water from graphene hydrogels selectively, which have large packing density (from 0.02 to 1.58 g cm-3) and outstanding volumetric capacitance (376 F cm-3) at almost no loss in specific surface area[32]. Xu et al. reported a simple mechanical compression method to substantially increase the packing density of graphene hydrogels from 0.012 g cm-3 to 0.71 g cm-3, resulting in a high volumetric capacitance of 212 F cm-3[33]. On the other hand, the poor cyclic stability is another factor limiting the development of 3D graphene hydrogels as electrode materials for supercapacitors. Theoretically, the charge and discharge times of carbon materials as supercapacitor electrode materials could be more than 100000 cycles[34,35]. However, the cycling numbers of 3D graphene hydrogel for most reported work has much lower than theoretical level. Xu et al. prepared 3D porous graphene hydrogel by hydrothermal method at 180 °C, the capacitance retention rate was 95% after 20000 cycles[33]. Wang and coworkers used Vc as reductant for prepareation of 3D graphene hydrogel, and the capacitance is not attenuated after 20000 cycles at the current density of 10 A g-1[31]. Sheng et al. prepared 3D interpenetrating graphene electrode by electrochemical reduction method, and the capacitance was maintained at 93% after 10000 cycles[37]. Jiang et al prepared 3D graphene hydrogel by chemical reducing GO with a reductant of acetaldehyde oxime in ammonia solution under mild conditions. The best sample exhibited stable 3

cyclibility with 95.3% capacitance retention after 5000 cycles[38]. Our group prepared the nitrogen doped 3D graphene hydrogel via hydrothermal reaction using amino acids as heteroatom source, the capacitance retention rate was just 92% after 1000 cycles of charge and discharge[39]. In this paper, we apply chemical reduction method to prepare self-assemble 3D graphene hydrogel using L-cysteine as reducing and functional agent by simple heating GO aqueous suspension at 95 °C under atmospheric pressure. The influence of reduction time on the structure and morphology, and subsequently the supercapcitance performance of the prepared samples has been investigated. The 3D graphene aerogel RGAs-8 that prepared after reduction for 8 hour exhibits the packing density as high as 1.44 g cm-3, and is therefore ideal electrode materials with high volumetric capacitance. The results show that RGAs-8 displays excellent gravimetric capacitance of 203.9 F g-1and volumetric capacitance of 293.6 F cm-3 at 0.5 A g-1 in 6 M KOH electrolyte. In addition, after 300000 cycles of charge and discharge the capacitance was no attenuation, showing the long cycle life and stability.

2. Experimental section 2.1. Preparation of RGAs In order to optimize the reaction temperature, we prepared the samples at different temperatures ( 65, 75, 85, 95 and 105℃) for 6h and their capacitive performance were tested (see supporting information). Hence, the temperature of 95℃was selected. Graphene oxide (GO) was prepared from graphite powder using a modified Hummers method[41]. In a typical procedure, L-Cysteine (C3H7NO2S, 40 mg) were added to GO suspension (20 mL) with a concentration of 2 mg/mL. Subsequently, the mixture was sonicated to form a homogeneous solution in the 25 mL bottle, and then placed into an oil bath at 95℃ for 2，4，6，8 and 10 h to form the reduced graphene hydrogels. After the solution was naturally cooled down to room temperature, the resulting cylindrical structure of the hydrogels were immersed in deionized water for seven days, during which deionized water was renewed once a day to remove any unreacted reagents. The obtained reduced graphene hydrogels were lyophilized to get the corresponding reduced graphene aerogels (RGAs), which were labelled as RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively. For comparison, the samples 4

synthesized without adding L-Cysteine and a series of samples prepared at different temperatures were also invested. (for details, see the preparation method, characterization and electrochemical performance testing (Fig S1, S2) in supporting information). 2.2. Materials Characterization The morphology and structure of the prepared samples were investigated by scanning electron microscopy (SEM) using a Hitachi S-4800 system and transmission electron microscopy (TEM, JEM-2010F). X-ray diffraction (XRD) measurements were performed with a D8 Bruker diffractormeter with a Cu Kα, (λ=1.5418 Å ) radiation. Raman spectra were recorded with a Bruker spectrometer with 532 nm laser, and X-ray photoelectron spectroscopy (XPS) measurements were carried out using a ESCALab220i-XL electron spectrometer (VG Scientific) using a 300W Al Kα radiation. Fourier transform infrared (FT-IR) spectra were recorded in a range of wavenumbers from 500 to 4000 cm−1. Nitrogen physisorption was carried out at 77 K with an ASAP 2020 Physisorption Analyzer. The electrical conductivity was carried out using an RTS-9 four-point probes resistivity measurement system. 2.3. Electrochemical measurements Slices were first cut from the as-prepared cylindrical hydrogels, and immersed subsequently in aqueous electrolyte (6.0 M KOH) for 12 h to ensure the internal water was exchanged by the electrolyte. The obtained RGHs slices solvated with aqueous was placed on the aluminium foils, and compressed using hydraulic press under 30 MPa for 10 min to form high density RGHs film with a thickness of about 20 μm, during which the squeezed electrolytes were removed by filter papers. Subsequently, the film was cut into RGHs wafer (area is 1 cm2) using a embossing device. Finally, the wet RGHs wafers were lyophilized to get the corresponding dry RGAs wafer. To determine the packing density (ρ), we calculated the volume by measuring the radius (r) and thickness (h) of the compressed RGAs wafer (V= h × π × r2). We can thus obtain the packing density by dividing the net mass by the volume of the dry RGAs wafer (ρ= m / V, m is the net weight of the RGAs wafer). Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and Electrochemical impedance spectroscopy (EIS) of RGAs wafer electrodes were performed with a CHI660D electrochemical workstation (Shanghai Chenhua) using a three electrode system. Furthermore, 5

the achieved high density RGAs wafer were placed on nickel foam first and compressed under a 30 MPa pressure for 30s to as a working electrode without any additional additives. Meanwhile, the counter and reference electrodes were employed by Pt foil and standard Hg electrode, respectively, and 6.0 M KOH aqueous solution as the electrolyte. CV and GCD test in the potential range of -1.0 to -0.2V was performed at different scan rate and current densities. In addition, the EIS dates were performed using a frequency range from 0.01 Hz to 100 kHz at an alternating voltage with 5 mV amplitude. The gravimetric (Cg) and volumetric (CV) capacitances were estimated using the equation

I  t m  V

(1)

CV  Cg  

(2)

Cg 

where I is the applied current (A), Δt is the discharging time (s), m is the net mass of the active material on working electrode (g) and ΔV is the voltage of discharging process (V), and ρ (g cm-3) presents the packing density of the as-achieved RGAs wafer. The symmetric supercapacitor (SC) was fabricated with 6.0 M KOH aqueous electrolyte to evaluate the energy density. Two same RGAs wafer placed on separate nickel foam were used as electrodes, and separated by an ion-porous separator (Celgard 3501) soaked with electrolytes. The potential range for CV and GCD tests was 0–0.8 V. Volumetric energy density (Ev) and volumetric power density (Pv) were estimated as

1 EV  CV  V 2 / 3600 2

(3)

PV  EV  3600 / t

(4)

where CV is the specific capacitance of the symmetric supercapacitor calculated according to the GCD curves based on the total mass of electroactive materials in two electrodes, Δt is the discharge time (s), ΔV is the operating voltage of symmetric capacitor (V).

6

3. Results and discussion Scheme 1 shows the formation process of 3D graphene hydrogel by using L-cysteine as reducing and functional agent. As the reaction progress, the π-conjugated structures and hydrophobicity of the reduced graphene oxide layer increased due to the reduction of L-cysteine. The restored conjugated structure of rGO sheets can induce the partial overlapping or aggregating of flexible rGO sheets via π-π interaction, and resulting in the formation of 3D graphene hydrogel. At the same time, redox reaction between -SH/-NH3 of L-cysteine and oxygen containing groups of GO layers lead to L-cysteine grafted onto the graphene layers, which not only improves the wettability of the prepared samples but also provides more active sites as supercapacitor electrode material[21,39]. Then the prepared RGAs wafer with a thickness of～20μm were obtained by mechanical compression and freeze drying. Fig. 1a shows the photo of series 3D RGAs cylinders using L-cysteine as reducing and functional agent at different reduction time. It is worth noting that the 3D RGAs volume is decreased gradually with the increase of reduction time. The reason is that with the increase of reduction time, the degree of reduction of graphene oxide increased and therefore the stacking of the π-π conjugate between the graphene sheets increased, resulting in the graphene hydrogel formed and the apparent volume shrinkage. Scanning electron microscopy (SEM) was applied to observe the interior microstructures of the obtained serial RGAs samples. As shown in Fig. 1b, RGAs-2 sample displays an interconnected 3D porous structure that was formed by the cross-linking of graphene sheets. With the increase of the reduction time, the SEM images (Fig. 1c-f) show folded and reunited graphene sheets and both the pore sizes and numbers of the samples gradually reduced. When the reaction time increased to 8h, the as-prepared sample RGAs-8 is composed of randomly crumpled sheets tightly connected with each other. Especially, RGAs-10 forms a tightly stacked solid, which suggests the serious aggregation occurred. SEM images of these prepared samples at different magnifications are shown in Fig. S3. The morphology changes can be attribute to the reducing action of the L-cysteine, as the –SH that L-cysteine contained can reduce the oxygen groups of GO. It is well known that GO sheets are negatively charged in aqueous solution due to the existence of carboxyl groups. The carboxylic groups can not be fully reduced in reduced graphene oxide (rGO), and the negatively charged nature still maintained for rGO. As a result, the 7

electrostatic repulsion between rGO sheets can prevent them from aggregation and thus the interconnected wrinkles with porous structure can be formed. With the increasing of the reaction time, the carboxylic groups was further reduced, the electrostatic repulsive force between the rGO sheets decreased, and ultimately lead to the aggregation of the prepared samples. The XRD patterns of GO and RGAs are shown in Fig. 2a. For the GO sample, the diffraction peak located at 2θ =10.4°C corresponding to the (001) reflection of exfoliated GO. After the reaction, a broad diffraction peak centered at around 2θ = 23.4 and 42.5 is observed for serial RGAs, which can be assigned to the (002) and (100) crystalline planes of graphite, respectively. In addition, the 002 and 100 peaks become strong and sharp with the increasing of reduction time, indicating that the stacking of GO layers become more serious, and the degree of reduction and graphitization increased gradually[15]. Furthermore, the disordered structural features of RGAs are also investigated by Raman spectroscopy analysis (Fig. 2b). It can be seen that all the spectra exhibit a D band (at ca. 1350 cm-1) and G band (at ca. 1595 cm-1). It is known that the D band is related to the disorder or defects in the carbon structure, while the G band is ascribed to the ordered graphitic structure. The ratio of the relative intensity of the D peak and G peak (I D/IG) can be denote as the degree of graphitization, defects or the domain size of graphitization in the carbon materials[43]. With the increase of reduction time, ID/IG value gradually increased from 0.88 for GO to 1.18 for RGAs-10 (Fig. 2b), indicating the defects of these samples increased. The increased defects can be attributed to the increasing of N and S contents in these as-prepared samples, which can be confirmed by EDS mapping and XPS tests. The elements of these samples are measured by EDS mapping, and C, O, N and S elements are detected. The density and distribution of these elements are displayed in the corresponding elemental maps (Fig. S4- S8). It can be seen that N and S elements are distributed homogeneously. The result of XPS measurement shown that with the increasing of reduction time, the N、S content of the serial RGAs samples increased (Table 1), implying more N、S functional groups were grafted onto the graphene sheets, and thus the defects degree of the serial RGAs samples increased. Furthermore, the electrical conductivity of these samples was tested and shown in Fig. 3c. For the sample of RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, the electrical 8

conductivity is 33.3, 57.7, 74.7, 86.6 and 108.3 S m-1, respectively. It can be seen that with the increase of reduction time, the conductivity of RGAs increases gradually, which is due to the decrease of oxygen-containing groups of graphene. The improved electrical conductivity will be beneficial for the electrochemical performance of the samples. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition of the serial RGAs aerogels. As shown in Fig. 3a, the full survey spectra reveal four characteristic peaks located at ca. 286, 532, 400 and 164 eV, corresponding to C 1s, O 1s, N 1s and S 2p respectively in the five samples. In addition, The XPS survey spectrum and the surface atomic concentrations of GO and L-Cysteine were shown in Fig. S9 and Table. S1, respectively. Fig. 3b shows the high-resolution spectrum of C 1s in these five samples, which could be fitted by several peaks corresponding to C–C/C=C (284.8 eV), C–O (286.3 eV), C=O (287.7 eV), and the particular peak located at 285.2 eV is C–N and C–S [44]. The deconvolution of the N 1s spectra yields three peaks in each of the five samples, i.e. pyridinic N (398.4 eV), pyrrolic N (400.1 eV) and graphitic N (401.3 eV) and were shown in Fig. 3c. It can be seen that the pyridine N and pyrrole N predominates. Similarly, a detailed scan of S 2p spectrum in each of the five samples in Fig. 3d can be fitted into three peaks, two of them at 163.9 eV and 165.0 eV are assigned to thiophene-S (–C–S–C–) and –C=S– owing to the spin-orbit coupling[45]. The thiophene-S indicates that the incorporated S atoms are mainly bonded with two C atoms at the edges or defects of ordered mesoporous carbon. The peak located at 168.3 eV could be assigned to the C–SOX–, which usually form at the edges of graphene species[46]. With the increase of reduction time, the N types changed from solely pyrrolic N to pyridinic N, pyrrolic N and graphitic N, and as for the S types, C–SOX– groups become ever more obvious. The variation of N and S types is largely because that, with the increase of reaction time, rGO was reduced more thoroughly and the redox reaction between -SH/-NH3 of L-cysteine and oxygen containing groups of GO layers lead to more L-cysteine grafted onto graphene layers. As a result, N and S substitute more oxygen-containing group of rGO surface. The surface atomic concentrations of C, O, N, S and C/O were derived from the corresponding peak areas of the XPS and the results are summarized in Table 1. It can be seen that surface atomic concentration of O decreased gradually, and C, N, S and C/O increased 9

gradually with the increase of reduction time, indicating the reduction degree of RGAs and the grafting amount of -SH, -NH groups increased, which is the reason for the increase of ID/IG in the characterization of Raman. In comparison with the FT-IR spectra of serial RGAs samples in Fig. 4, with the increase of reduction time, the absorption bands of carbonyl C=O at 1709 cm-1 and epoxy C-O-C at 1203 cm-1 and 1029 cm-1 of GO were obviously decreased, indicating the reductive degree of graphene sheets increased. The bands at 733 cm-1 in the product were the characteristic bending vibrations of C-S [47]. In addition, the peak at 2852 and 2912 cm-1 belong to the vibration absorption band of -NH3+ in all the five samples, which was the typical characteristic peak of -NH3+ in amino acids. The FT-IR spectra of GO and cysteine were shown in Fig. S9. These results also indicated that L-cysteine was grafted onto the graphene layers. The above-mentioned results show that the GO sheets can be self-assembled into RGAs due to the reduction of L-cysteine. In combination with the results of XPS and IR, we speculate the possible self-assembly and functionalization mechanism of RGAs through L-cysteine, and the possible reaction process is shown in Scheme 2. The thiol groups (-SH) in L-cysteine can react with the epoxy groups on the GO layers to form C-S covalent bonds (Scheme 2a) between the L-cysteine molecules and the GO sheets[42]. The amino group (-NH2) that L-cysteine contained can react with the carboxyl group on the GO layers to form amide-like structure, and then transform into the aromatic structure (C-N) by decarbonylation (Scheme 2b). With the increase of reaction time, the other structures, such as carbon nitrogen double bonds, can also be formed by a similar reaction process between carbonyl groups on GO and amino groups on L-cysteine (Scheme 2c) [48]. These reactions not only reduced the oxygen-containing groups on the GO layer, but also grafted the L-cysteine onto the graphene sheets. As a result, graphene sheets were simultaneously self-assembled into 3D graphene hydrogels due to the strong π-π interaction between the GO sheets, and meanwhile the reduced graphene sheets were functionalized via covalent graft or doping of L-cysteine. The specific surface area and porous properties of RGAs were investigated by the nitrogen adsorption/desorption experiments. As shown in Fig. 5a, all the curves show predominantly type-IV isotherm, implying the presence of predominant meso-pores. This is 10

consistent with the pore size distribution in Fig. 5b (mes: 2-24 nm). The specific surface areas (SSAs) calculated by using the Brunauer–Emmett–Teller (BET) model are summaried in Table 2. The SSAs of serial RGAs aerogels are 95.9, 94.8, 80.0, 38.7 and 37.9 m2 g-1, while the pore volumes are 0.76, 0.74, 0.51, 0.19 and 0.18 cm3 g-1 for RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively. Obviously, with the increase of reduction time, the SSAs and total pore volume of the samples decreased gradually, and the bulk density increased gradually. Fig. 6a-c shows the cross section of the three parallel images of RGAs-8 wafer. According to the SEM image, its average thickness is 22.2 μm. The calculated bulk density (ρ) of the RGAs-8 wafer is 1.44 g cm-3 according the equation of ρ=m/V. Following the same method, the bulk density (ρ) of RGAs-2, RGAs-4, RGAs-6, and RGAs-10 are 0.79, 0.81, 0.99 and 1.47 g cm-3, respectively. The bulk density of reported 3D graphene aerogels that prepared by chemical reduction are typically in the range of 0.67–1.58 g cm-3[31-33,49,50,54]. The bulk density of the prepared serial RGAs wafer, especially RGAs-8 and RGAs-10 are higher than most of the reported 3D graphene materials. As for the electrochemical performance of these samples, the prepared RGAs wafer were used as electrode material, and tested by using CV, GCD and EIS techniques with a three-electrode system in 6 mol L-1 KOH aqueous solution. Fig. 7a shows the CV curves of the serial RGAs at scan rate of 10 mV s-1. The voltammograms of the five samples show approximately rectangular shape, which are typical for double layer capacitors. The loop area of RGAs-6 is the greatest, followed by RGAs-8, RGAs-10, RGAs-4. RGAs-2 has the smallest area, which indicating that RGAs-6 has the highest capacitance, while RGAs-2 has lowest capacitance. Fig. 7b presents the GCD curves of the serials RGAs electrodes within a potential window of -1.0 to -0.2 V vs SCE at a current density of 1 A g-1. The curves show an approximately triangular from linearity form for the all five samples, and the time to accomplish one charge–discharge cycle for RGAs-6 is much longer than that of the other four samples, implying it has the highest gravimetric capacitance, which is consistent with the result of CV test. The CV curves at different scan rate and GCD curves at different current densities of the serial RGAs samples were displayed in Fig. S10-S14. The specific capacitance of RGAs electrodes is evaluated to be 164.4, 189.4, 212.5, 203.9 and 195.5 F g-1 11

at a discharge current density of 0.5 A g -1 for RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively (Table 2). As shown in Fig. 7c, the specific capacitances of RGAs electrodes are further investigated with the increasing of charge/discharge current density from 0.5 to 100 A g-1. It can be seen that the specific capacitances decrease at high charging/discharging current density. This is a common feature of real supercapacitor since there is no enough time for the electrolyte ions to diffuse into the entire pore surface, especially at higher charge current density[36]. However, RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10 can still maintain the specific capacitances of 100.0, 102.5, 125.6.3, 124.8 and 118.8 F g-1 at a very high current density of 50 A g-1, the retention rate are 60.8, 59.4, 59.2, 61.2 and 60.8%, respectively, indicating these samples have excellent rate performance. In addition, according to the high packing density of the prepared samples, we calculate the volume specific capacity of these five RGAs. As shown in Fig. 7d, the volume specific capacitance of RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10 are 130.4, 152.4, 210.8, 293.6 and 286.5 F cm-3 at the current densities of 0.5 A g-1, respectively, which is considerably higher than the previously reported values for carbon electrode materials in aqueous electrolytes (Table. S2). The electrochemical test results show that RGAa-6 has the largest gravimetric capacitance, which is due to the larger specific surface area and electrical conductivity of the RGAs-6 sample. For a double layer capacitor, the larger specific surface area of the electrode material can provide more desorption space for the electrolyte ion. High conductivity can accelerate the transmission rate of the charge. Although RGAs-2 and RGAs-4 has larger surface area than RGAa-6, their lower degree of reduction leads to lower conductivity, so their specific capacitance are lower. When the reduction time are 8h and 10h, the reduction degree of GO increases, which makes the conductivity of RGAs-8 and RGAs-10 increased. However, due to the serious stacking of graphene sheets, the specific surface area of RGAs-8 and RGAs-10 decreases sharply. Simultaneously, this also leads to the larger packing density of RGAs-8, as a result, the RGAs-8 sample has the largest volume specific capacitance calculated by the equation (2). EIS measurements was carried out to further investigate the capacitive performance and electrical conductivity of RGAs. Fig. 7e shows the Nyquist plots and the equivalent electrical circuit (insert of Fig. 7e) of the serial RGAs. Seen from the magnified image in the high 12

frequency region, the intercept of the semicircle on the real part represents the equivalent resistance (Rs), which are calculated to be 0.48, 0.46, 0.44, 0.43 and 0.41 Ω, corresponding to RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively. The decrease of Rs from RGAs-2 to RGAs-10 should be ascribed to the increasing of electrical conductivity, which are consistent with the results of conductivity test (Fig. 2c). In the high-to-medium frequency region, the compressive semicircle can be observed, which is the characteristic of charge transfer resistance (Rct). The Rct values of RGAs-2 and RGAs-4 are 0.33 Ω and 0.25Ω, respectively, and the Rct of RGAs-6, RGAs-8 and RGAs-10 are similar (about 0.16 Ω)，which are smaller than those of RGAs-2 and RGAs-4. Detailed data are shown in Table. 2. The much smaller resistance means a faster charge transfer and charge discharge rate. Simultaneously, RGAs-2 and RGAs-4 have a steeper slope in the low frequency region, suggesting the faster ion diffusion rate in the RGAs-2 and RGAs-4 electrode. This is may be due to RGAs-2 and RGAs-4 have higher specific surface area and more open cross-linked structure than other RGAs[51]. The low resistances of the serial RGAs, could be attributed to their 3D cross-linked structure and excellent electrical conductivity. From the bode phase diagrams (Figs. 7f), it can be seen that the phase angle of RGAs-8 is 85° and is near to that of an ideal capacitor 90°, which indicates the fast diffusion of electrolyte ions in the abundant mesoporous/macropores channels of RGAs-8[51]. The relaxation time constant (Τ0) is calculated by the equation of Τ0=1/f0, in which f0 is the frequency at angle of 45°. The calculated Τ0 of RGAs-8 is 0.56 s, which further confirms the fast adsorption and diffusion of electrolyte ions on the surface of RGAs-8 electrodes. This can be attributed to good electrical conductivity and porous structure of RGAs-8[51,52]. It is well known that cycling stability is a crucial factor required for supercapacitor electrodes[34-36], and thus RGAs-8 electrode was selected to test the cycling stability. As is shown in Fig 7g, after 300000 cycles, the specific capacitance of RGAs-8 was no attenuated, the coulombic efficiency always remains above 100%, and the specific capacitance can reach to 110.4% of the initial one, demonstrating a robust long-term stability. Table. S2 gives a comparison of the present electrochemical data with the reported works on supercapacitor performance, which further confirms the best cycling stability of the present electrode material among the reported carbon materials. Based on the above-mentioned structural and morphological results , the reason for the 13

excellent cycling stability of RGAs-8 could be contribute to the following points. First, RGAs-8 has the higher conductivity (86.6 s/m), which means low charge transfer resistance and fast charge transfer rate and it can better maintain the long-time reversible reaction and electrode integrity. Secondly, the strong 3D crosslinking structure and the heteroatom doping are not only facilitates fast ion diffusion through providing unimpeded channels during rapid charge/discharge processes, but also improving the wettability and utilization ratio of the material channels[53]. Furthermore, there is a very interesting phenomenon that the specific capacitance of RGAs-8 did not decrease in the 300000 cycles in 6 M KOH but actually having increased by 10.4%. This phenomenon may be caused by ‘‘electro-activation’’[60]. This ‘‘electro-activation’’ is suggested to occur because the graphene sheets would move to adjust to different electrolyte ions. The long time charging and discharging should also help the ions in the electrolyte intercalate into the spaces between the graphene layers and therefore, producing more surface area for the ions to access to. As a result, more active sites were exposed, which makes the grafts containing S functional groups providing more pseudocapacitive (Fig. 8a-b). This is attributed to the observation of increasing specific capacitance. To verify the reason for the increase in capacitance, the electrochemical performance of the obtained RGAs-8 was also performed after 300000 cycles in three electrode system. It can be seen that the obvious redox peaks appears in the CV curve at ca. -0.7 to -0.5 V of the anodic scan and ca. -0.8 to -0.6 V of the cathodic scan at scan rate of 30 mV s-1 after the cycle test (Fig. 8a). These redox peaks indicate the contribution of pseudocapacitance by the surface reaction of S functional groups[54]. The reaction process are shown by the following formula a and b.

Simultaneously, the pseudocapacitance also causes the distortion of the GCD curves. As shown in the Fig. 8b, the GCD curve showed a weak bend at about 0.63V, which is consistent with the results of the CV curve. This indicated that the exposed S functional groups as active 14

sites providing pseudo capacitors during long time charge and discharge processes[55]. Nyquist plots was shown in Fig. 8c, the smaller semicircle intercept in the high frequency region and the steeper slope in the low frequency region indicates that RGAs-8 after 300000 cycles has a smaller charge transfer resistance and faster ion transfer rate. In addition, in order to eliminate the potential of capacitor from the nickel foam after 300000 cycles, we tested the CV and CP of the nickel foam after 300000 cycles (remove the active materials by ultrasonic treated). After 300000 cycles, the CV curve of foamed nickel (Fig. 8a) shows a rather small area, which indicates that the foam nickel has a very small specific capacitance (18.3 F g-1 at 0.5 A g-1, Fig. S15). The specific capacitance (18.3 F g-1) is provided by the residual active materials, but higher than that of pure foam nickel (2.6 F g-1 at 0.5 A g-1). Furthermore, we tested the XRD of the nickel foam after 300000 cycles (remove the active materials by ultrasonic treated), the 002 peaks in the XRD spectrum (Fig. 8d) is the characteristic of the carbon. In addition, the XRD spectrum of foam nickel after 300000 cycles and pure nickel foam did not change, indicating that nickel foam did not contribute to the increase of capacitance after 300000 cycles. In order to further investigate the reliable capacitive performance of RGAs-8 electrode, it is employed to assemble symmetrical capacitors (RGAs-8//RGAs-8) in 6 mol L-1 KOH aqueous electrolyte. As showed in Fig. 9a, the GCD profiles also show highly linear at a current density of 0.5-10 A g-1, indicating excellent electrochemical reversibility and low equivalent series resistance in the RGAs-8//RGAs-8 symmetric supercapacitor. The calculated specific capacitance based on GCD curves was estimated to be 75.9, 71.3, 67.8, 64.4, 63.0 and 60.0 F g-1 at current densities of 0.5, 1, 2, 4, 8 and 10 A g-1, respectively. The volumetric energy density and power density of RGAs-8 are calculated and the Ragone plot is shown in Fig. 9b. And the values of volume energy density and power density of RGAs-8//RGAs-8 symmetric supercapacitor at different current densities were shown in table S3. It is worth noting that the assembled symmetric supercapacitor exhibits the largest volumetric energy density of 11.2 Wh L-1 at a power density of 125.0 W L-1. At a high power density of 12.5 kW L-1, it still maintains a high energy density of 7.7 Wh L-1, This volumetric energy density is much better than that those reported O/N co-doped graphene foams (8 Wh L-1 at 56 W L-1) [57], rGO hydrogel (6 Wh L-1 at 350 W L-1) [58], activated carbon nanospheres (6 Wh L-1 at 15

80 W L-1) [51], PPD-graphene film (0.8v) (7.3 Wh L-1 at 320 W L-1) [59], rGO/Mn3O4 nanocrystals hybrid fiber(4.05 Wh L-1 at 10 W L-1) [56], and hierarchical porous carbons materials (8.4 Wh L-1 at 13.9 W L-1) [52]. We tested the cycling stability of the symmetric supercapacitor. As shown in Fig.9c, after 100000 cycles, the specific capacitance of RGAs-8 ∥RGAs-8 was no attenuated, the coulombic efficiency always remains above 100%, and the specific capacitance can reach to 104.1% of the initial one, demonstrating a robust long-term stability. Fig. 9d is the schematic illustration of SC configuration. Particularly, the four RGAs-8//RGAs-8 SCs can even power 5 LEDs in series for 7 min (Fig. 9e). Therefore, the as-assembled SCs had high energy density in practical application.

4. Conclusions In conclusion, 3D reduced graphene aerogels (RGAs) with hierarchical pore structures have been prepared in a facile chemical reduction method by using L-Cysteine as reducing and functional agent. Electrochemical measurements demonstrate that RGAs have the capacitance of 164.4, 189.4, 212.5, 203.9 and 195.5 F g-1 at 0.5 A g-1 for RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively. They maintain high capacitances of 100.0, 102.5, 125.6, 124.8 and 118.8 F g-1 at a very high current density of 50 A g-1. Most important of all, these materials exhibit higher bulk density and volume specific capacitance. The volume specific capacitance of RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10 are 130.4, 152.4, 210.8, 293.6 and 286.5 F cm-3 at the current densities of 0.5 A g-1, respectively. Besides, the specific capacitance of RGAs-8 electrode still remained 110.4% and the coulombic efficiency remains above 100% of the initial one after 300000 cycles, clearly demonstrating an excellent long-term stability and reversibility. These results demonstrate that the asprepared RGAs presented here might be a promising candidate for the electrode materials of high-performance supercapacitors. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21561029, 21471127 and 21763023). Thanks for the Center of Testing and Analysis, Xinjiang University, about the XPS characterization. 16

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Figure caption: Scheme 1. Schematic diagram of the preparation of RGAs wafer. Fig. 1. (a) Photographs of the aerogels prepared by chemical reduction at different times. (b-f) SEM images of RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10 samples, respectively. Fig. 2. (a) XRD patterns, (b) Raman spectra and (c) Conductivity of RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively. Fig. 3. (a) XPS survey spectrum and (b-d) C1s, N1s, S2p region spectra of RGAs-2, RGAs-4, RGAs-6, RGAs-8 and RGAs-10, respectively. Fig. 4. IR spectra of RGAs-2, RGAs-4, RGAs-6, RGAs-8, RGAs-10. Fig. 5. (a) N2 adsorption-desorption isotherms and (b) Pore size distribution of RGAs-2, RGAs-4, RGAs-6, RGAs-8, RGAs-10. Scheme 2. Illustration of the controlled assembly of RGAs by L-cysteine. Inset (a) shows the possible reaction pathways for sulphur doping. Insets (b and c) show the possible reaction pathways for nitrogen doping. Fig. 6. (a-c) SEM image of cross-sectional of the compressed RGAs-8 wafer. Fig. 7 Electrochemical characteristics in the three-electrode system (a) Cyclic votammograms (CVs) curves measured at 10 mV s-1 and (b) Galvanostatic charge/discharge (GCD) curves tested at a current density of 1 A g-1 of RGAs-2, RGAs-4, RGAs-6, RGAs-8, RGAs-10, respectively. (c) Gravimetric and (d) Volumetric capacitances of the serial RGAs at different current densities. (e) Nyquist plots and diagram of equivalent circuit (inset) (f) Bode plots of phase angle versus frequency of the serial RGAs. (g) Cycling stability of the RGAs-8 at 4 A g-1 Fig. 8 Electrochemical characteristics of RGAs-8 in the three-electrode system before and after 300000 cycles: (a) CV curves measured at 30 mV s-1 (b) GCD curves tested at a current density of 1 A g-1 (c) Nyquist plots collected by EIS (d) XRD pattern of nickel foam after 300000 cycles and pure nickel foam. Fig. 9 Electrochemical characteristics of the RGAs-8//RGAs-8 symmetric supercapacitor in a two electrode 22

system: (a) GCD curves at different current densities. (b) Ragone plot of energy density versus power density (c) Cycling stability of symmetric supercapacitor at 8 A g-1 (d) Schematic illustration of SC configuration. (e) LED indicator lighted up by four SCs in series. Table 1 The concentrations of C, O, N, S and the C/O atomic ratio of RGAs samples by XPS analyses. Table 2 BET surface area, specific capacitance properties at 0.5 A g-1, electrical conductivity and resistance of the as-prepared RGAs.

Scheme 1.

23

Fig. 1.

Fig. 2.

24

Fig. 3.

25

Fig. 4.

Fig. 5.

26

Scheme 2.

Fig. 6.

27

Fig. 7.

28

Fig. 8.

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Fig. 9.

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Table 1. Different N functionalities(at %)

Different S functionalities(at %)

Sample

C

O

C/O

Total N (at %)

Npyridinic

Npyrrolic

Ngraphitic

Total S (at%)

-C-S

-C=S-

-SOx

RGAs-2

78.43

19.83

3.9

0.92

_

0.92

_

0.82

0.50

0.28

0.04

RGAs-4

83.90

14.25

5.8

0.96

0.03

0.93

_

0.89

0.48

0.33

0.08

RGAs-6

85.43

11.64

7.3

1.21

0.07

1.05

0.09

1.70

0.83

0.71

0.16

RGAs-8

85.75

10.94

7.8

1.23

0.37

0.52

0.34

2.10

0.95

0.84

0.31

RGAs-10

86.15

10.10

8.5

1.36

0.42

0.55

0.39

2.43

1.11

0.89

0.43

Table 2. SBET

Vtotal

ρ

Cg

Cv

[m2 g-1]

[cm3 g-1]

[g cm-3]

[F g-1]

[F cm-3]

RGAs-2

95.9

0.76

0.79

164.4

RGAs-4

94.8

0.74

0.81

RGAs-6

80.0

0.51

RGAs-8

38.7

RGAs-10

37.9

Sample

Electronic

Rs

Rct

conductivity(S/m)

(Ω)

(Ω)

130.4

33.3

0.48

0.33

189.4

152.4

57.7

0.46

0.25

0.99

212.5

210.8

74.7

0.44

0.16

0.19

1.44

203.9

293.6

84.6

0.43

0.16

0.18

1.47

195.5

286.5

108.3

0.41

0.16

Synopsis Three-dimensional (3D) reduced graphene aerogel (RGAs) with high volumetric capacitance and ultralong cycling stability were synthesized via simple chemical reduction method using L-cysteine as reducing and functional agent.

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Highlights 

3D graphene aerogel was prepared using L-cysteine as reducing and functional agent by simple heating GO aqueous suspension at 95 °C.



The prepared RGAs sample presented high volumetric capacitance and ultra long cycle stability when it was used as active electrode materials for the storage of energy in supercapacitors.



The bulk density of the prepared sample was up to 1.44g cm-3, the volumetric capacitance was 293.6 F cm-3, and the capacity retention rate could maintain 110.4% after 300000 cycles of charge and discharge.

32