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Research Article pubs.acs.org/journal/ascecg

Fabrication of Novel Ternary Three-Dimensional RuO2/ Graphitic‑C3N4@reduced Graphene Oxide Aerogel Composites for Supercapacitors Jiao Zhang,† Jie Ding,† Chuanqi Li,† Baojun Li,† Dan Li,† Zhongyi Liu,*,† Qiang Cai,‡ Jianmin Zhang,† and Yushan Liu*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, People’s Republic of China ‡ Key Laboratory for Advanced Materials of Ministry of Education and College of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, one modified electrostatic selfassembly strategy was proposed to achieve new ternary threedimensional (3D) graphene-based aerogel composite for smart supercapacitor (SC) candidates, where graphitic-C3N4 (gC3N4) was selected as a component of increasing the electrochemical active center. Herein, the precursor RuO2/gC3N4@graphene oxide was prepared by protonized g-C3N4, positively charged RuO2 colloid nanoparticles (NPs), and negatively charged graphene oxide (GO), which was subsequently transformed to the designed 3D RuO2/gC3N4@reduced graphene oxide (rGO) aerogel composite (RCGA) via simple hydrothermal and lyophilization processes. In the electrochemical study, RCGA exhibited superior capacitive performance with a high specific capacitance of 704.3 F g−1 at a current density of 0.5 A g−1 and excellent performance. Superior performance also was obtained in the asymmetric device. These results highlight the potential applications of this facile strategy in fabricating N-doped 3D rGO aerogel composite for high-performance SC. KEYWORDS: RuO2, g-C3N4 graphene, 3D aerogel, supercapacitor

1. INTRODUCTION During the recent years, graphene-based composite materials have displayed their muscle in the energy storage area, especially in supercapacitor (SC) and lithium ion batteries (LIB), because of their unique advantages.1−5 Many chemists and material scientists constantly researched new graphenebased composite systems to enrich this scaffold and develop their energy device capabilities.6−8 In comparison with the graphene-based binary composite material easily obtained,9−12 a more sophisticated multiple system has been gradually attracting increased attention.13−15 However, the integration of each component advantage and the weakness suppression in one multiple system to achieve the smart capability is commonly a tricky work that can be supported by meticulous design and clever preparation methods. As we know, both the RuO216−21 and graphene7,21−25 have been used as good candidates electrode material for SC, but the actual electrochemical performance is hindered by the serious self-aggregation of RuO2 nanoparticles (NPs)26,27 and the graphene nanosheets restacking in the preparation process.28,29 © 2017 American Chemical Society

The existence of RuO2/graphene binary composite material could eliminate these drawbacks, because of the synergistic effect between the two different components.17,30−32 Currently, one analogue of graphene, two-dimensional graphitic-C3N4 (gC3N4) with high N atom content, has been explored in SC electrode material for its unique property,33,34 where a tremendous number of electrochemical active centers are injected into the system to improve the electrochemical performance.35,36 However, in g-C3N4-based composite material, there is a major limitation, in which bad conductivity of g-C3N4 is not conducive for rapid electron movement.33,37 Herein, to construct a new RuO2/g-C3N4@graphene ternary composite system, is one attractive choice to select the excellent candidate of SC electrode material. In our previous work, we reported that graphene-based binary composites could be easily prepared by a facile electrostatic-assembly process.38 FurtherReceived: February 6, 2017 Revised: April 11, 2017 Published: May 3, 2017 4982

DOI: 10.1021/acssuschemeng.7b00358 ACS Sustainable Chem. Eng. 2017, 5, 4982−4991

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ACS Sustainable Chemistry & Engineering more, a colloidal electrostatic self-assembly method was improved to successfully obtain several types of graphenebased functional composites for different applications.38−40 In this paper, we displayed the expansion of this facile method to accomplish the new RuO2/g-C3N4@graphene ternary composite material. At first, both RuO2 colloidal NPs and protonated g-C3N4 were positively charged,37 which were easily captured by the negatively charged graphene oxide (GO).38,41 Because of strong electrostatic interaction, the selfaggregation of RuO2 NPs and the graphene nanosheets restacking in the reduction process could be effectively suppressed to obtain suitable arrangements of the three components, while these two componentsRuO2 NPs and g-C3N4also would enhance the entire specific surface area and electric double layer capacitance of graphene (EDLC). After the simple hydrothermal and lyophilization treatment processes, novel three-dimensional (3D) RuO2/g-C3N4/rGO aerogel composite (RCGA) was finally obtained. Compared with the other N-doping graphene techniques reported previously,42,43 the N atom could be introduced into the 3D graphene aerogel composite homogeneously via this facile assembly route. For the purpose of comparison, the binary reference 3D RuO2/rGO aerogel composite (RGA) was also prepared by the same procedure, only without the injection of g-C3N4. In the SC performance tests, high specific capacitances of 704.3 F g−1 for RCGA and 560.3 F g−1 for RGA were obtained at the current density of 0.5 A g−1, indicating the obviously improvement due to the existence of g-C3N4 in aerogel. In the asymmetric device fabricated by activated carbon (AC) cathode and 3D RCGA anode, a capacitance of 22.4 F g−1 with an energy density of 2.50 Wh kg−1 at 1 A g−1 and a power density of 4500 W kg−1 at 10 A g−1 was obtained. More importantly, these results would display a new strategy to construct the smart N-doped graphene-based multiple composite system.

within 5 h. A stable RuO2 colloidal suspension was obtained at room temperature. 2.3. Synthesis of RCGA, RGA, and GA. First, GO (0.0838 g) and protonated g-C3N4 (0.0186 g) were dissolved in distilled water (10 mL). In this prepared GO/g-C3N4 aqueous solution, 20 mL of a RuO2 colloidal suspension (0.0838 g) was added and stirred in 30 min. Such mixture was sealed in a 50 mL Teflon-lined autoclave at 150 °C for 20 h. When the autoclave was naturally cooled to room temperature, a black cylindrical hydrogel was obtained. Finally, the black cylinder was dried in the freeze dryer with the original shape. For comparison, RuO2/rGO aerogel composite (RGA) and pure oxide graphene aerogel (GA) were also prepared in the similar procedure for control experiment (see the Supporting Information). 2.4. Material Characterizations. The morphology and microstructure of samples were revealed by transmission electron microscopy (TEM) (JEOL, Model JEM-2100F) and scanning electron microscopy (SEM) (JEOL, Model JSM-6301F). The phase structures of products were detected using an X-ray diffraction (XRD) analysis system (Bruker, Model D8 Avance) with Cu Kα radiation (λ = 1.5418 Å). Herein, some structural information was illustrated by Raman spectra (Renishaw, Model RM-1000 with excitation from the 514 nm line of an Ar-ion laser with a power of ∼5 mW) and Fourier transform infrared (FT-IR) (Bruker Optics, Model IFS 66 v/S, in the wavelength range from 400 cm−1 to 4000 cm−1) spectra. Thermogravimetric analysis (TGA) was carried out using a TGA system (Netzsch, Model STA 449 F3) in an air atmosphere with a constant heating rate of 10 °C min−1 from 30 °C to 700 °C. X-ray photoelectron spectroscopy (XPS) was measured by a photoelectron spectrometer (Thermo Fisher, Model ESCALAB 250Xi). Brunauer−Emmett−Teller (BET) measurement was performed using an ASAP 2420 instrument (Micromeritics Instrument Corporation). The pore size distribution was measured using mercury intrusion porosimetry (Quantachrome, Model PM33GT-17). The electrical conductivities of the GA, RGA, and RCGA aerogel composite were measured using an SDY-4 fourpoint probe meter at room temperature, using the method on a pressed pellet that is described by the formula

2. EXPERIMENTAL SECTION

where σ refers to electrical conductivity, V is the voltage, I is the current, D is the diameter of the pellets, W is the thickness of the pellets, S is the average space between the probes, F(D/S) is the amendatory coefficient of the diameter of the pellets, F(W/S) is the amendatory coefficient of the thickness of the pellets, and Fsp is the amendatory coefficient of the space between the probes. The pellets were obtained by subjecting the powder sample to a pressure of 30 MPa. The reproducibility of the result was checked by measuring the resistance three times for each pellet. 2.5. Electrochemical Measurements. The RCGA, RGA, and GA electrodes were prepared in the same procedure. In detail, 80 wt % active material (3 mg), 15 wt % carbon black, and 5 wt % Nafion membrane were mixed in ethanol to form a slurry with the proper viscosity. Subsequently, the slurry was uniformly laid on a piece of Ni foam (∼1 cm2) and dried at 40 °C for 5 h. Before the electrochemical experiment, such composite Ni foam electrode was pressed for 1 min under 1.0 MPa. The electrochemical behavior of the electrode prepared as described above was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge−discharge (GCD). CV was performed using a Model CHI660E electrochemical workstation at different voltage scan rates (10, 20, 30, 40, and 50 mV s−1). The data of EIS were collected by an electrochemical workstation (CH Instruments, Model CHI660E) in a frequency range from 1 Hz to 100 kHz at open circuit potential. In the CV and EIS tests, a three-electrode glass cell was used that contained a work electrode prepared as described above, a platinum counter electrode, and a standard calomel reference electrode (SCE). The GCD measurements were conducted on a LAND battery system at different current densities (0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 10.0 A g−1).

⎛ 1 ⎞ ⎛V ⎞ ⎛D⎞ ⎛W ⎞ σ = ⎜ ⎟ = ⎜ ⎟ × F ⎜ ⎟ × F ⎜ ⎟ × Fsp ⎝S⎠ ⎝S⎠ ⎝ ρ⎠ ⎝ I ⎠

2.1. Chemicals. Graphite powder, hydrogen peroxide (H2O2, ≥30%) were provided by Sinopharm Chemical Reagent Co., Ltd. Sodium nitrate (NaNO3), concentrated sulfuric acid (H2SO4, 98%) and potassium permanganate (KMnO4) were analytical reagents purchased from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. Ruthenium(III) chloride hydrate (RuCl3·nH2O, 37%) was received from Kunming Institute of Precious Metals. Melamine was obtained from Tianjin Kermel Chemical Reagent Co., Ltd. Carbon black (acetylene, compressed, 99.9%), Nafion membrane (5 wt % in lower aliphatic alcohols and water) and activated black were purchased from Aldrich. All of the chemical reagents were analytical grade and used directly without any further purification. Deionized water was used throughout the entire experiment. 2.2. Synthesis of GO, g-C3N4, and RuO2 Colloidal NPs. GO was synthesized according to the modified Hummer’s method previously reported.38,41 Protonated g-C3N4 was prepared by the method reported previously.33,44 Bulky yellow g-C3N4 was prepared by heating melamine in 4 h at 550 °C with a ramp rate of 10 °C min−1, where the cooling process to room temperature in a muffle furnace. Subsequently, this product (1 g) was milled into powder and protonated by 37% HCl solution (10 mL) within another 3 h. Protonated g-C3N4 was finally obtained after centrifugal washing with water and drying at 105 °C in air overnight. A colloidal suspension of RuO2 NPs was obtained by the method reported in a previous paper.45 0.1308 g (0.63 mmol) RuCl3·3H2O was dissolved in 40 mL of distilled water. Then, 3 mL of 30% H2O2 (aqueous) was diluted with 20 mL of distilled water, which was added dropwise into the RuCl3 solution. As follows, such an obtained suspension was added in an oven at 95 °C 4983

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ACS Sustainable Chemistry & Engineering Scheme 1. Construction Process of RCGA

The average specific capacitance was estimated from the discharge slope, according to the method reported in the literature. The asymmetric SC device was assembled with active material as work electrode and AC was adapted as a counter electrode. A Celgard membrane (3501, 25 μm thick) was applied to separate the two electrodes. The AC electrode was prepared by mixing 80 wt % active carbon, 15 wt % carbon black, and 5 wt % Nafion membrane binder. The homogeneous slurry was then coated onto the Ni foam and dried at 40 °C for 5 h. The asymmetric electrochemical measurements were carried out in 6 M KOH aqueous solution using two electrodes and the separator pressed by organic glass.

Compared with the reference RGA sample (Figure 1a), the TEM image of RCGA (Figure 1e) also showed the sheetlike morphology that was in the range of several micrometers. At the same time, the ultratiny RuO2 NPs (Figures 1b and 1f, 1.5 ± 0.2 nm) were loaded onto the surface of rGO sheets or intercalated into rGO layers, which also was similar to that in the reference RGA. Obviously, no free RuO2 NPs, g-C3N4, or

3. RESULTS AND DISCUSSION In order to easily inject g-C3N4 into the graphene/RuO2 aerogel composite, RuO2 colloidal NPs and protonated gC3N4 were selected to capture graphene oxide (GO) prepared via the Hummers’ method,38,41 which was supported by the results of zeta potential experiments (RuO2 colloidal NPs, +33.3 mV; protonated g-C3N4, +31.4 mV; and GO, −43.7 mV) (see Figure S1 in the Supporting Information). At first, because of the strong electrostatic interaction between the oppositely charged components, the connection of protonated g-C3N4 and GO was tight,37,44 which could suppress the restacking of the graphene nanosheets, and inject a tremendous number of new electrochemical active centers into the composite. On the other hand, the subsequently added RuO2 NPs was synchronously pulled by the negatively charged GO and pushed by protonated g-C3N4.39−41,46 This process was not only effectively decreased the self-aggregation of RuO2 NPs, but also accomplished the suitable arrangement of three components. In addition, such arrangement could be further stabilized by other noncovalent interactions (van der Waals force and hydrogen bonding) and chemisorption process.38,39,41,46 When the yellowish-white gC3N4 dispersion and the black RuO2 colloid suspension were stepwise added into the yellow GO aqueous suspension, a brown mixture of RuO2/g-C3N4/GO was distinctly observed (see Scheme 1, as well as Figure S2a in the Supporting Information). Because of the excellent hydrophilicity and low density of RuO2, the RuO2/g-C3N4/GO suspension was sufficiently stable to form one hydrogel in the next treatment process (Figure S2b in the Supporting Information), which was slightly different from the examples reported previously in our laboratory.38,39 When this suspension was treated via the following hydrothermal and lyophilization processes, an RCGA aerogel composite was obtained (Figure S2c in the Supporting Information) and further confirmed by TEM, SEM, FT-IR, Raman, XRD, and XPS measurements. For comparison, reference RGA without g-C3N4 and pure GA were prepared via a similar procedure.

Figure 1. (a, b) TEM images of RGA (insets show the particle size distributions of RGA); (c) HRTEM image of RGA; and (d) SAED pattern of RGA. (e, f) TEM images of RCGA (insets shows the particle size distributions of RCGA); (g) HRTEM image of RCGA; and (h) SAED pattern of RCGA. 4984

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Figure 2. (a) SEM image (left) and EDS energy spectrum (right) of RGA. (b) SEM image (left) and EDS energy spectrum (right) of RCGA; an elemental map of the images of RCGA is shown along the bottom of the panel.

Figure 3. (a) TGA curves of GA, RGA, and RCGA; (b) XRD patterns of GA, RGA, and RCGA; (c) FT-IR spectra of GA, RGA, and RCGA; and (d) Raman spectra of GA, RGA, and RCGA. Inset in panel (d) shows an enlarged Raman spectra of RCGA in the range of 150−700 cm−1.

rGO was observed in the images of RCGA or RGA, which roughly indicated the effective combination of the three components via this facile self-assembly process. The g-C3N4 sheets are ∼50−100 nm in diameter, as shown in Figure S2 in the Supporting Information. The RuO2-sparse regions could be observed from Figure 1f, which might be deduced to the electrostatic repulsion force between RuO2 NPs and g-C3N4. As

shown in Figures 1c and 1g, the edges and wrinkles of graphene were observed directly from the HRTEM image of RCGA and the lattice distance of 0.22 and 0.20 nm matched well with the (200) and (210) planes of RuO2 (JPCS File No. 00-018-1139), respectively.45 On the other hand, the selected area electron diffraction (SAED) pattern (Figures 1d and 1h) further showed characteristic rings corresponding to (210) and (200) planes of 4985

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Figure 4. (a) XPS spectrum of GA, RGA, and RCGA; the high spectra of (b) Ru 3d, (c) Ru 3p, and (d) O 1s from the XPS spectrum of RCGA.

hydrated RuO245 and visible (002) plane of rGO,37 which were indicative of the combination of the two components in RGA and RCGA. Similarly, the SAED of sample RCGA (Figure 1h) did not only show the hydrated RuO2 ((210) and (200) plane signals) and rGO ((002) plane signal) characterstics, but also other weak diffraction rings were observed that were ascribed to (411) and (320) plane signals of g-C3N4,37 respectively. This is powerful evidence to indicate the combination of three components in RCGA, which also was supported by the HRTEM image of RCGA (Figure 1g). The morphology and elemental distribution of RGA and RCGA were further revealed by SEM analyses. As shown in Figures 2a and 2b, the thin layers of rGO sheets overlapped, intertwisted, and interconnected to construct a porous threedimensional (3D) macrostructure. In the energy-dispersive Xray spectroscopy (EDX) analyses, the existence of elemental C, O, and Ru was clear in RGA, whereas, except for the present elemental C, N, O, and Ru elements, the signal of N element also was visibly observed in RCGA, indicating the existence of g-C3N4 in the graphene aerogel composite.47 According to the results of element analysis, a nitrogen content of 5.93% was detected (see Table S1 in the Supporting Information) in RCGA, indicating the successful injection of g-C3N4 in the 3D sample. As we know, the combination of RuO2 NPs and GO facilitates the formation of nanopores and nanochannels, which may act as electrolyte reservoirs to impel the transportation of ions under the electrochemical redox-active conditions. The BET surface area of RCGA was evaluated to be 127 m2 g−1 in the N2 adsorption−desorption measurement. As shown in Figure S4 in the Supporting Information, the pore size distribution of the samples was measured by mercury intrusion porosimetry, indicating the existence of unique macroporous cavities in the sample besides the mesopore. The pore volume of RCGA was 0.62 cm3 g−1.

Briefly, the thermal stability of RuO2 composite, either RCGA or RGA, was relatively poorer than that of GA (Figure 3a), which remained stable until 600 °C. In detail, the weight losses in TGA curves of RCGA and RGA below 200 °C were attributed to the evaporation of physically adsorbed water in the aerogel composite. The weight loss between 200 °C and 400 °C in the curve of either RCGA or RGA was ascribed to the departure of water molecules of hydrous RuO2 in the composite, which was also the organization process of crystalline RuO2. As follows, such a process might trigger the subsequently large mass loss at 400 °C, which was the elimination of the graphene plane in either RGA or RCGA. From the TGA curves of RGA and RCGA, weight losses of ∼59% and 58% occurred because of the decomposition of rGO and C3N4 under an air atmosphere.40,48 The remaining stable curves indicated the RuO2 constitutes ∼41% and 42% in RGA and RCGA, respectively. In comparison with GA, the absent peak at 9.7° (001) corresponding to GO (Figure S5 in the Supporting Information), and the relatively broad characteristic peak at 25° in the curve of RGA or RCGA, implied the complete reduction of GO to rGO (see Figure 3b).49,50 This result was also proved by the FTIR and Raman analyses (Figures 3c and 3d). For example, either in RGA or RCGA, the absent peak at 3403 cm−1 indicated the reduction of GO to rGO.38,39,49,50 Furthermore, the peaks at 1584 and 1250 cm−1 were ascribed to the typical signal of aromatic C−N heterocycles (trigonal (N−(C)3) (full condensation) and bridging C−NH−C units (partial condensation)),33,51 indicating the presence of g-C3N4 in RCGA. For all three aerogel samples, the G-band corresponding to the sp2 hybridized carbon is observed at 1588 cm−1 and the D-band originating from the disordered carbon is observed at 1341 cm−1. In the Raman spectra of RGA and RCGA (Figure 2b), the intensity ratio between the D-band and the G-band is distinctly higher than that of GA (1.31), 4986

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Figure 5. (a, b) CV curves of RCGA and RGA at different scan rates, respectively; (c, d) GCD curves of RCGA and RGA at different current densities, respectively; (e) Nyquist plots of RCGA and RGA; and (f) cycling stability of RCGA and RGA.

529.3 and 530.7 eV. The presence of H2O molecules at 533 eV might be induced from the water adsorbed from the atmosphere before the XPS investigation. In this paper, the electrochemical performances of RCGA and the reference GA and RGA were investigated in detail at room temperature for the potential application of SC electrode material. As shown in Figures 5a and 5b, both RCGA and the reference RGA showed excellent rectangular-like shapes with broad redox peaks, which corresponded to a typical pseudocapacitive performance of hydrated RuO2,16,21,52 respectively. At the same time, the character of double layer capacitance at different scan rates (10, 20, 30, 40, 50 mV s−1) in the range of −0.1 V to 1.1 V (vs saturated calomel electrode, SCE) indicated the excellent capacitive behavior of RCGA (Figure 5a). Significantly, the current density of RCGA is larger than that of RGA at the same scan rate, because of the addition of g-C3N4 in the ternary aerogel hybrids. Herein, in 6 M KOH aqueous electrolyte (Figures 5c and 5d), the ideal linear symmetric triangular charge−discharge GCD curves of RCGA and RGA beautifully show the charge−discharge coulombic efficiency and reversibility during the electrochemical processes. No obvious IR drop was observed in the initial part of the discharge profile

which is consistent with the literature. The obvious increased D-band intensity of RCGA and RGA results from the apparent structure interaction between RuO2 NPs and rGO sheets. Especially, with the doping of hydrous RuO2 and g-C3N4, the intensity ratio was increased from 1.42 (RGA) to 1.48 (RCGA; see Figure 3d). Moreover, two weak bands at 402 and 640 cm−1 of RuO2 phase were observed in the RCGA sample.31 In comparison with the GA sample (Figure 4a), the Ru 3p peak of RGA indicated the successful incorporation of RuO2 in RGA, which also was observed in RCGA. Moreover, the obvious N 1s peak was clear information that indicated the presence of the g-C3N4 in RCGA. As shown in Figure 4b, the XPS data in the common C 1s region of RCGA is somewhat weird, because of the inclusion of the Ru 3d peaks. Herein, the peaks at 281.7 and 283.3 eV were assigned to Ru 3d5/2 of RuO2,17,18 while the peak located at 284.6, 286.2, and 288.6 eV were originated from C 1s.17,18,49,50 This partial overlap of C 1s and Ru 3d3/2 interfered with the quantitative estimation of oxidation states in the composite. In order to eliminate the interference of carbon, the Ru 3p fine spectrum was introduced (Figure 4c), where the peaks at 462.9 and 464.6 eV were ascribed to RuO2 and RuOH, respectively.17,18 In addition, the oxygen spectra (Figure 4d) showed the presence of RuO2 at 4987

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Figure 6. (a) CV curves of AC and the RCGA at a scan rate of 10 mV s−1; (b) CV curves at different scan rates; (c) GCD curves at different current densities; (d) Ragone plot of the RCGA//AC asymmetric supercapacitor; (e) Nyquist plots of RCGA//AC, (inset image shows an enlarged Nyquist plot of RCGA//AC in the range of 0−3.0 ohm; and (f) charge−discharge cycling test at a current density of 1 A g −1.

the better electrochemical performance of RCGA, compared to that of RGA, demonstrated the beneficial presence of g-C3N4 in this ternary composite.49 The planar g-C3N4 nanosheets with higher surface area and more active sites obtained from bulk gC3N4 through an ultrasonic exfoliation were easily combined with graphene sheets by the electrostatic assembly process. Except for this, because of high N atom content of g-C3N4, plenty of electrochemical active reaction sites could be easily linked onto the composite. Thus, RCGA would have higher capacitance, in comparison with that of RGA.33,36,37 The comparison of the electrochemical performance of RCGA with other RuO2 composite materials is shown in Table S3 in the Supporting Information. Except for the above inference, the low resistance also is advantageous to enhance the capacitance in RCGA. The electrical conductivities of the GA, RG, and RCGA aerogel were measured (Table S1). In comparison with GA, because of the existence of RuO2 and g-C3N4 in the aerogel composite, RCGA exhibited a relatively low conductivity. The EIS test (Figure 5e) was conducted to prove this possibility at the frequency range of 1 Hz to 100 kHz on the RCGA working

in either RCGA or RGA, suggesting their excellent electrical conductivity.16 As expected, RCGA showed a superior electrical performance with the specific capacitance of 704.3 F g−1 at a current density of 0.5 A g−1, which was 560.3 A g−1 under the same conditions for RGA. Interestingly, the specific capacitance of RCGA in the tested current range (594.9 F g−1 at 1 A g−1, 327.6 F g−1 at 2 A g−1, 299.3 F g−1 at 3 A g−1, 279.5 F g−1 at 4 A g−1, 273.6 F g−1 at 5 A g−1, 262.4 F g−1 at 10 A g−1) was always larger than that of RGA (535.1 F g−1 at 1 A g−1, 303.8 F g−1 at 2 A g−1, 277.7 F g−1 at 3 A g−1, 261.9 F g−1 at 4 A g−1, 255.2 F g−1 at 5 A g−1, 240.8 F g−1 at 10 A g−1). The coulombic efficiency of the RCGA supercapacitor was shown in Figure S10a in the Supporting Information at the current density of 1 A/g. It is also obviously higher than that of either the solely pure graphene or hydrated RuO2 (see Figures S6a and S6b in the Supporting Information). The excellent electrochemical performance of RCGA might result from the unique 3D crosslinked porous structure of the composite,53 where the uniformly dispersed ultrasmall RuO2 NPs and g-C3N4 layers were embedded into the 3D graphene framework. Meanwhile, 4988

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4. CONCLUSION In this work, we have successfully fabricated a new 3D RCGA aerogel composite through an expanded strategy of colloidal electrostatic assembly. When used as electrode material for SC, RCGA exhibited superior performance with high capacitance and extreme cycling stability. The larger capacitance of RCGA than RGA indicated the enhancement effect of g-C3N4 in the 3D graphene-based aerogel composite. Moreover, the good performance of asymmetric RCGA//AC SC device indicated its potential application in energy issue. Because of the compatibility and adaptability of this preparation strategy, we believe that this facile method may be a versatile platform for synthesizing other functional N-doped graphene/inorganic hybrids.

electrode (vs SCE), while the control experiments were executed in GA and RGA. As we know, the common Nyquist plot showed a semicircle in the high-frequency region, as well as a straight line in the low-frequency region. The diameter of the semicircle corresponds to the charge transfer resistance of the electrode/electrolyte interface, and the slope of the straight line represents the electrode diffusion resistance of the electrolyte in the electrode pores.54,55 As shown in Figure 5e, the straight line of RCGA was more vertical than that of either GA or RGA at lower frequency. Therefore, the electrolyte diffusion and ion transport were more facilitated in this N-doped RCGA. The relatively larger interfacial charge-transfer resistance of RCGA (Table S2 in the Supporting Information), in comparison with either RGA or GA, was possibly a result from the relatively poor electrical conductivity of RuO2 NPs and g-C3N4. More importantly, continuous charge−discharge cycling test (Figure 5f) showed that 98% specific capacitance of RCGA was preserved after 2000 cycles, which demonstrated the excellent stability of RCGA, which is a favorable feature for the highquality SC electrode material. An asymmetric SC device was assembled to investigate the practical performance of the RCGA. The mass ratio of AC electrode to RCGA electrode was 4.6:1, as determined by charge balance theory.56 The CV curves of the RCGA electrode and AC electrode at a scan rate of 10 mV s−1 with potential windows of −1 to 0 V and −1.1 to −0.1 V are shown in Figure 6a, respectively. The rectangular CV curve of AC exhibits its unique EDLC feature. Meanwhile, the roughly rectangular CV curve of RCGA electrode showed the pseudo-capacitive performance with good reversibility. The CV curves of the fabricated RCGA//AC device are shown in Figure 6b. With the scan rate increasing from 10 mV s−1 to 50 mV s−1, the rectangular-like shape were well preserved and no clearly redox peaks were observed, suggesting a high rate capability and ideal capacitive behavior. GCD curves of the RCGA//AC device are illustrated in Figure 6c. The symmetrical charge−discharge lines displayed perfect electrochemical reversibility. As shown in Table S3, the specific capacitances of a hybrid device based on two electrodes mass were calculated to be 23.2, 22.4, and 16.0 F g−1 at 0.5, 1.0, and 1 A g−1, respectively. The Coulombic efficiency of RCGA supercapacitor is shown in Figure S10b in the Supporting Information at the current density of 1 A g−1. The hybrid SC exhibited an energy density of 2.60 W h kg−1 at a power density of 225 W kg−1 (see Tables S4 and S5 in the Supporting Information). Figure 6d is the Ragone plot of the RCGA//AC asymmetric supercapacitor. It can deliver an energy density of 80.64 Kw h−1 at 450 W kg−1, which was much better than other hybrid device.57 In comparison with other RuO2 composite materials reported previously shown in Table S6, the energy density and power density of RCGA//AC were at similar levels. The EIS test (Figure 6) was conducted at the frequency range of 1 Hz to 100 kHz on the asymmetric RCGA//AC device. The common Nyquist plot showed a semicircle in the high-frequency region, as well as a straight line in the low-frequency region. The capacity retention even kept at 85% after 2000 cycles at a current density of 1 A g−1 (Figure 6f). The good cycling stability further demonstrated the superiority of the RCGA aerogel composite. The corresponding specific capacitances and electrochemical performance curves of RGA//AC asymmetric supercapacitor are shown in Table S2 and Figure S9 in the Supporting Information.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00358. Further characterization including zeta potential, TEM, XRD, N2 adsorption/desorption, and mercury intrusion measurements; electrochemical measurement, photographs, and electrical conductivities of related samples; comparisons of the energy density and power density with other RuO2 composite materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z. Liu). *E-mail: [email protected] (Y. Liu). ORCID

Baojun Li: 0000-0001-8325-3772 Zhongyi Liu: 0000-0003-1082-3433 Yushan Liu: 0000-0002-1773-5463 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 21543011, 21273205, 21401168). REFERENCES

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