Effects of graphene oxide addition on the synthesis and supercapacitor performance of carbon aerogel particles

Abstract

Graphene-containing carbon aerogel (CAG) particles with morphologies of spheres, irregular semi-spheres and wrinkle-capsules were synthesized by carbonization of graphene oxide (GO)-loaded resorcinol–formaldehyde aerogels prepared by an inverse emulsion method. The effects of GO content on the morphologies and structures of CAG were explored. It was found that the morphology and particle size could be controlled and adjusted by GO concentration and the stirring rate of the resorcinol–formaldehyde inverse emulsion system, respectively. The ambient drying CAG possesses a BET specific surface area of 488 m² g⁻¹ with a total pore volume of 0.379 cm³ g⁻¹ including 0.215 cm³ g⁻¹ of micropore volume. The electrochemical properties of the prepared CAG particles were investigated as supercapacitors. When the GO concentration is 0.75 wt%, the prepared wrinkle-capsules displayed a stabilized capacity of 123.6 F g⁻¹ under a current density of 0.1 A g⁻¹ and 113.9 F g⁻¹ under 1 A g⁻¹ after 2000 cycles, and a superior specific surface area capacitance of 0.23 F m⁻², indicating a good electrochemical performance and potential application in energy storage devices.

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1. Introduction

Resorcinol–formaldehyde (RF) aerogel and corresponding carbon aerogel (CA), firstly prepared by Pekala et al.^1,2 have been constantly studied for their low density and highly porous structures.^3,4 In recent years, CA-based composites have been studied for the developed structure and high performance applications in supercapacitors,^5–8 catalysts,^9,10 hydrogen storage,¹¹ thermal insulators¹² and high mechanical strength materials.¹³ The related reinforcement for CA composites includes carbon fibers and nanotubes,^14,15 metal or metal derivatives,^5,6,11 graphene,^16–23 organics²⁴ and some other addictives.^25–29

Graphene oxide (GO) and graphene, due to the two-dimensional plate-like structure and properties of good mechanical strength, good dispersibility in water and outstanding electrochemical properties, are good additives and reinforcements of polymer-based composites^18,30,31 and inorganic materials.³² Lee et al.³² synthesized graphene-decorated vanadium pentoxide nanowires (G-VONs) by simply mixing GO and V₂O₅ suspensions at room temperature and GO was proven to be vital for the growth of G-VONs as an oxidant; meanwhile, the G-VONs showed superior electrical conductivity compared with that of the pure VONs because of the insertion of reduced GO sheets into the V₂O₅ layered structure. The research showed that GO can influence the structure and synthesis of some inorganic materials. As for the application of GO in the synthesis of polymer-based composites, many researches have done this on RF carbon aerogels–GO composites. For example, Guo et al. synthesized GO and graphene-reinforced RF–CA composites to decrease the density and drying shrinkage of RF–CA and improve the corresponding mechanical and thermal performance.^16,17 Lee prepared (activated) graphene-containing CA using both polyethyleneimine-modified GO and pure GO. The BET surface area of the modified GO-containing CA was 792 m² g⁻¹ and the specific capacitance was 205 F g⁻¹.^18–20,23 Meng et al.²¹ synthesized NaOH-treated graphene oxide-catalyzed graphene/carbon composite aerogels with a BET surface area of 763 m² g⁻¹ and specific capacitance of 122 F g⁻¹. KOH-activated RF–GO composites were prepared by Zhang et al. with a GO content of 1–10%.²² The above researches all focused on the function of GO on improving the specific capacitance and mechanical or thermal performance of blocky CA. Few works have been done to study the effect of GO on the morphologies and capacitance of carbonized RF (CRF) aerogel particles.

Herein, we reported a simple process to synthesize CRF particles with controlled morphologies of spheres, irregular semi-spheres and wrinkle-capsules by carbonization of a RF aerogel precursor extracted from a low content GO-introduced RF sol–gel in an inverse emulsion system. The CRF morphology control was realized by simply adjusting the GO content. GO addition changed the morphology formation mechanism of the CRF particles, enhanced the utilization of micropores in the CRF structure and brought an increased specific capacitance when the prepared materials were used as electrodes for supercapacitors. When the GO concentration is 0.75 wt%, the morphology of the prepared CRF is wrinkle-capsules, the BET surface area is 488 m² g⁻¹ and the stabilized capacity is 123.6 F g⁻¹ under a current density of 0.1 A g⁻¹ and 113.9 F g⁻¹ under 1 A g⁻¹. The materials are promising as electrode materials for supercapacitors.

2. Experimental section

2.1 Preparation of CAG particles

2.1.1 Materials. Resorcinol (99%) and formaldehyde (37%) were produced by Tianjin Fuchen Chemical Reagents Factory and Xi Long Chemical Co. Ltd., respectively. Span-80, cyclohexane and acetone were purchased from Tianjin Fuchen and Beijing Chemical Corp. High purity graphite was produced by Qing Dao Tai Chang Graphite Co., Ltd and other reagents for the oxidation and final treatment of graphite include sulfuric acid (H₂SO₄), sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), hydrochloric acid (HCl) and hydrogen peroxide (H₂O₂, 30%, volume percentage). All the chemicals were analytical pure grade and used as received without further purification. The water was deionized before being used.

2.1.2 Preparation of GO. GO was prepared from high purity graphite powder by a modified Hummers method.^33,34 Graphite (5 g) was mixed with 2.5 g of NaNO₃ and 150 mL of H₂SO₄ in an ice bath. Then, 15 g of KMnO₄ was added slowly into the reactant solution under the action of stirring. After stirring for half an hour, the reaction temperature was adjusted to 35 °C. The reaction was continued for another 6 hours. Then, 300 mL water was added into the mixture for attenuation and the mixture was treated with H₂O₂ after attenuation until the colour changed to brilliant yellow. The mixture was then centrifuging at 4000 rpm with 0.1 mol L⁻¹ HCl aqueous solution to eliminate SO₄²⁻ and washed with water to eliminate residual acid. Finally, GO was purified by dialysis. The used GO solution was finally obtained by further dilution and sonication and the pH value was 2.9 when tested before being used.

2.1.3 Preparation of GO-loaded resorcinol–formaldehyde and the composite carbon aerogel particles. The GO-containing resorcinol–formaldehyde (RFG) aerogel precursor was synthesized by in situ sol–gel polymerization and inverse emulsion polymerization of resorcinol, formaldehyde and GO aqueous solution. The molar ratio of resorcinol and formaldehyde was 1 : 2 and the used resorcinol for each sample is 0.02 mol. The solid concentration of the RFG aerogel was controlled at 27% (estimated assuming full conversion of resorcinol and formaldehyde into aerogels) and the weight percentage of the GO additive was adjusted between 0.15 wt% (5 mg) and 0.75 wt% (15 mg). The sol–gel process was simply catalyzed by acid GO solution. The procedure for the preparation of RFG particles was as follows: resorcinol was dissolved in GO solution by stirring for 1 h, and then formaldehyde and water were added into the resorcinol–GO solutions. After continuously stirring at 300 rpm for 1 h, the reaction solution was added into the oil phase (50 mL, consisting of 5 mL Span-80 and 45 mL cyclohexane) under a stirring rate of 400 rpm in a 100 mL three-necked flask. After continuously stirring at 45 °C for 72 h, the mixture was washed with acetone to remove the oil phase and then dried under ambient pressure and room temperature. Pure Na₂CO₃ catalyzed RF particles without GO were synthesized as a comparison sample by the method described in the previous work.³⁵ The pH value of the GO-containing system is ∼2.87–3.04. The stirring rate of the inverse emulsion system was adjusted from 400 rpm to 1000 rpm to control the RFG particles size.

Carbonization of the RFG particles was proceeded at 700 °C under N₂ atmosphere for 2 h with a heating rate of 2 °C min⁻¹ in a tubular furnace to get graphene-containing carbon aerogel (CAG) particles. The RFG and CAG particles were named as RFG-x and CAG-x, where x represents the GO content.

2.2 Characterization

Chemical structure analysis of the RFG was investigated with a Fourier transform infrared spectroscopy (FT-IR) analyzer (iS-50 FT-IR, Nicolet). All the samples were mixed with KBr powder, tabulated, and tested in the wavenumber range of 4000 to 400 cm⁻¹ . X-ray photoelectron spectroscopy (XPS) was recorded on a ESCALAB 250 XPS spectrometer (Thermo Electron Corporation) using a monochromatized Al Kα radiation (1486.6 eV) with 30 eV pass energy in 0.5 eV step over an area of 650 μm × 650 μm to the samples. All binding energies were referenced to the C 1s peak at 284.6 eV. The samples was degassed before XPS measurement under high vacuum conditions (<10⁻⁷ Pa) to remove adsorbed oxygen and water.

The morphologies of the samples were studied by scanning electron microscopy (SEM, Hitachi S-4700). X-ray diffraction (XRD) patterns were obtained on an X-ray diffraction system (Rigaku D/max-2500B2+/PCX) operating with Cu Kα radiation (λ = 1.5406 Å). Nitrogen sorption isotherms were carried with ASAP 2020 (Micromeritics, USA) at 77 K and the samples were degassed before being tested under 250 °C for 6 h. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method. The microporous and mesoporous size distributions were estimated using both non-local density functional theory (NLDFT) and the Barrett–Joyner–Halenda (BJH) method.

The electrochemical performance was tested by a three-electrode setup. The prepared CAG electrode, nickel foil and Hg/HgO were applied as the working, the counter, and the reference electrodes, respectively and 30 wt% KOH aqueous solution was used as the electrolyte. The weight of the prepared CAG particles in every tested CAG electrode is around 3 mg and 3 samples were tested for each CAG sample. The galvanostatic charge–discharge test was processed using a battery program controlling test system (CT2001A, China-Land Com. Ltd.) under a potential ranging from 0.01 V to 0.9 V. The specific capacitance of the CAG electrodes was calculated from the slope of the discharge curves (dV/dt), where V is the potential during the discharge process and t represents the discharge time. The cyclic voltammetric measurements were processed on a CHI 660B electrochemical working station under varied sweep rates between 2 and 250 mV s⁻¹ in the a potential range of −0.9 V to 0 V. Electrochemical impedance spectroscopy (EIS) was performed within the frequency range of 1–10 kHz.

3. Results and discussion

3.1 Effect of GO content on the morphology of CAG

The effects of GO addition on the morphology of CAG were explored by SEM, and the results are shown in Fig. 1. Fig. 1 shows that the pure CRF particles without GO are spheres (Fig. 1a). With the increase of GO content, the morphologies of the prepared CAG particles change from spheres (Fig. 1b, 0.15 wt%) to irregular spheres (Fig. 1c, 0.45 wt%) and wrinkle-capsules (Fig. 1d, 0.75 wt%). Fig. 1e is a SEM image of the organic precursors of RFG-0.75, and the same morphologies with the corresponding CAG (Fig. 1d) indicates that the morphology of organic RFG can be completely preserved during carbonization. Fig. 1f shows the SEM image of the prepared GO sheets. GO changed the formation mechanism of RF aerogels. Fig. 1 suggests that: (a) the morphologies of CAG particles can be changed and controlled by GO addition in RFG, and (b) the morphologies of RFG can be completely preserved by further carbonization. The detailed explanation of the CAG preparation progress will be illustrated in the mechanism part.

Fig. 1 SEM images of CAG particles of: (a) pure CRF; (b) CAG-0.15; (c) CAG-0.45; (d) CAG-0.75; (e) RFG-0.75; and (f) prepared reduced GO sheets.

Fig. 2 shows the images of RFG-0.15 spheres under different stirring rates. The average particle size by calculating 50–100 particles under a stirring speed of 400, 600, 800 and 1000 rpm is 30, 20, 11, and 13 μm, respectively. When the stirring speed was adjusted between 400 rpm and 1000 rpm, the RFG-0.15 particle size decreased under a stirring speed of less than 800 rpm, and then increased. The result is consistent with our previous work and typical studies about emulsion and inverse emulsion systems:^35,36 a smaller reaction unit of the dispersed phase (RF–GO solution) and particle size were caused by the raised stirring speed in order to keep the balance between the interfacial tension and shearing stress. Agglomeration of small particles happened and the particle size increased to lower surface energy and improve the system stability when the stirring rate was increased to a certain value (>800 rpm in this study). On the other hand, Fig. 1 shows that the CAG particle size is larger compared with the pure CRF particles, and the difference might be caused by the enhanced strength of the RF–GO sol–gel skeleton before being added into the oil phase, which may draw increased resistance for the water phase to be tailored to be small spheres. In conclusion, the stirring rate of the inverse emulsion system is considered as a main effect factor for particle size control when preparing RFG particles with differing GO content.

Fig. 2 SEM images of RFG-0.15 particles under a stirring speed of (a) 400 rpm, (b) 600 rpm, (c) 800 rpm, and (d) 1000 rpm.

3.2 The effects of GO on the chemical structure of RFG particles and the pore structure of CAG particles

The chemical structures of the typical RF spheres, RFG particles and GO were analyzed by FT-IR spectroscopy and the FT-IR characterization is shown in Fig. 3a. As for the FT-IR curve of GO, the absorption peaks at 1060 cm⁻¹, 1630 cm⁻¹ and 1738 cm⁻¹ are assigned to the stretching vibration of the C–O bond, the skeleton vibration of C–C, and the stretching vibration of –COOH, respectively.^33,37 Compared to the GO line, the RFG-0.75 line dose not show the 1739 cm⁻¹ peak of –COOH, suggesting that a –COOH-related chemical reaction between RF and GO happened in this RF–GO system. When preparing RF aerogel, –CH₂–OH is a typical intermediate product of resorcinol and formaldehyde,^2,38 so esterification reactions may take place between –CH₂–OH and –COOH of GO, leading to the decrease or disappearance of –COOH.³³ After carbonization, the peak at 1381 cm⁻¹ (C–OH) disappears, implying the pyrolysis of oxygen-containing groups, the reduction of GO and the formation of graphene sheets.^33,39

Fig. 3 (a) FT-IR spectra of GO, RF spheres, RFG-0.75 and CAG-0.75; (b) XRD patterns of GO, RF spheres, RFG-0.75 and CAG particles; (c) C 1s XPS spectrum of RF spheres and RFG-0.75.

The XRD diffraction patterns of GO, pure RF spheres, RFG-0.75 and CAG particles are shown in Fig. 3b. GO exhibits a typical sharp diffraction peak at 2θ = 11.2°, corresponding to the (0 0 1) crystal face;^40,41 the characteristic diffraction peak of pure RF spheres appears around 2θ = 18.2°,¹⁷ and RFG-0.75 shows a wide diffraction peak at 19.1° close to that of the pure RF spheres. The disappearance of the characteristic peak of GO in the RFG-0.75 XRD diffraction pattern illustrates that GO sheets were uniformly dispersed in the RFG skeleton.¹⁷ As to the pattern of the prepared CAG particles, no sharp diffraction peaks can be detected, also confirming that the resulting pyrolytic GO is well dispersed in the CAG structure.¹⁷ The dispersion of GO is considered as one of the key factors for the formation of morphology-controlled RFG particles and will be discussed later in detail in the mechanism part.

XPS was employed to analyze the chemical composition of pure RF spheres and RFG-0.75. As shown in Fig. 3c, in the C 1s spectrum of RF spheres, the peaks centered at 284.6, 286.4 and 288.04 eV correspond to C–C/C=C of aromatic rings, C–O, and carbonyl, respectively.^21,22,42 The existence of a large amount of C–O bonds is consistent with the FT-IR analysis and typical RF aerogel composition; the resorcinol–formaldehyde sol–gel process is developed based on a large amount of hydroxymethyl and the corresponding condensation derivatives, methylene and methylene ether.^1,2 The C 1s spectrum of RFG-0.75 differs from that of the RF spheres by the 289.2 eV peaks of O=C–O,²¹ confirming the esterification of –CH₂–OH on the aromatic rings of resorcinol and the –COOH of GO.

The GO-related preparation mechanism of morphology control of CAG particles is proposed according to the above analysis and is shown in Fig. 4. Fig. 1 illustrates that the morphology of the CAG particles can be changed and controlled by GO addition: with the increasing GO content, the morphologies of the prepared CAG particles change from spheres (0.15 wt%), irregular spheres (0.45 wt%) and wrinkle-capsules (0.75 wt%). The semi-spheres and capsules are the solvent-released product of hollow particles during the drying process and the morphology control of the particles is based on the skeleton strength of the hollow particles.³⁵ The XRD diffraction patterns in Fig. 3 illustrate that the GO sheets were well dispersed in the RFG skeleton. The XPS and FT-IR analysis both confirmed that there is a chemical combination between RF and GO. In the GO-related RF sol–gel system, the resorcinol and formaldehyde sol–gel process tends to proceed on or near the GO sheets due to the chemical reaction or hydrogen bond¹⁶ between the additive (GO) and RF matrix, leading to the formation of RF–GO growth units. Meanwhile, GO is composed of a hydrophobic basal plane and hydrophilic functional groups,⁴³ which makes it possible for the RF–GO sol–gel process to proceed around the nuclear of the oil drop, promoting the formation of the hollow RFG structure. When the GO content is 0.15 wt%, GO sheets were scattered, the above function did not work obviously in the RF solution, and the morphology of RFG-0.15 is closed compared to that of pure RF. With the increasing GO content, RF–GO hollow particles formed for the RFG-0.45 sample, and the GO sheets are helpful to resist the capillary force during drying and the solvent release process, leading to the formation of irregular semi-spheres and capsules. When the GO content was increased to 0.75 wt%, the growth of RF proceeded mainly on the GO sheets and the skeleton of RFG was considered to be composed of RF–GO sheets, leading to the morphology of shrunken or wrinkle-capsules shown in Fig. 1d. This morphology-controlling mechanism is expected to be suitable for the RF sol–gel system with different amphiphilic additives.

Fig. 4 The synthetic mechanism model of CAG particles.

The pore structure analysis of the prepared CAG particles is shown in Fig. 5a and Table 1. In Fig. 5a, in the N₂ adsorption–desorption isotherms, the samples show a type-IV isotherm according to the classification of IUPAC, signifying a developed porous structure. Meanwhile, at a high relative pressure value on the isotherms, the hysteresis loop can be observed, which is typical for aggregates of plate-like particles of open large pores.⁴⁴ The large pores may be formed by the crossed GO sheets and RF skeleton. The fast increase of the adsorbed curve ranging in the low relative pressures indicates the presence of micropores.^18,44

Fig. 5 (a) Nitrogen adsorption–desorption isotherms of the prepared CAG particles; (b) pore size distribution obtained by the NLDFT method (N₂, 77 K). Inset of (b) shows the pore size (2–50 nm) distribution by applying the BJH model.

Table 1 Specific surface area and pore structure analysis of the prepared CRF and CAG particles

Samples	Specific surface area^a (m^2 g^−1)	t-Plot micropore area^b (m^2 g^−1)	t-Plot external surface area^b (m^2 g^−1)	Total pore volume^c (cm^3 g^−1)	t-Plot micropore volume^c (cm^3 g^−1)	Mesopore and macropore volume (cm^3 g^−1)
a Calculated by the BJH model (relative pressure 0.05–0.3).b Reckoned by the t-plot method using the Harkins and Jura standard isotherm with a thickness of 3.5–5.0 Å.c Calculated from the adsorbing capacity when P/P0 = 0.995.
CRF spheres	396	56	340	0.242	0.028	0.214
CAG-0.15	468	378	90	0.491	0.197	0.294
CAG-0.45	462	410	52	0.251	0.214	0.03
CAG-0.75	488	414	74	0.379	0.215	0.158

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The results of analysis about the BET surface area and pore characteristics of the prepared CRF and CAG particles are summarized in Table 1, illustrating that the CRF spheres without GO mainly contain mesopores and the CAG particles are mainly micropores. In general, the specific surface area and pore volume of the CAG particles are significantly larger than those of pure CRF spheres according to Table 1, confirming the enhancement and shrinkage stress resistance function of GO on the RF skeleton described in the preparation mechanism. The RF–GO sol–gel process proceeded under the strong acid condition of GO solution. The cross-linking of the RF sol solution was catalyzed by H⁺ and the cross-linking degree of the RF sol could be enhanced by a higher H⁺ concentration, inducing the increased proportion of micropores.⁴⁵ In this study, the CAG was mainly microporous due to the high degree of cross-linking and small skeleton particles. A similar situation has been reported by many researchers.^17,19,20,22 In addition, during the drying process, GO could act as an anti-shrinkage additive for the RF composites and increase the specific surface area of the RF aerogels;¹⁶ in this study, the addition of GO enhanced the anti-shrinkage properties of the RFG particles, which can improve the retention of pores in the RFG structure. After pyrolyzation, the specific surface area of the corresponding CAG particles is obviously larger than that of the pure CRF particles. The BET surface area of the graphene nanosheets contributed rarely because the initial GO content was very low.

Fig. 5b shows the NLDFT pore distribution and the inset of Fig. 5b shows the BJH mesopore distribution. As for the NLDFT curves, the microporous maximum pore distributions of CAG concentrated at 1.1–1.4 nm, with some mesopores concentrated at 2–5 nm and 41–45 nm (on the BJH pore distribution).

3.3 Electrochemical performance of CRF particles

The electrochemical performance of the prepared CRF and CAG was studied for use as materials for supercapacitors. The cyclic performance of the CRF spheres, CAG spheres (CAG-0.15), CAG irregular semi-spheres (CAG-0.45) and wrinkle-capsules (CAG-0.75) in the potential window from 0.01 to 0.9 V is shown in Fig. 6a. According to Fig. 6a, after the test cycles (1600 cycles for pure CRF and 2000 cycles for CAG particles), the reversible capacity of CRF is 81.9 F g⁻¹ while that of CAG-0.15, CAG-0.45, and CAG-0.75 is 87.3, 95.3 and 113.9 F g⁻¹, illustrating that the addition of GO can improve the electrochemical performance of CRF. According to Fig. 5 and Table 1, the high surface area ambient drying CAG particles are microporous due to the acidic sol–gel condition. As reported before, the micropores cannot be fully accessed by the electrolyte,^22,46 which limited the application of mainly microporous activated carbons. In this study, the microporous structure did not reduce the capacity of the CAG particles, and a higher capacity of CAG was obtained by increasing the GO content and the more developed microporous structure. In Fig. 6b, the potential–t curve (cycle 2–11) of the CAG-0.15 spheres shows symmetrical isosceles triangles, indicating the high efficiency electric double-layer capacitor characteristic of the prepared CAG particles. The capacitance retention–current density curve of the CAG particles is shown in Fig. 6c and the capacitance of the CAG particles was proven to be stable and higher than that of CRF particles. Fig. 6d shows the cyclic voltammograms for the prepared CRF electrodes at 5 mV s⁻¹ and the corresponding capacitances of the CRF spheres, CAG-0.15, CAG-0.45 and CAG-0.75 are 77 F g⁻¹, 80.7 F g⁻¹, 92.1 F g⁻¹ and 117.1 F g⁻¹, in accordance with the trend of specific surface area of micropores exhibited in Table 1.

Fig. 6 (a) Cycling performance of the CRF and CAG particles under a current density of 0.1–1 A g⁻¹; (b) cycle 2 to 11 potential–t curve of the sample CAG-0.15. The inset of (b) shows the potential–t curves of cycle 2 of the samples; (c) capacitance retention under different current densities; (d) cyclic voltammetry curves (5 mV s⁻¹) of the samples in 30% KOH; (e) the Nyquist plots of the samples with the changing GO content.

According to Table 1, the total surface area and mesopore area of CAG-0.45 is less than that of CAG-0.15 while the micropore area is 32 m² g⁻¹ higher than that of CAG-0.15. As for the electrochemical performance, CAG-0.45 shows a higher capacitance than that of CAG-0.15, tested by both the galvanostatic charge–discharge process and cyclic voltammetry, indicating that the micropores are helpful when the CAG particles are used as electrodes for supercapacitors. The calculated ratios of the specific capacitance and specific surface area (the area capacitance) of the CRF spheres, CAG-0.15, CAG-0.45 and CAG-0.75 are 0.21, 0.19, 0.21 and 0.23 F m⁻², illustrating that the contribution of specific surface area to capacitance of the CAG particles increased with a higher GO content, larger micropore specific surface area and micropore volume. The above conclusions confirm that, in this study, the micropores are helpful for improving electrochemical performance. The high utilization of micropores in the CAG electrodes can be attributed to the graphene sheets dispersed in the skeleton, which is proven to be helpful for the supercapacitor of the CAG samples because the linkage of the RF–GO skeleton by GO and the oxygen-containing surface functional groups was positive for the wettability of electrodes in the electrolyte,^22,47 and also, the GO sheets can enlarge the transmission path for the electrolyte. The comparison of specific surface area and supercapacitor performance of the prepared CAG particles with some reported relevant studies is shown in Table 2. The performance of the prepared CAG particles is superior to most of the previous works in area capacitance and with a simple preparing method according to Table 2. The capacitance of pyrolized GO in this study was tested as 84.4 F g⁻¹ at a current density of 0.1 A g⁻¹, nearly at that of the pure CRF particles, and the influence of loaded GO on the specific capacitance of CAG particles can be neglected for the extremely low content. The function of graphene in improving the electrical conductivity of both inorganic and organic materials has been reported widely.^32,48,49 GO addition may bring an improved electrical conductivity for CAG particles, which is conducive to the transmission of ions in the electrolyte and the electrochemical performance.

Table 2 Comparison of specific surface area and specific capacitance of prepared CAG with reported CA in relevant studies

Specific capacitance (F g^−1)	Specific surface area (m^2 g^−1)	Area capacitance (F m^−2)	Method/additives	Ref.
114 (1 A g^−1)	488	0.23	CAG-0.75	This work
85 (1 A g^−1)	733	0.12	CRF-2 wt% GO	19
221 (10 mV s^−1)	1158	0.19	CRF-2 wt% GO-K2CO3 activated	20
158 (10 mV s^−1)	792	0.2	CRF-1.5 wt% polyethyleneimine modified GO	18
122 (50 mA g^−1)	763	0.16	CRF-10 wt%-NaOH treated GO	21
55 (5 mV s^−1)	790	0.07	Pure CRF-supercritical carbon dioxide drying	50
86 (10 mV s^−1)	614	0.14	Cresol-RF precursor with 30 wt% cresol	51
107 (10 mV s^−1)	681	0.16	7 wt% Mn-doped CRF	52

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The Nyquist plots for the samples are shown in Fig. 6e and the inset of Fig. 6e shows the high frequency semi-circles. In the low frequency region, the Nyquist plots represent the impedance induced by charge transfer at the electrode/electrolyte interface, and the prepared CRF and CAG particles show a nearly vertical line parallel with the imaginary axis. The inset of Fig. 6e shows that the semi-circle loop of CAG-0.75 is smaller than that of the other samples, indicating that a lower inter-granular electrical resistance existed between the CAG skeleton particles.¹⁸ The x-intercept of the horizontal axis represents the ESR value of the tested electrode¹⁸ and the corresponding values for CRF, CAG-0.15, CAG-0.45 and CAG-0.75 are 0.30 Ω, 0.32 Ω, 0.28 Ω, and 0.33 Ω without significant differences. Fig. 6 confirms that the prepared CAG is promising in EDLC applications and CAG-0.75 performed the best among the samples with different original GO content.

4. Conclusions

A simple RF–GO sol–gel and inverse emulsion system was built to synthesize RFG particles. GO was proven to change the morphology of the CRF particles and enhance the utilization of micropores in the CRF structure, bringing a better electrochemical performance for the prepared materials. The morphologies of the prepared ambient drying RFG and corresponding CAG particles were easily controlled to be spheres, irregular semi-spheres and wrinkle-capsules by adjusting the original GO weight content. The particle size of the sample was proven to be controlled by changing the stirring rate of the inverse emulsion system. The ambient drying CAG particles are mainly microporous and the specific surface area was 488 m² g⁻¹, and the total pore volume was 0.379 cm³ g⁻¹ including 0.215 cm³ g⁻¹ of micropore volume. The additive GO was proven to be helpful for improving electrochemical performance. When the GO content is 0.75%, the morphology of the prepared CAG particles is wrinkle-capsules, with a stabilized capacity at 123.6 F g⁻¹ under a current density of 0.1 A g⁻¹, and 113.9 F g⁻¹ under 1 A g⁻¹, with a high area capacitance of 0.23 F m⁻². The CAG particles were promising in supercapacitors and some specific surface area and morphology relevant applications, such as desalination materials or templates.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51272016 and 51272019).

Notes and references

1. R. W. Pekala, J. Mater. Sci., 1989, 24, 3221.
2. R. W. Pekala and D. W. Schaefer, Macromolecules, 1993, 26, 5487.
3. H. Tamon, H. Ishizaka, T. Yamamoto and T. Suzuki, Carbon, 1999, 37, 2049.
4. H. Tamon, H. Ishizaka, T. Yamamoto and T. Suzuki, Carbon, 2000, 38, 1099.
5. G. R. Li, Z. P. Feng, Y. N. Ou, D. Wu, R. Fu and Y. X. Tong, Langmuir, 2010, 26, 2209.
6. X. Y. Wang, X. Y. Wang, L. H. Yi, L. Liu, Y. Z. Dai and H. Wu, J. Power Sources, 2013, 224, 317.
7. H. C. Chien, W. Y. Cheng, Y. H. Wang and S. Y. Lu, Adv. Funct. Mater., 2012, 22, 5038.
8. G. F. Lv, D. C. Wu, R. W. Fu, Z. B. Zhang and Z. J. Su, J. Non-Cryst. Solids, 2008, 354, 4567.
9. S. Wei, D. Wu, X. Shang and R. Fu, Energy Fuels, 2009, 23, 908.
10. C. Moreno-Castilla and F. J. Maldonado-Hódar, Carbon, 2005, 43, 455.
11. K. S. Lin, Y. J. Mai, S. W. Chiu, J. H. Yang and S. L. I. Chan, J. Nanomater., 2012, 2012, 5271.
12. J. Feng, C. Zhang, J. Feng, Y. Jiang and Z. Nan, ACS Appl. Mater. Interfaces, 2011, 3, 4796.
13. J. Yang, S. Li, Y. Luo, L. Yan, F. Wang, J. Yang, S. Li, Y. Luo, L. Yan and F. Wang, Carbon, 2011, 49, 1542.
14. J. Feng, C. Zhang and J. Feng, Mater. Lett., 2012, 67, 266.
15. M. A. Worsley, J. H. Satcher and T. F. Baumann, Langmuir, 2008, 24, 9763–9766.
16. K. Guo, H. Song, X. Chen, X. Du and L. Zhong, Phys. Chem. Chem. Phys., 2014, 16, 11603.
17. K. Guo, Z. Hu, H. Song, X. Du, L. Zhong and X. Chen, RSC Adv., 2015, 5, 5197.
18. Y. J. Lee, W. P. Hai, G. P. Kim, J. Yi and I. K. Song, Curr. Appl. Phys., 2013, 13, 945.
19. Y. J. Lee, H. W. Park, U. G. Hong and I. K. Song, J. Nanosci. Nanotechnol., 2013, 13, 7944.
20. Y. J. Lee, G. P. Kim, Y. Bang, J. Yi, J. G. Seo and I. K. Song, Mater. Res. Bull., 2014, 50, 240.
21. F. Meng, X. Zhang, B. Xu, S. Yue, H. Guo and Y. Luo, J. Mater. Chem., 2011, 21, 18537.
22. K. Zhang, B. T. Ang, L. L. Zhang, X. S. Zhao and J. Wu, J. Mater. Chem., 2011, 21, 2663.
23. Y. J. Lee, J. Lee, G. P. Kim, E. J. Lee, J. Yi and I. K. Song, J. Nanosci. Nanotechnol., 2014, 14, 8602.
24. H. An, Y. Wang, X. Wang, L. Zheng, X. Wang, L. Yi, L. Bai and X. Zhang, J. Power Sources, 2010, 195, 6964.
25. W. Yang, S. Chen and W. Lin, Int. J. Hydrogen Energy, 2012, 37, 942.
26. J. Zhou, B. H. Liu and P. L. Zhou, Solid State Ionics, 2013, 244, 23.
27. K. J. Huang, L. Wang, J. Z. Zhang and K. Xing, J. Electroanal. Chem., 2015, 752, 33.
28. Y. Tao, D. Noguchi, C. M. Yang, H. Kanoh, H. Tanaka, M. Yudasaka, S. Iijama and K. Kaneko, Langmuir, 2007, 23, 9155.
29. G. Rasines, P. Lavela, C. Macías, M. C. Zafra, J. L. Tirado, J. B. Parra and C. O. Ania, Carbon, 2015, 83, 262.
30. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197.
31. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. B. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558.
32. M. Lee, W. G. Hong, H. Y. Jeong, S. K. Balasingam, Z. Lee, S. Chang, B. H. Kim and Y. Jun, Nanoscale, 2014, 6, 11066.
33. M. Li, H. Song, X. Chen, J. Zhou and Z. Ma, Phys. Chem. Chem. Phys., 2015, 17, 3250.
34. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
35. Q. Lei, H. Song, D. Zhou, S. Zhang and X. Chen, RSC Adv., 2015, 5, 78526.
36. M. Nomura, M. Harada, W. Eguchi and S. Nagata, J. Appl. Polym. Sci., 1972, 16, 835.
37. Y. Xu, H. Bai, G. Lu, C. Li and G. Shi, J. Am. Chem. Soc., 2008, 130, 5856.
38. K. Barral, J. Non-Cryst. Solids, 1998, 225, 46.
39. S. Qu, M. Li, L. Xie, X. Huang, J. Yang, N. Wang and S. Yang, ACS Nano, 2013, 7, 4070.
40. H. Hu, Z. Zhao, W. Wan, Y. Gogotsi and J. Qiu, Adv. Mater., 2013, 25, 2219.
41. Y. Zhu, S. Murali, M. D. Stoller, A. Velamakanni, R. D. Piner and R. S. Ruoff, Carbon, 2010, 48, 2118.
42. O. C. Compton, D. A. Dikin, K. W. Putz, L. C. Brinson and S. B. T. Nguyen, Adv. Mater., 2010, 22, 892.
43. P. Guo, H. Song and X. Chen, J. Mater. Chem., 2010, 20, 4867.
44. D. Meloni, R. Monaci, V. Solinas, A. Auroux and E. Dumitriu, Appl. Catal., A, 2008, 350, 86.
45. N. Job, R. Pirard, J. Marien and J. Pirard, Carbon, 2004, 42, 619.
46. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520.
47. Z. B. Wen, Q. T. Qu, Q. Gao, X. W. Zheng, Z. H. Hu, Y. P. Wu, Y. F. Liu and X. J. Wang, Electrochem. Commun., 2009, 11, 715.
48. M. Li, C. Gao, H. Hu and Z. Zhao, Carbon, 2013, 65, 371.
49. H. Kim, Y. Miura and C. W. Macosko, Chem. Mater., 2010, 22, 3441.
50. A. Laheäär, A. L. Peikolainen, M. Koel, A. Jänes and E. Lust, J. Solid State Electrochem., 2012, 16, 2717.
51. W. Li, G. Reichenauer and J. Fricke, Carbon, 2002, 40, 2955.
52. Y. J. Lee, J. C. Jung, S. Park, J. G. Seo, S. H. Baeck, J. R. Yoon, J. Yi and I. K. Song, Curr. Appl. Phys., 2010, 10, 947.

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