ZIF-derived nitrogen-doped carbon/3D graphene frameworks for all-solid-state supercapacitors

Abstract

Nitrogen-doped porous carbons supported on three-dimensional (3D) graphene frameworks were prepared by decorating ZIF-8 or ZIF-67 onto graphene aerogels by a heterogeneous nucleation method and subsequent carbonization and etching processes. The resulting carbon composite exhibited a hierarchically porous structure and a high surface area of 207 m² g⁻¹. Due to the high electrical conductivity and high-rate transportation arising from graphene frameworks and nitrogen-doping, the ZIF-67 derived porous carbon/graphene composite (GA@CZIF-67-E) showed a high specific capacitance of 53 F g⁻¹ at a scan rate of 5 mV s⁻¹, and good cycle stability after 1000 cycles when applied in all-solid-state supercapacitors.

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Introduction

Nowadays, there is an urgent demand for a flexible, lightweight, and environmentally-friendly energy storage system to meet the growing needs of portable electronics or wearable devices such as laptops, digital cameras, smart watches, and glasses, etc.^1,2 Among all the energy storage devices, the supercapacitor is an emerging one due to its attractive characteristics such as high power density, long cycle stability, free maintenance and balanced energy density, and it is able to fill the gap between low-energy-density traditional electrolytic capacitors and low-power-density Li-ion batteries (LIBs).^3,4 At present, supercapacitors using liquid electrolyte have drawbacks of corrosive electrolyte leakage and low power-weight ratio.⁵ However, all-solid-state supercapacitors (ASSSs) employ solid electrolyte and therefore are safer and more reliable for lightweight and wearable electronics.⁶ Recent research efforts have been focused on improving the performance of the supercapacitor (i.e. capacitance, rate capability, cycling stability and mechanical integrity). In particular, the development of the electrode materials has been considered to be the most important task in enhancing the performance.^3,7,8

Graphene, a one-atom-thick sheet of sp²-bonded carbon atoms, is a prevailing electrode material with an extraordinary theoretical capacitance (550 F g⁻¹), as well as outstanding electrochemical stability, excellent electronic conductivity, remarkable thermal conductivity and mechanic properties.⁹ However, because of strong π–π stacking and van der Waals interactions, graphene irreversibly agglomerates or even restacks to form graphite, which greatly impedes its intrinsic performance.^10–12 Recently, researchers confirmed that three-dimensional (3D) graphene materials with interconnected macroscopic structures of graphene can prevent inter-sheet restacking.^6,13 Moreover, 3D frameworks provide multidimensional electron transport highway, which enable high-rate transport between the electrolyte ions and the electrode surfaces, resulting in excellent electrochemical performance, such as in electrocatalysts^14–16 and electrochemical storage electrodes.^17,18 On the other hand, graphene nanosheets could be modified with hydrophilic groups such as hydroxyl and carboxylic after oxidation, which are favored for the introduction of highly active materials, like carbon-based materials,¹³ metal oxides¹⁹ or polymers.²⁰ For instance, in 2013, Liu et al. developed 3D graphene/ordered mesoporous carbon materials, which showed good capacitance in all-solid-state supercapacitors.¹³

Metal–organic frameworks (MOFs), modularly synthesized by self-assembly of transition-metal clusters and organic molecules, are a class of materials having many fascinating properties, such as light density,²¹ ultrahigh surface area,²² large pore volume²³ and tunable pore size²⁴ in the nanometer regime. Till now they have attracted considerate attention as promising candidates for CO₂ capture,²⁵ gas separation,²⁶ nanoparticles encapsulation,²⁷ electrocatalysts²⁸ and energy storage.²⁹ As an important subclass of MOFs, zeolitic imidazolate frameworks (ZIFs) consist of transition metal ions (Zn²⁺, Co²⁺) and high content of nitrogen imidazole ligands. Direct carbonization is proved to be a straightforward route for preparing porous carbon materials possessing a high specific surface area and a hierarchically nanoporous structure with meso/micropores.³⁰ Recent research has proved that ZIF-derived nanoporous carbons exhibited high electrochemical activity, capacitance and excellent stability. For example, Yamauchi et al. used ZIF-8 as a precursor to synthesize nitrogen-doped carbon materials by direct pyrolysis at high temperatures.³¹ The materials showed a very high electrochemical capacitance in the aqueous acid electrolyte. Wang et al. recently reported a direct synthesis of N-doped carbon nanotube frameworks after calcining ZIF-67 in an Ar/H₂ atmosphere. The final materials showed remarkable performance even superior to commercial Pt/C electrocatalyst as bifunctional electrocatalyst for the ORR and the OER.³² In 2014, Qu et al. successfully incorporated ZIF-8 crystals onto graphene, which enhanced the capacitance of graphene after carbonization. The composite showed a high performance when employed as a liquid electrolytic supercapacitor, which proved practical to give much greater capacitance by hybridizing graphene with ZIFs.³³ However, till now few research papers have been focusing on comparing ZIF-8 and ZIF-67 as precursors for the preparation of nanocarbons at same testing conditions, especially for use in all-solid-state supercapacitors.

Herein, we report a heterogeneous nucleation growth method to synthesize hierarchically porous materials with macro/mesoporous structure and high specific surface area (i.e. 207 m² g⁻¹) by decorating ZIFs (ZIF-8, -67) onto 3D graphene aerogels, and following carbonization and etching processes. In this approach, graphene oxide (GO) was employed as a support, and ZIFs were used as nitrogen-doped carbon precursors. By choosing different kinds of ZIFs (ZIF-8, -67), the correlation between precursor and capacitance is investigated and hence further benefits to achieve higher performance. Due to the synergistic effects of graphene aerogels and the coordinative components,^28,34,35 the resultant composite (GA@CZIF-67-E) exhibited high capacitance and long cycle stability when employed as electrodes for all-solid-state supercapacitor.

Experimental

Synthesis of the materials

Graphene oxide (GO) was prepared by the oxidation of natural graphite flakes (Aldrich) according to a modified Hummers' method.³⁶ 3D GA monolith with macroporosity was synthesized by hydrothermal reaction of 1.5 mg mL⁻¹ GO aqueous solution at 160 °C for 12 h, followed by a freeze-drying process.

The resulting GA monolith was immersed into a 40 mL methanol (analysis, Merck) solution of 0.810 g of Zn(NO₃)₂·6H₂O (Sigma-Aldrich, 98%). After sonication for 15 min, a 40 mL methanol solution of 0.526 g of 2-methylimidazole (Hmim, Sigma-Aldrich, 98%) was added. The mixed solution was stirred for 30 min and kept still for 12 h. Afterward, the GA monolith was taken out and washed with methanol for three times. After drying at 80 °C in a vacuum oven for 8 h, the monolith was calcined at 800 °C for 3 h in Ar atmosphere to obtain GA@CZIF-8. GA@CZIF-8-E was prepared after etching by 2 M HCl solution for 12 h to remove Zn particles, followed by washing with methanol three times and drying at 80 °C for 8 h.

The preparation of GA@CZIF-67-E was similar to GA@CZIF-67-E except that GA monolith was immersed into a 40 mL methanol solution of 1.163 g Co(NO₃)₂·6H₂O (Sigma-Aldrich, 98%) and 0.600 g polyvinylpyrrolidone (M_w = 40 000, Sigma-Aldrich), followed by adding 40 mL methanol solution of 2.630 g 2-methylimidazole.

Characterization

X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex 600 diffractometer using Cu Kα radiation (40 kV, 15 mA). Field emission-scanning electron microscopy (FE-SEM) images were taken on an FEI Nova NanoSEM 450 microscope operating at 5 kV. Transmission electron microscopy (TEM) measurements were conducted on an FEI Tecnai G2 T20 and an FEI F20 (Japan) at an accelerating voltage of 200 kV. Samples are suspended in ethanol and transferred onto a Cu grid. Nitrogen sorption isotherms were measured at −196 °C with a Micromeritics ASAP 2020 analyzer. Before measurements, the samples were degassed under vacuum at 200 °C for at least 8 h.

Electrochemical measurements

Electrochemical measurements were conducted on a CHI 660E electrochemical workstation under ambient temperature. For the assembly of all-solid-state supercapacitors, a homogeneous slurry containing polytetrafluoroethylene (PTFE, 1 wt%) and active materials (weight ratio 1 : 9, in ethanol) was painted between two pieces of Cu foil with an area of 1 × 3 cm². The electrodes were dried at 100 °C in a vacuum oven for 12 h. Active materials loading on the electrode were calculated by the mass difference of the electrode before and after. Consequently, the electrodes were dipped into PVA/H₂SO₄ gel electrolyte for 5 min and followed by solidification for 12 h at room temperature. The PVA/H₂SO₄ electrolyte was prepared by stirring 6 g H₂SO₄ and 6 g PVA in 60 mL deionized water and thus heated up to 80 °C until the solution becomes clear. Finally, as-prepared two electrodes were symmetrically face-to-face integrated into one ASSSs device under a pressure of about 5 MPa for 1 min.

Calculation. The electrochemical performance of the GA@CZIFs-E electrode was examined by cyclic voltammetry (CV) diagram. The capacitance values were calculated from the CV data according to the following equation:
where C_device is denoted as the capacitance contribution from GA@ZIFs-E electrodes, m is the total electrode weight (in gram), v is the scan rate (in V s⁻¹), V_f and V_i are the integration potential limits of the voltammetric curve and I(V) is the voltammetric discharge current (in amperes).

Results and discussion

A heterogeneous nucleation growth method was introduced in the synthesis of GA@CZIFs-E as schematically illustrated in Fig. 1. In the first step, the 3D graphene aerogels were soaked in a methanol solution of metal ion (Zn²⁺, Co²⁺), followed by adding a ligand solution of 2-methylimidazole. Metal ions could be well adsorbed onto the graphene oxide nanosheets due to the incorporation interactions between the positively charged metal ions and negatively charged oxygen contained groups (such as epoxy and hydroxyl).³⁷ After nucleation and growth, the macroporous structure of GA maintained, and ZIFs crystal particles were uniformly deposited on graphene aerogels. Due to the deposition of ZIF particles, the black monolith of graphene aerogels became gray (GA@ZIF-8) or purple (GA@ZIF-67).

Fig. 1 Schematic illustration of the synthesis procedure for GA@CZIFs-E.

Upon calcination and etching of the metal particles generated during calcination, the hierarchically porous (mesoporous and macroporous) GA@CZIFs-E were obtained.

The specific surface area of the resulting carbon materials was measured using nitrogen sorption technique (Fig. 2). In their N₂ adsorption/desorption isotherms, all the samples showed a typical type IV curve with a combination of H2 and H4 hysteresis loop at relative pressures (P/P₀) of 0.45–1.0, indicating the existence of mesopores. The Brunauer–Emmett–Teller (BET) analysis revealed that the specific surface areas and the pore volumes for GA@CZIF-8-E and GA@CZIF-67-E were 239 m² g⁻¹, 0.54 cm³ g⁻¹ and 207 m² g⁻¹, 0.38 cm³ g⁻¹, respectively. The decrease of the specific surface area of GA@CZIF-67-E may be due to the incomplete etching of Co particles. The specific surface area of GA@CZIFs-E was lower than the CZIFs-E (Table 1), which was probably because BET is not capable of detecting the macropores (>100 nm) of graphene frameworks.¹³ Fig. 2b and d revealed that the pore size distributions calculated by density functional theory method indicated that both GA@CZIF-67-E (2.1 nm) and GA@CZIF-8-E (2.7 nm) had a mesoporous structure.

Fig. 2 N₂ adsorption–desorption isotherms of (A) GA@CZIF-8-E and (C) GA@CZIF-67-E. Pore distribution of (B) GA@CZIF-8-E and (D) GA@CZIF-67-E.

Table 1 Porosity and electrochemical performance of GA, GA@CZIFs-E, and CZIFs-E

	D^a [nm]	SBET^b [m^2 g^−1]	C^c [F g^−1]	C^d [F g^−1]	Capacitance retention^e
a D is the pore size diameter.b SBET is the BET surface area.c C is the calculated capacitance from cyclic voltammetry at scan rate 5 mV s^−1.d C is the calculated capacitance from cyclic voltammetry at scan rate 500 mV s^−1.e Capacitance retention after 1000 cycles.
GA@CZIF-67-E	2.1	207	53	24	91%
GA@CZIF-8-E	2.7	239	49	22	88%
GA	—	198	35	14	82%
CZIF-67-E	3.9	421	25	6	55%
CZIF-8-E	4.0	521	18	2	21%

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The SEM investigation revealed that the 3D interconnected macroporous structures had been maintained after deposition of ZIFs on graphene aerogels (Fig. S1,† 3a and 4a). The ZIF-8 nanocrystals by heterogeneous nucleation with a particle size of ∼150 nm were grown on 3D graphene aerogels uniformly (Fig. 3b), which was also observed in TEM image as shown in Fig. 3c. The carbonization of GA@ZIF-8 was performed in Ar atmosphere at 800 °C for 3 h. Fig. 4a and b showed the ZIF-67 with a particle size of around 400 nm was successfully deposited on the graphene nanosheets, which was also proved in TEM image as shown in Fig. 4c. After calcination, the carbonized ZIF-67 particles shrank to around 200 nm in size (Fig. 4d). This is because that the ZIFs particles have high surface area, highly porous structure and low density. During the carbonization progress, the Zn/Co species were reduced by surrounding carbon and formed Zn/Co particles.³⁸ Upon carbonization and etching, the ZIFs shrank to smaller sizes and finally transformed into porous carbon materials (Fig. 4d). In Fig. 4a and d, the content of CZIF-67 seemed decreased after carbonization. This was probably because ZIF-67 particles were so big, and it was difficult for the entire particles to remain on the graphene layer. Part of the particles may be lost during carbonization and etching. Comparing Fig. 4d with the SEM images of pure GA (Fig. S1†), it was obvious that the pure GA surface was smoother while the GA@CZIF-67-E layers were much rougher and thicker. This proved that while part of particle lost, the remaining part had still been anchored by the graphene layer. The uniform distribution of nitrogen and cobalt (Fig. 5), which could only derive from ZIF-67, also proved the successful and uniform deposition of CZIF-67 on graphene after carbonization and etching process. At the same time, for the GA@CZIF-67, the carbon surrounding Co, which derived from 2-methylimidazole was converted to graphitized carbon by the catalysis of Co at 800 °C.³⁴ The pores revealed and formed the mesoporous structure via etching.^34,39

Fig. 3 SEM images of (A and B) GA@ZIF-8, (D and E) GA@CZIF-8-E. TEM images of (C) GA@ZIF-8, and (F) GA@CZIF-8-E.

Fig. 4 SEM images of (A and B) GA@ZIF-67, and (D) GA@CZIF-67-E. TEM images of (C) GA@ZIF-67, (E and F) GA@CZIF-67-E.

Fig. 5 STEM images (A), corresponding element mapping images (B–D), and EDX images (E) of GA@CZIF-67-E.

Scanning transmission electron microscopy (STEM) images of GA@CZIF-67-E (Fig. 5) proved that the Co particles were embedded in the carbon matrix. The corresponding element mapping image showed a uniform distribution of the nitrogen and carbon elements. The energy-dispersive X-ray spectroscopy (EDX) image (Fig. 5E) revealed that the compositions of carbon and nitrogen elements were 93.8 and 2.0 wt%, respectively. The composition of cobalt element was 1.6 wt%, indicating a small amount of residual cobalt after acid etching because of graphite layer wrapping.³⁴

XRD patterns of GA@ZIF-8 and GA@ZIF-67 revealed a typical crystalline structure similar to ZIF-8 and ZIF-67, implying that the addition of GA in the solution of ZIF clusters did not impede the formation of the structure of ZIFs. The peaks of graphene were not observed owing to its lower intensity comparing with that of ZIFs. After carbonization, the ZIF-8, -67 characteristic peaks disappeared, and the graphitization peak of graphene emerged (Fig. 6), proving the deposition of ZIF particles and the main structure of graphene framework. No peaks related to Zn crystals were seen, showing the complete removal of Zn particles during the carbonization and etching. By comparison, GA@CZIF-67-E presented new peaks which could be related to Co phase. The Co nanoparticles of ∼10 nm may be wrapped by the graphitic carbon closely, and finally remained from acid etching (Fig. 7).

Fig. 6 XRD patterns of GA@ZIFs and GA@CZIFs.

Fig. 7 Comparison of ASSSs based on CZIF-8-E, CZIF-67-E, GA, GA@CZIF-8-E, and GA@CZIF-67-E. (A) CV curves obtained at the scan rate of 100 mV s⁻¹. (B) CV curves of GA@CZIF-67-E at different scan rates. (C) Specific capacitance retention as a function of scan rates from 5 to 500 mV s⁻¹. (D) Cycling stability obtained from CV curves at 50 mV s⁻¹ for 1000 circles.

The digital photos showed the GA@CZIFs-E had a light density (Fig. S6†). The composite as a monolith could be attached to a ruler by electrostatic force, indicating the main component of graphene aerogels.

Due to the uniform mesoporous carbon/graphene/mesoporous carbon sandwich-like structure, high specific surface area, and nitrogen-doping carbon framework, GA@CZIF-8-E, and GA@CZIF-67-E were used as the electrode materials for all-solid-state supercapacitors. To verify the performance, GA, CZIF-8-E, and CZIF-67-E were also employed as ASSSs for comparison. The electrodes of GA@CZIFs-E with the same weight were employed face-to-face to form an all-solid-state supercapacitor device (Fig. S6†).

As shown by Fig. S4,† all the samples exhibited the increment in the area of the CV with the increase of the scan rates, showing a typical EDLC behavior.⁴⁰ Moreover, it could be discerned that the GA, GA@CZIF-8-E and GA@CZIF-67-E revealed a larger capacitive response. Also, their CV curves showed nearly symmetrical response and rectangular shapes than pure CZIF-8-E, CZIF-67-E and maintained well even at the scan rate up to 500 mV s⁻¹, indicative of an ideal capacitive behavior.³ Specifically, the capacitance of GA@CZIF-67-E was about 53 F g⁻¹ at a scan rate of 5 mV s⁻¹, higher than those of GA (35 F g⁻¹), CZIF-8-E (18 F g⁻¹), CZIF-67-E (25 F g⁻¹), and GA@CZIF-8-E (49 F g⁻¹). The outstanding capacitance of GA@CZIF-67-E could be attributed to the graphitized carbon structure derived from ZIF-67 by calcination which lowered the electrochemical resistance and finally increased the rate capability and cycling stability.^41,42 The Nyquist plots (Fig. S5†) proved that GA@CZIF-67-E had a much lower impedance compared to GA@CZIF-8-E, CZIF-67-E, and CZIF-8-E both at low and high frequency area. In order to compare the capacitance retention of GA@CZIFs-E electrodes at high scan rates, the variation of specific capacitances at different scan rates were summarized (Table 1). Notably, GA@CZIF-67-E exhibited a high rate capability of 24 F g⁻¹ at a scan rate as high as 500 mV s⁻¹, greater than those of GA (14 F g⁻¹), CZIF-8-E (2 F g⁻¹), CZIF-67-E (6 F g⁻¹), and GA@CZIF-8-E (22 F g⁻¹). The interconnected graphene macropores within frameworks acted like reservoirs to minimize the diffusion distances from electrolyte to highly conductive graphene frameworks.⁴³ The cycling stability test showed that the GA@CZIF-67-E retained 92% of the initial capacitance after 1000 cycles, outperforming all others, indicating an excellent cycling stability. This was probably because the Co particles catalysed the surrounding carbon to a high crystalline state and finally made the carbon electrochemically stable during charging and discharging processes. The cycling stability of GA@CZIF-8-E increased and then dropped continuously to 88% within 1000 cycles, which was possibly due to the improvement of ion accessibility in 3D graphene frameworks in the first 100 cycles,⁴⁴ and the formation of the solid-electrolyte interface (SEI) on the surface of the electrode onwards.⁴⁵ Both GA@CZIF-8-E and GA@CZIF-67-E showed better performance than CZIF-8-E and CZIF-67-E, proving the importance of graphene cooperation and uniform nitrogen-doping, which increased the capacitance through active sites electrosorption of electrolyte ions.^46–49 Compared to all-solid-state supercapacitors published by other groups, GA@CZIF-67-E showed a higher or comparable performance due to the 3D structure and nitrogen and cobalt doping.^6,13

Based on the above analyses, we assumed the schematic illustration of the ion transportation mechanism as shown in Fig. 8. The highly porous CZIFs particles attached to the graphene sheets provided numerous mesoporous tunnels in the GA@CZIFs, which were conducive to the accessibility of the electrolyte and the rapid diffusion of electrons.⁵⁰ The 3D graphene frameworks and the nitrogen-doped carbon derived from ZIFs acted as nano-current collectors which endowed the electrode high rate capability. Moreover, the high conductive graphene provided electron transport highway and the nitrogen-doped carbon enhanced the capacitance. When in the case of GA@CZIF-67-E, because of the catalysis effect of cobalt, the surrounding carbon was graphitized and endowed the supercapacitor a high conductivity and electrochemical stability, which finally enhanced the capacitance performance further more.

Fig. 8 Schematic illustration of the ion transportation mechanism.

Conclusions

In summary, heterogeneously nucleated ZIFs nanocrystals supported on 3D graphene aerogels were prepared by a heterogeneous nucleation growth method. By the incorporation of mesopores derived from ZIFs particles and the 3D interconnected macroporous structure of GA, the obtained GA@CZIFs-E with hierarchical pore structures showed high electrochemical capacitance, excellent rate capability, and good cycling stability. Due to the Co catalyst effect on carbon graphitization, the GA@CZIF-67-E exhibited the highest capacitance of 53 F g⁻¹ at 5 mV s⁻¹, best rate capability and maintained 92% capacitance after 1000 cycles in the series. This work provided an efficient method to synthesize light-weight porous graphene frameworks with high capacitance. Attributed to the diversity of the available MOFs with different pores and active species, it is expected that this work will open the pathway to the graphene composites with MOFs for high-performance flexible all-solid-state supercapacitors.

Acknowledgements

The authors thank Dr Jun Wang from the University of Wollongong for the help with XPS measurement and the staff of Monash Centre for Electron Microscopy for their assistance in SEM and TEM. This work is supported by the Australian Research Council (Discovery Project No. DP150100765).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16153g

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