Constructing nitrogen-doped nanoporous carbon/graphene networks as promising electrode materials for supercapacitive energy storage

Abstract

Nitrogen-doped nanoporous carbon/graphene networks (NPC/G) have been prepared by zeolitic imidazolate frameworks (ZIF-8) homogeneously encapsulated in graphene oxide (GO) networks, which were carbonized at high-temperature under a N₂ atmosphere and washed to get rid of impurities. The obtained NPC/G composites are highly graphitizing and exhibit a high specific surface area up to 703 m² g⁻¹. By optimizing the annealing conditions, the nanoporous frameworks derived from ZIF-8 can be effectively maintained, which are beneficial for electrolyte ion adsorption and transportation. Furthermore, the NPC/G composites are used as electrode materials for supercapacitive energy storage and show a high specific capacitance of 235 F g⁻¹ at a current density of 1 A g⁻¹, which is larger than that of NPCs and rGO. This provides a new route for designing novel structures to improve electrochemical performance. More interestingly, these electrode materials also present excellent rate capability and 85% retention of its initial capacitance after 1000 cycles in 1 M KOH aqueous solution.

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1. Introduction

Nowadays, with the depletion of fossil fuels and increasing environmental pollution, there are major challenges for constructing efficient, clean, and sustainable sources of energy, as well as developing new technologies associated with energy conversion and storage. Supercapacitors, as a renewable electrochemical device, have attracted wide attention due to their high power density, fast charge–discharge rate, long cycle life and less environmental pollution.^1–5 Currently, electrical double-layer capacitance (EDLC) and pseudocapacitance are two main types of supercapacitors based upon charge storage mechanism: EDLC generally arises from the charge separation at the electrode/electrolyte interface and no electrochemical reaction happens in this process, whereas pseudocapacitance is ascribed to faradaic charge transfer through redox reactions at the electrode/electrolyte interface.^6–9 It is well known that the development of high-quality electrode materials for improving supercapacitive performance is still a great challenge.

In the last decade, graphene has shown promising applications in high-performance electrochemical energy storage devices owing to its excellent electronic properties, high specific surface area (2630 m² g⁻¹), and chemical and mechanical stability.^10–14 However, it was repeatedly demonstrated that the face-to-face restacking behavior of graphene nanosheets hindered the charge transport and thereby limited its application for supercapacitors. To prevent restacking of graphene nanosheets and increase their efficient electrical conducting paths, different kinds of spacer materials were reported for improving the performance of supercapacitors.^15–17 For example, incorporating pseudocapacitive nanostructured transition metal oxides or hydroxides (MnO₂, Ni(OH)₂, etc.)^18–22 into graphene sheets was adopted to avoid the stacking of nanosheets for improving the capacitance. Unfortunately, the so-called pseudocapacitors, even though it is possible to store more energy than EDLC, suffer from drawbacks such as a lower power density and lower stability during cycling.^1,10

Recently, nanoporous carbons have been shown as promising electrode candidates applied in EDLCs.^23–28 In order to prepare highly nanoporous carbons with excellent features for supercapacitors, metal-organic frameworks (MOFs) are often considered as precursor or sacrificial templates owing to their highly specific surface areas, good thermal stability, tailoring pore structures and controlled channels.^29–33 Therefore, it is anticipated that constructing uniform MOF derived nanoporous carbons embedded into graphene nanosheets to obtain nanoporous carbon/graphene networks is highly desirable for supercapacitors owing to the combination of nanopore structures and electron transfer. Moreover, the imidazole-based ligands of ZIF-8 will yield N-doped porous carbons, which can enhance the properties of the porous carbons. For example, N-doped carbons may afford pseudocapacitance and improve the electronic conductivity of carbon-based materials.^34–36

In this study, the NPC/G networks were prepared by ZIF-8 nanocrystals homogeneously encapsulated in GO nanosheets, followed by calcining and washing processes. The obtained NPC/G networks have a high specific surface area, hierarchical pore structure and high nitrogen content, which are beneficial for electrolyte ion adsorption and transportation. Furthermore, the NPC/G as electrode materials were applied in supercapacitive energy storage for the first time and exhibited a high specific capacitance, excellent rate capability and 85% retention of its initial capacitance after 1000 cycles in 1 M KOH aqueous solution.

2. Experimental section

2.1 Materials

The graphite powder, potassium permanganate, zinc nitrate, 2-methylimidazole, sulfuric acid, hydrochloric acid, hydrogen peroxide, methanol and ethanol employed in the current research were commercially available analytical-grade products. N-Methyl-2-pyrrolidone (C₅H₉NO, 99%) and polyvinylidene fluoride (PVDF, 98%) were purchased from Aldrich and used as received without further purification.

2.2 Synthesis of NPC/G, NPCs and rGO

Graphene oxide (GO) was prepared by an improved Hummers’ method^37–40 (for the detailed synthetic procedure, see the ESI†). In a typical synthesis of NPC/G composites, the given amount of the as-prepared GO was dispersed in 30 mL of water, while 0.376 g of Zn(NO₃)₂·6H₂O and 0.815 g of 2-methylimidazole were dissolved in 10 mL of methanol, respectively. Then, the Zn(NO₃)₂ solution and the 2-methylimidazole solution were in turn added into the GO solution. After stirring for 24 h, the gray precipitate was collected by centrifugation and washed with methanol at least three times. Finally, the freeze dried product was calcined at 800 °C for 5 h under a N₂ atmosphere, and was immersed in 2 M HCl aqueous solution for 12 h. The NPC/G was collected by centrifugation, washed with water and dried at 50 °C. Additionally, NPC/G-700 and NPC/G-900 samples were also synthesised under the same experimental conditions, but they were calcinied at 700 °C (NPC/G-700) and 900 °C (NPC/G-900) for 5 h under a N₂ atmosphere. The synthesis procedure of NPCs was the same as that of NPC/G but without the addition of GO. The rGO was obtained by calcining GO at 800 °C for 5 h under a N₂ atmosphere.

2.3 Characterization

Powder X-ray diffraction (XRD) patterns were obtained using a SCX mini diffractometer with Cu-Kα radiation (λ = 1.54056 Å). The Fourier-transform infrared (FT-IR) spectrum was obtained on a Vertex 7.0 spectrometer with a pressed KBr pellet in the range of 4000–400 cm⁻¹. Raman spectroscopy was carried out using a LabRAM HR with a 532 nm laser source. Elemental analysis was performed using a vario MICRO elemental analyzer. Transmission electron microscopy (TEM) images were obtained using a F20 transmission electron microscope. Scanning electron microscopy (SEM) was performed on a JSM-6700 scanning electron microscope. The thickness of the GO sheets was analyzed with a Nanoscope IIIa Multimode SPM atomic force microscope (AFM) (Veeco instruments) under tapping mode. The X-ray photoelectron spectroscopy (XPS) measurement was carried out with an ESCALAB 250Xi spectrometer using Al Kα (1486.6 eV) X-ray. The nitrogen adsorption isotherm was measured at 77 K using a Micromeritics ASAP2020 analyzer. The pore size distribution was calculated from the density functional theory (DFT) method with the slit pore model.

Electrochemical experiments were performed on a CHI 660e electrochemical workstation (Chenhua, Shanghai, China) with a standard three-electrode cell in 1 M KOH aqueous solution. A platinum foil (1.0 cm²) and mercuric oxide electrode were used as counter and reference electrode, respectively. The working electrode was prepared by coating a slurry of sample onto a Ni-foam electrode (1 × 1 cm²) and drying in a vacuum oven at 333 K overnight. Typically, the NPC/G was uniformly mixed with polyvinylidene difluoride (PVDF) binders in a weight ratio of 95 : 5 and made as a slurry using N-methyl-2-pyrrolidone. To evaluate the electrochemical characteristics of the materials as electrodes in aqueous systems, cyclic voltammetric (CV) and galvanostatic charge–discharge (GC) measurements were performed. Cyclic stabilities were characterized using galvanostatic charge–discharge measurements over 1000 cycles at a charge–discharge rate of 3 A g⁻¹. The specific capacitance C_g was calculated based on the following equation:

C_g = I × Δt/(m × ΔV)

where I, Δt, ΔV and m are indexed to the applied current, discharge time, voltage change and the mass of the active material, respectively.

3. Results and discussion

The NPC/G networks were prepared using ZIF-8 nanocrystals homogeneously encapsulated in GO nanosheets followed by calcining and washing processes, as illustrated in Fig. 1. Firstly, a Zn(NO₃)₂·6H₂O methanol solution was added to the GO aqueous solution with stirring, and zinc ions were adsorbed onto the oxygen containing functional groups of the GO layers through an electrostatic interaction. Then, a 2-methylimidazolate methanol solution was added to the mixture solution, resulting in homogenous nucleation of ZIF-8 nanocrystals. The small ZIF-8 nanocrystals were embedded into the GO nanosheets and acted as a protective layer to prevent the severe agglomeration of reduced graphene oxide (rGO) during high-temperature treatment. Finally, NPC/G networks with a nitrogen-doped carbon framework were obtained by calcination of ZIF-8/GO at 800 °C under a N₂ flow and acid washing with 2 M HCl.

Fig. 1 Schematic illustration for the preparation of NPC/G.

Graphene oxide (GO) was prepared by an improved Hummers’ method^37–40 (for the detailed synthetic procedure, see the ESI†). The as-prepared GO was characterized by XRD and FT-IR spectroscopy (see Fig. S1 and S2, in the ESI†). The XRD result showed that there was a larger interlayer spacing of d₀₀₂ (9.5 Å) than that of graphite (3.35 Å), attributed to the successful integration of a large amount of hydroxyl and epoxide functional groups in the GO nanosheets. The IR spectrum of GO prepared by this routine was essentially consistent with that reported in the literature,^37,41,42 and the following functional groups were identified: O–H stretching vibrations (3432 cm⁻¹), C=O stretching vibration (1737 cm⁻¹), C=C from sp² CC bonds (1632 cm⁻¹), C–OH vibrations (1400 cm⁻¹), and C–O vibrations (1227 cm⁻¹). Further, the surface morphology and texture of the GO samples were investigated by TEM and AFM measurements. As shown in Fig. S3,† a lamellar nanostructured architecture is observed. The obtained nanosheets are further examined by AFM analyses in Fig. S4† which reveal an average thickness of approximately 0.8 nm for GO.

ZIF-8 was chosen as the precursor in the presence of GO for preparing ZIF-8/GO nanocomposites. The evolution of the structure during the synthesis of ZIF-8/GO and NPC/G was demonstrated using XRD. As revealed by the X-ray diffraction (XRD) pattern shown in Fig. 2, all diffraction peaks with a strong intensity for ZIF-8/GO match well with the simulated ZIF-8, indicating a pure phase and high crystallinity. After the heat treatment, the diffraction peak of NPC/G at around 25° was assigned to the interplane (002) reflection of graphite carbon, and there were no diffraction peaks of impurities (Zn species), indicating thorough carbonization of ZIF-8 and that the zinc species were fully removed by acid washing.

Fig. 2 XRD patterns of ZIF-8 (simulation), GO/ZIF-8 and NPC/G.

The SEM and TEM observations shown in Fig. 3a and c demonstrate that the ZIF-8 nano-polyhedrons, with a size of around 150 nm, were successfully embedded into the GO networks. To our knowledge, this is the first example of a novel architecture consisting of nanoporous ZIF-8 particles embedded in a microsized graphene network. Such a type of structure could effectively prevent the agglomeration of graphene during high-temperature treatment. In addition, in the absence of GO, Zn salts in methanol were added to imidazole solution for preparing ZIF-8. As revealed by the SEM image shown in Fig. 3b, ZIF-8 exhibited a nano-polyhedron structure with a size of around 500 nm, which is larger than that of those in ZIF-8/GO. This result suggests that GO can confine the growth of ZIF-8 nanoparticles, which is similar to the report by Wang’s group.⁴³

Fig. 3 SEM images of (a) ZIF-8/GO and (b) ZIF-8. (c) TEM image of ZIF-8/GO. (d) Schematic illustration for ZIF-8/GO.

After high-temperature calcination, the ZIF-8/GO nanocomposites can be transformed to derivative NPC/G with size and morphological retention as revealed by the SEM images shown in Fig. 4a and b. To further investigate the morphology and structure of NPC/G, TEM characterization has been carried out. As presented in Fig. 4c, the NPC particles homogeneously encapsulated in the graphene nanosheets maintained the structural integrity and prevented the agglomeration of graphene nanosheet. Moreover, the high-resolution TEM image in Fig. 4d further confirms that NPC/G has a nanoporous structure, suggesting that the optimized annealing conditions can prevent the collapse of the structure. In the Raman spectra of NPC/G before and after calcination, as shown in Fig. S5,† there are two dominant peaks at 1350 and 1584 cm⁻¹, corresponding to the D band and G band. Notably, the I_D/I_G value of NPC/G (I_D/I_G = 0.99) was higher than that of ZIF-8/GO (I_D/I_G = 0.91), indicating the formation of abundant defects and disordered carbon during the heat-treated process.

Fig. 4 (a) and (b) SEM images and (c) and (d) TEM images of NPC/G.

The surface area and pore size distribution of NPC/G were characterized by N₂ adsorption–desorption measurements. As shown in Fig. 5a, the NPC/G displayed a type-IV isotherm with a distinct hysteresis loop in the relative pressure region of 0.43 < P/P₀ < 1.0, indicating a hierarchically porous structure with micropores and mesopores.^44,45 The BET surface area of NPC/G was up to 703 m² g⁻¹. The pore size distribution, calculated from the density functional theory (DFT) method, revealed that the NPC/G possessed micro- (0.47 nm, 1.3 nm), meso- (2–50 nm) and macropores (50–100 nm), as shown in Fig. 5b. Compared with pore size distribution of NPC/G, the NPCs exhibited a similar distribution in micropores, and an obvious difference in mesopores and macropores (Fig. S6†). This result revealed that the micropores might originate from the carbon framework by calcination, and the mesopores and macropores might be mainly attributed to the stacking of carbon frameworks and graphene.⁴⁴ The proportion of hierarchical pore distribution endowed NPC/G with a large electrode/electrolyte contact area and facilitated diffusion of electrolyte ions.

Fig. 5 (a) Nitrogen adsorption (○)–desorption (●) isotherm of the NPC/G networks at 77 K. (b) Pore size distribution of NPC/G.

Nowadays, various synthesis methods and characterization techniques for N-graphene have been explored. For example, Pumera’s group presented an optimized, scalable technique for fabrication of large quantities of highly nitrogen doped (>7 at%) graphene.⁴⁶ The charge distribution and electron doping in nitrogen-doped graphene were investigated by Joucken et al.,⁴⁷ and individual nitrogen dopants in monolayer graphene were characterized by Pasupathy et al.⁴⁸ The nitrogen doping effects on the structure of graphene were studied by Sun’s group.⁴² In addition, different synthesis and characterization methods for nitrogen-substituted graphene were reviewed by Wang et al.⁴⁹ Different from the above strategy for nitrogen directly doping into graphene frameworks, in this study, zeolitic imidazolate frameworks (ZIF-8) were adopted to embed into GO nanosheets for yielding N-doped porous carbon/graphene composites. Nitrogen doping of porous carbons can enhance the capacitive property by affording pseudocapacitance and improving the electronic conductivity of carbon-based materials.³⁴ Therefore, X-ray photoelectron spectroscopy (XPS) was used to determine the nitrogen content and the nitrogen type in NPC/G. As shown in Fig. 6, the N 1s peak of NPC/G was deconvoluted to four different peaks located at 398.4, 400.0, 401.1, and 405.4 eV, corresponding to pyridinic nitrogen (N-1), pyrrolic nitrogen (N-2), quaternary nitrogen (N-3) and pyridinic N⁺–O⁻, respectively.^36,43 N-1 and N-2 can provide faradaic pseudocapacitance, whereas N-3 can effectively improve the conductivity of porous carbons. The N-1, N-2 and N-3 content was 4.7%, 4.2% and 4.0%, respectively, indicating that the NPC/G may have an excellent capacitive behavior. In addition, elemental analysis showed N content of 11.17% (mass percent), which was close to the result of XPS.

Fig. 6 High-resolution N 1s spectra for NPC/G; inset shows four different configurations for nitrogen atoms in the N-doped carbon materials ((N-1) pyridinic N; (N-2) pyrrolic N; (N-3) graphitic N; (N-4) pyridinic N⁺–O⁻).

Motivated by the advantages of a hierarchically porous structure and high content of nitrogen,^50–54 the capacitance behavior has been investigated by cyclic voltammetric (CV) and galvanostatic charge–discharge (GC) measurements. Compared with NPCs and rGO, as shown in Fig. 7, the NPC/G has a larger covered area in the CV curve at a scan rate of 25 mV s⁻¹, indicating that NPC/G has a larger specific capacitance than that of NPCs and rGO, suggesting that this novel nanoporous architecture would be beneficial for improving the performance of supercapacitors.

Fig. 7 CV curves of NPC/G, NPCs and rGO in 1.0 M KOH aqueous solution at a scan rate of 25 mV s⁻¹.

As displayed in Fig. 8a, comparing the CV curves of materials obtained at different calcinating temperatures (700, 800, and 900 °C), it was found that the NPC/G networks carbonized at 800 °C exhibit the largest CV area, which may be attributed to that the NPC/G can effectively maintain the nanoporous framework derived from ZIF-8, and possess a high surface area.⁴⁴ The electrochemical performance of NPC/G was further studied carefully. The CV curves of NPC/G showed quasi-rectangular shapes at different scan rates as presented in Fig. 8b. With increasing scan rates from 5 mV s⁻¹ to 100 mV s⁻¹, the specific capacitances decreased from 200 F g⁻¹ to 136 F g⁻¹. The more detailed data are listed in Table S1.† In general, the current density tends to increase with the increase of the scanning rate.^27,30,32 The CV curves retained their shape even at high scan rates, indicating a good high-rate performance of the active material. As presented in Fig. 8c, the GC curves of NPC/G displayed almost symmetrical triangular shapes at different current densities ranging from 1 to 10 A g⁻¹, implying the excellent electrochemical reversibility of NPC/G. The specific capacitances based on the GC curves are exhibited in Fig. 8c. NPC/G has specific capacitances of 235, 201, 186, 176 and 168 F g⁻¹ at current densities of 1, 2, 3, 4 and 5 A g⁻¹, respectively. For comparison, the CV, GC curves and specific capacitance of NPCs are shown in Fig. S7† and 9. The NPCs hold the specific capacitances of 171, 141, 131, 124 and 119 F g⁻¹ at current densities of 1, 2, 3, 4 and 5 A g⁻¹, respectively. Obviously, the NPC/G networks showed a higher specific capacitance than that of the NPCs. This result indicated that the construction of NPC/G networks was beneficial for improving the specific capacitance. Meanwhile, this would also provide a new route for designing novel structures to improve the electrochemical performance.

Fig. 8 The electrochemical characterization of all of the samples in 1 M KOH aqueous solution at room temperature. (a) CV curves of the NPC/G, NPC/G-700 and NPC/G-900 at a scan rate of 25 mV s⁻¹; (b) CV curves of NPC/G at different scan rates; (c) GC curves of NPC/G at different current densities ranging from 1 A g⁻¹ to 10 A g⁻¹; (d) specific capacitance as a function of current density for NPC/G.

Fig. 9 The electrochemical characterization of NCPs in 1 M KOH aqueous solution at room temperature. (a) GC curves at different current densities ranging from 1 A g⁻¹ to 10 A g⁻¹; (b) specific capacitance as a function of current density.

To our knowledge, some nanoporous carbons derived from MOFs have been used as electrode materials for supercapacitors with high performances, as listed in Table 1. The NPC/G networks show a high specific capacitance of 235 F g⁻¹ at current density of 1 A g⁻¹, which is comparable to some materials derived from MOFs reported previously.

Table 1 Capacitance of carbon materials prepared using MOFs as precursors

Material	Specific capacitance (F g^−1)	Reference
NPC/G	235 (1 A g^−1) or 200 (5 mV s^−1)	This work
Z-800	130 (50 mV s^−1)	27
NPC-800	238 (20 mV s^−1)	24
C800	188 (5 mV s^−1)	55
CZIF69a	168 (5 mV s^−1)	56
AS-ZC-800	211 (10 mV s^−1)	32
NPC800	151 (0.05 A g^−1)	57
MC-A	222 (0.25 A g^−1)	58
HPC	166 (0.1 A g^−1)	59
C-S700	182 (2 mV s^−1)	60
C-MOF-2	170 (10 mV s^−1)	61
C-Zn-BTC	134 (10 mV s^−1)	61

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Moreover, the cyclic stability of the NPC/G composites was measured at a current density of 3 A g⁻¹, as shown in Fig. 10. The specific capacitance retention was 85% after 1000 cycles. The degradation of 15% of the specific capacitance may be caused by two factors as follows. One is that the hetero-atoms (N or O) in the framework or as functional groups on the surface are not stable during long cycling and a gradual decrease of capacitance is observed.^62,63 The other is structural breakdown and decay of electrical conductivity during the charging/discharging process.⁶⁴

Fig. 10 Specific capacitance retention versus cycle number for NPC/G.

4. Conclusions

In summary, nitrogen-doped nanoporous carbon/graphene networks (NPC/G) have been prepared by zeolitic imidazolate frameworks (ZIF-8) homogeneously encapsulated in graphene oxide (GO) nanosheets, carbonized at high-temperature under a N₂ atmosphere and washed to get rid of impurities. The obtained NPC/G composites are highly graphitizing and exhibit a high specific surface area up to 703 m² g⁻¹. Furthermore, the NPC/G composites are used as electrode materials for supercapacitive energy storage and show a high specific capacitance of 235 F g⁻¹ at a current density of 1 A g⁻¹. For comparison, the NPC/G networks show a higher specific capacitance than NPCs. This result indicates that the construction of NPC/G networks is beneficial for improving the specific capacitance. Meanwhile, this would also provide a new route for designing novel structures to improve electrochemical performance. More interestingly, these electrode materials exhibit excellent rate capability and 85% retention of its initial capacitance after 1000 cycles in 1 M KOH aqueous solution.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21273236) and partly supported by Science and Technology Planning Projects of Fujian Province of China (2014H2008 and 2015I0008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedure, XRD, FT-IR spectroscopy, TEM image of GO nanosheet, AFM scan of GO sheet, pore size distribution of NPCs, Raman spectra of NPC/G before and after calcination, CV curves of NCPs at different scan rates, and specific capacitance at different scan rates for NCP/G. See DOI: 10.1039/c6ra01623e

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