Low-temperature preparation of macroscopic nitrogen-doped graphene hydrogel for high-performance ultrafast supercapacitors

Abstract

In this work, a simple method by using ammonia at a relatively low temperature (90 °C) to prepare nitrogen-doped graphene hydrogel (N-GH) is demonstrated. Ammonia is not only a nitrogen source but also a modification to the 3D structure of graphene hydrogel. The structure and morphology characterization of the materials was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The resulting N-GH has a high atomic percentage of N up to 13.44%. It has been found that the supercapacitor performance of the resulting N-GH could be significantly improved. At the constant current density of 1 A g⁻¹, the specific capacitance was estimated as 217.8 F g⁻¹. Even at the constant current density of 200 A g⁻¹, the electrode still has a specific capacitance of 189.8 F g⁻¹. The material also has a retention rate of 95.8% of its initial capacitance after 1000 cycles at a current density of 20 A g⁻¹. Moreover, the N-GH has excellent electrochemical stability and a good performance for the rate property even at an ultrafast charge/discharge rate; thus, they may have potential applications as ultrafast supercapacitors.

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1. Introduction

Supercapacitors, also known as electrochemical capacitors, have long been regarded to be a kind of advanced energy storage systems, due to the long cycle lifetime, high power density and rapid charge/discharge rate.^1–4 It is of significant interest to increase the use of supercapacitors in a wide and growing range of applications such as electric vehicles, electric utilities, and backup power sources.^5–7 Various materials have been used as electrode materials for supercapacitors, including carbon materials, metal oxides or conducting polymers.^5,7,8,31,32 In fact, carbon in various forms is the most common electrode material for supercapacitor applications that require high capacitance and high power density. Among all kinds of carbon, graphene has become the candidate for potential applications in supercapacitors.^9,10 However, the graphene hydrogel (GH) has recently been the focus of many studies because of its high specific surface area, high conductivity, nano-micropore structure and multidimensional electron transport pathways.^11–14 Although great progress has been made in graphene hydrogel, it remains unexplored and highly desirable to control the assembly of graphene hydrogel in order to further utilize the excellent properties of graphene macroscopic assemblies.^{14–18,39–43}

Nitrogen doping is an important means developed in recent years to produce graphene hydrogel with high capacitance. It was found to be an effective method to alter the properties of graphene hydrogel because nitrogen chemically binds to the carbon lattice of graphene hydrogel.^19–21,31 Moreover, nitrogen has a comparable atom size and high eletronegativity compared to carbon.^8,19,20 Although some methods have recently been reported to synthesize nitrogen-doped graphene hydrogel (N-GH), such as CVD method and arc discharge method,^21–23 these methods require complicated experimental conditions. Compared with these methods, the low-temperature method reported in this work has the merit of using mild conditions.

In this work, we report a low-temperature approach to prepare the nitrogen-doped graphene hydrogel. The doping process can be successfully realized at the temperature as low as 90 °C by heating graphene oxide (GO), sodium sulfide (Na₂S) and ammonia in a 20 mL sample vial. The reaction between NH₃ and oxygenic groups in GO is facilitated by Na₂S. Simultaneously, Na₂S ensures the formation of the hydrogel. The 3D structure of the graphene hydrogel provides the interconnected frameworks with a macroporous architecture, which are in favor of ion diffusion and electron transport.^24,25,40 Moreover, the nitrogen doping in carbon networks can facilitate a charge transfer between neighboring carbon atoms, and thus enhance the electrochemical performance of carbon materials.^26–28 Eventually, the resulting N-GH has not only a high atomic percentage of N, up to 13.44%, but also high capacitance and excellent electrochemical stability.

2. Experimental

2.1. Sythesis of GO, GH and N-GH

The graphene oxide (GO) was synthesized from natural flake graphite following a modified Hummers method.^29,31,32

To synthesize graphene hydrogel (GH), a 2 mg mL⁻¹ solution of graphene oxide was prepared at first. Then, 1 mL GO solution and 52 μL Na₂S were placed in the oven and heated at 90 °C for 8 hours. After cooling to room temperature, the GH samples were obtained.

To synthesize nitrogen-doped graphene hydrogel (N-GH), a 2 mg mL⁻¹ solution of graphene oxide was prepared at first. Then, 1 mL GO solution and ammonia were placed together with the weight ratio of NH₃·H₂O/GO = 1 : 2, 1 : 4, 1 : 8, which are denoted as N-GH2, N-GH4, N-GH8, respectively. Subsequently, 52 μL Na₂S was added into the solution. Finally, the samples were placed in the oven and heated at 90 °C for 8 hours. After cooling to room temperature, the N-GH samples were obtained.

To characterize the morphology and structure of the samples, sample preparation is necessary. The samples need to be washed by the deionized water, and then freeze-dried for 24 hours at −50 °C.

2.2. Sample characterization

The morphology and structure of the samples were characterized by a Hitachi S-4800 field emission scanning electron microscope (SEM). The crystallographic structures of the samples were examined by a Bruker D8 ADVANCE (using Cu Kα radiation) X-ray diffractometer (XRD), Raman spectra were recorded by a Thermo Fisher DXR raman spectrometer, EDS patterns were characterized by a Hitachi S-4800 field emission scanning electron microscope, and X-ray photoelectron spectra (XPS) were recorded by on a Kratos AXIS Ultra DLD spectrometer.

2.3. Eletrochemical measurement

The supercapacitor samples were assembled according to the following method. Two slices of N-GH were immersed in 6 M aqueous KOH for 10 hours. Nickel foams were used as the current collectors. The N-GH electrode materials was pressed on the nickel foam, separated by a glass fibre filter paper and finally placed in a CR2032 coin cell.

All tests were carried out in a two-electrode cell system to study the electrochemical properties by galvanostatic charge/discharge, the cycle voltammogram (CV) and electrochemical impedance spectroscopy (EIS) on a CHI660C electrochemical workstation.

3. Result and discussion

3.1. Structure and morphology characterization

Images of N-GH and GH are shown in Fig. 1. Fig. 1a indicates that the typical product is 9 mm in diameter and 1.5 mm in height. Fig. 1b show the different features between N-GH and GH. It can be clearly seen that the samples have different diameters, which indicate that ammonia played a role in this test.

Fig. 1 Images of N-GH and GH. (a) Images of the typical N-GH4. (b) Images of the N-GH4 and GH.

In this reaction, ammonia reacts with carboxylic acid species and forms an amide-like structure, and then decarbonylate to form stable aromatic structures. Ammonia can also react with other oxygen-containing moieties to reduce graphene oxide. Na₂S reacts with the remaining oxygen-containing moieties to hinder the π–π stacking in the single graphene sheet, which helps to complete the reduction reaction.

Fig. 2 shows the SEM images of a typical N-GH. The SEM images (Fig. 2a–d) show that N-GH has an interconnected three-dimensional porous network. The pore size ranges from a few hundred nanometers to several micrometers. Note that NH₃ reacts with oxygen containing moieties, and during this process, the reduced graphene sheets, which are in high concentration, will assemble into hydrogel due to the π–π stacking interactions. The reaction of NH₃ and GO can impede the π–π stacking in the single graphene sheet and expand the space between the graphene sheets. Consequently, compared with the GH, there are bigger porous networks in N-GH. Fig. S1 and Table 1† indicate that the reaction between NH₃ and oxygenic groups in GO has been completed and results in a high atomic percentage of N up to 13.44% in N-GH. The resulting NG sheets have relatively high capacitance and excellent electrochemical stability.

Fig. 2 SEM images of the typical N-GH microstructure.

Fig. 3 shows the XRD results of GO, GH and the different weight radio N-GH. The XRD pattern of GO exhibits an intense and sharp peak at around 2θ = 11.38°, indicating that the interlayer distance is approximately 0.777 nm. This value is much larger than that of graphite (0.336 nm),³⁰ owing to the amounts of oxygen-containing functional groups inserted into the graphite layers. After reduction with the Na₂S at 90 °C, the diffraction peak at 2θ = 11.38° disappears and a broad peak at 2θ = 25° is found in the XRD pattern of the GH, indicating the interlayer distance is approximately 0.365 nm. The XRD pattern of N-GH2, N-GH4, N-GH8 also shows a major peak at 2θ = 25°, which corresponds to an average interlayer spacing of 3.74, 3.72 and 3.68. These spacing values are still higher than that of graphite (0.336 nm) due to doping with oxygen and nitrogen. The XRD results indicate that the GH, N-GH2, N-GH4 and N-GH8 are successfully prepared from graphene oxide.^39,42,43

Fig. 3 XRD spectra of GO, GH, N-GH4, N-GH8.

Fig. 4 shows the XPS results of GH and N-GH. The XPS spectra of GH and N-GH are shown in Fig. 4a. For the GH samples, only the C1s peaks (about 285.1 eV) and O1s (about 531.5 eV) peaks are detected, while an additional peak at 400.0 eV ascribed to N1s can be observed for N-GH samples. The figure clearly shows the appearance of a peak at about 400 eV, which corresponds to nitrogen. Fig. 4b shows the C1s peaks and high resolution results of N-GH. For a high resolution of C1s spectrum of N-GH, the dominant peak at 284.5 eV corresponds to sp²-hybridised graphitic carbon atoms, which indicates that most of the carbon atoms in the N-GH are arranged in a conjugated honeycomb lattice. The small peak at 288.0 eV corresponds to C=O configurations, which revealed that most of the oxygen groups had been removed. The new peak at 285.9 eV can be indexed to N-sp², and it appeared because of the doping of nitrogen atoms. Fig. 4c show the N1s peaks and its high resolution results of N-GH. It is clear that nitrogen is observed in N-GH, confirming its incorporation into graphene hydrogels. In general, there are mainly four nitrogen functional groups in nitrogen-doped carbon. The peak at 398.7 eV corresponds to pyridinic-N, which refers to nitrogen atoms at the edge of the graphene planes. The peak at 400.4 eV corresponds to pyrrolic-N, which refers to nitrogen atoms that are bonded to two carbon atoms and contribute to the system with two p-electrons. The peak at 401.2 eV corresponds to graphitic-N. Nitrogen atoms are incorporated into the graphene layer and replace carbon atoms within a graphene plane. The peak at 405.1 eV corresponds to oxidized nitrogen. The XPS results indicate that N-graphene contains all these three functional groups (pyridinic-N, pyrrolic-N, and graphitic-N).^41–43

Fig. 4 XPS results of N-GH2 and GH. (a) XPS spectra for GH and N-GH. (b) C1s peaks and high resolution results of N-GH. (c) N1s peaks and high resolution results of N-GH.

3.2. Electrochemical measurements

A two-electrode system was used to investigate the electro-chemical performance of the supercapacitor test cells of the N-GH in an aqueous electrolyte. Fig. 5 shows the supercapacitor performance of the typical N-GH. Fig. 5a shows the cyclic voltammetry (CV) measurement data. The CVs are rectangular in shape at different scan rates, especially at 500 mV s⁻¹, which indicates an excellent capacitive behavior over a wide range of voltage scan rates. Fig. 5b shows the comparison of the cyclic voltammetry curves between N-GH and GH at a scan rate of 50 mV s⁻¹. It is easy to observe that the CV curve of N-GH is more rectangular with mirror symmetry, indicating a more excellent reversibility. The area under N-GH's curve is larger than that of GH, which reflects a higher capacitance, due to a remarkable contribution of nitrogen doping in raising the capacitance.

Fig. 5 Supercapacitor performance of the typical N-GH. (a) CV curves of N-GH4 at different scan rates. (b) CV curves of N-GH4 and GH at a scan rate of 500 mV s⁻¹. (c)–(e) The galvanostatic charge/discharge curves of N-GH4 at different current densities. (f) The galvanostatic charge/discharge curves of N-GH4 and GH at the current density of 1 A g⁻¹ and 100 A g⁻¹.

The galvanostatic charge/discharge curves at different current densities are shown in Fig. 5c–e. The shape of the curves is almost typical isosceles triangle and highly linear, indicating that the electrode material has ideal capacitive characteristic and excellent electrochemical reversibility. The specific capacitance values of the N-GH electrode are calculated using galvanostatic charge/discharge curves. The equation is:

C_s = 2IΔt/ΔVm

where I is the charge/discharge current, Δt is discharge time, ΔV is potential drop and m is the mass of active material in a single electrode.^33,35 At a constant current density of 1 A g⁻¹, the specific capacitance was estimated as 217.8 F g⁻¹; moreover, at a constant current density of 10 A g⁻¹, the specific capacitance was also estimated as 198.1 F g⁻¹. Furthermore, at a constant current density of 100 A g⁻¹ and 200 A g⁻¹, the electrode still has a specific capacitance of 182.8 F g⁻¹ and 189.8 F g⁻¹, respectively. As is shown in Fig. 5f, the specific capacitance of N-GH is larger than GH because of the longer discharge time at the same current density.

The bigger porous network in N-GH after nitrogen-doping is in favor of the ion diffusion and electron transport. The improved capacitance by nitrogen doping is maintained well for the higher current densities. As is shown in Fig. 6a, the N-GH has the ultrafast charging rate and a good performance for the rate capability even at a high current density.

Fig. 6 (a) Gravimetric capacitance of N-GH2, N-GH4, N-GH8 and GH at a series of current densities. (b) Nyquist plot of a typical N-GH and GH supercapacitor. (c) The cycling life of N-GH4 at a current density of 20 A g⁻¹.

Electrochemical impedance spectroscopy (EIS) measurements at a frequency range of 100 kHz to 0.01 Hz were carried out in Fig. 6b. The Nyquist plots for N-GH and GH show a semicircle over the high-frequency range, followed by a straight sloped line in the low-frequency region. The straight line is ascribed to the diffusive resistance of the electrolyte in the electrode pores and proton diffusion in the host materials.^34–38 At a low frequency, the line of the N-GH sample is more vertically straight than that of the GH sample, which indicates the faster ion diffusion behavior of the porous samples. Therefore, the electrode resistance of the N-GH is much lower than that of GH. In addition, at a current density of 20 A g⁻¹, the N-GH's capacitance remained at 95.8% after 1000 cycles (Fig. 6c).

4. Conclusions

In summary, the N-GH can be achieved by a simple method using graphene oxide and ammonia as the precursors at a low temperature. The resulting N-GH not only has a relatively high capacitance and excellent electrochemical stability but also a good performance for the rate property even at an ultrafast charge/discharge rate. Consequently, the typical N-GH may have potential applications as capacitors in vehicles, lifts and other devices at a high rate.

Acknowledgements

We acknowledge support from the National Basic Research Program 973 of China (Grants no. 2011CB932700 and no. 2011CB932703), the Chinese Natural Science Fund Project (Grants no. 61335006, no. 61378073, and no. 61077044), and the Beijing Natural Science Fund Project (Grant no. 4132031).

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Footnote

† Electronic supplementary information (ESI) available: The summary of methods to prepare the typical GO, GH and N-GH; EDS images of the GH and N-GH. (Fig. S and Table 1) See DOI: 10.1039/c4ra09497b

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