Effect of drying conditions on the structure of three-dimensional N-doped graphene and its electrochemical performance

Abstract

Freeze drying is a general method for preparing three dimensional (3D) graphene based materials, however, in this paper, we used N-doped graphene as a model material and found that if the N-doped graphene hydrogel was pre-frozen, instead of an interconnected porous structure, a loosely packed layered structure was obtained. Furthermore, the effects of drying conditions on the pore and chemical structure have been discussed. Results demonstrated that when the graphene hydrogel was directly freeze dried, an interconnected porous structure can be obtained, whereas, heat dried and pre-frozen samples had layered structures. The structure determines the performance; the sample prepared by direct freeze drying with an interconnected porous structure had the highest specific capacitance of 218 F g⁻¹ when the current density was 0.01 A g⁻¹, and the specific capacitance still remained at about 85% of its initial value even at a high current density (1 A g⁻¹). The excellent electrochemical performance is mainly attributed to the fast electrolyte diffusion paths provided by the interconnected porous channels.

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1 Introduction

Three-dimensional (3D) graphene with excellent electrical conductivity, large surface area and 3D cross-linked porous structure has attracted much attention.^1–4 The combination of the 3D porous structure and excellent inherent characteristics of graphene provides the 3D graphene with fast mass and electron transport and unblocked electrolyte transfer in supercapacitors (SCs), hence it is considered an ideal active material for SCs.^5–11

There are several methods to assemble graphene into 3D graphene, including the CVD method,^12,13 hydrothermal method,^14,15 and directly carbonizing the composite of carbon precursor and NH₄Cl.¹⁶ The self-assembly of graphene oxide into 3D graphene by traditional hydrothermal method is the most popular one because of the controllable pore size, ease of doping and low cost. Up to now, many efforts have been made to optimize the preparation conditions and explore the applications in supercapacitors of 3D graphene prepared by hydrothermal method. For example, Xu et al. prepared 3D graphene hydrogel using 2 mg ml⁻¹ graphene oxide as precursor by freeze drying, the specific capacitance of the sample can reach 160 F g⁻¹ at the current density of 1 A g⁻¹.¹⁷ Another improvement was made by Wang et al., they used carbon nanotube as spacer to prevent the aggregation of graphene oxide during hydrothermal process, the specific capacitance of the material improved to 181 F g⁻¹.¹⁸ On the other hand, the preparation of nitrogen rich graphene is also a hot point. In which N can not only alter the electron donor–acceptor properties of the graphene layer, but also provide pseudocapacitance related to Faradaic charge transfer reactions between the nitrogen atoms and the electrolytes.^19–22 This synergistic effect lowers electrical conductivity and improved the capacitance as well. Recently, numerous important reports have focused on the preparation of N-doped graphene foams as the promising electrode materials for SCs. Yang Zhao et al. prepared an ultralight N-doped graphene framework by hydrothermal treatment of graphene oxide and pyrrole (Py), because of the 3D porous structure and high conductivity, the sample demonstrated high electrochemical performance.²³ Subsequently, compression tolerant supercapacitor based on polypyrrole mediated graphene foam was also synthesized by the same group.²⁴ Similar researches have also been reported by other groups.^25,26 In a word, the combination of continuous porous structure, high conductivity and pseudocapacitance provided by nitrogen improved the electrochemical performance. To further optimize the performance, You et al. reported that 3D N-doped graphene–CNT networks (NGCs) can be obtained through hydrothermal treatment, freeze-drying and subsequent carbonization, the resulting NCGs showed high specific capacitance, good rate capability and cycle stability.²⁷ Very recently, Ye and coworkers fabricated a three-dimensional hierarchical graphene/polypyrrole aerogel (GPA) using graphene oxide (GO) and already synthesized one-dimensional hollow polypyrrole nanotubes (PNTs), the supercapacitor electrodes based on such materials exhibited excellent electrochemical performance, including a high specific capacitance up to 253 F g⁻¹, good rate performance, and outstanding cycle stability.²⁸ Although much progress had been made in the preparation of 3D graphene and their application in supercapacitor,^29,30 all the samples were prepared by freeze drying, in fact, porous structure will be changed if the sample was pre-frozen, the related results have not been reported.

In this paper, we used 3D N-doped graphene hydrogel (NGH) as a model material and dried it by three different drying conditions, including pre-frozen drying (PFD), direct freeze drying (DFD) and heat drying at (100 °C) (HD). The influence of drying conditions on the pore and chemical structure of the samples, and further on the electrochemical performance was investigated.

2 Experimental

2.1 Synthesis of 3D N-doped graphene

The precursor is graphene oxide (GO) and Py. GO was synthesized from natural graphite powders by a modified Hummers method.³¹ 3D N-doped graphene was prepared by hydrothermal method. In a typical process, 35 ml of 2 mg ml⁻¹ GO solution was mixed with Py with a mass ratio of 1 : 2, after agitation for 1 h, the mixture was transferred to a Teflon lined stainless-steel autoclave, hydrothermally treated at 180 °C for 20 h, then cooled freely. The obtained samples were dried at three conditions separately, One was firstly frozen at −60 °C, then freeze dried, the sample obtained was labeled as PFD; the other was put into freeze dryer directly to freeze dry, the sample was labeled as DFD; the last one was heat dried at 100 °C, which was labeled as HD.

2.2 Characterization

Morphology of the as-prepared samples was observed by scanning electron microscopy (SEM) (FEI Quanta FEG). Functional groups and chemical bonds of samples were determined by X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA) and Fourier transform infrared (FTIR) spectroscopy (P.E. Spectrum 100). X-Ray diffraction (XRD) was measured on a Bruker D8 Advance X-ray diffractometer. Raman spectra was recorded using a Raman Station (P.E., 400/400F) fitted with a 785 nm laser. Specific surface areas of the samples were measured by nitrogen adsorption–desorption isotherms performed at 77 K on a Micromeritics ASAP-2020 volumetric adsorption system, and the specific surface area (SSA) was calculated by the Brunauer–Emmett–Teller (BET) method.

2.3 Electrochemical measurements

All electrochemical measurements were carried out in a conventional two-electrode system using an electrochemical workstation (CHI 750D, Chenhua Instruments, China) with 6 M KOH solution as the electrolyte. The electrodes were prepared by mixing the as-prepared samples and poly (tetrafluoroethylene) (PTFE) in a mass ratio of 9 : 1, a small amount of ethanol was added to obtain a homogenous slurry, which then rolled to a uniform film, the film was then subsequently punched into electrodes with a diameter of 6 mm and dried at 60 °C for 24 h in a vacuum oven. The mass loading of each electrode was about 2.5–3.0 mg. All the electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out. CV tests were carried out with 5, 10, 20, 50, and 100 mV s⁻¹ scan rates in a potential window of −0.2–0.8 V. Galvanostatic charge–discharge measurement was performed for various current densities.

3 Results and discussion

Fig. 1 was the interior microstructure of the samples dried at different conditions.

Fig. 1 SEM images of PFD (a and b), DFD (c and d) and HD (e and f) with low (a, c and e) and high magnification (b, d and f).

As was shown in Fig. 1(a) and (b), the sheets of PFD were loosely stacked layer by layer, and the distance between each layer was about 100 nm. From the thickness of Fig. 1(b) it can be deduced that each layer of PFD contained several layers of graphene sheets. On the contrary, when the sample was freeze dried directly, 3D interconnected frameworks with randomly opened pores were formed, which can be vividly viewed from Fig. 1(c) and (d). When the sample was heat dried, tightly packed layers were formed, as shown in Fig. 1(e) and (f).

When the hydrogel was frozen firstly, water in the hydrogel turned into ice, then the volume was expanded, graphene layers were squeezed together, after the subliming of the ice, large space was formed, therefore, loosely packed layered structure of PFD appeared. Conversely, when the sample was freeze dried directly, only small ice crystals were formed, the original structure of the hydrogel would be maintained. On the other hand, when the hydrogel was directly heat dried, water would evaporate quickly, and the volume shrank seriously, the sheets of HD tended to stack layer by layer because of van der Waal force. In a word, drying condition of hydrogel influenced the pore structure greatly, when the hydrogel was pre-frozen, loosely packed layered structure was formed; the interconnected 3D porous structure can be obtained when the hydrogel was directly freeze dried; compactly stacked layered structure was formed when the hydrogel was heat dried.

Pore structures of the samples were further detected by nitrogen adsorption. As shown in Fig. 2(a), there were obvious hysteresis loops of N₂ adsorption–desorption isotherms for all the samples, which indicated that there were abundant mesopores. The pore size distribution of Fig. 2(b) verified the deduction from N₂ adsorption–desorption isotherm. There was a sharp peak appearing at about 4 nm for HD, which was due to the tight compression during heat drying process. The curves of the other two samples showed high broad peaks at about 27 and 63 nm. All the results indicated that different drying conditions led to different pore structure. More porous properties of samples were summarized in Table 1. BET surface areas of PFD, DFD and HD were 284, 361 and 236 m² g⁻¹, respectively. The BET surface areas of PFD and HD were lower than that of DFD, these results might be assigned to the collapse and/or shrinkage of pores during freeze or heat process. It was clear that DFD had the highest BET surface area, largest quantities of mesopores and largest total pore volume, demonstrating that the interconnected 3D porous structure obtained by direct freeze drying was more effective for preventing graphene sheets from restacking. Furthermore, such a porous structure with high specific surface area and mesopores maybe favorable for improving the capacitance of the material since the hydrated ions in the electrolyte were easily accessible to the external and internal surface of the pores.

Fig. 2 (a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions of the samples.

Table 1 Pore characters of the samples

Sample	SBET (m^2 g^−1)	Vtotal (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Vmeso (cm^3 g^−1)	Dave (nm)
PFD	284	0.71	—	0.69	9.99
DFD	361	0.95	—	0.94	10.52
HD	236	0.28	0.01	0.25	4.72

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XRD patterns of the samples were recorded in Fig. 3. A diffraction peak centered at 26° of d (002) plane was observed for HD, and a peak at about 43.5° indexed to (101) plane of graphite could also be observed in the samples, which indicated that the graphene sheets were restacked to graphite layered structures during heat drying. On the contrary, a broad diffraction peak centered at about 25° was observed for PFD and DFD, suggesting the graphene sheets were randomly oriented.

Fig. 3 XRD patterns of PFD, DFD and HD.

Raman spectroscopy played an important role in the study of graphitic materials, including graphene based materials. As was shown in Fig. 4, the samples represented two typical bands indexed at around 1314 cm⁻¹ and 1588 cm⁻¹, which were attributed to the well-documented D and G bands of hydrothermally reduced GO. It was notable that no any perceptible peaks were observed for the samples in Raman spectrum, suggesting that Py had reacted with graphene during hydrothermal process, and nitrogen was mainly doped in the 3D graphene structure, which was consistent with the results obtained from FTIR and XPS.

Fig. 4 Raman spectra of PFD, DFD and HD.

As was discussed above, drying condition played an important role on the pore structure of the 3D porous graphene, the influence on the chemical structure was also detected by FTIR and XPS. Fig. 5 was the FTIR spectra of the sample dried at different conditions, the three samples had the same FTIR spectra, indicating the drying condition had little influence on the chemical structure of the samples. The peak at 2982 cm⁻¹ and 2906 cm⁻¹ could be attributed to the asymmetric stretching and symmetric vibrations of –CH₂.^32,33 The peak located at 1557 cm⁻¹ represented the C=C stretching vibration of benzene ring.³⁴ The peak at 1038 and 757 cm⁻¹ were assigned as C–OH stretching vibration and C–H bending vibration, and peaks at 1692 cm⁻¹ was attributed to C–N stretching vibration.^35,36

Fig. 5 FTIR spectra of PFD, DFD and HD.

XPS analysis confirmed the results of FTIR, Fig. 6 showed the XPS survey scan and C1s spectra of the samples. From Fig. 6(a) it can be seen that all the samples contained C, N and O, the intensity of each element was almost equal, indicating the similar surface chemical compositions. C1s peak of the three samples indicated the presence of four carbon bonds, 284.6 eV for C–C, 285.4 eV for C–N, 286.5 eV for C–O, and 287.7 eV for C=O, respectively.³⁷ This results further confirmed that Py had reacted with GO during hydrothermal process, and the influence of drying condition on the chemical structure of the samples can be ignored.

Fig. 6 (a) XPS survey scan of the samples. C1s spectra of (b) PFD, (c) DFD and (d) HD.

Since the pore structures were significantly influenced, and the chemical properties changed little after graphene hydrogels being treated by different drying conditions, the prepared 3D graphene should be an ideal model for discussing the relationship between the structure and performance as supercapacitor electrode. To compare the electrochemical performance of the samples, cyclic voltammetry, galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy were measured in 6 M KOH solution. Fig. 7(a)–(c) showed the cyclic voltammograms (CVs) of PFD, DFD and HD at the scan rate from 5 mV s⁻¹ to 100 mV s⁻¹. The CV curves of all the samples were rectangular at low scan rate, which became a fusiform shape for PFD and HD when the scan rate increased, but the rectangular shape kept well for DFD, which implied that DFD had lower internal resistances and better rate capabilities. Compared with PFD and HD, DFD had 3D interconnected porous structure, which was more convenient for the diffusion of electrolyte ions and therefore increased the ion accessible surface area, resulted in the lower resistance as electrode and higher performance in supercapacitor. Fig. 7(d) compared the GCD curves of the three samples at the current density of 0.5 A g⁻¹, all the samples demonstrated a symmetric triangle feature, indicating the good capacitance behavior of the electrode. The detailed information can be seen from the discharge curve, at the initial it can be seen a sharp IR drop, which reflected the resistance of the electrode. The IR drop of DFD was smaller than that of the other two samples, proving the higher electrical conductivity of DFD. Calculated from the slope of the GCD curves, the specific capacitance was 162, 179, and 163 F g⁻¹ for PFD, DFD and HD, respectively. Fig. 7(e) showed the rate performance of the samples, at the low current density, the three samples had the similar specific capacitance, when the current density increased, the performance decreased greatly for PFD and HD, the electrode made from PFD and HD only retained about 70% of their initial capacitance, but DFD retained about 85%, which meant that the DFD had a better rate performance than that of PFD and HD. At the lower current density, electrolyte ions had sufficient time to diffuse in the inner pores of the PFD and HD, but when the current density increased, the samples cannot provide fast ions transfer paths because of the tightly packed structure, which resulted in the higher resistance and lower capacitance. On the other hand, DFD can provide unblocked fast shuttle paths for ions, which made more accessible areas, then decreased the resistance and increased the specific capacitance. EIS analysis was another principal method to investigate the fundamental behavior of electrode materials. The resulting Nyquist plots in Fig. 7(f) further confirmed the favorable performance of DFD electrode. Obviously, DFD exhibited the smallest semicircle in the high-frequency region. This indicated that DFD had the lowest charge-transfer resistance and excellent conductivity due to the particular pore structure. In the low-frequency region, the slope of the samples was close to 90°, indicating a capacitive performance of the samples, however, which was steeper for DFD than PFD and HD, affirming that better capacitive behavior of DFD.

Fig. 7 (a) CV curves of PFD, (b) DFD and (c) HD at different scan rates. (d) GCD curves of PFD, DFD and HD at 0.5 A g⁻¹. (e) Specific capacitance of PFD, DFD and HD as a function of current density. (f) Nyquist plots of PFD, DFD and HD (inset: the expanded high-frequency region of the plots).

4 Conclusion

In summary, pore structure of 3D nitrogen doped graphene can be tuned by changing the drying conditions of graphene hydrogels. PFD had a loosely packed layered structure, and interconnected porous structure can be obtained by direct freeze drying, when the hydrogel was heat dried, a tightly packed layered structure was obtained. As expected, DFD demonstrated better electrochemical performance because the interconnected pore structure provided fast electrolyte ions transfer path, which led to lower electrolyte ions diffusion resistance and higher effective areas.

Acknowledgements

We acknowledge the financial support of National Nature Science Foundation of China (51102168, 51102169, 51272157); Key Basic Research Program of Shanghai Municipal Science and Technology Commission (13NM1401102); Innovation Program of Shanghai Municipal Education Commission (14YZ084); The Hujiang Foundation of China (B14006); The Innovation Fund Project For Graduate Student of Shanghai (JWCXSL1401).

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