Porous reduced graphene oxide paper as a binder-free electrode for high-performance supercapacitors

Abstract

A facile template approach is developed to prepare highly conductive and flexible reduced graphene oxide (rGO) paper with hierarchical porous structures. When used as an electrode for a capacitor, these porous carbon layers demonstrate an enhanced electrochemical performance due to their large ion-accessible surface area and efficient electrolyte-ion transport pathways. The porous rGO electrode delivers a specific capacity of 173.5 F g⁻¹, almost 1.8 times higher than 95.1 F g⁻¹ of packed rGO. More importantly, the porous electrode also exhibits a good electrochemical performance with an energy density of 28.5 W h kg⁻¹ and power density of 4000 W kg⁻¹, as well as excellent cycling stability with 101.5% of its initial capacitance after 2000 charge–discharge cycles at a current density of 5 A g⁻¹.

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1. Introduction

The limited storage and higher consumption rates of fossil fuels have stimulated intense research on sustainable energy technologies. Supercapacitors, a fast-rising class of electrical energy storage devices with ultrahigh power density, fast charging and long cycling life, have received considerable attention for emerging renewable energy applications.^1–5 Recently, free-standing thin film materials with excellent mechanical properties have been of great interest. Various carbon-based materials (carbon nanotubes, carbon fibers and graphene) are employed to construct thin and flexible electrodes.^6–8 Among them, graphene, a monolayer two-dimensional carbon nanostructure, with good mechanical properties, excellent electrical conductivity, and high specific surface area, is of current interest.^9–11 However, the strong aggregation tendency of graphene sheets during utilization as electrode usually leads to severe decrease in the electrochemically active surface area and the ion diffusion rate, resulting in a real value much lower than its intrinsic capacitance and charge–discharge rates.¹² For instance, the reported surface area for closely packed reduced graphene oxide (rGO) nanosheets, is only 14.9 m² g⁻¹ and much lower than the theoretical value of 2630 m² g⁻¹, as well as a specific capacity of only 66.0 F g⁻¹ at 2 mV s⁻¹.¹³ Therefore, the fabrication of graphene into porous structures with optimum porosity is one of the most promising strategies to solve above problems. Various kinds of porous graphene materials, such as macroporous graphene frameworks, thin films and hydrogels have been prepared as high-performance electrochemical energy storage electrodes.^14–17

Here, we employ a template approach to fabricate free-standing porous rGO framework. On the basis of our previous work,¹³ CuO nanosheets/rGO hybrid lamellar film was prepared firstly, then immersed in an aqueous solution of HCl at room temperature to etching away the CuO nanosheets template and generated porous rGO paper. When used as an electrode for supercapacitor, this porous rGO paper shows an enhanced surface area and unique porous structure, which provide efficient ions transport pathway within the electrode and facilitate the fast diffusion and transport of electrolyte ions during charge–discharge process. Therefore, the unique porous rGO exhibited excellent electrochemical properties, including a specific capacity of 173.5 F g⁻¹ at 2 mV s⁻¹, 101.5% capacity retention upon a current increase to 5 A g⁻¹ after 2000  cycles, high energy density of 28.5 W h kg⁻¹ and power density of 4000 W kg⁻¹. This process is a facile, simple and large-scale production approach for preparing binder-free porous rGO electrode with enhanced electrochemical performance.

2. Experimental

2.1 Synthesis of CuO nanosheets

The preparation process of CuO nanosheets was described in our previous work.^18–20 Briefly, a certain volume of 2 mM Cu(NO₃)₂ aqueous solution was mixed with an equal volume of 1.6 mM NH₂CH₂CH₂OH aqueous solution with stirring for 1 minute. Then maintaining the mixture at 25 °C for 24 hours, the mesoporous CuO nanosheets colloidal solution with a concentration of nearly 0.053 mg mL⁻¹ was obtained.

2.2 Synthesis of porous rGO paper

For the preparation of porous rGO paper, GO nanosheets were firstly synthesized from graphite powder according to the well-established modified Hummer's method.¹³ Then 20 mg of the as-prepared GO was dispersed ultrasonically in distilled water (with a concentration of 0.2 mg mL⁻¹) and the pH of the suspension was adjusted close to 6.2. Then, the above GO suspension was quickly mixed with 375 mL mesoporous CuO nanosheets colloidal solution under vigorous stirring. After collecting the flocculent suspension by filtration, the CuO/GO lamellar films were treated with a diluted hydrazine (15 mM) solution for 15 min, followed by the treatment with a diluted HCl solution (5%) to remove the CuO nanosheets. Finally, the porous structure was obtained and further annealed at 600 °C for 3 h in a N₂ flow. The synthetic process is illustrated in Scheme 1. Moreover, packed rGO film without combing with CuO was also prepared for comparison.

Scheme 1 Schematic of the synthesis procedure of porous rGO paper.

2.3 Electrode preparation

Porous rGO paper was binder-freely used as the work electrode and sandwiched between two pieces of foam Ni sheets. Square model electrodes were cut off from the obtained free-standing film with sides 1 cm, the mass of porous rGO is 1.6 mg, the mass of packed rGO is 1.4 mg. The capacitance measurement was conducted by a CHI 660D electrochemical workstation in a 1 M Na₂SO₄ aqueous solution with a platinum counter electrode and an Ag/AgCl reference electrode. Cyclic voltammetry (CV), constant-current galvanostatic (GV) charge–discharge curves, and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical performances of porous rGO papers.

2.4 Characterization

The phases of the as-prepared products were characterized by X-ray diffraction (XRD) at room temperature using an X'Pert PRO (PANalytical, Netherlands) instrument with Cu Kα radiation. Raman spectra were conducted on the Renishaw inVia Raman microscope under the excitation length of 532 nm. The morphologies and structures were characterized using scanning electronic microscopy (SEM) (Hitachi S-4800). The specific surface area was calculated by Brunauer–Emmett–Teller (BET) method using a Quantachrome Autosorb-1 apparatus.

3. Results and discussion

Fig. 1a shows the XRD patterns of the as-prepared CuO/rGO and porous rGO paper. After the treatment with a diluted HCl solution, there are no peaks of CuO nanosheets were observed, indicating the CuO nanosheets have been removed completely, which resulted in the formation of porous structure. Fig. 1b is the Raman spectra of CuO/rGO and porous rGO. Two characteristic peaks (D and G bands) of graphene based materials are observed. But the peaks of CuO at 281 (A_g) and 612 cm⁻¹ (B_g)^21,22 disappeared from porous rGO, which is consisted with the results of XRD, further confirming the formation of pure porous rGO. Pore characterization of the porous structure was achieved by BET adsorption/desorption measurements as shown in Fig. 1c and d. The BET surface area of porous rGO paper is 87.3 m² g⁻¹, much higher than the 14.9 m² g⁻¹ of packed rGO as reported,¹³ due to the porous morphology between the rGO sheets as observed in Fig. 2f. Most pores of the porous rGO are centered at 3.7 nm. This porous structure is beneficial for electrolyte-ion access and suitable for the electrical double layer formation.²¹

Fig. 1 (a) XRD patterns, (b) Raman spectra of CuO/rGO and porous rGO films, respectively. (c) Nitrogen adsorption and desorption isotherms of porous rGO paper and (d) the corresponding pore size distribution plots.

Fig. 2 Cross-section SEM images of packed rGO (a and b), CuO/rGO composite (c and d), porous rGO (e and f).

Fig. 2a and b show the cross-section SEM images of packed rGO. It is clearly seen that the rGO layers are tightly packed, which not good for electrolyte diffusion and applicable for capacitor. The SEM images of CuO/rGO precursor at different magnifications are shown in Fig. 2c and d. The inter-layer space between rGO sheets is clearly seen and spaced by the CuO nanosheets. After the treatment with a diluted HCl solution, as shown in Fig. 2e and f, the porous rGO film was obtained. Numerous porous structures of the fabricated rGO paper are favorable for the access of electrolyte and rapid diffusion of electrolyte ions, which is difficult to achieve in closely packed rGO paper.

The porous rGO presented here have a unique porous structure and good electronic conductivity, therefore, they present the appropriate characteristics for advanced electrochemical capacitors. To confirm this, the capacitive performance of the porous rGO and packed rGO papers were tested in a three-electrode configuration with a Pt counter electrode and an Ag/AgCl reference electrode in a 1 M Na₂SO₄ aqueous electrolyte, respectively.

Fig. 3a and b present the CV curves over a voltage range from −1 to 0 V for the porous rGO and packed rGO papers at various scan rates. The shapes of the CV reveal that the capacitance characteristic is normally close to an ideal rectangular shape and consistent with the electric double-layer capacitance. The capacitance calculated from the CV curves at different scan rates are shown in Fig. 3c. It is obviously porous rGO shows higher capacity than packed rGO at different scan rates. At the scan rate of 2 mV s⁻¹, the specific capacity of porous rGO is 173.5 F g⁻¹, while the capacity of the packed rGO is only 95.1 F g⁻¹. Even at high scan rate of 100 mV s⁻¹, the capacity of porous rGO still remains 101.6 F g⁻¹ but only 13.5 F g⁻¹ for packed rGO. Moreover, the CV curve for the porous rGO electrode (at 100 mV s⁻¹) still retains nearly rectangular shape without obvious distortion, implying that an efficient electric double layer is established in the electrode.

Fig. 3 (a) and (b) are the CV curves of porous rGO and packed rGO at different scan rates, respectively. (c) Specific capacity of porous rGO and packed rGO at different scan rates derived from CV curves. (d) Constant-current charge–discharge performance of porous rGO and (e) the corresponding capacitance plot from (d). (f) The corresponding specific energy and power density.

The capacitances CV curves were calculated from the eqn (1):²¹

where C (F g⁻¹), m (g), v (V s⁻¹), V_c and V_a, and I (A) are the specific capacitance, the mass of the active materials in the electrode, potential scan rate, high and low potential limit of the CV tests, and the instant current on CV curves, respectively.

The capacitive performance for porous rGO was further tested with galvanostatic charge–discharge experiments. The GV curves of porous rGO at different current densities are shown in Fig. 3d. It can be seen the charge–discharge cycling curves are highly linear and have a very symmetric nature in shape, indicating that the electrode has a good electrochemical capacitive characteristic and excellent electrochemical reversibility. The specific capacity derived from the discharging curves at different current densities is shown in Fig. 3e. The capacitance of porous rGO at the current density of 1 A g⁻¹ was calculated to be 321.2 F g⁻¹, which is comparable with those of corrugated graphene sheets (227 F g⁻¹),¹⁵ holey graphene frameworks (310 F g⁻¹),¹⁶ porous activated rGO (∼300 F g⁻¹),¹⁷ 3D micro-porous conducting carbon (<254 F g⁻¹),²¹ porous graphene-like nanosheets (276 F g⁻¹),²³ hierarchical porous rGO (<200 F g⁻¹),²⁴ graphene hydrogels (222 F g⁻¹) and other carbon based materials as shown in Table 1.^{15,16,21–30} But still lower than graphene aerogels (366 F g⁻¹)³¹ and sandwiched porous carbon layer/graphene hybrids (481 F g⁻¹),³² this may relate to the surface area, diffusible electrolyte and conductivity of the active materials.

Table 1 Supercapacitor performance of various porous rGO-based materials

Materials	SSA (m^2 g^−1)	Electrolyte	Specific capacity (F g^−1)	Specific capacity retention/cycles
Porous activated rGO^17	3100	KOH	∼300	95%/2000
Holey graphene frameworks^16	830	EMIMBF4/AN	310	95%/20 000
Corrugated graphene sheets^15	518	KOH	227	108%/8000
3D micro-porous conducting carbon^21	1327	H2SO4	<254	90%/5000
Porous graphene-like nanosheets^23	1874	KOH	276	99.5%/5000
Hierarchical porous rGO^24	540	H2SO4	<200	98%/1000
Macroporous bubble graphene^25	128	H2SO4	58	—
Graphene hydrogels^26	951	KOH	222	92%/2000
Mesoporous graphene nanofibers^27	1280	EMIMBF4	193	98.8%/5000
Porous graphene^28	794	KOH	233	—
Activated 3D porous graphene^29	3523	TEABF4/AN	202	99%/5000
Activated microporous carbonNanoplates^30	2557.3	H2SO4	264	93.2%/10 000
Graphene aerogels^31	463	KOH	366	—
Sandwiched porous carbon layer/graphene hybrids^32	2937	KOH	481	97.2%/5000
Porous rGO (this work)	87.3	Na2SO4	321.2	101.5%/2000

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The capacitances of samples derived from galvanostatic curves were calculated from the eqn (2):²¹

C = IΔt/mΔV (2)

where C (F g⁻¹), I (A), Δt (s), m (g) and ΔV are the specific capacitance, the discharge current, the discharge time, the mass of the active materials in electrode, and the potential window, respectively.

To further evaluate the practical potential of our materials, energy density and power density were calculated from the GV curves at different current density according to the following equations:²¹

E = 1/2C(ΔV)² (3)

P = E/t (4)

where E (W h kg⁻¹) is the specific energy density, P (W kg⁻¹) is the power density, t is charging or discharging time. Fig. 3f shows a Ragone Plot of the corresponding specific E versus P values. The material has the capability to deliver energy density of 28.5 W h kg⁻¹ with a power density of 400 W kg⁻¹ whereas the power density increases to 4000 W kg⁻¹ when the energy density decreased to 6.9 W h kg⁻¹. The obtained energy density (28.5 W h kg⁻¹) of porous rGO is comparable to highly corrugated graphene sheets (30.4 W h kg⁻¹),¹⁵ and much higher than those of chemically reduced graphene (11.5 W h kg⁻¹)³³ and graphene film electrode (15.4 W h kg⁻¹).³⁴ This improved performance can be ascribed to the unique porous structure of rGO paper, which effectively prevented the graphene sheets from agglomeration and restacking during the testing process, ensuring full utilization of the surface of rGO nanosheets. Furthermore, the numerous mesopores remarkably shorten the ion diffusion lengths and allowed efficient and fast ion diffusion through the three-dimensionally structure, and provided continuous electron pathways and served as a double layer capacitor.

Long cycling life is an important requirement for supercapacitor electrodes. The long-term stability of the porous rGO paper was investigated by repeating the GV charge–discharge test between −0.8 and 0 V at the current density of 5 A g⁻¹. As shown in Fig. 4, it is worth noting that the specific capacity increases after the initial 800 cycles (104.9% of the initial cycle) and then decreases slowly for the subsequent cycles. The increase of the specific capacity during the first 800 cycles is probably due to the activation process of the electroactive material.^35,36 After 2000 continuous cycles, the porous rGO displays excellent long cycle life along with an approximately 101.5% of the specific capacitance can still retain due to the improved wettability of the electrode and the reduction of oxygen groups,³⁷ implying its excellent reversibility in the repetitive charge–discharge cycling. Besides, the last 10 cycles remained almost the same shape of charge–discharge curves with the first 10 cycles (insets in Fig. 4a), illustrating the excellent long-term cyclability of the electrode. The superior long-term electrochemical stability is important for the practical application. Furthermore, the cross-section SEM image of porous rGO paper after cycling tests was shown in Fig. 4b. The porous morphology of the lamellar structure almost remains after the charge–discharge cycles, which is consisted with the cycling test.

Fig. 4 (a) Cycling performance of porous rGO at the current density of 5 A g⁻¹. Inset shows the galvanostatic charge–discharge cyclic curves of the first and last 10 cycles. (b) is the cross-section SEM image of porous rGO paper after cycling tests.

As reported recently,² due to the relatively strong intersheet van der Waals interactions of graphene sheets, densely stacking structure usually makes the resulting films less porous and electrolyte infiltration more difficult when used as electrochemical electrodes. Thus, we have further probed the ion diffusion within porous rGO paper using electrochemical impedance spectroscopy (EIS). Fig. 5 depicts the Nyquist plots tested of porous rGO and packed rGO at the frequency of 100 kHz to 0.01 Hz. The Nyquist plot expanded in the high frequency region is given in the inset. The projected length of the Warburg-type line on the real axis reflects the electrolyte diffusion process from solution into the intersheet region of the graphene papers. Porous rGO exhibits the shorter Warburg-type line, suggesting the faster ion diffusion in the porous rGO paper, since porous rGO is less aggregated and the more porous structure is favourable for electrolyte diffusion. In contrast, packed rGO paper displays a longer transitional curve, mainly due to its densely stacked structure, which limited the electrolyte diffusion. The vertical lines at lower frequencies indicate a pure capacitive behaviour. The more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor.³⁸ The superior electrochemical performance is mainly due to its small internal resistance, enhanced specific surface area and excellent electrolyte accesses to the porous rGO through their unique porous structure.

Fig. 5 Nyquist plot of the EIS of porous rGO and packed rGO papers.

4. Conclusion

In conclusion, flexible porous rGO paper was synthesized by a facile template approach and evaluated for the supercapacitor electrode. The porous structures provided rapid pathways for ionic and electronic transport and showed remarkable performance, such as nearly 1.8 times higher specific capacity than that for packed rGO. Furthermore, the capacitor exhibited good cycling stability along with 101.5% capacitance retention after 2000 cycle tests. It also showed high energy density of 28.5 W h kg⁻¹ and power density of 4000 W kg⁻¹. Therefore, these porous rGO papers hold tremendous potential for energy storage.

Acknowledgements

This work was supported by the National Natural Science Foundations of China (NSFC 21271154), the National Basic Research Program of China (2015CB655302), Natural Science Foundation for Outstanding Young Scientist of Zhejiang Province, China (LR14E020001), Doctoral Fund of Ministry of Education of China (20110101110028).

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