Facile synthesis and high electrochemical performance of porous carbon composites for supercapacitors

Abstract

A facile and green method was used to synthesize Fe₂O₃/mushroom derived porous carbon (Fe₂O₃/MPC) nanocomposites with high capacitive properties. The resultant materials were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) measurements. It is shown by the BET surface area measurements that MPC has a high specific surface area and abundant porosity, which are conducive to improve the electrochemical performance. The Fe₂O₃/MPC nanocomposite exhibits a maximum specific capacitance of 367.39 F g⁻¹ at a current density of 0.5 A g⁻¹ and good capacitance stability over 1500 cycles at a current density of 5 A g⁻¹. These attractive results indicate that the Fe₂O₃/MPC nanocomposite could be a potential electrode material for supercapacitors.

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1 Introduction

Increasing energy demands and uncertainties in energy supply urge researchers to develop efficient energy storage systems. Supercapacitors are becoming attractive power sources because of their excellent potential performance. As energy storage devices, supercapacitors can be classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors based on their underlying charge-storage mechanism.¹ Compared with conventional capacitors, supercapacitors have been more widely concerned owing to their high power density, short charging time and long cycling life.^2–5 However, the practical applications of supercapacitors are still limited because of unsatisfactory performance of the electrode materials and the high production cost of noble-metal oxides based materials.^6,7 Therefore, the main challenge for researchers is to develop high performance and low-cost materials.

Recent studies are shown that activated carbons are suitable candidates for EDLCs on account of their high surface area, excellent electrochemical stability, good electrical conductive properties, etc.⁸ However, conventional carbon precursors like fossil fuels of petroleum coke⁹ and coal tar pitches¹⁰ are not the ideal materials for supercapacitors on account of their unsustainability and non-renewable. Recently, the use of biological materials as precursors for activated carbons has been attracting researchers' attention because of low cost, accessibility, renewable, porous structures and complex morphologies.^11,12 Most biological materials have porous structures, such as sugarcane bagasse,¹³ fish scale,¹⁴ neem leaves,¹⁵ peanut shell,¹⁶ etc. But the flaw in EDLCs shows low energy density and capacitance. To overcome this problem, researches are focused on introducing transitional metal oxides (e.g., RuO₂, MnO₂, NiO, Fe₂O₃, Cu₂O) to porous carbon materials. These transitional metal oxides can exhibit large capacitance and high energy density due to their reversible multielectron redox faradaic reactions.¹⁷ Comparing with the other transitional metal oxides, Fe₂O₃ has higher theoretical capacitance, lower production costs and abundance.^18–20 So it has become a suitable material for supercapacitors. However, two main shortcomings limit its application: low conductivity and rapid capacity decay.^21,22

Mushroom, a kind of readily available goods in the market, has porous structures observed in the scanning electron microscopy (SEM) analysis, may be an excellent biological materials for developing supercapacitors. In this study, we report a facile and green method to fabricate Fe₂O₃/MPC nanocomposite from iron nitrate and inexpensive mushroom derived porous carbon material for supercapacitors via a simple calcination method under gas protection.

2 Experimental section

2.1 Materials

The beech mushrooms were purchased from market in Shanghai. Potassium hydroxide, ethanol, hydrochloric acid, nitric acid, sulfuric acid, graphite powder were purchased from Shanghai Chemical Reagents Co., Ltd. Polytetrafluoroethylene (PTFE) solution was purchased from Shanghai 3F New Materials Co., Ltd. Nickel foil was purchased from Shanghai Hongxiang Plant. Pure nitrogen was purchased from Shanghai BOC Special Gases Sales Service Co., Ltd. Deionized water was used throughout the experiments.

2.2 Preparation of Fe₂O₃/MPC composite

The mushrooms were washed with deionized water and dried at 80 °C. The cleaned beech mushrooms were subsequently pre-carbonized at 500 °C for 2 h under argon atmosphere. Then the pre-carbonized materials were ground to powder, mixed with KOH (m_KOH : m_c = 2), ultrasonic treatment for 30 min, and dried at 120 °C for 12 h. After that, the mixtures were heated in a ceramic crucible from room temperature to 800 °C at a rate of 5 °C min⁻¹ and the final temperature was kept for 2 h under argon atmosphere. Finally, the dark products were washed with excess 1 M HCl solution and then washed several times with deionized water. The remainders were dried at 80 °C to obtain the final product of MPC.

0.5 g MPC was impregnated with 50 mL Fe(NO₃)₃·9H₂O solution. After ultrasonic treatment, the mixture was keeping at room temperature for 6 h. Then, the resultant solution was filtered, and the samples were dried at 60 °C. The obtained powders were annealed at 800 °C for 3 h without oxygen to get the final product of Fe₂O₃/MPC nanocomposite.

2.3 Preparation of the electrodes

The working electrode was prepared by mixing the composite (85 wt%), graphite power (10 wt%) and PTFE (5 wt%), using deionized water as solvent. Then, the slurry was applied to a nickel foam (1 cm × 1 cm) at 6 MPa and dried at 80 °C. Electrochemical measurements were performed with a three electrode experimental setup, nickel foam and Ag–AgCl electrode were used as the counter and reference electrodes in a 6 M KOH solution, respectively.

2.4 Characterization techniques

Scanning electron microscopy (SEM, Phenom Pro-G2, Netherlands), energy dispersive spectroscope (EDS) and transmission electron microscopy (TEM, JEOL JEM-2100, Japan) were used to characterize the surface morphology and the elemental compositions of the Fe₂O₃/MPC nanocomposites. The adsorption amounts were obtained from the Micromeritics Tristar 3000 gas adsorption analyzer. The specific surface area was calculated by using a single-point Brunauer–Emmett–Teller (BET) method. Thermogravimetric analysis was carried out on a NETZSCH STA409 PC thermal analysis instrument, and the heating rate was 5 °C min⁻¹. Powder X-ray diffraction (XRD) patterns were performed on Bruker Focus D8 diffractometer with Cu Kα radiation (40 kV, λ = 0.15418 nm) between 5 and 70°. Electrochemical measurements were tested by a CHI 660E electrochemical workstation (ChenHua Corp., Shanghai, China).

3 Results and discussion

3.1 Sample characterization

Scheme 1 illustrates a schematic diagram of formation process of MPC and Fe₂O₃/MPC nanocomposite. The fresh mushroom was pre-carbonized at 500 °C, and further carbonized at 800 °C with KOH as activating agent. After the above steps, there are abundant micropores inside mushroom-derived porous carbon, which provide a place to store iron nitrate solution. During the drying and calcination processes, iron nitrate finally decomposed into ferric oxide and formed the Fe₂O₃/MPC nanocomposite.

Scheme 1 Formation mechanism of MPC and Fe₂O₃/MPC.

The surface morphology and structure of the prepared materials were examined using SEM and TEM techniques. Fig. 1A shows that the surface of carbonized mushroom has lax and porous structure. The abundant micropores are formed due to KOH activation in pre-carbonized mushroom (Fig. 1B). Fig. 1C and D show the morphology of Fe₂O₃ nanoparticles anchored on MPC and the uniform dispersion of Fe₂O₃ nanoparticles. The synthesis of Fe₂O₃ on MPC can be inferred as follows:

Fig. 1 SEM images of (A) MPC and (C) Fe₂O₃/MPC. TEM images of (B) MPC and (D) Fe₂O₃/MPC.

Fig. 2A shows the chemical compositions of MPC are consist of C and O, which indicates the MPC materials do not have other impurity elements. Two weak peaks are observed in Fig. 4B which confirm the existence of a small amount iron in Fe₂O₃/MPC nanocomposite. The EDS result proves that the experimental method of this paper is effective in loading iron element on MPC.

Fig. 2 EDS of elements of (A) MPC and (B) Fe₂O₃/MPC.

Fig. 3A shows the XRD patterns of MPC and Fe₂O₃/MPC. It is distinctly that MPC material possess a typical graphitic stacking peak around at 22°, which should enhance the electrochemical performance.^23,24 For sample Fe₂O₃/MPC, the characteristic peaks such as (012), (104), (110), (113), (024) (116) (214) and (300) can be indexed to Fe₂O₃ (JCPDS File no. 33-0664). The peak related to carbon can not be observed in the diffraction patterns, which means that the carbon is present in the composite in the amorphous state.²⁵ It is entirely consistent with the previous result of EDS analysis. Fig. 3B shows a thermogravimetric curve of the sample Fe₂O₃/MPC in the air. The weight loss was 6% between 20 and 250 °C, because of the evaporation of physically absorbed water. There is no more weight loss at more than 600 °C, and the remains are Fe₂O₃ nanoparticles (about 68%). The result indicates that the amount of Fe₂O₃ nanoparticles is 72% in the nanocomposites.

Fig. 3 (A) X-Ray diffraction patterns of MPC and Fe₂O₃/MPC; (B) thermogravimetric curve of Fe₂O₃/MPC.

In order to obtain the surface area and pore size distribution of the as-prepared Fe₂O₃/MPC nanocomposites, the liquid nitrogen cryosorption measurements were conducted. Fig. 4A shows the nitrogen adsorption–desorption isotherms of two samples could be type I characteristics with a H4 hysteresis loop,²⁶ indicating the typical microporous character of the samples.^27,28 Pore size distribution of the samples is shown in Fig. 4B, which shows that synthesized samples have the similar pore size distributions in the range of 2–5 nm. The specific porous properties of the samples are detailed in Table 1. The specific surface area of MPC and Fe₂O₃/MPC are 1639.04 and 1361.98 m² g⁻¹ with the volumes of 0.83 and 0.69 cm³ g⁻¹, respectively. Moreover, the high specific surface area and abundant porosity of sample MPC facilitates effective access of electrolyte ions to the electrode surfaces, which are conducive to improve the electrochemical properties of supercapacitors.²⁹ As displayed in Table 1, the micropore volume (V_micro) are 0.76 and 0.63 (cm³ g⁻¹) for MPC and Fe₂O₃/MPC, respectively. Furthermore, the specific surface area and pore volume of micropore decreased slightly after Fe₂O₃ nanoparticles filled into the micropores of the MPC.

Fig. 4 (A) N₂ adsorption–desorption isotherms and (B) pore-size distributions of samples MPC and Fe₂O₃/MPC.

Table 1 Pore characteristics of MPC and Fe₂O₃/MPC

Samples	SBET [m^2 g^−1]	Smico [m^2 g^−1]	Smeso [m^2 g^−1]	SLangmuir [m^2 g^−1]	Vpore [cm^3 g^−1]	Vmicro [cm^3 g^−1]	Vmeso [cm^3 g^−1]	Daver [nm]
MPC	1639.04	1604.27	34.77	2253.52	0.83	0.76	0.07	2.03
Fe2O3/MPC	1361.98	1335.31	26.67	1867.68	0.69	0.63	0.06	2.03

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3.2 Electrochemical properties

Cyclic voltammetry (CV) study. The electrochemical behavior of MPC and Fe₂O₃/MPC were investigated by three-electrode system in a 6 M KOH electrolyte and the voltage range from −1 to −0.2 V. Fig. 5A and B display the CV curves of MPC and Fe₂O₃/MPC at different scan rates (1 to 20 mV s⁻¹). For MPC electrode material, the CV curves still keep rectangular shape for scan rates up to 20 mV s⁻¹, indicating an ideal material for supercapacitors. For Fe₂O₃/MPC electrode material, the redox peaks are observed in Fig. 5B, which shows that the capacitance of Fe₂O₃/MPC are consist of two parts: the redox pseudocapacitance derives from Fe₂O₃ and the EDL capacitance of MPC.^30,31 The specific capacitance are calculated using the following equation:
where C_s is the specific capacitances (F g⁻¹), I is the response current (A), ν is the scan rate (mV s⁻¹), ΔV is the potential window (V), and m is the mass of the electrode (g). According to the above formula, the specific capacitances of MPC and Fe₂O₃/MPC are found to be 268.74 and 171.51 F g⁻¹ at 1 mV s⁻¹, respectively. Fig. 5C lists all specific capacitances with different scan rate for MPC and Fe₂O₃/MPC. In present study, the capacitance of Fe₂O₃/MPC is much higher than a-Fe₂O₃/multi-walled carbon nanotubes³² and graphene/Fe₂O₃,³³ reported previously. The high electrochemical performance of Fe₂OS₃/MPC could be due to the high specific surface area with abundant porosity and Fe₂O₃ loading effects.

Fig. 5 CV curves of (A) MPC and (B) Fe₂O₃/MPC at different scan rate; specific capacitance of working electrode as according to scan rate ((C) MPC and Fe₂O₃/MPC).

The galvanostatic charge–discharge study. The galvanostatic charge–discharge curves of MPC and Fe₂O₃/MPC at the different current densities are shown in Fig. 6A and B, respectively. The specific capacitance is calculated from the following formulate:
where C_s is the specific capacitance (F g⁻¹), I is the discharge current (A), t is the discharge time (s), m is the active materials, ΔV is the potential difference (V). As described in Fig. 6, the discharge time of Fe₂O₃/MPC much longer than MPC at the same current density owing to the pseudocapacitance of Fe₂O₃. Table 2 shows the specific capacitance at different current densities ranged from 0.5 to 5 A g⁻¹. When the current density at 0.5 A g⁻¹, Fe₂O₃/MPC exhibits the specific capacitance of 367.39 F g⁻¹, which means the existence of Fe₂O₃ nanoparticles is effective to improve the specific capacitance of carbon material. As listed in Table 2, Fe₂O₃/MPC still remained specific capacitance of 213.94 F g⁻¹ at a current density of 5 A g⁻¹. But with the increase of current density, the specific capacitance of Fe₂O₃/MPC dropped apparently. This trend of the capacitance illustrates that the redox reaction can not keep high rate at a high current density.³⁴

Fig. 6 Charge–discharge curves of (A) MPC and (B) Fe₂O₃/MPC at a different current density.

Table 2 Specific capacitance of MPC and Fe₂O₃/MPC computed from charge–discharge curves

Electrode	Current density
	0.5 A g^−1	1 A g^−1	2 A g^−1	3 A g^−1	4 A g^−1	5 A g^−1
MPC (F g^−1)	199.36	177.94	167.60	163.68	161.40	160.13
Fe2O3/MPC (F g^−1)	367.39	290.13	248.44	235.18	221.95	213.94

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The cyclic stability study. The cyclic stability of Fe₂O₃/MPC was investigated at a current density of 5 A g⁻¹ within a potential window from −1 to −0.2 V. Fig. 7 shows the retention of specific capacitance over 1500 cycles of Fe₂O₃/MPC. As shown in Fig. 7, it retains above 90% of the initial specific capacitance up to 400 cycles. The long cycle performance maintains at about 82.7% of the initial capacitance over 1500 cycles. The decrease in capacitance of Fe₂O₃/MPC may be on account of irreversible reaction between electrode and electrolyte.³⁵ Wu et al.^36,37 have reported the capacitance retention of about 70% over 500 cycles for a-Fe₂O₃ nanosheets. The above results indicate that porous carbon material can withstand from structural change, and Fe₂O₃ nanoparticles can improve the specific capacitance of composite.

Fig. 7 Cyclic behavior of Fe₂O₃/MPC at constant current density of 5 A g⁻¹.

The electrochemical impedance study. Electrochemical impedance spectroscopy (EIS) analysis is a tool to examine the conductivity of the electrode materials. Fig. 8 shows Nyquist plots of MPC and Fe₂O₃/MPC nanocomposites with a frequency range from 10⁻¹ to 10⁵ Hz in 6 M KOH solution. Generally, the Nyquist plots consist of both a high-frequency semicircle and a low-frequency sloping straight line. The semicircle is related to the charge transfer resistance of the electrode and the charge transfer resistance is the Faraday resistance. In the low frequency range, the Fe₂O₃/MPC hybrid supercapacitor plot had a 45° phase angle, which corresponds to the Warburg region and indicates a high ion diffusion resistance.³⁸ In high frequency range, the sample Fe₂O₃/MPC has a bigger semicircle than the sample MPC. This is due to the low electronic conductivity and intrinsically higher charge-transfer resistance of Fe₂O₃ nanoparticles.³⁹ The above result of study indicates that the Fe₂O₃ nanoparticles act in the composite electrodes by faradaic reactions, inducing pseudocapacitance.

Fig. 8 Nyquist plots of MPC and Fe₂O₃/MPC nanocomposites with a frequency range from 10⁻¹ to 10⁵ Hz.

4 Conclusion

Fe₂O₃/MPC nanocomposite for supercapacitors was successfully synthesized through a facile and green method. Owing to the loading of Fe₂O₃, the specific capacitance of Fe₂O₃/MPC nanocomposite is greatly improved compared with MPC. The Fe₂O₃/MPC electrode material exhibits the maximum specific capacitance of 367.39 F g⁻¹ at a current density of 0.5 A g⁻¹ and good capacitance stability (82.7%) over 1500 cycles at a current density of 5 A g⁻¹. The results indicate that the Fe₂O₃/MPC nanocomposite is a potential ideal material for supercapacitors.

Acknowledgements

The authors are grateful to the financial support of the National Natural Science Foundation of China (no. 91122025, 21103127, 21101118), the State Major Research Plan (973) of China (no. 2011CB932404), the Nano-Foundation of Shanghai in China (no. 11nm0501300), the State Key Laboratory of Fine Chemicals (no. KF1103) and the Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials (no. 2012MCIMKF03).

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