Facile synthesis of Fe₃O₄ nanorod decorated reduced graphene oxide (RGO) for supercapacitor application

Abstract

The development of electrode materials capable of delivering high electrochemical performance is a major challenge. Herein, we demonstrate a facile approach for the synthesis of rod-shaped Fe₃O₄ nanostructures anchored on the reduced graphene oxide (RGO) surface and its application as an active electrode material for supercapacitors. The RGO–Fe₃O₄ nanocomposite was prepared by the spontaneous deposition of the rod-like FeOOH nanostructure onto the self-reduced GO surface followed by a thermal annealing process. The physical characterizations demonstrate the decoration of the rod-like Fe₃O₄ nanostructure over the RGO surface. Morphology analysis demonstrates that Fe₃O₄ nanorods with an average size of 150 nm are distributed over the RGO surface. The surface area analysis demonstrates that the as-synthesized RGO–Fe₃O₄ nanorod nanocomposite has 186 m² g⁻¹ specific surface area, which is higher compared to the Fe₃O₄ nanorods. As an active electrode material, the RGO–Fe₃O₄ nanocomposite shows excellent electrochemical performance compared to Fe₃O₄ nanorods. On the RGO–Fe₃O₄ nanocomposite based electrode a specific capacity of 315 C g⁻¹ was observed at 5 A g⁻¹ current density. Additionally, the RGO–Fe₃O₄ nanocomposite based electrode displayed excellent cycling stability with 95% specific capacity retention after 2000 cycles. The electrochemical results demonstrates that the RGO–Fe₃O₄ nanocomposite could be a promising material for energy conversion and storage.

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1 Introduction

The energy crisis and environmental pollution due to the massive consumption of fossil fuels demand the production of energy by cleaner means. To address this issue, in recent years, supercapacitors have emerged as a potential candidate because of their high power density, long cycle life and better performance than existing energy storage devices.^1–3 Considering the charge storage mechanism, supercapacitors are of two types; (i) electrical double layer capacitor (EDLC), in which only charge accumulation occurs at the electrode/electrolyte interface and (ii) pseudocapacitor, involves fast and reversible redox reactions at the electrode materials surface.^4,5 Typically, transition metal oxides and conducting polymers are the examples of active materials that have been widely used in the development of pseudocapacitors. On the other hand, the carbon based materials e.g., graphene, carbon aerogels, and carbon nanotubes etc. have been used in EDLCs.⁶ Compared to EDLCs, pseudocapacitors deliver much higher specific capacitance and energy density.^7,8 Therefore, in recent years development various pseudocapacitive electrode materials has become a thrust research area in order to achieve much enhanced specific capacitance.⁹ As per pseudocapacitive electrode materials are concerned, RuO₂ has been considered as a potential electrode material in the fabrication of pseudocapacitor and using this much enhanced capacitance has been achieved^10,11 but it's high cost and toxicity has insisted the scientists to search for other alternative cheaper electrode materials which could deliver similar performance as RuO₂. For example, MnO₂,¹² NiO,¹³ SnO₂,¹⁴ Co₃O₄,¹⁵ V₂O₅,¹⁶ MoO₃,¹⁷ TiO₂,¹⁸ CuO¹⁹ and ZnO²⁰ etc. have been widely used as active electrode materials in the fabrication of pseudocapacitors. Apart from the aforementioned metal oxides, in recent years, Fe₃O₄ a new type of pseudocapacitive material has emerged as an active electrode material in the development of pseudocapacitors considering its low-cost, and eco-friendly nature^21,22 using which better supercapacitive performance has been achieved. In principle, further enhancement in the supercapacitive performance of Fe₃O₄ is possible by integrating it with different conductive supports which in turn can be a promising material for supercapacitor application.²³ In the recent past, several attempts have been made to integrate Fe₃O₄ with highly conductive carbonaceous materials to enhance the supercapacitive performance.^23–25 Among the carbonaceous materials, graphene-pseudocapacitive material nanocomposite has been widely used for this purpose considering the exceptional physiochemical properties of graphene^26,27 and better electrochemical performance has been achieved.^28,29 For example, Shi et al. demonstrated the synthesis of Fe₃O₄ nanoparticle decorated on RGO sheets by a solvothermal process and the as-synthesized nanocomposites delivered a specific capacitance of 480 F g⁻¹ at 5 A g⁻¹ current density.³⁰ In another study, Karthikeyan et al. prepared RGO–Fe₃O₄ nanocomposite by urea assisted microwave method and as an active electrode material the as-synthesized RGO–Fe₃O₄ nanocomposite delivered a specific capacitance of 415 F g⁻¹ at 2 A g⁻¹ current density.³¹ Recently, Wasinski et al. synthesized RGO–Fe₃O₄ nanocomposite by hydrothermal approach and used this nanocomposite for supercapacitor application.³² Qu et al. demonstrated the synthesis of rod-like Fe₃O₄ nanoparticle decorated over the RGO surface by hydrolysis followed by electrochemical transformation and as an active material the RGO–Fe₃O₄ nanorod delivered a specific capacitance of 326 F g⁻¹ at 0.5 A g⁻¹ current density.³³ Mishra and Ramaprabhu demonstrated the synthesis of Fe₃O₄ nanoparticle decorated RGO and the as-synthesized RGO–Fe₃O₄ nanocomposite delivered a specific capacitance of 180 F g⁻¹ at 10 mV s⁻¹ scan rate.³⁴ Zhang et al. demonstrated the synthesis RGO–Fe₃O₄ nanocomposite by solvothermal method and the as-synthesized RGO–Fe₃O₄ nanocomposite delivered a specific capacitance of 358 F g⁻¹ at 1 A g⁻¹ current density.³⁵ Qi et al. prepared RGO–Fe₃O₄ nanocomposites through hydrolysis and subsequent annealing process and as an electroactive material the as-synthesized RGO–Fe₃O₄ nanocomposite delivered a specific capacitance of 350.6 F g⁻¹ at 1 mV s⁻¹ scan rate.³⁶ In another study, Khoh and Dong demonstrated a layer by layer self-assembly approach for the synthesis of Fe₃O₄ and RGO film on indium tin oxide (ITO) substrate and achieved a specific capacitance of 151 F g⁻¹ at 0.9 A g⁻¹ current density.³⁷ Wang et al. demonstrated hydrothermal method for the synthesis of RGO–Fe₃O₄ nanocomposite and using this nanocomposites they have achieved a specific capacitance of 220 F g⁻¹ at 0.5 A g⁻¹ current density.³⁸ In another study, Ghasemi and Ahmadi demonstrated the synthesis of RGO–Fe₃O₄ nanocomposite by electrophoretic deposition (EPD) method followed by electrochemical reduction and as an active electrode material the as-synthesized RGO–Fe₃O₄ delivered a specific capacitance of 154 F g⁻¹ at 1 A g⁻¹ current density.³⁹ Recently, Muthukannan et al. synthesized RGO–Fe₃O₄ nanocomposite using waste iron ore tailings (IOTs) by hydrothermal approach and using this nanocomposites they have achieved a specific capacitance of 154 F cm⁻² at 0.2 A g⁻¹ current density.⁴⁰ In another study, Yan et al. synthesized hexagonal Fe₃O₄ nanoplate decorated RGO by colloid electrostatic self-assembly strategy followed by a heat treatment approach and the as-synthesized RGO–Fe₃O₄ delivered a specific capacitance of 193 F g⁻¹ at 0.3 A g⁻¹ current density.⁴¹ Recently, Li et al. synthesized ultra small Fe₃O₄ nanoparticle decorated RGO by hydrothermal method and as an active material the RGO–Fe₃O₄ nanocomposite delivered a specific capacitance of 241 F g⁻¹ at 1 A g⁻¹ current density.⁴² Recently, Pardieu et al. developed a layer-by-layer method for the decoration of raspberry like Fe₃O₄ nanoparticles on the few layer graphene and as an active material the as-synthesized nanocomposites delivered a specific capacitance of 153 F g⁻¹ at 0.1 A g⁻¹ current density.⁴³ In another study, Liu et al. demonstrated a facile colloid electrostatic self-assembly process followed by hydrothermal treatment for the synthesis of cubic Fe₃O₄ nanoparticle decorated RGO and the as-synthesized RGO–Fe₃O₄ nanocomposite delivered a specific capacitance of 216.7 F g⁻¹ at 0.5 A g⁻¹ current density.²³ It is well-known that, the morphology of Fe₃O₄ largely influences the supercapacitive performance.⁴⁴ In principle, if we could fine tune the morphology of Fe₃O₄ over the RGO surface then better supercapacitive performance could be achieved.

To address this issue, herein, we have attempted for the shape-controlled synthesis Fe₃O₄ nanostructures over RGO sheets and tracked out its supercapacitive performance. Though a handful of reports are available on the synthesis of RGO–Fe₃O₄ nanocomposites and their application in the fabrication of supercapacitor electrode, yet, to the best of our knowledge, there is no report available on the synthesis of Fe₃O₄ nanorod decorated RGO for supercapacitor application by the method demonstrated in the current investigation. The current synthetic approach does not require any harsh conditions and costly equipments. Additionally, as an electrode material, the as-synthesized RGO–Fe₃O₄ nanocomposite delivered much better electrochemical performance compared to Fe₃O₄ nanorods.

2 Experimental section

2.1 Materials

The chemicals used in this investigation are of analytical grade and used as received. Graphite powder, N-methyl-2-pyrrolidone (NMP), PVDF, FeCl₂·6H₂O, and carbon black were purchased from Sigma-Aldrich (Germany). NH₄OH, KOH, H₂SO₄, KMnO₄ and H₂O₂ were purchased from Duksan Pure Chemicals Co., Ltd. Electrochemical measurements were carried out using 1 M KOH as supporting electrolyte. Millipore water (Milli-Q system) was used for the preparation of solutions.

2.2 Synthesis of RGO–Fe₃O₄ nanorod nanocomposite

In a typical synthesis, 100 mg of GO was dispersed in 30 mL de-ionized water via ultrasonication. Then, 0.5 g of solid FeCl₂·4H₂O was added slowly into the above solution with stirring and the pH of this reaction mixture was maintained at ∼9 by the drop wise addition of NH₄OH. The reaction mixture was stirred for 24 h and the products were collected by filtration followed by washing with water and ethanol alternatively for several times to remove the unreacted reactant and impurities. This product was dried, followed by calcinations at 450 °C for 4 h under an argon atmosphere with a ramping rate of 2 °C min⁻¹ using a programmable furnace. For comparative study, bare Fe₃O₄ nanoparticle was synthesized following the same approach in the absence of GO. The synthesis of RGO–Fe₃O₄ nanorod nanocomposite is schematically represented in Scheme S1.†

3 Results and discussion

The as-synthesized Fe₃O₄ and RGO–Fe₃O₄ nanocomposite were characterized by XRD measurement and is shown in Fig. 1. In the case of Fe₃O₄ nanoparticles six peaks were noticed at 30.40°, 35.76°, 43.28°, 53.92°, 57.28° and 62.88°, corresponding to (220), (311), (400), (422), (511) and (440) facets of the face centered cubic (fcc) structure, respectively (Fig. 1(a)) supporting the formation of Fe₃O₄ nanoparticles.⁴⁵ It is interesting to note that, the six peaks that were noticed in the case of Fe₃O₄ nanoparticles along with an additional hump at ∼25.93° was observed in the case of RGO–Fe₃O₄ nanocomposite (Fig. 1(b)), demonstrating the presence of Fe₃O₄ nanoparticles on the surface of RGO. This type of XRD profile for RGO–Fe₃O₄ nanocomposite is well known.⁴⁶ The XRD results obtained in the present case confirms the formation of RGO–Fe₃O₄ nanocomposite.

Fig. 1 X-ray powder diffraction patterns obtained for (a) Fe₃O₄ and (b) RGO–Fe₃O₄ nanocomposite.

Raman spectroscopy is a powerful tool capable of providing the information about the structural features and composition of the samples. To evaluate the structural features, samples were characterized by Raman spectroscopy and are shown in Fig. 2. As can be seen in the Fig. 2, the Raman spectrum of RGO shows two bands at ∼1355 and ∼1583 cm⁻¹ corresponding to the D and G bands, respectively.⁴⁷ The D band is related to the breathing mode of A_1g symmetry, where as the G band is related to the E_2g symmetry of sp² carbon atoms. In the case of RGO–Fe₃O₄ nanocomposite, two bands that were present in RGO were observed along with other two bands at 285 and 603 cm⁻¹. The bands at 285 and 603 cm⁻¹ are believed to be originated from the Fe–O vibration of E_g and A_1g modes in Fe₃O₄,⁴² suggesting the presence of Fe₃O₄ on the RGO surface.

Fig. 2 Raman spectra obtained for (a) RGO, (b) Fe₃O₄ and (c) RGO–Fe₃O₄ nanocomposite.

Electrochemical performance is dependent on the specific surface area of the electrode material.⁴⁸ Therefore, specific surface area of the as-synthesized Fe₃O₄ nanorod and RGO–Fe₃O₄ nanorod nanocomposite were investigated by BET measurement and is shown in Fig. 3. In the case of Fe₃O₄, type III isotherm was noticed (Fig. 3(a)) and the surface area was found to be 53 m² g⁻¹ and the pore volume was 0.19 cm³ g⁻¹. On the other hand, RGO–Fe₃O₄ has different type isotherm as compared to Fe₃O₄. In this case, type IV isotherm was observed (Fig. 3(b)). The specific surface area of RGO–Fe₃O₄ nanocomposite is measured to be 186 m² g⁻¹ and the pore volume was 0.47 cm³ g⁻¹. Since, RGO–Fe₃O₄ nanocomposite has high surface area, so in principle, it could offer a large number of active sites to take part during the electrochemical reaction. Due to the high pore volume in the case of RGO–Fe₃O₄ nanocomposite, the electrolyte ions could be accessed to the electrode surface easily during the charge–discharge storage process resulting excellent electrochemical performance.⁴⁹

Fig. 3 Nitrogen adsorption/desorption isotherms of (a) Fe₃O₄ and (b) RGO–Fe₃O₄ nanocomposite.

The as-synthesized Fe₃O₄ and RGO–Fe₃O₄ nanocomposites are further characterized by SEM measurements and are shown in Fig. 4. As can be seen in Fig. 4(a), the Fe₃O₄ nanoparticles have rod-like morphology with an average size of 100 nm. The close examination of the SEM image of RGO–Fe₃O₄ nanocomposite (Fig. 4(b)) demonstrates that a large number of rod-like Fe₃O₄ nanoparticles are distributed over the surface of RGO sheets, confirming the formation of RGO–Fe₃O₄ nanocomposite. To further confirm the formation of RGO–Fe₃O₄ nanocomposite, energy dispersive X-ray absorption spectroscopic (EDS) study was performed. The EDS study confirmed that the as-synthesized nanocomposite is composed of carbon, iron and oxygen (Fig. S1†). The ratio of carbon and Fe₃O₄ in the RGO–Fe₃O₄ nanocomposite is found to be 2 : 3. It is well known that the anchoring of metal/metal oxide nanoparticles with RGO sheet is capable of preventing the re-stacking of RGO sheets with each other as a result the enhancement in the active surface area occurs which is essential for better electrochemical performance. In the present case, loading of rod-like Fe₃O₄ nanoparticles onto RGO sheet could also hinder the restacking of RGO during the synthesis of nanocomposite as a result faster electron transport can be achieved which is essential for the enhancement in electrochemical performance.

Fig. 4 SEM image obtained for (a) Fe₃O₄ nanorods and (b) RGO–Fe₃O₄ nanocomposite.

The morphology of the as-synthesized Fe₃O₄ and RGO–Fe₃O₄ nanocomposite was further investigated by TEM measurement and is shown in Fig. 5. The TEM image of Fe₃O₄ nanoparticles shows rod-like morphology with an average size of 100 nm (Fig. 5(a)). HRTEM image demonstrates a fringe spacing of 0.25 nm (inset in Fig. 5(a)) corresponding to the (311) plane of Fe₃O₄.⁵⁰ The selected area electron diffraction (SAED) pattern shows ring pattern (Fig. 5(b)), indicating the polycrystalline nature of the nanoparticles. The TEM image obtained for RGO–Fe₃O₄ nanocomposite (Fig. 5(c)) shows a random distribution of rod-like Fe₃O₄ nanoparticles over the RGO surface. The HRTEM image of RGO–Fe₃O₄ nanocomposite (Fig. 5(d)) shows a fringe spacing of 0.25 nm corresponding to the (311) plane of Fe₃O₄ nanoparticles along with the presence of few layered RGO sheets. This result further confirms the presence of Fe₃O₄ nanoparticles over RGO surface. The rod-like Fe₃O₄ nanoparticles can act as the spacer avoiding the restacking of RGO sheets as a result the enhancement in the active surface area can be achieved which in turn may lead to the achievement in the better electrochemical performance.

Fig. 5 TEM image (a) and SAED pattern (b) of Fe₃O₄ nanorods and TEM (c) and (d) HRTEM image of RGO–Fe₃O₄ nanocomposite.

4 Electrochemical performance

The capacitive behaviour of RGO, Fe₃O₄ nanorods and RGO–Fe₃O₄ nanorod nanocomposite were investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements in 1 M KOH electrolyte. Fig. 6, shows the cyclic voltammograms obtained on Fe₃O₄ nanorods and RGO–Fe₃O₄ nanorod nanocomposite based electrodes at a scan rate of 20 mV s⁻¹ in the potential range from −1.2 to −0.4 V. The voltammograms obtained on the RGO–Fe₃O₄ nanocomposite based electrode shows a quasi-rectangular voltammogram consisting of redox peaks. The origin of quasi-rectangular voltammogram consisting of redox peaks in the case of RGO–Fe₃O₄ nanorods nanocomposite are ascribed to the integration of electrical double layer capacitance of RGO with the redox activity of the Fe²⁺/Fe³⁺ redox couple.⁵¹ This result suggests the formation of RGO–Fe₃O₄ nanorod nanocomposite by the method described in the present investigation. The examination of the voltammograms of Fe₃O₄ nanorods and RGO–Fe₃O₄ nanorod nanocomposite reveal that they all have different area at same scan rate. In the case of RGO–Fe₃O₄ nanorod nanocomposite, much wider area was observed compared to Fe₃O₄ nanorods, indicating that RGO–Fe₃O₄ nanorod nanocomposite could deliver highest specific capacity, since specific capacity is dependent on the area of the voltammogram.³⁶ The specific capacity values of Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite based electrode were calculated according to the method reported elsewhere.⁵² Recently, a new concept has been developed which clearly describes about whether the electrochemical behaviour of the electrode material will be considered as pseudocapacitive or not.⁵³ At the same time, the authors of that paper have clearly described in which case the term “capacitance” (F) and “capacity” (coulomb, C, or mAh) should be used.⁵³ Inspired by this concept, Gu et al. has performed a couple of investigations on the active electrode materials which has battery-like behaviour and have demonstrated that the term “capacity” (C g⁻¹) is appropriate to describe the supercapacitive performance of such materials.^54,55 In the present case, since our active electrode material RGO–Fe₃O₄ nanocomposite showed battery-like behaviour, so the performance was evaluated in terms of specific capacity. The specific capacity values for Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite were calculated to be 108 and 458 C g⁻¹ at the scan rate of 20 mV s⁻¹. The specific capacity value indicates that as an active material, the RGO–Fe₃O₄ nanocomposite has better electrochemical performance than Fe₃O₄ nanorods. The electrochemical performance of Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite were further investigated with respect to scan rate (5–20 mV s⁻¹) in the potential range from −1.2 to −0.4 V. As can be seen in the Fig. 7(a), irrespective of the scan rate the voltammetric profile retains its quasi-rectangular shape, indicating the excellent capacitive behavior of the RGO–Fe₃O₄ nanocomposite. At the scan rate of 5, 10, 15 and 20 mV s⁻¹ the specific capacity was found to be 458, 420, 395, and 371 C g⁻¹, respectively. Similarly, the Fe₃O₄ nanorod based electrode retains its voltammetric profile shape irrespective of the scan rate (Fig. S2†). The plot of specific capacity vs. scan rate (Fig. 7(b)) shows that, compared to Fe₃O₄ nanorods, the RGO–Fe₃O₄ nanocomposite has highest specific capacity value in all scan rates. At 5 mV s⁻¹ scan rate, the specific capacity value was 458 C g⁻¹. However, further increase in the scan rate to 20 mV s⁻¹, the specific capacity value decreased to 371 C g⁻¹. The decrease in the specific capacity value with the 4 fold increase in scan rate was ascribed to the diffusion effect.⁵⁶ It is well established that, at low scan rate, most of the inner active sites of the active electrode material can take part in the electrochemical reaction leading to the achievement in high performance. However, at high scan rate, the active participation of most of the inner active sites of the active electrode material is not possible.⁵⁷ The same trend was also noticed in the case of Fe₃O₄ nanorod based electrodes.

Fig. 6 Cyclic voltammetric response of (a) Fe₃O₄ nanorods and (b) RGO–Fe₃O₄ nanocomposite based electrode in 1 M KOH. Scan rate: 20 mV s⁻¹.

Fig. 7 (a) Cyclic voltammetric response of RGO–Fe₃O₄ nanocomposite based electrode in 1 M KOH at different scan rates and (b) plot of specific capacity vs. scan rate.

Furthermore, the electrochemical performance of Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite based electrodes was investigated by galvanostatic charge/discharge technique at 5 A g⁻¹ current density and is shown in Fig. 8(a). The specific capacity value was calculated from the charge/discharge profiles according to the method described elsewhere.⁵² As can be seen in Fig. 8(a), the RGO–Fe₃O₄ nanocomposite based electrodes has longest discharge time compared to Fe₃O₄ nanorod based electrodes. The RGO–Fe₃O₄ nanocomposite based electrodes delivered a specific capacity of 315 C g⁻¹. On the other hand, at the same current density of 5 A g⁻¹, the Fe₃O₄ nanorod based electrodes delivered a specific capacity of 205 C g⁻¹. The enhancement in the specific capacity value in the case of RGO–Fe₃O₄ nanocomposite was ascribed to the synergetic effect of RGO and Fe₃O₄. The specific capacity value obtained from the galvanostatic charge/discharge measurement follows the same trend as obtained in the case of cyclic voltammetric results. It is difficult to compare our result with the earlier reported literatures describing the supercapacitive performance of RGO–Fe₃O₄ nanocomposite, since, in those works, the active electrode materials have been wrongly considered to be a pseudocapacitive material and their performance has been evaluated in terms of specific capacitance (F g⁻¹). However, we have attempted to compare the performance of the RGO–Fe₃O₄ nanorod nanocomposite roughly with some reported literature (Table S1†). Based on the stability, the RGO–Fe₃O₄ nanorod nanocomposites showed high stability with highest % of retention compared to some reported literature.^{23,35,36,39,40,42} The origin of better supercapacitive performance in the present case was ascribed to the effect of Fe₃O₄ nanostructure morphology. It is well known that, the morphology of Fe₃O₄ nanostructure has profound effect on the supercapacitive performance.⁵⁸

Fig. 8 (a) Galvanostatic charge–discharge curves of Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite based electrode at 5 A g⁻¹ current density, (b) galvanostatic charge–discharge curves obtained for RGO–Fe₃O₄ nanocomposite based electrode at different current densities and (c) plot of specific capacity vs. current density obtained on Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite based electrodes.

Further we have studied the effect of current density on the galvanostatic charge/discharge profile as well as specific capacity value of RGO–Fe₃O₄ nanocomposite based electrode (Fig. 8(b)). As can be seen in Fig. 8(b), as the applied current density was increased (5 to 12.5 A g⁻¹) the discharge time decreased. In this case, the specific capacity values are calculated to be 315, 266, 227 and 155 C g⁻¹ at the current density of 5, 7.5, 10, and 12.5 A g⁻¹, respectively. The decrease in the specific capacity at higher current density is ascribed to the limited migration of electrolyte ions into the inner side of the active material.⁵⁹ The same trend was also noticed in the case of Fe₃O₄ nanorod based electrode (Fig. S3†). The plot of specific capacity vs. current density (Fig. 8(c)) obtained using the specific capacity value delivered by the Fe₃O₄ nanorod and RGO–Fe₃O₄ nanocomposite based electrode shows that, the RGO–Fe₃O₄ nanocomposite based electrode has highest specific capacity value in all current density. The close examination of this plot indicates that, the Fe₃O₄ nanorod based electrode delivered 205 C g⁻¹ specific capacity at 5 A g⁻¹ current density and this value reached to 70 C g⁻¹ at 12.5 A g⁻¹ current density which is 34% of the value that observed at 5 A g⁻¹. It is interesting to note that, the RGO–Fe₃O₄ nanocomposite delivered 315 C g⁻¹ specific capacity at 5 A g⁻¹ current density and this value reached to 155 C g⁻¹ at 12.5 A g⁻¹ which is 49% of the value that observed at 5 A g⁻¹, demonstrating that the RGO–Fe₃O₄ nanocomposite has a good rate of capability.

The electrochemical property of Fe₃O₄ nanorods and RGO–Fe₃O₄ nanocomposite were further investigated using electrochemical impedance spectroscopy (EIS) technique and is shown in Fig. 9. Fe₃O₄ nanorod based electrode shows that the impedance plot consists of a semicircle and straight line in high and low-frequency region, respectively. The semicircle represents the charge-transfer resistance while the straight line represents diffusion or transport of ions from the electrolyte to the electrode surface.^60,61 Since the Fe₃O₄ nanorod based electrode shows a large semicircle therefore it is believed that this active electrode material has poor electrical conductivity. On the other hand, in the case of RGO–Fe₃O₄ nanocomposite based electrode, only a straight line was observed indicating that this active electrode material has high electrical conductivity compared to the Fe₃O₄ nanorod. The increase in the electrical conductivity in the case of RGO–Fe₃O₄ nanocomposites was ascribed to the synergistic effect of the integration of Fe₃O₄ nanorod with highly conductive RGO.

Fig. 9 Electrochemical impedance spectra obtained for (a) Fe₃O₄ nanorods and (b) RGO–Fe₃O₄ nanocomposite.

Investigation on the long-term cyclic stability of the electrode material is an important parameter for its practical applications in supercapacitor. We had investigated the cyclic stability of RGO–Fe₃O₄ nanocomposite based electrode using galvanostatic charge/discharge technique in the potential range from −1.2 and −0.4 V applying at a current density of 5 A g⁻¹ for 2000 cycles. Fig. 10, shows the specific capacity of Fe₃O₄ nanorod and RGO–Fe₃O₄ nanocomposite as a function of cycle number. After 2000 CD cycles the specific capacity value decreased from 315 C g⁻¹ to 298 C g⁻¹. Only 5% decrease in the specific capacity value after 2000 CD cycles demonstrates that the RGO–Fe₃O₄ nanocomposite based electrode has excellent long-term cyclic stability. The excellent performance of RGO–Fe₃O₄ nanocomposite was attributed to be originating from the synergistic effect between RGO and Fe₃O₄ nanorods.

Fig. 10 Charge capacity of (a) Fe₃O₄ nanorods and (b) RGO–Fe₃O₄ nanocomposites with respect to cycle number.

5 Conclusions

In summary, we have developed a facile two step synthetic protocol for the synthesis of RGO–Fe₃O₄ nanorod nanocomposite for supercapacitor application. Our method involves initial spontaneous in situ deposition of rod-like FeOOH nanoparticles onto the self-reduced GO surface and its subsequent annealing to get Fe₃O₄ nanorods decorated RGO. As an active electrode material, the as-synthesized RGO–Fe₃O₄ nanorod nanocomposite delivered a high specific capacity compared to Fe₃O₄ nanorods. On the RGO–Fe₃O₄ nanocomposite based electrode a specific capacity of 315 C g⁻¹ was observed at 5 A g⁻¹ current density. The proposed RGO–Fe₃O₄ nanorod nanocomposite based electrode has excellent cycling stability. The origin of better electrochemical performance in the case of RGO–Fe₃O₄ nanorod nanocomposite was ascribed to the synergetic effect between the single component RGO and Fe₃O₄ nanorods. The electrochemical results demonstrate that RGO–Fe₃O₄ nanocomposite could be a promising electrode material for supercapacitor application.

Acknowledgements

This research was supported by the Priority Research Center Program (NRF-2014R1A6A1031189) and the Korea–China International Cooperation Program (NRF-2015K2A2A7053101) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education and the Ministry of Science, ICT, and Future Planning, respectively.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of graphene oxide (GO), fabrication of working electrode. See DOI: 10.1039/c6ra23665k

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