Introduction

With the improvement of the economy, environmental stress has been increasing, and it is extremely urgent to develop a new, low cost, environmentally friendly alternative energy source and energy storage system [1,2,3,4]. Supercapacitors are one of the most important as promising candidate material for energy storage because of their advantages such as high power density, excellent rate capacitance, and long cycle life [5,6,7]. Recently, a variety of transition-metal oxides, such as RuO2, Co3O4, NiO, Fe2O3, Fe3O4, and MnO2, have been studied for supercapacitor applications [8,9,10]. Iron oxides including Fe3O4 [11] and Fe2O3 [12] are considered as one of the most promising pseudocapacitor electrode materials due to its abundance, low cost, environmental harmlessness, and high electrochemical performance. However, their serious disadvantages of limited electrode applications due to their poor electrical conductivities and the synthesis of these novel structures are quite complex and not easy to control. Another option that can improve the electrochemical performance is to combine α-Fe2O3 with other components with good conductivity to facilitate the electron transfer between the active materials and the current collector.

Recently, graphene is a unique single layer of carbon atoms tightly packed into a two-dimensional honeycomb sp2 carbon lattice and has paying fabulous attention in energy storage devices as a result of its huge specific surface area, good chemical stability, and remarkable electrical conductivity [13]. Many literatures have reported that Fe2O3/graphene nanocomposites for supercapacitor applications. For example, Dewei Wang et al. [14] have prepared Nanostructured Fe2O3–graphene composite via facial hydrothermal method and prepared electrode material large specific capacitance (151.8 F g−1 at 1 A g−1), good rate capability (120 F g−1 at 6 A g−1), and excellent cyclability. Hang Zhang et al. [15] have prepared solvothermal-induced ɑ-Fe2O3/graphene nanocomposite by hydrothermal method. The results showed that capacitances are admirable 2310 F g−1 at 5 mV s−1 and 615 F g−1 at the amazing high current density of 100 A g−1. Attempt to further improvement in the electrochemical performance of these nanostructure materials, this work we have prepared Fe2O3/graphene nanocomposites via one step hydrothermal method and systematically investigated the thermal, structural, morphological, and electrochemical performances. The results showed that Fe2O3/G nanocomposites showed high specific capacitance, good rate capability, and excellent cyclability compared to pure Fe2O3. To the best of the author’s knowledge, this is the first preliminary report about high performance electrochemical behavior of Fe2O3/G nanocomposite electrodes via a one step facile hydrothermal method.

Experimental procedure

Materials

Iron(III) nitrate [Fe (NO3)2], purity-99.97%, urea [CO (NH2)2], and liquid ammonium hydroxide solution (NH4OH-purity-99.97%) were used as the precursor solutions for Fe2O3 nanostructures. All the chemicals used were of analytical grade, purchased from Merck, India, and without any further purification.

Preparation GO

Graphite oxide (GO) was synthesized from pristine graphite powder based on a modified Hummers method. In a typical process, 25 mg GO and 50 mL of water were ultrasonicated in a beaker for 90 min in order to fully exfoliate the graphite oxide to GO sheets. Finally, a homogeneous GO aqueous dispersion (0.5 mg mL−1) was obtained.

Preparation Fe2O3/G nanocomposites

The Fe2O3/G nanocomposite was prepared by hydrothermal process using urea as fuel agent. In a typical experimental procedure, 0.1 M Fe (NO3)2 was dissolved in 50 mL distilled water under strong magnetic stirring until completely dissolved. Then, the prepared GO powder was mixed with the above solution. The ratio of Fe (NO3)2 and GO is 1:1. The urea was added in the above solution and stirred for 30 min. The ratio of Fe (NO3)2 and urea is 1:2. Then, 25 ml aqueous solution of NH4OH was introduced into above aqueous solution to make the desired alkaline concentration until the pH value was attain 10. During this reaction, the drop rate was must be controlled to achieve the chemical homogeneity. Then, the solution was transferred into a Teflon lined stainless steel autoclave, which was sealed and maintained at 150 °C for 24 h. After completing the hydrothermal reaction, the mixture was centrifuged to get the precipitate out and washed with ethanol and distilled water and then dried in air in a laboratory hot air oven at 100 °C for 10 h. Pure Fe2O3 particles were also prepared in the same conditions without the presence of GO.

Characterization details

The thermal analysis was performed on SDT Q600 V8.3 Build 101 TGA system. The heating rate of 10 °C min−1 in nitrogen flow (40 ml min−1) and the weight of the sample taken in the experiment were around 5 mg. Identification of the crystalline phase and size of the Fe2O3/G nanocomposite was performed using powder X-ray diffraction (Bruker diffractometer) using the Cu Kα wavelength of 1.5406 Å and operating at 35 kV and 30 mA. PXRD patterns were recorded in the range 20–80° at the scan rate of 0.50 s. Morphology and particle size of the prepared nanoparticles were studied, employing transmission electron microscope that was recorded with accelerating voltage of 200 kV. The elemental analysis of the samples was analyzed by EDS spectra (JEOL model JED-2300) operated at 15 kV. The Raman spectra of the samples were recorded using BRUKER RFS 27: stand-alone FT-Raman Spectrometer at a resolution of 0.2 cm−1. The specific surface areas and porous nature of materials were further investigated by nitrogen adsorption/desorption measurements on nova 2200e XPS was analyzed by using a Thermo Scientific K-alpha surface analysis instrument.

Electrode preparation

The electrochemical properties of the samples were investigated with cyclic voltammetry (CV), Galvanostatic charge/discharge analysis, and impedance tests by using software-controlled conventional three electrode electrochemical cell (CHI 660C electrochemical workstation), consisting of the glassy carbon electrode (GCE) as the working electrode (WE), Ag/AgCl as the reference electrode, and the platinum wire as the counter electrode. The working electrode was prepared by mixing active material, carbon black, and polyvinylidene fluoride (PVDF) in 10% solution of N-methyl-2-pyrrolidone with a mass ratio of 75:20:5. Then, the prepared slurry was coating on Ni foil (1 cm × 1 cm) and dried at 80 °C for overnight. The mass of the electrode material is 0.8 mg. All the electrochemical measurements were performed in a 2-M KOH aqueous electrolyte solution at room temperature. The CV measurements were performed at scan rate of 10, 50, and 100 mV s−1. A potential window in the range from − 0.4 to 0.2 V was used in all the measurements.

Results and discussion

TGA analysis

Thermal properties of nanocomposites were determined by thermogravimetric analysis. The TGA spectra of pure and Fe2O3/G nanocomposite samples are shown in Fig. 1. The TGA curve of the composite displays a significant loss of weight at approximately 250 °C. As can be seen from it, the weight loss over the temperature range up to 200 °C is about 1%. This might be due to the loss of residual water in the sample. The major weight loss between 350 and 450 °C is about 30%, which represents the amount of graphene [16]. The second weight loss was observed around 600 °C (4.5%) which could result from detachment of oxygen installed on graphene surface. The total weight loss was about 70–73%, and then the remaining material is composed of Fe2O3 only. This TG analysis confirms again the formation of Fe2O3 as the product of the deposition.

Fig. 1
figure 1

TGA curve of pure and Fe2O3/G nanocomposite samples

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XRD analysis

X-ray diffraction is used to identify its structure and average grain size of the samples. Figure 2 shows the powder XRD pattern of pure GO, Fe2O3, and Fe2O3/G nanocomposite. In pure graphene, the broad peak in range of 20–30° is corresponded to the (002) diffraction of graphene. It can be seen that all the diffraction peaks could be readily indexed as a pure rhombohedral phase of α-Fe2O3 (hematite, space group: R3c) with lattice constants a = 05.11 Å and c = 013.21 Å, which are good agreement with the values given in the standard card (JCPDS no. 33–0664). Moreover, no other impurities phased were detected, which suggests the high purity of the as-prepared product. At the same time, characteristic diffraction peaks of Fe2O3 are also revealed in the XRD pattern of Fe2O3/graphene composite, demonstrating the formation of Fe2O3 on surface of graphene. [17]. Compared to pure Fe2O3, diffraction peaks of Fe2O3/G composite are relatively broad, indicating that the crystals constituting the products are of small sizes. The average crystalline sizes of the pure and Fe2O3/G nanocomposite electrodes were calculated by using Scherrer’s equation [18].

Fig. 2
figure 2

Powder XRD pattern of pure GO, Fe2O3, and Fe2O3/G nanocomposite samples

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$$ d=\frac{K\lambda}{\beta \cos \theta } $$

where d is the mean crystallite size, K is the shape factor taken as 0.89, λ is the wavelength of the incident beam, β is the full width at half maximum, and θ is the Bragg angle. The average crystalline size was calculated as 35 nm and 25 nm for pure and Fe2O3/G nanocomposite respectively.

TEM and EDS analysis

The morphology, particle size, and crystallinity of the samples have been investigated from TEM images. Figure 3a shows the TEM image of pure graphene; it consists of nanosheet with transparent and wrinkle. Figure 3c, d shows the TEM images of pure and Fe2O3/G nanocomposite samples respectively. It was clear evidence that both the samples showed mainly spherical nanoparticles with homogeneously anchored to the graphenes with uniform sizes less than 35 nm. Enlarged HRTEM image of Fe2O3 (Fig. 3c) clearly indicates the high-quality crystalline character of α-Fe2O3. The measured distinct spacing of periodic lattice fringes is 0.24 nm, assigned to the interspacing of (110) planes of hematite α-Fe2O3, exactly matching with the XRD results. In order to confirm the chemical composition of the samples, EDAX measurement was carried out. Figure 3e, f shows the TEM-based EDAX spectra of pure and Fe2O3/G nanocomposite sample. In pure graphene, the result shows the presence of C (65.38 at Wt%) and O (34.62 at Wt%) peaks, whereas in Fe2O3/G composite, the samples mainly composed of Fe, C, and O elements, which confirm the purity of the samples. The Cu element was found in the composition, due to the grid used for EDS measurements. The weight percentages of graphene were estimated to be 58.42 wt% for Fe2O3–graphene composite. This value is slightly less than the theoretical value (66.7 wt%), which is calculated based on the experimental conditions.

Fig. 3
figure 3

TEM images of a pure graphene, b Fe2O3, c Fe2O3/G nanocomposite, d HRTEM image of Fe2O3, e EDS analysis of pure GO, and f Fe2O3/G nanocomposites

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Raman spectra analysis

Raman spectroscopy can be used to gain information about the structural changes in carbonaceous materials. The Raman spectra of pure and Fe2O3/G nanocomposite samples were shown in Fig. 4. It was noted that the fundamental Raman scattering peaks for Fe2O3 are observed at 221, 282, 379, and 496 cm−1 corresponding to the Ag1(1), Eg2 + Eg3, Eg4, and A1g (2) modes [19], respectively. In the case of Fe2O3/G composite, in addition to the bands corresponding to Fe2O3, the characteristic D and G bands of graphene also appear clearly, respectively. These results further suggest the good integration of graphene and Fe2O3 nanoparticles.

Fig. 4
figure 4

Raman spectra of pure and Fe2O3/G nanocomposite samples

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XPS analysis

XPS measurements were employed to further characterize the elemental components of the pure Fe2O3 and Fe2O3/G nanocomposites. The XPS survey spectrum was carried out for bare Fe2O3 and Fe2O3/G composite and depicts in Fig. 5a along with the core level spectra of Fe 2p, O 1s, and C 1s peaks that are shown in Fig. 5b–d, respectively. In the magnified spectrum for pure Fe2O3 (Fig. 5b), the binding energies of Fe2p1/2 and Fe2p3/2 are 725.1 and 711.8 eV, respectively, whereas in Fe2O3/G nanocomposite, two peaks that appeared at 726.3 eV and 713.23 eV are exhibited in the Fe 2p spectrum (Fig. 5b), which correspond to the Fe 2p1/2 and Fe 2p3/2 spin–orbit peaks of Fe2O3 [20]. The O 1s peak of pure Fe2O3 is observed at 530.5 eV and for Fe2O3/G nanocomposite; the peak that appeared at 531.1 eV in the O 1s spectra corresponds to the oxygen species in the metal oxide (Fig. 5c). In the C 1s spectrum (Fig. 5d), the peaks at 282.2 and 286.3 eV belong to the sp2-C and sp3-C bonds of rGO, respectively. XPS results again supported that successful loading of Fe2O3 nanoparticles in the graphene surface.

Fig. 5
figure 5

a The full-range XPS survey spectrum of Fe2O3 and Fe2O3/G and the high-resolution spectrum of b Fe 2p, c O 1s, and d C 1s (Fe2O3/G nanocomposite)

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BET surface analysis

The specific surface area and pore size distribution are the important factors that can intensely show the performance of the electrodes. The surface area of the samples was analyzed by using Brunauer–Emmett–Teller (BET). From the N2 adsorption–desorption isotherm (Fig. 6a), it is observed that both the samples exhibit an irreversible type IV isotherm in the Brunauer classification that corresponds to the typical mesoporous feature. The specific BET surface area was found to be 76 and 91 m2 g−1 for pure and Fe2O3/G nanocomposite electrodes respectively. The resulting increase in surface area of Fe2O3/G than Fe2O3 clearly endorses the TEM and XRD analysis results. The corresponding BJH pore size distribution of the samples is shown in Fig. 6b and found to be 10 and 17 nm for pure and Fe2O3/G nanocomposite electrodes respectively.

Fig. 6
figure 6

a N2 nitrogen adsorption and desorption and b pore size distribution spectra of pure and Fe2O3/G nanocomposite samples

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Electrochemical analysis

CV analysis

CV and galvanostatic charge–discharge measurements were examined in a half-cell configuration with three electrode mode to evaluate the electrochemical properties of the Fe2O3–graphene composites. Figure 7a, b shows the representative CV curves of the Fe2O3 and Fe2O3/G composite electrodes at different scan rates in 2 M KOH electrolyte with a potential window between − 0.4 and 0.2 V (vs. Ag/AgCl). The curve shape of both samples that showed shapes are obviously not rectangular, indicating pseudocapacitive character. Moreover, the shape of the CV curves was retained even at the very high scan rate of 100 mV s−1. At an even higher scan rate of 100 mV s−1, the rectangularity of the CV deteriorated but was still logically good. The results suggest that Fe2O3/G electrode is a tremendous electrochemical reversibility, negligible internal resistance, and uniform working potential. This could be due to high charge transport/transfer rates and uniform distribution of Fe2O3/G nanocomposite electrodes achieved by using the graphene. The excellent rate capability of the electrode can be further investigated by the galvanostatic charging/discharging analysis at different current densities.

Fig. 7
figure 7

CV curve of a Fe2O3 and b Fe2O3/G nanocomposite with different scan rates

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Galvanostatic charge/discharge analysis

Figure 8a, b shows the charge–discharge behavior of the as-prepared Fe2O3 and Fe2O3/G electrodes at different current densities in 2 M KOH electrolyte with a potential window between − 0.4 and 0.2 V (vs. Ag/AgCl). The specific capacitance value of all of the electrode materials was again determined from charge/discharge profiles by using the following equation [21].

$$ \mathrm{Cs}=\frac{\boldsymbol{i}\Delta \boldsymbol{t}}{\boldsymbol{m}\Delta \boldsymbol{V}}\kern0.5em $$
(1)

where Cs is the specific capacitance (F g−1), I is the specific current (A), ∆t is the discharge time (s), m is the mass of the active material (g), and ∆V is the potential window (V). The discharge specific capacitance of the Fe2O3/graphene electrodes at the current density of 2 A g−1 is 315 F g−1, which is higher than the already reported literature values [22]. The detailed specific capacitance value of both Fe2O3 and Fe2O3/G composites is shown in Table. 1. The variation of specific capacitance as a function of different current density is also shown in Fig. 8c, d. Moreover, the galvanostatic charge discharge (GCD) results demonstrate that Fe2O3/G composite shows a specific capacitance as high as 315 F g−1 at a discharge current density of 2 A g−1. Even at the current density of 10 A g−1, the specific capacitance is still as high as 185 F g−1. The remarkably increased capacitance of Fe2O3/G may be caused by the combination of electric double-layer capacitance of G and faradic pseudocapacitance of Fe2O3. The decrease in specific capacitance with the increase of current densities (2 to 10 A g−1) is attributed to the relatively insufficient faradaic redox reaction of the active materials under higher discharge current densities.

Fig. 8
figure 8

Charge–discharge behavior of a Fe2O3 and b Fe2O3/G electrodes at different current densities and c, d variation-specific capacitance values as a function current density

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Table 1 Summary of recent reports of Fe2O3/G-based material for supercapacitors
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Cyclability test

The cycle life is another important factor for the supercapacitor materials. The electrochemical cycle stability of the as-prepared Fe2O3 and Fe2O3/G electrodes was evaluated at a current density of 2 A g−1 for 2000 cycles as shown in Fig. 9a, b and inset. The inset shows the last 500 cycles of the electrodes. As shown in Fig. 9a, b, the capacitance retention rate of the composite electrode remained at a very high value of 98%, after 2000 cycles, revealing the excellent electrochemical stability of the Fe2O3/G electrode, while that of the α-Fe2O3 electrode dropped to 88%. The excellent capacitive performance of the Fe2O3/G composite electrode is probably attributed to the constructive synergistic effects between the Fe2O3 and graphene. Graphene also provides a highly conductive network for electron transport during the charge and discharge processes, thus reducing the polarization of the electrodes, which results in enhance the electrochemical performance of Fe2O3 [28]. According to the specific capacitance of the supercapacitors, the energy density (E, in W h kg−1) and power density (P, in W kg−1) could be further calculated using the equations, E = 1/2CU2 and P = E/t, respectively [29]. Moreover, the proposed electrodes showed superior electrochemical performance than that of previous reports and shown in Table 1 [23,24,25,26,27]. The overall electrochemical results suggest that Fe2O3/G is superior electrochemical performance than pure Fe2O3. This could be due to the existence of Fe2O3 nanoparticles and graphene sheets that enables fast electronic and ionic transport through the active materials to charge collector and make the charge–transfer resistance of Fe2O3/G electrode decrease and thus improve the electrochemical performance, which results in specific capacitance of the Fe2O3/graphene electrodes at the current density of 2 A g−1 is 315 F g−1. Moreover, Fe2O3 nanoparticles on the surface of graphene sheets can act as spacers to efficiently prevent the aggregation of the nanoparticles as well as the restacking of graphene sheets, avoiding/weakening the loss of their high active surface area. Moreover, the large specific surface area could significantly improve the electrochemical reactive sites and better penetration of the electrolyte in to the whole electrode materials, which will generate the high supercapacitive performance.

Fig. 9
figure 9

Cycling stability of the electrodes for 2000 cycles at a current density of 2 A g−1. Bottom inset is the charge–discharge curves of the last 500 cycles. a Fe2O3. b Fe2O3/G

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EIS tests

Complementary to CV and galvanostatic charge/discharge measurements, EIS provides useful information on the electrochemical frequency of a system for the measurement of redox reaction resistance. For further understanding, the impedance of Fe2O3 and Fe2O3/G composite electrodes was measured in the frequency range of 100–0.1 kHz at open circuit. Figure 10a shows the Nyquist plots of Fe2O3 and Fe2O3/G electrodes. It can be seen that both the electrode exhibited a vertical capacitive line in the low-frequency region, semicircle in the high-frequency region and a straight line inclined at a constant phase in the mid-frequency region. The high-frequency semicircle region is due to the charge transfer reaction at the interface of the electrolyte/oxide electrode, and it corresponds to the charge transfer resistance (Rct) [30]. It is clear proof that the Rct of Fe2O3/G (2.01 Ω) is smaller than that of Fe2O3 (2.43 Ω), indicating that the electrochemical reaction on the electrode/electrolyte interface is the more facile for Fe2O3/G. This confirms that the formation of Fe2O3 nanoparticles really makes the electronic resistance of the electrode decrease. These results prove that the formation of Fe2O3 nanoparticles on G can significantly improve the electrical performance due to the intrinsic resistance change of the active material and less aggregation of the graphene. To further evaluate the Fe2O3/G electrode for practical applications, an asymmetric supercapacitor was manufactured by using Fe2O3/G as the positive electrode and AC as the negative electrode. Figure 10b shows CV curves of the Fe2O3/G @AC asymmetric device performed at different scanning rates ranging from 5 to 100 mV s−1. It can be seen that the potential range could be max to 2 V, and all CV curves exhibited redox peaks, revealing the combined contribution of both EDLC and battery-type electrodes on the total CV behaviors. The GCD curves at different current densities are shown in Fig. 10c, and the shapes of all curves are triangular, further confirming excellent capacitive performance of the Fe2O3/G@AC asymmetric device. As shown in Fig. 10d, the specific capacitance of the Fe2O3/G @AC ASC is calculated from GCD curve and a maximum capacitance of 98 F g−1 at a discharge current density of 2 A g−1. The cyclic durability of the Fe2O3/G@AC ASC is further evaluated at 10 A g−1 for 2000 cycles, and it can be seen that the ASCs retains 88% of the initial capacitance, proving its good cycling performance (Fig. 10e). Further, two important electrochemical parameters like power density and energy density are measured to get the information about energy storage performance of supercapacitors. The plot consisting of energy density and power density is presented in Fig. 10f. The energy density (W h kg−1) and power density (kW kg−1) are calculated by using the equations reported literature [31]. The results show high-energy density of 23.5 W h kg−1 at a power density of 405 W kg−1. Even at high power density of 885 W kg−1, relative high-energy densities of 14.5 W h kg−1 are achieved. The values of energy and power densities obtained in the present work are higher than compared with some reported similar Fe2O3, such as FeOOH//MnO2 (12 W h kg−1) [32], Fe3O4//MnO2 (8 W h kg−1) [33], MnO2//Fe2O3 (19.4 W h kg−1) [34], and graphite foam-CNT@Fe2O3//graphite foam-CoMoO4 (1.4 W h kg−1) [35].

Fig. 10
figure 10

a Nyquist plots of Fe2O3 and Fe2O3/G electrodes with a frequent range of 0.1 Hz–100 kHz in 1 M KOH electrolyte and equivalent circuit (inset). b CV curves at different scan rates. c GCD curves at various current densities. d Variation of specific capacitance with different current densities. e Cycling stability of the Fe2O3/G//AC device for 2000 cycles at the current density of 10 A g−1. f Ragone plots

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Conclusions

In summary, we have successfully developed a facile and one-step hydrothermal method for the synthesis of pure and αFe2O3/G nanocomposite, which can be scaled up in a large quantity. XRD, TEM, Raman, and XPS analyses confirm that well-crystalline Fe2O3 nanoparticles in rhombohedral phase of α-Fe2O3 (hematite, space group: R3c) with 25–35 nm in size are highly encapsulated in a graphene matrix. The electrochemical property of Fe2O3 was significantly improved by graphene matrix. The Fe2O3/G nanocomposite electrode shows a specific capacitance of 315 F g−1 at a current density of 2 A g−1 and results in good long-term cyclic stability 98% capacitance retention after 2000 cycles. This material takes the advantage both of the electrical double layer capacitor from G and pseudocapacitance of Fe2O3 nanoparticles. Hence, the present α-Fe2O3/G nanocomposite electrodes proved to be an excellent anode material for fabrication of neutral aqueous asymmetric supercapacitors.