rGO


May 8, 2017 - 0.1 M hydrochloric acid was slowly added into the solution until its pH ... 4:1), and the corresponding composites were named α-Fe2O3/r...
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Research Article pubs.acs.org/journal/ascecg

Construction and Performance Characterization of α‑Fe2O3/rGO Composite for Long-Cycling-Life Supercapacitor Anode Yuanyuan Zhu,† Shuang Cheng,*,† Weijia Zhou,† Jin Jia,† Lufeng Yang,† Minghai Yao,† Mengkun Wang,† Jun Zhou,† Peng Wu,† and Meilin Liu*,†,‡ †

Guanzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong 510006, China ‡ School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332-0245, United States S Supporting Information *

ABSTRACT: Thumb-ring-like α-Fe2O3 and reduced graphene oxide (rGO) composites, α-Fe2O3/rGO, have been synthesized via a simple hydrothermal method accompanied by surface potential tuning. The obtained samples exhibit good electrochemical performance with a wide negative potential window of −1−0.2 V vs Ag/AgCl when serving as supercapacitor electrodes. There is only about 10% decay after 11 000 cycles of galvanostatic charge−discharge (GCD) test. After a few tens of cycles of cycling activity, the capacitance achieved a stable value of 255 F g−1 at 0.5 A g−1 and 174 F g−1 at 5 mV s−1; 75% of the capacitance was retained when the scan rate increased to 200 mV s−1, indicating satisfactory power density. Most attractively, along with cycling, the α-Fe2O3 particles begin to be well-wrapped by rGO gradually from prior stacked structure, which is supposed to be the key factor for the exceptional high cycling stability. KEYWORDS: rGO-wrapped a-Fe2O3, Anode material, Supercapacitor, Excellent rate capability, High cycling stability



Co3O4,11,12 TiO2,13 WO3−x,14,15 polyaniline (PANI),16 and so forth. These are only used as cathode materials with an operating voltage window smaller than 1 V. To broaden this voltage window and realize high energy density in aqueous electrolyte, to date, the construction and designment of asymmetric supercapacitors seem to the best choice.17 Therefore, it is quite meaningful to explore anode materials with excellent electrochemical performance. For instance, some negative electrodes have been reported, such as Fe2O3,18−23 MoO3−x,24,25 VN,26,27 Mo3N2,28 TiN,29 polypyrrole (PPY),30,31 polythiophene,32 and so on. Among these negative electrode materials, α-Fe2O3 (hematite), the most stable iron oxide with n-type semiconducting properties, has become pretty promising due to its high theoretical specific capacitance, low cost, abundance, and environmental harmlessness.33−35 However, αFe2O3-based electrodes always suffer from low power density due to the poor electrical conductivity and low cycling stability resulting from their large crystal size and volume change during charge/discharge which would cause severe particle aggregation or structure fragmentation. To overcome these disadvantages of α-Fe2O3-based electrodes, catering to the demands for high-

INTRODUCTION As always, owing to desirable characteristics of high power density, long cycling stability, and low maintenance cost, supercapacitors, which have also been termed as electrochemical capacitors or ultracapacitors, have been extensively explored as energy storage devices for various applications, such as portable electronics, electric vehicles, and renewable energy systems.1−4 However, scientists are still lining up to explore new electrode materials due to the unsatisfactory energy density. In order to further improve the energy density (E = 1/ 2(CV2))5,6 of a supercapacitor device, the capacitance (C) and the operating voltage (V) urgently needed to be enhanced. One of the effective approaches is to use organic electrolytes, which can provide a much wider voltage window (up to 3 V).7 However, organic electrolytes are usually believed to be have high cost, low ionic conductivity, and high perniciousness and/ or flammability, which could lead to environmental and safety issues and inferior power density. However, neutral aqueous electrolyte is a much better choice so far, being nonflammable and environmentally friendly with cheap resources and high ionic conductivity.5,6 Over the recent years, in aqueous electrolyte, electrochemically active materials with high pseudocapacitance of transition metal oxides and electronically conducting polymers have been comprehensively investigated, such as RuO2,8 MnO2,9,10 © 2017 American Chemical Society

Received: February 12, 2017 Revised: May 1, 2017 Published: May 8, 2017 5067

DOI: 10.1021/acssuschemeng.7b00445 ACS Sustainable Chem. Eng. 2017, 5, 5067−5074

Research Article

ACS Sustainable Chemistry & Engineering

was slowly dropped under magnetic stirring. Finally, the mixed solution was stirred at room temperature for 12 h and transferred into a 50 mL Teflon-lined stainless-steel autoclave for hydrothermal treatment at 180 °C for 12 h. Final product was obtained by centrifugation and several washes with deionized water, then freezedried. The weight ratio of α-Fe2O3 to GO was varied (1:1, 2:1, and 4:1), and the corresponding composites were named α-Fe2O3/rGO (1:1), α-Fe2O3/rGO (2:1), and α-Fe2O3/rGO (4:1), respectively. Characterization. Phase compositions of the sample were determined by D8 Advance (Germany Bruker) X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA at a scanning rate of 0.02°/s. X-ray photoelectron spectroscopic (XPS) measurement was performed using a PHI X-tool instrument (UlvacPhi). Morphologies of the sample were observed by a field-emission scanning electron microscope (FESEM, MERLIN Compact, Carl Zeiss) and a transmission electron microscope (TEM, a JEM-2100F Field Emission Electron Microscope, JPN) at an acceleration voltage of 200 kV. Raman spectra were obtained by a LabRAM HR800 spectrometer at an Ar ion laser (λ = 514.5 nm) and a long working distance 50× objective lens. Electrochemical Measurements. The electrochemical measurements of the electrodes were carried out in a standard three-electrode electrolytic cell with 1 M Na2SO4 aqueous solution serving as electrolyte at room temperature. Working electrodes were prepared by mixing active materials (α-Fe2O3 or α-Fe2O3/rGO), acetylene black, and binder (60 wt % polytetra-fluoroethylene suspension) in a mass ratio of 8:1:1, then coating it on carbon fiber paper (Hesen, Shanghai Electric. Co.) that served as current collector. The working electrodes had a geometric area of 1 cm2, and mass loading of α-Fe2O3/rGO composites was tuned to be 0.8−2 mg. A Pt mesh of 1 cm2 and Ag/ AgCl (in saturated KCl) were used as counter and reference electrode, respectively. The electrochemical measurements, including cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS), were carried out using a CHI 660E electrochemical workstation. EIS was measured at a frequency ranging from 0.1 Hz to 100 kHz with amplitude voltage of 5 mV at open-circuit potential. The cycle life was measured through an Arbin testing system (MSTAT) in 1 M Na2SO4 aqueous solution at room temperature.

performance supercapacitors, ongoing research efforts have been made. It has been confirmed that modifying α-Fe2O3 with conducting medium, such as conducting polymers,34,36 carbon nanotubes,37,38 carbon nanofibers,39 as well as graphene,40−42 is an effective strategy to adjust its electrochemical properties. Compared with other conductive materials, graphene or chemically modified graphene with their unique planar structure, high electrical conductivity, large specific surface area, highly flexible nature, and especially excellent structural and chemical stability make an undeniably superior support for metal oxides.43 Here, it was decorated by a hydrothermal process to obtain a reduced graphene oxide (rGO) rich with surface functional groups, due to its advantages of binding ability with metal oxides and better hydrophilic feature.44 Herein, thumb-ring-like α-Fe2O3 was fabricated first through the introduction of phosphate ions and sulfate ions to effectively tune the size and shape of nanorings and control well the growth of hematite crystals.45,46 Then, composites with rGO were synthesized by a two-step hydrothermal procedure and a surfactant modification. The sample not only exhibited a good rate performance (75% retention ability for the capacitance when the scan rate increased 40 times, from 5 to 200 mV s−1) but also a high cycling stability (only about 10% decay of the initial capacitance after 11 000 cycles) at a rather wide potential window (−1 to 0.2 V vs Ag/AgCl). For the initial cyclic voltammetry curves (CVs), there are weak redox pairs and obvious irreversible discharge capacitances which can be attributed to the irreversible reaction of Fe(III) to Fe(II). After a few tens of cycles, CV shapes changed to rectangular, and the specific capacitance can achieve a stable value of 174 F g−1 at 5 mV s−1. In addition, it was found that α-Fe2O3 began to be gradually wrapped by rGO during cycling test, which should be a crucial factor for the high cycling stability, while pure αFe2O3 electrode emerged with much lower specific capacitances without redox pairs under a same test condition (obvious redox pairs only appear when the negative scan potential is extended to −1.4 V and the corresponding capacity will be largely enhanced). Hence, it can be speculated that rGO not only served as conductive pathway but also effectively promoted the Faradaic reactions of α-Fe2O3 and protected the α-Fe2O3 from possible mass loss or aggregation.





RESULTS AND DISCUSSION Phase compositions of the as-obtained sample were determined by XRD first, as shown in Figure 1. The diffraction pattern displayed by blue line was indexed to α-Fe2O3, which is in good agreement with that of the standard card (Powder Diffraction File (PDF) No. 33−0664, Joint Committee on Powder Diffraction Standards (JCPDS), [1981]). All the diffraction peaks were very sharp, and no other peaks were detected, indicating a high crystallinity and high-purity. However, after

EXPERIMENTAL SECTION

Preparation of α-Fe2O3. α-Fe2O3 powder was synthesized via a hydrothermal treatment as reported elsewhere.45,46 First, 0.4 g of FeCl3·6H2O, 2 mg of NaH2PO4·2H2O, and 6 mg of Na2SO4 were dissolved in 80 mL of deionized water with vigorous stirring for 0.5 h. Then, the obtained yellow solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave for hydrothermal treatment at 220 °C for 48 h. After cooling to room temperature naturally, red precipitate was collected by centrifugation, washed with deionized water several times, and dried under vacuum at 60 °C. Preparation of α-Fe2O3/rGO Composite. GO was synthesized from fake graphite by a modified Hummers method.47 First, 40 mg of the obtained α-Fe2O3 powder was dispersed in 40 mL of deionized water with ultrasonication for 10 min. Then, 0.1 mL of 3aminopropyltriethoxysilane (APTES) was added under magnetic stirring for 2 h. The final solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave for hydrothermal treatment at 100 °C for 24 h. Powder product was obtained by centrifugation and washed with deionized water several times after the autoclave was taken out. Then, the powder was ultrasonic dissolved in 20 mL of deionized water for 10 min to ensure good dispersion. Subsequently, 0.1 M hydrochloric acid was slowly added into the solution until its pH value reached 3 before negatively charged GO suspension (2 mg/mL)

Figure 1. XRD patterns of the α-Fe2O3 and α-Fe2O3/rGO (2:1) powder. 5068

DOI: 10.1021/acssuschemeng.7b00445 ACS Sustainable Chem. Eng. 2017, 5, 5067−5074

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ACS Sustainable Chemistry & Engineering

Figure 2. SEM images of the (a) α-Fe2O3 and (b) α-Fe2O3/rGO (2:1) powder; (c) TEM and (d) HRTEM images of the α-Fe2O3/rGO (2:1) powder.

being composited with GO, all the diffraction peaks still belonged to α-Fe2O3, and no typical diffraction peaks aroused from GO were observed, shown as the red curve. However, the relative intensity of the peak at 24° increased, which could be attributed to the overlap of the corresponding diffraction peaks of α-Fe2O3 and rGO at 24°,48 indicating a reduced process of GO through hydrothermal treatment. Besides, XPS measurement of to the composite also confirms the formation of Fe2O3 (Figure S1a),49 while C 1s spectrum (Figure S1b) demonstrates the existence of some C−OH and CO which are the possible interaction sites of α-Fe2O3 and rGO. To confirm the microstructure of the obtained sample, SEM and TEM were employed. SEM images of the α-Fe2O3 samples without and with rGO are shown in Figure 2a,b, respectively. As exhibited in Figure 2a, large-scale thumb-ring-like α-Fe2O3 with uniform size and morphology was obtained, with outer diameters of ∼150 nm, inner diameters of ∼80 nm, and heights of ∼100 nm. This kind of hollow structure with open ends will be of benefit for the fast cations’ transportation and mitigate the volume expansion during charging/discharging. Through a modulation process to surface charges, α-Fe2O3 was well-mixed with GO which was reduced to be rGO via a hydrothermal treatment, presented in Figure 2b,c. It can be seen that α-Fe2O3 still retained nanoring structures after 2 rounds of hydrothermal treatment and were uniformly adhered to the surface of rGO. The high-resolution TEM image (Figure 2d) with a clear lattice structure that can be indexed to the (110) plane of α-Fe2O3

further confirms the phase of the ring-like structure, while the thin pieces around it can be indexed to rGO with an interlayer distance of about 0.34 nm. To realize optimum use of α-Fe2O3 and achieve good electrochemical performance, the balance of the mass ratio of α-Fe2O3 and rGO in the composite is critical, and the composite electrode with a mass ratio of 2:1 (α-Fe2O3: rGO) emerged as having the best electrochemical performance (Figure S2). Therefore, electrochemical behaviors of the αFe2O3/rGO (2:1) electrode were further monitored by CV and GCD in detail. For the initial CV curves (Figure S3), there are weak redox pairs, and obvious irreversible capacities that may be concerned with irreversible reaction of Fe(III) to Fe(II). After several cycles of activity at 5 mV s−1, the CV shapes changed to be rectangular and the irreversible parts almost disappeared. As shown in Figure 3a, after activity the CV curves of the α-Fe2O3/rGO composite (2:1) at different sweep rates from 5 to 200 mV s−1 exhibit rectangular shapes in a negative potential window from −1 to 0.2 V vs Ag/AgCl, signifying an ideal capacitive response. Meanwhile, there was no obvious voltage drop from the GCD curves (Figure 3b) at various discharge current densities, indicating low resistance and fast charge/discharge ability of the electrode. The corresponding specific capacities calculated from the CV and GCD results were also shown here in Figure 3c (detailed calculation method was described in the Supporting Information and shown in Figure S4). Specific capacitance (Csp) of the α-Fe2O3/rGO 5069

DOI: 10.1021/acssuschemeng.7b00445 ACS Sustainable Chem. Eng. 2017, 5, 5067−5074

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. (a) CV and (b) GCD curves of the activated α-Fe2O3/rGO electrode (2:1) at different scan rates and current densities with a potential window from 0.2 to −1 V (vs Ag/AgCl) in 1 M Na2SO4; (c) corresponding specific capacitances were calculated based on CV (black) and GCD (red) curves. (d) Cycle performance of the α-Fe2O3/rGO electrode at a current density of 10 A g−1 and the α-Fe2O3 electrode at a scan rate of 50 mV s−1.

electrode can reach 174 F g−1 at a scan rate of 5 mV s−1 and 255 F g−1 at a current density of 0.5 A g−1. Even when the scan rate was increased to 200 from 5 mV s−1, Csp was still as high as 131 F g−1, and more than 75% was retained. However, Csp (the red curve) was 132 F g−1 at a high discharge current density of 50 A g−1. There was not much drop at a high discharge rate, implying high power density of the composite electrode (a high energy density of ∼51 Wh kg−1 at the power density of ∼0.3 kW kg−1 and still high as ∼29 Wh kg−1 at ∼6 kW kg−1, Figure S5). To explore the energy storage behavior of pure α-Fe2O3, the sample was tested at different potential windows, as shown in Figure S6. The CV area enclosed is very small when scanned in a potential range below −1 V, corresponding to low capacity. However, when the potential window was extended to a more negative voltage (−1.4 V), the CV area was largely enhanced, and even obvious redox pairs can be observed, indicating high capacity. However, though the capacity will be largely enhanced when the voltage goes to more negative, the cycling stability will drop dramatically even when composited with graphene (behavior similar to that in alkali electrolyte, Figure S7). Hence, a suitable cutoff voltage should be used to realize long cycling life, a very important characteristic for a supercapacitive electrode. Electrochemical performance of pure α-Fe2O3 with a scanning window of −0.8 to 0 V was monitored. Its CV curves exhibited semirectangular shapes, and GCD curves emerged with high asymmetry (see Figure S8). The corresponding specific capacitances of the α-Fe2O3/rGO composites are much higher than that of the pure α-Fe2O3 electrode, which could be ascribed to the activity Faradaic reactions of α-Fe2O3 in the composite due to the formation of new defect energy lever during the composite process and the interaction between α-Fe2O3 and rGO. Furthermore, cycling

stability is also a vital parameter of supercapacitors for practical applications. Figure 3d exhibits long-term cycling performance of the α-Fe2O3/rGO (2:1) composite at a current density of 10 A g−1 in the potential window ranging from 0.2 to −1 V with 11 000 cycles and that of the pure α-Fe2O3 electrode at a scan rate of 50 mV s−1 from 0 to −0.8 V with 5000 cycles. The specific capacitance of α-Fe2O3 declined rapidly with less than 50% left after 5000 cycles. On the contrary, α-Fe2O3/rGO composite electrode exhibited much better cycling stability: The performance dropped sharply (about 5%) during the first 50 cycles; after that, there was only about 5% loss in the subsequent 1000 cycles, then a little gain, and finally barely any drop in the following 10 000 cycles, which can be attributed to an initial activation process for better contact of rGO and αFe2O3 and then a stable well-wrapped process. Before cycling, α-Fe2O3 was adhered to the surface of rGO, and rGO was made up of large blocks and highly stacked (Figure 2b). Along with cycling, under the action of electrostatic force own to the charge aggregation, prior stacked rGO with weak interaction and abundant surface groups could be easily reassembled, thinned, and even encapsulated the α-Fe2O3 particles gradually. The great improvement of the electrochemical performance should be addressed to the compounding of α-Fe2O3 with rGO which holds high conductivity and may induce new defects during the composite process. In addition, an asymmetric supercapacitor has also been assembled using the α-Fe2O3/rGO as anode and MnO250 as cathode (Figure S9). The asymmetric supercapacitor demonstrates a high working voltage of 2 V, an excellent cycling stability with 90% retention of the initial capacitance after 7000 cycles at 4 A g−1, and high specific capacitance of about 60 F g−1 at a scan rate of 5 mV s−1 based on total active mass of the two electrodes. 5070

DOI: 10.1021/acssuschemeng.7b00445 ACS Sustainable Chem. Eng. 2017, 5, 5067−5074

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ACS Sustainable Chemistry & Engineering

Figure 4. (a) XRD and (b) Raman spectra of the α-Fe2O3/rGO (2:1) electrode before and after cycling.

After activity cycling, the composite electrode exhibited high capacity and excellent stability. To figure out what happened during the cycling, XRD and Raman measurements were employed to characterize the possible chemical composition changes of the α-Fe2O3/rGO composite before and after cycling. After long time cycling, no obvious change can be detected from the XRD pattern except for the peaks from carbon current collector (PDF no. 26−1080, JCPDS, [1988]), shown in Figure 4a. Three typical features for carbon-based material can be detected in Raman spectra (Figure 4b) of the cycled sample was used after being carefully peeled off from the carbon current collector: the G band (1580 cm−1) that corresponded to the first-order scattering of the E2g mode of C sp2 atoms, the D band (1350 cm−1) that was attributed to edge planes and disordered structures, and its overtone band of G′ (also called 2D in some literatures).51−53 Compared with the as-prepared sample, the G and D bands of the cycled composite red-shifted to 1580 and 1349 cm−1, respectively. Additionally, the area ratio of the D band to G band (AD/AG = 1.58) for the long-term cycled α-Fe2O3/rGO composite is much lower than that before cycling (AD/AG = 2.02), indicating that the degree of disorder and defect sites decreased after longterm cycling. Moreover, after cycling, the G′ band was much enhanced, and the relative intensity ratio (IG/IG′ = 4.78) obviously decreased compared with that before cycling (IG/IG′ = 10.11), implying a possible change of the rGO stacklayers.53,54 To further investigate the evolution of morphologies and structures of the electrode after long time cycling, SEM and TEM analysis were carried out, as shown in Figure 5. The large rGO blocks disappeared, and only uniform particles without obvious agglomeration were left, as shown in the SEM image of Figure 5a. When seen closely (Figure 5b), it can be found that the particles were near-spherical with a diameter of about 100 nm and were surrounded by some light and thin materials, which were speculated to be α-Fe2O3 nanoparticles wrapped by rGO. Superimposed elemental mapping images of the aftercycling composite peeled off from the current collector has also been captured, as shown in Figure S10. Carbon signal from rGO and oxygen and iron signals from α-Fe2O3 were overlapped uniformly across the entire sample. The red color representing C is likely on the surface of the green particles representing Fe, indicating a uniform rGO coating on the αFe2O3 surface. To confirm this, HRTEM was employed. The red arrow in Figure 5c exhibits a nanocrystal with an interplanar spacing of 0.371 nm, which can be assigned to the (012) crystal plane of α-Fe2O3, while the interlayer spacing of 0.348 nm (the

Figure 5. SEM and HRTEM images of the α-Fe2O3/rGO electrode (2:1) after long time cycling. (a) SEM and (b) TEM images implying the solid particles were surrounded by something; (c) HRTEM image, indicating a nanocrystal with an interplanar spacing of 0.371 nm that can be assigned to (012) crystal plane of α-Fe2O3 and an interlayer spacing of 0.348 nm corresponding to rGO.

white arrows in Figure 5c corresponds to rGO. Hence, it can be confirmed that the near-spherical particles and light and thin materials in Figure 5b were α-Fe2O3 and rGO, respectively. Along the red dashed line, it can be seen that the α-Fe2O3 crystal was well-covered by discontinuous rGO with a thickness of about 5−9 layers. From the SEM and TEM results, it can be seen that the initial ring-like hollow structure of α-Fe2O3 was collapsed to be near-spherical particles and the large rGO blocks were reassembled to be thinner layers and well-covered the surface of α-Fe2O3 during charge/discharge cycling. (A schematic diagram to illustrate this structural evolution is given in Figure S11.) Though the energy storage mechanism of αFe2O3 is unclear, so that the surface adsorption/desorption of sulfate ions and the identity of the charge cations and sulfate ions in the redox reaction are still undetermined,55,56 it is shown that α-Fe2O3 were involved into the energy storage process speculated from these morphologies change. After being wrapped by rGO, the conductivity improved, and the αFe2O3 particles were well-protected for agglomeration, which 5071

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should be responsible for its good power density and excellent cycle stability discussed above. The resistance change of the composite electrodes after cycling was also carried out in a frequency range from 100 kHz to 0.1 Hz at open circuit potential using perturbation amplitude of 5 mV. As shown in Figure 6, Nyquist plots of the before- and

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00445. XPS results and TEM image of the α-Fe2O3/rGO (2:1) composite electrode; electrochemical performance of the α-Fe2O3/rGO composites and pure α-Fe2O3 electrodes; electrochemical behavior of the α-Fe2O3/rGO//MnO2 asymmetric supercapacitor; SEM image and elemental mapping image accordingly, for the α-Fe2O3/rGO (2:1) composite electrode after long time cycling; and calculation methods (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] Tel./Fax: + 86-020-39380525. *E-mail: [email protected] Tel.: 404-894-6114. Fax: 404-894-9140. ORCID Figure 6. Nyquist plots of α-Fe2O3/rGO electrode before and after 10 000 cycles at a frequency range from 100 kHz to 0.1 Hz. Inset is the electrical equivalent circuit used for fitting impedance spectra.

Shuang Cheng: 0000-0001-6301-175X Weijia Zhou: 0000-0003-4339-0435 Meilin Liu: 0000-0002-6188-2372

after-cycling samples were analyzed by fitting the equivalent electrical circuit using the software of ZSimpWin, which consists of the equivalent series resistance (Rs), charge transfer resistance (Rct), the double-layer capacitance (Cdl), Warburg element (W), and the limit capacitance (CL).6 The Rs of the electrode can be obtained from the real axis (Z′) intercept of the Nyquist plot, and its value (2.5 Ω) had almost no change before and after cycling. At high frequencies, the semicircle represents the charge transfer resistance at the electrode/ electrolyte interface. After cycling, the calculated charge transfer resistance was slightly increased to 1.12 from 0.39 Ω, which is consistent with the SEM and HRTEM results showing that αFe2O3 was fully covered by rGO which can prevent the electrolyte ions’ transmission to the surface of α-Fe2O3.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The research was supported by National Science Foundation for Young Scientists of China (no. 21403073), the Fundamental Research Funds for Central Universities of SCUT, China (no. 2015ZZ118), and Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200).



REFERENCES

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CONCLUSIONS α-Fe2O3/rGO composites served as anode material for supercapacitor have been fabricated which exhibited good electrochemical performance with high specific capacitance of 255 F g−1 at 0.5 A g−1, excellent rate capability, and good cycling life (about 10% decay in the first 1000 cycles and then barely any drop during the following 10 000 cycles) at a rather wide potential window (1.2 V) in neutral aqueous electrolyte. Moreover, the morphology and structure changes before and after cycling were exposed by SEM, HRTEM, and Raman to explore cycling behavior. First, α-Fe2O3 with ring-like hollow structure was just mixed with mass rGO. During charge/ discharge cycling, it gradually began to be wrapped by rGO. After long-term cycling, no obvious rGO blocks can be found, and the α-Fe2O3 nanorings collapsed into nanoparticles and were wrapped well by rGO, which prevented the mass loss of αFe2O3 and the agglomeration of both α-Fe2O3 and rGO. Though the energy storage behavior of α-Fe2O3 is unclear, it can be concluded that α-Fe2O3 is involved in the energy storage process via Faradaic reaction. This work indicates that αFe2O3/rGO composite is a promising candidate material as a negative electrode in asymmetric aqueous supercapacitor for practical applications. 5072

DOI: 10.1021/acssuschemeng.7b00445 ACS Sustainable Chem. Eng. 2017, 5, 5067−5074

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DOI: 10.1021/acssuschemeng.7b00445 ACS Sustainable Chem. Eng. 2017, 5, 5067−5074