rGO Paper as an Effective

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Hybrid Fe2O3 nanoparticle clusters/rGO Paper as an Effective Negative Electrode for Flexible Supercapacitors Yating Hu, Cao Guan, Qingqing Ke, Zhen Feng Yow, Chuanwei Cheng, and John Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02585 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Chemistry of Materials

Hybrid Fe2O3 nanoparticle clusters/rGO Paper as an Effective Negative Electrode for Flexible Supercapacitors ‡ † Yating Hu,† Cao Guan,† Qingqing Ke, Zhen Feng Yow, Chuanwei Cheng,§ John Wang*, †

†

Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, 117576, Singapore ‡

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634 §

Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, P.R. China

ABSTRACT: Flexible asymmetric supercapacitors (ASCs) are promising energy storage devices as they provide much higher energy density and wider applications than symmetric or non-flexible supercapacitors. While various positive electrode materials have been demonstrated with good performances, suitable and matching negative electrode materials are still desperately in need. In this work, Fe2O3 nanoparticle clusters are rationally coupled with reduced graphene oxide (rGO) sheets, leading to a highly flexible thin paper with excellent mechanical stability and electrical conductivity. The well dispersed and porous Fe2O3 nanoparticle clusters are composed of much smaller nanoparticles (~30 nm), which provide a large electrode-electrolyte interface. When tested in 3 M KOH aqueous electrolyte, the hybrid Fe2O3 nanoparticle clusters/rGO paper demonstrates much improved volumetric capacitance in the negative voltage range, compared to pristine rGO paper (178.3 compared to 106.2 F cm-3 at 1 mV s-1 scan rate in cyclic voltammetry test). In addition, when assembled into ASC, both a high volumetric energy and a power density are obtained (0.056 Wh cm-3 and 6.21 W cm-3), which are much higher compared with that of the ASC used rGO paper as the negative electrode. The systematic investigations in understanding the synergistic effects of electrochemical behavior between the Fe2O3 nanoparticle clusters and rGO paper provide insights to future hybrid-type supercapacitor design.

Asymmetric supercapacitors (ASCs) have been found to be an effective approach to meet the ever-increasing demand of high energy density supercapacitors, through providing larger voltage window.5, 6 ASCs make use of two electrochemical materials that have bridging, yet very different voltage windows, e.g., combination of a carbonbased material as negative electrode (voltage window from -1.0 V to 0 V) with a pseudocapacitive MnO2-based positive electrode (voltage window from 0 to 1 V) can extend the potential window to up to 2 V in aqueous electrolytes.6, 7 There are several types of materials that have been studied for positive electrode

INTRODUCTION

Supercapacitors have been studied extensively and their performances have been improved greatly over the past decade.1, 2 They can store up to three orders of magnitude greater charge than conventional capacitors. However, to meet the future energy storage demand, development for supercapacitors of high energy density with their high power density and long cycling stability retained are highly desirable. As the energy density is proportional to operative voltage window of the device, one apparent and effective way to boost energy density is to increase the voltage window.3, 4

1 ACS Paragon Plus Environment


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materials, mainly transition metal oxides/hydroxides or their hybrid materials, such as RuO2,8 MnO2,9 CoO,10 and Ni(OH)2.11 However, there are less materials to choose from for negative electrodes. Currently ASC researches mainly make use of carbonaceous materials (porous carbons, carbon nanotubes and graphenes),12, 13 while very little researches has focused on metal oxides, such as V2O5 and MoO3 and their nanocomposites.3 Other than these materials, Fe2O3-based materials have also been studied, showing great electrochemical performance in the negative potential, in addition to their low cost and environmental friendliness.5,

. In this work, we have successfully devised a new strategy to enhance the volumetric capacitance for the negative electrode by developing a class of hybrid Fe2O3 nanoparticle clusters/rGO papers using a facile hydrothermal method and electrochemical reduction. The Fe2O3 nanoparticle clusters act as spacers to prevent the restacking of rGO sheets, as well as provide the desired pseudocapacitance for higher electrochemical performance. The hybrid structure gave rise to the high electrical conductivity and structural flexibility. Systematic electrochemical characterizations are carried out to understand the synergic effect of the Fe2O3 nanoparticle clusters and the rGO sheets in the hybrid paper. A flexible ASC is developed by coupling the hybrid Fe2O3 nanoparticle clusters/rGO paper with a Mn3O4-based flexible positive electrode, and the volumetric capacitance, energy and power density can be much improved compared to the performance by using conventional rGO paper as the negative electrode.17

14-17

To align with the huge market potential of portable and flexible devices, mechanically flexible energy storage devices are being extensively studied very recently.18-20 Flexible supercapacitors require both good electrochemical performance and integrated mechanical flexibility.21 One way of making the desired flexible electrodes is to make use of the flexible carbon material substrates, e.g., graphene or reduced graphene oxide (rGO) papers.22-24 The graphene or rGO could be assembled into light and flexible substrates with high mechanical strength to withstand bending, stretching, folding or rolling. In addition, they could deliver excellent electrical conductivity, which is much preferred for supercapacitors. Among the various synthesis methods for graphene, e.g, chemical vapor deposition (CVD) and epitaxial growth on electrically insulating surfaces such as silicon carbide (SiC),25, 26 the reduction of GO is facial and easy-to-scale up. Using GO suspension as the starting material, followed by reduction to restore the graphene structure, is a much more convenient way to obtain graphene than the CVD on metallic catalysts or the direct exfoliation of graphite to graphene.27 One convenient reduction method that has been recently investigated is the electrochemical reduction.17, 28 By adjusting the external power source, this method can effectively modify the electronic states by changing the Fermi energy level of the graphene (as working electrode).28 By applying an appropriate voltage (e.g. negative potential -1.5V), the energy barriers for oxygen functionalities is overcame. Thus, these electrochemically unstable oxygen groups are eliminated.

EXPERIMENTAL SECTION

Material synthesis. Pyrrole (≥98%), FeCl3 ( ≥99.99 %), Na2SO4 (≥99%), KOH (≥85%) and KBr (≥99%) were purchased from Sigma-Aldrich. Graphene oxide (GO) aqueous dispersion of 5 g L-1 was purchased from Graphene Laboratories Inc. Reference electrode (Ag/AgCl) was purchased from Bio-Logic EC-Lab®. Deionized water was used as aqueous solvent. Pyrrole was added into 2 M FeCl3 (aq) drop wise. The ratio of pyrrole to FeCl3 is 1 mol to 40 mol. A gel was then formed. The gel is washed three times with deionized water to remove the unreacted FeCl3, and polypyrrole formed from the redox reaction. The mixture was then stirred for at least 12 hours and undergone hydrothermal reaction of 195 ◦C for 12 hours. GO aqueous dispersion of 5 g L-1 was added into the thus-obtained Fe2O 3 nanoparticle clusters. The solution is stirred well and dispersed by 30 mins ultrasonic bath before undergoing vacuum filtration. For the GO paper, GO dispersion was diluted by distilled water (5 times dilution) and then undergone vacuum filtration. The papers thus obtained from vacuum filtration then undergo thermal treatment in N2 atmosphere at 150 ◦C for 30 mins to eliminate water and to enhance mechanical strength.

There have been considerable efforts on developing ASCs by using rGO paper or 3D graphene foam as the negative electrode material.17, 29, 30 However, due to the aggregation and restacking problems, the areal or volumetric capacitance of these carbon-based materials is rather limited, usually not more than 100 mF cm-

Electrochemical reduction. Electrochemical reduction of the GO or hybrid Fe2O3 nanoparticle clusters/GO paper as the working electrode was performed in 0.1 M Na2SO4 (aq) at the potential of –1.5 V for a few cycles, each cycle for 600 s. A Pt plate and Ag/AgCl electrode were used as the counter electrode and the reference electrode,


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respectively. The reduced paper was then washed with deionized water and dried.

PVA/KOH solid state electrolyte test. Scan rates of 1, 5, 10, 25, and 50 mV s−1 were used for CV, while the current densities of 1, 2, 5 and 10 A cm−3 were used for charge-discharge measurement.

Materials Characterizations. X-ray diffraction (XRD) patterns were collected on a Bruker AXS X-ray powder diffractometer (D8 Advance, Cu Kα, λ = 0.154 nm). X-ray photoelectron spectroscopy (XPS) studies were conducted with a Kratos Axis Ultra DLD photoelectron spectrometer. Transmission electron microscopy (JOEL 2010F, 200 kV) and field-emission scanning electron microscopy (SUPRA 40 ZEISS, Germany) were used to examine the morphology of all samples. Raman scattering spectra were recorded on a LABRAMHR Raman spectrometer excited with 514.5 nm Ar+ laser. Mass content were measured by thermal gravimetric analysis (SDT Q600). FT-IR spectra of the powder samples (prepared in KBr pellet) were obtained using a Varian 3100 Excalibur Series FTIR Spectrophotometer.

RESULTS AND DISCUSSION

Fe2O3/rGO characterizations. Firstly, Fe2O3 gels were obtained by a gel formation reaction, followed by the growth of crystalline Fe2O3 nanoparticle clusters during hydrothermal reaction and mixing with GO suspension. The mixture is then assembled into a hybrid paper through facile vacuum filtration. A series of hybrid Fe2O3 nanoparticle clusters/GO papers were electrochemically reduced into hybrid Fe2O3 nanoparticle clusters/rGO (FG) papers.17 Sample rGO is also electrochemically reduced from vacuum filtered GO paper for comparison purposes. The Fe2O3 nanoparticle clusters obtained from hydrothermal reaction contains almost no residuals, as shown from the thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR) results (Figure S1). Scanning electron microscopy (SEM) image shows the morphology of the hybrid FG paper, which is composed of numerous layers of rGO sheets (Figure 1a). The Fe2O3 nanoparticle clusters, which are about 500 nm in dimensions, uniformly lay among the individual rGO sheets, which is marked out in yellow (Figure 1a and 1b). The nanoparticle clusters are composed of smaller nanoparticles, which are of 30-40 nm in dimension (Figure 1c). Thus the surface of these Fe2O3 nanoparticle clusters are porous and penetrable by the electrolyte. Figure 1d shows the corresponding high resolution TEM image, demonstrating the presence of crystalline fringes, corresponding to the (0 1 2) plane and (1 0 4) plane of the hematite structure (α-Fe2O3, in accordance with the X-ray diffraction (XRD) results). By inserting Fe2O3 nanoparticle clusters among the rGO sheets, the nanoparticle clusters act as spacers, thus, eliminate the restacking problem and promote faster charge transfer.

Electrochemical Measurements. The reduced graphene oxide (rGO) and hybrid Fe2O3 nanoparticle clusters/rGO (FG) papers were characterized in standard Swagelok®2-electrode cell configuration in 3 M KOH aqueous electrolyte and the area used for all samples is 1 cm2. For ASC tests, a piece of Mn3O4/rGO (MG) paper was used as the positive electrode material and rGO paper or FG paper were used as the negative electrode material, respectively. The detailed synthesis method and full characterization as positive electrode material of this MG paper are described in our recently published work.17 After matching their capacitance (through adjusting the size of the MG, rGO or FG paper), the ASC were firstly characterized in the standard Swagelok®2electrode cell configuration in 3 M KOH aqueous electrolyte and then in PVA/KOH solid-state electrolyte to demonstrate its flexibility. To prepare PVA/KOH electrolyte, 6M KOH (aq) was added drop wisely into hot PVA solution (13 wt. % PVA solution at 80 °C,). The mixture is stirred and a thin layer is dispersed on glass to form the separator after solidification. The MG, rGO or FG papers were dipped into the hot PVA/KOH mixture to form a thin layer coving the electrodes. After solidification, the PVA/KOH covered electrodes and separator are then assembled together to form the free standing solid-state ASC. Cyclic Voltammetry (CV), Galvanostatic chargedischarge measurement and Electrical Impedance Spectroscopy (EIS) were conducted using Solartron System 1470E and 1400A, respectively. For rGO and FG papers, the measurement voltage range is controlled at 0 to 1.0 V for both CV and charge-discharge. For ASC, the measurement voltage was controlled in the range of 0.1 to 1.9 V for aqueous electrolyte test and 0 to 1.7 V for



correspond to Fe2O3.31 The XRD pattern of the hybrid FG paper (Figure 2b) shows that the Fe2O3 nanoparticle clusters retained their Hematite after electrochemical structure (α-Fe2O3) reduction (the XRD pattern for hybrid Fe2O3 nanoparticle clusters/GO paper is shown in Supporting information, Figure S2a). The broad peak at 2θ of ~26.6° indicates the formation of graphene, after the electrochemical reduction of GO.32 XPS was performed to confirm the chemical states of Fe and C in the hybrid FG paper and to evaluate the effectiveness of electrochemical reduction (Figure. 2c and 2d). The XPS spectrum of Fe 2p shows three strong peaks including Fe 2p3/2 at 711.7 eV, Fe 2p1/2 at 725.3 eV and a satellite at 719.1 eV, which is consistent with the presence of α-Fe2O3. In general, Fe2O3 exhibits an inverse spinel structure which has both Fe3+ and Fe2+ in its lattice. Hence, the two sub-peaks at 711 eV and 724.5 eV indicate that there is also a small amount of Fe2+ ions.33 The XPS spectrum of C 1s shows that after reduction, the -C-O- to -C-C ratio is largely reduced (as compared to the ones for the hybrid Fe2O3 nanoparticle clusters/GO paper, Figure S2b), indicating the elimination of oxygencontaining functional groups. The -C-C- peak becomes dominating, indicating that the GO is effectively reduced into graphene.7

Figure 1. (a) SEM image of the cross section view of hybrid FG paper; (b) Transmission electron microscopy (TEM) image of the Fe2O3 nanoparticle clusters in rGO sheet; (c) Higher magnification TEM image of Fe2O3 nanoparticle clusters, showing individual nanoparticles; (d) High resolution TEM image showing the lattice fringes.

The electrochemically reduced hybrid papers are further characterized using Raman, XRD and X-ray photoelectron spectroscopy (XPS) to identify the structures and chemical states. The Raman spectra shows the co-existence of graphene and Fe2O3 (Figure 2a). The set of six phonon modes at 225, 241, 291, 403, 493 and 610 cm-1

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Figure 2. (a) Raman spectra pattern of the hybrid FG paper; (b) XRD pattern of the hybrid FG paper; XPS spectra and peak fitting of the hybrid FG paper for (c) Fe 2p and (d) C 1s.


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Electrochemical characterizations of rGO and FG papers. Electrochemical characterizations were conducted for sample rGO and three different FG samples: hybrid FG paper with thickness of 30 µm and Fe2O3 nanoparticle clusters loading of 1.2 g cm-3 (sample FG-t1); FG paper with thickness of 20 µm and Fe2O3 loading of 1.2 g cm-3 (sample FG-t2) and FG paper with thickness of 20 um and high Fe2O3 loading of 2.5 g cm-3 (sample FG-HL). Thickness was estimated from crosssection SEM images (Figure S3) The samples were tested in standard Swagelok® 2-electrode cell configuration in 3 M KOH aqueous electrolyte. Figure 3(a) and (b) show the cyclic voltammetry (CV) curves for sample rGO and FG-t2, at scan rates of 1 mV s-1 to 50 mV s-1. Typical rectangular CV curves are observed for both sample rGO and FG-t2. The CV curves of the FG-t2 electrode remain the rectangular shape without distortion under all scan rates, indicating an ideal capacitive behavior. Rectangular or semi-rectangular CV curves for Fe2O3 materials in neutral aqueous electrolyte have been reported,5, 34, 35 as well as in alkaline electrolyte.36, 37 However, there are also some works that have reported Fe2O3 materials showing obvious peaks in both neutral and alkaline electrolyte.31, 38, 39 Although it is not clear

under what condition will Fe2O3 show CV curves without redox peaks, one suspects that it is due to the coexistence of Fe3O4 in some of these Fe2O3 materials that causes the electrode to have successive multiple surface redox reactions, which is similar to manganese oxide.40 To clarify, the CV and charge-discharge performances of pristine Fe2O3 nanoparticle clusters powders dipped on Ni foam is also tested in the same electrolyte, showing similar rectangular CV curve and symmetric charge-discharge curves, but slightly smaller voltage window (Figure S4). Thus, it is confirmed that the Fe2O3 nanoparticle clusters itself exhibits a rectangular CV curve in 3 M KOH aqueous electrolyte, in this case. Figure 3(c) and (d) show the charge-discharge curves for sample rGO and FG-t2 at the current density from 1 A cm-3 to 10 A cm-3. The charge-discharge curves for sample FG-t2 shows perfect symmetric straight lines, which results into high column efficiency of 98.1% at the scan rate of 10 A cm-3. This indicates the capability of the electrode for superior reversible redox reactions, even under fast chargedischarge.41 It is notable that by incorporating Fe2O3 nanoparticle clusters in, the shape of CV curves and charge-discharge did not change much.



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Figure 3. Electrochemical characterizations in 3 M KOH aqueous electrolyte: CV curves at various scan rates (1, 5, 10, 25, and 50 mV s-1) for (a) sample rGO, and (b) sample FG-t2; Charge-discharge curves under various current -3 densities (1, 2, 5and 10 A cm ) for (c) sample rGO, and (d) sample FG-t2; (e) CV curve comparison for sample rGO -1 and FG-t2 at scan rate of 10 mV s ; (f) Comparison of volumetric capacitance of sample rGO and FG-t2 at different scan rates.

In order to better understand the roles of Fe2O3 nanoparticle clusters and rGO in electrochemical process, systematic electrochemical impedance spectroscopy (EIS) studies are carried out. Figure 4(a) shows the Nyquist plots comparison of sample rGO and FGt2. It is obvious that the high frequency region of the Nyquist plot for sample rGO consists of only one semicircle as in Figure 4(b), while the one for sample FG-t2 has an elongated and distorted semicircle shape-curve (Figure 4(c)). The single semicircle observed in Nyquist plot for sample rGO indicates it is a pure EDLC type capacitor in which electrolyte only interact with rGO sheets (single type of interface).42 In the Series Layer Model for impedance spectroscopy,43 the impedance spectra for a two-phase mixture that

are assumed to be stacked in parallel layers consists of two semicircles, for example, those as shown in Figure 4(c). This two-phase mixture correlates to the hybrid structure of Fe2O3 nanoparticle clusters incorporated among rGO sheets, in which the electrolyte interacts not only with the rGO sheets, but also with the Fe2O3 nanoparticle clusters (two types of interfaces). Thus, sample FG-t2 is fitted using two ConstantPhase Elements (CPE), where CPE1 and CPE2 correspond to the lower frequency and higher frequency semicircles, respectively. Figure 4(d) shows the equivalent circuits for sample rGO and FG-t2, and the fitted CPE values of the two circuits. CPE is defined by CPE-T and CPE-P, in which CPE-P is determined by the slope of the straight line at the middle to low frequency range.


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A CPE-P’s value that is close or equals to 1 indicates an ideal capacitor system with negligible resistance of the capacitor. In this case, CPE-T’s value represents the capacitance value of the CPE.44 Nevertheless, the relative capacitance could be estimated from the following equation:

‫(ܴ = ܥ‬

భష೙ ) ೙

భ

‫ ܧܲܥ‬ቀ೙ቁ

equivalent circuit (using Equation 1 and values from Figure 4d). As has been mentioned, the two semicirles of sample FG-t2’s impedance profile are coresponding to different interfaces. Through comparison with impedance profile of pristine Fe2O3 nanoparticle clusters, one can conclude that, the higher frequency semicirle of sample FGt2 arises from the interface of electrolyte and Fe2O3 nano-clusters. Similarly, through comparions between the impedance profile of sample FG-t2 and sample rGO, it is apparent that the lower frequency one arises from the interface of electrolyte and rGO sheets. Thus, it is further confirmed that the incorpration of Fe2O3 nanoparticle clusters does not affect the electrical conductivity of the rGO sheets and the overall impedance profile significantly, but only has a small contribution as the electrolyte-electrode (e.g. Fe2O3 nanoparticle clusters) interfacial resistance which is evidenced in the higher frequency semicircle shown in Fig 4(c).

(1)

where R is the resistance parallel to the CPE, n is the value of CPE-P and CPE is the value of CPET.45 In the fitting results, the CPE-P’s values for both samples are quite high (from 0.67 to 0.87), thus we can conclude that sample rGO and FG-t2 are both highly capacitive. When the Fe2O3 nanoparticle clusters are tested alone, there is only one semicircle (Figure S5a) which has same size and close CPE-T value with the ones for CPE2 of sample FG-t2. In addition, the capacitance estimated for sample rGO’s equivalent circuit has a very close value with the capacitance corresponding to CPE1 in sample FG-t2’s

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Figure 4. (a) Nyquist plots comparison of sample rGO and FG-t2; High frequency parts of Nyquist plots for (b) sample rGO and (c) sample FG-t2; (d) equivalent circuits and fitting results for sample rGO and FG-t2.

Figure 3(e) shows the CV curves comparison for sample rGO and FG-t2 under 10 mV s-1 scan rate. Figure 3(f) shows the volumetric capacitance calculated from different scan rates of CV tests. It is obvious that sample FG-t2 has a significant improvement in volumetric capacitance as compared to sample rGO (178.3 compared to 106.2 F cm-3 at 1 mV s-1 scan rate). The volumetric capacitance for different FG samples are plotted in Figure S5(b). Sample FG-t1 has the same Fe2O3 nanoparticle clusters loading amount per unit volume with sample FG-t2, but a higher thickness (30 µm vs. 20 µm). Although their capacitance

values at low scan rate are very close (171.5 and 178.3 F cm-3 at 1 mV s-1 scan rate), the capacitance of sample FG-t1 drops faster than that of sample FG-t2 when the scan rate increases from 1 mV s-1 to 50 mV s-1 (54.8% compared to 69.4% retained). Impedance test shows that FG-t1 has smaller slope in low frequency region of the nyquist plot, indicating a higher ion diffusion resistance than that of FG-t2 (Figure S5c),46 which could be well due to less effective reduction when the paper thickness becomes higher. Sample FG-HL shows the lowest capacitance and a much lower conductivity than that of sample FG-t1 and FG-t2



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(Figure S5b and 5c), indicates that too high the loading amount of Fe2O3 nanoparticle clusters hinders the conductivity of the hybrid paper and reduces the capacitance (Figure S3c and S3d shows the cross section SEM images of sample FG-HL). Sample FG-t2 with a suitable amount of loading and thickness shows much improved capacitance and retain rate compared to sample rGO. The improved capacitance is therefore arising from the additional pseudocapacitanc contributed by Fe2O3 nanoparticle clusters and the high electrical conductivity retained by the optimized structure. In addition, Fe2O3 nanoparticle clusters acting as spacers among rGO sheets have hindered the restacking problem, making the hybrid paper more suitable for high scan rates (69.4% compares to 55.0% retained from 1 mV s-1 to 50 mV s-1).

PVA/KOH electrolyte is lower than that in aqueous electrolyte (Figure S6c). In addition, the voltage window in PVA/KOH solid state electrolyte is lower than that in 3 M KOH aqueous electrolyte e.g. 1.7 vs. 1.8 V (Figure S6a and 6b shows the CV curves under 10mV s-1 scan rate in different electrolytes). Therefore, future optimization of the solid state electrolyte is needed in order to improve further for applications (e.g. the conductivity of the PVA/KOH solid state electrolyte could well be improved by optimizing the ratio of PVA to KOH and the preparation method of the ASC). The free standing rGO paper eliminates the necessity of using a substrate. Comparing with carbon clothe,5 Ni foam47 and stainless steel substrate,48 rGO paper based hybrid electrode has greatly reduced the thickness of the electrode from a few hundred µm to ~20 µm, therefore greatly increased the volumetric capacitance, energy and power density. The volumetric energy and power densities for ASC MG/rGO and ASC MG/FG-t2 measured in 3 M KOH aqueous electrolyte and PVA/KOH solid state electrolyte are calculated based on the volume including positive electrode, negative electrode and the separator. These values are plotted in the Ragone plot in Figure 5(f) together with the values from other recent reports of supercapacitors (most works used MnOx based positive electrode materials, as in this work).49-56 The highest volumetric energy and power density obtained with ASC MG/FG-t2 in 3 M KOH aqueous electrolyte are 0.056 Wh cm-3 (at the corresponding power density of 0.32 W cm-3) and 6.21 W cm-3 (at the corresponding energy density of 0.020 Wh cm-3), respectively. When the power density is increased by almost 10 fold, 50% of the energy density is still retained. By introducing the pseudocapacitive Fe2O3 nanoparticle clusters, which also act as spacers, into the rGO paper, the hybrid FG paper provides higher energy and power than rGO paper within the same volume of the electrode, thus has improved the volumetric energy and power density.

Assembly of Asymmetric Supercapacitor and Electrochemical Tests. Upon optimizing the paper thickness and Fe2O3 nanoparticle clusters’ loading amount of the FG papers, an ASC is assembled using hybrid FG or rGO paper as the negative electrode and hybrid Mn3O4/rGO (MG) hybrid paper as the positive electrode.17 The ASC is firstly tested in 3 M KOH aqueous electrolyte for electrochemical characterizations. Figure 5(a) and (b) show the CV curves and charge-discharge curves of ASC MG/FG-t2, with a voltage window of 1.8 V. The ASC MG/FG-t2 shows a much higher volumetric capacitance than that of ASC MG/rGO, and a cycling retention of 83.1% after 10 000 cycles charge-discharge, under 1 A cm-3 current density (Figure 5c). The ASC is then assembled in solid state supercapacitor using PVA/KOH electrolyte to demonstrate its mechanical flexibility and strength to withstand bending and twisting even during charging-discharging (Figure 5d and insert of Figure S6b). Figure 5(e) shows the schematic illustration of the flexible ASC device based on hybrid MG paper as the positive electrode and hybrid FG paper as the negative electrode in PVA/KOH solid state electrolyte. The solid state ASC also shows high cycle ability (Figure S6d). However, due to lower electrical conductivity of the PVA/KOH electrolyte, the volumetric capacitance of the ASC MG/FG-t2 tested in


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Figure 5. (a) CV curves of ASC MG/FG-t2 at different scan rates, and (b) Charge discharge curves at different current -3 densities in 3 M KOH aqueous electrolyte; (c) Cycling ability results at 1 A cm of charge-discharge test for ASC MG/rGO and MG/FG-t2 in 3 M KOH aqueous electrolyte; (d) Picture of ASC MG/FG-t2 assembled in PVA/KOH solid state electrolyte being slightly twisted during electrochemical testing; (e) Schematic illustration of the flexible ASC device based on hybrid MG paper as the positive electrode and hybrid FG paper as the negative electrode in PVA/KOH solid state electrolyte. (f) Ragone plot of the ASC devices tested in 3 M KOH aqueous electrolyte and PVA/KOH solid state electrolyte and the values reported for other supercapacitor devices are compared within the graph.

CONCLUSION

nanoparticle clusters provide high pseudocapacitance without compromising the high conductivity of the overall hybrid paper. These studies provide new insights into future design of the hybrid-type of electrode materials for energy storage.

In summary, a highly flexible hybrid Fe2O3 nanoparticle clusters/rGO paper is designed and prepared by a convenient gel formation reaction followed by hydrothermal growth, vacuum filtration and electrochemical reduction methods, which is scalable and applicable to other hybrid type of electrode materials. Compared with the pristine rGO paper, the hybrid FG paper exhibits excellent volumetric capacitance, and energy and power density when assembled into ASC with the same positive electrode material. A high energy density of 0.056 Wh cm-3 is obtained at the corresponding power density of 0.32 W cm-3, making the hybrid FG paper a promising negative electrode material candidate for the flexible solid state ASC. Additionally, the electrochemical studies of the hybrid paper has been thoroughly conducted and revealed the key roles played by both Fe2O3 nanoparticle clusters and rGO sheets during electrochemical process. In the structural optimized hybrid paper, the rGO sheets act as conductive, yet flexible host and the Fe2O3

ASSOCIATED CONTENT Supporting Information Additional SEM images, XPS, TGA, FT-IR results and detailed CV data and Nyquist plots of rGO and FG papers. AUTHOR INFORMATION Corresponding Author *Phone: +65 65163325. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by MOE, Singapore Ministry of Education (Tier 2, MOE2012-T2-2-102), conducted at the National University of Singapore.



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TOC figure


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rGO Paper as an Effective

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