Designed Construction of a Graphene and Iron Oxide Freestanding

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Designed construction of Graphene and Iron Oxide Freestanding Electrode with Enhanced Flexible Energy Storage Performance Meng Li, Feng Pan, Eugene Shi Guang Choo, Yunbo Lv, Yu Chen, and Jun Min Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10853 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on February 29, 2016

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ACS Applied Materials & Interfaces

Designed construction of Graphene and Iron Oxide Freestanding Electrode with Enhanced Flexible Energy Storage Performance Meng Li1, Feng Pan1, Eugene Shi Guang Choo2, Yunbo Lv1,Yu Chen1 and Junmin Xue1* 1

Department of Materials Science and Engineering, National University of Singapore,

Singapore, 117573 2

Carl Zeiss Pte. Ltd., Microscopy Business Group, Singapore, 415926

* Corresponding author: Tel: +65 65164655 Email address: [email protected] (Dr J.M. Xue)

Keywords: Graphene, Iron Oxide, Freestanding film, Hybrid supercapacitor, Flexible Device.

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ABSTRACT In this work, bendable graphene@iron oxide hybrid film (GFeF) electrode was fabricated through filtration-assited self-assembly method. The morphological characterizations of GFeF revealed uniform distribution of iron oxide nanoparticles between graphene nanosheets. The surface chemical characterizations confirmed that graphene oxide in the as-prepared hybrid film was effectively reduced after thermal reduction. The electrochemical performance of GFeF half-cell vs. Li/Li+ exhibited high gravimetric capacity (855.2 mAh g-1 at 0.02 A g-1), high volumetric capacity (1949.9 mAh cm-3 at 0.02 A g-1) and superior cycling stability (93% capacitance retention after 500 cycles). Based on such bendable electrode, hybrid Li-ion supercapacitor that offer operation voltage of 3.5 V and deliver high energy density (129.6 Wh kg-1) like a Li-ion battery combined with high power density (1870 W kg-1) like a supercapacitor was fabricated. In addition to the superior energy storage capability, the asfabricated prototype pouch-cell also exhibited excellent mecahnical flexiblity and stable electrochemical performances under dynamic bending. The viability of such energy storage device provides a possible design pathway for future wearable electronics.


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INTRODUCTION The emergence of wearable electronic devices requires their batteries to not only be charged within a short period but also with new feature of flexibility. Such devices include nextgeneration display1, energy-harvesting devices2-3, sensors4-5, light-emitting diodes6, and circuits7-8. In order to power these wearable electronics, new flexible energy storage devices are being developed9-11. Bendable freestanding films, as one example of flexible electrodes, have attracted considerable amount of research in the field of flexible energy storage devices due to their dense structure without the need for added binders and conductive additives, while achieving high volumetric capacity12-14. Graphene is thought as one of the most suitable candidate to fabricate such flexible film electrodes because of its numorous advanced properties15-17. However, there remains a few challenges that restrict the further development of fabricating high performance flexible film electrodes by using graphene. The greatest one lies in the restacking of graphene in fabrication process due to the

stacking and strong

van der Waals attraction, where the advantages of individual graphene sheets, such as high flexibility and surface area, are lost. In an effort to address this challenge, we recently developed a 3D expressway-like film electrode with optimized composite structure of graphene and “hard nano-spacer” via an extended filtration assisted self-assembly method17. In this film electrode, graphene sheets were densely packed, and Ni(OH)2 nanoplates were intercalated in between the graphene sheets. Although such single electrode offered the impressive gravimetric and volumetric capacitances of ~ 573 F g-1 and ~ 655 F cm-3 in aqueous electrolyte, the energy density (~18 Wh kg-1) of the flexible supercapacitor on the basis of this film electrode was poor due to the narrow potential window of 1.6 V. According to the equation of energy density (E): E=1/2·CV2 (where C is the total capacitance of the cell, and V is the cell voltage), increasing the cell voltage is more effective to enhance the energy density because of the quadratic dependence of the voltage18-19.



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To address the aforementioned drawback of narrow potential for aqueous-based asymmetric supercapacitor, hybrid electrochemical capacitor (HEC) coupling an electrochemical double layer capacitor (EDLC)-type electrode and a Li-ion battery-type electrode have been designed and fabricated by employing Li+ containing organic solvent as electrolyte20. HECs offer high potential windows up to 4 V and ultimately result in higher energy density without sacrificing power density. Such hybrid approach also overcomes the capacitive limitation of carbonaneous electrodes in traditional EDLCs due to the employment of battery-type electrode at one side. As a result, a variety of metal oxides, such as titanium oxide21-24, iron oxide25-28, cobalt oxide29-30, tin oxide31-32 and vanadium oxide33 were employed as a batterytype electrode in HECs in the view of their relatively large theoretical capacities and low potentials. Based on these battery-type electrodes, a series of Li-ion HECs with high energy densities were fabricated: Fe3O4@Graphene//3DGraphene hybrid supercapacitor with ultrahigh energy density of 147 Wh kg-1 at power density of 150 W kg-1;26 TiO2//Ordered Mesoporous Carbon hybrid supercapacitor with energy density of 25 Wh kg-1 at power density of 3000 W kg-1.22 However, none of these studies have attempted to develop flexible HECs with both high gravimetric and volumetric performance. Herein we report the fabrication of bendable freestanding electrode with optimized structure of graphene and iron oxide for the application of flexible HEC. In the designed construction of this freestanding electrode (Figure 1a), graphene sheets were densely packed by layers, and iron oxide nanoparticles were intercalated in between the graphene intersheet. Compared with other metal oxide aforementioned, iron oxide is one of the best and chosen here in terms of three reasons: i) high theoretical capacity of 924 mAh g-1 and relatively low potential of ~ 0.8 V vs. Li/Li+;34-35 ii) easy fabrication process of iron oxide nanoparticles with readily controllable particle size and morphology;28, 36

37-39

iii) natural abundance and compatibility with overall

synthesis. Asymmetric configuration was adopted in the designed HEC. In the asymmetric configuration, composite film electrode of graphene@iron oxide was used as negative 4 ACS Paragon Plus Environment

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electrode, and flexible aluminum foil coated by porous graphene was prepared as positive electrode. Such hybrid configuration is expected to combine the advantages of both EDLCs and Li-ion batteries. The designed HEC would offer an enhanced energy density and maintained power density as compared to traditional supercapacitor. EXPEFIMENTAL SECTION Preparation of Iron Oxide nanoparticles: In a typical synthesis, 0.8 ml of 0.5 M FeCl3·6H2O was dissolved in 58.84 ml distilled water to form a yellow transparent solution. 0.36 ml of 0.02 M NH4H2PO4 solution was subsequently added drop by drop under magnetic stirring condition. The mixed solution was then transferred into a 125 ml Teflon-lined autoclave, sealed and maintained at 220 °C for 48 h. After the heating completion and aircooling to room temperature, the red powder was collected and washed several times with distilled water. The as-obtained iron oxide nanoparticle solution was then sonicated using a bench-top ultrasonic homogenizer for 30 minutes prior to the freeze-drying. Finally, the powder of iron oxide nanoparticles was stored at room temperature. Preparation of graphene@iron oxide freestanding film: To prepare graphene@iron oxide hybrid films (GFeF), a homogeneous suspension of graphene oxide and iron oxide nanoparticles was carefully prepared. 25 ml of GO aqueous (1 mg/ml) was sonicated for 30 min to form a stable solution. Subsequently, 25 ml of iron oxide nanoparticle aqueous (1 mg/ml) was mixed with GO aqueous and sonicated for another half hour to form homogenous dispersion. The GO@iron oxide suspension was vacuum filtered through a mixed cellulose ester filter membrane (200 nm pore size) to obtain hybrid films. More experimental details are available in Supporting Information.



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RESULTS AND DISCUSSION

Figure 1: a) schematic illustrating 3D structure of GFeF; b~d) cross sectional SEM image of GFeF from high magnification to low magnification, inset demonstrating its flexibility; e) side view of the FIB etched GFeF, inset showing FIB notch; f) GFeF 3D imaging of FIB etched one cross-section slice. The morphology and structure characterizations of the as-prepared GFeF are shown in Figure 1. The film composed of graphene sheets and iron oxide nanoparticles is synthesized by an extended filtration assisted self-assembly method17. The SEM image in Figure S1 (Supporting Information) shows the as-synthesized bare iron oxide nanoparticles, were uniformly spherical morphology with an average diameter of 55±8 nm. The narrow size distribution of iron oxide nanoparticles guarantees a uniform intercalation into the dense graphene layers. The cross-sectional SEM image in Figure 1b of GFeF clearly shows its layered structure, and the closely packed graphene sheets became much more porous when more iron oxide nanoparticles were incorporated. The larger frame SEM image in Figure 1c directly shows the uniform distribution of the nanoparticles between 2D nanosheets. The lower magnified SEM 6 ACS Paragon Plus Environment

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image in Figure 1d indicates the thickness of the obtained film is around 6.6 µm. The inset photograph of GFeF shows its good flexibility. Moreover, the in-plane view of the film (Figure S2a) further indicates the uniform distribution of iron oxide nanoplates within the film. To more accurately observe the internal structure of the film, the FIB milling was performed with the notch drilled in the film of 20 µm in width (Figure 1e inset). The cross-sectional SEM image of the notch in Figure 1e shows the uniform distribution of iron oxide nanoparticles between the graphene layers, with the graphene layers being the “continue phase” acting as the electron transpoter. Such coninous graphene framework is very important for the conductivity of the hybrid films. Furthermore, 3D imaging of FIB in Figure 1f shows the inner structure in three dimensions (blue dots represent the iron oxide nanoparticles), clearly representing the uniform dispersion of iron oxide nanoparticles within the graphene stack. The complete 3D imaging aligned and reconstructed by 274 cross-section slices in Supplementary Movie 1 (Supporting Information) vividly displays the distribution and inner compositions of the film electrode. To confirm the crystal phase of iron oxide nanoparticles, the GFeF sample was studied using high-resolution transimission electron microscopy (HRTEM, Figure S2b) and X-ray diffraction (XRD, Figure S2c), where the peaks in XRD line profile could be indexed to the mixture of Fe2O3 (JCPDF: 33-0664) and Fe3O4 (JCPDF: 653107). One fringe with interlayer distance of about 0.25 nm and 0.29 nm were observed in HRTEM images of Figure S2b, which are corresponds to (110) planes of α-Fe2O3 and (220) planes of Fe3O4, respectively.



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Figure 2: a) FTIR spectra of hybrid films (GOFeF represents the GFeF sample without annealing treatment) and pristine GO; carbon core level XPS spectra of b) GOFeF sample and c) GFeF sample; d) pore-size distribution of GFeF. The evolution of the chemical compositions of the hybrid films were characterized by Fourier Transform Infrared Spectroscopy (FTIR), as shown in Figure 2a. For comparison, pristine graphene oxide (GO) was also tested. The FTIR result of GO exhibites typical peaks which are assigned to the functional groups of O-H bonding (ca. 3461 cm-1), C=O bonding (ca. 1735.6 cm-1), C=C bonding (ca. 1626.3 cm-1) and C-O bonding at the bands from ca. 1394.3 cm-1 to ca. 1080.9 cm-1. The sample of GOFeF (GFeF without annealing treatment) shows a similar absorption peaks, suggesting the self-assembly process does not involve any chemical reactions. In contrast, after annealing treatment in N2 environment at 300 oC, a majority of these functional groups in GFeF sample were eliminated, indicating the effecitve reduction of GO. The X-ray photoelectron spectroscopy (XPS) spectrums of GFeF and GOFeF are shown in Figure 2b and 2c,. In the sample of GOFeF (Figure 2b), its XPS spectrum of C 1s was 8 ACS Paragon Plus Environment

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deconvoluted into three peaks (colored lines) with different relative carbon contents: sp2 bonded carbon at 284.5 eV (C−C, 41%), hydroxyls at 286.5 eV (C−O, 46%), carbonyls at 288.1 eV (C=O, 13%)17, suggesting its high O percentage before recuction. In contrast, the CO and C=O peaks in GFeF sample were dramatically decreased to 13% and 6%, respectively, and the content of sp2 bonded carbon was increased to 81% (Figure 2c). It is concluded that GO in this composite is converted into reduced GO after thermal reduction. This conclusion was further confirmed by the resutls of film conductivity test. The conductivity of as-prepared hybrid films were measured by four-point probe method, and the results show the conductivity of GOFeF was around 0.5 S m-1 while GFeF was as high as 2264 S m-1. The surface area of GFeF was determined by isothermal N2 adsorption-desorption method with the value of 375 m2/g. The curve of pore size distribution in Figure 2d suggestes the presence of mesopores and macropores, and average pore size is around 33.2 nm.



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Figure 3: Electrochemical characterizations of GFeF based half-cell (voltage limits 0.01~3 V, vs. Li/Li+). a) CV profiles in the first cycle and reversible steady-state cycles measured at scan rate of 2 mV s-1; b) Galvanostatic charge-discharge curves at the current density of 0.02 A g-1 in the first 3 cycles. Inset shows the CD profiles at the stable cycles; c) Variation of capacities (based on the total mass of iron oxide and graphene) as a function of current density; d) Cyclability of the GFeF based half-cell. Inset showing the photograph of GFeF freestanding electrode. The electrochemical tests of the as-prepared hybrid film were first performed in a half-cell vs. Li/Li+ in the potential range 0.01-3 V. Figure 3a shows the cyclic voltammetry (CV) curves of GFeF electrode in the first cycle and reversible steady-state cycles at the scan rate of 2 mV s-1. In the first cycle, a sharp peak at around 0.74 V in the cathodic sweep and a smooth peak at around 1.8 V in the anodic sweep were consistent with previous reports26, 40. These redox peaks correspond to the mixed Faradaic reactions of iron oxide: Fe3O4 + 8Li+ + 8e3Fe0 + 4Li2O and Fe2O3 + 6Li+ + 6e-

2Fe0 + 3Li2O.

28, 41

In the later steady-state

cycles, both cathodic and anodic peaks were slightly shifted due to the polarization of the electrode in the first cycle42. Additionally, the CV curve of GFeF electrode in the 50th cycle almost keeps the same profile with that of the 5th cycle, indicating a good reversibility of the electrode. The galvanostatic charge/discharge profiles in the first three cycles at the current density of 0.02 A g-1 are presented in Figure 3b. A voltage plateau was observed at about 0.76 V in the first discharge process, and shifts to around 0.92 V in the later cycles, which corresponds to the reduction of Fe2+/Fe3+ to Fe0. Meanwhile, a tilted voltage plateau due to the oxidation of Fe was found around 1.8 V. The voltage plateaus in galvanostatic test are in good agreement with the peak positions in CV test. In addition, the discharge and charge capacities in the first cycle were 1506.1 and 1107.4 mAh g-1, respectively, with a first-cycle Columbic efficiency of 73.5%. The relatively low Columbic efficiency could be attributed to the formation of solid electrolyte interface (SEI)43-44. However, this value is much higher than our previous works of 59% and 64.9% for the similar Fe3O4@C electrodes28, 45, suggesting a highly reversible capacity of the GFeF electrode. In the second cycle, GFeF delivered discharge and charge capacities of 857.2 and 856.1mAh g−1, with the coulombic efficiency of 10 ACS Paragon Plus Environment

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99.8%. Moreover, the galvanostatic charge-discharge curve at the same current density (Figure 3b inset) exhibits almost a symmetrical shape, revealing its capacitor-like behavior due to the presence of physically ironic adsorption of the electrode. To further evaluate the rate performance of GFeF electrode, a series of galvanostatic charge-discharge tests were performed at different current densities, and the variation of gravimetric and volumetric capacities (based on the total mass of graphene and iron oxide without any additives) as a function of current density are presented in Figure 3c. It is noted that the electrochemical behavior of GFeF based half cell is battery-type system. Thus “capacity” in “C” or “mAh” are recommended as the most appropriate and meaningful metric to use in such case46. As can be seen (left Y axis in Figure 3c), at the current density of 0.02 A g-1 GFeF electrode could yield a high gravimetric capacity of 855.2 mAh g-1. Besides, its capacity remained at 790.4 mAh g-1 (7.6% capacity loss) while the current density was increased to 0.2 A g-1. Further increasing current density to 2 A g-1, the electrode was still able to delivery capacity of 348.0 mAh g-1. Although the gravimetric performance is a critical indicator that reflects the intrinsic energy storage ability of the electrode materials, numerous recent studies pointed out volumetric performance has become the focus over gravimetric performance due to the fast development of electric vehicles and smart devices47-50. Therefore, the as-prepared GFeF electrode was further evaluated in volumetric performance (right Y axis in Figure 3c). According to our previous study, the densely packed graphene/oxide electrodes were expected to have a high volumetric performance owing to their optimized structures. In Figure 3c, the volumetric capacity of GFeF electrode is as high as 1949.9 mAh cm-3 at a current density of 0.02 A g-1, which is quite comparable to the recent literatures. For example, Cheng et al fabricated carbon nanotube/sulfur composite electrode in which aligned carbon nanotubes served as interconnected conductive scaffolds to accommodate sulfur, and the as-prepared electrode 11 ACS Paragon Plus Environment


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resulted in volumetric capacity of 1116.0 mAh cm-3 under the measurement of 2-electrode half cell51. The superior volumetric performance of the GFeF electrode material highlights the advantages of the designed construction, which can be accounted for that the layered structure not only provides sufficient place to store energy but also effectively reduce the unnecessary voids in micrometer level. The test of cycle stability for GFeF half-cell was also carried out and the corresponding result is shown in Figure 3d. As expected, the capacitance retention was remained high over the long term cycling. There was a 27% increase in the capacitance retention during the first 100 cycles, which could be due to ions gradually penetrating the films more freely in the internal regions that were not accessed initially43, 45. After 500 cycles, the capacitance retained 93% of its initial value, suggesting its superior cycling stability.

Figure 4: a) illustration of hybrid structure with volume-effect accommodation during Li+ extraction/insertion; b) illustration of the expressway-like structure with bicontinuous ion/electron transport pathways and large electrode-electrolyte contact area; c,d) morphology comparison of cross-sectional SEM images for GFeF before lithiation (c) and after 500 cycles of charge/discharge (d).


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The aforementioned electrochemical results clearly demonstrated an important role of the well-organized interleaved structure of the graphene@metal oxide composite in improving the lithium storage performance. We believe the void space between iron oxide nanoparticles and graphene sheets allowed the iron oxide particles to expand and also accommodate more lithium ions as illustrated in Figure 4a. The layered graphene sheets were also able to significantly buffer the stress and strain induced by the volume contraction/expansion of metal oxide during long term cycling. Actually, the similar accommodated volume-effect ideas such as yolk-shell structures52-53, pomegranate-inspired structure54, semihollow bicontinuous graphene scrolls55 and self-adaptive strain-relaxation structure56 have been well demonstrated by previous researchers. Large amount of voids (Figure 4b) provide a high contact area between electrode and electrolyte, and each iron oxide nanoparticle is fixed in position by graphene layers. This well-organized interleaved structure can provide rapidly bicontinuous ion and electron pathways, resulting in outstanding lithium storage performance57. The internal structure of the GFeF electrode before and after charge/discharge cycling were also compared. It is clear to observe that the graphene sheets are packed by layer (yellow dash line in Figure 4c) before lithiation. After 500 charge/discharge cycles, the GFeF electrode still possessed interclated structure with only slightly disordered graphene sheets (Figure 4d). The SEM images at low magnifications (Figure S3a and S3d; S3b and S3e) for GFeF before and after cycling indicate retention of good structure integrity. Moreover, the higher magnification SEM images at 60 000 times (Figure S3c and S3f) for GFeF before and after cycling show the iron oxide nanoparticles remained spherical without cracks, suggesting that this structure is quite effective to accommodate their volume expansions during the harsh cycling process. In order to realize the high-power characteristic of hybrid supercapacitor, an asymmetric configuration was adopted wherein porous graphene was employed as EDLC side (positive 13 ACS Paragon Plus Environment


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electrode). The electrochemical evaluation of porous graphene is performed in a half-cell vs. Li/Li+ in the potential range 1.5-4.5 V. As can be seen in Figure S4a, the porous graphene electrode exhibits a quasi-rectangular shaped CV at different sweep rates, suggesting its favorable capacitive behavior. Besides, the symmetric galvanostatic charge-discharge profiles in Figure S4b indicate its typical EDLC storage mechanism, and the calculated specific capacitance from discharge curves was 133 F g-1 at the current densities of 0.05 A g-1. The result of cycling stability is shown in Figure S4c. After 1000 cycles, 95 % of its initial capacitance was maintained, suggesting its good cycling stability. These superior performance suggests the as-prepared porous graphene foil is an ideal positive electrode for the Li-ion HEC.

Figure 5: Electrochemical performance of GFeF//pG HEC(GFeF as negative electrode and porous graphene as positive electrode with mass ratio of GFeF/pG = 4.5). a) CV measured at different sweep rates; b) Galvanostatic charge-discharge curves at the current density of 0.02 A g-1; c) Cycle stability; d) Ragone plots, and results comparison with literatures: TiO2 14 ACS Paragon Plus Environment

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tube//porous C22; Nb2O5–CNT//AC58; V2O5–CNT//AC59; C-LiTi2(PO4)3//AC60; Fe3O4-G//pG26; Li4Ti5O12//RGO61. Shadow area belong to Li-ion battery and EDLCs. As discussed above, the superior electrochemical performances of both GFeF and porous graphene (pG) electrodes are expected to result in high performance Li-ion HEC. In order to investigate the capacitive performance of GFeF freestanding electrode in a full cell set-up, a Li-ion HEC GFeF//pG was assembled, in which pre-lithiated GFeF (-) without any additives was used as anode and pG (+) coated on aluminum foil was served as cathode. In order to achieve high energy density for this HEC, the charge balance should follow the relationship of q+=q–17,62. Accordingly, the optimized mass ratio between the cathode and anode should be m(pG)/m(GFeF) = 4.5 in the hybrid full cell. Figure 5a illustrates the CV curves of the asfabricated Li-ion HEC in a wide potential window of 3.5 V. Difference from the sharp redox peaks presented in GFeF half-cell, the full cell hybrid capacitor shows a quasi-rectangular CV curve, suggesting the synergistic effect of two different energy storage mechanisms of EDLC and battery25-26, 63. By means of comparing the area of the largest rectangle in CV curve with the area of remaining part, the percentages of EDLC and pseudocapacitance are estimated in Figure S5. The galvanostatic charge-discharge profile at 0.02 A g-1 in Figure 5b reveals symmetrical shape with a quick I-V response and approximately linear variatio. This result is in good agreement with CV results, suggesting its capacitor-like behavior. Based on the total mass of the cathode and anode, the calculated specific gravimetric capacitances are 83.2 and 42.3 F g-1 at 0.01 and 0.1 A g-1, respectively (Figure S6). The volumetric capacitances (Figure S6) based on the total volume of both electrodes are 69.9 and 35.5 F cm-3. It should be noted that the gravimetric capacitances of the as-prepared hybrid Li-ion capacitor are not the best value as compared with the previous reports25-26, 63. However, the obtained capacitance values are a reliable performance of the electrodes without additional contribution of carbon additives such as carbon black or active carbon. The GFeF//pG HEC also exhibited a good cycle stability with the capacitance retention of 73.9 % after 3000 cycles at high current 15 ACS Paragon Plus Environment


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density of 0.1 A g-1 (Figure 5c). The Ragone plot is shown in Figure 5d. At power density of 18.7 W kg-1 the corresponding energy density calculated based on the both electrodes is 129.6 Wh kg-1. At a high power density of 1870 W kg-1 (1 A g-1), the corresponding energy density remains at 59.5 Wh kg-1. It is useful to compare the performance of as-prepared GFeF//pG HEC with previously reported HECs. As shown in Figure 5d, various HECs of TiO2 tube//porous C22, Fe3O4-G//pG26, Nb2O5–CNT//AC58, V2O5–CNT//AC59, C-LiTi2(PO4)3//AC60 and Li4Ti5O12//RGO61 delivered comparable energy densities as our GFeF//pG HEC. However, given that the addition of conductive agents and binders for those common systems would result in “dead weight” and “dead volume” for real devices, our GFeF//pG HEC on the basis of freestanding film electrode still offer some advantages. Furthermore, the feasibility to realize flexible energy storage devices using GFeF electrode with high volumetric capacitance is another advantage as compared to the common reported HEC electrode materials. As the real energy density of total package account for up to 60% of calculated one, the energy density of GFeF//pG HEC (estimated) show in Figure 5d is expected to almost achieve a Liion battery level energy density and upholding EDLC power delivery64. These results clearly demonstrated an important role of synergistic effects of Li-ion batteries and EDLCs 65.


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Figure 6: Demonstration on flexible properties of GFeF//pG HEC. a) photographs of asprepared flexible pouch-cell battery in different bending states (left), and corresponding CV curves (right). Inset scheme illustrating the definition of bending angle; b) A light-emitting diode (LED) powered by as-prepared flexible pouch-cell battery. The flexible property of the hybrid Li-ion battery was further demonstrated in a bendable pouch-cell prototype (Figure 6). The electochemical test at differnet bending angles was carried out for the prototype. As shown in Figure 6a, even bending to 180°, no obvious change of CV curve was observed. Another demonstration in Figure 6b shows that the asprepared pouch-cell capacitor was used to light a LED lamp under flat, static 180° bending and dynamic bending. The lighting of the LED was not affected under different bending conditions, suggesting the superior flexibility of the pouch-cell battery. Figure S7 (Supporting Information) shows that HEC prototype exhibits good bending cycle stability with 87.5% capaciatnce rentation after 100 bending cycles. Moreover, the flexible HEC prototype also demonstrate the ability to be charged within 10 seconds and discharged to power up a 3 V mini-fan for up to 1 minute under the static 180° bending status (Supplementary Movie 2 Test 1). Besides, the mini-fan was able to last for 20 seconds under continuously dynamic bending 17 ACS Paragon Plus Environment


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(Supplementary Movie 2 Test 2). The energy drop was attributed to the self-discharge of the pouch-cell under dynamic bending condition and bending fatigue effect66. These results suggest that the flexibility of GFeF//pG hybrid Li-ion capacitor can be opened to a wide range of electronic applications. CONCLUSIONS In summary, bendable GFeF electrode was fabricated by combining graphene sheets and iron oxide nanoparticles through filtration-assited self-assembly method. The structural characterizations of SEM and FIB 3D-reconstructed images confirm that the oxide nanoparticles were homogeneously intercalated in the hybrid films, serving as effective energy storage cells in between the graphene layers. The electrochemical performances of GFeF half-cell vs. Li/Li+ exhibit high gravimetric capacity (855.2 mAh g-1 at 0.02 A g-1), high volumetric capacity (1949.9 mAh cm-3 at 0.02 A g-1) and superior cycling stability (93% capacitance retention after 500 cycles). A hybrid Li-ion capacitor was assembled by integrating GFeF as battery-type anode and porous graphene coated foil as capacitor-type cathode. This hybrid energy storage device showed satisfactory energy density and power density. Finnaly, it was demonstrated the excellent flexibility and relatively stable electrochemical performances of the hybrid Li-ion capacitor even when the as-prepared pouch cell underwent multiple bending deformations. We believe the flexbile pouch-cell prototype could be a promising design to fabricate high energy density storage devices for the progress of wearable electronics in the near future. ASSOCIATED CONTENTS The Supporting Information provides the detailed experimental procedures and supporting results. The Supplementary Movie 1 shows reconstructed 3D imaging of hybrid film and Supplementary Movie 2 shows the discharging demo for flexible HEC prototype. These materials are available free of charge via the Internet at http://pubs.acs.org. ACKNOLEDGMENTS The author thanks the financial support provided by Singapore MOE Tier 1 funding R-284000-124-112. The author also thanks Carl Zeiss Advanced Imaging Center for FIB-SEM imaging applications support. 18 ACS Paragon Plus Environment

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Designed Construction of a Graphene and Iron Oxide Freestanding

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