Asymmetric Supercapacitors Assembled by Dual Spinel Ferrites

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Asymmetric Supercapacitors Assembled by Dual Spinel Ferrites@Graphene Nanocomposites as Electrodes Hongfei Wang, Yuqing Song, Xuxu Ye, Heng Wang, Weishuai Liu, and Lifeng Yan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00433 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Energy Materials

Asymmetric Supercapacitors Assembled by Dual Spinel Ferrites@Graphene Nanocomposites as Electrodes Hongfei Wang, Yuqing Song, Xuxu Ye, Heng Wang, Weishuai Liu, and Lifeng Yan*

CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Chemical Physics, iCHEM, University of Science and Technology of China, Hefei, 230026, P.R.China

KEYWORDS: Supercapacitors, asymmetric, electrode, graphene, spinel ferrites.

Corresponding author: Lifeng Yan, Tel/Fax:+86-551-63606853, e-mail: [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT: Spinel-type ferrites are potential active materials for their highly theoretical capacity as electrodes of supercapacitors. Here, CoFe2O4@graphene nanocomposites have been synthesized by a facile hydrothermal method and worked as cathode material. It was found that the performance of the electrode depend on the weight ratio of ferrites to graphene. The specific capacitance can be significantly increased to 579 F g-1 at 1 A g-1 when the content of graphene is 40% in the composite. Next an asymmetric supercapacitor (ASC) was fabricated by using the as-prepared Fe3O4@graphene nanocomposites as anode material. The ASC with 1.7 V working voltage delivers a high specific capacitance of 114.0 F g-1 and a promising energy density of 45.5 Wh kg-1 at a power density of 840 W kg-1, along with a highly cycling stability of 91% capacitance retention after 5000 cycles. It provides a new change to fabricate

high

performance

ASCs

using

spinel

ferrites

based

graphene

nanocomposites as electrodes.

2


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INTRODUCTION Supercapacitors are recognized as an attractive type of energy storage device nowadays.1-3 Compared with traditional chemical supplies, supercapacitors have several prominent advantages such as high power density, long cycle life and wide working temperature range.4-6 According to the charge storage mechanism, supercapacitors can be generally divided into electric double layer capacitors7 (EDLCs) and pseudocapacitors8-9 (PCs). The charge is stored in EDLCs at the interface of electrolyte and electrode, which provide high power density but suffer from limited specific capacitances. In contrast, PCs based on the reversible faradic reaction on the electrode surface offer much higher specific capacitances than those of EDLCs.10 On account of the superiority in capacitances, many new material systems of PC electrodes are proposed to achieve larger energy density and higher rate performance. 11 Recently, mixed valence oxides of transition metals,12-15 especially for spinel-type ferrites with a form of CoxFe3-xO4 (x = 0, 1, 2, 3),16 have attracted considerable attention owing to their high chemical stability and rich redox chemistry, making them promising alternatives for PC electrode materials. Among all this kind of materials, CoFe2O4 (x = 1) with a highly theoretical capacity of 916 mAh g-1 have been extensively investigated in other energy storage systems like lithium-ion batteries (LIBs).17 Likewise, it is probable for CoFe2O4 to be explored as a candidate for PCs.18 Gao et al. showed that the as-fabricated hierarchically porous CoFe2O4 nanosheets on Ni foam delivered a specific capacitance of 503 F g-1 at 2 A g-1.19 3



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Kumbhar et al. synthesized CoFe2O4 nano-flakes with a specific capacitance of 366 F g-1 at 5 mV s-1.20 Xiong et al. prepared

CoFe2O4/graphene/polyaniline

nanocomposites to increase the specific capacitance up to 1133.3 F g-1 at 1 mV s-1.21 However, like many other transition metal oxides, CoFe2O4 is prone to aggregate during the charge/discharge process,22 which seriously degrades its electrochemical activity and stability. Thus, it is necessary to design well-organized nanostructures subtly solving the aggregation problems. Here, graphene sheets with intriguing properties were introduced to serve as the conductive substrates for loading CoFe2O4 nanoparticles uniformly in a facile hydrothermal method.23-24 The distribution of CoFe2O4 can be controlled by adjusting the weight contents of graphene, which shows great influence on the electrochemical behavior of composites as the cathode in PCs. Among all the spinel ferrites, Fe3O4 (x = 0) is an ideal anode for ASCs, due to its wide operational voltage window (-1.2 to 0.25 V ), high theoretical capacitance (926 mAh g-1) and low cost.25 To further investigate the electrochemical perforamance of CoFe2O4 and Fe3O4 in ASCs, we design a novel pair of electrode combinations, CoFe2O4/rGO||Fe3O4/rGO. The resultant ASC based on dual spinel materials demonstrates a remarkable specific capacitance, high energy density and great cycling stability.

EXPERIMENTAL SECTION Materials. Graphite power, natural briquetting grade, ~100 mesh，99.9995% (metal basis) was purchased from Alfa Aesar. Analytical grade KMnO4, 98% H2SO4, 30% 4


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H2O2, 30% HCl, Fe(NO3)3·9H2O, CoCl2·6H2O, absolute ethanol, NH3·H2O, FeCl2·4H2O, FeCl3, NaOH, vitamin C, were purchased from Shanghai Chemical Reagents Company.

Synthesis of graphene oxide. Graphene oxide (GO) was obtained through the modified Hummers’ method.26 In brief, 50 mL of 98% H2SO4 was added to 1 g of graphite powder. Next 5 g of KMnO4 was added slowly in an ice-water bath to form a dark green solution, which was stirred for 24 h. After oxidation, the solution mixture was disposed with 50 mL of distilled water and 30 mL of 3% H2O2 respectively. The resulting solution was washed by 1M HCl (2 times) and water (4-5 times) under high speed centrifugation. The washed GO product was finally freeze-dried for 72 h.

Synthesis of CoFe2O4 nanoparticles on graphene nanosheets. Firstly, 20 mg of GO, 0.104 g of Fe(NO3)3·9H2O (0.4 mmol) and 0.0476 g of CoCl2·6H2O (0.2 mmol) were dispersed in 60 mL of absolute ethanol. Subsequently, 5 mL of NH3·H2O was added at room temperature. The mixture was stirred at 80 oC for 12 h, followed by the reduction reaction in the Teflon-lined stainless steel autoclave at 180 oC for 5 h. After that, the resulting composites were centrifuged, washed with ethanol (3 times), and freeze-dried for 12 h. A series of CoFe2O4@graphene nanocomposites were prepared by adjusting the mass of GO, denoted as CG-30% (m = 20 mg), CG-40% (m = 31 mg), CG-50% (m = 47 mg), CG-60% (m = 70 mg). The pure reduced graphene oxide is denoted as rGO. 5



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Synthesis of 3D Fe3O4@graphene hydrogel. The colloidal suspension of Fe3O4 was first prepared. Typically, 1.3 g of FeCl3 (8 mmol), 0.75 g of FeCl2·4H2O (4 mmol) were successively added to 12.5 mL of deoxygenated ultrapure water, which was mixed with 0.22 mL of 12 M HCl previously. The black precipitation of Fe3O4 appeared after the above solution was added dropwise to 62.5 mL of 1.5 M NaOH. Ultrapure water was added again to wash the precipitation (3 times) with the aid of centrifugation. In order to neutralize the anionic charges on the surface of the Fe3O4 particles, 125 mL of 0.01 M HCl was added to the above precipitation. After separated by centrifugation, the cationic Fe3O4 nanoparticles were peptized with a little water to form a stable and homogeneous hydrosol. To fabricate Fe3O4@graphene hydrogel, 2.5 mL of Fe3O4 sol and 50 mL of 1.5 mg mL-1 GO solution were mixed under mild ultrasonication, followed by the addition of 0.22 g of vitamin C. The hydrogel formed gradually while the mixture was water-bath treated in 95 oC without stirring for 3 h. At the end, the product was dialyzed over a week.

Electrochemical measurements. CHI660D electrochemical workstation was used to carry out all the electrochemical measurements. Both CoFe2O4@graphene and Fe3O4@graphene active materials were loaded and pressed onto nickel foam as working electrode. For the electrochemical performance of individual electrodes mentioned above, 1 M KOH served as the aqueous electrolyte, a platinum electrode as the counter electrode, and an Ag/AgCl electrode as the reference electrode to make up 6


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a three-electrode system. The supercapacitor device was fabricated by using CoFe2O4@graphene as cathode and Fe3O4@graphene as anode, which were immersed in 1 M KOH electrolyte and separated by polytetrafluoroethylene (PTFE) membrane.

Characterization. The crystallographic structures of different samples were investigated by using XRD (D/MAX-1200, Rigaku Denki Co. Ltd., Japan). The morphology and microstructures were measured by employing SEM (XL30 FEG-SEM Philips), TEM (JEM-2010), and HRTEM. The chemical composition and surface analysis were determined by XPS (VG Scientfic Ltd., United Kingdom). The thermal analysis was performed on a thermogravimeter (TGA, DTA-50, Shimazu) under an air atmosphere at a heating rate of 10.00 oC min-1. The specific surface areas and pore size distributions were obtained by the BET method on an automatically microporous instrument (ASAP 2020 M+C, Micromeritics). Raman spectra were acquired using a RAMAMLOG 6 (Spex, USA) with a 514.5 nm laser excitation.

RESULTS AND DISCUSSION The

synthesis

processes

of

CoFe2O4@graphene

and

Fe3O4@graphene

nanocomposites for high performance supercapacitor electrodes are illustrated in Scheme 1，including: (a) solution-phase self-assembly of spinel CoFe2O4 or Fe3O4 and GO nanosheets; (b) reduction treatment for GO to further embed nanoparticles and facilitate electron transport. Figure 1 shows the X-ray diffraction (XRD) patterns of GO, CoFe2O4 and 7



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CoFe2O4@graphene. By comparison, the typical peak of GO (001) disappears in the XRD pattern of CoFe2O4@graphene, indicating the reduction of GO after the hydrothermal treatment. Meanwhile, the broad diffraction peak appearing at about 25º is assigned to the (002) plane of graphene.27 In addition, other characteristic peaks of CoFe2O4@graphene nanocomposites can be well indexed to the (220), (311), (400), (422), (511), (440) and (533) planes of pure CoFe2O4 (JCPDS No. 22-1086).28 Figure 2a shows the Raman spectra of pure graphene and CoFe2O4@graphene. For the spectrum of CoFe2O4@graphene, there are a series of peaks at around 600 cm-1, indicating the existence of CoFe2O4.29 Besides, the two peaks at 1358 and 1594 cm-1 are the typical D and G bands of graphene, respectively.30 The result also indicates that the D and G bands in CoFe2O4@graphene are red-shifted comparable with pure graphene, which is ascribed to the charge transfer between graphene and CoFe2O4. Figure 2b presents the TGA curve of CG-40%. The loss of water happened in the range from room temperature to 360 oC. In terms of the weight loss between 360 oC and 800 oC, the CoFe2O4 content can be estimated to be about 58 wt% (neglecting the oxidation of Co2+), which is in good agreement with the raw materials ratio. The HRTEM images of the as-prepared CoFe2O4@graphene are shown in Fig. 3a and 3b, which shows the size of nanopaticles is about 10 nm. It’s clear to obtain the similar result in the magnified SEM image (Figure S1). Figure 3b exhibits the well-resolved lattice fringes with a lattice spacing value of 0.29 nm corresponding to the (220) plane of spinel phase. The inset is the selected area electron diffraction (SAED) pattern, reflecting the crystallinity of CoFe2O4. The four bright rings can be 8


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well corresponded to the (220), (311), (400) and (511) planes of spinel CoFe2O4.31 In Figure 4a-e, the dark-field STEM elemental mapping analysis reveals that Fe, Co and O elements all are uniformly distributed on the graphene sheets. Furthermore, the elemental composition CoFe2O4@graphene is obtained from energy dispersive X-ray spectroscopy (Figure 4f) to confirm that Fe, Co, O and C elements are homogeneous distribution in the composites, and the atomic ratio of O/Fe/Co (4.8:1.7:1) is also basically consistent with the molecular formula of CoFe2O4. In order to investigate the link between the CoFe2O4 content in composites and its distribution on graphene nanosheets, a set of comparison TEM images are presented in Fig. 5a-d. When 40 wt% CoFe2O4 is introduced (Fig.5a), all nanoparticles are evenly attached to the graphene sheets. When the content is increased to 50 wt% (Fig.5b), the CoFe2O4 distribution is denser. Further increasing the content to 60 wt% (Fig.5c), the utilization of the areas on graphene sheets is maximized with slight aggregation. But when the content reaches 70 wt% (Fig.5d), the problem of aggregation is very severe. It can be concluded that graphene provides excellent substrates for uniform anchoring of CoFe2O4 with an appropriate content. In the ensuing discussion, the loading of CoFe2O4 ultimately has a dramatic effect on the electrochemical property of the material. N2 sorption measurements were conducted to characterize the specific surface area of CoFe2O4@graphene. Figure 6a presents the type IV isotherm with a hysteresis loop in the P/P0 value between 0.5 and 1.0 for all samples, suggesting the existence of mesoporous structure in nanocomposites. In particular, the as-prepared samples own 9



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the values of the BET specific surface areas and total pore volumes in the range of 112-191 m2 g-1 and 0.322-0.446 cm3 g-1 (Table 1), which are larger than those of pure CoFe2O4 (55 m2 g-1, 0.222 cm3 g-1).32 Figure 6b displays a broad pore size distribution, indicating the co-existence of meso- and macroporous nature. The above result also confirms that graphene sheets play a vital role in the inhibition of aggregation and increasing of specific surface area. Figure 7a exhibits the XPS survey spectra of the as-prepared CoFe2O4@graphene, which proves the presence of Co, Fe, C and O elements. For the Co2p spectrum (Fig.7b), two peaks at 781.1 and 796.9 eV are caused by Co 2p1/2 and Co 2p3/2. Accordingly, there are another two shake-up satellite peaks at 786.9 and 802.8eV. The observed two pair of peaks are defined as the existence of Co2+ in the high-spin state. In the Fe 2p spectrum (Fig.7c), two main peaks located at 712.4 and 724.4eV correspond to Fe 2p1/2 and Fe 2p3/2, which can be attributed to the presence of Fe3+.24 The C 1s spectrum (Fig.7d) can be deconvoluted into 284.8 and 286.0eV, assigned to C=C and C-C. The peaks for other oxygen-containing species of GO are very insignificant. This also indicates that GO is sufficiently reduced during the formation of CoFe2O4@graphene composites.33 To evaluate the electrochemical properties of CoFe2O4@graphene as electrode materials, cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements were employed in a three-electrode system. For comparison, all samples with different graphene contents (CG-30%, CG-40%, CG-50%, CG-60%) were tested under the same condition. As indicated in Fig.8a and Fig.S2a-c, the CV 10


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curves of individual electrodes show pseudocapacitive characteristics with a pair of well-defined redox peaks in the potential window of 0-0.6 V. It should be noted that the redox reaction is related to the transition of Co3+/Co2+ and Fe3+/Fe2+.34 Since both redox potential is very close, only one pair of distinct redox peaks appear in the CV curve.35 Meanwhile, the enlargement of scan rate leads to the position of oxidation peak and reduction peak moving towards higher potentials and lower potentials separately, which is due to the electric polarization.34As for specific capacitances, Figure 8b shows the GCD curves of every sample at the current density of 1 A g-1, in which CG-40% also exhibits the longest discharge time. In addition, Figure S3a-d displays the GCD curves of CG-30%, CG-40%, CG-50%, CG-60% at various current densities. Specifically, the specific capacitance of CG-40% is 579.3 F g-1 at 1 A g-1, while the values of CG-30%, CG-50%, CG-60% are only 514.2 F g-1, 548.8 F g-1, 449.3 F g-1, respectively (Table 1). More visually, the specific capacitances of samples as a function of current densities are plotted in Fig.8c. Notably, CG-40% can still deliver a high specific capacitance of 545 F g-1 even at a current density of 8 A g-1 with 94% capacitance retention, surpassing the other samples. To further compare the electrochemical behavior of all electrodes, the Nyquist plot recorded from 0.01 to 100000 Hz is shown in Fig.8d. In the low frequency region, the slope of the line stands for the diffusive resistance of ion from the electrolyte to the electrode. Clearly, CG-40% exhibits an almost vertical line relative to the other samples, showing the lowest diffusive resistance. In the high frequency region, The semicircle corresponds to the charge-transfer resistance (Rc) at the interface of the electrode and electrolyte. 11



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Similarly, the Rc value of CG-40% (0.4 Ω) is smaller than CG-30% (1.0 Ω), CG-50% (1.1 Ω) and CG-60% (0.6 Ω). While the intercept on the real axis represents the total ohmic resistance, also known as equivalent series resistance (Rs), which is composed by electrolyte resistance, intrinsic resistance of material and the contact resistance of active material with current collector. From the plot, CG-40% displays a Rs value of 2.1 Ω lower than CG-30% (3.0 Ω), CG-50% (2.8 Ω) and CG-60% (2.3 Ω), which is consistent with the previous result that uniform distribution of CoFe2O4 in CG-40% further contributes to improve the electrical conductivity.36 To construct an ASC, a suitable anode material is urgently needed to pair with the cathode. Herein, Fe3O4@graphene is designed to act as anode. XRD pattern of the sample is shown in Fig.S4, which clearly exhibits the broad peak of graphene and several diffraction peaks of spinel Fe3O4.37 Both the SEM (Fig.S5a) and TEM (Fig.S5b) images verify that graphene sheets are coated homogeneously by Fe3O4 particles with a diameter of about 10 nm. As shown in Fig.S5c, the HRTEM image of Fe3O4@graphene exhibits the crystal structure with an interplanar spacing of 0.25 nm, assigned to the (311) plane of Fe3O4. The Fe3O4 content in nanocomposites is measured to be about 70% by TGA (Fig.S6). As can be observed in Fig.S7, the N2 adsorption-desortion isotherm is also a type IV curve. Based on the analysis of BET method, a high specific surface area of 190 m2 g-1 and a total pore volume of 0.362 cm3 g-1 are obtained for Fe3O4@graphene. The C 1s, O 1s and Fe 2p peak signals are illustrated in Fig.S8a. Furthermore, the Fe 2p spectrum (Fig.S8b) shows the peaks of Fe 2p1/2 at 711.4 eV and Fe 2p3/2 at 724.7 eV for Fe3O4.38 The CV curves of 12


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Fe3O4@graphene electrode at different scan rates in the potential range from 0 to -1.2 V are illustrated in Fig.S9a. There are also a pair of weak redox peaks at -0.8 and -1.1 V representing the pseudocapacitive behavior of Fe3+/Fe2+.39 The GCD tests (Fig.S9b) were further carried out to evaluate the capacitance of anode. By calculation, the electrode delivers a maximum specific capacitance value of 278.5 F g-1 at 1 A g-1. With the increasing of current density, a high specific capacitance value of 197.0 F g-1 can also be available at 8 A g-1, suggesting good rate performance. Given

the

outstanding

electrochemical

performance,

the

ASC

with

CoFe2O4@graphene as cathode, Fe3O4@graphene as anode and 1 M KOH as aqueous electrolyte was assembled in this work. Figure 9a shows the CV curves of both cathode and anode materials in separate potential windows of 0-0.6 V and -1.2-0 V at a scan rate of 10 mV s-1 in a three-electrode configuration. Therefore, superimposing the voltage interval of positive and negative electrodes, the total device voltage of ASC will reach 1.8 V theoretically. In practice, the working voltage only can be extended to 1.7 V owing to the interference of polarization. In order to balance the charge storage capacities of two electrodes, the mass ratio of CoFe2O4@graphene to Fe3O4@graphene was calculated by the equation below: Q+ =Q- → m+ /m- = C-ΔE-/ C+ Δ E+, where C is the specific capacitance tested from the three-electrode configuration, ΔE is the operating potential window, and m is the mass of the active material. The optimal mass ratio was obtained to be m+ /m- = 0.55 (CG-30%), 0.46 (CG-40%), 0.50 (CG-50%), and 0.61 (CG-50%). For example, if the mass loading of CG-50% was 1 mg cm-2, the corresponding value of the negative electrode would be 2 13



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mg cm-2. In the ASC, the CV curves of CG-40% shown in Fig.9b at different scan rates can be divided into two parts, indicating the contribution of electric double layer capacitance (0 to 0.8 V) and pseudocapacitance

(0.8 to 1.7 V) reaction.40 For all

samples, the shape of CV curves (Fig.9b and Fig.S10a-c) kept undeformed basically with increasing scan rates demonstrates good rate performance. Evidently, the ASC with CG-40% as electrode exhibits the largest CV area compared with CG-30%, CG-50% and CG-60%. To obtain the specific capacitance precisely, the GCD profiles of every ASC are shown in Fig.10a-d. Based on the above results, the plots of the specific capacitances for all ASCs at different current densities have been presented in Fig.11a. Interestingly, the values of CG-40% are higher than those of the other ASCs. Therein, a maximum specific capacitance of 114.0 F g-1 at 1A g-1 in the ASC containing CG-40% (Table 1), which still retains 77.2 F g-1 at 8 A g-1. In order to investigate the overall performance of supercapacitors, energy density and power density are calculated to form the Ragone plot in Fig.11b. Similarly, the ASC from CG-40% manifests a maximum energy density of 45.5 Wh kg-1 at a power density of 840 W kg-1. When the power density is increased to 6172 W kg-1, a high energy density of 30.8 W kg-1 can also be achieved. As it can be seen in Fig.12, these obtained values are significantly higher than those of recent reports, such as ZnFe2O4 ‖Mn3O4 (28 Wh kg-1),41 MnO2/CNT‖MnFe2O4/rGO (25.9 Wh kg-1),42 CoFe2O4‖AC (22.8 Wh kg-1),19 FeCo2O4‖AC (23 Wh kg-1),40 MnO2‖Fe3O4 (17.3 Wh kg-1),43 Mn3O4‖rGO (17.8 Wh kg-1),44 Co3O4/NF‖CA(17.9 Wh kg-1).45 To evaluate the cycle life, the ASC based on CG-40% was examined over 5000 cycles at 14


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a high current density of 8 A g-1 in Fig.13. Eventually, 91% of initial specific capacitance is retained, demonstrating good cycling stability. The superior electrochemical capacitive performance of 1.7 V ASC can be ascribed to the synergistic effect of dual spinel-type nanocomposites in anode and cathode.21 Firstly, porous structures observed in both CoFe2O4 and Fe3O4 provide a large number of channels to facilitate electrolyte ions into the inner surface of electrodes, promoting the full utilization of active materials capacitance.46 Secondly, the highly conductive graphene framework is in close contact with CoFe2O4 and Fe3O4, accelerating electron transfer and improving rate capability. However, whether the nanoparticles are well dispersed on graphene sheets also should be taken seriously.47 In both the measurements of the three-electrode system and the supercapacitor device, the electrochemical performance of CG-40% totally prevails over CG-30%, CG-50% and CG-60%. Combined with the analysis of TEM images, CG-30% with lower content of graphene shows severe CoFe2O4 particles aggregation, inhibiting ion diffusion and increasing the resistance. Nonetheless, the contents of CG-50%, CG-60% are relatively higher, which can’t maximize the specific capacitance of composites. Therefore, a suitable graphene content of 40% not only addresses the aggregation issue, but also enhances ions transfer between electrode and electrolyte to optimize the capacitance performance.

Conclusions In summary, CoFe2O4@graphene nanocomposites have been prepared by a simple 15



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hydrothermal method. When adjusting the weight content of graphene in composites to 40%, the CoFe2O4@graphene electrode exhibites a maximum specific capacitance of 579 F g-1 and good rate capability. To construct a supercapacitor device, Fe3O4@graphene nanocomposites are synthesized as the anode. The ASC with a 1.7 V working voltage demonstrates a maximum energy density of 45.5 Wh kg-1 at a power density of 840 W kg-1 and a long-time cycling life.

ORCID Lifeng Yan: http://orcid.org/0000-0002-6063-270X

Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51673180 and 51373162).

Notes. There are no conflicts to declare.

ASSOCIATED

CONTENT Supporting

Information

Available:

SEM of

CoFe2O4@graphene, CV and curves of nanocomposites with various CGs at various scan

rates

and

current

densities,

XRD

and

XPS

patterns,

Nitrogen

adsorption-desorption isotherm, TGA curve, SEM, TEM and HRTEM images, and its electrode performances of Fe3O4@graphene.

16


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17



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Scheme 1. Illustration of the synthesis process of CoFe2O4@graphene and Fe3O4@graphene

18


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Figure 1. XRD patterns of GO, CoFe2O4 and CoFe2O4@graphene.

19



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Figure 2. Raman spectra of graphene and CoFe2O4@graphene (a), TGA curve of CG-40% (b)

20


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Figure 3. HRTEM images of CoFe2O4@graphene (a, b). Inset in (b) is the selected area electron diffraction (SAED) pattern of CoFe2O4@graphene.

21



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Figure 4. Dark-filed STEM image of CoFe2O4@graphene (a). EDS mapping of C (b), O (c), Fe (d), Co (e). EDX spectrum of CoFe2O4@graphene (f).

22


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Figure 5. TEM images of CG-30% (a), CG-40% (b), CG-50% (c), and CG-60% (d).

23



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Figure 6. Nitrogen adsorption-desorption isotherm (a), and pore size distribution (b) for all CoFe2O4@graphene nanocomposites.

24


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Figure 7. XPS survey spectra of CoFe2O4@graphene (a), and relative Co 2p pattern (b), Fe 2p pattern (c), and C 1s pattern (d).

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Figure 8. CV curves of CG-40% at different scan rates (a), GCD curves of CG-30%, CG-40%, CG-50%, CG-60% measured at a current density of 1 A g-1 (b), Specific capacitance retention of all samples with increasing current densities (c), and Nyquist plots of all electrodes (d).

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Figure 9. CV curves of the anode (Fe3O4@graphene) and cathode (CoFe2O4@graphene) in a three-electrode configuration at 10 mV s-1 (a), CV curves of the ASC based on CG-40% at various scan rates (b).

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Figure 10. GCD curves of the ASC base on CG-30% (a), CG-40% (b), CG-50% (c), andCG-60% (d) at various current densities.

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Figure 11. Comparison of ASCs based on CG-30%, CG-40%, CG-50% and CG-60%: Specific capacitance retention with increasing current densities (a), Ragone plot of the ASCs (b).

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Figure 12. Ragone plot of the as-fabricated ASC based on CG-40% compared with other previous work.

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Figure 13. Cycle performance of the ASC based on CG-40% at a current density of 8 A g-1.

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Table 1. a) SBET is the BET specific surface area; b) V is the total pore volume; c) Celectrode is the electrode specific capacitance calculated from the GCD curves in the three-electrode configuration at 1 A g-1; d) Cdevice is the ASC specific capacitance calculated from the GCD curves in the two-electrode configuration at 1 A g-1. SBETa)

Vb)

Celectrodec)

Cdeviced)

(m2 g-1)

(cm3 g-1)

(F g-1)

(F g-1)

CG-30%

112

0.322

514.2

87.2

CG-40%

158

0.337

579.3

114.0

CG-50%

172

0.364

548.8

108.3

CG-60%

191

0.446

449.3

84.5

Sample

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TOC

39


Asymmetric Supercapacitors Assembled by Dual Spinel Ferrites

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