In situ encapsulated Fe3O4 nanosheet arrays with

Showcasing a study on the design and synthesis of graphene-encapsulated Fe3O4 nanosheets as anode materials for asymmetric supercapacitors by Dr Jinghuang Lin, Prof. Junlei Qi and Prof. Jian Cao at State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology.

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In situ encapsulated Fe3O4 nanosheet arrays with graphene layers as an anode for high-performance asymmetric supercapacitors The highly conductive graphene layers in situ encapsulating Fe3O4 nanosheets not only benefits the transfer of electrons, but also protects the active Fe3O4 from degradation during cycles to obtain outstanding cycling performance. Further, as fabricated CuCo2O4//G@Fe3O4 hybrid asymmetric supercapacitor shows a high energy density.

See Junlei Qi, Jian Cao et al., J. Mater. Chem. A, 2017, 5, 24594.

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In situ encapsulated Fe3O4 nanosheet arrays with graphene layers as an anode for high-performance asymmetric supercapacitors† Jinghuang Lin,a Haoyan Liang,a Henan Jia,a Shulin Chen,a Jiale Guo,a Junlei Qi, Chaoqun Qu,b Jian Cao,*a Weidong Feia and Jicai Fenga

Received 30th August 2017 Accepted 3rd October 2017

Published on 06 October 2017. Downloaded on 06/12/2017 00:44:31.

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*a

DOI: 10.1039/c7ta07628b rsc.li/materials-a

Energy density of asymmetric supercapacitors (ASCs) is greatly limited by the electrochemical performance, especially low specific capacitance and poor cycling stability, of anode materials. To achieve highperformance ASCs, herein, we designed and synthesized a new anode material of Fe3O4 nanosheet arrays, which were encapsulated in situ by graphene layers (G@Fe3O4) through plasma enhanced chemical vapor deposition. Vertical-standing G@Fe3O4 nanosheet arrays directly on the conductive substrates can facilitate electrolyte diffusion and reduce the internal resistance. Furthermore, the highly conductive graphene layers in situ encapsulating the Fe3O4 nanosheets not only could provide fast ion and electron transport pathways, but could also maintain a stable structure for G@Fe3O4. When used as electrodes, G@Fe3O4 exhibited highest capacitance (up to 732 F g1), better rate capability, and cycling stability as compared to pristine Fe3O4. Furthermore, an asymmetric supercapacitor device synthesized using G@Fe3O4 as an anode and CuCo2O4 as the cathode showed a high energy density of up to 82.8 W h kg1 at a power density of 2047 W kg1 and good cycling stability (88.3% capacitance after 10 000 cycles).

1. Introduction Supercapacitors, including electrical double-layer capacitors (EDLCs) and pseudocapacitors, have attracted signicant attention as new energy storage devices due to their higher power density, long cycle life, and fast charge–discharge rate.1–4 In fact, pseudocapacitors can provide a higher capacitance than EDLCs due to their fast and reversible redox reactions.3–5 Therefore, great efforts have been devoted to enhance the electrochemical performance of pseudocapacitors.5–8 However, conventional pseudocapacitors are limited by their low energy a

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. E-mail: [email protected]; [email protected]; Fax: +86-451-86418146; Tel: +86-451-86418146

b

Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China † Electronic supplementary 10.1039/c7ta07628b

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density when compared with lithium-ion batteries.9,10 Based on the equation of the energy density E ¼ 1/2CV2, two strategies have been proposed to achieve higher energy density: improvement of the specic capacitance of electrode materials (C) and extension of the working voltage window (V).11–15 Currently, cathode materials are being extensively studied; this has led to the development of high-performance cathodes for aqueous supercapacitors, and some of them have even achieved their theoretical capacitance.16–20 To achieve a higher energy density, current research is mainly focused on the construction of asymmetric supercapacitors (ASCs) and extension of the operating potential window in aqueous electrolytes.13–15 Despite the wide potential window in ASCs, the limited performance of anode materials has been a bottleneck for high energy density in ASCs.21–23 Due to their low specic capacitance, the commonly used carbon nanomaterials as an anode cannot match well with high-performance cathode materials; this may greatly limit the performance of ASCs. Undoubtedly, it is vital and urgently needed to design and synthesize highperformance anode materials to achieve higher energy densities. Currently, great efforts have been devoted to develop pseudocapacitive anode materials, such as FeOx, MoOx, VOx, Bi2O3, and FeOOH, for ASCs.24–28 Among these anode materials, Fe3O4 is a promising candidate for ASCs due to its high theoretical capacitance, wide operating potential window, low cost, and eco-friendly nature.29,30 Although signicant progress has been achieved for Fe3O4 anode materials, most of the reported Fe3O4 materials still suffer from relatively low specic capacitance, poor cycling stability, and inferior rate capability.31,32 One promising approach to improve the electrochemical performance is to incorporate Fe3O4 nanostructures with carbonbased materials.31–36 For example, Sun et al. fabricated honeycomb Fe3O4 integrated on functionalized exfoliated graphite that showed a good specic capacitance of up to 327 F g1 and remained about 83% of the initial capacitance aer cycling.33 Moreover, Liu et al. synthesized monodisperse Fe3O4 nanoparticles on graphene as an electrode, that showed a high

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capacitance of up to 368 F g1, much higher than that of the pristine Fe3O4 (only 157 F g1).34 However, conventional hybridization approaches oen involve the addition of organic binders, which can impede ion diffusion and show high contact resistance.35 To solve this problem, a merging concept is the growth of nanomaterials directly on the conductive substrates as binderless electrodes, which can supply a short ion transport pathway and low contact resistance. Furthermore, conventional hybridization approaches cannot lead to strong connections between the carbon material and Fe3O4; this will lead to the degradation of electrodes.34,36 Therefore, it is still a great challenge to develop and design Fe3O4-based electrodes with good electrochemical performance for high-performance ASCs. In the current study, vertical-standing Fe3O4 nanosheets encapsulated in situ by graphene layers (G@Fe3O4) directly on conductive substrates have been developed and designed, which exhibit enhanced electrochemical performance. The asfabricated G@Fe3O4 electrode shows a high specic capacitance of 732 F g1 and good cycling stability, making it a perfect anode candidate for ASCs. To further investigate the electrochemical performance of the G@Fe3O4 anode in ASCs, we also designed a suitable battery-type cathode, CuCo2O4, using a hydrothermal process. The CuCo2O4//G@Fe3O4 hybrid asymmetric supercapacitor shows a high energy density of about 82.8 W h kg1 at a power density of 2047 W kg1 and good cycling.

2.

Experimental

2.1 Synthesis of G@Fe3O4 hybrid nanosheet arrays as an anode in ASCs To protect the Ni foam, we conducted a chemical vapor deposition (CVD) process to coat a thin layer of graphene on the surface of Ni foam (g-Ni foam). The whole synthesis process was in accordance with the study reported byCheng et al.37. Fig. 1 shows a schematic of the fabrication process of verticalstanding Fe3O4 nanosheets encapsulated in situ by graphene layers (G@Fe3O4); G@Fe3O4 have been directly synthesized on substrates without the use of any organic binders. Initially, 2 mmol of Fe(NO3)3$9H2O, 2 mmol of NH4F, and 10 mmol of CO(NH2)2 were dissolved in 60 mL of deionized water and ultrasonically treated for 5 min. Then, the obtained solution was transferred to a Teon-lined stainless-steel autoclave containing g-Ni foam. The hydrothermal process was conducted at 120 C for 6 h. Then, the obtained samples were cleaned and dried overnight at 80 C. Aer this, the obtained samples were heated to 350 C during plasma enhanced chemical vapor deposition (PECVD). The conditions for plasma enhanced chemical vapor deposition

Fig. 1

Journal of Materials Chemistry A

were as follows: Ar/CH4 (Ar: 95 sccm, CH4: 5 sccm) at a power of 200 W for 1 min. Finally, the samples were annealed in Ar/H2 (Ar: 95 sccm, H2: 5 sccm) at 350 C for 1 h to obtain [email protected] For comparison, pristine Fe3O4 nanosheets were also obtained via only the annealing process. 2.2 Synthesis of CuCo2O4 nanowire arrays as a cathode in ASCs The synthesis process of CuCo2O4 nanowire arrays mainly involved the hydrothermal and annealing processes. A mixed solution of CuCl2$2H2O (3 mmol), CoCl2$6H2O (6 mmol), CO(NH2)2 (28 mmol), and deionized water (75 mL) was transferred to a Teon-lined stainless-steel autoclave containing g-Ni foam. Aer being heated at 120 C for 10 h, the obtained precursor was cleaned and dried overnight at 80 C. Then, the cleaned precursor was annealed at 400 C for 2 h in air to obtain CuCo2O4. 2.3

Material characterization

The microstructure and morphology of the synthesized samples were characterized by eld-emission scanning electronic microscopy (FE-SEM, Helios Nanolab 600i) and transmission electron microscopy (TEM, Tecnai G2 F30). The surface chemical composition and phase analysis of the synthesized samples were examined using X-ray photoelectron spectroscopy (XPS, Thermo Fisher) and X-ray diffraction (XRD, D8 Advance), respectively. In addition, we conducted Raman spectroscopy (Renishaw inVia) to investigate the carbon materials in the synthesized samples. 2.4

Electrochemical measurements

The electrochemical performance was investigated in a threeelectrode cell using 2 M KOH as the electrolyte via CHI 760E and PARSTAT 4000A electrochemical workstations. The synthesized samples, Pt foil, and Hg/HgO electrode served as the working, counter, and reference electrodes, respectively. The specic capacitance (F g1) was calculated using the following equation: C ¼ I Dt/(m DV) where I (A), Dt (s), m (g), and DV (V) represent the current density, discharge time, the mass loading, and potential window, respectively. 2.5

The fabrication process of ASCs

An asymmetric supercapacitor device was fabricated using CuCo2O4 as the cathode and G@Fe3O4 as the anode. The power

A schematic of the fabrication process for G@Fe3O4 nanosheets.


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density (P, W Kg1) and energy density (E, W h kg1) of the asymmetric supercapacitor were calculated using the following equations: C ¼ I Dt/(mt DV) ð E ¼


P ¼

IV ðtÞdt 3:6 3600 E Dt

where C (F g1), mt (g), DV (V), V(t) (V), dt (s), and Dt (s) represent the capacitance of the asymmetric supercapacitors, mass loading of the active materials in the ASC, potential window, device voltage, time differential, and discharge time, respectively.

3.

Results and discussion

Fig. 2 shows the SEM and TEM images of the Fe3O4 and G@Fe3O4 nanosheet arrays. As shown in Fig. 2a, the Fe3O4 nanosheets grow vertically on the current collector, which are similar to the FeOOH nanosheets (see Fig. S2†). Furthermore, the TEM images, as shown in Fig. 2b and c, show that the Fe3O4 nanosheets have a porous structure due to the evacuation of gaseous contents during the annealing process. The highresolution TEM (HRTEM) image in Fig. 2d shows fringes with a lattice spacing of about 0.14 nm and 0.21 nm, which are indexed to the (531) and (400) planes of Fe3O4, respectively.38,39

Fig. 2

The XRD pattern shown in Fig. S3a† further conrms the presence of Fe3O4 (JCPDS no. 19-0629).39 These results also suggest that the synthesized samples are polycrystalline Fe3O4 nanosheets. Similarly, the G@Fe3O4 samples aer the PECVD and annealing process maintained their nanosheet morphology, as shown in Fig. 2e. Interestingly, the TEM images shown in Fig. 2f and g showed that the G@Fe3O4 nanosheets have a rougher appearance. The HRTEM images of G@Fe3O4 are shown in Fig. 2h. A lattice spacing of about 0.21 nm was measured, corresponding to the (400) plane of Fe3O4. It is observed that the graphene layers (a measured lattice spacing of about 0.34 nm) encapsulating the outermost surface of the Fe3O4 nanoparticles can be easily identied.40,41 Furthermore, the graphene layer numbers were about 3–5. In addition, the XRD pattern shown in Fig. S3b† can be indexed to the Fe3O4 phase (JCPDS no. 190629).39 However, no obvious peaks for the carbon materials can be observed; this may be due to the presence of small amount of carbon nanomaterials in G@Fe3O4. Further evidence for the structural characterization and chemical composition of the as-prepared samples was provided via Raman spectroscopy and XPS, as shown in Fig. 3. As shown in the Raman spectra of the Fe3O4 and G@Fe3O4 samples (Fig. 3a), the peaks at about 542 cm1 and 668 cm1 were indexed to the T2g and A1g vibration modes of Fe3O4, respectively.42,43 Obviously, there are two additional sharp peaks at about 1350 and 1580 cm1 in G@Fe3O4, which have been indexed to the disorder-induced D band and graphitic G band.44–46 This suggests that the encapsulated materials are carbon nanomaterials. Based on the TEM results, the presence of few-layer graphene can be inferred. The sharp and high G band demonstrates the high crystallinity of the graphene layers

SEM images of the (a) Fe3O4 and (e) G@Fe3O4 nanosheets. TEM and HRTEM images of the (b–d) Fe3O4 and (f–h) G@Fe3O4 nanosheets.

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Fig. 3


The (a) Raman spectra and (b) XPS Fe 2p spectra of Fe3O4 and G@Fe3O4. The (c) C 1s and (d) O 1s XPS spectra of G@Fe3O4.

in [email protected] In the high-resolution Fe 2p spectra (Fig. 3b), both the Fe3O4 and G@Fe3O4 samples show two distinct peaks for Fe 2p3/2 (714.9 eV) and Fe 2p1/2 (728 eV), which are consistent with those reported in the literature for Fe3O4.47 Furthermore, as shown in Fig. 3c, the high-resolution C 1s spectrum of the G@Fe3O4 sample could be deconvoluted into four peaks at 284.4 (C]C), 285.5 (C–O–C), 286.3 (C–OH), and 287.8 eV (OH– C]O).48 Obviously, it was found that the C 1s spectrum was mainly composed of the C]C peak. This suggests that the graphene layers in the G@Fe3O4 sample show a highly crystalline nature, in agreement with the HRTEM and Raman results. As shown in Fig. 3d, the high-resolution O 1s spectrum of the G@Fe3O4 sample could be deconvoluted into four peaks at 533.0 (C–O), 531.5 (Fe–O–C), 530.7 (Fe–O), and 530.0 eV (Fe– O).48 It can be inferred that Fe–O–C is formed between the Fe3O4 and graphene layers. According to the previous research, the Fe–O–C bond has a similar mechanism of formation as previously reported Co–O–C and Ni–O–C.49,50 In other words, the strong chemical connection between Fe3O4 and the graphene layer was successfully achieved via the PECVD process, which was benecial for improving the conductivity and cycling performance of the hybrid electrodes.50 Based on the abovementioned results, it can be conrmed that we successfully synthesized the G@Fe3O4 sample, which was encapsulated in situ by the graphene layers to maintain its nanosheet morphology, via the PECVD process. The structural characteristics of the G@Fe3O4 samples can be summarized as follows: rst, vertical-standing G@Fe3O4 nanosheet arrays directly on the conductive substrates without organic binders can facilitate electrolyte diffusion and reduce the resistance between the current collectors and active materials. Second, the graphene coating with moderate layers is a favorable factor for the accommodation of mechanical strain and facilitation of electron transfer, which can be benecial for enhancing the electrochemical properties of Fe3O4. Finally, the highly conductive graphene layers in situ encapsulating the Fe3O4 nanosheets can form a strong connection between the graphene layers and Fe3O4. Therefore, we believe that these unique graphene-Fe3O4 hybrid nanosheet arrays show great potential as anodes for ASCs. The electrochemical performance of the Fe3O4 and G@Fe3O4 electrodes was tested in a three-electrode conguration in the operating window from 1.0 to 0 V (vs. Hg/HgO). A comparison of the cyclic voltammetry (CV) curves at a scan rate of 50 mV s1


is shown in Fig. 4a. Based on the integral area of the CV curves, a signicantly enhanced performance of the G@Fe3O4 electrode as compared to that of Fe3O4 was clearly seen. This can be ascribed to the graphene layers formed in situ on the Fe3O4 nanosheets. Fig. 4b shows the CV curves of the G@Fe3O4 electrode at various scan rates. As the scan rate increases, the CV curves obtained for G@Fe3O4 still retain a denite rectangular shape without any obvious distortion. The shapes of the CV curves indicate that the capacitance of the G@Fe3O4 nanosheet electrode can be attributed to the EDLC via the surface adsorption of electrolyte ions and the pseudocapacitance of Fe3O4 via the redox couple of Fe2+/Fe3+.11,21,51 The typical galvanostatic charge–discharge (GCD) curves were plotted for the G@Fe3O4 electrode at various current densities, as shown in Fig. 4c. Based on the GCD curves, the calculated specic capacitances of the Fe3O4 and G@Fe3O4 electrodes are exhibited in Fig. 4d. It can be seen that the G@Fe3O4 electrode delivers the highest specic capacitance of 732 F g1 at 2 A g1, whereas only 371 F g1 is obtained for Fe3O4. Even at a high current density of 50 A g1, the G@Fe3O4 electrode can still retain a large specic capacitance of 397 F g1, demonstrating good rate capability. In addition, we compared the G@Fe3O4 electrodes formed with different PECVD times (the detailed experiments and discussions are shown in Fig. S5–S7†). Furthermore, the specic capacitance of the G@Fe3O4 electrode was competitive among those of the previously reported Fe-based electrodes (see Table S1†). Long cycling stability is also a signicant factor for anode materials. As shown in Fig. 4e, it was noted that the G@Fe3O4 electrode possessed a good cycling stability aer 10 000 cycles. Electrochemical impedance spectroscopy (EIS) from 0.1 to 100 kHz with an amplitude of 5 mV was further conducted to investigate the electrochemical properties of the obtained Fe3O4 and G@Fe3O4 electrodes, as shown in Fig. 4f. The AC impedance equivalent circuit is shown in Fig. S8.† In both Nyquist plots, there was a semicircle in the high-frequency region and a straight line in the low-frequency region, related to the charge transfer resistance (Rct) and the capacitive behavior of the electrode, respectively.52–55 It was noted that G@Fe3O4 showed a much lower Rct (about 1.2 U) than Fe3O4 (about 3.0 U) due to the graphene layer in situ encapsulation of Fe3O4; this permitted ultra-fast electron transfer reaction kinetics. The almost vertical line in the low frequency further conrms the ideal capacitive behavior of the G@Fe3O4 electrode.56 Furthermore, G@Fe3O4 shows a lower internal resistance (ESR), which can be estimated

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Fig. 4 (a) The CV curves obtained for the Fe3O4 and G@Fe3O4 electrodes at a scan rate of 50 mV s1. (b) The CV curves and (c) GCD curves obtained for the G@Fe3O4 electrodes. (d) The calculated specific capacitance of the Fe3O4 and G@Fe3O4 electrodes at various current densities. (e) The cycling stability of the Fe3O4 and G@Fe3O4 electrodes tested using GCD at a current density of 20 A g1. (f) The Nyquist plots obtained for the Fe3O4 and G@Fe3O4 electrodes.

by the intersection of the plot and the x-axis.53 These lower Rct and ESR are benecial for enhancing the electrochemical performance of the G@Fe3O4 electrode. The remarkable electrochemical performance of the G@Fe3O4 anode can be attributed to the following effects: (i) the G@Fe3O4 nanosheets directly grown on the current collector without any organic binder ensure fast ion diffusion and low contact resistance; (ii) the vertical-standing nanosheet arrays can facilitate fast reaction kinetics and boost the effective surface area of the active material; (iii) the graphene coating with moderate layers not only benets electron transfer, but also protects active Fe3O4 from degradation during cycling to obtain outstanding cycling performance; (iv) the formation of Fe–O–C bonds can lead to a strong chemical connection between Fe3O4 and the graphene layer that is benecial for improving the conductivity and cycling performance of the hybrid electrodes. All the aforementioned merits generate the remarkable electrochemical performance of the G@Fe3O4 electrode. To investigate the feasibility of the G@Fe3O4 electrode as an anode in ASCs, we also synthesized CuCo2O4 nanowires as the cathode. As shown in Fig. 5a and b, the high-density CuCo2O4 nanowire arrays were uniformly and vertically grown on the substrates. The TEM image of a typical CuCo2O4 nanowire with a diameter of about 100 nm is shown in Fig. 5c. Furthermore, the XRD pattern in Fig. 5d shows that all the diffraction peaks are indexed to the diffraction planes of CuCo2O4 (JCPDS card no. 78-2177).57 Fig. 5e shows the CV curves obtained for the CuCo2O4 electrode at various scan rates. Overall, a pair of redox peaks can be observed, demonstrating a typical pseudocapacitive behavior.57 Based on the GCD curves shown in Fig. 5f, we

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calculated the specic capacitance of CuCo2O4 at various current densities. As shown in Fig. 5g, the CuCo2O4 electrode delivers a specic capacitance of 608 F g1 at 2 A g1 and retains about 73% of its capacitance at 20 A g1. Additionally, the synthesized CuCo2O4 electrode exhibits a capacitance retention of about 90.2% aer 10 000 cycles. Therefore, we believe that the as-synthesized CuCo2O4 electrode is an excellent cathode material for the design of ASCs that can offer high energy and power densities and outstanding cycling performance. To investigate the practical energy storage device properties, we fabricated an ASC using the G@Fe3O4 electrode as the anode and synthesized CuCo2O4 as the cathode in a 2 M KOH electrolyte, as illustrated in Fig. 6a. The CV curves obtained for the as-fabricated ASC at various voltage windows at the scan rate of 50 mV s1 are shown in Fig. 6b. It can be found that a stable potential window of the as-fabricated ASC can be reached at 1.6 V. Thus, we have chosen the potential window of 0–1.6 V to further investigate the electrochemical performance of the CuCo2O4//G@Fe3O4 ASC. Fig. 6c shows the CV curves obtained for the ASC at various scan rates in the potential window from 0 to 1.6 V. The CV curves have a non-rectangular shape with a couple of broad reversible redox peaks, which indicate that the capacitance mainly originates from the redox reactions. The non-linear GCD curves shown in Fig. 6d further conrm the contribution from the faradaic redox reaction, which is in accordance with the CV curves (Fig. 6c). Based on the GCD curves at various current densities from 2 to 20 A g1, the calculated specic capacitance of the ASC is shown in Fig. S9a.† The ASC delivers a specic capacitance of 182 and 140 F g1 at a current density of 2 and 20 A g1, respectively, demonstrating


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Fig. 5 (a and b) SEM and (c) TEM images, and (d) XRD pattern of the CuCo2O4 nanowire arrays. (e) The CV and (f) GCD curves obtained for the CuCo2O4 electrode. (g) The calculated specific capacitance of the CuCo2O4 electrode at various current densities. (h) The cycling stability of the CuCo2O4 electrode investigated using GCD at a current density of 20 A g1.

good rate capability. Fig. S9b† shows the long-term cycling stability of the ASC, which retains about 88.3% of its initial specic capacitance aer 10 000 cycles. The energy and power density of the ASCs were calculated from the GCD curves and are plotted in the Ragone plot (Fig. 6e). The energy density of the as-fabricated ASC can reach about 82.8 W h kg1 at a power density of 2047 W kg1. In particular, the energy density of our

fabricated ASC device was superior to those of the previously reported ASCs such as CuCo2O4/CuO//RGO/Fe2O3 (33.0 W h kg1),58 MnCo–LDH@Ni(OH)2//AC (47.6 W h kg1),59 meso-NiO/Ni//carbon nanocage (19.1 W h kg1),20 Co(P,S)//CC (39 W h kg1),60 and Co3O4 nanosheets//MnO@C nanosheets (59.6 W h kg1)61 (see Table S2†). The prominent performance of the CuCo2O4//G@Fe3O4 device indicates that the G@Fe3O4

(a) A schematic of the ASC device configuration. (b) The CV curves obtained for the ASC device at different potential windows at a scan rate of 50 mV s1. The (c) CV and (d) GCD curves of the as-fabricated ASC. (e) The Ragone plot of the ASC.

Fig. 6


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electrode is an ideal anode candidate for energy storage in practical supercapacitors.


4. Conclusions In conclusion, we have successfully developed and synthesized a G@Fe3O4 anode for ASCs. The in situ formation of graphene layers on the Fe3O4 nanosheets can facilitate electron transfer and maintain the overall nanosheet structure, leading to signicantly enhanced electrochemical performance. A highest specic capacitance of 732 F g1 at 2 A g1 was attained for the G@Fe3O4 anode, which was superior to those previously reported for Fe-based hybrid materials. Furthermore, the G@Fe3O4 hybrid electrode shows excellent cycling stability due to the in situ formation of graphene layers on the Fe3O4 nanosheets. Remarkably, the as-fabricated CuCo2O4//G@Fe3O4 device exhibits an excellent energy density and good cycling stability. The present investigation provides a novel route towards the development of anode materials in energy storage devices for practical applications.

Conflicts of interest There are no conicts to declare.

Acknowledgements The support received from the National Natural Science Foundation of China, China (Grant No. 51575135 and U1537206) is highly appreciated.

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