One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices

CoFe2O4/reduced graphene oxide (CoFe2O4/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach. The three-dimensional (3D) network structure consists of well-dispersed CoFe2O4 nanoparticles on the surfaces of graphene sheets. As a binder-free electrode material for supercapacitors, the electrochemical properties of the CoFe2O4/rGO hybrid hydrogel can be easily adjusted by changing the concentration of the graphene oxide (GO) precursor solution. The results indicate that the hybrid material made using 3.5 mg mL−1 GO solution exhibits an outstanding specific capacitance of 356 F g−1 at 0.5 A g−1, 68% higher than the pure CoFe2O4 counterpart (111 F g−1 at 0.5 A g−1), owing to the large specific surface area and good electric conductivity. Additionally, an electrochemical energy storage device based on CoFe2O4/rGO and rGO was assembled, which exhibits a high energy density of 17.84 W h kg−1 at a power density of 650 W kg−1 and an excellent cycling stability with 87% capacitance retention at 5 A g−1 after 4000 cycles. This work takes one step further towards the development of 3D hybrid hydrogel supercapacitors and highlights their potential application in energy storage devices.


Introduction
In recent years, the conversion and storage of energy originating from green and renewable resources has becoming increasingly in demand. Supercapacitors (also known as electrochemical capacitors), as new energy storage systems, have attracted huge attention due to their fast charge/discharge rate, superior power density and long cycle life. [1][2][3][4][5][6] However, the low energy density (normally less than 10 W h kg À1 ) has limited their commercial applications. 1,6 In the past few years, numerous researchers have already made many efforts to improve the specic capacitance by developing nanostructured electrode materials or increasing the operating voltage of supercapacitors without sacricing their power density and cycle life. [7][8][9][10][11] The electrochemical performance of supercapacitors depends mainly on the structure and properties of the electrode materials. To date, various materials such as transition metal oxides, metal hydroxides and carbon-based materials have been extensively explored. [12][13][14][15][16][17][18][19] Among them, one kind of binary metal oxide, CoFe 2 O 4 , has been widely used as an electrode material for lithium ion batteries and supercapacitors, due to its high theoretical specic capacitance, good chemical stability, and good electrical and magnetic properties. [20][21][22][23] However, the practical application of pure CoFe 2 O 4 is limited because of its poor electrical conductivity. To address this problem, many efforts have been devoted to developing nanocomposites based on CoFe 2 O 4 and carbon materials. [24][25][26][27][28][29] For instance, a ternary cobalt ferrite/graphene/polyaniline composite was prepared via a combination of hydrothermal and polymerization processes, and was found to exhibit a high specic capacitance of 1133.3 F g À1 , superior rate capability and excellent cycling stability. 24 Similarly, He et al. reported that the specic capacitance and cycling stability of reduced graphene oxide-CoFe 2 O 4 composites synthesized using a co-precipitation method were greatly improved compared with those of the pure CoFe 2 O 4 electrode. 30 Such composite materials not only can improve the specic capacitance, but also contribute to an improved rate capability. Thus, it is advantageous to introduce carbonaceous species into nanostructured CoFe 2 O 4 -based electrode materials to enhance their electrochemical properties.
Graphene, a typical two-dimensional (2D) one-atom-thick carbon material, has attracted considerable attention because of its appealing features including huge surface area, superb thermal conductivity, electronic conductivity, and mechanical properties. [31][32][33][34][35] Interestingly, Shi's group recently reported a selfassembled graphene hydrogel in a 3D network via a convenient one-step hydrothermal method, which exhibited good electrical conductivity, mechanical strength and thermal stability, showing promise for high performance supercapacitors. 36 Many researchers have carried out intensive studies on various electrode materials based on graphene hydrogels. [37][38][39][40][41][42] For example, Wang et al. reported that NiOOH nanosheet/graphene hydrogelbased electrode materials, obtained via combined solvothermal and hydrothermal reactions, delivered a high capacitance of 1162 F g À1 at 1 A g À1 and 981 F g À1 at 20 A g À1 . 38 Zhu et al. assembled a hierarchical and interconnected reduced graphene oxide/b-MnO 2 nanobelt hybrid hydrogel via a hydrothermal route, which exhibited higher specic capacitance and better cycling stability than pure b-MnO 2 nanobelts. 39 Nevertheless, the synthesis of CoFe 2 O 4 nanostructures with a self-assembled 3D graphene hydrogel composite for use in supercapacitors has rarely been reported.
Herein, novel CoFe 2 O 4 nanoparticles/reduced graphene oxide (CoFe 2 O 4 /rGO) hybrid hydrogels were synthesized via a facile one-pot solvothermal method. The attachment of uniformly-distributed CoFe 2 O 4 nanoparticles on the surfaces of graphene sheets is benecial for providing more electroactive sites for faradic redox reactions. Interestingly, the CoFe 2 O 4 /rGO hybrid hydrogels possess a 3D mesoporous network, which can offer easy diffusion of the electrolyte and efficient pathways for electron transfer and ion transport. As binder-free electrode materials for supercapacitors, the CoFe 2 O 4 /rGO hybrid hydrogels exhibit excellent electrochemical performance with high capacitance, good rate capability and remarkable cycling stability. In addition, an electrochemical energy storage device was assembled, using CoFe 2 O 4 /rGO hybrid hydrogel as the positive electrode and rGO hydrogel as the negative electrode, and was found to deliver high energy density and cycling stability. The charming electrochemical properties demonstrate the potential application of CoFe 2 O 4 /rGO hybrid hydrogels in high performance supercapacitors.
GO was prepared according to a modied Hummers' method. [40][41][42][43] The CoFe 2 O 4 /rGO hybrid hydrogel was prepared using a solvothermal method. In a typical procedure, GO powder was dispersed in 30 mL of deionized water to form a homogeneous aqueous solution (2 mg mL À1 ). 1 mmol of Co(NO 3 ) 2 $6H 2 O, 2 mmol of Fe(NO 3 ) 2 $6H 2 O and 5 mmol of sodium acetate were dissolved into 30 mL of ethylene glycol. Then the above solutions were mixed together and vigorously stirred for 1 h. Subsequently, the mixed solution was transferred into an 80 mL Teon-lined stainless-steel autoclave and kept at 180 C for 12 h. Aer cooling down to room temperature naturally, the CoFe 2 O 4 /rGO hydrogels were rinsed with deionized water several times. For further characterization, freezedrying technology under vacuum was employed to obtain aerogels. Four different CoFe 2 O 4 /rGO hybrid hydrogels were prepared by changing the concentration of the GO precursor solution (2, 3, 3.5 and 4 mg mL À1 ). For comparison, a pure CoFe 2 O 4 sample was fabricated by following the same solvothermal process in the absence of GO. Meanwhile, a pure rGO hydrogel was prepared by using the same solvothermal process without inorganic metal salts as precursors.

Structural characterization
The morphology and microstructure of these samples were characterized using scanning electron microscopy (SEM, JEOL Hitachi S-4700, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM200CX, JEOL). Powder Xray diffraction (XRD, Bruker D8 Advance diffractometer with Cu-Ka radiation) experiments were performed to study the crystallographic information of the samples. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientic, USA) was performed using an ESCALab MKII spectrometer with Al Ka (1.4866 keV) as the X-ray source. The specic Brunauer-Emmett-Teller (BET) surface areas of the hydrogels were measured by analyzing the nitrogen adsorption and desorption isotherms at À196 C, obtained using a Micromeritics Model ASAP 2020 sorptometer.

Electrochemical measurements
Electrochemical properties were measured on an IVIUM electrochemical workstation in 6 M KOH solution as the electrolyte. A three-electrode system was employed, where Pt and Hg/HgO were used as the counter electrode and the reference electrode, respectively. The pure CoFe 2 O 4 electrode was prepared by mixing 80 wt% CoFe 2 O 4 powder with 10 wt% acetylene black and 10 wt% polytetrauorene ethylene (PTFE) binder together. Then, drops of isopropyl alcohol were used to form slurries thoroughly. Aer drying in a vacuum oven at 60 C for 8 h, the mixture was pressed onto nickel foam current collectors (1 Â 1 cm 2 ). The CoFe 2 O 4 / rGO hybrid hydrogel electrode was prepared by cutting a piece of freeze-dried hydrogel, and then the patch was pressed onto the nickel foam current collectors (1 Â 1 cm 2 ). Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements were carried out to test the electrochemical properties of the asprepared electrodes. Electrochemical impedance spectroscopy (EIS) was preformed over a frequency range from 10 5 to 0.01 Hz at an amplitude of 5 mV. The capacitive performance of the electrodes in a two-electrode conguration was also assessed using CV and GCD, and a LAND battery system was used to examine the cycling stability. A battery-capacitor hybrid device was fabricated using CoFe 2 O 4 /rGO hybrid hydrogel as the positive electrode and rGO hydrogel as the negative electrode. The capability of the hybrid devices (CoFe 2 O 4 /rGO//rGO) (two single devices were connected in series) was evaluated by powering a light-emitting-diode (LED 3 V, 0.06 W).

Structural characterization
The morphology and crystalline structure of the CoFe 2 O 4 /rGO hybrid hydrogel were investigated by SEM and XRD. Fig. 1a and b show the SEM images of the CoFe 2 O 4 /rGO hybrid hydrogel aer freeze-drying treatment under vacuum, displaying a 3D porous network with pore sizes ranging from sub-micrometers to several micrometers. This network is benecial for enhancing the electrochemical properties, as it not only provides an efficient pathway for electron transport but also reduces the ion diffusion resistance of the electrolyte for charge storage reactions. 36,38 In addition, the 3D porous network provides abundant active sites to load CoFe 2 O 4 nanoparticles. As shown in the elemental mapping images (Fig. 1c), C, Fe and Co are uniformly distributed in the composite, suggesting that the CoFe 2 O 4 nanoparticles are homogeneously dispersed on the surface of the graphene sheets.  44 Aer forming a composite with rGO, the highly crystalline nature of the CoFe 2 O 4 /rGO hybrid sample did not change. However, the conventional graphene-sheet stacking peak at 2q ¼ 24.5 assigned to the (002) plane cannot be obviously seen, indicating that the graphene sheets might be restacked during the reduction process, resulting in the amorphous nature. 26 To further investigate the details of the morphology and composition, TEM, HRTEM and EDX were employed. The TEM images ( Fig. 2a and b) conrm that the CoFe 2 O 4 nanoparticles are well-dispersed and rmly anchored on the graphene sheets. The HRTEM image in Fig. 2c reveals that the clear lattice fringe with an interplanar spacing of 0.25 nm corresponds well to the (311) plane of CoFe 2 O 4 . The EDX results (Fig. 2d) show the elements Co, Fe, O and C, which are in accordance with the nanocomposite and a Co/Fe atom ratio of about 1 : 2, further demonstrating the successful synthesis of the CoFe 2 O 4 /rGO hybrid hydrogel.
The chemical structure and valence states were studied using X-ray photoelectron spectroscopy (XPS) as shown in Fig. 3. The survey scan XPS spectrum of the CoFe 2 O 4 /rGO composite shows the existence of the elements Fe, Co, C and O. The highresolution C1 XPS spectrum in Fig. 3b demonstrates plenty of functional groups on the graphene sheets. For example, the C 1s peaks at 284.7 and 286.5 eV can be attributed to the C]C and C-O groups of rGO in the composite, respectively. 42 The high resolution Co 2p spectrum (Fig. 3c) is deconvoluted into two spin-orbit doublets located at 781.6 and 796.7 eV, corresponding to the electronic states of Co 2p 3/2 and Co 2p 1/2 , respectively, accompanied by two shakeup satellites centered at 784.8 and 803.8 eV. The main spin-energy separation of 15.1 eV is a signature of the Co 2+ oxidation state. 45 As shown in Fig. 3d, in addition to the satellite peak at 716.7 eV, the peaks at 711.8 and 725.6 eV correspond to Fe 2p 3/2 and Fe 2p 1/2 spin-orbit peaks, respectively, which are attributed to the dominant states of Fig. 1 (a and b)    Paper Fe 3+ . 44,46,47 The 2p 3/2 -2p 1/2 separation and satellite structures feature a broad peak in the sample, which is characteristic of high-spin octahedral cations, indicating the existence of some Fe 2+ components. 48 As indicated by XPS analysis, CoFe 2 O 4 /rGO hybrid hydrogels were successfully synthesized. Fig. 4 shows the Raman spectra of rGO and the CoFe 2 O 4 /rGO hybrid hydrogel. Both display a D-band ($1350 cm À1 ) related to the breathing mode of sp 2 carbon atoms and activated by the existence of oxygen-containing groups, 49 and a G-band ($1590 cm À1 ) ascribed to the edge planes and disordered structure. Interestingly, the 2D-band is missing in both samples, possibly because the rGO hydrogel processes a network structure instead of a monolayer graphene structure. 50 The intensity ratio of D-band to G-band (I D /I G ¼ ca. 1.2) indicates that several functional groups still exist in the graphene in the pure rGO sample, 51 which is benecial for composite formation with CoFe 2 O 4 . Unfortunately, due to the large amount of rGO in the hybrid hydrogel sample, the spinel CoFe 2 O 4 phase cannot been clearly seen.
Nitrogen physisorption measurements were conducted to characterize the porous features of the pure CoFe 2 O 4 and CoFe 2 O 4 /rGO composite. As presented in Fig. 5, the specic BET surface area of the CoFe 2 O 4 /rGO composite was calculated to be 614.4 m 2 g À1 , while that of the pure CoFe 2 O 4 nanoparticle sample is only 179.7 m 2 g À1 . This indicates that aer the reaction with the rGO hydrogel, the obtained CoFe 2 O 4 /rGO composite maintains a 3D structure with a remarkably increased surface area, which is 3.4 times larger than that of pure CoFe 2 O 4 , suggesting that the construction of a 3D framework via a hydrothermal route is an effective way to achieve nanocomposites with a high surface area. As displayed in Fig. 5a, the CoFe 2 O 4 /rGO composite exhibits a typical type-IV hysteresis loop at a relative pressure of between 0.4 and 0.9, and the pore size distribution is centered at 3.5 nm (inset in Fig. 5a), while the pore size distribution for the pure CoFe 2 O 4 sample is mainly located at $10 nm (inset in Fig. 5b). The higher surface area and much smaller mesopores of the CoFe 2 O 4 /rGO composite would provide a more convenient channel for ion diffusion and electron transfer, leading to a higher electrochemical capacity. Moreover, the introduction of the rGO hydrogel not only reduces the agglomeration of CoFe 2 O 4 nanoparticles, but the attachment of CoFe 2 O 4 nanoparticles can also weaken the strong interaction between the rGO sheets, which is benecial for the formation of the porous rGO-based aerogel. 42

Electrochemical characterization
The electrochemical properties of the pure CoFe 2 O 4 nanoparticle sample and the four CoFe 2 O 4 /rGO hybrid hydrogels with different GO precursor solution concentrations were rstly evaluated in a three-electrode cell, and the results are shown in Fig. 6. The cyclic voltammetry (CV) curves in Fig. 6a were measured at a sweep rate of 30 mV s À1 within the potential window of 0-0.5 V in 6 M KOH aqueous solution. A pair of symmetric redox peaks can be clearly observed on each CV curve, indicating that the capacitance characteristics are mainly governed by surface faradaic redox mechanisms. 20 The reaction of the conversion in the electrolyte between different cobalt and iron oxidation states can be described by the following equation: Furthermore, the area of the CV curves for the CoFe 2 O 4 /rGO composite synthesized with a GO precursor solution concentration of 3.5 mg mL À1 (denoted as CoFe 2 O 4 /rGO(3.5)) is larger than that of the other three hybrid samples, showing the highest specic capacitance. They follow the order: CoFe 2 O 4 / rGO (3.5

) > CoFe 2 O 4 /rGO(3) > CoFe 2 O 4 /rGO(4) > CoFe 2 O 4 /rGO(2).
Galvanostatic charge-discharge (GCD) measurements at a current density of 1 A g À1 within a potential window of 0-0.5 V (vs. SCE) were conducted to further estimate the best ratio of CoFe 2 O 4 and rGO (Fig. 6b). A distinct potential plateau region and nearly symmetric curves for all of the electrodes are observed, suggesting the superb coulombic efficiency of the charge-discharge process. The specic capacitance of the different hybrid hydrogel samples can be calculated according to eqn (2): where C is the specic capacitance, i is the discharge current, t is discharge time, m is the mass of the active materials and Dn is the potential window.   (CoFe 2 O 4 /rGO (4)). It is noteworthy that the hybrid sample CoFe 2 O 4 /rGO(3.5) delivers the largest specic capacitance, which is in accordance with the area results from the CV curves in Fig. 6a. To further explore the electrochemical performance of the hybrid hydrogels, the CoFe 2 O 4 /rGO(3.5) sample was examined using CV at different scan rates and GCD at different current densities. Similarly, the CV curves measured at scan rates from 5 to 50 mV s À1 , as shown in Fig. 6c, exhibit a pair of symmetric redox peaks on each CV curve and the shapes are slightly different. The oxidation peak shis to a higher potential, while the reduction peak shis to a lower potential as the scan rate increases. A high symmetry at different current densities from 0.5 to 10 A g À1 is displayed in the GCD curves for the CoFe 2 O 4 /rGO(3.5) electrode (Fig. 6d). The specic capacitance values were calculated to be 356, 310.7, 298, 280, 265 and 244 F g À1 at current densities of 0.5, 1, 2, 3, 5 and 10 A g À1 , respectively. The hybrid hydrogel electrode maintains capacitance retention as high as 68.5% aer increasing the current density 20-fold, indicating good rate capability. The comparison of the specic capacitance for different composite samples with various current densities is plotted in Fig. 7a. With increasing current density, the specic capacitance for all the electrodes decreases gradually. This phenomenon is possibly attributed to the insufficient electroactive material involved in the faradaic reactions at higher current densities. These samples follow the trend: CoFe 2 O 4 /rGO(3.5) > CoFe 2 O 4 /rGO(3) > CoFe 2 O 4 /rGO(4) > rGO > CoFe 2 O 4 /rGO(2) > CoFe 2 O 4 , which is in good agreement with the results in Fig. 6a and b. This indicates that the introduction of an appropriate amount of rGO into CoFe 2 O 4 nanoparticles is helpful for enhancing the specic capacitance. Meanwhile, varying the loading amount of the excellent conductive material rGO greatly affects the electrical conductivity of the composites, as indicated in the EIS spectra in Fig. 7b. The Nyquist plots for all the electrode samples display a depressed semicircle in the highfrequency region, corresponding to the charge transfer resistance, and a straight line in the low-frequency region, reecting the diffusion of the electroactive species. The CoFe 2 O 4 /rGO (3.5) electrode demonstrates the smallest semicircles, suggesting that it has the smallest charge transfer resistance, thus leading to the largest specic capacitance.
To further investigate the electrochemical properties of the CoFe 2 O 4 /rGO hybrid hydrogel electrodes in a full-cell conguration, an electrochemical energy storage device was assembled using the optimal CoFe 2 O 4 /rGO(3.5) sample as the positive electrode and the reduced graphene oxide (rGO) hydrogel as the negative electrode. To balance the charge storage between the positive electrode and negative electrode, the masses of the two electrodes were calculated according to eqn (3):    where m + and m À are the masses of the positive and negative electrodes (g), respectively, and the specic capacitance C SÀ , C S+ (F g À1 ), and the voltage range Dn À , Dn + (V) are for the negative and positive electrodes, respectively. 39 The specic capacitance of CoFe 2 O 4 /rGO(3.5) is 310.7 F g À1 , while that of rGO is 197.5 F g À1 at a current density of 1 A g À1 (Fig. 7a). The potential windows of the CoFe 2 O 4 /rGO hybrid electrode and the pure rGO electrode are 0-0.5 V and À0.8-0 V, respectively. Therefore, the mass ratio m þ m À between the two electrodes is 1.02. Fig. 8a Fig. 8b. The curves of each GCD exhibit a good symmetrical shape. In particular, the potential window of the hybrid device is beyond the thermodynamic limit of $1.2 V and thus greatly improves the energy density. 44,48,52 The potential window of 0-1.3 V was chosen for further investigation with CV and GCD tests. Fig. 8c shows the CV curves of the CoFe 2 O 4 /rGO//rGO hybrid device at scan rates varying from 20 to 50 mV s À1 . With increasing scan rate, the peak current increases. Moreover, the CV curves show two relatively weak redox peaks, revealing synergistic contributions from both the electric double-layer capacitance and the pseudocapacitance. The specic capacitances calculated from the GCD curves in Fig. 8d of the hybrid device are 76, 60.1, 52.7, 48.6 and 46 F g À1 at a current density of 1, 2, 3, 4 and 5 A g À1 , respectively (Fig. 8e). The energy density and power density were calculated according to eqn (4) and (5), respectively: where C is the specic capacitance, DV is the potential window, and Dt is the discharge time. The hybrid device delivers a maximum energy density of 17.84 W h kg À1 at a power density of 650 W kg À1 and a maximum power density of 3250 W kg À1 at an energy density of 10.8 W h kg À1 (Fig. 8f).
The cycling performance of the CoFe 2 O 4 /rGO//rGO hybrid device was also tested for charge-discharge cycles at a high current density of 5 A g À1 , and the specic capacitance was found to still be stable at 40.2 F g À1 with 87% retention of the initial value aer 4000 cycles (Fig. 9). Additionally, the coulombic efficiency remains near 92% aer 4000 cycles, further conrming the excellent cycling stability and reversibility of the faradic reactions. Table 1 lists the main electrochemical parameters of the CoFe 2 O 4 /rGO hybrid hydrogel electrode compared with the related electrode materials in the literature. It is obvious that the specic capacitance of the novel hydrogel electrode material is comparable with the best values obtained for related CoFe 2 O 4 -based electrode materials that possess excellent stability. Moreover, two CoFe 2 O 4 /rGO//rGO devices connected in series can power a light emitting diode (LED) with a voltage of 3.0 V for several seconds (inset (right) in Fig. 9), conrming their promising application in energy storage devices.

Conclusions
In summary, we have presented a scalable strategy to construct a unique architecture comprising 3D porous rGO hydrogelsupported CoFe 2 O 4 nanoparticles, through a facile one-pot solvothermal process. The as-prepared CoFe 2 O 4 nanoparticles are evenly distributed on the surfaces of graphene sheets. When used as a binder-free supercapacitor electrode material, CoFe 2 O 4 /rGO synthesized using 3.5 mg mL À1 GO precursor solution shows a high capacitance of up to 356 F g À1 at 0.5 A g À1 , which is 3.2 times higher than the pure CoFe 2 O 4 nanoparticle electrode, due to the large specic surface area and excellent electric conductivity. Moreover, the CoFe 2 O 4 /rGO//rGO electrochemical energy storage device exhibits a high energy density of 17.84 W h kg À1 at a power density of 650 W kg À1 and 87% capacitance retention at 5 A g À1 aer 4000 cycles. The excellent electrochemical performance indicates that CoFe 2 O 4 / rGO hybrid hydrogels hold great promise for high-performance energy storage devices.

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