A Facile Microwave Hydrothermal Synthesis of ZnFe2O4/rGO Nanocomposites for Supercapacitor Electrodes

As a typical binary transition metal oxide, ZnFe2O4 has attracted considerable attention for supercapacitor electrodes due to its high theoretical specific capacitance. However, the reported synthesis processes of ZnFe2O4 are complicated and ZnFe2O4 nanoparticles are easily agglomerated, leading to poor cycle life and unfavorable capacity. Herein, a facile microwave hydrothermal process was used to prepare ZnFe2O4/reduced graphene oxide (rGO) nanocomposites in this work. The influence of rGO content on the morphology, structure, and electrochemical performance of ZnFe2O4/rGO nanocomposites was systematically investigated. Due to the uniform distribution of ZnFe2O4 nanoparticles on the rGO surface and the high specific surface area and rich pore structures, the as-prepared ZnFe2O4/rGO electrode with 44.3 wt.% rGO content exhibits a high specific capacitance of 628 F g−1 and long cycle life of 89% retention over 2500 cycles at 1 A g−1. This work provides a new process for synthesizing binary transition metal oxide and developing a new strategy for realizing high-performance composites for supercapacitor electrodes.


Introduction
Due to the severe worldwide energy crisis and global environmental degradation, it is necessary to explore renewable and clean energy sources in combination with finding suitable energy storage devices [1] among various energy storage devices, such as secondary batteries, fuel cells, and supercapacitors. Compared with secondary batteries and fuel cells, supercapacitors (SCs) have gained significant attention in recent years owing to their high power density, quick charge/discharge capability, and long cycle life in recent years [2][3][4][5].
Supercapacitors can be divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors arising from different energy storage mechanisms. The EDLCs store charges by electrostatic adsorption/desorption at the interface of the electrodes/electrolytes. Activated carbon, carbon aerogels, graphene, and carbon nanotubes (CNTs) are widely used in EDLCs due to their high specific surface area and high conductivity [6][7][8]. The researchers prepared an activated carbon as a supercapacitor electrode, and it exhibits a high specific capacitance of 333.8 F g −1 and long cycle life of 96% retention over 38,000 cycles [8]. Although EDLCs have high power density and long cycle life due to the physical adsorption/desorption, while the low specific capacitance and energy density severely limits their broader application [9,10]. Different from EDLCs, pseudocapacitors usually store charges by fast and reversible surface or near-surface redox reactions [11]. Conducting polymers and transition metal oxides are often used as electrodes of pseudocapacitors, which possess high theoretical specific capacitance [12,13]. However, the undesirable aggregation or volume expansion and poor electrical conductivity of such conducting polymers and transition metal oxides during the charge and discharge process leads to low specific capacitance, poor rate capability, and poor cycle life which limits their practical applications.
Compared with single transition metal oxides, binary transition metal oxides usually possess better electrical conductivity, a different redox potential, and a high specific surface area, which is beneficial for realizing high electrochemical performance [14][15][16]. For example, Zate et al. synthesized Ni x Mn 1-x Fe 2 O 4 film and used it as the electrode of a supercapacitor which showed a specific capacitance of 185 F g −1 at 5 mV s −1 for Ni 0.8 Mn 0.2 Fe 2 O 4 [17]. Giri et al. prepared nitrogen-doped rGO-NiMnO 3 by a hydrothermal method, which delivered a specific capacitance of 749.2 F g −1 at 0.5 A g −1 [18]. The above results demonstrate the feasibility of binary transition metal oxides as electrode materials for supercapacitors.
As a typical binary transition metal oxide, ZnFe 2 O 4 gained much attention due to its high theoretical specific capacitance (~2600 F g −1 ), but is still restricted by poor conductivity and severe agglomeration during charge and discharge process, causing low specific capacitance and cycle life [19,20]. Numerous studies have confirmed that synthesizing composites with carbonaceous material is an effective way to enhance the capacitance and lifespan of ZnFe 2 O 4 for supercapacitors [21][22][23][24]. Due to the superior electrical conductivity and large theoretical specific surface area, graphene is the perfect candidate for dispersed nanoparticles. Therefore, designing composites by spreading binary transition metal oxides onto a graphene surface is feasible to improve electrode performance. Li et al. prepared ZnFe 2 O 4 and nitrogen-doped graphene composites via the solvothermal method, which delivered specific capacitances of 244 F g −1 at a current density of 0.5 A g −1 : much higher than pure ZnFe 2 O 4 nanoparticles [25].
Furthermore, the material morphology and the particle size also have an important influence on the electrochemical properties of supercapacitors [26][27][28]. For instance, when a size-controllable ZnFe 2 O 4 /rGO hybrid was prepared through a solvothermal process, the high-performance supercapacitor electrode was achieved by controlling the ZnFe 2 O 4 particle size [16]. In addition, a pure ZnFe 2 O 4 deposited on nickel foam produced a specific capacitance of 552 F g −1 at a scan rate of 5 mV s −1 as a binder-free supercapacitor material [29]. Compared with the traditional heating process, microwave hydrothermal is a uniform heating process that occurs within the material, and it can achieve the synthesis of nano-sized particles within a few minutes [30][31][32][33]. Some researchers have made a great deal of progress in synthesizing ZnFe 2 O 4 using the microwave hydrothermal method. For example, Wang et al. synthesized ZnFe 2 O 4 /rGO nanomaterials for the removal of methylene blue contamination by using a microwave hydrothermal technique, and achieved successful removal (contamination removal rate close to 100%) [34]. However, there is still limited research on using this facile microwave hydrothermal process to synthesize ZnFe 2 O 4 /rGO nanocomposites for supercapacitor electrodes.
Considering the above information, ZnFe 2 O 4 /rGO nanocomposites with nano-sized ZnFe 2 O 4 were prepared using this facile microwave hydrothermal method. The generation of ZnFe 2 O 4 nanoparticles and the reduction of GO were accomplished simultaneously. The effects of rGO content on morphology, chemical, and crystalline structure were investigated by powder X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM) tests. In addition, the electrochemical performance of ZnFe 2 O 4 /rGO nanocomposites with different rGO contents was evaluated with a series of electrochemical tests. We believe that this work can provide a new perspective for designing high-performance electrode materials for supercapacitors.

Materials Synthesis
The modified Hummers' method was used to prepare graphene oxide (GO). The microwave hydrothermal process was selected to synthesize ZnFe 2 O 4 /rGO nanocomposites. First, 50 mg freeze-dried GO powder was dissolved in 50 mL distilled water and sonicated for 1 h to obtain a homogeneous suspension. Subsequently, ZnCl 2 and FeCl 3 ·6H 2 O were added into the GO suspension with a molar ratio of 1:2 and stirred for another 1 h. The pH value of the solution was then adjusted to 8 with the addition of NaOH (1 mol L −1 ). Next, the mixture was transferred into a sealed reactor with microwave heating for 7 min at a microwave power of 700 W and microwave frequency of 2450 MHz. Finally, the reactor was cooled down to room temperature naturally. To obtain the final product, the obtained solution was washed with ethanol and distilled water several times and freeze-dried for 40 h (denoted as ZnFe 2 O 4 /rGO−2). In addition, ZnFe 2 O 4 was prepared under the same conditions without the addition of GO, and ZnFe 2 O 4 /rGO−1 and ZnFe 2 O 4 /rGO−3 were prepared by adjusting GO additions for comparison.

Materials Characterization
Powder X-ray diffraction (XRD) experiments using a Rigaku Smart-lab diffractometer equipped with Cu-Kα radiation (λ = 0.15418) from 20 • to 70 • at a scan rate of 10 • /min were used to examine the crystal structures. A Thermo Scientific K-Alpha electron spectrometer was used for the X-ray photoelectron spectrometer (XPS) tests. A confocal laser micro-Raman spectrometer (LABRAMHR, JY Co., Parise, France) was used to record the Raman spectra. The morphology and microstructure of the obtained materials were examined on a field-emission scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (FE-SEM, Zeiss Sigma 500). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on a JSM-2100Plus (JEOL, Tokyo, Japan). On TG/DTA7200, thermogravimetric analysis (TGA) was carried out under an air atmosphere from 20 • C to 800 • C with a heating rate of 10 • C/min. The N 2 adsorption and desorption isotherm was measured at 77 K on a Quantachrome ASiQwin AUTOSORB IQ device. The Brunaure Emmett Teller (BET) model and Barrett Joyner Halenda (BJH) model were used to analyze the specific surface area and pore size distribution.

Electrochemical Measurements
Active materials (80 wt%), carbon black (10 wt%), and polytetrafluoroethylene (10 wt%) were mixed to prepare the slurry. The obtained slurry was coated with nickel foam with dimensions of 1 cm × 1 cm evenly. The foam was dried in a vacuum drying oven at 60 • C overnight, and then pressed at 10 MPa to obtain the working electrode. The loading of active materials for each electrode was around 0.8 mg. The three-electrode system was used to conduct electrochemical measurements at room temperature in a 2 mol L −1 KOH aqueous electrolyte. The platinum electrode and Hg/HgO electrode were used as the counter electrode and the reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (Zennium Pro, Zahner, Kronach Germany). EIS was carried out with an AC amplitude of 5 mV in a frequency range of 10 mHz to 100 KHz. The cycling performance of ZnFe 2 O 4 /rGO−2 was performed on the LAND battery test system (CT2001A).

Results and Discussion
The crystal structure and particle size of as-prepared ZnFe 2 O 4 and ZnFe 2 O 4 /rGO nanocomposites were analyzed by X-ray diffraction (XRD). As Figure 1a shows, the cubic ZnFe 2 O 4 (JCPDS No. 22-2012) lattice planes (220), (311), (400), (422), (511), and (440) were detected at the peaks of 29.9 • , 35.3 • , 42.8 • , 53.1 • , 56.6 • , and 62.2 • , respectively. No other peaks were found, which proves that this facile microwave hydrothermal process synthesized ZnFe 2 O 4 with high purity. It is worth noting that the peak belonging to rGO disappeared, which can be explained by the highly crystalline nature of the ZnFe 2 O 4 nanoparticles [35]. Moreover, in order to determine the particle size, the lateral crystal size (D) of the (311) plane of ZnFe 2 O 4 was calculated by Scherrer's equation [36]: where K is the Scherrer constant equal to 0.9; λ = 1.5418 Å; β is the full width at half maximum (FWHM) of the diffraction peak; and θ is the corresponding Bragg angle of the (311) plane. The particle size was calculated to be 7.4, 7.6, 7.2, and 6.5 nm for ZnFe 2 O 4 , ZnFe 2 O 4 /rGO−1, ZnFe 2 O 4 /rGO−2, and ZnFe 2 O 4 /rGO−3, respectively. These results indicate that the nano-sized particles in pure ZnFe 2 O 4 and ZnFe 2 O 4 /rGO nanocomposites were successfully synthesized by this facile microwave hydrothermal process.   The specific surface area and pore size distribution of the ZnFe 2 O 4 /rGO nanocomposites were investigated by N 2 adsorption and desorption experiments, and the results are shown in Figure 3. All of the as-prepared samples exhibited simple type IV hysteresis IUPAC lines, indicating the existence of mesoporous structure. The specific surface area of ZnFe 2 O 4 /rGO−1, ZnFe 2 O 4 /rGO−2, and ZnFe 2 O 4 /rGO−3 were calculated to be 150, 180, and 170 m 2 g −1 , respectively. This demonstrates that the addition of rGO has effects on the specific surface area of the sample. The ZnFe 2 O 4 nanoparticles interspersed on the graphene sheets form more pore structures, and the pore size distribution is shown in Figure 3a through Figure 3d. According to the Barrett Joyner Halenda (BJH) model, the average pore diameters of ZnFe 2 O 4 /rGO−1, ZnFe 2 O 4 /rGO−2, and ZnFe 2 O 4 /rGO−3 were 4.11 nm, 5.59 nm, and 4.28 nm, respectively. The highest specific surface area and optimal pore size distribution can increase the electrochemically active sites for redox reactions and provide ionic diffusion channels to enhance ion transportation.

Morphology Analysis
A field-emission scanning electron microscope (FE-SEM) was used to characterize the detailed morphology and microstructure of ZnFe 2 O 4 and ZnFe 2 O 4 /rGO nanocomposites. Figure 4a shows the serious aggregation of ZnFe 2 O 4 nanoparticles prepared without GO addition. When less GO was added, Figure 4b exhibits that the aggregation of ZnFe 2 O 4 nanoparticles was partially improved. The ZnFe 2 O 4 /rGO−2 (Figure 4c) with medium content of rGO shows that the ZnFe 2 O 4 nanoparticles were uniformly anchored onto the rGO surface without obvious aggregation. However, when the amount of GO continues to increase (Figure 4d), the nanoparticles cannot fully occupy the surface of rGO and the large specific surface area of rGO cannot be effectively utilized. From the FE-SEM elemental mapping images and elemental content of ZnFe 2 O 4 /rGO−2 nanocomposites (Figure 4e), the element distribution of C, O, Fe, and Zn can be observed. Figure 4f shows the elemental contents of ZnFe 2 O 4 /rGO−2 at the selected area. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterization were utilized to investigate detailed microstructure features of pure ZnFe 2 O 4 and ZnFe 2 O 4 /rGO nanocomposites (results shown in Figure 5). The TEM pictures showed the same results as the SEM pictures, wherein the aggregation of ZnFe 2 O 4 nanoparticles was reduced as the content of rGO increased. As shown in Figure 5c, the particle size of ZnFe 2 O 4 nanoparticles in ZnFe 2 O 4 /rGO−2 was approximately 8 nm, which is close to the XRD results. Figure 5e shows the HRTEM of ZnFe 2 O 4 /rGO−2. The lattice spacing of ZnFe 2 O 4 was 0.254 nm and 0.298 nm, corresponding to the spacing of the (311) and (220) crystalline planes, which indicates the same results as XRD (Figure 1a).

Electrochemical Performance of Supercapacitor
For comparison, reduced graphene oxide(rGO) was prepared using the same method as described above, and the electrochemical properties of ZnFe 2 O 4 , rGO, and ZnFe 2 O 4 /rGO−X (X = 1, 2, 3) were explored. Figure 6a shows the CV curves of ZnFe 2 O 4 , rGO, and ZnFe 2 O 4 /rGO nanocomposites at 20 mV s −1 . An approximately rectangular shape can be seen on the CV curve of rGO, which is a common electric double-layer capacitance characteristic. The ZnFe 2 O 4 and ZnFe 2 O 4 /rGO nanocomposites showed obvious redox peaks, which can be explained that by the predominance of the pseudocapacitance contribution. In addition, the ZnFe 2 O 4 /rGO−2 shows the biggest area among all of the ZnFe 2 O 4 and ZnFe 2 O 4 /rGO nanocomposites, indicating the highest specific capacitance. This can be attributed to the synergistic interaction between ZnFe 2 O 4 and rGO, where ZnFe 2 O 4 inhibits rGO selfstacking and rGO also effectively restrains ZnFe 2 O 4 agglomeration, thereby supplying more active sites for the electrochemical process, increasing the electrochemical performance and charge storage capacity. The CV curves of the ZnFe 2 O 4 /rGO−2 composites electrode are shown in Figure 6b with scan rates from 5 to 100 mV s −1 at a potential window of −0.9 to 0 V. The oxidation peaks and reduction peaks during the redox reaction can be explained by the following equation [40]:  Figure 6c shows the charge and discharge profiles of ZnFe 2 O 4 , rGO, and ZnFe 2 O 4 /rGO nanocomposites at 1 A g −1 . According to the following equation, the longest discharge time indicates the highest specific capacity, hence the ZnFe 2 O 4 /rGO−2 nanocomposites possess the highest specific capacity, which is the same as the CV results: where C s is the specific capacitance (F g −1 ), I is the discharge current (A), ∆t is the discharge time (s), m is the mass of active materials (g) and ∆V is the voltage change during the discharging process (V). Figure 6d shows the GCD curves of ZnFe 2 O 4 /rGO−2 at different current densities. The specific capacitance of ZnFe 2 O 4 /rGO−2 was calculated to be 628 F g −1 at 1 A g −1 , and it can remain at 156 F g −1 when the current density increases to 10 A g −1 , which is higher than all of the ZnFe 2 O 4 , rGO, ZnFe 2 O 4 /rGO−1, and ZnFe 2 O 4 /rGO−3 nanocomposites (Figure 6e). The apparent decrease of specific capacitance of all samples can be attributed to the fact that as the current density increases, the electrode-electrolyte interface absorbs a large number of electrolyte ions, which leads to a rapid drop in the electrolyte ion concentration at, or near, the interface, combined with the increase of electrode polarization.
where Cs is the specific capacitance (F g −1 ), i is the corresponding current (mA), V 1 and V 2 are the two working potential limits (V), m is the mass of active materials (g), ∆V is the voltage window (V), and v is the scan rate (mV s −1 ). According to the calculated results ( Figure S1), in comparison to rGO and ZnFe 2 O 4 , ZnFe 2 O 4 /rGO composites exhibit high specific capacitance. It is worth mentioning that ZnFe 2 O 4 /rGO−2 has a specific capacitance of 554 F g −1 at 5 mV s −1 , maintains a specific capacitance of 261 F g −1 at 100 mV s −1 , and has a specific capacitance of 628 F g −1 at 1 A g −1 . ZnFe 2 O 4 /rGO−2 is assumed to have the best performance since it has the highest specific surface area, the best pore size distribution, a uniform ZnFe 2 O 4 dispersion, and the fastest redox reaction on the surface even when subjected to high currents.
The essential performance of ZnFe 2 O 4 , rGO, and ZnFe 2 O 4 /rGO−X (X = 1, 2, 3) nanocomposite electrodes were further explored with Nyquist plots. The Nyquist plots can be divided into three parts. In the low-frequency region, the line corresponds to the diffusion impedance. Thus, a large slope indicates a faster diffusion capability. The semi-circle existing in the medium-frequency region corresponds to charge transfer resistance (Rct) and the intercept of the plot with the Z -axis in the high-frequency region indicates the contact resistance (Rc). As shown in the inset of Figure 6f, the Nyquist plots of ZnFe 2 O 4 /rGO are a semicircle in the high-frequency region and straight in the low-frequency region. The data of the electrochemical impedance spectroscopy is fitted by the equivalent resistance plot. The ZnFe 2 O 4 /rGO−2 electrode shows intrinsic resistance (Rs) and charge transfer resistance (Rct) 0.73 Ω and 0.84 Ω. The ZnFe 2 O 4 /rGO−2 exhibits the smallest charge transfer resistance, comparable intrinsic resistance, and good diffusion capability among ZnFe 2 O 4 and ZnFe 2 O 4 /rGO composites. The largest specific capacitance of ZnFe 2 O 4 /rGO−2 can be attributed to its improved diffusion and high conductivity.
An asymmetrical supercapacitor was assembled using ZnFe 2 O 4 /rGO−2 electrodes, 2M KOH as the electrolyte, and glass fiber (GF/D) as the separator. Figure 7a shows the CV curves of symmetric supercapacitor at various scan rates from 0 to 0.9 V. There is no obvious redox peak that closely resembles the rectangles. The CV curve remained fairly rectangular in shape without significantly changing as the scan rate was raised from 5 mV s −1 to 100 mV s −1 . Figure 7b shows plots of constant current charge and discharge at various current densities. The triangular charge and discharge curves show good capacitive performance. Figure 7c shows the rate performance of ZnFe 2 O 4 /rGO−2 for a symmetrical supercapacitor. The specific capacitance of 150 F g −1 was achieved at 0.5 A g −1 , which can still maintain 86.7% at 5 A g −1 . The equivalent resistance plot, which represents the electrochemical AC impedance of a symmetrical supercapacitor, exhibits intrinsic resistance (Rs) and charge transfer resistance (Rct) 0.91 Ω and 0.57 Ω (Figure 7d). Cyclic stability is a crucial consideration for a candidate electrode material for the supercapacitor's practical application. In Figure 7e, the capacitance retention rate of ZnFe 2 O 4 /rGO−2 is 89% after 2500 cycles at 1 A g −1 . Table 1 exhibits the higher or comparable electrochemical performance of ZnFe 2 O 4 /rGO−2, indicating the advantages of this facile microwave hydrothermal process. Furthermore, when compared with previous reports, our work also shows superior or comparable specific capacitance. For exploring the high conductivity of rGO, the comparison of pure rGO and carbon black/rGO was provided. As shown in Figure S2, pure rGO shows the nearly similar electrochemical performance to carbon black/rGO, the results indicate the he high conductivity of synthesized rGO by microwave hydrothermal process.

Conclusions
In conclusion, a facile microwave hydrothermal process was applied to synthesize ZnFe 2 O 4 /rGO nanocomposites. The as-prepared ZnFe 2 O 4 /rGO nanocomposites showed uniform distribution of ZnFe 2 O 4 less than 10 nm on the rGO surface, and higher specific capacitance and rich pore structure. The ZnFe 2 O 4 /rGO nanocomposites with 44.3 wt.% rGO show the highest specific capacitance (628 F g −1 at 1 A g −1 ), which is much higher than the pure ZnFe 2 O 4 (68 F g −1 at 1 A g −1 ). When ZnFe 2 O 4 /rGO−2 was prepared as the electrode of the symmetric supercapacitor, it exhibited a capacitance retention of 89% for 2500 cycles. These results demonstrate that this facile microwave hydrothermal process could be used in preparing other binary transition metal oxide and carbon material nanocomposites for supercapacitors electrodes.