Facile Synthesis of CoSe/Co3O4-CNTs/NF Composite Electrode for High-Performance Asymmetric Supercapacitor

Electrode materials are key factors for supercapacitors to endow them with excellent electrochemical properties. Here, a novel hybrid structure of a CoSe/Co3O4-CNTs binder free composite electrode on nickel foam was prepared via a facile flame method, followed by an electrodeposition process. Benefitting from the synergetic effects of the multicomponent (with low resistances of 1.542 Ω cm2 and a moderate mesoporous size of 3.12 nm) and the enlarged specific surface area of the composite material (77.4 m2 g−1), the CoSe/Co3O4-CNTs composite electrode delivers a high specific capacitance of 2906 F g−1 at 5 mV s−1 with an excellent rate stability. The fabricated CoSe/Co3O4-CNTs/NF//AC ASC exhibits a high energy density of 43.4 Wh kg−1 at 0.8 kW kg−1 and a long cycle life (92.7% capacitance retention after 10,000 cycles).


Introduction
Highly efficient energy storage devices are one of the important means to satisfy the increasing energy demand. As an important piece of equipment for electrochemical energy storage, supercapacitors (SCs) have been a wide concern in the field of hybrid vehicles, consumer electronics, and industrial power management, because of their long maintenance-free lifetime, high power density, environmental protection, safety and reliability [1][2][3].
Among them, transition metal selenides have been shown to possess higher electronic conductivity than oxides and sulfides and relatively high theoretical specific capacities, implying that transition metal selenides have great potential to be used as advanced electroactive materials for SCs [20,21]. However, the addition of binders usually decreases the electronic conductivity of the electrode, resulting in the deterioration of electrochemical

The Synthesis of Co 3 O 4 -CNTs/NF
NF (with the thickness of 1.0 mm, surface density = 34.2 mg cm −2 , pore per inch = 110) was immersed into a 3 M HCl solution for 1 min to remove the oxides and impurities on the surface firstly, then the precleaned NF was washed with acetone, ethanol, and DI water under the assistance of ultrasound. Finally, it was dried in a vacuum oven at 60 • C overnight.
Co 3 O 4 -CNTs loaded on the skeleton of NF was prepared via a facile flame method. Concisely, an ethanol solution of Co(acac) 3 (with a concentration of 10 mg/mL) was prepared. Then, a piece of NF (with the area of 1 × 1 cm 2 ) was immersed into the above solution for about 5 s to make the solution attach onto the skeleton of NF. After that, the NF was ignited and burned under ambient condition to gain the NF decorated by Co 3 O 4 -CNTs. To increase the mass loading of Co 3 O 4 -CNTs, the immersing and flaming process could be executed for more times. In our research, the mass of Co 3 O 4 -CNTs loaded on NF were about 0.1, 0.2, 0.4, 0.9, 1.5 mg, corresponding to the various immersing and flaming times (1, 2, 5, 10, 15).

The Synthesis of CoSe
CoSe particles were deposited on Co 3 O 4 -CNTs via electrodeposition process. The bath for electrodeposition consists of 25 mL DI water, 0.2 mmol CoCl 2 •6H 2 O, 0.2 mmol SeO 2 and 200 mg LiCl•H 2 O. After being stirred evenly, a homogeneous pink solution was obtained. Through a typical three-electrode system on an electrochemical workstation (CHI 660C) (CH Instruments, Inc., Shanghai, China), the constant current electrodeposition procedure was realized by using Co 3 O 4 -CNTs/NF as working electrode, Pt foil and saturated calomel electrode (SCE) as the counter electrode and the reference electrode, respectively. The electrodeposition was conducted at 15 mA cm −2 for 1200 s. Pure CoSe nanoparticles were also deposited on NF without the flame process for comparison. The mass of CoSe/Co 3 O 4 -CNTs, Co 3 O 4 -CNTs and CoSe loaded on NF were 0.7, 0.4 and 1 mg, respectively.

Characterization
A Rigaku Miniflex600 X-ray diffraction (XRD) system (Rigaku, Tokyo, Japan) with Cu K α irradiation was applied to analyze the crystal structures of samples. The chemical state of the synthetic elements was determined by X-ray photoelectron spectrometer (Thermo Scientific K-Alpha+) (Thermo Fisher Scientific, Waltham, MA, USA). A FEI Verios 460 scanning electron microscopy (SEM) was used to observe the morphologies and microstructures. The Brunauer-Emmett-Teller (BET) (Kubo X1000) analysis (Beijing Builder Electronic Technology Co., Ltd., Beijing, China) was used to study the specific surface area and pore size distribution of the sample. Raman spectra were obtained with an excitation wavelength of 514 nm, using a Horiba Scientific LabRAM HR Evolution. (HORIBA Scientific, Kyoto, Japan).

Electrochemical Measurements
The three-electrode system in 3 M KOH was introduced to investigate the electrochemical properties of the as-synthesized materials on NF, where an Hg/HgO electrode and a Pt-wire were used as the reference electrode and the counter electrode, respectively. Cyclic voltammetry (CV) measurements were tested, ranging from 0 to 0.6 V at different sweep rates (5-100 mV s −1 ), while galvanostatic charge-discharge (GCD) was measured within the potential window of 0-0.5 V at different current densities (1-10 A g −1 ). Electrochemical impedance spectroscopy (EIS) measurement was performed at a frequency between 0.01 and 100 kHz, where the amplitude was set to be 5 mV. The ASCs were assembled in 3 M KOH with the CoSe/Co 3 O 4 -CNTs/NF (or CoSe/NF) electrode as the positive electrode and the AC electrode as the negative electrode. The cycling stability was investigated by a Neware battery test instrument (BST-3008) (Neware Technology Limited, Shenzhen, China) in a voltage range of 0-1.6 V at 2 A g −1 .

Characterization of Materials
Scheme 1 illustrates the fabrication process of the CoSe/Co 3 O 4 -CNTs/NF electrode. The forming mechanism of Co 3 O 4 -CNTs is that the hydrocarbon of cobalt acetylacetonate are catalytic-decomposed to Co + C (Co 2 C, Co 3 C) over NF, then cobalt nanoparticles combined with oxygen atoms (air atmosphere) and Co 3 O 4 -CNTs is the final product [28][29][30]. Cobalt nanoparticles act as the catalyst for in situ formation of CNTs and oxidized to finally form Co 3 O 4 . The effect of different loading amounts on the morphology and properties of Co 3 O 4 -CNTs is shown in Figures S1 and S2. The SEM image with five flame cycles show the appropriate load amount. In Figure S2, the NF electrodes have a lower performance and almost no contribution to electrochemical performance. The Co 3 O 4 -CNTs electrodes with different flame cycles show extremely low capacitance (180 F g −1 ). After consideration, the Co 3 O 4 -CNTs/NF-0.4 (0.4 stans for the loading amount of 0.4 mg cm −2 ) was used for the next electrodeposition. In the process of cathodic current reduction, the SeO 2 will be reduced stepwise to Se 2− and then reacted with the metal ions to form metal selenides. The electrodeposition mechanism can be interpreted by the following chemical equations [31,32]: Co 2+ + Se 2− → CoSe (4) Scheme 1. Schematic diagram of the process to prepare the CoSe/Co3O4-CNTs/NF electrode. Figure 1 shows the SEM image of CoSe/NF, Co3O4-CNTs/NF and CoSe/Co3O4-CNTs/NF. The morphology of CoSe/NF with a homologous nanoparticle architecture with less pores is shown in Figure 1a,b, showing the typical SEM image of the Co3O4-CNTs/NF sample, indicating that Co3O4-CNTs coated on the surface of NF shows a loose structure with abundant micro-voids. This superhydrophilic architecture with excellent conductivity is believed to provide more nucleation sites for the subsequent electrodeposition to expand the specific surface area of composites, which is beneficial to promote the electrochemical properties of the electro-active materials [33,34]. XRD pattern of Co3O4-CNTs is shown in Figure S3. Three diffraction peaks of NF substrate (JCPDS no. 04-0850), which are located at 44.7°, 52.1° and 76.6°, can be seen clearly. The other four weak diffraction peaks at 31.4°, 36.9°, 59.6°, and 65.4° are assigned to Co3O4 (JCPDS no.74-1657). However, no diffraction peaks of CNTs are observed, which maybe imply the poor crystallinity of CNTs. The Raman spectrum in Figure S4 further indicated the relatively good graphitization of Co3O4-CNTs. The ID/IG of about 0.96 suggests this carbon material possesses rich defects [35]. The TEM image ( Figure S5) indicates the diameter of the CNTs is about 18 nm, and Co3O4(2-4 nm) nanoparticles adhere on the surface of CNTs and encapsulate at the tip. Figure 1c shows the SEM micrographs of the CoSe/Co3O4-CNTs/NF sample, which reveals a uniform nanoparticle-like architecture with plenty of pores on its surface. As mentioned above, this porous structure is beneficial for the infiltration of electrolyte and the improvement of electrochemical properties of CoSe. N2 adsorptiondesorption technique was employed to analyze the characteristics of pores quantitatively. The specific surface area (SSA) and the pore diameter distribution of Co3O4-CNTs/NF, CoSe/NF and CoSe/Co3O4-CNTs/NF are displayed in Figure S6. As can be seen, all of them show type IV curves with mesoporous feature. The SSA of CoSe/Co3O4-CNTs/NF (77.4 m 2 g −1 ) is much larger than that of CoSe/NF (17.1 m 2 g −1 ) and Co3O4-CNTs/NF (21.5 m 2 g −1 ). The insert patterns display Barrett-Joyner-Halenda (BJH) pore size distribution curves. Among them, CoSe/Co3O4-CNTs/NF displays the main pores of 3.21 nm in a range of 1-100 nm. The larger SSA and moderate mesoporous size feature contribute to the fast ion migration/diffusion during redox reaction [36]. Actually, double-layer capacitance (CDL) or Gaussian fitting of the SEM or TEM images method can also be used to analyze the pore diameter distributions, which were more convenient [37,38].   Figure 1a,b, showing the typical SEM image of the Co 3 O 4 -CNTs/NF sample, indicating that Co 3 O 4 -CNTs coated on the surface of NF shows a loose structure with abundant micro-voids. This superhydrophilic architecture with excellent conductivity is believed to provide more nucleation sites for the subsequent electrodeposition to expand the specific surface area of composites, which is beneficial to promote the electrochemical properties of the electro-active materials [33,34]. XRD pattern of Co 3 O 4 -CNTs is shown in Figure S3. Three diffraction peaks of NF substrate (JCPDS no. 04-0850), which are located at 44.7 • , 52.1 • and 76.6 • , can be seen clearly. The other four weak diffraction peaks at 31.4 • , 36.9 • , 59.6 • , and 65.4 • are assigned to Co 3 O 4 (JCPDS no.74-1657). However, no diffraction peaks of CNTs are observed, which maybe imply the poor crystallinity of CNTs. The Raman spectrum in Figure S4 further indicated the relatively good graphitization of Co 3 O 4 -CNTs. The I D /I G of about 0.96 suggests this carbon material possesses rich defects [35]. The TEM image ( Figure S5) indicates the diameter of the CNTs is about 18 nm, and Co 3 O 4 (2-4 nm) nanoparticles adhere on the surface of CNTs and encapsulate at the tip. Figure 1c shows the SEM micrographs of the CoSe/Co 3 O 4 -CNTs/NF sample, which reveals a uniform nanoparticle-like architecture with plenty of pores on its surface. As mentioned above, this porous structure is beneficial for the infiltration of electrolyte and the improvement of electrochemical properties of CoSe. N 2 adsorption-desorption technique was employed to analyze the characteristics of pores quantitatively. The specific surface area (SSA) and the pore diameter distribution of Co 3 O 4 -CNTs/NF, CoSe/NF and CoSe/Co 3 O 4 -CNTs/NF are displayed in Figure S6. As can be seen, all of them show type IV curves with mesoporous feature. The SSA of CoSe/Co 3 O 4 -CNTs/NF (77.4 m 2 g −1 ) is much larger than that of CoSe/NF (17.1 m 2 g −1 ) and Co 3 O 4 -CNTs/NF (21.5 m 2 g −1 ). The insert patterns display Barrett-Joyner-Halenda (BJH) pore size distribution curves. Among them, CoSe/Co 3 O 4 -CNTs/NF displays the main pores of 3.21 nm in a range of 1-100 nm. The larger SSA and moderate mesoporous size feature contribute to the fast ion migration/diffusion during redox reaction [36]. Actually, double-layer capacitance (CDL) or Gaussian fitting of the SEM or TEM images method can also be used to analyze the pore diameter distributions, which were more convenient [37,38].  can be assigned to the NF, while no other diffraction peaks can be observed, implying that the CoSe grown on the Co3O4-CNTs/NF mainly exists in amorphous form. The composition and chemical valence of the CoSe/Co3O4-CNTs/NF sample was further investigated by using XPS analysis (as shown in Figure S7). The full XPS survey spectrum of CoSe/Co3O4-CNTs/NF represents the coexistence of Co and Se elements ( Figure S7a). Two major peaks at 795.92 and 780.48 eV in the high-resolution XPS spectrum of Co 2p ( Figure S7b) correspond to the Co 2p1/2 and 2p3/2, with two satellites at 802.51 and 784.73 eV, respectively [34,[39][40][41][42][43], which suggests the presence of Co 2+ . Figure S7c shows the detailed analysis of Se 3d high-resolution XPS spectra at a lower energy. Although there is interference on the Co 3p line on 59.1 eV, the emerged peaks at 55.8 and 54.8 eV can be appointed to the Se 3d3/2 and 3d5/2 [41,44], suggesting that the Se elemental mainly exists in Se 2− . XPS results verify the successful formation of CoSe on the Co3O4-CNTs/NF substrate. In order to obtain the detailed morphology of samples, CoSe was stripped from the substrate via ultrasonic vibration slightly to avoid structural damage. The TEM image in Figure 2b shows that the CoSe displays a connected nanosheet-like structure construction with ultrathin thickness and a rough surface. Furthermore, no clear lattice fringes are observed in the corresponding high-resolution TEM pattern (Figure 2c), which means that the as-synthesized CoSe possesses a low-crystalline characteristic. This result is consistent with the result of XRD. Active materials with higher SSA and low-crystalline have been proven to be beneficial to provide more active sites and withstand the distortion of structure during the charge/discharge process [45], resulting in a higher specific capacitance and longer cycling stability. Therefore, we deduced that the as-synthesized CoSe/Co3O4-CNTs/NF will exhibit excellent electrochemical properties.

Electrochemical Performances of CoSe/Co3O4-CNTs/NF
Electrochemical performances of the CoSe/Co3O4-CNTs/NF electrodes are shown in Figure 3. For the CV curves in Figure 3a, a pair of redox peaks clearly show the faradaic   Figure S7). The full XPS survey spectrum of CoSe/Co 3 O 4 -CNTs/NF represents the co-existence of Co and Se elements ( Figure S7a). Two major peaks at 795.92 and 780.48 eV in the high-resolution XPS spectrum of Co 2p ( Figure S7b) correspond to the Co 2p 1/2 and 2p 3/2 , with two satellites at 802.51 and 784.73 eV, respectively [34,[39][40][41][42][43], which suggests the presence of Co 2+ . Figure S7c shows the detailed analysis of Se 3d high-resolution XPS spectra at a lower energy. Although there is interference on the Co 3p line on 59.1 eV, the emerged peaks at 55.8 and 54.8 eV can be appointed to the Se 3d 3/2 and 3d 5/2 [41,44], suggesting that the Se elemental mainly exists in Se 2− . XPS results verify the successful formation of CoSe on the Co 3 O 4 -CNTs/NF substrate. In order to obtain the detailed morphology of samples, CoSe was stripped from the substrate via ultrasonic vibration slightly to avoid structural damage. The TEM image in Figure 2b shows that the CoSe displays a connected nanosheet-like structure construction with ultrathin thickness and a rough surface. Furthermore, no clear lattice fringes are observed in the corresponding highresolution TEM pattern (Figure 2c), which means that the as-synthesized CoSe possesses a low-crystalline characteristic. This result is consistent with the result of XRD. Active materials with higher SSA and low-crystalline have been proven to be beneficial to provide more active sites and withstand the distortion of structure during the charge/discharge process [45], resulting in a higher specific capacitance and longer cycling stability. Therefore, we deduced that the as-synthesized CoSe/Co 3 O 4 -CNTs/NF will exhibit excellent electrochemical properties.   Figure S7). The full XPS survey spectrum of CoSe/Co3O4-CNTs/NF represents the coexistence of Co and Se elements ( Figure S7a). Two major peaks at 795.92 and 780.48 eV in the high-resolution XPS spectrum of Co 2p ( Figure S7b) correspond to the Co 2p1/2 and 2p3/2, with two satellites at 802.51 and 784.73 eV, respectively [34,[39][40][41][42][43], which suggests the presence of Co 2+ . Figure S7c shows the detailed analysis of Se 3d high-resolution XPS spectra at a lower energy. Although there is interference on the Co 3p line on 59.1 eV, the emerged peaks at 55.8 and 54.8 eV can be appointed to the Se 3d3/2 and 3d5/2 [41,44], suggesting that the Se elemental mainly exists in Se 2− . XPS results verify the successful formation of CoSe on the Co3O4-CNTs/NF substrate. In order to obtain the detailed morphology of samples, CoSe was stripped from the substrate via ultrasonic vibration slightly to avoid structural damage. The TEM image in Figure 2b shows that the CoSe displays a connected nanosheet-like structure construction with ultrathin thickness and a rough surface. Furthermore, no clear lattice fringes are observed in the corresponding high-resolution TEM pattern (Figure 2c), which means that the as-synthesized CoSe possesses a low-crystalline characteristic. This result is consistent with the result of XRD. Active materials with higher SSA and low-crystalline have been proven to be beneficial to provide more active sites and withstand the distortion of structure during the charge/discharge process [45], resulting in a higher specific capacitance and longer cycling stability. Therefore, we deduced that the as-synthesized CoSe/Co3O4-CNTs/NF will exhibit excellent electrochemical properties.

Electrochemical Performances of CoSe/Co3O4-CNTs/NF
Electrochemical performances of the CoSe/Co3O4-CNTs/NF electrodes are shown in Figure 3. For the CV curves in Figure 3a

Electrochemical Performances of CoSe/Co 3 O 4 -CNTs/NF
Electrochemical performances of the CoSe/Co 3 O 4 -CNTs/NF electrodes are shown in Figure 3. For the CV curves in Figure 3a faradaic pseudocapacitive characteristics of the battery-like electrode, corresponding to the following redox reactions [46,47]: relatively high surface area ( Figure S6), and its capacitance is 180 F g at 1 A g . To evaluate the charge transfer rate of the hybrid electrode, EIS tests were conducted (Figure 3c). The intercepts at the real axis are associated with Rs (contributions of the ionic resistance of the electrolyte, intrinsic resistance, and contact resistance between the active material and the current collector). In addition, the semicircles represent Rct (chargetransfer resistance) of electrode materials. The slopes of the impedance line in the lowfrequency region denote W (Warburg impedance), which indicates the ion diffusion in the interface of electrode material/electrolyte. The corresponded fitting results of Nyquist plots (Table S1) reveal that CoSe/Co3O4-CNTs/NF has a lower charge-transfer resistance (Rct = 0.89 Ω cm 2 ) and solution resistance (Rs = 0.73 Ω cm 2 ), indicating its superior charge mobility. We also calculate the resistances of Co3O4-CNTs/NF, CoSe/NF and CoSe/Co3O4-CNTs/NF electrodes from the voltage drop in GCD plots at the higher current densities (100 to 500 A g −1 ) ( Figures S8 and Table S2). The electrode of CoSe/Co3O4-CNTs/NF has the lower resistance 1.542 Ω cm 2 . The outstanding impedance characteristics of CoSe/Co3O4-CNTs/NF arise from the synergetic effect of Co3O4-CNTs and CoSe materials, in which the Co3O4-CNTs winded with each other can provide good conductivity and stability. Meanwhile, the larger SSA and moderate mesoporous size of CoSe/Co3O4-CNTs/NF can offer excellent infiltration of the electrolyte [36]. The distinct peaks may be attributed to the redox features of Co 2+ /Co 3+ and Co 3+ /Co 4+ . Additionally, the CoSe/Co 3 O 4 -CNTs/NF electrode has a much larger integrated area than the CoSe/NF electrode, indicating that the CoSe/Co 3 O 4 -CNTs/NF electrode will provide a satisfying energy storage ability. Figure 3b displays the compared GCD curves of CoSe/Co 3 O 4 -CNTs/NF, CoSe/NF and Co 3 O 4 -CNTs/NF electrodes at 1 A g −1 . Apparently, the CoSe/Co 3 O 4 -CNTs/NF delivered a higher specific capacitance of 2906 F g −1 , which was two times larger than that of the CoSe/NF electrode (1440.8 F g −1 ). Co 3 O 4 -CNTs/NF has a relatively high surface area ( Figure S6), and its capacitance is 180 F g −1 at 1 A g −1 .
To evaluate the charge transfer rate of the hybrid electrode, EIS tests were conducted (Figure 3c). The intercepts at the real axis are associated with R s (contributions of the ionic resistance of the electrolyte, intrinsic resistance, and contact resistance between the active material and the current collector). In addition, the semicircles represent R ct (charge-transfer resistance) of electrode materials. The slopes of the impedance line in the low-frequency region denote W (Warburg impedance), which indicates the ion diffusion in the interface of electrode material/electrolyte. The corresponded fitting results of Nyquist plots (Table S1) reveal that CoSe/Co 3 O 4 -CNTs/NF has a lower charge-transfer resistance (R ct = 0.89 Ω cm 2 ) and solution resistance (R s = 0.73 Ω cm 2 ), indicating its superior charge mobility. We also calculate the resistances of Co 3 O 4 -CNTs/NF, CoSe/NF and CoSe/Co 3 O 4 -CNTs/NF electrodes from the voltage drop in GCD plots at the higher current densities (100 to 500 A g −1 ) ( Figure S8 and Table S2). The electrode of CoSe/Co3O4-CNTs/NF has the lower resistance 1.542 Ω cm 2 . The outstanding impedance characteristics of CoSe/Co 3 O 4 -CNTs/NF arise from the synergetic effect of Co3O4-CNTs and CoSe materials, in which the Co 3 O 4 -CNTs winded with each other can provide good conductivity and stability. Meanwhile, the larger SSA and moderate mesoporous size of CoSe/Co 3 O 4 -CNTs/NF can offer excellent infiltration of the electrolyte [36].
The CV cures of the CoSe/Co 3 O 4 -CNTs/NF electrode at various scan rates are depicted in Figure S9a. Notably, with the increasing of scan rates, the anodic and cathodic peaks shift to higher and lower voltage, respectively, suggesting the increased internal diffusion resistance of the electrode [48]. Meanwhile, the perfect linear relationship of i and v 1/2 (Figure 3d) indicates the redox reactions between electrolyte and electrode interface at the electrolyte/electrode interface is related to a quasi-reversible and diffusion control process [24].
The specific capacitance and rate capacity of the CoSe/Co 3 O 4 -CNTs/NF electrode are further investigated by GCD test at different current densities ( Figure S9b and Figure 3e).

Conclusions
In summary, a free-standing and highly active CoSe/Co3O4-CNTs hybrid material was synthesized by electrodepositing CoSe nanoparticles on a forest of density and uniform Co3O4-CNTs supported by NF, which was used as a positive electrode of supercapacitor directly. Due to the synergist effects of the multicomponent, nanoporous structure and the enlarged SSA of the CoSe/Co3O4-CNTs composite material, the hybrid electrode delivered a high specific capacitance of 2906 F g −1 , at 5 mV s −1 and 1362.8 F g −1 , at 50 mV s −1 . ACS assembled using CoSe/Co3O4-CNTs/NF and AC as positive and negative electrodes achieved a high energy density of 43.4 Wh kg −1 at 0.8 kW kg −1 with an excellent capacitance retention of 92.7% after 10,000 cycles. Meanwhile, it is worth noting that, due to the facile fabrication process, the novel hybrid electrode structure can be scaled up by supercapacitor manufacturers easily. This work presents an effective strategy to design a hybrid composite electrode for a high-performance asymmetric supercapacitor.

Conclusions
In summary, a free-standing and highly active CoSe/Co 3 O 4 -CNTs hybrid material was synthesized by electrodepositing CoSe nanoparticles on a forest of density and uniform Co 3 O 4 -CNTs supported by NF, which was used as a positive electrode of supercapacitor directly. Due to the synergist effects of the multicomponent, nanoporous structure and the enlarged SSA of the CoSe/Co 3 O 4 -CNTs composite material, the hybrid electrode delivered a high specific capacitance of 2906 F g −1 , at 5 mV s −1 and 1362.8 F g −1 , at 50 mV s −1 . ACS assembled using CoSe/Co 3 O 4 -CNTs/NF and AC as positive and negative electrodes achieved a high energy density of 43.4 Wh kg −1 at 0.8 kW kg −1 with an excellent capacitance retention of 92.7% after 10,000 cycles. Meanwhile, it is worth noting that, due to the facile fabrication process, the novel hybrid electrode structure can be scaled up by supercapacitor manufacturers easily. This work presents an effective strategy to design a hybrid composite electrode for a high-performance asymmetric supercapacitor.
Author Contributions: Conceptualization, Q.Z., X.X. and Y.Z.; methodology, C.Y.; investigation, X.C.; writing-original draft preparation, Y.W.; writing-review and editing, X.Z. All authors have read and agreed to the published version of the manuscript.