Optimized 3D interconnected foam-like NiO@rGO composite as a binder-free electrode for high-performance asymmetric supercapacitor

Bonding of transition metal oxides and highly conductive carbon materials to exploit the synergistic effect of both materials has been proven to be an efficient means to develop high-capacity electrode materials. A unique interconnected foam-like NiO@rGO structure was constructed by loading the NiO nanoparticles onto rGO frameworks as a binder-free supercapacitor electrode via three steps including hydrothermal reaction, electrodeposition and heating treatment. The morphology and crystallinity were tuned by controlling the electrodeposition time and heating temperature, and the electrochemical properties of the NiO@rGO composites were systematically investigated. The optimized NiO@rGO-250 composite showed excellent electrochemical properties (1399 F g−1 at 1 A g−1) and superior cycling stability. Furthermore, an asymmetric supercapacitor using NiO@rGO and active carbon as two electrodes achieved a high specific energy of 40.4 Wh k g−1 at a specific power of 750 W k g−1.


Introduction
In view of the depletion of fossil fuels and the increased environmental pollution in recent years, there is a critical need for the efficient use of renewable energy sources, as well as the development of their energy storage components [1][2][3].Supercapacitors (SCs) have generated extensive research due to their fast storage capacity and excellent cycling stability [4][5][6].Depending on the type of charge-storage mechanism, it can be classified into double layer capacitors (EDLCs) and pseudocapacitors.In EDLCs, ions in the electrolyte cling to the surface and store charge due to electrostatic attraction, thus forming two charged layers on the electrode surface.Unlike EDLCs, pseudocapacitors store charge by reversible redox reactions, which occur at the interface of the electrode/electrolyte [7].Although EDLCs and pseudocapacitors have the advantage of high specific power, their inherent low specific energy hinders their wide commercial application [8].In order to overcome the obstacle of low specific energy, one of the most commonly used methods is to develop innovative composite electrode materials by the virtues of the two different types of materials.The transition metal oxides, such as NiO [9,10], Co 3 O 4 [11], MnO 2 [12] and their composites, have become a research hotspot because of their rapid reversible redox reaction.
Nickel oxide/high conductive carbon composites are promising electrode materials because of the high theoretical specific capacitance of NiO (2583 F g −1 ) [13].However due to their inherently low conductivity (about 10 −4 S cm −1 ), NiO-based electrodes only reach a fraction of their theoretical capacitance [14,15].Many recent studies have demonstrated that the composites exhibit excellent electrochemical properties by growing nickel oxide on highly conductive carbon materials such as graphene, carbon nanotubes, and carbon fibers [16][17][18][19][20][21].Lai et al synthesized CNT/NiO electrodes with a stable layered structure by hydrothermal treatment, showing excellent electrochemical performance (713.9F g −1 at 2 mV s −1 ) [19].Kahimbi and his colleagues reported the preparation of NiO/rGO electrode materials with excellent electrochemical properties (590 F g −1 at 1 A g −1 ) using an one-step ball milling method [20].Alshoaibi et al used chemical bath deposition to synthesize transition metal/graphene oxide nanocomposites and obtained a maximum specific capacitance value of 1280.48F g −1 (at 1 A g −1 ) [21].The reduced graphene oxide (rGO) has consistently been found to be an excellent electrode material for SCs owing to the high electrical conductivity and high specific surface area [22], facilitating efficient electrochemical loading of NiO.
Many studies revealed the excellent properties of NiO/rGO composites, but the crucial of exploiting the synergetic enhancement of both materials is the selection of an appropriate structure/bonding method [23][24][25].Nowadays, most of the preparations of NiO/rGO composites are generally time-consuming or have complex preparation processes.Electrodepositing NiO onto the rGO framework directly to form a interconnected foamlike structure prevents the use of a binder and is expected to be simple and well-controlled [26,27].Besides, this structure has many advantages: First, the 3D cross-linked rGO frameworks has a large specific surface area and a porous foam-like structure, contributing to the deposition of NiO and electrolyte infiltration.Second, the rGO nanosheet acts as an electron conducting path for the NiO nanoparticles, increasing the specific capacitance of the composite.Third, the growth and morphology of NiO nanocrystals can be easily and precisely controlled by electrodeposition, effectively shortening the ion transport path and facilitating the reversible redox reaction [28].Furthermore, to our knowledge the growth of NiO structures on rGO using electrodeposition has rarely been reported and are expected to further improve the capacitive properties of the nickel oxide/graphene composites.
In this work, a unique interconnected foam-like NiO@rGO composites were synthesized via a three-step procedure consisting of hydrothermal, electrodeposition and calcination methods.The surface morphology and electrochemical properties of the NiO@rGO composites were tuned by varying the electrodeposition time and heating temperature.The electrochemical properties of NiO@rGO composites were investigated using CV, GCD, and cycling performance tests.The optimized NiO@rGO-250 composite showed excellent electrochemical properties (1399 F g −1 at 1 A g −1 ).Finally, asymmetric supercapacitors (ASCs) were assembled from NiO@rGO composites and active carbon, achieving a specific energy of 40.4 Wh kg −1 at a high specific power of 750 W kg −1 .

Experimental
2.1.Synthesis of the NiO@rGO composite All chemicals and solvents are analytically pure and used without purification.Ni(NO 3 ) 2 •6H 2 O was purchased from Alfa Aesar, Graphite powder was purchased from Tianjin Kermel Chemical Reagent Co., Ltd., and ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd.Graphene oxide (GO) was prepared by a Hummer method.First, 60 mg of graphene oxide was dissolved in 30 mL water, forming a brown transparent graphene oxide solution.The pre-cleaned nickel foam (1 × 1 cm 2 ) and the above solution were transferred to a 50 ml PTFE-lined autoclave, then temperature was raised to 150 °C for 2 h.After that, the nickel foam with rGO loaned on its surface was washed with water and freeze-dried.
The NiO@rGO composite was prepared by the electrodeposition method.The above-mentioned nickel foam loaded with rGO was used as the working electrode, and the platinum and the Ag/AgCl electrode as the reference electrode and counter electrode, respectively.The electrodeposition solution was an 100 ml of water/ ethanol (1:1 volume ratio) mixture containing Ni(NO 3 ) 2 •6H 2 O (2 mM).The typical electrodeposition time was 5 min, and the potential was −1.2 V. Finally, the product was calcined at 250 °C for 2 h in an Ar atmosphere with a rate of 3 °C min −1 , and the final product NiO@rGO was obtained.The average mass of the NiO@rGO electrodes was 2 mg cm −2 on each piece (1 × 1 cm 2 ).

Characterization
The composites were characterized by scanning electron microscopy (SEM, FlexSEM1000) and transmission electron microscopy (TEM, FEI Talos F200X) .Identification of the crystalline phases was carried out by x-ray diffractometry (XRD, Smartlab SE).The elemental composition and chemical valence of the materials were investigated by x-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) tests and ICP measurement (Agilent 7800 ICP-MS).

Electrochemical measurements
The electrochemical performance of NiO@rGO composites were carried out using the electrochemical workstation (CHI 660E).A three-electrode system was applied in the tests, with NiO@rGO as the working electrode and Pt and Hg/HgO electrodes as counter and reference electrodes, in a 6 M KOH electrolyte.The  cycling performance was tested using the battery test system (LAND CT2001A).The average mass of the NiO@rGO electrodes was 2 mg cm −2 .

Structure characterization
The products of different synthetic steps are shown in figure 1, and their morphology was explored using SEM (figure 2).As shown in figure 2(a), the rGO sheets are thin and interconnected foam-like.After  electrodeposition, the NiO precursors were grown uniformly on the rGO frameworks as shown in figure 2(b).Then the NiO@rGO composite retains its porous structure after calcination, facilitating electrolyte infiltration (figure 2(c)).The crystalline phases of the composite were investigated by XRD analysis in figure 2(d).The pattern of graphene oxide (GO) shows that a distinct diffraction peak appears at 10.4°, corresponding to the (001) plane of graphene oxide [29].After hydrothermal treatment, the diffraction peak at 23°indicate the successful synthesis of rGO [30].The XRD pattern of the precursor/rGO shows several weak diffraction peaks, which corresponds to the electrodeposition products of the previous studies [31].For the XRD pattern of NiO@rGO, the diffraction peaks at 37.2°, 43.3°, and 62.9°are attributed to the (111), (200), and (220) planes of the cubic NiO phase (JCPDS no.47−1049) [32].
In addition, NiO@rGO composites with different electrodeposition times (1 min, 2 min, 5 min, 10 min, 15 min, and 20 min) were prepared in order to obtain the optimum conditions and the morphologies of different samples were investigated by SEM images (figure 3).It can be seen that the deposited layer on the rGO surface gradually becomes thicker as time increases, indicating that NiO can be effectively controlled using the electrodeposition.
TEM and energy spectroscopy were used for the study of the NiO@rGO composites.Figure 4(a) shows the lamellar structure of rGO nanosheets.As seen in figure 4(b), NiO is evenly distributed on the rGO substrate.Figure 4(c) illustrates a high-resolution image.The lattice striations of NiO can be clearly seen with a lattice spacing of 2.41 Å, which corresponds to the (111) plane of the cubic NiO phase [32].Figure 4(d) shows the EDS elemental mapping distribution of the NiO@rGO composite.Noticeably, the elements C, O, and Ni are uniformly scattered, indicating the uniform loading of the NiO nanoparticles on the rGO sheet.According to the ICP-MS analysis, the Ni content of the NiO@rGO composite is 0.72 mmol g −1 or 43.2 wt%.
The effects of heating temperatures on the morphology of NiO@rGO composites were further investigated by TEM (figure 5).The NiO@rGO precursors are shown in figure 5(a), which shows no visible particles.As  shown in figures 5(b)-(d), NiO nanoparticles were loaded on the surface of rGO, and the particles became larger with increasing temperature.Figure S1 shows the XRD patterns of the NiO@rGO composites at different heating temperatures.All XRD curves show the diffraction peaks of NiO phases.The NiO lattice parameters were calculated according to the Scherer formula.The grain sizes of NiO were calculated to be 5.2 nm, 7.4 nm, and 8.9 nm for heating temperatures at 250 °C, 300 °C and 350 °C, respectively (table 1).It indicates that the NiO crystal grain size of the NiO@rGO composites increases with the rising heating temperature.
The valence and elemental information of the NiO@rGO composites were determined using XPS analysis.Figure 6

Electrochemical performance test
In order to investigate the effect of temperatures on the electrochemical properties, NiO@rGO electrodes with heating temperatures of 250 °C, 300 °C and 350 °C were prepared and systematically tested (shown in figure 7).The cyclic voltammetry (CV) measurements of the NiO@rGO electrodes were taken at the scan rate of 10 mV s −1 , as shown in figure 7(a).As can be seen, the CV curves of all NiO@rGO electrodes consist of a pair of redox peaks.The oxidation and reduction peaks of NiO@rGO-250 electrode located at 0.48 V and 0.33 V, respectively, correspond to the transition between Ni(II) and Ni(III) ions [38].To further determine its specific capacitance, the galvanostatic charge/discharge (GCD) curves of NiO@rGO electrodes were tested, as shown in figure 7(b).It can be found that the NiO@rGO-250 electrode has the longest discharge time, showing a 1399 F g −1 discharging capacity.By comparing the three curves, it was found that the specific capacitance is reduced with the increasing heating temperature of the electrodes.The GCD performances of the NiO@rGO electrodes with different electrodeposition times (calcined at 250 °C) were also produced and shown in figure S2.It was concluded that the NiO@rGO electrode with a deposition time of 5 min showed the best electrochemical performance, which might be because excessive loading of NiO breaks the porous structure and is not beneficial to electrolyte infiltration.
The cycling performance of NiO@rGO electrodes at 2 A g −1 for 10000 times is illustrated in figure 7(c).As can be seen, the maximum specific capacitance were 1459 F g −1 (NiO@rGO-250), 1340 F g −1 (NiO@rGO-300) and 1206 F g −1 (NiO@rGO-350), respectively.The specific capacitance of the NiO@rGO electrodes decreased with increasing heating temperature, in according with the GCD test results.However, after 10000 cycles, their capacitance retention was 86.2% (250 °C), 88.6% (300 °C) and 83.4% (350 °C), respectively.The EIS results are shown in figure 7(d).The intersection with the x-axis in the high-frequency region reflects the electrolyte resistance (Rs), while the diameter of the semicircle represents the charge transfer resistance (Rct).The Rs values for the five samples are 0.345, 0.376, 0.309, 0.268, and 0.36 Ω, respectively and the Rct values for the five samples are 0.292, 0.197, 0.212, 0.141, and 0.255 Ω, respectively In order to explore the electrochemical rate capability of the NiO@rGO-250 electrode, the GCD curves were measured at different current densities, as shown in figure 8(a).The GCD curves exhibited good electrochemical reversibility.The specific capacitance calculated from the GCD curves for different current densities is shown in figure 8(b), obtaining a specific capacitance of 1399 F g −1 at a current density of 1 A g −1 .When the current density was increased to 10 A g −1 , the capacitance was retained by 76.4%, indicating the good rate capability of the NiO@rGO electrode.The type of electrochemical reaction of the NiO@rGO electrode was investigated by means of CV curves of the NiO@rGO-250 electrode at different scan rates in the voltage range 0-0.6 V (figure 8(c)).It can be observed that there are no significant deformations in the CV curves as the scan rate increases.The 1/2 power of the scan rate and the current value of the oxidization peak was fitted linearly, and the result is shown in figure 8(c).The variance of the fit is closer to 1, which indicates a well-defined linear relationship, as shown in the following equation, where i is current, v is scan rate, a and b both are adjustable parameters: It satisfies b = 1/2, which indicates that the electrochemical reaction process of the NiO@rGO electrode is a redox reaction controlled by diffusion [39].Besides, the specific capacity calculated from the CV patterns for different scan rate is shown in figure 8(d).

Electrochemical performance of NiO@rGO//AC asymmetric supercapacitor
The asymmetric supercapacitors (ASCs) assembled through using NiO@rGO-250 electrode as the anode and active carbon (AC) electrodes as the cathode.The ratio of anode and cathode masses is determined according to the balance of charges, as seen in the supporting information.The CV of the anode and cathode in their respective voltage ranges is shown in figure 9(a).It can be seen that at a scan rate of 10 mV s −1 , the NiO@rGO electrode exhibits good reversibility in the range of 0 to 0.6 V. Similarly, the activated carbon exhibits a suitable double-layer capacitance in the voltage range of −0.9 to 0 V, and the shape is similar to a rectangle.The voltage range of the ASCs is given as 0−1.5 V.As shown in figure 9(b), the CV curves of the ASC can be found to show good electrochemical activity at a high scan rate of 1000 mV s −1 .
Figure 9(c) shows the GCD curves at different current densities.The specific capacitance at different current densities can be obtained and shown in figure 9(d).It can be seen that the specific capacitance of the NiO@rGO//AC ASCs is 129.3 F g −1 , 124.8 F g −1 , 117.0 F g −1 , 110.8 F g −1 , 105.5 F g −1 and 100.3 F g −1 at current densities of 1 A g −1 , 2 A g −1 , 4 A g −1 , 6 A g −1 , 8 A g −1 and 10 A g −1 .The capacitance was retained by 82.7% when the current density was varied from 1 A g −1 to 10 A g −1 with excellent rate capability.The cycling performance of the NiO@rGO//AC ASCs was shown in figure 9(e), revealing a good capacity retention of 79.1% over 10000 cycles.
Based on the specific capacitance, the specific energy of NiO@rGO//AC asymmetric supercapacitors are calculated at 750 W k g −1 , 1500 W k g −1 , 3000 W k g −1 , 4500 W k g −1 , 6000 W kg −1 and 7500 W k g −1 when the specific energy can reach 40.4 Wh k g −1 , 39.0 Wh k g −1 , 36.5 Wh k g −1 , 34.6 Wh k g −1 , 32.9 Wh k g −1 and 31.4Wh kg −1 , respectively.Table 2 exhibited the electrochemical performances of the recent reported NiObased supercapacitors, showing a obvious advantages of this work.

Conclusion
A unique interconnected foam-like NiO@rGO structure was fabricated by loading the NiO nanoparticles onto rGO frameworks via a three-steps procedure including hydrothermal reaction, electrodeposition and heating treatment.The impacts of deposition time and heating temperatures on the properties and electrochemical performances of NiO@rGO composites were investigated.It was found that the specific capacitance of NiO@rGO composites decreased with increasing heating temperature.The NiO@rGO electrode with 5 min deposition time and 250 °C heating temperature has the largest specific capacitance (1399 F g −1 at 1 A g −1 ), and the capacitance was retained by 86.2% after 10000 cycles.Furthermore, NiO@rGO//AC ASCs were assembled with NiO@rGO-250 as the anode and AC as the cathode.When the specific power is 750 W k g −1 , the specific
(a) shows the C 1 s fine spectrum of NiO@rGO composites.After fitting, it consists of three peaks, C 1 , C 2 , and C 3 .Among them, C 1 at 284.8 eV attributes to C-C, C 2 at 286.2 eV to C-O-C, and C 3 at 288.3 eV to C-O

Figure 7 .
Figure 7. Electrochemical performance of the three NiO@rGO electrode at different heating temperatures (a) CV curves at 10 mV s −1 , (b) GCD curves at 1 A g −1 and (c) cycling stability at a constant current density of 2 A g −1 and (d) EIS patterns.

Figure 6 (
c) shows the fine spectrum of Ni 2p.It can be seen that the 2p spectrum of Ni consists of two spin peaks and two accompanying peaks.The electron binding energy peaks at 854.1 eV and 871.1 eV correspond to Ni 2p 3/2 and Ni 2p 1/2 , respectively.The 2p 1/2 peak corresponds to Ni (II) ions [37], indicating the presence of divalent nickel ions in the product.

Figure 8 .
Figure 8. Electrochemical properties of NiO@rGO-250 electrode (a) GCD curves at different current densities, (b) specific capacitance at different current densities, (c) CV curves at different scan rates and (d) fitting graph of scan rate 1/2 with peak current values.

Figure 9 .
Figure 9. (a) CV curves of AC and NiO@rGO-250 electrodes at scan rates of 10 mV s −1 ; electrochemical properties of NiO@rGO// AC asymmetric supercapacitor (b) CV curves at different scan rates, (c) GCD curves under different current densities, (d) specific capacitance at different current densities and (e) cycling performance at current density of 10 A g −1 .

Table 1 .
Unit cell parameters and grain size of NiO at different thermo-treatment temperatures.