Carbon nanotube branch-grown nickel nanoparticles/graphene composites for a high-capacitance electrode

Despite the high capacitance and low cost, transition metal oxides have the limitation of low electrical conductivities and structural instability. In order to resolve these problems, herein, we propose a one-pot facile synthesis approach to construct a hierarchically structured nanohybrid material, where carbon nanotube (CNT) branches encapsulate NiO nanoparticles inside the tubes and interconnect them with steam-activated reduced graphene oxide. This unique hierarchical structure is attributed to large accessible surface areas, rapid electronic conduction, fast ion diffusion, and buffering effects. Moreover, the mixed Ni and NiO particles acts as catalysts to grow CNT branches and high capacitance redox active materials. In particular, the resulting composite electrode deliver a high specific capacitance of up to 1605.81 F g−1 at a current density of 1 A g−1 as well as, an excellent cycle stability with 71.56% capacitance retention after more than 10 000 cycles. Consequently, this research provides a rational material design chemistry to construct hierarchical architectures and multiple compositions of CNT/graphene/metal oxide nanoparticle hybrids for high-capacitance electrodes of composite capacitors.


Introduction
Since the signing of the Paris Climate Agreement in 2015, there have been a renewed efforts to develop appropriate energy storage systems compatible with CO 2 emission regulations and to meet the worldwide increase in the demand for renewable energy [1,2]. An energy storage system stores excess power in a smart grid for later transmission during low energy production, and maximizes the power usage efficiency [3,4]. Currently, electrochemical energy storage is predominantly achieved through batteries and supercapacitors [4]. Li-ion batteries are the most widely used owing to their large energy density and low self-discharge rate in various applications, from small devices such as mobile phones to large devices such as electric vehicles [5,6]. However, the power density, charging rate, and cyclic stability of the Li-ion batteries are much poorer than electrochemical capacitors [7,8]. Collectively, these factors have motivated active research on electrochemical capacitors, which are classified as electrical double-layer capacitors and pseudocapacitors depending on the existence of Faradaic redox reaction on the surface [3,9]. They are characterized by fast response, high current charge/discharge, semi-permanent excellent charge/discharge cycles and inherent safety over a wide temperature range [10][11][12][13][14]. However, their applicability is severely restricted by their energy density of approximately 20 Wh kg −1 , which is one order of magnitude lower than that of Li-ion batteries [15][16][17][18]. To increase energy density for practical applications of electrochemical capacitors [19,20], high-capacitance pseudocapacitive materials using the principle of redox reactions have been developed [19][20][21][22][23][24].
Transition metal oxide-based electrode materials possess excellent capacities and are relatively easy to synthesize, making them promising high-capacitance electrode materials [25,26]. However, most of metal oxides are exhibiting the Faradaic reaction have lower electrical conductivities and less structural stability than carbon-based materials, limiting their rate performance and cycling stability [18,[21][22][23][24][25]. To mitigate these challenges, the hybridization of transition metal oxides with carbon-based materials (possessing excellent conductivity and large surface area), such as graphene and carbon nanotubes (CNTs), has been extensively studied [3,26]. Carbon-based materials are suitable for improving both, the conductivity and stability, because they improve the bonding properties between carbon and inorganic materials, reduce the charge transfer resistance, and prevent the volume expansion of battery-type materials through an intermediate buffer layer when forming composites [18,26,27]. In particular, their hierarchical structure, which is composed of a transition metal oxide and nanostructured carbon, introduces a synergistic effect as each material possesses differing structural and compositional characteristics [28,29]. The strategy of simultaneously using one-dimensional (1D) CNTs and two-dimensional (2D) graphene to compositeize with the transition metal oxides is very useful for constructing the hierarchical architecture of carbon-based composites with multiple compositions [28,30]. 1D CNTs and 2D graphene can be assembled into the three-dimensional (3D) porous structure that provides large access areas, rapid charge transfer, fast ion diffusion, and buffering effects [31]. Although various studies have explored the effectiveness of compositeizing redox-active transition metal oxides with nanostructured carbon [31,32], transition metal nanoparticles encapsulated in tubes of hierarchically constructed 1D CNTs/2D reduced graphene oxide (rGO) hybrids have yet to be explored as a means to synthesize high-capacitance electrode materials [33].
Herein, we introduced a NiO nanoparticle precursor and steam treated rGO (srGO) to synthesize NiO@srGO/CNTs with a hierarchical structure using microwave irradiation, for electrochemical capacitors. Microwave irradiation is a simple method of transferring energy directly to the material and growing metal oxide nanoparticles and CNTs branches at relatively high reaction rates [5,34]. Owing to microwave irradiation, the Ni precursor rapidly decomposes, and the reaction proceeds to generate NiO without a separate side reaction for a high capacitance [35]. Moreover, the CNTs branch provides percolated conduction paths between the graphene nanosheets and NiO nanoparticles, augmenting the accessible surfaces to form a more complex hierarchy. Therefore, the NiO@srGO/CNTs composite achieves a high specific capacitance and an excellent cycle stability owing to the hierarchical structure and multiple composition.

srGO
To obtain srGO, graphene oxide (GO) was first reduced into rGO using the modified Hummers method [36]. Specifically, a solution obtained by mixing distilled water (15.9 ml) with a GO solution (1 g) at a concentration of 2 mg ml −1 was uniformly dispersed through a bath sonication for 1 h. Iodine (I 2 , Daejung Chemicals and Metals Co. Ltd, 99.0%) was then completely dissolved through active stirring in hypophosphoric acid (H 3 PO 2 , Daejung Chemicals and Metals Co. Ltd, 50 wt%) and mixed thoroughly with the previously prepared GO solution. After mixing the GO and iodine solutions in a vortex mixer for 15 min, the mixture was placed in an oven at 80 • C and stored for 12 h. Following gel formation, the pH of the product was adjusted to neutral; the neutralized gel was completely frozen by immersion in liquid nitrogen for 1 h, transferred to a freeze dryer, and thawed for 72 h. After obtaining rGO in the form of a black sponge, srGO was synthesized by annealing the lyophilized sample in a tube furnace to construct 3D porous structure. Freeze-dried rGO was transferred into the tube, where 10 ml of deionized water was injected for 1 h. During this time, the sample was heated to 900 • C at a heating rate of 5 • C min −1 , held at the temperature for 1 h, and gradually cooled to 25 • C to yield srGO.

NiO@srGO/CNTs
Microwave synthesis was carried out by mixing the previously prepared srGO with azodicarboxamide (ADC), nickelocene (bis-cyclopentadiene-nickel (II)), and acetonitrile (ACN). The srGO powder (10 mg) was mixed with 1 mg of ADC and vigorously stirred in 3 ml of ACN solution for 30 min. The Ni precursor, nickelocene, was then introduced into the mixture, which was thoroughly mixed by stirring for an additional 30 min. As agitation progressed and the mixture assumed a gel form, was transferred to a domestic microwave oven and irradiated at 900 W for 10 min. Nickelocene, which functions as a Ni precursor, decomposes into Ni particles and cyclopentadienyl when exposed to microwave irradiation [28]. The Ni-containing precursor formed on the srGO surface served as a foundation for subsequent CNTs growth [37]. In addition, cyclopentadienyl provide the carbon required to produce CNTs. In some cases, ADC was introduced into the reaction mixture as additives. Following microwave irradiation, ADC decomposes into CO, CO 2 , and N 2 gases, to make the reaction mixture inert, peeling off the srGO sheet, and preventing any moisture contamination [28,38]. To quantify the effect of the Ni precursor, nickelocene-to-srGO ratios (by weight) 1:1, 1:3, and 1:5 were used; the thus-obtained samples were represented as NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5, respectively.

Material characterization
The surface morphology of all materials used in the experiment was analyzed using a scanning electron microscope (SEM, MERLIN (Carl Zeiss)), and crystallographic information of the material was observed with a transmission electron microscope (TEM, JEM ARM 200F). In addition, x-ray diffraction (XRD) (Bruker D8 High-resolution XRD) was used to cross-check the crystal structure, and CuKa x-rays with a wavelength of 1.5406 Å were used. To reveal the chemical structure of the material synthesized in this study, it was analyzed using x-ray photoelectron spectroscopy (XPS, ESCA2000). BET and BJH analyzes were performed to obtain specific surface area and pore information of the synthesized material (Belsorp-Mini II (BEL Japan)). All electrochemical data measured in this study were measured through EC-Lab (NEO Science VMP3).

Electrochemical measurement
All electrochemical data conducted in this study were conducted through EC-Lab (NEO Science VMP3), and 6 M KOH aqueous electrolyte was used. The NiO@srGO/CNTs electrode used in this experiment was obtained by mixing an active material, a conductive material (carbon black), and a binder (poly(vinylidene fluoride)) in a weight ratio of 8:1:1. After that, the electrode slurry was uniformly coated on the surface of the pretreated nickel foam (area of coating: 1 cm 2 ), and it was allowed to remove NMP for 12 h in an oven at 80 • C. The active material loading amounts of the NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 electrodes are 0.87, 1.14, and 1.27 mg cm −2 , respectively. A three-electrode system was employed for electrochemical measurement, the previously synthesized NiO@srGO/CNTs electrode was used as the working electrode, Pt was used as the counter electrode, and the Hg/HgO electrode for base was used as the reference electrode. The gravimetric capacitance (C g ) of the electrode was calculated using the equation described below [39]: In the above equation, I d is the current density used for discharging, and t d is the time taken until the discharging is completed. And V term indicates that operating voltage range. In addition, ν (mV s −1 ) denotes the scan speed, and I (v) indicates the current response value. The mass (m) means the mass of the active material excluding the nickel foam used as the current collector and is expressed by the following formula, whereas V i and V f are initial and final potentials of an electrode during charging: In the above formula, m b means the pristine nickel foam mass before the electrode is coated, and m a means the mass after coating the active material on the nickel foam surface. In order to reflect the reversible characteristics of the all-terrain electrode, the energy efficiency (E e ) was calculated. And the following formula was used: In the above formula, I c (mA) indicates that a constant charge current density, and E d (E c ) is an integral current region under a galvanostatic charge/discharge (GCD) curve during discharging (charging).

Results and discussion
As shown schematically in figure 1(a), the NiO@srGO/CNTs electrode is synthesized using a simple microwave method. The srGO support, Ni precursor, and ADC additive are combined in an ACN solution to form a homogeneous mixed solution; the reaction proceeds under microwave irradiation for approximately 10 min. As a result, NiO@s-rGO/CNTs nanoparticles in the form of a black powder are obtained. The SEM images of srGO, NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 are shown in figures 1(b)-(i). As shown in the SEM images of the srGO in figures 1(b) and (c), the 3D microporous structure is constructed with the crumpled rGO nanosheets, which include some distortions introduced through steam treatment. In comparison with the image of rGO presented in figure S1, the srGO surface possesses more distortions. Even after steam treatment, srGO still retains the macropore framework derived from rGO despite the formation of micropores [40]. As described above, the numerous defect regions present on the surface of srGO serves as active sites into which the Ni precursor can be inserted [34].
show that the NiO nanoparticles are uniformly deposited onto the srGO sheet due to the microwave irradiation of the reaction volume to facilitate a rapid reaction [5]. It was further demonstrated that the mixing ratio of srGO and the Ni precursor had a significant effect on the growth of CNTs on the surface. Figures 1(d) and (e) illustrate that when srGO and the Ni precursor are mixed in a ratio of 1:1, NiO particles are uniformly decorated on the surface of srGO, but CNTs growth is not achieved. As mentioned previously, nickelocene, the Ni precursor decomposes to produce NiO nanoparticles and cyclopentadienyl; cyclopentadienyl in turn provides the carbon required for the growth of CNTs [37]. A 1:1 ratio of srGO and Ni precursors does not yield sufficient carbon to support the growth of CNTs branches. In contrast, the use of excess Ni precursor induces the formation of a distortion region covering the entire surface of srGO, while an excess of carbon from cyclopentadienyl increases the thickness of the CNTs branch. Complete coverage of the surface by thickly formed CNTs branches limits the availability of ion channels by reducing the effective surface area, making fast ion transport impossible. Instead, randomly deposited NiO nanoparticles (using an optimal srGO-to-NiO precursor ratio of 1:3) are well distributed without the formation of large local agglomerates on the mesoporous surface of srGO, while long and thin CNTs branches grow in abundance. Through figure S2, the SEM mapping reveals a uniform distribution of Ni, O and C elements. Intense C and O peaks originating from srGO serving as a substrate and CNTs densely grown on the surface can be identified. In addition, it was confirmed that the Ni particles were evenly located throughout the srGO substrate having a porous structure.
To confirm the crystal structure of the synthesized NiO@srGO/CNTs, XRD pattern analysis was performed and the results of which are presented figure 2(a). The peaks at 2θ = 37.  [42]. The XRD pattern analysis shows that Ni and NiO phases are mixed, and most of them exist as NiO phases. The peak near 45 • corresponding to the NiO (200) plane exists over a wide range owing to the small crystalline size, and also includes a section of NiO (111) that is partially out of the scan range. Based on the full width at half maximum value, it was confirmed that the concentration was lower than that of the nickel precursor, but still a distinct peak was maintained. These results suggest that the Ni precursor changes into Ni and NiO phases when microwaves are irradiated, which is consistent with high-resolution TEM (HR-TEM) results. Finally, the peak at 26.4 • coincides with the (002) plane of carbon (JCPDS 41-1487) [43]. In figure S3, for NiO@srGO/CNT1, the amount of Ni precursor is relatively insufficient compared to the amount of srGO; thus, the peaks corresponding to the Ni and NiO phases are understandably weak. In contrast, NiO@srGO/CNT5 shows distinct Ni and NiO peaks owing to the excess availability of Ni precursor.
To further validate the structure, NiO@srGO/CNT3 was extensively analyzed using transmission electron microscope (TEM). Figure 2(b) confirms that the Ni particles are located both on the surface of srGO and encapsulated inside the grown CNTs. In addition, the srGO and CNTs component can be distinguished in the NiO@srGO/CNTs composite owing to their different geometries. The CNTs protrude from the NiO particles embedded in the srGO surface, providing a large specific surface area. It is known that metal particles may exist in both, a metal oxide phase and a metal particle phase when irradiated with microwaves, with observable differences in the crystal morphologies associated with each [44]. Therefore, the Ni particles generated by the decomposition of the Ni precursor (nickelocene) are adsorbed onto the srGO surface and converted into NiO and Ni particles, while also functioning as a catalyst for the growth of CNTs. As a result, Ni particles are observed primarily on the surface of srGO, while NiO phases are mainly observed inside the CNTs. In the proximity of srGO, as illustrated in figure 2(c), the Ni (200) crystal plane (corresponding to 0.18 nm) is verified as an index for the Ni nanoparticle. In the CNTs regions illustrated in figure 2(d), a crystal plane interval of 0.2 nm corresponds to NiO (200). The Ni on the surface of the srGO and NiO particles on the branches of CNTs, significantly improves the electronic properties to effectively facilitate interfacial electron transport during electrochemical reactions. Furthermore, the size of the NiO particles in the NiO@srGO/CNT3 composite is determined to be approximately 20 nm. In figure S4, the energy dispersive spectroscopy elemental mapping of NiO@srGO/CNT3 presents the elemental distributions of C, O, and Ni. The C signal in proximity to the srGO and CNTs branches can be confirmed over a wide region; the Ni signal is attributable not only to the srGO surface but also to the regions where CNTs grow. These results further support the notion that NiO particles function as catalysts for nucleation and growth of CNTs branches directly on the surface of srGO [37].
The specific surface areas and surface pore properties of the NiO@srGO/CNT3 composites were analyzed using nitrogen adsorption measurements. The nitrogen adsorption and desorption isotherms of NiO@srGO/CNT3 and the Barrett (Joyner) Halenda (BJH) pore distribution curves are shown in figure 3(a). The nitrogen adsorption and desorption isotherms are consistent with type IV hysteresis, indicating the existence of mesopores and macropores. Brunauer-Emmett-Teller (BET) analysis was conducted to determine the specific surface area of the NiO@srGO/CNT3 hybrid, which was measured to be 784.9 m 2 g −1 .
In addition, by performing BJH analysis, it was confirmed that NiO@srGO/CNT3 possessed a pore volume of 4.47 cm 3 g −1 and pore diameters in the range of 10-200 nm. The srGO shown in figure S5 possesses a relatively large specific surface area of 1556 m 2 g −1 . When rGO is initially converted to srGO through steam activation, numerous small pores (<4 nm) are generated, increasing the surface area significantly; as synthesis progresses, Ni particles and CNTs block some pores of the steam-activated rGO structure. The presence of pores present in large quantities in srGO not only acts as an active site that can adsorb ions, but also plays an important role in the growth of CNTs by serving as a root for nickel particles to grow [45]. In the case of NiO@srGO/CNT1, Ni particles can be located on distortions on the surface of srGO and decorated. A relatively large-sized Ni particles blocks the pores of srGO, resulting in relatively pore blocking. As shown in the BET results, the specific surface area of NiO@srGO/CNT1 reaches 277.04 m 2 g −1 owing to pore blocking, while the NiO particles decorate the srGO surface. In the case of NiO@srGO/CNT5, the excess Ni particles cover the surface of srGO completely and clog the pores, and coinciding with the formation of thick CNTs to yield mesopores and macropores, and thereby contributing to a relatively low specific surface area (224.79 m 2 g −1 ).
XPS analysis was performed to confirm the chemical bonding and composition of NiO@srGO/CNTs. The XPS survey spectrum in figure S6 shows typical peaks of NiO@srGO/CNT3 containing Ni, C, and O. In the high-resolution Ni 2p XPS profile shown in figure 3(b), the distinct peak observed at 852 eV is attributed to metallic Ni, while the peaks at 853, 856, 872 and 873 eV are attributed to Ni 2+ in the Ni-O bond [46]. These peaks confirm the presence of the Ni 2+ oxidation state, providing that high-capacitance NiO nanoparticles are present on the graphene surface. In addition, two pairs of satellites are observed with binding energies several electron volts higher than those of the main peak of the spectrum [47]. It can be confirmed that the satellite peaks induced by the 2p 3/2 main peak at 853.9 eV and 2p 1/2 peak at 872.3 eV occur at 862.3 eV and 879.7 eV, respectively, which is further evidence of the existence of NiO. In addition, a distinct C1s XPS profile can be observed in figure 3(c), owing to the presence of srGO and CNTs in Ni@srGO/CNT3. At 285.1 eV and 288.1 eV, peaks appear due to the bond between C atoms and the bond between C and O atoms, respectively [48]. Furthermore, a C-OH peak generated by the adsorption of water molecules is detected at 286 eV [48]. Finally, in the O1s spectrum of figure 3(d), peaks 1, 2, and 3 of NiO@srGO/CNT3 correspond to those produced by metal-oxygen bonds (O-Ni), hydrolyzers of water molecules adsorbed on the surface, and O-C=O bonds, respectively [49]. These results confirm that microwave irradiation induces strong interactions between the NiO and srGO surfaces.
The electrode performance of the NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 materials was evaluated based on a three-electrode analysis in a 6 M KOH aqueous electrolyte. The cyclic voltammetry (CV) profiles of the srGO, NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 electrodes are shown in figure 4. The electrochemical measurement confirms that srGO, which does not contain a Ni precursor, does not exhibit any distinct redox peaks and does not affect the capacitance significantly. In contrast, the CV curve of the NiO@srGO/CNTs electrode containing the Ni component exhibits a unique redox peak and the profile of a typical battery-type electrode. In figure 4(a), a redox peak generated by the reversible reaction of Ni 2+ /Ni 3+ with NiO can be clearly observed. The electrochemical reaction of a NiO compound/graphene complex structure (NiO@srGO/CNTs) electrode on which CNTs are grown in an alkali electrolyte is represented by the following equation [50]: The redox peaks of the NiO@srGO/CNT3 electrode are more clear than those of the other two electrode materials measured in this study, indicating that its electrochemical performance is superior. The CV profile of the NiO@srGO/CNTs hybrid is presented in figures S7(a)-(c) and was measured at various scan rates (1-100 mV s −1 ). Each of the NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 electrodes exhibits a clear redox peak at a voltage of 0.4-0.6 V, regardless of the scan rates. In addition, this redox reaction is maintained even when the scan rate is increased to 100 mV s −1 . The distinct redox peaks of the NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 electrodes are attributed to the electrochemical reaction of the NiO compound identified in the XPS analysis [51]. The distinct oxidation/reduction peak indicates that the high-capacitance Ni particles grow stably on the graphene surface. This implies that the Ni@srGO/CNTs electrodes enable rapid redox reactions and produce additional ion transfer networks to enable rapid material transfer.
In figure 4(b), the specific capacitances of the NiO@srGO/CNT1, NiO@srGO/CNT3, and NiO@srGO/CNT5 electrodes are evaluated using the aforementioned capacitance formula. The capacitances measured at a scan rate of 2 mV s −1 are 1607.65 F g −1 , 1455.105 F g −1 , and 1335.54 F g −1 for NiO@srGO/CNT3, NiO@srGO/CNT5, and NiO@srGO/CNT1, respectively. Relative to the other electrodes, the NiO@srGO/CNT3 electrode exhibited a relatively high capacitance, which was maintained even at high scan speeds. These results demonstrate the advantages of combining srGO and NiO to promote excellent electron conductivity and high redox reactivity associated with the growth of CNTs on the surface of the material. However, as can be observed from the characterization results of NiO@srGO/CNT5, if the CNTs grow to a very large size, their conductivity decreases, which is unfavorable for high-speed charge transfer [52]. Figure 4(c) presents the GCD results of all the samples measured at a current density of 5 A g −1 . The nonlinearity of each GCD curve corresponds to a Faraday-dominant battery-type electrochemical response within the electrode. These GCD results are in good agreement with the CV profile presented above. In addition, the discharge time of the GCD profile decreases in the following order: NiO@srGO/CNT3 > NiO@srGO/CNT5 > NiO@srGO/CNT1. The excellent electronic conductivity of NiO@srGO/CNT3 translates to a high specific capacitance. As shown in figures S7(d)-(f), the NiO@srGO/CNTs electrodes undergoes a significant oxidation/reduction reaction at voltages of 0.4-0.6 V, regardless of the variation in the current density from 1 to 20 A g −1 . This is consistent with the CV results, which indicate distinct oxidation/reduction peaks despite the increased scanning speed.
The capacitance calculated as a function of the discharge time is presented in figure 4(d). Consistent with the result calculated through the CV profile, the calculated capacitance from the GCD measurements decreases according to the following sequence: NiO@srGO/CNT3 > NiO@srGO/CNT5 > NiO@srGO/ CNT1; this corresponds to capacitances of 1605.82 F g −1 , 1441.37 F g −1 , and 1357.79 F g −1 (at a current density of 1 A g −1 ), respectively. The NiO particles are battery-type materials and provide excellent capacitance, and the CNTs branches surrounding them accelerate electron transfer, causing a remarkable redox reaction even at high current densities. At a high current density as high as 20 A g −1 , the gravimetric capacities are 1312.98, 1012.07 and 849.56 F g −1 for NiO@srGO/CNT3, NiO@srGO/CNT1, and NiO@srGO/CNT5, respectively. The capacitance retention decreases from 96.6% to 85.56%, from 95% to 81.79%, and from 93.75% to 73.39% for NiO@srGO/CNT3, NiO@srGO/CNT1, and NiO@srGO/CNT5, respectively, as shown in figure 4(e).
In figure 4(f), when the current density increases from 2 A g −1 to 20 A g −1 , the energy efficiency tends to decrease from 84.31% to 77.91%, and from 80.36% to 75.94% and 78.78% to 70.15% for NiO@srGO/CNT3, NiO@srGO/CNT1, and NiO@srGO/CNT5, respectively. The superior capacitance of the NiO@srGO/CNT3 nanocomposite relative to the two controls is achieved through the combination of srGO and CNTs with excellent electron conductivity and NiO with a high redox reactivity. In addition, the support offered by the CNTs frames grown on the srGO surface prevents the agglomeration and restacking of metal particles, allowing the maximum utilization of redox-active NiO. The large surface areas of srGO and CNTs for pore and ion transport not only result in a short diffusion distance, but also functions as a charge transport path [53]. Thus, this not only increases the charge storage capacitance but also improves interface and structure control for the reversible use of an active material for energy preservation during fast charging and discharging, thereby contributing to the improvement of the electrochemical performance.
An electrochemical impedance spectroscopy study investigated fast charge transport (electrons and ions) and diffusion limiting material accessibility, which are key parameters to confirm the synergistic effects of interface and structure control. The ohmic resistance, which is a real impedance axis fragment, represents the ion conductivity of the electrolyte with electrolyte resistance Rs. The subsequent semicircle reflects the charge transfer resistance (R ct ), and is formed when charges move at the electrode interface. The Warburg impedance (Z w ), which appears in the low frequency region, provides information on ion diffusion in energy storage devices [54]. The closer the slope is to 90 • , the lesser the ideal diffusion observed; the closer the slope is to 45 • , the greater the extent of the diffusion-dominated charge transport reactions. A slope between 45 • and 90 • corresponds to diffusion control [55]. The resistance behavior is represented by the Nyquist plot in   figure 5(a). The internal resistance, which is a combination of the bulk electrolyte and equivalent series resistances, has values of 1.118, 1.107, and 1.137 for NiO@srGO/CNT1, NiO@srGO/CNT3, NiO@srGO/CNT5, respectively. The NiO@srGO/CNT3 electrode exhibits the lowest internal resistance, and the diameter of its semicircle is the smallest of the three electrodes. These measurements are based on the acceleration of charge storage dynamics induced by the numerous CNTs of the NiO@srGO/CNT3 electrodes, which act as 3D ion transport channels, facilitating faster ion transport and providing easy access to active sites. Figure 5(b) confirms the cycle stability over 10 000 charging/discharging cycles. For NiO@srGO/CNT3, NiO@srGO/CNT1, and NiO@srGO/CNT5, the capacitance retention rates are 71.56%, 69.48%, and 55.35%, respectively, after 10 000 charge-discharge cycles. In the case of NiO@srGO/CNT1, the capacitance rapidly decreases owing to the initial loss in capacitance after 1000 cycles due to the structural collapse caused by the reaction of NiO on the surface [56]. Thereafter, the capacitance is maintained at approximately 69.48% after 10 000 cycles. In contrast, CNTs branches growing on the NiO@srGO/CNT3 and NiO@srGO/CNT5 electrodes surfaces act as pillars to prevent restacking and structural collapse; thus, the initial irreversibility is relatively small. The capacitance retention rate of the NiO@srGO/CNT3 electrode is 71.56% relative to the initial capacitance following 10 000 cycles of charging and discharging and 55.35% for the NiO@srGO/CNT5 electrode. Because the CNTs branch successfully prevented structural collapse [57], all three measured electrodes recorded remarkably high coulomb efficiency. However, for NiO@srGO/CNT5 electrodes, Coulombic efficiency fluctuated and decreased relatively due to the damage to some of the excessively grown CNTs branches over the long cycle.
As displayed in figure 6, excess CNTs detached from the srGO surface after electrochemical cycling do not function reliably when used in NiO@srGO/CNT5 electrodes. On the other hand, in the case of the NiO@srGO/CNT3 electrode, the CNTs maintain their initial shape on the srGO surface even after electrochemical measurements, showing excellent stability. Collectively, these findings validate that the NiO active phase combined with the srGO support material enables a synergetic effect that promotes the excellent electrochemical performance of the material as a battery-type electrode. Figure S8 shows the XRD results for NiO@srGO/CNT3 electrode after a long period of time. The nickel foam used as a currency collector can check the strong peaks that appear at 2θ = 44 • and 51 • . Relatively weak peaks were being observed due to the high strength peak of the crystalline nickel foam board and the washing of the electrode after the long-term cycle measurement. However, due to the NiO peak observed at 37.2 • and the carbon peak appeared at 26.4 • , the CNTs-grown structure is well maintained even after long-term cycling, and these results are in good agreement with the SEM results measured after long-term cycling.

Conclusion
In this study, we have demonstrated a simple one-pot method of synthesizing a NiO@srGO/CNTs composite structure through a microwave irradiation. Through this process, the NiO nanoparticles were randomly distributed onto the srGO sheet, while the CNTs branches grown on the surface of the srGO acted as a pillar to prevent restacking simultaneously providing 3D porous ion passage, percolated electron conducting pathway, and large accessible surface area. This unique hierarchical structure with composite composition was attributed to the significantly improved capacitive performance. Consequently, the NiO@srGO/CNTs electrode synthesized in this study exhibited a distinct redox peak and superior capacitance of 1605.82 F g −1 (at 1 A g −1 ) and indicated its suitability as a high capacitance battery-type electrode of hybrid capacitor. It also maintained a capacitance retention rate of 71.56% over 10 000 charge-discharge cycles. This study is applicable as a design of energy storage material that utilizes the synergy between the carbon-based hybrid structure and metal oxide nanoparticles.

Data availability statement
The data cannot be made publicly available upon publication because they contain commercially sensitive information. The data that support the findings of this study are available upon reasonable request from the authors.