Controlled-synthesis of hierarchical NiCo2O4 anchored on carbon nanofibers mat for free-standing and highly-performance supercapacitors

In this work, a versatile one-step hydrothermal technique was used to produce a hybrid standalone electrode of NiCo2O4 hierarchical nanostructures anchored on CNFs mat for highly-performance and substrate-free supercapacitors. The CNFs mat was working as a conductive and a three-dimensional template for the deposition of the hierarchical NiCo2O4 nanostructures at the same time. The morphological and structural data analysis revealed a pure spinel NiCo2O4 nanostructures with a unique surface morphology comprising interconnected ultrathin nanoneedles and nanoflowers were successfully anchored to the CNFs mat. Real supercapacitors consist of two-symmetrical hybrid electrodes with different NiCo2O4 loading ratios were assembled and tested. The electrochemical performances of the assembled devices in terms of specific capacitance, energy, and power densities were systematically evaluated. Increasing the NiCo2O4 loading on the CNFs mat had shown a positive impact on improving the overall electrochemical performance of the assembled supercapacitors. A hybrid electrode loaded with NiCo2O4 twice as much as CNFs possess a specific capacitance value of 540 F g−1 along with an energy density of 30 Wh kg−1 at a power density of 515.6 W kg−1. In addition, the device showed excellent cycling stability and high capacitance retention against 6000 charge–discharge cycles at a charging current of 1.0 A g−1.


Introduction
The upcoming times will witness more spreading of flexible and wearable electronics for various oriented applications such as medical treatment purposes, smart assistance devices, and also the remote sensing devices [1,2]. This would indeed be accompanied by increasing the demands on highly-performance and flexible energy storage devices to meet the electric energy requirements of such applications. Nevertheless, lightweight, environmentally safe, long cyclic life in addition to fast recharge capability are prerequisites for these types of power sources [3]. A supercapacitor (SC) is an electrochemical storage device that combines the merits of the high-power from the traditional dielectric capacitors and the high-energy-density from conventional batteries together in one device [4].
Recently, the flexible SCs became more attractive as power sources than micro-batteries owing to its unique qualities of flexibility and high rate capabilities [5]. Flexible SC devices built from various electrode materials have been already fabricated and tested. Many carbon nanostructures in the form of stand-alone and on flexible substrates such as carbon nanotubes (CNTs) [6,7], graphene [8,9], mesoporous carbon [10], and carbon nanofibers (CNFs) [11] have been explored for a long time. Although carbon allotropes are chemically stable and non-toxic materials, however, and due to its intrinsic electric double layer (EDL) storing mechanism the devices are usually suffered from low specific capacitances (commonly lower than 400 F g -1 ) and poor power density [12][13][14].
Alternatively, SC electrodes from materials relies on the pseudocapacitance storing mechanism had been proven to attain much higher specific capacitances and power densities than EDL carbonaceous materials [15]. Among a large number of the fabricated and tested pseudocapacitive materials, flexible SCs based on metal oxides electrodes such as RuO 2 [16], MnO 2 [17][18][19], NiO [20], and Co 3 O 4 nanostructures [21,22] were got attention due to their outstanding performances and not to contain toxic elements such as sulfides and selenides. Although the promising performance of RuO 2 Flexible SC (as high as 700 F g -1 ), its large-scale application is restricted by its rareness in nature [23][24][25][26]. On the other side, the large-scale applications of earth-abundant metal oxides (MnO 2 , NiO, Co 3 O 4 , … etc.) in flexible SC devices are hindered by their inferior electric conductivity, low specific capacitances than RuO 2 besides the relatively short cycling life compared to carbon-based devices [27,28]. Hence, there was an exigent necessity for developing robust and scalable approaches to fabricate hybrid electrodes for flexible SCs bridging the preferable qualities of abundance, chemical stability, and also excellent electrochemical storage performance from both metal oxides and carbon nanostructures.
As a newly leading material, nickel cobaltite (NiCo 2 O 4 ) has got considerable attention as a pseudocapacitive electrode material because it possesses better electrochemical activity and electrical conductivity at least two orders of magnitude higher than pure Co 3 O 4 and NiO [12,29]. Despite the promising results the NiCo 2 O 4 -based SCs were achieved, the reported studies considering NiCo 2 O 4 nanostructures in real two-electrode flexible SC devices still very rare. For instance, Jun Du et al. electrodeposited ultrathin porous NiCo 2 O 4 nanosheets on a flexible carbon fabric and recorded a 2658 F g -1 specific capacitance using a three-electrode system [30]. Also, F. Deng and his coworkers reported in 2015 the solvothermal synthesis of mesoporous NiCo 2 O 4 nanosheets on carbon fiber paper and they achieved as high as 1036 F g -1 specific capacitance, but it was also using a three-electrode system [29,31]. Recently, W. Luoa and H. Xue succeeded to fabricate hybrid nanocomposite from NiCo 2 O 4 embedded CNFs (NiCo 2 O 4 -CNFs) using the one-step electrospinning method followed by thermal treatment [29]. The hybrid (NiCo 2 O 4 -CNFs) nanocomposite showed promising supercapacitance performance reached 836 F g -1 with an acceptable retention rate of 80.9% after 2000 testing cycle. However, the samples were not tested for real two-electrode SC device and the authors were able only to prepare spherical NiCo 2 O 4 nanoparticles embedded into CNFs that will provide low specific surface area compared to other nanostructures such as nanorods and nanoflowers.
In this regard, this study introduces a scalable and readily approach for the fabrication of highly-performance, free-Standing, and flexible SC real devices based on hybrid nanocomposites from a CNFs mat decorated by hierarchical NiCo 2 O 4 nanostructures. Herein, the conductive CNFs mats are served as a free-standing flexible electrode as well as the porous framework for the deposition of the NiCo 2 O 4 nanostructures. The CNFs mats were firstly obtained by the high-temperature carbonization of electrospun polyacrylonitrile nanofiber mats. NiCo 2 O 4 nanostructures with different loading mass ratios were grown onto the CNFs electrode using the facile hydrothermal method. Superior to other coating methods such as chemical bath deposition (CBD), spin-coating, and even dip-coating, the hydrothermal method is a well-controlled deposition method that can easily tune the shape and morphology of the prepared nanostructure materials via adjusting the recipe and the media of the hydrothermal bath. Furtherly, it is a green synthesis method that is convenient for large-scale synthesis, saving chemicals used with minimum raw materials wastage as well as it can precisely load the desired amounts of the nanostructures on the target template. Herein, the free-standing SCs based on NiCo 2 O 4 :CNFs hybrid electrodes prepared based on the hydrothermal method attained remarkable electrochemical performances those far exceeding the devices from bare CNFs, NiO:CNFs, and also Co 3 O 4 :CNFs electrodes. The electrochemical measurements were performed on a real two-electrode SC device fabricated based on the flexible NiCo 2 O 4 :CNFs hybrid nanocomposite showed a very high specific capacitance that reached 540 F g -1 . This was also accompanied with a maximum energy density of 30 Wh kg -1 at a remarkable power density of 515.6 W kg -1 as well as excellent cycling stability. The significance electrochemical performances of the hybrid nanocomposites were carefully interpreted and evaluated from the structural and morphological characterization employed in this study.

Preparation of free-standing CNFs mat
The freestanding CNFs mat was prepared according to our previous procedure [32]. In a typical procedure, a PAN nanofibers mat was first prepared to form a polymeric solution of 8 wt. % PAN in DMF using an electrospinner (Nanon-01 electrospinner, MECC, Japan). The electrospinning process was performed at a constant feed rate of 1.5 ml h -1 and an applied voltage of 25 kV. The distance between the spinneret and the collecting drum was kept fixed at 15 cm, while the drum speed was constant at 500 RPM. At the end of the electrospinning process, the PAN nanofibers mats were collected and dried at 50°C for at least 12 h under vacuum. In order to obtain the conductive CNFs mat, the carbonization process of the PAN nanofibers mat was carried out at two stages. Firstly, is the oxidative stabilization stage in which the PAN mat was annealed in air at 280°C for 3 h at a heating rate of 1.0°C min -1 . Secondly, is the carbonizing stage in which the stabilized PAN mat was annealed under argon flow at 1100°C for one hour with a heating rate of 4.0°C min -1 in a tubular furnace. The renewed argon flow was used to prevent the oxidation as well as eliminate the impurities and byproducts such as N 2 , CO 2 , and H 2 O. The free-standing CNFs mat preparation procedure is schematically summarized in Fig. 1.

Synthesis of NiCo 2 O 4 nanoparticles coated carbon nanofiber
The synthesis of NiCo 2 O 4 nanostructures anchored on CNFs mat with different loadings (NiCo 2 O 4 @-CNFs) ratio were synthesized using a simple hydrothermal method [33]. The reaction vessel is a 100 ml Teflon-lined stainless-steel autoclave reactor containing 75 ml of the reactant solution and the CNFs mat. In this work and unlike organic solvents usually used for solvothermal preparations, distilled water was used as a green solvent, byproducts-free solvent, and non-toxic reaction media for hydrothermal synthesis of the

Materials characterization
The surface morpho1ogy of the prepared hybrid electrodes were investigated using a scanning electron microscope technique (SEM) (JEOL JSM-6010LV (Tokyo, Japan). A high-resolution transmission electron microscope (HR-TEM) (JEOL JEM-2010F) was also used for further morphology characterization. The crystal structure of the samples was investigated by X-Ray Diffractometer (XRD) (Shimadzu Xlab 6100, Japan) using Cu Ka radiation. Fourier Transform Infrared (FT-IR) spectrometer (Bruker Vertex 70, Germany) was used to investigate the IR vibration modes of the samples over the range of 400-4000 cm -1 . Raman scattering spectra from the hybrid electrodes were collected using a Thermo Raman spectrometer (Fisher Scientific Inc., MA, USA).

Electrochemical measurements
The electrochemical performance of the supercapacitor (SC) device based on hybrid NiCo 2 O 4 :CNFs nanocomposites electrode was examined in the form of symmetric two electrodes system using cyclic vo1tammetry (CV), galvanic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests [25,37,38]. Two pieces 2 cm 9 2 cm from the hybrid NiCo 2 O 4 :CNFs electrode were assembled using a spacer and 1 M KOH electrolyte solution. The standalone NiCo 2 O 4 :CNFs mats were used here as the supercapacitor electrode as well as the current collector in the same time, whereas the electric connections were made from a bare terminal without deposited NiCo 2 O 4 . All e1ectrochemical measurements were performed using potentiostat/galvanostat (PARSTAT 4000 ? workstation).
The specific capacitance (C s , Fg -1 ) was ca1cu1ated from the GCD measurement in accordance with Eq. (1).
where I is the discharge current (0.25-7.0 mA), DV is the voltage window in volts during the discharging process, Dt is the discharge time in seconds. The power density (P, W Kg -1 ) and energy density (E, Wh Kg -1 ) of the fabricated devices were calculated using Eq. (2) and (3): 3 Results and discussion

Structural characteristics of the NiCo 2 O 4 :CNFs hybrid nanocomposites
The hydrothermal approach proposed here is a onestep technique intended for the controlled formation of high-quality and highly-performance standalone hybrid NiCo 2 O 4 :CNFs electrodes for flexible supercapacitor devices. Figure 2 is a schematic representation that suggests the NiCo 2 O 4 nanostructures formation mechanism on the CNFs mat. Simply, the dissociation of the metal nitrates in water releases plenty of Ni 2? and Co 2? metallic cations. When the CNFs mats were soaked into this solution, Ni 2? and Co 2? were electrostatically attached to its surface and also within the CNFs network which is also driven by the action of the sonication power. During the hydrothermal process and as the temperature of the reacting bath is increasing, the urea granules were slowly decomposing and produce a large number of OHions: The OHions start to react with the metallic Ni 2? and Co 2? ions triggering the nucleation and the growth of mixed double hydroxides [(Ni,Co)(OH) 6 ] on the surface of the CNFs mats. Increasing the reaction temperature to 160°C led to increasing the concentration of the reactants which are two factors responsible for the increment of the deposited mixed double hydroxides on the surfaces of the CNFs mats: The hydrothermally grown of (Ni,Co)(OH) 6 onto CNFs substrates were converted to NiCo 2 O 4 :CNFs electrodes by the calcination process that was performed at 360°C in the presence of the oxygen environment. The calcination process also helped in releasing the bonded water molecules out of the hybrid electrodes: The XRD patterns collected from the prepared NiCo 2 O 4 :CNFs hybrid electrode are presented in Further confirmations on the phase purity of the NiCo 2 O 4 nanostructure and its attachment quality on the CNFs mat were conducted by Raman scattering and FTIR spectroscopic measurements as depicted in Fig. 3b, c, respectively. In the Raman spectrum of NiCo 2 O 4 2:1 CNFs nanocomposite, Fig. 3b, the peak appeared at 658 cm -1 wavenumber besides the D and G peaks of the CNFs was assigned to the A 1g vibrational active mode of the NiCo 2 O 4 spinel oxide [39]. Also, the noticeable increase in the intensity of the G-band with respect to the D-band after loading with NiCo 2 O 4 is observed. Such that, the (I D /I G ) ratio was decreased from 0.97 in the case of neat CNFs mat to be 0.78 in the case of the NiCo 2 O 4 2:1 CNFs nanocomposites. This is firmly attributed to the enhancements in the CNFs graphitization degree by the action of hydrothermal and the calcination treatments [40]. Along the same lines, both FTIR spectra of the neat CNFs mat and the NiCo 2 O 4 :CNFs nanocomposites in Fig. 3c showed the same FTIR bands as indicated except that, the spectrum for NiCo 2 O 4 2:1 CNFs composite revealed the presence of two strong bands at lower frequencies of 557 and 652 cm -1 . Those observed new peaks were assigned to the stretching vibrations of the Ni-O and Co-O bonds in the nickel-cobalt oxide, respectively [41]. These results confirm to a large extent the formation of well-anchored and pure spinel NiCo 2 O 4 nanostructures on the CNFs mat.

Morphological characteristics of the NiCo 2 O 4 :CNFs hybrid nanocomposites
The surface morphology of the neat CNFs mat and the hybrid NiCo 2 O 4 :CNFs electrodes were inspected by means of SEM and the captured micrographs are shown in Fig. 4. The low-magnification micrograph of the neat CNFs mat in Fig. 4a revealed a continuous carbon fibers network having an average diameter of * 300 nm, with a smooth surface and beads free. Whereas, the high-magnification micrograph in Fig. 4a1 shows that the CNFs are fused together and formed a conductive porous network. This unique structure of the CNFs mat is not only providing a large exposed area for the deposition of the pseudoactive oxides and electrolyte infiltration but also offers plenty of conductive channels through the electrode to facilitate the swift charge carriers kinetics. Furthermore, the rigid CNFs conductive network affords a robust mechanical template that satisfies the flexibility and conductivity required for a reliable standalone supercapacitor electrode.    Fig. 5 give a closer look at the nature of the NiCo 2 O 4 nanostructures coated on the CNFs mat. The TEM micrograph of the neat CNFs in Fig. 5a confirms the excellent morphology of the CNFs without any beads or cracks and also shows the perfect cross-linked sites. After the NiCo 2 O 4 hydrothermal deposition, the micrograph of the NiCo 2 O 4 1:2 CNFs electrode in Fig. 5a shows that the plain surface of the CNFs network was coated with the NiCo 2 O 4 nanoparticles. The TEM images of the NiCo 2 O 4 1:1CNFs and NiCo 2 O 4 2:1 CNFs samples with higher loadings given in Fig. 5c, d show spinal NiCo 2 O 4 nanoparticles connected together to form either hierarchical nanoflowers or nanorods, respectively. The enlarged inset views of both samples disclose the ultra-porous structure of the hierarchical NiCo 2 O 4 nanostructures those having an average pores diameter of 2-4 nm.
In brief, hybrid electrodes composed of the NiCo 2 O 4 nanostructures anchored on CNFs conductive for flexible SCs were facilely synthesized. Based on the 3D hierarchical structure of NiCo 2 O 4 :CNFs nanocomposites synthesized with high mass loading of the pure and crystalline NiCo 2 O 4 electrochemical catalyst, a remarkable electrochemical performance is anticipated to be achieved for many reasons. First, the CNFs mat represents an ideal 3D network composed of highly conductive nanofibers that served as an excellent open scaffold to receive as high as concentrations from NiCo 2 O 4 in the form of ultrathin nanoneedles. This is suggested to facilitate the electrons and ions transport along the surface and within the hybrid electrodes. Second, the uniform distribution of NiCo 2 O 4 nano-flower with interconnected nano-needles structure provides high exposed surface area rich with superabundant active sites ready for participating in enormous Faradic reactions. This is in turn suggested leading to high electrochemical capacitance and satisfying rate capability. Lastly, the in-situ growth of NiCo 2 O 4 nanostructure directly on the CNFs gave us the opportunity to avoid the usage of additional conductive additives or even the usage of any charge-transport-hindering polymeric binders. Thus, high mechanical stability and an improved electrical conductivity are additional expected advantages of our prepared hybrid electrodes than other similar hybrid nanocomposites reported previously in the literature [42,43].

Electrochemical performance of the NiCo 2 O 4 : CNFs hybrid nanocomposites
The electrochemical performances of the NiCo 2 O 4 :-CNFs hybrid electrodes for standalone flexible supercapacitors were examined using CV, GCD, and EIS measurements performed on two-symmetrical electrodes supercapacitor configuration. As schematically shown in Fig. 6a, each device was composed of two rectangular symmetric pieces of NiCo 2 O 4 :CNFs as device electrodes, 1 M KOH aqueous electrolyte, and polyethylene /polypropylene (PP/ PE) separator. In the assembled devices, the CNFs mat acts as the standalone substrate and the current collectors at the same time. Worthwhile to mention that, the NiCo 2 O 4 nanostructure was carefully removed from the edges before the assembly to ensure making good contact between the device and the potentiostat leads. After the assembly, the devices were let for about one hour before performing the measurements to permit the electrolyte to completely diffused and wet the electrodes. Figure 6b displays the CV voltammograms acquired for all fabricated SC devices measured at 50 mV s -1 . Noticeably, all the CV voltammograms of the NiCo 2 O 4 :CNFs devices yielded higher current densities and attained a larger enclosed area than the neat CNFs device. This observation affirms the effective pseudocapacitive role the NiCo 2 O 4 nanostructures played in enhancing the electrochemical capacitance of the devices. This was also noticed from the quasi-rectangular CV voltammograms exhibited by all the curves of the NiCo 2 O 4 :CNFs devices, unlike the rectangular voltammogram of the neat CNFsbased device. The transformation to quasi-rectangular was increased by increasing the NiCo 2 O 4 mass loading ratio on the CNFs electrode. Furtherly, the enclosed area of these voltammograms, which qualitatively corresponds to the specific capacitance (C s ) of the devices, increases with increasing the loading ratio of the NiCo 2 O 4 on the CNFs. These observations indicate the enhancements in the overall devices' capacitances with increasing the NiCo 2 O 4 loading on the CNFs electrode.
A comparison between the galvanic charge/discharge (GCD) profiles for the three NiCo 2 O 4 :CNFs devices with different loading ratios acquired at a current density of 1A g -1 is given in Fig. 6b. All the GCD profiles show slightly asymmetric shapes, indicating the quasi-reversible characteristic and nonlinear discharge profile. This behavior is mainly attributed to the integration of the Faradaic characteristic of the NiCo 2 O 4 nanostructures with the EDL characteristic of the CNFs electrode. Moreover, increasing the loading ratio of the NiCo 2 O 4 increased the asymmetry of the discharge profiles and also increase the time required to complete one discharge cycle. These results reflect the battery-like behavior of the NiCo 2 O 4 :CNFs devices fabricated from the hybrid electrodes especially those fabricated from the high NiCo 2 O 4 loadings. In addition to the previous results, the IR drop during the discharging was decreasing with increasing the NiCo 2 O 4 mass loading. Thus, a preferred interfacial charge transfer is inferred to exist at the NiCo 2 O 4 :CNFs/electrolyte interface.
For the evaluation of the high-capacitive capability of the proposed hybrid electrodes, the specific capacitances C s of the fabricated SC devices were calculated at different galvanic current densities and plotted in Fig. 6d. The C s values calculated at 1 A g -1 are and summarized in Table 2. The hybrid NiCo 2 O 4 2:1 CNFs electrode achieved a superiorly specific capacitance of 540 F g -1 at 1 A g -1 current density that is higher than 330 F g -1 of the NiCo 2 O 4 1:1 CNFs device, 250 F g -1 of the NiCo 2 O 4 1:2 CNFs device, and much higher than 17 F g -1 of bare CNFs device. Additionally, the flexible SC device built from the hybrid NiCo 2 O 4 2:1 CNFs electrode maintained its superiority at the overall applied GCD current densities and keep showing higher C s values than the other devices. These significant enhancements in the specific capacitance and the overall performance of the NiCo 2 O 4 2:1 CNFs based device is foremost attributed to the unique 3D open structure of NiCo 2 O 4 nanoflowers with needle-like petals. This offers a larger surface area for the Faradaic reaction to occur as well as for larger EDL ions adsorption/ desorption process at the electrode. However, the observed reduction in C s value with increasing the GCD current is usually explained in the literature to the decrease in ions adsorption and diffusion rate capability from the electrolyte to the electrode at high charging-discharging rates [44].
The electrochemical impedance spectroscopy (EIS) measurement is an effective tool employed here to get an insight assessment on the charge transfer resistances and the interfacial dynamics of the fabricated devices. Figure 6e displays the EIS Nyquist plots recorded for the NiCo 2 O 4 :CNFs hybrid devices with different loading ratios measured in the frequency range of 10 MHz to 100 kHz. All the fabricated SC devices showed typical Nyquist plots for electrochemical capacitor those consists of a highfrequency small semicircle followed by a straight line in the low-frequency region. The steeper the lowfrequency line, the more EDL capacitive nature of a porous electrode. Also, the intercept of the high-frequency small semicircle with the real impedance axis at the high-frequency gives the equivalent series resistance (ESR), whereas its diameter represents the charge transfer resistance (R CT ). The source of the R CT of the SC devices is originated from both the EDL effect and Faradic reactions at the hybrid electrode surface. The electrical equivalent circuit used to fit the EIS data can be found in Fig. S1 in the supporting information and the interpreted data are given in Table 2. [45][46][47].  As seen from Table 2, The NiCo 2 O 4 2:1 CNFs hybrid electrode showed the lowest ESR *17.4 X, whilst the higher ESR equals 26.4 X was obtained from the NiCo 2 O 4 1:2 CNFs electrode. This means the NiCo 2 O 4 2:1 CNFs possess high intrinsic electric conductivity than the other two electrodes. Furtherly, the very small semicircles were observed for all devices at high-frequency regions, suggesting a fast ion transport and favorable charge transfer across the NiCo 2 O 4 /electrolyte interface. However, the lowest R CT was obtained for the NiCo 2 O 4 2:1 CNFs device. Again, this is mainly attributed to the unique 3D open structure of the NiCo 2 O 4 that provides better electrolyte accessibility and abundant conductive channels. Finally, the steeper low-frequency line of the two devices with high mass loadings of the NiCo 2 O 4 indicates their higher electrochemical capacitances, in a good agreement with the results from the GCD measurements.
To further clarify the high rate capability and dynamic capacitive behavior of the NiCo 2 O 4 2:1 CNFs electrode for standalone SCs application among the other electrodes, Fig. 7a, b display the CV voltammograms and GCD profiles measured at different voltage scan rate (5-50 mV s -1 ) and current densities (1-7 A g -1 ), respectively. Rectangular CV voltammograms were almost recorded for the NiCo 2 O 4 2:1 CNFs SC device at the low voltage scan rates (5 and 10 mV s -1 ) as seen in Fig. 7a. This referring to the higher contribution of the EDL capacitance mechanism of the CNFs in the overall capacitive performance of the nanocomposite than the faradic capacitance at the low scan rate. Contrarily, ellipsoidal CV voltammograms were obtained by the device at high voltage scan rates and dominant faradic capacitance contribution can be inferred. This can be explained on the basis of the dynamics of electrolyte ions transportation and redox reactions. Under slow scan rate conditions, and due to the slow ions dynamics, the ions completely penetrate into the pores of the CNFs mat and the EDL capacitance is effectively contributes to the overall capacitance of the device. Whilst, under high voltage scan rate conditions, the electrolyte ions are not having enough time to completely infiltrate deeply the pores of the CNFs mat similar to what likely to happen in the case of the slow scan rates. Instead, the faradic electrochemical reactions fastly occurred at the surfaces of the CNFs those completely coated by the NiCo 2 O 4 nanostructures and nicely wetted with the KOH electrolyte and ellipsoidal CV were attained.
The excellent rate capability of the NiCo 2 O 4 2:1 CNFs SC device can be seen from the GCD profiles in Fig. 7b, such that the device was able to complete one charging-discharging cycle at a very high current density of 7 A g -1 . This was accompanied by maintaining small IR drops under all tested current densities. Besides, a high coulombic efficiency and high reversibility of the device can be easily noticed from the symmetry between the charging and discharging branches of each recorded GCD profile.
The energy density (E, Wh kg -1 ) and the power density (P, KW Kg -1 ) are the two essential factors that must be provided for clear judging the energy capacity and power deliverability of any storage device. The P and E of the SC devices assembled from the NiCo 2 O 4 :CNFs electrode with different loading ratios are calculated using Eqs. (3) and (4) and tabulated in Table 2 Table 3 shows a comparison between our results and some recently reported results for NiCo 2 O 4 based SCs prepared by different techniques. As mentioned earlier, the reported flexible SC devices fabricated from two symmetric NiCo 2 O 4 nanostructure electrodes are very rare. As seen from Table 3, all high C s values were reported with either three-electrode configuration and high electrolyte concentration. This lead to being that the P and E values were not reported as long as the real devices were not fabricated and measured. However, our NiCo 2 O 4 :-CNFs device possesses comparable Cs, P, and E values and sometimes higher to some real devices fabricated from NiCo 2 O 4 nanostructures such as NiCo 2 O 4 /NiMoO 4 core/shell [48,49] and Porous NiCo 2 O 4 Microspheres electrodes. This was besides the easiness, scalability, and controllability advantages of our two-step approach.
Finally, the charging and discharging cycling stability for the NiCo 2 O 4 2:1 CNFs SC device tested at a current density of 1 A g -1 is shown in Fig. 8, After 6000 testing cycles, the NiCo 2 O 4 2:1 CNFs SC retained * 93.1% of its initial specific capacitance. Such behavior is considered a very satisfactory performance for a prototype electrochemical storage device assembled with liquid electrolytes and subjected to this large number of charging and discharging cycles at such a slow rate. The assembly with gel electrolyte and using the proper casing are suggested to improve the lifetime and cyclability of the NiCo 2 O 4 :CNFs hybrid SC device but maybe at the expense of the device performance (Fig. 9).

Conclusions
In summary, we successfully prepared highly-performance NiCo 2 O 4 and CNFs hybrid electrodes for flexible and substrate-free supercapacitor applications. The facile hydrothermal technique was used to attach spinel NiCo 2 O 4 nanostructures on the free-standing CNFs electrode of different morphologies and loadings. The CNFs electrode was used as a 3D porous template for the NiCo 2 O 4 deposition and as a current collector for the devices. The best electrochemical performance for the devices is only achievable when the CNFs electrode was loaded with the utmost NiCo 2 O 4 nanostructure mass loading. The device fabricated by the hybrid electrode that contains twice NiCo 2 O 4 as much as CNFs (NiCo 2 O 4 2:1CNFs) achieved superior electrochemical performance in terms of low charge transfer resistances, higher rate capability, larger specific capacitance, and also longer discharging time. A specific capacitance of 540 F g -1 and an energy density of 30 Wh kg -1 at a power density of 515.6 W kg -1 besides the excellent cycling stability is recoded for the NiCo 2 O 4 2:1CNFs supercapacitor. This superior performance of the hybrid nanocomposite is attributed not only to the Power density (W Kg -1 ) Fig. 8 The energy and power densities of the fabricated twosymmetric electrodes SC devices based on the different metal oxides:CNFs hybrid electrodes synergistic integration of EDLC and pseudocapacitive materials together but also to the highly-exposed active surface area, the ultrathin and porous features, shorter ion pathways alongside the fast electron transport provided by the hierarchical NiCo 2 O 4 nanostructure tightly anchored on the conductive CNFs electrode.