In Situ Grown Mesoporous Structure of Fe-Dopant@NiCoOX@NF Nanoneedles as an Efficient Supercapacitor Electrode Material

In this study, we designed mixed metal oxides with doping compound nano-constructions as efficient electrode materials for supercapacitors (SCs). We successfully prepared the Fe-dopant with NiCoOx grown on nickel foam (Fe-dopant@NiCoOx@NF) through a simple hydrothermal route with annealing procedures. This method provides an easy route for the preparation of high activity SCs for energy storage. Obtained results revealed that the Fe dopant has successfully assisted NiCoOx lattices. The electrochemical properties were investigated in a three-electrode configuration. As a composite electrode for SC characteristics, the Fe-dopant@NiCoOx@NF exhibits notable electrochemical performances with very high specific capacitances of 1965 F g−1 at the current density of 0.5 A g−1, and even higher at 1296 F g−1 and 30 A g−1, respectively, which indicate eminent and greater potential for SCs. Moreover, the Fe-dopant@NiCoOx@NF nanoneedle composite obtains outstanding cycling performances of 95.9% retention over 4500 long cycles. The improved SC activities of Fe-dopant@NiCoOx@NF nanoneedles might be ascribed to the synergistic reactions of the ternary mixed metals, Fe-dopant, and the ordered nanosheets grown on NF. Thus, the Fe-dopant@NiCoOx@NF nanoneedle composite with unique properties could lead to promising SC performance.


Introduction
These days, there is a growing insistence on greener and more renewable energy sources, including hydro-energy and solar power, to replace fossil fuel applications, which have negative environmental effects [1,2]. Superior electrochemical storing devices that are safe and eco-friendly with energy durability are therefore needed [3][4][5]. Supercapacitors (SCs), as currently recommended energy-storing equipment, have been vitally improved for electric motors, portable electrical devices, and networks through virtue of great power densities, ultrafast charging/discharging, and prolonged life cycles [6,7]. It is well known that operational sample electrodes are important components of SCs. Thus, three representative anode electrode materials-denoted graphite electrodes [8][9][10], transitional capabilities even after 4500 long cycles that were derived from the synergetic contributions of Ni, Fe, and Co oxides.

Materials
All the reagents are analytically graded and used with no further purification. Typical nickel foam substrates were successfully pretreated with aqueous 3 M HCl solution, ethanol, and DI water before utilization.

Fabrication of the Fe-dopant@NiCoO x @NF Nanoneedles
Initially, 1 mmol Co(NO 3 ) 2 ·6H 2 O and 0.5 mmol Ni(NO 3 ) 2 ·6H 2 O were added to 150 mL deionized water (DI) with constant stirring for 45 min to give a light pink precursor. In addition, Fe-dopant reagent and 43 mg FeCl 3 ·6H 2 O were were added to the mixture. Then, 70 mg PVP and 350 mg urea were also added to the above precursors. Light pink colored solutions were formed through vigorous stirring. The described solution was put under ultrasonication for 25 min, moved to an autoclave, and optimized at 110 • C for 15 h. Afterwards, the procured black powders were cleansed with DI water and ethanol various times and dried at 50 • C overnight.

Fabrication of the NiCoO x @NF Nanowires
The NiCoO x @NF nanowires were prepared using an easy hydrothermal technology with the combination of an annealing process. Additionally, pure NiCoO x nanowires without Fe-dopant sources were also produced to be comparable with the Fe-dopant@NiCoO x @NF samples. Eventually, the accumulated precursors were optimized at 350 • C for 180 min with ramping speeds of 3 • C·min −1 to obtain NiCoO x @NF nanowires and Fe-dopant@NiCoO x @NF nanoneedles with abundant nanosheets. The mass loading of active material NiCoO x @NF nanowires and Fe-dopant@NiCoO x @NF on Ni foam substrate was calculated to be 4.6 mg cm −2 and 6.9 mg cm −2 by subtracting the mass of bare Ni foam substrate from the mass of active material loaded onto Ni foam.

Measurements and Characterizations
The as-synthesized product sample was analyzed by X-ray diffraction (XRD, Bruker D8 Advance, Bruker AXS LTD., Busan, Republic of Korea) with Cu K radiation (1.5406 Å), and the sample was scanned in the 2q range from 10 • to 90 • in steps of 0.02 • with a voltage of 40 kV and a current of 200 mA using a high resolution transmission electron microscope (TEM, JEM-2100F, JEOL LTD., Busan, Republic of Korea) and a field emission scanning electronic microscope (FE-SEM, JSM-7800F, JEOL LTD., Busan, Republic of Korea) instrumented with energy-dispersive spectra (EDS) and X-ray photoelectronic spectroscope analyzers with an energy analyzer (XPS; ESCCALAB 250Xi, Thermo Scientific LTD., Busan, Republic of Korea). Brunauer-Emmett-Teller (BET) was recorded at 77 K to examine the specific surface area and pore size utilizing a Micromeritics ASAP 2010 (Busan, Republic of Korea) absorption analyzer.

Electrochemical Measurements
A qualitative 3-electrode setup was created in 3 M KOH aqueous electrolytes, and all the analyses were maintained on electrochemical workstations (Bio-Logic, SP-150, Busan, Republic of Korea). The as-fabricated electrodes, Ag/AgCl, and platinum (Pt) foil electrodes were proposed as the working electrode, reference electrode, and counter electrodes, respectively. The CV measurements were conducted at different scan rates ranging from 5 to 100 mV s −1 within the potential window of 0-0.4 eV. The GCD measurement was carried out at different current densities ranging from 1 to 20 A g −1 . The EIS measurements were carried out using Nyquist plots in the frequency range of 0.001 Hze100 KHz. Therefore, GCD results from varying current densities were procured to quantify the spe-cific capacitances of electrodes. The specific capacitances were measured according to the following equation [37,38]: wherein I is the current density, t is the discharge time, ∆V is the window range for the GCD procedures, and m denotes the mass of the samples.

Results and Discussion
The synthesis process of the unique Fe-dopant@NiCoO x @NF nanoneedle composite is demonstrated in Figure 1. Initially, the vertical NiCoO x nanoparticles composed of nickel foam were successively organized and produced by the hydrothermal route. After that, the Fe-dopant was well decorated and dispersed on NiCoO x nanowires and on nickel foam and was successively prepared through facile hydrothermal followed by annealing.
ried out at different current densities ranging from 1 to 20 A g −1 . The EIS measurements were carried out using Nyquist plots in the frequency range of 0.001 Hze100 KHz. Therefore, GCD results from varying current densities were procured to quantify the specific capacitances of electrodes. The specific capacitances were measured according to the following equation [37,38]: C = (I × Δt)/(m × ΔV) (1) wherein I is the current density, t is the discharge time, △V is the window range for the GCD procedures, and m denotes the mass of the samples.

Results and Discussion
The synthesis process of the unique Fe-dopant@NiCoOx@NF nanoneedle composite is demonstrated in Figure 1. Initially, the vertical NiCoOx nanoparticles composed of nickel foam were successively organized and produced by the hydrothermal route. After that, the Fe-dopant was well decorated and dispersed on NiCoOx nanowires and on nickel foam and was successively prepared through facile hydrothermal followed by annealing. The micro-morphologies and the structural characteristics of the as-designed NiCoOx@NF and Fe-dopant@NiCoOx@NF product samples were analyzed using an FE-SEM microscope. As shown in Figure 2a, the NiCoOx nanowires were vertically developed and homogeneously formed on the nickel foam skeleton structure. Figure 2b,c demonstrated that the NiCoOx morphologies possess a nanowire formation with steady sizes between 3-5 μm formulated from abundant nanowires grown on the NF skeleton surface area. As demonstrated in Figure 2c, all nanowires consist of diameters of ranges between 100 and 200 nm with a span length of 1-2 μm. Impressively, all wire-shaped structures are attributed to the many minor internally connected crystals that possess uniform sizes (Figure 2c). Figure 2d-f corresponds to the Fe-dopant@NiCoOx@NF nanoneedle composite in FE-SEM images. The influence of Fe-dopant on NiCoOx morphology is clearly seen, and the diameters of the nanowires gradually decreased to be in the range of 40-60 nm, while the lengths were preserved due to the effect of the Fe-dopant on the NF foam substrate (Figure 2f). It is seen that the mesoporous property of NiCoOx was forcibly affected by the Fe-dopant. The Fe-dopant@NiCoOx@NF nanoneedle composite possesses rough interfaces that substituted for the smoother surfaces of the nanoneedles after annealing. The micro-morphologies and the structural characteristics of the as-designed NiCoO x @NF and Fe-dopant@NiCoO x @NF product samples were analyzed using an FE-SEM microscope. As shown in Figure 2a, the NiCoO x nanowires were vertically developed and homogeneously formed on the nickel foam skeleton structure. Figure 2b,c demonstrated that the NiCoO x morphologies possess a nanowire formation with steady sizes between 3-5 µm formulated from abundant nanowires grown on the NF skeleton surface area. As demonstrated in Figure 2c, all nanowires consist of diameters of ranges between 100 and 200 nm with a span length of 1-2 µm. Impressively, all wire-shaped structures are attributed to the many minor internally connected crystals that possess uniform sizes ( Figure 2c). Figure 2d-f corresponds to the Fe-dopant@NiCoO x @NF nanoneedle composite in FE-SEM images. The influence of Fe-dopant on NiCoO x morphology is clearly seen, and the diameters of the nanowires gradually decreased to be in the range of 40-60 nm, while the lengths were preserved due to the effect of the Fe-dopant on the NF foam substrate (Figure 2f). It is seen that the mesoporous property of NiCoO x was forcibly affected by the Fe-dopant. The Fe-dopant@NiCoO x @NF nanoneedle composite possesses rough interfaces that substituted for the smoother surfaces of the nanoneedles after annealing.
TEM was used to study the microstructure of the as-developed unique composites. It can be clearly identified in Figure 3a,b that the formation of individual particles is explicit and is similar to that found in FE-SEM, with regular particles 3-5 µm in size whose diameters were around 30 to 50 nm. Impressively, these nanoparticles can be attributed to many uniform crystals, resulting in their mesoporous behavior. Therefore, the TEM results are in agreement with the SEM outcomes. The hierarchical and novel NiCoO x @NF nanowire microstructures and their successive ornamentation using Fe-dopant nanosheets could boost the electrical kinetics and the active sites. The selected area electron diffraction (SAED) image (shown in the inset of Figure 3b) displays multiple diffraction rings with varied diameters that are indexed with the (220), (311), and (400) phases of the NiCo 2 O 4 spinel [25]. The TEM was used to study the microstructure of the as-developed unique composites. It can be clearly identified in Figure 3a,b that the formation of individual particles is explicit and is similar to that found in FE-SEM, with regular particles 3-5 μm in size whose diameters were around 30 to 50 nm. Impressively, these nanoparticles can be attributed to many uniform crystals, resulting in their mesoporous behavior. Therefore, the TEM results are in agreement with the SEM outcomes. The hierarchical and novel NiCoOx@NF nanowire microstructures and their successive ornamentation using Fe-dopant nanosheets could boost the electrical kinetics and the active sites. The selected area electron diffraction (SAED) image (shown in the inset of Figure 3b) displays multiple diffraction rings with varied diameters that are indexed with the (220), (311), and (400) phases of the NiCo2O4 spinel [25]. The SAED results reveal the polycrystalline characteristics of the sample. To further distinguish the elemental distributions found in the synthesized Fe-doped@NiCoOx@NF nanosheets, EDS maps were also created, as demonstrated in Figure 3c-g. EDS mappings of the sample showed that Fe, Ni, Co, and O elements uniformly covered the entire nanoneedle, indicating that Fe-dopant particles were successively decorated on NiCoOx. XRD was used to investigate the phases of NF, NiCoO x @NF, and Fe-dopant@NiCoO x @NF samples. As depicted in Figure 4a, the diffraction peaks allocated at 2θ values were well ascribed to the (220), (311), (222), (400), (422), (511), and (440) spaces of the spinel NiCo 2 O 4 angle (JCPDS No.00-020-0781) [39]. Moreover, the two representative angles at 44.53 • and 51.68 • could be attributed to (111) and (200) phases of NF crystals (JCPDS card no. 01-089-7128) [40,41]. The XRD patterns do not display any peaks of other phases, thus manifesting the unalloyed phases of NiCo 2 O 4 . These characteristics can affect the electrochemical performances of NiCo 2 O 4 composites via the formation of sufficient electroactive sites for the redox mechanism procedures [29].
The distribution of pores and surface areas of the designed NiCoO x -based electrode samples were extensively investigated by the BET. The N 2 absorption-desorption isotherms are shown in Figure 4b. All the fabricated electrodes displayed type-IV hysteresis with a loop at high pressure that indicates the mesoporous behaviors. The BET surface area of Fe-dopant@NiCoO x @NF was detected to be around 118.6 m 2 ·g −1 . Hence, the electrochemically active sites were distinctly improved because of the Fe-dopant. From the BJH image, as depicted in Figure 4c, the pore distribution sizes of the Fe-dopant@NiCoO x @NF composite sample displayed mesoporous distribution characteristics [42]. As illustrated in Figure 4c, the Fe-dopant@NiCoO x @NF sample consists of large meso/micro-pore volumes.
The comprehensive survey of XPS spectra of the Fe-dopant@NiCoO x @NF composite sample that were ascribed to Fe 2p, Ni 2p, Co 2p, O1s, and C1s are shown in Figure 5a. Various chemical states were extensively analyzed, and the outcomes are depicted in Figure 5b-f. The elevated spectra of Fe 2p signals can be manually deconvoluted into two existing species, Fe 2p 1/2 (at 726 eV) and Fe 2p 1/2 (at 710.5 eV), proving that Fe 3+ ions were successively doped in the spinel species of the NiCo 2 O 4 sample and outright oxidational states [28,43]. The Ni 2p spectra (Figure 5c) of elevated resolutions possess 2 sub-peaks at 855.5 and 873.4 eV, corresponding to Ni 2p 3/2 and Ni 2p 1/2 spin-orbit doublets, respectively. These spin-orbit deconvolution characteristics revealed the existence of 2 shake-up satellite peaks at 880.3 and 861.4 eV. All the spin-orbits and shake-up satellite signals were closed to Ni 3+ and Ni 2+ . Moreover, the Co 2p spectra deconvolution confirmed the existence of 2 spinal-orbit doublets, Co 2p 1/2 (at 795.4 eV) and Co 2p 3/2 (at 780.6 eV). These spinal-orbit doublets consist of 2 supplemented shake-up species at 803.3 and 786.9 eV, corresponding to Co 2+ and Co 3+ (shown Figure 5d), respectively. Additionally, the sub-peaks at 794.6 and 779.1 eV are associated with Co 2+ , and those at 796.1 and 780.4 eV were related to Co 3+ . The existence of Ni 2+ , Ni 3+ , Co 2+ , and Co 3+ peaks was also reported in recent studies [32]. In addition, the elevated fitting of O1s spectra (Figure 5e) revealed the occupancy of one strong peak. The main one at 529.6 eV was ascribed to the metal binding oxides [44] that consist of the oxygen defection (OH − groups) site in the Fe-dopant@NiCoO x @NF interface. The C1s elevated spectra (Figure 5f) were further examined to reveal the interactions among the Fe-dopant NiCo 2 O 4 and NF and to prove the reduction degrees of NF. In conclusion, all XPS spectra proved the existence of Fe 3+ , Ni 3+ , Ni 2+ , Co 2+ , Co 3+ , and O 2− , which agree well with the analyzed phases of the Fe-dopant@NiCoO x @NF nanoneedle composite [45]. XRD was used to investigate the phases of NF, NiCoOx@NF, and Fe-dopant@NiCoOx@NF samples. As depicted in Figure 4a, the diffraction peaks allocated at 2θ values were well ascribed to the (220), (311), (222), (400), (422), (511), and (440) spaces of the spinel NiCo2O4 angle (JCPDS No.00-020-0781) [39]. Moreover, the two representative angles at 44.53°and 51.68° could be attributed to (111) and (200) phases of NF crystals (JCPDS card no. 01-089-7128) [40,41]. The XRD patterns do not display any peaks of other phases, thus manifesting the unalloyed phases of NiCo2O4. These characteristics can affect the electrochemical performances of NiCo2O4 composites via the formation of sufficient electro-active sites for the redox mechanism procedures [29].
The distribution of pores and surface areas of the designed NiCoOx-based electrode samples were extensively investigated by the BET. The N2 absorption-desorption isotherms are shown in Figure 4b. All the fabricated electrodes displayed type-IV hysteresis with a loop at high pressure that indicates the mesoporous behaviors. The BET surface area of Fe-dopant@NiCoOx@NF was detected to be around 118.6 m 2 ·g −1 . Hence, the electrochemically active sites were distinctly improved because of the Fe-dopant. From the BJH image, as depicted in Figure 4c, the pore distribution sizes of the Fe-dopant@NiCoOx@NF composite sample displayed mesoporous distribution characteristics [42]. As illustrated in Figure 4c, the Fe-dopant@NiCoOx@NF sample consists of large meso/micro-pore volumes.  The comprehensive survey of XPS spectra of the Fe-dopant@NiCoOx@NF composite sample that were ascribed to Fe 2p, Ni 2p, Co 2p, O1s, and C1s are shown in Figure 5a. Various chemical states were extensively analyzed, and the outcomes are depicted in Figure 5b-f. The elevated spectra of Fe 2p signals can be manually deconvoluted into two existing species, Fe 2p1/2 (at 726 eV) and Fe 2p1/2 (at 710.5 eV), proving that Fe 3+ ions were successively doped in the spinel species of the NiCo2O4 sample and outright oxidational states [28,43]. The Ni 2p spectra (Figure 5c) of elevated resolutions possess 2 sub-peaks at Ni 3+ , Ni 2+ , Co 2+ , Co 3+ , and O 2− , which agree well with the analyzed phases of the Fe-dopant@NiCoOx@NF nanoneedle composite [45].

Electrochemical Properties of Electrode Materials
The CV and GCD tests of NF, NiCoO x @NF nanowires ( Figure S1), and Fe-dop-ant@NiCoO x @NF nanoneedle composite electrodes were conducted using the setup of 3 electrodes in 3 M KOH aqueous solution. Figure 6a comprises the CV tests (plotted at 10 mV s −1 ) of NF, NiCoO x @NF nanowires, and Fe-dopant@NiCoO x @NF nanoneedle composite electrodes. A redox couples (oxidation-reduction peaks) can be clearly observed, indicating the Faradic type behavior of the supercapacitor electrode. It can be observed that the integral regions of CV plots for NF, NiCoO x @NF nanowires, and Fe-dopant@NiCoO x @NF nanoneedle composite electrodes successively increase, revealing that Fe-dopant@NiCoO x @NF nanoneedle composite electrodes have the greatest area specific discharges. To quantify the specific capacitance data, GCD analysis was successively performed from a 0 to 0.5 V potential window relative to Ag/AgCl. The GCD curves of NF, NiCoO x @NF nanowires, and Fe-dopant@NiCoO x @NF nanoneedle composite electrodes at 0.5 A g −1 are depicted in Figure 6b. Interstingly, the specific discharge capacitance of Fe-dopant@NiCoO x @NF nanoneedle composite electrodes (at 0.5 A g −1 ) is nearly twice that of the NiCoO x @NF nanowire electrodes and about triple that of the NF sample, indicating the superior storage charge ability and excellent energy storage performance of the as-grown composite. Thus, electrolyte ions would powerfully and suitably penetrate the inside surfaces of the active sample to prodcue the Faradic reaction.

Electrochemical Properties of Electrode Materials
The CV and GCD tests of NF, NiCoOx@NF nanowires ( Figure S1), and Fe-dopant@NiCoOx@NF nanoneedle composite electrodes were conducted using the setup of 3 electrodes in 3 M KOH aqueous solution. Figure 6a comprises the CV tests (plotted at 10 mV s −1 ) of NF, NiCoOx@NF nanowires, and Fe-dopant@NiCoOx@NF nanoneedle composite electrodes. A redox couples (oxidation-reduction peaks) can be clearly observed, indicating the Faradic type behavior of the supercapacitor electrode. It can be observed that the integral regions of CV plots for NF, NiCoOx@NF nanowires, and Fe-dopant@NiCoOx@NF nanoneedle composite electrodes successively increase, revealing that Fe-dopant@NiCoOx@NF nanoneedle composite electrodes have the greatest area specific discharges. To quantify the specific capacitance data, GCD analysis was successively performed from a 0 to 0.5 V potential window relative to Ag/AgCl. The GCD curves of NF, NiCoOx@NF nanowires, and Fe-dopant@NiCoOx@NF nanoneedle composite electrodes at 0.5 A g −1 are depicted in Figure 6b. Interstingly, the specific discharge capacitance of Fe-dopant@NiCoOx@NF nanoneedle composite electrodes (at 0.5 A g −1 ) is nearly twice that of the NiCoOx@NF nanowire electrodes and about triple that of the NF sample, indicating the superior storage charge ability and excellent energy storage performance of the as-grown composite. Thus, electrolyte ions would powerfully and suitably penetrate the inside surfaces of the active sample to prodcue the Faradic reaction. Figure 6. Electrochemical characteristic of the as-synthesized electrodes in a three-electrode system: (a) CV curves for NF, NiCoOx@NF nanowires, Fe-dopant@NiCoOx@NF nanoneedle composite electrodes at 10 mV·s −1 ; (b) GCD plots for NF, NiCoOx@NF nanowires, and Fe-dopant@NiCoOx@NF nanoneedles at 0.5 A g −1 ; (c) full CV tests for Fe-dopant@NiCoOx@NF nanoneedle composite electrodes at several applied scan rates; and (d) full GCD tests for Fe-dopant@NiCoOx@NF nanoneedle composite electrodes at several applied currents. Figure 6. Electrochemical characteristic of the as-synthesized electrodes in a three-electrode system: (a) CV curves for NF, NiCoO x @NF nanowires, Fe-dopant@NiCoO x @NF nanoneedle composite electrodes at 10 mV·s −1 ; (b) GCD plots for NF, NiCoO x @NF nanowires, and Fe-dopant@NiCoO x @NF nanoneedles at 0.5 A g −1 ; (c) full CV tests for Fe-dopant@NiCoO x @NF nanoneedle composite electrodes at several applied scan rates; and (d) full GCD tests for Fe-dopant@NiCoO x @NF nanoneedle composite electrodes at several applied currents.
The existence of an obvious pair of Faradic lines indicates the redox reactions and the lines are clearly shown in Figure 6c. As demonstrated in Figure 6c, the symmetric triangular formation of GCD curves exists at the greatest applicable current value of 30 A g −1 , reflecting its notable Faradic reactions under the GCD charge mechanisms. As can be seen in Figure 6d, the storage mechanisms could be classified into two paths: the first one consists of the Faradic reaction contributions from the GCD effects, and the second path possesses the capacitance contribution reactions from the EDLC characteristic. The assistance of the Fe-dopant has been further studied by quantifying the GCD plots with numerous Fe amounts. The electrochemical storage mechanisms and the kinetic reactions were investigated by CV tests. Two pairs of redox species were obtained under the CV curves in the applied potential range of 0.0-0.5 V, which were mainly interconnected with the reversible Fe 3+ , Co 3+ /Co 2+ , and Ni 3+ /Ni 2+ redox reactions [46][47][48]. Consequently, the electrochemical storage mechanisms of Fe-dopant@NiCoO x @NF nanoneedle composite samples can be explained via the equations [49][50][51]: The plots of GCD-specific discharge capacitances versus the specific current of the Fe-dopant@NiCoO x @NF and NiCoO x @NF samples are shown in Figure 7a. Apparently, the Fe-dopant@NiCoO x @NF nanoneedle composite electrodes achieve the largest discharge times at all current values compared with the binary NiCoO x @NF nanowire electrodes. The Fe-dopant@NiCoO x @NF nanoneedle composite exhibited a high specific capacitance of 1965 F g −1 at 0.5 A g −1 , which is much larger than the values found in recent reports (shown in Table 1). When current values were increased from 0.5 to 30 A g −1 , the capacitance of the Fe-dopant@NiCoO x @NF nanoneedle composite decreased from 1965 to 1289 F g −1 , whereas the NiCoO x @NF nanowire electrodes achieved 1391 F g −1 at 0.5 A g −1 . Particularly, the Fe-dopant@NiCoO x @NF nanoneedle composite achieved high rate capabilities (89.4%) when compared with pristine NiCoO x @NF nanowires (78.7%), indicating the effect of iron dopant on the electrical conductivity characteristics and the good construction of the Fe-dopant with NiCoO x nanowires on NF. EIS was further conducted to estimate the conductivity characteristics. All EIS plots displayed two principal categories. As seen in Figure 7c,d, the semi-circle diameters represent the charge transfer resistances (Rct), while the internal resistance, Rs, is obtained from the intersection of Nyquist curves with the Z′-axis. Moreover, the angles in the lower frequency region determine the kinetic mass transfer procedures. The Rct of NiCoOx@NF nanowires and Fe-dopant@NiCoOx@NF nanoneedle composite electrodes were approximated to be 0.57 and 0.36 Ω, respectively, revealing that Fe-dopant@NiCoOx@NF nanoneedle composites possess rapid kinetic charge transfers and low resistance internals. Such characteristics favor the paths for ions affected by the Figure 7. Electrochemical behavior of the as-developed electrodes in a three-electrode configuration: (a) Specific discharge capacity for as-fabricated NF, NiCoO x @NF nanowires, and Fe-dopant@NiCoO x @NF nanoneedle composite electrodes measured at diverse specific current, (b) cyclic stability properties, (c) EIS plots of the prepared electrodes (inset: fitted equivalent circuit), and (d) EIS plots of the enlarged high-frequency region. Cycling stability is an additional crucial constituent for evaluating the electrochemical capabilities of our samples. Cycling stability was quantified under GCD tests at 4 A g −1 over 4500 long cycles (Figure 7b). Figure 7b depicts the cycling plots of NiCoO x @NF nanowires and Fe-dopant@NiCoO x @NF nanoneedle composite electrodes, respectively. It is detected that the Fe-dopant@NiCoO x @NF composite could be stable at 95.9% of the starting capacitance over 4500 cycles. On other hand, the capacitance of the NiCoO x @NF nanowire samples slightly diminishes to 82.6% of the prior data. Thus, hydrothermal preparations of Fe-dopant anchored on more stable NiCoO x nanowires successively enhance the stability properties of composites, indicating excellent electrochemical stability behaviors.
EIS was further conducted to estimate the conductivity characteristics. All EIS plots displayed two principal categories. As seen in Figure 7c,d, the semi-circle diameters represent the charge transfer resistances (R ct ), while the internal resistance, R s , is obtained from the intersection of Nyquist curves with the Z -axis. Moreover, the angles in the lower frequency region determine the kinetic mass transfer procedures. The R ct of NiCoO x @NF nanowires and Fe-dopant@NiCoO x @NF nanoneedle composite electrodes were approximated to be 0.57 and 0.36 Ω, respectively, revealing that Fe-dopant@NiCoO x @NF nanoneedle composites possess rapid kinetic charge transfers and low resistance internals. Such characteristics favor the paths for ions affected by the Fe-dopant particles to the internal zone of Fe-dopant@NiCoO x @NF nanoneedle composites [37,38]. Impressively, the Fe-dopant@NiCoO x @NF nanoneedle composites sample delivered a smaller circled line with possible vertical slope lines, indicating the ultra-fast rate of ion transport diffusion and adequate connections between electrolytes and electrode samples.

Conclusions
In summary, we have designed an earth-abundant, cost-effective transition metal pair composed of Fe-dopant and NiCoO x nanowires for SC energy storage performance. The Fedopant with NiCo 2 O 4 nanowires was effectively anchored and occupied on the NF skeleton, and the impact of synergistic reactions of greater redox activities was observed. Thus, the prepared Fe-dopant@NiCoO x @NF nanoneedle composite nanostructures resulted in the inclusion of energy storage active sites and enhanced charge transportation, and it lessened the transportation pathways for ion diffusion in the electrolyte. The outcomes revealed that Fe-dopant@NiCoO x @NF nanoneedle composites demonstrate notable electrochemical reactions as unique electrode materials for SCs. Consequently, the Fe-dopant@NiCoO x @NF nanoneedle composites exhibited a superior specific capacitance of 1965 F g −1 at 0.5 A g −1 .
In addition, the leading rate capability and the outstanding cycling stability were distinctly reflected under the capacitance retentions of 95.9% over 4500 cycles. Thus, preparing a unique nanostructure concept with transition metals will lead to progress in creating efficient SCs.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano13020292/s1, Figure S1. Electrochemical characteristic of the as-synthesized electrodes at a three-electrode system: (a) Full CV tests for NiCoO x @NF nanowires electrode at everal applied scan rates, and (b) Full GCD tests for NiCoO x @NF nanowires electrode at several applied currents.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.