Embedding Metal–Organic Frameworks for the Design of Flexible Hybrid Supercapacitors by Electrospinning: Synthesis of Highly Graphitized Carbon Nanofibers Containing Metal Oxide Nanoparticles

Electrospun carbonaceous fibers have emerged as promising electrode materials for application in energy storage devices. However, their relatively poor electrical conductivity (due to their amorphous carbon structures) and low capacitive performance lead to poor prospects for their further application. Herein, a universal synthesis of highly graphitized carbon nanofibers, containing various metal oxide nanoparticles (e.g., Fe2O3, NiO), by the pyrolysis of metal–organic framework (MOF)‐embedded electrospun nanofibers, is reported. The resulting carbon nanofibers exhibit large mesopore volumes, contain large quantities of Faradic metal oxide nanoparticles, and are highly graphitized. The fibers also have excellent mechanical flexibility, provide fast ion transfer characteristics, and a large pseudocapacitance combined with excellent electrical conductivity, leading to large specific capacitances. Consequently, asymmetric flexible hybrid supercapacitors assembled from Fe2O3‐embedded highly graphitized carbon nanofibers (FOCNF) and NiO‐embedded highly graphitized carbon nanofibers (NOCNF) exhibit a high energy density of 43.1 Wh kg−1 at a power density of 412.5 W kg−1 and possess excellent flexibility (capacitance retention of 94.4% at 180° bending and 96.2% at 30° twisting) with superior cycling stability. This strategy provides a new MOF‐based approach for the design and synthesis of multifunctional flexible carbonaceous materials and might lead to their further application in flexible energy storage devices.

Electrospun carbonaceous fibers have emerged as promising electrode materials for application in energy storage devices. However, their relatively poor electrical conductivity (due to their amorphous carbon structures) and low capacitive performance lead to poor prospects for their further application. Herein, a universal synthesis of highly graphitized carbon nanofibers, containing various metal oxide nanoparticles (e.g., Fe 2 O 3 , NiO), by the pyrolysis of metal-organic framework (MOF)-embedded electrospun nanofibers, is reported. The resulting carbon nanofibers exhibit large mesopore volumes, contain large quantities of Faradic metal oxide nanoparticles, and are highly graphitized. The fibers also have excellent mechanical flexibility, provide fast ion transfer characteristics, and a large pseudocapacitance combined with excellent electrical conductivity, leading to large specific capacitances. Consequently, asymmetric flexible hybrid supercapacitors assembled from Fe 2 O 3 -embedded highly graphitized carbon nanofibers (FOCNF) and NiO-embedded highly graphitized carbon nanofibers (NOCNF) exhibit a high energy density of 43.1 Wh kg À1 at a power density of 412.5 W kg À1 and possess excellent flexibility (capacitance retention of 94.4% at 180°bending and 96.2% at 30°twisting) with superior cycling stability. This strategy provides a new MOF-based approach for the design and synthesis of multifunctional flexible carbonaceous materials and might lead to their further application in flexible energy storage devices.
the graphitization of carbon nanofibers during the electrospinning synthesis process is effective for the improvement of their electrical conductivity. Frequently used strategies are often based on the ultrahigh-temperature (typically > 2000°C) thermal treatment of ECNFs under reduced pressure, [15] although this process requires complicated and rigorous procedures/operations. Catalytic graphitization of ECNFs involving the introduction of transition metal species can lead to graphitized carbon nanofibers at lower temperatures (<1000°C). This process can also be used to generate Faradic/battery-type nanoparticles where there is a significant electrochemical contribution to the total capacitance. [16,17] However, the diameters and distribution of the engineered particles cannot be controlled during this process, and the porosity of the resulting nanofibers is also decreased.
Metal-organic frameworks (MOFs) consisting of organic linkers and metal ions provide an emerging platform for the controllable production of materials with Faradic nanoparticles contained at the interiors of graphitic carbon frameworks. [18] This process can be recognized as a space-confined effect in MOF nanocages involving pyrolysis. Also, the feasibility of MOF electrospinning has already been demonstrated as a method for the synthesis of functional ECNFs. [19] Following on from these achievements, here we propose the MOF electrospinning nanoarchitectonic concept for the preparation of highly graphitized carbon nanofibers containing different Faradic metal oxide nanoparticles. In detail, MIL-88(Fe) nanorods or MOF-74 (Ni) nanospheres have been embedded in nanofibers by electrospinning polyacrylonitrile (PAN) precursor solutions containing dispersed nanoparticles of MIL-88(Fe) or MOF-74(Ni), respectively. After carbonization and oxidization, highly graphitized carbon nanofibers embedded with Fe 2 O 3 and NiO, respectively, denoted as FOCNF and NOCNF, were obtained. It is worth mentioning that both Fe and Ni can catalyze the graphitization of ECNFs during the thermal treatment process, thus improving the degree of graphitization of the resulting ECNFs. In this unique structure, Fe 2 O 3 and NiO serve, respectively, as battery-type active centers in the anode and cathode of the flexible hybrid supercapacitors (FHSCs). Furthermore, in contrast to the metal oxide nanoparticle/graphite ECNFs reported previously, [20] our strategy, the MOF-derived graphitic carbon microstructure connecting ECNFs to the oxide nanoparticles, not only promotes fast electron transfer for Faradic reactions, [21] but also protects and maintains the oxide nanoparticles in an appropriate state. Finally as a proof of concept, an FHSC assembled from FOCNF and NOCNF nanomaterials exhibits excellent electrochemical performance with high energy density and superior cycling stability.

MIL-88(Fe) Nanorods
FeCl 3 ·6H 2 O (1.35 g) and fumaric acid (0.58 g) were added to deionized water (50 mL) with vigorous stirring. The solution was then transferred to a Teflon autoclave and heated at 100°C for 4 h. The reactor was allowed to cool to room temperature, the resulting precipitate was collected by centrifugation, and washed with deionized water. After repeating several times, the brick red product was dried at 60°C for 24 h yielding MIL-88(Fe).

MOF-74(Ni) Nanospheres
Tribenzoin acid (0.3 g) and PVP K30 (2 g) were added to a solution of Ni(NO 3 ) 2 ·6H 2 O (0.58 g) in DMF (60 mL). The resulting mixture was transferred to a Teflon autoclave and heated at 150°C for 6 h. The reactor was allowed to cool to room temperature and the product was collected by centrifugation and washed several times with DMF. The resulting product was dried at 60°C for 24 h to obtain MOF-74(Ni).

Preparation of FOCNF and NOCNF
FOCNF was prepared using an electrospinning method. Typically, an aliquot of MIL-88(Fe) (0.5, 1, or 1.5 g) was added to DMF (10 g). After stirring for 12 h, PAN (1 g) was added into each of the three dispersed solutions followed by stirring at 80°C until complete dissolution to obtain three precursor solutions. Each of the precursor solutions was subjected to electrospinning at a voltage of 17 kV and at velocity 1 mL h À1 . The distance between roller and stainless steel needle was maintained at 15 cm and humidity was maintained at about 30%. The resulting nanofibers were collected and stabilized at 280°C for 2 h with a ramping rate of 5°C min À1 . Subsequently, the samples were carbonized at 800°C for 2 h with a ramping rate of 5°C min À1 . After carbonization, the products were oxidized at 300°C for 12 h with a ramping rate of 2°C min À1 . The samples obtained using 0.5, 1, and 1.5 g of MIL-88(Fe) are referred to as FOCNF-0.5, FOCNF-1, and FOCNF-1.5, respectively.
The preparation procedure for NOCNF was similar to that for FOCNF except that MIL-88(Fe) was replaced with MOF-74 (Ni). The samples obtained using 0.5, 1, and 1.5 g of MOF-74 (Ni) are referred to as NOCNF-0.5, NOCNF-1, and NOCNF-1.5, respectively. For comparison, ECNF was also prepared according to a similar procedure without adding MIL-88(Fe)/MOF-74 (Ni) and oxidizing and is referred to as PANC.

Characterizations
Field-emission scanning electron microscopy (FESEM, Hitachi S4800) with energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM, Hitachi H-8100) were used to investigate the morphology, elemental composition, and structure of the samples. A Raman spectrometer (Horiba, HR-800) with a laser of 532 nm in wavelength was used to obtain Raman spectra. X-Ray powder diffraction (XRD, Rigaku SmartLab) patterns were recorded to analyze the crystal structure of the samples. The specific surface area (SSA) and pore size distribution measurements were carried out using a physical adsorbent (Micromeritics ASAP 2020) involving N 2 adsorption-desorption based on Brunauer-Emmett-Teller (BET) and density functional theory (DFT) methods, respectively. X-Ray photoelectron spectroscopy (XPS) analyses were performed using a Thermo Fisher ESCALAB 250Xi equipment with an Al Kα excitation source to determine the elemental composition of the samples. For electrochemical testing, a traditional three-electrode system was used with Pt as counter electrode and Ag/AgCl electrode as reference electrode. The third working electrode was made from the sample materials cut to suitable dimensions (1 Â 2 cm 2 , about 2 mg) without any binder or conductive agent assembled using a Pt electrode clamp. KOH solution (2 M) was used as the electrolyte. Electrochemical impedance spectroscopy (EIS) was used in the frequency range 0.01 Hz-100 kHz with a 5 mV AC bias voltage. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were performed at different scan rates and current densities, respectively. Specific capacitance (C s , F g À1 ) was calculated from CV data according to the following equation is voltage window, and m (g) is mass of the active material. The specific capacitance (C c , F g À1 ) could also be calculated from GCD data based on the following equation where Δt (s) is discharge time. FHSC was assembled using a section of FOCNF-1 as the anode and a section of NOCNF-1 as the cathode. Carbon cloth was used as the current collector by contacting with the active materials. A section of nonwoven fabric, which was soaked in PVA/KOH gel electrolyte, was placed between the FOCNF-1 and NOCNF-1 components to prevent short circuiting. The gel electrolyte was prepared by dissolving KOH (3 g) and PVA (3 g) in deionized water (30 mL). The detailed structure of the FHSC is shown in Figure 1c. The mass of anode (m À ) and cathode (m þ ) was determined according to the following equation where ΔV À and ΔV þ are the voltage windows of anode and cathode and C À and C þ are the specific capacitances of anode and cathode, respectively. The energy density (E, Wh kg À1 ) and power density (P, W kg À1 ) of the FHSC device were calculated according to the following equations www.advancedsciencenews.com www.small-structures.com Figure 1a,b shows the preparation procedure of FOCNF and NOCNF, respectively. Following electrospinning, nanofibers embedded with MIL-88(Fe) nanorods ( Figure S1, Supporting Information) and MOF-74(Ni) nanospheres ( Figure S2, Supporting Information) were obtained. The composite structures can be clearly observed from the FESEM images of FOCNF-1 precursor and NOCNF-1 precursor shown in Figure S3 and S4, Supporting Information. Both MIL-88(Fe) and MOF-74(Ni) are coated with PAN, have porous surfaces, and are bunched as PAN nanofibers. After carbonization at high temperature, MIL-88(Fe) is transformed to Fe 3 C and porous carbon. It should be noted that Fe can also catalyze the graphitization of ECNFs at high temperature, leading to improved conductivity. Ultimately, Fe 3 C is oxidized to Fe 2 O 3 . Ni in NOCNF ( Figure 1b) can similarly also catalyze the graphitization of ECNFs. After carbonization and oxidization, NiO is formed. Figure 1c shows the structure of FHSC based on FOCNF-1 anode and NOCNF-1 cathode, having a flexible sandwich-like structure.

Results and Discussion
FESEM was used to observe the morphologies of the samples. As shown from the FESEM images of PANC ( Figure    contents exhibit a nanosphere-embedded carbon nanofiber skeleton structure. These nanospheres with a diameter of about 1 μm are MOF-74(Ni)-derived composites of Ni, NiO, and carbon with high degree of nanographitization. Similar to FOCNF, NiO and carbon nanospheres synergistically contribute high specific capacitance and excellent stability for supercapacitors. It should be noted that residual Ni is also beneficial for improving the conductivity of ECNFs. Figure S8, Supporting Information displays the photographic images of FOCNF-1 ( Figure S8a, Supporting Information) and NOCNF-1 ( Figure S8b, Supporting Information), illustrating their excellent flexibility; both can be wrapped around a thin glass rod. From the low-resolution TEM image of FOCNF-1 shown in Figure 2e, MIL-88(Fe)-derived carbon nanorods and carbon nanofibers with a porous structure can be clearly observed. The materials contain Fe 2 O 3 nanoparticles of different dimensions uniformly embedded in the carbon nanorods. As observed from the high-resolution TEM image in Figure 2f, the lattice fringe with identifiable lattice spacing of 0.25 nm can be ascribed to the (110) plane of α-Fe 2 O 3 or the (311) plane of γ-Fe 2 O 3 , [22,23] while that having a spacing of 0.34 nm corresponds to the (002) plane of nanographite. [24] Figure 2g,h shows the low-and highresolution TEM images of NOCNF-1, respectively. As shown in Figure 2g, MOF-74(Ni)-derived carbon nanospheres are combined with ECNFs, and both carbon nanospheres and ECNFs exhibit porous structures. NiO nanoparticles are distributed largely in the carbon nanospheres. The high-resolution TEM image (Figure 2h) displays obvious lattice fringes with interplanar spacings of 0.242 nm and 0.209 nm, corresponding to the (111) and (200) planes of NiO, [25] respectively, while an interplanar distance of 0.34 nm corresponds to the (002) plane of nanographite. In addition, the porous amorphous carbon is located in regions distinct from Fe-or Ni-containing regions ( Figure S9, Supporting Information), which is favorable for ion transfer. The elemental mapping of FOCNF-1 is shown in Figure 2i. Clearly, C, N, and O are distributed uniformly at the surface of FOCNF-1. However, in the case of Fe, the presence of high-contrast dots indicates that Fe 2 O 3 nanoparticles are distributed within the carbon nanorods. Figure 2j shows elemental mapping of NOCNF-1. In this case, C, O, and N are uniformly distributed at the surfaces of NOCNF-1, while Ni is distributed mostly at the surfaces of nanospheres, due to the fact that Ni exists largely as MOF-74 (Ni). Notably, both Fe 2 O 3 and NiO are coated with graphitic carbon. Compared with PANC, the EDS spectra shown in Figure S10, Supporting Information further prove the presence of Fe and Ni in FOCNF and NOCNF, respectively. Moreover, at increased MOF contents, the atomic ratios of N and O decrease dramatically (Table S1, Supporting Information), suggesting fewer defects in the phase structure of carbon. These results further support the enhanced catalytic graphitization based on the higher Fe and Ni elemental contents. These unique structures not only facilitate electron transfer but also maintain the oxides in an active state during long-term cycling. Figure 3a shows the N 2 adsorption-desorption isotherms of PANC, FOCNF, and NOCNF. The isotherm for PANC has a typical type I form indicating that porosity is due largely to micropores. [26] Such structures do not facilitate ion migration and diffusion, resulting in poor rate capabilities. For FOCNF and NOCNF, typical hysteresis loops in the range of 0.5-1 (P/P 0 ) can be observed. These data represent type IV isotherms, indicating that there are considerable quantities of mesopores present in the carbon nanofibers. [27] In the low relative pressure range (0-0.01 P/P 0 ), the adsorption curves undergo a sharp increase due to the presence also of micropores. [28] Therefore, both FOCNF and NOCNF have hierarchically micro-/ mesoporous structures, as also demonstrated by their pore size distribution curves shown in Figure 3b. Clearly, compared with PANC, both FOCNF and NOCNF contain components of their pore size distributions in the mesoporous range (2-50 nm). The SSAs and pore volumes of the samples were calculated and the results are listed in Table 1. It can be seen that with the increase in MIL-88(Fe) and MOF-74(Ni) contents in the precursors, the SSAs of FOCNF and NOCNF gradually decrease. However, FOCNF-1 and NOCNF-1 display the largest mesopores. Such a feature favors ion migration and diffusion in the Faradic reaction and electric double-layer (EDL) adsorption-desorption processes.
XRD patterns of the samples are shown in Figure 4. PANC ( Figure 4a) displays a broad peak at around 24.8°, corresponding to the (002) lattice plane of an amorphous carbon structure. [29] For FOCNF, a sharp diffraction peak at around 26.1°can be   [30] Relative to the diffraction patterns of FOCNF-1 and FOCNF-1.5, that of FOCNF-0.5 has stronger Fe 3 C diffraction peaks, which is probably due to the higher carbon content of FOCNF-0.5, which prevents the oxidation of Fe 3 C to iron oxides. In Figure 4b, similar to FOCNF, an obvious peak at 26.1°can be observed, which is again ascribed to the graphitic (002) plane. In addition, several peaks at 37.3°, 43.2°, and 62.9°are observed, corresponding to the (111), (200), and (220) planes of NiO (JCPDS no. 47-1049), respectively. It should be noted that the characteristic peaks of Ni (JCPDS no. 04-0850) at around 44.5°(111) and 51.8°(200) exist due to residual elemental Ni resulting from heating in a reducing environment with protection from oxidation to NiO by encapsulation in carbon. [31] The presence of Ni might improve conductivity and cycling stability of the resulting electrode materials. [32] Obviously, based on these XRD results, transition metal oxides have been successfully incorporated into nanographite-embedded ECNFs, which will be beneficial to maximize their respective advantages in reaching high specific capacitances.
The structures of the samples were further characterized by Raman spectroscopy. As shown in Figure 5, all samples show two peaks at around 1348 and 1577 cm À1 , corresponding to D band and G band of carbon, respectively. Generally, D band refers to disordered carbon while the G band represents graphitic carbon or sp 2 -bonded carbon. [33] The intensity ratio of D band to G band (I D /I G ) can be used to estimate the degree of graphitization of carbon materials. The I D /I G values of the samples are listed in Table 1. Evidently, both FOCNF and NOCNF exhibit lower I D /I G values than PANC, indicating higher degrees of graphitization for FOCNF and NOCNF over PANC. With increasing quantity of MIL-88(Fe) and MOF-74(Ni) in the    precursors, the value of I D /I G decreases, indicating that the degree of graphitization is improved due to catalytic enhancement of graphitization caused by the increased amounts of Fe or Ni in the precursors. [34,35] For FOCNF-1, FOCNF-1.5, NOCNF-1, and NOCNF-1.5, weak 2D peaks at around 2685 cm À1 are present, indicating the existence of graphitic carbon within the carbon nanofibers, [36] which is also consistent with the XRD results. For FOCNF, the characteristic Raman peaks at around 217 cm À1 can be assigned to the A 1g modes with peaks at 281 and 383 cm À1 originating from the E g modes of Fe 2 O 3 . [37] The Raman spectrum of NOCNF contains obvious peaks at around 355, 515, 673, 837, and 1063 cm À1 . The Raman peaks at 355 and 515 cm À1 can be ascribed to the NiO vibrational first-order phonon (1 P) modes including transverse optical (TO) and longitudinal optical (LO) modes, while the peaks at 673, 837, and 1063 cm À1 correspond to the secondorder phonon (2 P) modes 2P TO , 2P TOþLO , and 2P LO . [38,39] XPS analyses were also performed to acquire the elemental valence and chemical composition at the surfaces of the samples. Figure 6a shows the XPS full-survey spectra of PANC, FOCNF, and NOCNF. It can be seen from Figure 6a that all samples   Figure 6b-d and S11, Supporting Information show high-resolution N 1s spectra of the samples, all of which exhibit four nitrogen types at the binding energies of 398.5, 399.6, 400.3, and 401.3 eV, corresponding respectively to N-6 (pyridinic-like nitrogen), N-5 (pyrrolic-like nitrogen), N-Q (graphitic-like nitrogen), and N-X (oxidate-like nitrogen). [40] It should be noted that N-6 and N-5 can contribute to pseudocapacitance through Faradic reactions. In Figure S12, Supporting Information, the C 1s spectra of the samples contain four types of carbon atoms at the binding energies of 284.7, 285.9, 286.5, and 289 eV. The peak at 284.7 eV is owing to C─C/C&dbond;C, peaks at 285.9 and 286.5 eV are related to C─N and C─O, and the peak at 289 eV is ascribed to O─C&dbond;O bonding. [41] The O 1s spectrum of PANC ( Figure S13a, Supporting Information) shows three types of oxygen atoms at 531.6, 532.5, and 533.7 eV, corresponding to C&dbond;O, C─O, and C─OOH, respectively. For FOCNF and NOCNF, there are four types of oxygen atoms ( Figure S13, Supporting Information) at 530.2, 531.6, 532.5 eV, and 533.7 eV, corresponding to Fe(Ni)─O, C&dbond; O, C─O, and C─OOH, respectively. [42] For FOCNF, there is an obvious Fe 2p peak in its XPS spectrum (Figure 6a). Figure 6e and S14, Supporting Information show the high-resolution Fe 2p spectra of FOCNF. Peaks at 710.8 and 712.2 eV in the 2p 3/2 component and at 724.8 and 726.6 eV in the 2p 1/2 component correspond respectively to Fe 2þ and Fe 3þ . Satellite peaks at 720.2 and 731.8 eV can also be observed. [43,44] For NOCNF, there is an obvious Ni 2p peak in its XPS spectrum ( Figure 6a) and results of fitting are shown in Figure 6f and S15, Supporting Information. XPS peaks at 853.8 and 871.4 eV correspond to Ni 2p 3/2 and Ni 2p 1/2 of Ni 0 , respectively. There are also two characteristic peaks at 855.5 eV and 872.5 eV with two satellite peaks at 860.9 and 879.4 eV, corresponding to Ni 2p 3/2 and Ni 2p 1/2, respectively, indicating the presence of Ni 2þ in the samples. [39] The elemental compositions of PANC, FOCNF, and NOCNF are listed in Table 2. With the increase in MOF amount in the precursors, both Fe and Ni contents increase. The electrochemical performances of PANC, FOCNF, and NOCNF were measured in the three-electrode mode (Figure 7). Figure 7a shows the Nyquist plots of the samples. All curves display a sloping line in the low-frequency region and a semicircle in the high-frequency region. Generally, the diameter of the semicircle represents the charge transfer resistance (R ct ), reflecting the charge transfer ability between electrolyte and electrode surface. [45] The R ct values of the samples were fitted according to the equivalent circle model ( Figure S16, Supporting Information) and the results are listed in Table 2. PANC exhibits the largest R ct value of 1.77 Ω, which is probably due to its rich micropore structure, although that is disadvantageous for ion transfer. When MOFs were added to the precursors, R ct values decrease dramatically and reach the lowest values for FOCNF-1 and NOCNF-1, suggesting that these materials undergo fastest ion migration and diffusion. This can be ascribed to the increased mesopore volumes of NOCNF and FOCNF over PANC and the fact that mesopores promote ion migration and diffusion more effectively than micropores. However, excessive MOF in the precursors decreases the SSA, leading to poor charge transfer ability and high R ct value (FOCNF-1.5 and NOCNF-1.5). Figure 7b shows CV curves at a scan rate of 10 mV s À1 . For PANC, an approximately rectangular CV curve indicates that no obvious Faradic reactions occur. For FOCNF, typical cathodic and anodic peaks can be observed during the charge-discharge process, corresponding to the reversible Faradic conversion between Fe 2 O 3 and Fe(OH) 2 according to the following equation. [46] For NOCNF, a pair of cathodic and anodic peaks respectively at around 0.33 and 0.25 V suggest significant battery-type electrochemical behavior of the samples. The redox peaks can be ascribed to the following reaction equation. [47] NiO þ OH À ↔ NiOOH þ e À CV curves of the samples at different scan rates are displayed in Figure S17, Supporting Information. It can be seen that, with increasing scan rate, the anodic and cathodic peaks shift to increasingly negative or positive regions. This phenomenon can be attributed to polarization induced by the accumulation of ions as the scan rate increases. [48] During this process, the peak profile remains basically unchanged, indicating the good reversibility of these processes. Furthermore, all specific capacitances decrease as the scan rate increases due to the occurrence of polarization phenomenon ( Figure S17h, Supporting Information). FOCNF-1 and NOCNF-1 possess the highest specific capacitance at any scan rate, respectively, revealing that PAN:MOF ¼ 1:1 is the optimum mass ratio. The specific capacitances of FOCNF and NOCNF are improved over PANC, demonstrating that the addition of MIL-88(Fe) or MOF-74(Ni) is an effective method to improve the specific capacitance of flexible ECNFs.
GCD curves were also examined at various current densities from 0.5 to 20 A g À1 , as shown in Figure 7c and S18, Supporting Information. For PANC, an approximately isosceles triangleshaped trace was obtained at all current densities studied, suggesting that EDL capacitive behavior is dominant, which coincides with its CV curves shown in Figure 7b. In Figure 7c, the GCD curves of FOCNF reveal an obvious voltage plateau at about À0.8 V during charging and a voltage plateau at around À1.1 V during discharging, corresponding to the redox peaks at around À0.8 and À1.1 V in the CV curves ( Figure 7b) according to Equation (6). In addition, the observable over-discharge phenomenon of the as-obtained FOCNF might be due to the inevitable side reactions occurring during the discharge process. The GCD curves of NOCNF with obvious charging and discharging plateaus at %0.28 and 0.25 V can be observed respectively in Figure 7c, suggesting primary battery-type electrochemical behavior based on Equation (7). The specific capacitances of PANC, FOCNF, and NOCNF at different current densities were calculated according to Equation (2) and the results are displayed in Figure 7d. It can be seen that all specific capacitances decrease with increasing current density as the inner active sites cannot complete the required number of redox transitions, and both FOCNF and NOCNF possess much higher specific capacitances than PANC at any current density. The specific capacitances of the samples were calculated according to the discharge curves at a current density of 1 A g À1 and the results are listed in Table 2.
Clearly, compared with PANC (151 F g À1 ), specific capacitances are largely improved to 523 F g À1 for FOCNF-1 and 468 F g À1 for NOCNF-1. This can be attributed to the synergistic effect of the superior conductivity of the carbon nanofiber substrate and the excellent redox capabilities of Fe 2 O 3 and NiO. It is worth mentioning that the specific capacitances at a current density of 20 A g À1 for PANC, FOCNF-1, and NOCNF-1 are 17, 157, and 246 F g À1 , respectively, which are 9.3%, 27.5%, and 48.9% of their values at 0.5 A g À1 , revealing the improved rate performances of FOCNF and NOCNF relative to PANC. This results from the lower R ct values and high conductivities of FOCNF-1 and NOCNF-1, leading to the immediate reaction of ions. The cycling stability of FOCNF-1 and NOCNF-1 was investigated at a current density of 10 A g À1 for 6000 cycles. There resulting capacitance decays are only 9.3% and 6.5%, respectively, for FOCNF-1 and NOCNF-1 even after long-cycling tests ( Figure S19, Supporting Information), demonstrating their superior stabilities. The electrochemical performances of the asymmetric FHSC (FOCNF-1//NOCNF-1) device were examined in the twoelectrode mode, as shown in Figure 8. Figure 8a presents the FHSC structure and its discharge process. FHSC has a typical asymmetric structure with freestanding FOCNF-1 anode and NOCNF-1 cathode. During the discharge process, ions adsorbed at the surfaces of electrodes are released to the electrolyte, contributing EDL capacitance. At the same time, Fe(OH) 2 and NiOOH are converted to Fe 2 O 3 and NiO, respectively, establishing pseudocapacitance. CV curves at different scan rates with a stable voltage window of 1.65 V are shown in Figure 8b. Obvious redox peaks are present at all scan rates, indicating pseudocapacitive contribution. As the scan rate increases, the shapes of the CV curves change only slightly, revealing the excellent rate performances of the FHSC. GCD curves at various current densities with a voltage window of 1.65 V were measured to further  characterize the electrochemical performances of the FHSC, as shown in Figure 8c. The GCD curves display apparent plateaus at all current densities, indicating the occurrence of pseudocapacitive reactions. Furthermore, the as-assembled hybrid device also displays satisfactory charge efficiency at any current density ( Figure S20, Supporting Information), suggesting excellent electrochemical reversibility. Specific capacitances at different current densities were calculated according to Equation (2) and the results are presented in Figure 8d. The FHSC shows a high specific capacitance of 114 F g À1 at a current density of 0.5 A g À1 , while a capacitance of 44.3 F g À1 remains even at a high current density of 20 A g À1 , which is 38.9% of the initial value and suggests a superior rate performance of the FHSC. This can be ascribed to the superior conductivity and large mesopore volume of FOCNF-1 and NOCNF-1, which establish a fast ion response in the charge-discharge process.
To determine the stable operating voltage of FOCNF-1// NOCNF-1, CV curves in various voltage windows of 1, 1.2, 1.4, 1.65, 1.8, and 2 V were measured. As shown in Figure 8e, the CV curves indicate that the reversible charge-discharge process is maintained even over a high voltage window of 1.8 V, indicating a stable working voltage of FOCNF-1//NOCNF-1 of up to 1.8 V in the aqueous KOH/PVA gel electrolyte. The stability of the FHSC was examined by GCD measurement at a current density of 20 A g À1 , as shown in Figure 8f. The capacitance remains at 81.3% of its initial value after 10 000 cycles, indicating the good cycling stability of FOCNF-1//NOCNF-1. The superior cycling stability of FOCNF-1//NOCNF-1 can be attributed to its unique structure of nanographite-coated oxide which resists destruction even during long-cycle usage. Figure 9a illustrates the flexibility of FOCNF-1//NOCNF-1. The FHSC material can be recovered in its initial state after bending or twisting. Figure 9b shows the twisting and bending angles. To characterize the effect of twisting and bending deformations of FOCNF-1//NOCNF-1 on its capacitive properties, CV tests at a scan rate of 100 mV s À1 were carried out under bending or twisting, as shown in Figure 9c. The CV curves at bending angles of 60°, 120°, and 180°a s well as at twisting angle of 30°are similar to that at 0°. Specific capacitances at 180°bending and 30°twisting are 94.4% and 96.2% of the initial value, respectively, indicating the excellent flexibility of this device under operation. To demonstrate the practicality of  FOCNF-1//NOCNF-1, two FOCNF-1//NOCNF-1 devices were connected in series to light a commercial blue light emitting diode (LED) light with a start-up voltage of 2.7 V. As shown in Figure 9d,e, the LED light is successfully illuminated by the device even in a bent state. The Ragone plot of FOCNF-1//NOCNF-1 is shown in Figure 9f. The FHSC can achieve a high energy density of 43.1 Wh kg À1 at the power density of 412.5 W kg À1 and retains 16.8 Wh kg À1 even at the high power density of 16 500 W kg À1 . The electrochemical performances of Fe 2 O 3 and/or NiO-based asymmetric supercapacitors reported in the literature are shown in Figure 9f for comparison. Clearly, compared with the values of Fe 2 O 3 @N-doped carbon//NiO@N-doped carbon (14.1 Wh kg À1 at 1500 W kg À1 ), [49] reduced graphene oxide/ Fe 2 O 3 /carbon nanotubes//reduced graphene oxide/Fe 2 O 3 /carbon nanotubes (35.6 Wh kg À1 at 166.7 W kg À1 ), [50] Fe 2 O 3 /graphene// Co 3 O 4 /graphene (21.6 Wh kg À1 at 750 W kg À1 ), [51] Fe 2 O 3 @ PPy//MnO 2 (10.6 Wh kg À1 at 16326.4 W kg À1 ), [52] Fe 2 O 3 / MXene//MnO 2 (32.2 Wh kg À1 at 900.6 W kg À1 ), [53] MXene/ Fe 2 O 3 -C-MoS 2 /MXene//MXene/Fe 2 O 3 -C-MoS 2 /MXene (10.76 Wh kg À1 at 9201 W kg À1 ), [54] and ZnO@C@NiO//graphene (16 Wh kg À1 at 2704.2 W kg À1 ), [55] FOCNF-1//NOCNF-1 exhibits the higher energy density at the same power density, suggesting its excellent energy storage ability.

Conclusion
In summary, we have successfully prepared novel flexible ECNFs embedded with Fe 2 O 3 @nanographite nanorods or NiO@nanographite nanospheres. Fe and Ni present in the precursors not only catalyze the graphitization of ECNFs, but also improve the mesopore volume of the products. In the unique structures of FOCNF and NOCNF, ECNFs act both as a flexible substrate and establish rapid electron transfer for Faradic reactions. FOCNF and NOCNF exhibit high specific capacitances of 523 and 468 F g À1 , respectively, at a current density of 1 A g À1 . FHSC based on an FOCNF-1 anode and an NOCNF-1 cathode shows a high energy density of 43.1 Wh kg À1 at a high power density of 412.5 W kg À1 and maintains 16.8 Wh kg À1 even at the higher power density of 16 500 W kg À1 . The hybrid device also exhibits superior flexibility and excellent cycling stability with a capacitance retention of 81.3% after 10 000 chargedischarge cycles. FOCNF and NOCNF are promising flexible electrode materials for applications in FHSCs.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Queensland node of the Australian National Fabrication Facility, established under the National Collaborative Research Infrastructure Strategy, to provide nano-and microfabrication facilities for Australia's researchers.
Open access publishing facilitated by The University of Queensland, as part of the Wiley -The University of Queensland agreement via the Council of Australian University Librarians.