CoMnO2-Decorated Polyimide-Based Carbon Fiber Electrodes for Wire-Type Asymmetric Supercapacitor Applications

In this work, we report the carbon fiber-based wire-type asymmetric supercapacitors (ASCs). The highly conductive carbon fibers were prepared by the carbonized and graphitized process using the polyimide (PI) as a carbon fiber precursor. To assemble the ASC device, the CoMnO2-coated and Fe2O3-coated carbon fibers were used as the cathode and the anode materials, respectively. Herein, the nanostructured CoMnO2 were directly deposited onto carbon fibers by a chemical oxidation route without high temperature treatment in presence of ammonium persulfate (APS) as an oxidizing agent. FE-SEM analysis confirmed that the CoMnO2-coated carbon fiber electrode exhibited the porous hierarchical interconnected nanosheet structures, depending on the added amount of APS, and Fe2O3-coated carbon fiber electrode showed a uniform distribution of porous Fe2O3 nanorods over the surface of carbon fibers. The electrochemical properties of the CoMnO2-coated carbon fiber with the concentration of 6 mmol APS presented the enhanced electrochemical activity, probably due to its porous morphologies and good conductivity. Further, to reduce the interfacial contact resistance as well as improve the adhesion between transition metal nanostructures and carbon fibers, the carbon fibers were pre-coated with the Ni layer as a seed layer using an electrochemical deposition method. The fabricated ASC device delivered a specific capacitance of 221 F g−1 at 0.7 A g−1 and good rate capability of 34.8% at 4.9 A g−1. Moreover, the wire-type device displayed the superior energy density of 60.2 Wh kg−1 at a power density of 490 W kg−1 and excellent capacitance retention of 95% up to 3000 charge/discharge cycles.


Introduction
In recent years, the energy storage devices, such as Li ion batteries (LIBs) and electrochemical capacitors/supercapacitors (SCs) are becoming more and more important as environmentally clean and sustainable energy sources [1][2][3]. In particular, SCs are the most promising energy storage device owing to their higher power densities, longer cycle life, fast charging/discharging capability, non-toxic nature, and low-cost maintenance compared to the LIBs. In general, the charge storage mechanism of supercapacitors is divided in two ways, either by forming electrical double layer charge accumulation (electrical double layer capacitors; EDLCs) or by using faradaic reactions (pseudocapacitor) at the interface between the electrode and electrolyte [2,4]. So far, various efforts have been made to meet  Figure 1i,j), respectively. It can be clearly seen that hierarchical CoMnO 2 nanostructures were successfully formed on the surface of PICFs. While the bare PICFs showed smooth surface morphologies, CoMnO 2 @PICFs exhibited the porous hierarchical interconnected nanosheet structures, which depended on the added amounts of APS. Insets in Figure 1d-j show the higher magnified SEM images. We have also measured the elemental mapping and corresponding EDX spectrum using FE-SEM equipped with EDX measurement (Figure 1k). The results confirmed the presence of C, Co, Mn, and O elements, supporting the successful deposition of CoMnO 2 onto PICFs.

Results and Discussion
Molecules 2020, 25, x FOR PEER REVIEW 3 of 15 EDX measurement (Figure 1k). The results confirmed the presence of C, Co, Mn, and O elements, supporting the successful deposition of CoMnO2 onto PICFs. , and CoMnO2@PICF-9 (i,j), and inset shows the higher magnified images. The elemental mapping and EDX spectrum (k) of CoMnO2@PICF-6. Figure 2a presents CV curves of pure PICFs, CoMnO2@PICF-1, CoMnO2@PICF-3, CoMnO2@PICF-6, and CoMnO2@PICF-9 at the scan rate of 20 mV s −1 within the potential window 0.0 to + 0.6 V vs. (Ag/AgCl). The shape of CV curves for CoMnO2@PICFs was nearly rectangular. No redox peaks were clearly detected, attributed to the fast, reversible successive surface redox reactions [29]. Moreover, it clearly showed that the integrated area of CoMnO2@PICF-6 was larger than those of CoMnO2@PICF-1, CoMnO2@PICF-3, and CoMnO2@PICF-9, suggesting feasible enhancement in the electrochemical activity, probably due to its porous morphology and good conductivity. On the other hand, the contribution of pure PICFs to the capacitance was almost negligible, as seen in Figure 2a, suggesting that the added CoMnO2 significantly enhanced the capacitance. Further, as increasing the scan rate the CV curves of CoMnO2@PICFs well maintained symmetrical shape ( Figure S1), demonstrating the reversible electrochemical redox reaction and excellent rate capability of the electrode material [30], which is one of important parameters in the pseudocapacitive electrodes.
Capacitive performance of the CoMnO 2 @PICFs electrodes was further studied by the GCD measurements at the current density of 1 A g −1 within the potential range from 0 to + 0.5V (Figure 2b). The fabricated CoMnO 2 @PICF-1, CoMnO 2 @PICF-3, CoMnO 2 @PICF-6, and CoMnO 2 @PICF-9 electrodes delivered the specific capacitances of 362, 634, 928, and 688 F g −1 at the current density of 1 A g −1 , respectively. Figure 2c shows the relationship of specific capacitance vs. current density of the CoMnO 2 @PICF electrodes. The specific capacitance (~928 F g −1 @1 A g −1 ) of the CoMnO 2 @PICF-6 electrode was about 2.6, 1.5, and 1.3 times higher than those of the CoMnO 2 @PICF-1 (~362 F g −1 @1 A g −1 ), CoMnO 2 @PICF-3 (~643 F g −1 @1 A g −1 ), and CoMnO 2 @PICF-9 (~688 F g −1 @1 A g −1 ) electrodes at the current density of 1.0 A g −1 , respectively. Moreover, the capacitance of the CoMnO 2 @PICF-6 electrode decreased by 29 % at higher current density of 5 A g −1 , proving the excellent high-rate capability. The EIS was exploited to investigate the ion diffusion and electron transfer of the CoMnO 2 @PICF electrodes. Figure 2d presents the Nyquist plots for the pure PICF, CoMnO 2 @PICF-1, CoMnO 2 @PICF-3, CoMnO 2 @PICF-6, and CoMnO 2 @PICF-9 electrodes in the frequency range of 0.1 to 100 kHz with an amplitude of 10 mV. The EIS spectrum was composed of a small semicircle in a high-frequency range and a linear curve in the low-frequency range. The internal resistance (R S ) is the sum of the ionic resistance of the electrolyte. The intrinsic resistance of the active material and the contact resistance at the active material/current collector interface can be derived from the intercept of the plots on the real axis. The semicircle of Nyquist plot corresponds to the Faradic reaction and its diameter represents the interfacial charge transfer resistance (R CT ) [31]. The fabricated CoMnO 2 @PICFs electrodes showed a semicircle at higher frequency region. The calculated R CT values were 11.7 Ω, 27.1 Ω, 14.1 Ω and 23.8 Ω for the CoMnO 2 @PICF-1, CoMnO 2 @PICF-3, CoMnO 2 @PICF-6, and CoMnO 2 @PICF-9 electrodes, respectively. The values were tabulated in inset of figure. As a result, it was found that the CoMnO 2 @PICF-6 showed best electrochemical activity. Although the value of R CT (14.1 Ω) of the CoMnO 2 @PICF-6 electrode was a rather large, we could see the lower R S value (9.2 Ω) of the CoMnO 2 @PICF-6 electrode at the intersection of the real axis, indicating a low internal resistance. ionic resistance of the electrolyte. The intrinsic resistance of the active material and the contact resistance at the active material/current collector interface can be derived from the intercept of the plots on the real axis. The semicircle of Nyquist plot corresponds to the Faradic reaction and its diameter represents the interfacial charge transfer resistance (RCT) [31]. The fabricated CoMnO2@PICFs electrodes showed a semicircle at higher frequency region. The calculated RCT values were 11.7 Ω, 27.1 Ω, 14.1 Ω and 23.8 Ω for the CoMnO2@PICF-1, CoMnO2@PICF-3, CoMnO2@PICF-6, and CoMnO2@PICF-9 electrodes, respectively. The values were tabulated in inset of figure. As a result, it was found that the CoMnO2@PICF-6 showed best electrochemical activity. Although the value of RCT (14.1 Ω) of the CoMnO2@PICF-6 electrode was a rather large, we could see the lower RS value (9.2 Ω) of the CoMnO2@PICF-6 electrode at the intersection of the real axis, indicating a low internal resistance. In order to further improve the interfacial properties of the fabricated CoMnO2@PICFs electrode, the additional Ni as a seed layer was deposited on the PICF before CoMnO2 deposition.  Figure 3b). Furthermore, CoMnO2/N20@PICF-6 showed the similar surface morphology to the CoMnO2@PICF-6 ( Figure 3d). We have checked the crystal structure of pure PICF, N20@PICF, and CoMnO2/N20@PICF-6. As seen in Figure 4a, bare PICF showed a broad peak around 25.4 o , corresponding to the graphitic carbon peak [32]. The Ni20@PICF showed the three obvious diffraction peaks appearing at 44.6 o , 52.0 o , and 76.7 o , which were well indexed to the (111), (200), and (220) crystal planes of the face-centered cubic structure of nickel/nickel oxide (JCPDS No. 87-0712) [33] originated from the 3D-Ni metal skeleton [21] and coated Ni seed , relationship between capacitance and current density (c), and electrochemical impedance spectroscopy (EIS) curves (d) of pure PICFs, CoMnO 2 @PICF-1, CoMnO 2 @PICF-3, CoMnO 2 @PICF-6, and CoMnO 2 @PICF-9.
In order to further improve the interfacial properties of the fabricated CoMnO 2 @PICFs electrode, the additional Ni as a seed layer was deposited on the PICF before CoMnO 2 deposition. Figure Figure 3b). Furthermore, CoMnO 2 /N20@PICF-6 showed the similar surface morphology to the CoMnO 2 @PICF-6 ( Figure 3d). We have checked the crystal structure of pure PICF, N20@PICF, and CoMnO 2 /N20@PICF-6. As seen in Figure 4a, bare PICF showed a broad peak around 25.4 o , corresponding to the graphitic carbon peak [32]. The Ni20@PICF showed the three obvious diffraction peaks appearing at 44.6 o , 52.0 o , and 76.7 o , which were well indexed to the (111), (200), and (220) crystal planes of the face-centered cubic structure of nickel/nickel oxide (JCPDS No. 87-0712) [33] originated from the 3D-Ni metal skeleton [21] and coated Ni seed layer. The CoMnO 2 /N20@PICF-6 showed the diffraction peaks at 12.7 • , 18.9 • , 28.7 • , 36.   To confirm the chemical states of CoMnO2/N20@PICF-6, XPS studies were carried out ( Figure  4b-f). The survey spectrum shows the presence of Ni, Co, Mn, O, and C without other impurity elements. The observed Ni peak is ascribed to the Ni seed layer on the PICF. The deconvoluted Co 2p spectrum suggested the existence of Co (0) , Co 2+ , and Co 3+ at the 778.  To confirm the chemical states of CoMnO 2 /N20@PICF-6, XPS studies were carried out (Figure 4b-f). The survey spectrum shows the presence of Ni, Co, Mn, O, and C without other impurity elements. The observed Ni peak is ascribed to the Ni seed layer on the PICF. The deconvoluted Co 2p spectrum suggested the existence of Co (0) , Co 2+ , and Co 3+ at the 778. In order to investigate the effect of Ni seed layer on the electrochemical properties of CoMnO 2 @PICF-6, CV and EIS tests of bare PICF, CoMnO 2 @PICF-6, and CoMnO 2 /N20@PICF-6 were carried out in three-electrode configuration. As seen in Figure 5a, the CoMnO 2 /N20@PICF-6 electrode showed clearly large integrated area than those of other samples, suggesting an enhanced electrochemical storage ability. As expected, the CoMnO 2 /N20@PICF-6 displayed the longer discharge time (Figure 5b), as compared to the CoMnO 2 @PICF-6. The specific capacitances calculated from the discharge curves were plotted as a function of current density (Figure 5c). The CoMnO 2 /N20@PICF-6 electrode delivered excellent specific capacitances of 1206, 996, 876, 776, 700, 624, 574, 512, 468, and 420 F g −1 at the current densities of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 A g −1 , respectively, which were clearly higher than those of the CoMnO 2 @PICF-6 and even N20@PICF electrodes. The electrochemical performances of the CoMnO 2 -decorated PICF electrodes (CoMnO 2 @PICF-6, CoMnO 2 /N20@PICF-6) were further investigated by EIS. Compared to the CoMnO 2 @PICF-6 (14.1 Ω), the CoMnO 2 /N20@PICF-6 showed the lower R CT value (3.4 Ω), suggesting that Ni seed layer provided a good interfacial contact between CoMnO 2 and PICF, giving the lower interfacial resistance and a fast reversible redox reaction. In order to investigate the effect of Ni seed layer on the electrochemical properties of CoMnO2@PICF-6, CV and EIS tests of bare PICF, CoMnO2@PICF-6, and CoMnO2/N20@PICF-6 were carried out in three-electrode configuration. As seen in Figure 5a, the CoMnO2/N20@PICF-6 electrode showed clearly large integrated area than those of other samples, suggesting an enhanced electrochemical storage ability. As expected, the CoMnO2/N20@PICF-6 displayed the longer discharge time (Figure 5b), as compared to the CoMnO2@PICF-6. The specific capacitances calculated from the discharge curves were plotted as a function of current density (Figure 5c). The CoMnO2/N20@PICF-6) were further investigated by EIS. Compared to the CoMnO2@PICF-6 (14.1 Ω), the CoMnO2/N20@PICF-6 showed the lower RCT value (3.4 Ω), suggesting that Ni seed layer provided a good interfacial contact between CoMnO2 and PICF, giving the lower interfacial resistance and a fast reversible redox reaction. To evaluate the real capacitance of the fabricated electrodes, an ASC full-cell was assembled with cathode and anode materials. To fabricate the full-cell, Fe2O3-decorated N20@PICF with Ni seed layer (Fe2O3/N20@PICF) was prepared by hydrothermal method and then used as anode material. FE-SEM image showed a uniform distribution of porous Fe2O3 nanorods over the surface of PICF, as can be seen in inset of Figure 6a. Figure 6b shows the survey XPS spectrum of Fe2O3/N20@PICF sample which contained Fe, O, Ni, and C elements. The Fe 2p spectrum (Figure 6c) exhibited two distinct peaks of Fe 2p 1/2 and Fe 2p 3/2 at the binding energies of 724.6 and 710.9 eV, respectively. The two satellite peaks were also observed, indicating the existence of Fe 3+ in Fe2O3 [37]. Figure 6d presented the XPS spectrum of O 1s level of the sample. The peak can be deconvoluted into two peaks at 529.8 and 531.6 eV. The peak at 529.8 eV was due to the lattice oxygen of the Fe2O3, and the other peak at the 531.6 eV represented the presence of other components such as OH, H2O, and carbonate species adsorbed onto the surface [38]. The porous structures in electroactive materials also plays an important role in electrochemical energy storage devices. Figure S2 shows the N2 adsorption/desorption curves of CoMnO2/N20@PICF-6 electrode. It exhibited a type IV isotherm with an H3 hysteresis loop at high relative pressure, indicating the presence of mesoporous structure. The BET surface area and total pore volume were measured to be 27.54 m 2 ·g −1 and 0.048 cm 3 ·g −1 . The result showed that the major volume of the pores was the mesopore, ranging from of 1.9 to 4.2 nm. Therefore, the large specific surface area and the mesoporous architecture of the CoMnO2/N20@PICF-6 electrode can be expected to provide enormous electroactive sites at the electrode/electrolyte interface and shorten the ion diffusion length for an excellent electrochemical performance [39]. To test the feasibility of the CoMnO2/N20@PICF-6 and Fe2O3/N20@PICF electrodes for a real application, we have assembled the ASC device by sandwiching the CoMnO2/N20@PICF-6 as a cathode and Fe2O3/N20@PICF as an anode material with PVA-KOH as a polymer electrolyte. The optimal mass ratio (m + /m − ) of the cathode and anode materials was calculated by using Equation (2), To evaluate the real capacitance of the fabricated electrodes, an ASC full-cell was assembled with cathode and anode materials. To fabricate the full-cell, Fe 2 O 3 -decorated N20@PICF with Ni seed layer (Fe 2 O 3 /N20@PICF) was prepared by hydrothermal method and then used as anode material. FE-SEM image showed a uniform distribution of porous Fe 2 O 3 nanorods over the surface of PICF, as can be seen in inset of Figure 6a. Figure 6b shows the survey XPS spectrum of Fe 2 O 3 /N20@PICF sample which contained Fe, O, Ni, and C elements. The Fe 2p spectrum (Figure 6c) exhibited two distinct peaks of Fe 2p 1/2 and Fe 2p 3/2 at the binding energies of 724.6 and 710.9 eV, respectively. The two satellite peaks were also observed, indicating the existence of Fe 3+ in Fe 2 O 3 [37]. Figure 6d presented the XPS spectrum of O 1s level of the sample. The peak can be deconvoluted into two peaks at 529.8 and 531.6 eV. The peak at 529.8 eV was due to the lattice oxygen of the Fe 2 O 3 , and the other peak at the 531.6 eV represented the presence of other components such as OH, H 2 O, and carbonate species adsorbed onto the surface [38]. The porous structures in electroactive materials also plays an important role in electrochemical energy storage devices. Figure S2 shows the N 2 adsorption/desorption curves of CoMnO 2 /N20@PICF-6 electrode. It exhibited a type IV isotherm with an H3 hysteresis loop at high relative pressure, indicating the presence of mesoporous structure. The BET surface area and total pore volume were measured to be 27.54 m 2 ·g −1 and 0.048 cm 3 ·g −1 . The result showed that the major volume of the pores was the mesopore, ranging from of 1.9 to 4.2 nm. Therefore, the large specific surface area and the mesoporous architecture of the CoMnO 2 /N20@PICF-6 electrode can be expected to provide enormous electroactive sites at the electrode/electrolyte interface and shorten the ion diffusion length for an excellent electrochemical performance [39].
To test the feasibility of the CoMnO 2 /N20@PICF-6 and Fe 2 O 3 /N20@PICF electrodes for a real application, we have assembled the ASC device by sandwiching the CoMnO 2 /N20@PICF-6 as a cathode and Fe 2 O 3 /N20@PICF as an anode material with PVA-KOH as a polymer electrolyte. The optimal mass ratio (m + /m − ) of the cathode and anode materials was calculated by using Equation (2), and it turned out to be 1:0.7. The CV profiles of the CoMnO 2 /N20@PICF6 and Fe 2 O 3 /N20@PICF electrodes were shown in Figure 7a. The potential windows of the anode and cathode materials were −1.0 to 0 V and 0 to 0.6 V, respectively. Accordingly, the maximum operation potential for the wire-type ASC device was anticipated to reach 1.5 V. The typical CV curves of the ASC device in a potential window of 0.0-1.5 V at different scan rates were presented in Figure 7b. It displayed a rectangular-like shape, indicating that the device showed a fast charge/discharge behavior and high rate ability. The triangular-shaped GCD curves in Figure 7c revealed the satisfactory electrochemical reversibility and capacitive characteristics of the ASC device. The calculated specific capacitance was 221 F g −1 at the current density of 0.7 A g −1 . Moreover, the capacitance value started to decrease as the current density further increased. The ASC device showed stable electrochemical performances at different voltage windows, as seen in Figure 7d. The CV profiles remained rectangular-like shape up to 1.4 V. The charge-discharge profiles at different voltage ranges also exhibited triangular-shaped curves up to 1.4 V (Figure 7e). With the extension of voltage windows, the calculated electrical performance of the device slightly increased from 159.6 F g −1 to 177 F g −1 . The Nyquist plot (Figure 7f) showed the estimated R S and R CT values of 23.2 Ω and 16.9 Ω, respectively, which derived from the low resistance of the conductive carbon fiber substrate and the compact cell assembly. At low frequencies, the ASC device showed vertical behavior, which indicates that samples possess capacitor characteristics. Thus, the ASC device had good capacitive properties. In addition, the CoMnO 2 /N20@PICF-6//Fe 2 O 3 /N20@PICF device exhibited higher capacitance retention rate of 95% after 3000 charge/discharge cycles at 2.8 A g −1 than that (retention rate of 82%) of the CoMnO 2 @PICF-6//Fe 2 O 3 @PICF device without Ni seed layer, ascribed to the reduced interfacial contact resistance by an introduced Ni layer (Figure 7g). Furthermore, the rate capability was enhanced by the electrodeposition of Ni seed layer (inset in Figure 7g). The assembled CoMnO 2 /N20@PICF-6//Fe 2 O 3 /N20@PICF device delivered a maximum energy density of 60.2 Wh kg −1 at power density of 490 W kg −1 , as shown by Ragone plot (Figure 7h). Compared to other devices, these values were higher than other energy storage devices, such as NiCoMn-TH/AEG//CFP-S (23.5 Wh kg −1 at 427 W kg −1 ) [40], NiCoMn-OH//AC (43.2 Wh kg −1 at 790 W kg −1 ) [41], CoMn-HW/RGO10//AC (38.3 Wh kg −1 at 8000 W kg −1 ) [42], CoMn LDH/PPy//MLG (29.6 Wh kg −1 at 500 W kg −1 ) [43], NCM//AC (23.7 Wh kg −1 at 2625 W kg −1 ) [44], Ni-Mn LDH/rGO//AC (33.8 Wh kg −1 at 850 W kg −1 ) [45], and Co/Mn-ZIF//AC (52.5 Wh kg −1 at 1080 W kg −1 ) [46], as summarized in Table S1. As a result, the fabricated wire-type ASC device exhibited the excellent electrochemical performance with good electrical conductivity.
Molecules 2020, 25, x FOR PEER REVIEW 9 of 15 electrodeposition of Ni seed layer (inset in Figure 7g). The assembled CoMnO2/N20@PICF-6//Fe2O3/N20@PICF device delivered a maximum energy density of 60.2 Wh kg −1 at power density of 490 W kg −1 , as shown by Ragone plot (Figure 7h). Compared to other devices, these values were higher than other energy storage devices, such as NiCoMn-TH/AEG//CFP-S (23.  [46], as summarized in Table S1. As a result, the fabricated wire-type ASC device exhibited the excellent electrochemical performance with good electrical conductivity.

Fabrication of Transition Metal Oxides-Coated PICF Electrodes
At first, 4.5 mmol Co(CH 3 COO) 2 ·4H 2 O and 4.5 mmol Mn(CH 3 COO) 2 ·4H 2 O were dissolved in 30 mL of DI water and stirred for 1 h to make a clear bright red solution. Then the PICF was immersed in the above solution and sonicated slightly to remove the microbubbles of the solution. Afterwards, APS solution with various concentrations of 1, 3, 6, and 9 mmol was further added and reacted at 60 • C for 12 h (via a chemical oxidation reaction) and labelled as CoMnO 2 @PICF-1, CoMnO 2 @PICF-3, CoMnO 2 @PICF-6, and CoMnO 2 @PICF-9, respectively. The obtained CoMnO 2 -decorated PICFs (CoMnO 2 @PICFs) as cathode material was washed with DI water gently, and dried at 60 • C. The mass loading of CoMnO 2 metal oxide deposited onto PICF was about 2 mg. For the preparation of anode material, 6 mmol FeCl 3 ·6H 2 O was dissolved in 60 mL of DI water and stirred for 1 h at room temperature to make a homogeneous solution. Then, PICF and the above solution were transferred into 80 mL Teflon-lined stainless steel autoclave, and hydrothermal treatment was carried out at 140 • C for 12 h in an oven. The resultant product was carefully rinsed with DI water and dried at 60 • C for 12 h. Finally, the obtained sample was further calcinated at 350 • C for 2 h at a heating rate of 2 • C min −1 to achieve the Fe 2 O 3 -decorated PICF (Fe 2 O 3 @PICF). The mass loading of Fe 2 O 3 metal oxides deposited onto PICF was about 1 mg.
Molecules 2020, 25, x FOR PEER REVIEW 11 of 15 Scheme 1. Schematic illustration for the preparation of a wire-type asymmetric supercapacitor by using CoMnO2-and Fe2O3-coated carbon fibers as cathode and anode materials.

Characterization
The surface morphologies were examined by field emission scanning electron microscopy (FE-SEM, JEOL JSM-5900) along with energy dispersive X-ray (EDX) system. The X-ray diffraction studies were performed using Rigaku diffractometer, CuKα radiation operating at 40 keV/40 mA at a scanning rate of 15° per min in the 2θ range from 10° to 90°. The chemical state of the elements linked with the surface chemical composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB250, Al Kα radiation). The specific surface area (SSA) and pore size of Scheme 1. Schematic illustration for the preparation of a wire-type asymmetric supercapacitor by using CoMnO 2 -and Fe 2 O 3 -coated carbon fibers as cathode and anode materials.

Electrodeposition of Ni Seed Layer on PICF
For the deposition of Ni seed layer, 2M NH 4 Cl and 0.1M NiCl 2 ·6H 2 O as a supporting electrolyte were dissolved in 100 mL of DI water, and used as the solution for electrodeposition. The PICF and Pt wire were used as working and reference electrodes, respectively. The Ni seed layer deposition was carried out with a constant current density of 0.25 A cm −2 (by adjusting 7 V and 0.5 A) at a deposition time of 20 s (denoted as Ni20@PICF) using a DC power supply. After deposition, the samples were washed with DI water and dried at 60 • C for 12 h. The schematic illustration for the preparation of CoMnO 2 @PICFs was represented in Scheme 1.

Characterization
The surface morphologies were examined by field emission scanning electron microscopy (FE-SEM, JEOL JSM-5900) along with energy dispersive X-ray (EDX) system. The X-ray diffraction studies were performed using Rigaku diffractometer, CuK α radiation operating at 40 keV/40 mA at a scanning rate of 15 • per min in the 2θ range from 10 • to 90 • . The chemical state of the elements linked with the surface chemical composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB250, Al K α radiation). The specific surface area (SSA) and pore size of the samples were computed using the Brunauer-Emmett-Teller (BET) equation.

Electrochemical Measurements
The electrochemical performance was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD). The CV and EIS measurements were carried out in a three-electrode system at room temperature using an electrochemical workstation (Princeton Applied Research, Versatat 4). Here, Pt wire was used as the counter electrode, Ag/AgCl as the reference electrode and the prepared samples as the working electrode, respectively. The CV curves were recorded within the potential window from 0 to 0.6 V (vs. Ag/AgCl) in 1M KOH electrolyte at various scan rates (5, 10, 20, 50, and 100 mV s −1 ). The GCD tests were carried out within the potential range of 0 to 0.5 V in 1M KOH. The specific capacitance of the fabricated electrodes was calculated from the discharge curves using the following Equation (1).
where C is the specific capacitance (F g −1 ), I is the discharge current (mA), m is the mass (mg) of the electroactive material, ∆t and ∆V are the discharge time (s) and potential window (V). The EIS measurement was carried out at open circuit potential in the frequency range of 0.1 Hz to 100 kHz. The ZView software was employed to fit the EIS data. The impedance data are presented in the form of Nyquist plot, representing the resistive and capacitive behavior of electrodes. The values of the charge transfer resistance (R CT ) and internal resistance (R S ) were determined using Zsimpwin software simulations. For the practical applications, the asymmetric supercapacitor (ASC, CoMnO 2 /N20@PICF// Fe 2 O 3 /N20@PICF) device was constructed using CoMnO 2 /N20@PICF and Fe 2 O 3 /N20@PICF as cathode and anode materials, respectively. The PVA/KOH gel electrolyte was used for both electrolyte and separator in ASC device. To prepare the PVA/KOH gel electrolyte, 2 g PVA and 2g KOH were added to the 20 mL DI water and it was kept under stirring and then heated slowly to 90 • C until it became clear and transparent. After it was cooled down to room temperature, the CoMnO 2 /N20@PICF and Fe 2 O 3 /N20@PICF electrodes were coated with PVA/KOH gel electrolyte and carefully assembled together and then sealed in the plastic tube to fabricate the wire-type ASC device. To obtain optimum energy and power densities, the optimal mass ratio of both electrodes was determined based on the charge balance relationship (q + and qare the charges acquired by the cathode and anode materials) provided by the following Equation (2).
where m + , m − , C + , C − , V + , and V − signifies the mass (g), specific capacitance (F g −1 ) and working potential window (V) for the cathode and anode materials, respectively. The energy (E, Wh kg −1 ) and power densities (P, W kg −1 ) of as-assembled ASC device were computed by the following Equations (3) and (4). E = C × ∆V 2 /2 × 3.6 (3) P = E × 3600/∆t (4) where C indicates the specific capacitance of ASC device (F g −1 ), ∆V is the working potential window (V) and ∆t is the discharging time (s).

Conclusions
We have prepared the carbon fiber-based wire-type asymmetric supercapacitors (ASCs). The nanostructured and porous CoMnO 2 -coated and Fe 2 O 3 -coated carbon fibers were used as the cathode and the anode materials to produce the asymmetric CoMnO 2 /N20@PICF-6//Fe 2 O 3 /N20@PICF supercapacitor device. Such porous hierarchical interconnected nanosheet structures were confirmed by FE-SEM analysis. The electrochemical properties of the CoMnO 2 -coated carbon fiber electrode (CoMnO 2 @PICF-6) with the concentration of 6 mmol APS presented the enhanced electrochemical activity, due to its porous morphologies and good conductivity. Moreover, additional Ni seed layer provided a good interfacial contact between CoMnO 2 and PICF, giving the lower interfacial resistance and a fast reversible redox reaction. The fabricated ASC device delivered a specific capacitance of 221 F g −1 at 0.7 A g −1 and good rate capability of 34.8% at 4.9 A g −1 . The wire-type device displayed the superior energy density of 60.2 Wh kg −1 at a power density of 490 W kg −1 and excellent capacitance retention of 95% up to 3000 charge/discharge cycles.