Construction of α-MnO2 on Carbon Fibers Modified with Carbon Nanotubes for Ultrafast Flexible Supercapacitors in Ionic Liquid Electrolytes with Wide Voltage Windows

In this study, α-MnO2 and Fe2O3 nanomaterials are prepared on a carbon fiber modified with carbon nanotubes to produce the nonbinder core–shell positive (α-MnO2@CNTs/CC) and negative (Fe2O3@CNTs/CC) electrodes that can be operated in a wide voltage window in ultrafast asymmetrical flexible supercapacitors. MnO2 and Fe2O3 have attracted wide research interests as electrode materials in energy storage applications because of the abundant natural resources, high theoretical specific capacities, environmental friendliness, and low cost. The electrochemical performance of each electrode is assessed in 1 M Na2SO4 and the energy storage properties of the supercapacitors consisting of the two composite electrodes are determined in Na2SO4 and EMImBF4 electrolytes in the 2 V and 4 V windows. The 2 V supercapacitor can withstand a large scanning rate of 5000 mV S−1 without obvious changes in the cyclic voltammetry (CV) curves, besides showing a maximum energy density of 57.29 Wh kg−1 at a power density of 833.35 W kg−1. Furthermore, the supercapacitor retains 87.06% of the capacity after 20,000 galvanostatic charging and discharging (GCD) cycles. The 4 V flexible supercapacitor shows a discharging time of 1260 s and specific capacitance of 124.8 F g−1 at a current of 0.5 mA and retains 87.77% of the initial specific capacitance after 5000 GCD cycles. The mechanical robustness and practicality are demonstrated by physical bending and the powering of LED arrays. In addition, the contributions of the active materials to the capacitive properties and the underlying mechanisms are explored and discussed


Introduction
Supercapacitors are energy storage devices based on both physical adsorption and desorption, as well as electrochemical reactions, and have attracted extensive attention because of advantages such as the fast charging and discharging capability, high power density, robust cycling stability, and environmental friendliness [1,2]. The electrode materials, structure, and electrolyte determine the properties of the supercapacitors [3][4][5]. With regard to the electrode materials, various forms of carbonaceous materials, conductive polymers, and transition metal oxides are commonly used as the active materials in supercapacitors [6][7][8][9]. In particular, manganese oxide (MnO 2 ) originated in the battery industry and has been widely used in the area of batteries and supercapacitors that promote our in-depth understanding and research on this material [10]. It is interesting because of the natural abundance, environmental friendliness, high theoretical specific capacity, low cost, and compatibility with other materials in energy storage applications [10][11][12].
However, MnO 2 electrodes are prone to severe agglomeration and the volume expansion during electrochemical cycling and the rate capability are low because of the poor conductivity [13][14][15]. Therefore, the optimization of MnO 2 -based materials is crucial for improving the properties and the power density while maintaining the good energy density.
The integration of MnO 2 -based electrodes with carbonaceous materials such as graphene and carbon nanotubes can improve the properties of supercapacitors [16][17][18][19]. Carbon nanotubes can improve the conductivity, specific capacitance, and ion migration efficiency of the active materials, and increase the mechanical adhesion strength between the active materials and the substrate [20][21][22][23]. For example, a mild one-pot reaction has been demonstrated to synthesize the MnO 2 /CNTs composite which has a capacity of 201 F g −1 and does not show obvious decay after 10,000 cycles at a current density of 1 A g −1 [17]. The capacity of the MnO 2 /CNTs-CNFS composite electrode prepared by Wang et al. is 374 F g −1 and 94% of its capacitance is retained after 1000 cycles [24]. The needle-shaped MnO 2 -CNTs-CFC composite fabricated by Li et al. by the two-step electrophoretic deposition shows a specific capacity of 381.74 F g −1 and an 85% capacitance retention after 1000 cycles [18]. Therefore, the MnO 2 @CNTs composite is a potential electrode for supercapacitors. Although MnO 2 @CNTs have been studied, there are few reports on the preparation of this structure on flexible carbon fiber cloth, and the research of asymmetric flexible supercapacitors prepared based on this structure has just begun. Furthermore, a better understanding of the mechanism pertaining to the electrochemical kinetics and effects of different electrolytes on the electrochemical characteristics of MnO 2 @CNTs-based supercapacitors are required in the development of high-performance electrodes [5].
In this study, a novel structure composed of α-phase ultrathin MnO 2 films prepared on a piece of carbon fiber cloth modified with carbon nanotubes (CNTs/CC) is prepared. The CNTs offer advantages such as a large surface area, efficient ion diffusion, large active substance loading, and excellent substrate conductivity to improve the rate as well as the energy and power densities of the electrode. The α-MnO 2 @CNTs/CC positive electrode and Fe 2 O 3 @CNTs/CC negative electrodes are prepared ( Figure 1) [25] and the electrochemical properties of the two electrodes are determined in 1 M Na 2 SO 4 . An asymmetrical supercapacitor (ASCs) consisting of the α-MnO 2 @CNTs/CC as the positive electrode, Fe 2 O 3 @CNTs/CC as the negative electrode, and 1 M Na 2 SO 4 as the electrolyte is assembled to provide a 2 V window (α-MCNTs//FCNTs-2V). In order to demonstrate the practicality, a flexible asymmetrical supercapacitor (FASC) with the same electrode combination but different ionic electrolyte of ionic liquid is constructed for the 4 V window (α-MCNTs//FCNTs-4V). The electrochemical performance and mechanism of the electrodes and devices are investigated and discussed. Nanomaterials 2022, 12, x FOR PEER REVIEW 2 of 17 capacity, low cost, and compatibility with other materials in energy storage applications [10][11][12]. However, MnO2 electrodes are prone to severe agglomeration and the volume expansion during electrochemical cycling and the rate capability are low because of the poor conductivity [13][14][15]. Therefore, the optimization of MnO2-based materials is crucial for improving the properties and the power density while maintaining the good energy density.
The integration of MnO2-based electrodes with carbonaceous materials such as graphene and carbon nanotubes can improve the properties of supercapacitors [16][17][18][19]. Carbon nanotubes can improve the conductivity, specific capacitance, and ion migration efficiency of the active materials, and increase the mechanical adhesion strength between the active materials and the substrate [20][21][22][23]. For example, a mild one-pot reaction has been demonstrated to synthesize the MnO2/CNTs composite which has a capacity of 201 F g −1 and does not show obvious decay after 10,000 cycles at a current density of 1 A g −1 [17]. The capacity of the MnO2/CNTs-CNFS composite electrode prepared by Wang et al. is 374 F g −1 and 94% of its capacitance is retained after 1000 cycles [24]. The needle-shaped MnO2-CNTs-CFC composite fabricated by Li et al. by the two-step electrophoretic deposition shows a specific capacity of 381.74 F g −1 and an 85% capacitance retention after 1000 cycles [18]. Therefore, the MnO2@CNTs composite is a potential electrode for supercapacitors. Although MnO2@CNTs have been studied, there are few reports on the preparation of this structure on flexible carbon fiber cloth, and the research of asymmetric flexible supercapacitors prepared based on this structure has just begun. Furthermore, a better understanding of the mechanism pertaining to the electrochemical kinetics and effects of different electrolytes on the electrochemical characteristics of MnO2@CNTs-based supercapacitors are required in the development of high-performance electrodes [5].
In this study, a novel structure composed of α-phase ultrathin MnO2 films prepared on a piece of carbon fiber cloth modified with carbon nanotubes (CNTs/CC) is prepared. The CNTs offer advantages such as a large surface area, efficient ion diffusion, large active substance loading, and excellent substrate conductivity to improve the rate as well as the energy and power densities of the electrode. The α-MnO2@CNTs/CC positive electrode and Fe2O3@CNTs/CC negative electrodes are prepared ( Figure 1) [25] and the electrochemical properties of the two electrodes are determined in 1 M Na2SO4. An asymmetrical supercapacitor (ASCs) consisting of the α-MnO2@CNTs/CC as the positive electrode, Fe2O3@CNTs/CC as the negative electrode, and 1 M Na2SO4 as the electrolyte is assembled to provide a 2 V window (α-MCNTs//FCNTs-2V). In order to demonstrate the practicality, a flexible asymmetrical supercapacitor (FASC) with the same electrode combination but different ionic electrolyte of ionic liquid is constructed for the 4 V window (α-MCNTs//FCNTs-4V). The electrochemical performance and mechanism of the electrodes and devices are investigated and discussed.

Materials Preparation
The fabrication process and structure of the α-MnO 2 @CNTs/CC positive electrode, Fe 2 O 3 @CNTs/CC negative electrode, and α-MCNTs//FCNTs-4V flexible supercapacitor are shown in Figure 1. The CNTs were prepared on a carbon fiber cloth by chemical vapor deposition and then the MnO 2 and Fe 2 O 3 were fabricated on the CNTs hydrothermally. The chemicals used were analytical grade.
The carbon fiber cloth was cut into 1 × 1 cm 2 pieces and cleaned with a nitrogen plasma (200 W) for 10 min in vacuum. Fe as the seed for carbon nanotubes was electrodeposited on the carbon fiber cloth in 20 mM Fe(NO 3 ) 2 at a current density of 3 mA and CNTs/CC was then prepared by chemical vapor deposition under nitrogen and methane at 800 • C for 2 h.
The α-MnO 2 film was deposited on CNTs/CC hydrothermally. Potassium permanganate (0.158 g) was dissolved in 20 mL of deionized water and the clean 1 × 1 cm 2 CNTs/CC was placed in the solution in a container made of Teflon. After sealing, the container was placed in a reactor and heated to 180 • C for 12 h. After natural cooling to room temperature, the product was rinsed with deionized water several times and dried at 70 • C for 12 h. Fe 2 O 3 was prepared fabricated on CNTs/CC by the same method a the α-MnO 2 film except that 2 mmol Fe(NO 3 ) 2 and 15 mmol urea were used in the precursor solution and the hydrothermal reaction proceeded at 120 • C for 8 h. The α-MnO 2 /CC and Fe 2 O 3 /CC electrodes were produced by the same method by direct deposition of the active materials on the carbon fiber cloth.

Preparation of Asymmetrical Supercapacitors
Asymmetrical supercapacitors consisting of the α-MnO 2 -based positive electrode, Fe 2 O 3 -based negative electrode, and the nonwoven fabric separator were prepared. An aqueous electrolyte was used in the 2 V supercapacitor of α-MCNTs//FCNTs-2V (α-MnO 2 @CNTs/CC as the positive electrode and Fe 2 O 3 @CNTs/CC as the negative electrode). The pretreatment involved soaking the separator and electrodes in 1 M NaSO 4 for 10 min and the separator and electrodes were assembled into a CR2032 shell as a sandwiched structure. The same method used for α-MCNTs//FCNTs-2V was adopted to fabricate α-M//F-2V (α-MnO 2 /CC as the positive electrode and Fe 2 O 3 /CC as the negative electrode) and CNTs//CNTs (CNTs/CC as both the positive and negative electrode) for comparison. To prepare the ultrafast α-MCNTs//FCNTs-4V flexible supercapacitor with a large voltage window, the nonwoven fabric separator and electrodes were dipped in an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4)) for 10 min. The positive and negative electrodes were separated and encapsulated with the polyimide tape. Again, the same technique employed to prepare α-MCNTs//FCNTs-4V was implemented to fabricate α-M//F-4V (α-MnO 2 /CC as the positive electrode and Fe 2 O 3 /CC as the negative electrode) for comparison.

Materials Characterization
The morphology and microstructure were examined by scanning electron microscopy (SEM) on the 7500F (JEOL, Tokyo, Japan) and the elemental states and composition were determined by X-ray photoelectron spectroscopy (XPS) using the ESCALAB-250 (Thermo Fisher Scientific, Waltham, MA, USA). The crystal structure was determined by X-ray diffraction (XRD, D/max-2550, Rigaku, Tokyo, Japan) and the morphology and lattice of α-MnO 2 @CNTs and Fe 2 O 3 @CNTs were examined by the transmission electron microscopy (TEM) on the JEOL2100 (JEOL, Tokyo, Japan) at 200 kV.

Electrochemical Measurements
The electrochemical assessment was carried out on the CHI660E electrochemical workstation. In the configuration with 1.0 M Na 2 SO 4 electrolyte, CC, CNTs/CC, and α-MnO 2 @CNTs/CC or Fe 2 O 3 @CNTs/CC were the working electrodes, whereas a saturated calomel was the reference electrode and platinum wire was the counter electrode.
Galvanostatic charging/discharging (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were performed in addition to self-discharging and monitoring of the leakage currents. For the asymmetrical supercapacitor containing 1.0 M Na 2 SO 4 and ionic liquid as the electrolytes, respectively, GCD, CV, and EIS were performed on the electrochemical workstation and long-time cycling was carried out on the LAND supercapacitor testing system.
The specific capacitance of the composite electrodes C s was calculated by Equation (1) and the energy density (E) and power density (P) were calculated by Equations (2) and (3), respectively [26,27]: Here, C s (F g −1 ) is the specific capacitance, I(A) represents the current or current density during the GCD process, m(g) is the mass of active materials on the surface of the electrodes, ∆t(s) denotes the time in the discharging process of the GCD test, and ∆V(V) stands for the voltage window of the electrodes or asymmetrical supercapacitor in the GCD test.  [29]. The XPS survey spectrum of Figure 2b shows the existence of C, O, and Mn in α-MnO 2 @CNTs/CC and C, O, and Fe in Fe 2 O 3 @CNTs/CC. Figure 2c shows the Mn 2p spectrum of α-MnO 2 @CNTs/CC, revealing peaks at 654.2 eV for Mn 2p 1/2 and 642.5 eV for Mn 2p 3/2 of MnO 2 . The satellite peak (sat) is related to α-MnO 2 , consistent with previous studies and further confirming the formation of MnO 2 [28]. The H-O-H (531.9 eV) and Mn-O-H (530.2 eV) peaks in Figure 2d of α-MnO 2 @CNTs/CC are attributed to the hydroxyl groups and absorbed water, respectively [30][31][32]. The Fe 2p peaks at 724.58 eV and 710.86 eV in Figure 2e are the Fe 2p peaks of Fe 2 O 3 @TiN/CC representing Fe 2p 1/2 and Fe 2p 3/2 in α-Fe 2 O 3 [28]. As shown in Figure 2f, the peaks at 530.0, 531.4, and 532.5 eV of Fe 2 O 3 @CNTs/CC arise from O 2− , OH − , and O-C=O, respectively [25], and that at 532.6 eV corresponds to the H-O-H of the adsorbed water molecules [33]. The peak at 531.4 eV reflects the O-H of the surface or the internal hydroxyl groups and the chemically adsorbed oxygen [34], and that at 530.0 eV stems from the O 2− in Fe 2 O 3 [35].   (Figure 3a-c) to produce a conductive network for the α-MnO2 nanosheets and Fe2O3 nanodots. The carbon nanotubes are connected in a staggered and dense way to promote the transfer of electrons, ion extraction and insertion, and the effective area for the active materials. As shown in Figure 3d-f, the CNTs form a 3D porous skeleton and provide abundant nucleation sites for uniform MnO2 deposition, while preventing the agglomeration of the active substances during the electrochemical reaction [27]. Figure 3f shows many micropores in the α-MnO2@CNTs/CC electrode for enhanced electrolyte circulation in the electrode and reduced contact impedance between the electrode and electrolyte. According to the SEM images in Figure 3g-i, nanoscale Fe2O3 is deposited on CNTs/CC to form the Fe2O3@CNTs/CC negative electrode and the morphological and structural characteristics of the negative electrode are similar to those of the positive electrode. In the ionic electrolyte, the α-MnO2@CNTs/CC positive electrode and the Fe2O3@CNTs/CC electrode broaden the voltage window of the flexible supercapacitor to 4 V, thereby improving the power density without compromising the energy density of the device [36].   (Figure 3a-c) to produce a conductive network for the α-MnO 2 nanosheets and Fe 2 O 3 nanodots. The carbon nanotubes are connected in a staggered and dense way to promote the transfer of electrons, ion extraction and insertion, and the effective area for the active materials. As shown in Figure 3d-f, the CNTs form a 3D porous skeleton and provide abundant nucleation sites for uniform MnO 2 deposition, while preventing the agglomeration of the active substances during the electrochemical reaction [27]. Figure 3f shows many micropores in the α-MnO 2 @CNTs/CC electrode for enhanced electrolyte circulation in the electrode and reduced contact impedance between the electrode and electrolyte. According to the SEM images in Figure 3g-i, nanoscale Fe 2 O 3 is deposited on CNTs/CC to form the Fe 2 O 3 @CNTs/CC negative electrode and the morphological and structural characteristics of the negative electrode are similar to those of the positive electrode. In the ionic electrolyte, the α-MnO 2 @CNTs/CC positive electrode and the Fe 2 O 3 @CNTs/CC electrode broaden the voltage window of the flexible supercapacitor to 4 V, thereby improving the power density without compromising the energy density of the device [36]. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 17 The α-MnO2@CNTs and Fe2O3@CNTs core-shell structures were stripped from the composite electrodes ultrasonically to analyze by TEM. As shown in Figure 4a, the Fe2O3 nanoparticles are densely and uniformly deposited on the carbon nanotubes, forming the Fe2O3@CNTs nanocomposite with a diameter of about 40 nm. The high-resolution image in Figure 4b indicates lattice spacings of 0.22 nm and 0.25 nm matching the (120) and (110) planes of Fe2O3 [37]. Figure 4c,d show that α-MnO2@CNTs composite has a diameter of about 50 nm, which is different from that of Fe2O3@CNTs, but the general morphology of the carbon nanotubes is preserved. The high-resolution image of the α-MnO2@CNTs coreshell structure in Figure 4e reveals lattice spacings of 0.14 nm, 0.31 nm, and 0.24 nm corresponding to the (300), (101), and (110) planes of α-MnO2, respectively [27,38]. The coreshell structure comprising the MnO2 film and CNTs is confirmed by the elemental maps of α-MnO2@CNTs in Figure 4f   The α-MnO 2 @CNTs and Fe 2 O 3 @CNTs core-shell structures were stripped from the composite electrodes ultrasonically to analyze by TEM. As shown in Figure 4a, the Fe 2 O 3 nanoparticles are densely and uniformly deposited on the carbon nanotubes, forming the Fe 2 O 3 @CNTs nanocomposite with a diameter of about 40 nm. The high-resolution image in Figure 4b indicates lattice spacings of 0.22 nm and 0.25 nm matching the (120) and (110) planes of Fe 2 O 3 [37]. Figure 4c,d show that α-MnO 2 @CNTs composite has a diameter of about 50 nm, which is different from that of Fe 2 O 3 @CNTs, but the general morphology of the carbon nanotubes is preserved. The high-resolution image of the α-MnO 2 @CNTs core-shell structure in Figure 4e reveals lattice spacings of 0.14 nm, 0.31 nm, and 0.24 nm corresponding to the (300), (101), and (110) planes of α-MnO 2 , respectively [27,38]. The core-shell structure comprising the MnO 2 film and CNTs is confirmed by the elemental maps of α-MnO 2 @CNTs in Figure 4f [39]. The eight surfaces of these double chains share the apex with the neighboring double chains to form a [2 × 2] tunnel, and this large tunnel can accept cations with a radius of around 0.15 nm, such as Ba 2+ , K + , Pb 2+ , Na + , and NH4 + , as well as H2O molecules [40]. In the charge and discharge process of the α-MnO2 electrode material, the conversion of manganese atoms is between +3 valence and +4 valence, forming two energy storage mechanisms: one is ion adsorption and desorption on α-MnO2 surface [41]:

Materials Characterization
the other is ion insertion and extraction in the inter tunnel of α-MnO2: The first energy storage mechanism comes from the high specific surface area of α-MnO2; meanwhile, the second is based on the effective structural tunnel of manganese dioxide.   Figure 4f [39]. The eight surfaces of these double chains share the apex with the neighboring double chains to form a [2 × 2] tunnel, and this large tunnel can accept cations with a radius of around 0.15 nm, such as Ba 2+ , K + , Pb 2+ , Na + , and NH 4 + , as well as H 2 O molecules [40]. In the charge and discharge process of the α-MnO 2 electrode material, the conversion of manganese atoms is between +3 valence and +4 valence, forming two energy storage mechanisms: one is ion adsorption and desorption on α-MnO 2 surface [41]: the other is ion insertion and extraction in the inter tunnel of α-MnO 2 : Nanomaterials 2022, 12, 2020 8 of 17 The first energy storage mechanism comes from the high specific surface area of α-MnO 2 ; meanwhile, the second is based on the effective structural tunnel of manganese dioxide.

Electrochemical Properties of The Fabricated Electrodes
As shown in Figures 5a-c and S1, the electrochemical properties of CNTs/CC, α-MnO 2 /CC, and α-MnO 2 @CNTs/CC are similar, but α-MnO 2 @CNTs/CC shows a longer discharging time, a larger CV area, a smaller contact resistance, and a higher ion diffusion efficiency than CNTs/CC and α-MnO 2 /CC. According to Figure S3, Table S1, and Equation (1), the MnO 2 loading of α-MnO 2 on the carbon fiber cloth is 0.55 mg per cm 2 , and that of α-MnO 2 on the carbon fiber cloth with CNTs is 0.73 mg per cm 2 . The specific capacity of α-MnO 2 @CNTs/CC is 367.44 F g −1 at a current density of 2 mA cm −2 , which is superior to that of 188.55 F g −1 of α-MnO 2 /CC. Hence, the carbon nanotubes play a significant role in improving the electrochemical activity. The characteristics of the three electrodes shown in Figure 5a,b are consistent with the Nyquist curves in Figure 5c, and the detailed fits of the Nyquist curves are shown in Table S2. As shown in Figure 5d, the cyclic voltammograms of α-MnO 2 @CNTs/CC between 80 mV s −1 and 2500 mV s −1 show the typical rectangular shape without obvious deformation, even at a large scanning rate [27]. As shown in Figure 5e, the GCD curves of α-MnO 2 @CNTs/CC from 0.25 mA cm −2 to 8.0 mA cm −2 exhibit a similar shape of an isosceles triangle, consistent with the characteristics of the pseudocapacitance. The corresponding specific capacitances are listed in Table S1.

Electrochemical Properties of The Fabricated Electrodes
As shown in Figures 5a-c and S1, the electrochemical properties of CNTs/CC, α-MnO2/CC, and α-MnO2@CNTs/CC are similar, but α-MnO2@CNTs/CC shows a longer discharging time, a larger CV area, a smaller contact resistance, and a higher ion diffusion efficiency than CNTs/CC and α-MnO2/CC. According to Figure S3, Table S1, and Equation (1), the MnO2 loading of α-MnO2 on the carbon fiber cloth is 0.55 mg per cm 2 , and that of α-MnO2 on the carbon fiber cloth with CNTs is 0.73 mg per cm 2 . The specific capacity of α-MnO2@CNTs/CC is 367.44 F g −1 at a current density of 2 mA cm −2 , which is superior to that of 188.55 F g −1 of α-MnO2/CC. Hence, the carbon nanotubes play a significant role in improving the electrochemical activity. The characteristics of the three electrodes shown in Figure 5a,b are consistent with the Nyquist curves in Figure 5c, and the detailed fits of the Nyquist curves are shown in Table S2. As shown in Figure 5d, the cyclic voltammograms of α-MnO2@CNTs/CC between 80 mV s −1 and 2500 mV s −1 show the typical rectangular shape without obvious deformation, even at a large scanning rate [27]. As shown in Figure 5e, the GCD curves of α-MnO2@CNTs/CC from 0.25 mA cm −2 to 8.0 mA cm −2 exhibit a similar shape of an isosceles triangle, consistent with the characteristics of the pseudocapacitance. The corresponding specific capacitances are listed in Table S1. To assess the matching between the negative and positive electrodes, electrochemical tests are performed on Fe2O3@CNTs/CC, as shown in Figure 5f-i. Figure 5f shows the CV results at scanning rates between 80 mV s −1 and 2500 mV s −1 in the voltage window from To assess the matching between the negative and positive electrodes, electrochemical tests are performed on Fe 2 O 3 @CNTs/CC, as shown in Figure 5f-i. Figure 5f shows the CV results at scanning rates between 80 mV s −1 and 2500 mV s −1 in the voltage window from 0 to −1 V. The results reflect the abundant active sites boding well for rapid ion extraction and insertion. GCD curves are acquired from Fe 2 O 3 @CNTs/CC from 0.25 mA cm −2 to 8 mA cm −2 (Figure 5g), and the time duration of GCD increases with the decreasing current densities. The CV and GCD curves of CNTs/CC are shown in Figure S2 for comparison. At a current density of 0.5 mA cm −2 , CNTs/CC shows a discharging time of 260.1 s, whereas Fe 2 O 3 @CNTs/CC shows a discharging time of 958 s, confirming that the Fe 2 O 3 @CNTs composite shows enhanced performance. Figure 5h exhibits the stability curves of the α-MnO 2 @CNTs/CC, α-MnO 2 /CC, and Fe 2 O 3 @CNTs/CC electrodes for 10,000 GCD cycles. The specific capacitance of α-MnO 2 @CNTs/CC is 361.8 F g −1 at a current density of 3 mA cm −2 , and after 10,000 cycles, the attenuation is only 2.6% to 350.9 F g −1 , which is better than the observed 89.36% of α-MnO 2 /CC. The impedance data of the Fe 2 O 3 @CNTs/CC negative electrode before and after 10,000 cycles in Figure 5i are consistent with the cycling tests showing 93.54% capacity retention. The detailed analysis and fits of the Nyquist curves in Figure 5i are listed in Table S2. The leakage current and self-discharging are important parameters. To measure the leakage current, the composite electrode is the working electrode in the three-electrode system, and pulses of 1 V at 2 mA are applied. The leakage currents of the α-MnO 2 @CNTs/CC electrode versus time are shown in Figure 6a, disclosing a quick drop to 0.063 mA in a short time before stabilization in the next 2 h. The leakage current of the positive electrode is less than 0.34 mA of the 0.086 mA of 3D-NCS-3//N-rGO [42], Ni-Mn LDH/MnO 2 [43], and 0.08 mA of a-NENCs [44]. The small leakage current indicates an insignificant electrolyte diffusion on the electrode surface and fewer side reactions caused by impurities on the electrode [45]. The self-discharging process of the α-MnO 2 @CNTs/CC electrode shown in Figure 6b is monitored by measuring the potentials of the electrode under open circuit conditions for 21 h [46]. Owing to the leakage current, the potentials of the electrode decrease gradually with time and the potential drop is significant in the first 2-3 h before moderation in the subsequent 10 hours. All in all, the potential time curves in Figure 6b show a stable output potential of 467 mV after 4 h and it remains at about 385 mV after 21 h, which is comparable to previous studies [46].  (Figure 5g), and the time duration of GCD increases with the decreasing current densities. The CV and GCD curves of CNTs/CC are shown in Figure S2 for comparison. At a current density of 0.5 mA cm −2 , CNTs/CC shows a discharging time of 260.1 s, whereas Fe2O3@CNTs/CC shows a discharging time of 958 s, confirming that the Fe2O3@CNTs composite shows enhanced performance. Figure 5h exhibits the stability curves of the α-MnO2@CNTs/CC, α-MnO2/CC, and Fe2O3@CNTs/CC electrodes for 10,000 GCD cycles. The specific capacitance of α-MnO2@CNTs/CC is 361.8 F g −1 at a current density of 3 mA cm −2 , and after 10,000 cycles, the attenuation is only 2.6% to 350.9 F g −1 , which is better than the observed 89.36% of α-MnO2/CC. The impedance data of the Fe2O3@CNTs/CC negative electrode before and after 10,000 cycles in Figure 5i are consistent with the cycling tests showing 93.54% capacity retention. The detailed analysis and fits of the Nyquist curves in Figure 5i are listed in Table S2.
The leakage current and self-discharging are important parameters. To measure the leakage current, the composite electrode is the working electrode in the three-electrode system, and pulses of 1 V at 2 mA are applied. The leakage currents of the α-MnO2@CNTs/CC electrode versus time are shown in Figure 6a, disclosing a quick drop to 0.063 mA in a short time before stabilization in the next 2 h. The leakage current of the positive electrode is less than 0.34 mA of the 0.086 mA of 3D-NCS-3//N-rGO [42], Ni-Mn LDH/MnO2 [43], and 0.08 mA of a-NENCs [44]. The small leakage current indicates an insignificant electrolyte diffusion on the electrode surface and fewer side reactions caused by impurities on the electrode [45]. The self-discharging process of the α-MnO2@CNTs/CC electrode shown in Figure 6b is monitored by measuring the potentials of the electrode under open circuit conditions for 21 h [46]. Owing to the leakage current, the potentials of the electrode decrease gradually with time and the potential drop is significant in the first 2-3 h before moderation in the subsequent 10 hours. All in all, the potential time curves in Figure 6b show a stable output potential of 467 mV after 4 h and it remains at about 385 mV after 21 h, which is comparable to previous studies [46].  The relationship between impedance data and frequency of complex capacitance model is essential to analyze the supercapacitor electrodes [28]. Figure 6c depicts the relationship between the real part of the capacitance C (ω) with the frequency of CNTs/CC, α-MnO 2 /CC, and α-MnO 2 @CNTs/CC, according to Equations S1, S2, and S3. The capacitance change is the one commonly described in the supporting information [47]: when the frequency decreases, C (ω) sharply increases, then tends to be less frequency-dependent and can be displayed by the change available of stored energy. This is characteristic of the electrode structure and the electrode/electrolyte interface and compares with CNTs/CC and α-MnO 2 /CC, while α-MnO 2 @CNTs/CC is more like an ideal capacitor [48]. As shown in Figure 6d and calculated from Equations (S1), (S2), and (S4), the time constants of CNTs/CC, α-MnO 2 /CC, and α-MnO 2 @CNTs/CC are 3.83 s, 2.15 s, and 1.21 s, respectively, so that α-MnO 2 @CNTs/CC shows a faster energy storage speed of nearly twice that of α-MnO 2 /CC and nearly three times that of CNTs/CC [47].
The mechanism of the α-MnO 2 @CNTs/CC electrode is explored by calculating the pseudocapacitance ratio based on the CV plots in Figure 7a and k1 ( Figure 7b shows the images corresponding to six groups of random voltages during charge-discharge) [28] by Equation (6): where i(V) represents the current at the selected voltage (V) according to the CV curves, v is the CV scanning rate, k 1 is equal to the slope of the line obtained by fitting the connection points in the same voltage in Figure 7b, and k 1 v determines the current of pseudocapacitance at different potentials. The pseudocapacitance (red area) is obtained using k 1 v as the ordinate and the corresponding voltage as the abscissa, and the calculated Faraday pseudocapacitance accounts for the total capacitance (red area divided by the total CV area) from 1 mV s −1 to 5 mV s −1 , as shown in Figure 7c,d. The calculated proportions of the pseudocapacitance are about 77.18% at 1 mV s −1 and 88.31% at 5 mV s −1 , indicating that the pseudocapacitance makes the main contribution to the energy storage characteristics of α-MnO 2 @CNTs/CC. The pseudocapacitance contributions of α-MnO 2 /CC, derived from previous studies [28], show a 62.4% Faraday pseudocapacitance, accounting for the total capacitance at 1 mV s −1 , which is lower than that of α-MnO 2 @CNTs/CC. The detailed pseudocapacitance contribution of the α-MnO 2 /CC electrode from 1 mV s −1 to 5 mV s −1 are summarized in Figure 4d of [28].

Electrochemical Performance of α-MnO2-Based Supercapacitor Devices with 1 M Na2SO4 Electrolyte
To assess the performance of the electrodes systematically, an asymmetrical supercapacitor is assembled with a 1 M Na2SO4 electrolyte, the positive electrode of α-MnO2@CNTs/CC, as well as a negative electrode of Fe2O3@CNTs/CC separated by a

Electrochemical Performance of α-MnO 2 -Based Supercapacitor Devices with 1 M Na 2 SO 4 Electrolyte
To assess the performance of the electrodes systematically, an asymmetrical supercapacitor is assembled with a 1 M Na 2 SO 4 electrolyte, the positive electrode of α-MnO 2 @CNTs/CC, as well as a negative electrode of Fe 2 O 3 @CNTs/CC separated by a nonwoven fabric membrane. The supercapacitors of α-MnO 2 @CNTs/CC//Fe 2 O 3 @CNTs/CC (α-MCNTs//FCNTs-2V) and α-MnO 2 /CC//Fe 2 O 3 /CC (α-M//F-2V), shown in Figures 8 and S5, are assembled in the CR2025 cell. The CV curves of the α-MnO 2 @CNTs/CC positive electrode and the Fe 2 O 3 @CNTs/CC negative electrode at 300 mV s −1 are drawn in Figure 8a for comparison. The two CV curves have a similar shape, area, voltage difference, and rapid response, corroborating the electrochemical properties and the same EDLC behavior. Figures 8b,c and S6 show the electrochemical properties of the α-M//F-2V asymmetrical supercapacitor for comparison, and α-MCNTs//FCNTs-2V shows an obviously longer discharging time and larger CV area. The CV and GCD curves of α-MCNTs//FCNTs-2V acquired at different voltage windows are presented in Figure 8d,e. In the 0-2.2 V window, both the CV and GCD curves do not exhibit obvious deformation, showing that the positive and negative electrodes match well to broaden the working potential of the device. With increasing scanning rates, the CV curves do not change, even at 5000 mV s −1 , which is indicative of the rapid pseudocapacitive reaction on the electrode, as shown in Figure 8f. The GCD curves do not show much difference for the single electrode (Figure 8g), confirming the good electrochemical reversibility, pseudocapacitive characteristics, and I-V response. At a current of 0.5 mA, the specific capacitance is 103.27 F g −1 , which is comparable to those of similar MnO2-based devices reported in the literature, as shown in Table S3. Figure 8h discloses The CV and GCD curves of α-MCNTs//FCNTs-2V acquired at different voltage windows are presented in Figure 8d,e. In the 0-2.2 V window, both the CV and GCD curves do not exhibit obvious deformation, showing that the positive and negative electrodes match well to broaden the working potential of the device. With increasing scanning rates, the CV curves do not change, even at 5000 mV s −1 , which is indicative of the rapid pseudocapacitive reaction on the electrode, as shown in Figure 8f. The GCD curves do not show much difference for the single electrode (Figure 8g), confirming the good electrochemical reversibility, pseudocapacitive characteristics, and I-V response. At a current of 0.5 mA, the specific capacitance is 103.27 F g −1 , which is comparable to those of similar MnO 2 -based devices reported in the literature, as shown in Table S3. Figure 8h discloses that, even at a current of 10 mA, the asymmetrical supercapacitor has excellent cycling ability for 20,000 cycles, and it should be emphasized that such outstanding features are rarely observed from asymmetrical devices made of MnO 2 and Fe 2 O 3 . Owing to the excellent properties of CNTs, the α-MCNTs//FCNTs-2V device retains 87.06% of its initial capacity, that is better than α-M//F-2V (49.46%) and most other devices shown in Table S3. According to Equations (2) and (3), the energy and power densities of α-MCNTs//FCNTs-2V are calculated and shown in Figure 8i and compared to other Mn-based supercapacitors in Figure 8i and

The Performance of 4 V Voltage Window α-MnO 2 -Based Flexible Supercapacitor with Ionic Liquid Electrolyte
To demonstrate the practical aspects, the ultrafast α-MCNTs//FCNTs-4V flexible supercapacitor with a large voltage window is fabricated, as described in Section 2.2 and shown in Figure 9. In the 0-4 V window, the GCD and CV curves do not exhibit obvious deformation, as shown in Figure 9a,b, confirming that the flexible supercapacitor can adapt to the broader 4 V window. The CV curves in Figure 9c indicate that the flexible device has good pseudocapacitive characteristics even at 2400 mV s −1 , in addition to a fast I-V response with a good CV shape. The GCD curves are acquired up to 4 V at different currents (Figure 9d), and the discharging duration at the same current is more than three times that of the supercapacitor of α-MCNTs//FCNTs-2V in the aqueous electrolyte. Compared to the α-M/F-4V flexible supercapacitor in Figure S6, α-MCNTs//FCNTs-4V shows a larger CV area ( Figure 9e) and a longer discharging time (Figure 9f) due to the carbon nanotubes. At a current of 0.5 mA, the discharging time of α-MCNTs//FCNTs-4V is 1261 s, whereas that of α-M//F-4V is only 200 s. Figure 9g discloses that the α-MCNTs//FCNTs-4V flexible device retains 87.77% of the initial specific capacitance after 5000 cycles at a GCD current of 2 mA, thus faring better than α-M//F-4V (78.95%) and most other devices listed in Table S4.  Table 1, and the results confirm that α-MnO 2 @CNTs/CC with the EMImBF4 electrolyte produce high energy and power densities [58][59][60][61].
pared to the α-M/F-4V flexible supercapacitor in Figure S6, α-MCNTs//FCNTs-4V shows a larger CV area ( Figure 9e) and a longer discharging time (Figure 9f) due to the carbon nanotubes. At a current of 0.5 mA, the discharging time of α-MCNTs//FCNTs-4V is 1261 s, whereas that of α-M//F-4V is only 200 s. Figure 9g discloses that the α-MCNTs//FCNTs-4V flexible device retains 87.77% of the initial specific capacitance after 5000 cycles at a GCD current of 2 mA, thus faring better than α-M//F-4V (78.95%) and most other devices listed in Table S4.  Figure S6 shows the Nyquist plots of α-M//F-4V before and after 5000 cycles. After 5000 cycles, the axial intercept increases from 6.32 Ω to 6.69 Ω. The radius of the semicircle in the first half of the impedance spectrum becomes larger, indicating that the specific capacitance decreases with the increasing charge transfer resistance. The slope of the straight line in the second half becomes smaller, implying that the charge diffusion resistance becomes smaller and the manganese dioxide structure collapses after charging and discharging. Figure 9i and Figure S7 show that the flexible supercapacitor can tolerate large mechanical bending at a large angle, and that different bending angles have little effect on the device performance. After charging at a high current for 10 s, two flexible devices in a series can light up the LED array consisting of electronic circuits and 11 × 44 small LEDs. The results unambiguously demonstrate that the ultrafast flexible supercapacitor has a wide voltage window and large commercial potential in energy storage systems.

Conclusions
Nanoscale MnO 2 and Fe 2 O 3 are fabricated on a conductive carbon fiber cloth modified with CNTs to form flexible electrodes for high-performance supercapacitors. The 2 V supercapacitor comprises the positive α-MnO 2 @CNTs/CC electrode and the negative Fe 2 O 3 @CNTs/CC in 1 M Na 2 SO 4 , whereas the 4 V ultrafast flexible supercapacitor uses the EMImBF4 electrolyte. The electrochemical characteristics and mechanisms in the different electrolytes are evaluated systematically. The results reveal the fast I-V response, outstanding pseudocapacitive characteristics, and excellent electrochemical reversibility due to the enhanced ion transfer efficiency between the electrodes and electrolytes, as well as the mitigated agglomeration of the nanomaterials. The α-MnO 2 @CNTs/CC and Fe 2 O 3 @CNTs/CC electrodes with excellent properties have a large potential in energy applications.

Supplementary Materials:
The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12122020/s1, Figure S1. Electrochemical properties of α-MnO 2 /CC: (a) CV curves acquired at different scanning rates and (d) GCD curves obtained at different current densities; Figure S2. (a) CV curves obtained at a scanning rate of 600 mV s −1 and (b) GCD curves acquired at a current density of 0.25 mA cm −2 from CNTs/CC and Fe 2 O 3 @CNTs/CC; Electrochemical properties of CNTs/CC: (c) CV curves acquired at different scanning rates and (d) GCD curves obtained at different current densities; Figure S3. Specific capacitances of α-MnO 2 /CC and α-MnO 2 @CNTs/CC at different current densities; Figure S4. Electrochemical properties of the α-M//F-2V supercapacitor in 1 M Na 2 SO 4 electrolyte: (a) CV curves acquired at a scanning rate of 300 mV s −1 , (b) GCD curves obtained at a current of 3 mA with different upper cut-off voltages, (c) CV curves acquired at different scanning rates, and (d) GCD curves obtained at different currents; Electrochemical properties of CNTs/CC//CNTs/CC in 1 M Na 2 SO 4 electrolyte: (e) CV curves acquired at different scanning rates and (f) GCD curves obtained at different currents; Figure S5. Electrochemical properties of the α-M//F-4V flexible supercapacitor in EMImBF4 electrolyte: (a) GCD curves, (b) CV curves for different upper cut-off voltages, (c) CV curves acquired at different scanning rates, and (d) GCD curves obtained at different currents; Figure S6. Nyquist plots of the flexible hybrid supercapacitor for the 4 V voltage window before and after 5000 cycles; Figure S7. (a) CV plots, (b) GCD curves, and (c) Nyquist plots with different bending angles; Table S1. Specific capacitances of α-MnO 2 /CC and α-MnO 2 @CNTs/CC in 1 M Na 2 SO 4 ; Table S2. Important EIS parameters of the electrodes in Figure 5c,j; Table S3. Comparison of the electrochemical properties of MnO 2 -based supercapacitors. References [47,[49][50][51][52][53][54][55][56]62] are cited in the supplementary materials.