Study on Electrochemical Performance of MnO@rGO/Carbon Fabric-Based Wearable Supercapacitors

In this work, we reported the electrochemical performance of a type of carbon fabric-based supercapacitor by coating MnOx@rGO nanohybrids on carbon fabric with a simple one-step hydrothermal method. We studied the mass ratio of MnOx to rGO on the electrochemical properties of the carbon fabric-based supercapacitors. We found that as the mass ratio is 0.8:1 for MnO@rGO, the supercapacitor with a loading of 5.40 mg cm−2 of MnO@rGO nanohybrids on carbon fabric exhibits a specific capacitance of 831.25 mF cm−2 at 0.1 mA cm−2 current density. It also shows long-term cycling capacitance retention of 97.2% after 10,000 charge–discharge cycles at a current density of 0.4 mA cm−2. We speculate that the high electrochemical performance results from the strong interfacial bonding between the hierarchical architecture of MnO@rGO nanohybrids and carbon fabric.

One major study on wearable supercapacitors is focused on the design and preparation of flexible carbon-based materials. Carbon fabrics are composed of a large number of carbon fibers possessing high surface area and an inter-fibers porous structure along with rough and absorptive surface properties [9,10]. Though carbon fabrics display poor electrochemical properties [11,12], they are still promising as an attractive alternative to flexible electrochemical electrodes owing to their light weight, good chemistry stability, high porosity, and flexibility [13][14][15]. In recent years, numerous researchers have led investigations on fabricating supercapacitors [16,17] by using carbon fabrics as both electrodes and mechanical support. Up to now, research has been concentrated on graphene-functionalized carbon fabric composites for supercapacitor applications. For example, Joaquín Artigas-Arnaudas et al. [18] reported structural supercapacitors fabricated from graphene nanoplatelets combined with PVDF and PVA binders on the carbon fiber fabrics surface with a mass-specific capacitance of 5.2 mF g −1 at 0.02 mA g −1 current density. Recently, Zehan Yao et al. [19] prepared flexible carbon fabric supercapacitors fabricated by vertical graphene and carbon fabric, with 13.2 mW cm −2 power density and 86.6 µWh cm −2 energy density. However,

Materials
The graphene oxide (GO) was bought from the Sixth ElementMaterials Technology Co., Ltd. (Changzhou, China). Potassium permanganate (KMnO 4 ), anhydrous ethanol, and PVA 1788 (88% hydrolysis) were purchased from Shanghai Lingfeng Chemical Reagent Company (Shanghai, China). Manganese acetate tetrahydrate (Mn(CH 3 COO) 2 ·4H 2 O) was purchased from J&K Scientific (Shanghai, China). The Na 2 SO 4 was purchased from Shanghai Macklin Biochemical Company (Shanghai, China). Silver paste was purchased from the Shenzhen Sunflower Electronic Company (Shenzhen, China). The PTFE filament was purchased from Shantou Mingda Textile Company (Shantou, China). These materials were used as received without further purification or treatment.

Preparation of MnOx@GO Slurry
The MnOx@GO slurry was prepared via a simple, one-step hydrothermal method: Firstly, 15 mL of GO suspension (0.5 mmol) was subjected to ultrasonic vibration for 30 min. The Mn(CH 3 COO) 2 ·4H 2 O (0.025 mmol, 20 mL) was added to the GO solution and then underwent further ultrasonic mixing for 30 min. It was then mixed with KMnO 4 (0.2 mmol, 10 mL) and stirred at 85 • C for 1 h. The MnO x @GO slurry was concentrated after washing with water using vacuum filtration several times. The mass ratio of MnO x to GO in MnO x @GO composites was controlled to be 0:1, 0.5:1, 0.8:1, and 1:1 by changing the molar concentrations of Mn(CH 3 COO) 2 ·4H 2 O and KMnO 4 .

Preparation of MnOx@rGO/C
MnO x @rGO/C was prepared by the following three-step method. First, the above MnO x @GO slurry was thermally concentrated to a suitable concentration of 5 mM. Then, the as-received carbon fabric was immersed into the condensed MnOx@GO slurry and dried in a vacuum oven. The procedure was repeated 6-7 times. Finally, the MnO x @rGO/C sample was obtained by carbonizing the MnO x @GO/C in a high-temperature vacuum tubular furnace (OTF-1200X) at 800 • C for 3 h under an Argon (Ar) atmosphere with a heating rate of 10 • C/min. In comparison, the as-received carbon fabric was named C.

Preparation of MnO x @rGO/C Electrodes and Supercapacitors
One piece of MnO x @rGO/C with the mass ratio of MnO x to GO of 0:1, 0.5:1, 0.8:1, and 1:1 with the area of 1 cm × 1.1 cm was fixed on the plastic chuck to form the working electrode and soaked in 1 ml 1 M Na 2 SO 4 aqueous electrolyte. The all-solid-state MnO x @rGO (0.8:1) /C supercapacitors were prepared as described next. One gram of PVA powder was added to the mixture of 9 mL of DI water. Then, the mixture was heated to 90 • C under vigorous stirring for 30 min until the solution was cooled down to room temperature. Then, 1.42 g of Na 2 SO 4 crystalline particles were added to the mixture of 1 mL of DI water. Following this, 1 mol Na 2 SO 4 solution was added to the above PVA solution and stirred thoroughly to prepare the PVA/Na 2 SO 4 gel electrolyte. The assembled MnO x @rGO/C supercapacitors consist of two MnO x @rGO/C electrodes in parallel with the PVA/Na 2 SO 4 gel electrolyte in between.

Characterizations
A scanning electron microscope (SEM, HITACHI, TM3000 and SU5000) and transmission electron microscope (TEM, JEOL, JEM-2100) were used to characterize the surface morphologies of the electrodes. The characteristic elements were detected using an energydispersive X-ray spectrometer (EDS, Bruker, Billerica, MA, USA). Raman spectroscopy (Renishaw Micro-Raman/Photoluminescence System) was used to analyze the carbon structure of as-produced GO/C, MnO x @rGO/C and reduced MnO x @rGO/C using a 633 nm He-Ne laser. The XRD patterns of samples were obtained with an X-ray diffractometer with Cu Ka radiation (Υ = 1.54056 • A), (D/Max-2550 PC, Rigaku, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi) was used to characterize the element valence states and contents. The four-probe method (Scientific Equipment & Services) was used to measure the electrical conductivity of C, rGO@C, and MnO/rGO@C samples.

Characterizations of Electrochemical Performances
Electrochemical performance of C, rGO/C, and MnO x @rGO/C electrodes, including cyclic voltammetry (CV) tests, galvanostatic charge/discharge (GCD) tests, and electrochemical impedance spectra (EIS, 0.01 Hz to 100 kHz), were carried out on an electrochemical workstation (CHI 660E, CH Instruments Inc., Shanghai, China)with a three-electrode cell that contained a working electrode, a counter electrode (Pt foil), and a reference electrode (Ag/AgCl electrode). CV and GCD curves of the electrode were performed in a potential window of 0 to 0.8 V and the area capacitance of the supercapacitor (C A ) was calculated from GCD curves at different current densities.

Results and Discussion
The synthesis roadmap of MnO x @rGO/C was shown in Figure 1. Briefly, a MnO x @GO slurry was prepared using a one-step hydrothermal method, where MnO x nanoflakes were grown in situ on GO sheets, forming tight interactions. Next, C was repeatedly immersed in the MnO x @GO slurry. After drying, MnO x @GO/C was obtained. Finally, MnO x @rGO/C was formed by thermal carbonization under Ar purge for heat reduction of GO to rGO. For electrochemical measurements, one piece of MnO x @rGO/C electrode was immersed into Na 2 SO 4 aqueous electrolyte.
were grown in situ on GO sheets, forming tight interactions. Next, C was repeatedly immersed in the MnOx@GO slurry. After drying, MnOx@GO/C was obtained. Finally, MnOx@rGO/C was formed by thermal carbonization under Ar purge for heat reduction of GO to rGO. For electrochemical measurements, one piece of MnOx@rGO/C electrode was immersed into Na2SO4 aqueous electrolyte.  [41]. Figure 2b shows the C 1s spectrum of MnOx@rGO(0.8:1)/C; the spectrum reveals the presence of C=C (284.9 eV), which is consistent with the literature data [42]. The Mn 2p spectrum of MnOx@rGO(0.8:1)/C (Figure 2c) shows that the peaks of Mn 2p3/2 and Mn 2p1/2 are located at 642 eV and 653.9 eV, respectively, with an energy separation of 11.9 eV, which is in good agreement with the reported literature data of Mn 2p3/2 and Mn 2p1/2 in MnO [31]. Figure 2d reveals the O 1s spectrum of MnOx@rGO(0.8:1)/C, while the peaks located at 532 eV reveal the presence of an O-Mn bond [43]. In summary, the above XPS results further demonstrated the successful product of MnO. The compositional analysis of the C, MnO x @GO (0.8:1) /C, and MnO x @rGO (0.8:1) /C was characterized by XPS and the corresponding results are presented in Figure 2. As shown in Figure 2a, the XPS spectrum of C only presents two elements, namely, C and O. The sample of MnO x @GO (0.8:1) /C and MnO x @rGO (0.8:1) /C contains Mn, C, and O elements, proving the successful deposition of MnO on C. In contrast, the signal of manganese (Mn 2p) emerges in the XPS survey spectrum of MnO x @GO (0.8:1) /C and MnO x @rGO (0.8:1) /C, illustrating the attachment of MnO x on the surface of rGO sheets [41]. Figure 2b shows the C 1s spectrum of MnO x @rGO (0.8:1) /C; the spectrum reveals the presence of C=C (284.9 eV), which is consistent with the literature data [42]. The Mn 2p spectrum of MnO x @rGO (0.8:1) /C ( Figure 2c) shows that the peaks of Mn 2p 3/2 and Mn 2p 1/2 are located at 642 eV and 653.9 eV, respectively, with an energy separation of 11.9 eV, which is in good agreement with the reported literature data of Mn 2p 3/2 and Mn 2p 1/2 in MnO [31]. Figure 2d reveals the O 1s spectrum of MnO x @rGO (0.8:1) /C, while the peaks located at 532 eV reveal the presence of an O-Mn bond [43]. In summary, the above XPS results further demonstrated the successful product of MnO.
Next, Raman spectroscopy was utilized to investigate the graphitization and defect degree of the as-prepared sample. Figure 3 shows the Raman spectra of C, GO/C, rGO/C, MnO@GO (0.8:1) /C, and MnO@rGO (0.8:1) /C. The G band is associated to the highly ordered graphite of the samples. For GO/C, the G band at 1585 cm −1 is characteristic of sp 2hybridized C-C bonds [44]. The D band is attributed to the structural defects of the samples. Meanwhile, the D band located at 1329 cm −1 is corresponding to the defects and disorder carbon in the graphite layers [45]. The I D /I G values of GO/C, rGO/C, MnO@GO (0.8:1) /C, and MnO@rGO (0.8:1) /C are 1.016, 0.970, 1.074, and 1.054, respectively. Obviously, the defect degree of carbon materials in rGO/C is lower than that of GO/C. Additionally, the I D /I G values of MnO@GO (0.8:1) /C are larger than that of MnO@rGO (0.8:1) /C, and the adhesion between MnO and C in MnO@rGO (0.8:1) /C is relatively more tight than that in MnO@GO (0.8:1) /C. Next, Raman spectroscopy was utilized to investigate the graphitization and defect degree of the as-prepared sample. Figure 3 shows the Raman spectra of C, GO/C, rGO/C, MnO@GO(0.8:1)/C, and MnO@rGO(0.8:1)/C. The G band is associated to the highly ordered graphite of the samples. For GO/C, the G band at 1585 cm −1 is characteristic of sp 2 -hybridized C-C bonds [44]. The D band is attributed to the structural defects of the samples. Meanwhile, the D band located at 1329 cm −1 is corresponding to the defects and disorder carbon in the graphite layers [45]. The ID/IG values of GO/C, rGO/C, MnO@GO(0.8:1)/C, and MnO@rGO(0.8:1)/C are 1.016, 0.970, 1.074, and 1.054, respectively. Obviously, the defect degree of carbon materials in rGO/C is lower than that of GO/C. Additionally, the ID/IG values of MnO@GO(0.8:1)/C are larger than that of MnO@rGO(0.8:1)/C, and the adhesion between MnO and C in MnO@rGO(0.8:1)/C is relatively more tight than that in MnO@GO(0.8:1)/C.   Figure 4 depicts the XRD spectra of GO, rGO, and MnO@rGO(0.8:1). The X-ray diffraction spectrum of GO reflects a characteristic peak at 2θ = 11.4°, corresponding to (002) crystal plane of graphite [46]. In the case of rGO, a characteristic broad peak around 2θ = 24.5° was observed, which corresponds to the (002) plane of the graphite structure [47]. The diffraction peaks of MnO@rGO(0.8:1) at 2θ of 34.9°, 40.5°, 58.5°, 70.2°, 74°, and 88° cor-  Figure 4 depicts the XRD spectra of GO, rGO, and MnO@rGO (0.8:1) . The X-ray diffraction spectrum of GO reflects a characteristic peak at 2θ = 11.4 • , corresponding to (002) crystal plane of graphite [46]. In the case of rGO, a characteristic broad peak around 2θ = 24.5 • was observed, which corresponds to the (002) plane of the graphite structure [47].   Figure 4 depicts the XRD spectra of GO, rGO, and MnO@rGO(0.8:1). The X-ray tion spectrum of GO reflects a characteristic peak at 2θ = 11.4°, corresponding t crystal plane of graphite [46]. In the case of rGO, a characteristic broad peak arou 24.5° was observed, which corresponds to the (002) plane of the graphite structu The diffraction peaks of MnO@rGO(0.8:1) at 2θ of 34.9°, 40.5°, 58.5°, 70.2°, 74°, and 8 respond to the (111), (200), (220), (311), (222), and (400) lattice planes, respectivel cating the presence of MnO nanocrystals (JCPDs no. 07-0230) [26,31]. The peak rGO between rGO and MnO@rGO can be attributed to the reduced space betwe layers.   [48] can be seen in Figure 5d. Evenly distributed rGO [49] were well-connected on the surface of MnO@rGO (0.8:1) /C, as shown in Figure 5f. More MnO nanoparticles uniformly gathered and grew in situ on the surface of C with the aid of rGO [26].
Elemental mapping by EDS was used to illustrate the distribution of C, Mn, and O elements in MnO@GO (0.8:1) /C and MnO@rGO (0.8:1) /C, respectively, as shown in Figure 6a,b. 5a-e, respectively. Figure 5a,b shows the SEM images of C without any attached MnO@GO nanoparticles, while MnO@GO(0.8:1)/C ( Figure 5c) and MnO@rGO(0.8:1)/C ( Figure  5e) possess rougher surfaces compared to that of C. Moreover, GO nanosheets with clean and wrinkled surfaces [48] can be seen in Figure 5d. Evenly distributed rGO [49] were well-connected on the surface of MnO@rGO(0.8:1)/C, as shown in Figure 5f. More MnO nanoparticles uniformly gathered and grew in situ on the surface of C with the aid of rGO [26].    Figure 7a illustrates multi-layered wrinkled GO sheets, while wrinkles seem to have disappeared in rGO, as shown in Figure 7b. The high-resolution TEM image (Figure 7c) shows the periodic lattice fringe with an interplanar distance of about 0.26 nm, corresponding to the MnO (111) face [43]. It confirms the nanoflake morphology of MnO in MnO@rGO (0.8:1) /C. In addition, the element mapping (Figure 7d) shows an even distribution of C, Mn, and O elements in MnO@rGO (0.8:1) /C. We used TEM characterizations to further verify the structure of MnO@rGO(0.8:1)/C. Under the low magnification of GO, Figure 7a illustrates multi-layered wrinkled GO sheets, while wrinkles seem to have disappeared in rGO, as shown in Figure 7b. The highresolution TEM image (Figure 7c) shows the periodic lattice fringe with an interplanar distance of about 0.26 nm, corresponding to the MnO (111) face [43]. It confirms the nanoflake morphology of MnO in MnO@rGO(0.8:1)/C. In addition, the element mapping (Figure 7d) shows an even distribution of C, Mn, and O elements in MnO@rGO(0.8:1)/C.   The electrochemical performance as a function of the mass ratio of MnO to rGO in MnO@rGO/C composites at relatively low mass loading of active material (1.08 mg cm −2 ) is illustrated in Figure 9a-d. The typical CV curves of the prepared electrodes at 5 mV s −1 scan rate are presented in Figure 9a. Obviously, all the shapes of CV curves demonstrate near-rectangular shapes, which indicates good symmetrical reversible electrochemical reactions. Meanwhile, the CV curves of the MnO@rGO/C electrodes show indistinct redox peaks within the working voltage range, which may be caused by the electrical double layer charge storage of rGO, along with fast surface redox reactions between Na + from Na2SO4 electrolyte and MnO nanoflakes [50]. The areas of CV curves of MnO@rGO(0.5:1)/C,  The electrochemical performance as a function of the mass ratio of MnO to rGO in MnO@rGO/C composites at relatively low mass loading of active material (1.08 mg cm −2 ) is illustrated in Figure 9a-d. The typical CV curves of the prepared electrodes at 5 mV s −1 scan rate are presented in Figure 9a. Obviously, all the shapes of CV curves demonstrate near-rectangular shapes, which indicates good symmetrical reversible electrochemical reactions. Meanwhile, the CV curves of the MnO@rGO/C electrodes show indistinct redox peaks within the working voltage range, which may be caused by the electrical double layer charge storage of rGO, along with fast surface redox reactions between Na + from Na 2 SO 4 electrolyte and MnO nanoflakes [50].  GCD curves of the C, rGO/C, MnO@rGO (0.5:1) /C, MnO@rGO (0.8:1) /C, and MnO@rGO (1:1) /C electrodes have good charge-discharge performance and share nearly symmetrical triangular shapes, suggesting a high reversibility. The charge-discharge curves of C, rGO/C, and MnO@rGO/C electrode materials are approximately symmetrical, but the symmetry of their triangular charge-discharge curves becomes worse after the electrode materials are loaded with MnO. Moreover, the discharge time for MnO@rGO (0.8:1) /C is 1446 s, which is the longest of all the electrodes. IR drops are detectable in MnO@rGO/C electrodes, as low as 1.7 mV for the MnO@rGO (0.8:1) /C electrode, which is much smaller than that of the C electrode (21.3 mV), rGO/C electrode (11.3 mV), and for MnO@rGO (0.5:1) /C (2.8 mV) and 2 mV for MnO@rGO (1:1) /C (2 mV), respectively. The areal capacitance (C A ) value calculated based on the GCD curve reaches a maximum of 180.75 mF cm −2 for th4 MnO@rGO (0.8:1) /C electrode, which is twenty-one times that of the rGO/C electrode (8.35 mF cm −2 ). Both the low C A value and high IR drop of the rGO electrode may result from the stacking of rGO sheets, which impedes charge transfer and ion diffusion in rGO. However, for MnO@rGO (0.8:1) /C, MnO nanoflakes are well-distributed on rGO, which increases the spaces between rGO sheets, effectively alleviating the stacking problem and improving the charge storage ability.
The C A calculated from discharge curves is shown in Figure 9c. The maximum areal specific capacitances of the C, rGO/C, MnO@rGO (0.5:1) /C, MnO@rGO (0.8:1) /C and MnO@rGO (1:1) /C were 3.45 mF cm −2 , 8.35 mF cm −2 , 36.42 mF cm −2 , 180.75 mF cm −2 , and 135.71 mF cm −2 , respectively. The MnO@rGO (0.8:1) /C has a better area-specific capacitance than the other samples. This is believed to be caused by MnO on the carbon fiber completely covering the carbon fiber, forming a nanosheet structure that increases the specific surface area, thereby improving the electrode performance to collect electrons and increasing C A .
The electrochemical behavior of the C, rGO/C, MnO@rGO (0.5:1) /C, MnO@rGO (0.8:1) /C, and MnO@rGO (1:1) /C supercapacitors was further characterized by EIS. At a high frequency, the electron transfer limited avenue can be reflected, while at a low frequency, the diffusion process is displayed [51]. In Figure 9d, the MnO@rGO (0.8:1) /C displays a higher slope in the low-frequency region, which suggests that the MnO@rGO (0.8:1) /C has ideal charge storage capacitive behavior compared to that of the rGO/C, MnO@rGO (0.5:1) /C, MnO@rGO (0.8:1) /C, and MnO@rGO (1:1) /C. A close-up observation of the high-frequency region of the Nyquist plots revealed a semicircle with a small diameter followed by a short 45 • Warburg region, indicating low charge transfer resistance and fast ion diffusion within the MnO@rGO/C electrodes, which could be attributed to the high electrical conductivity and unique structure of MnO@rGO/C. Furthermore, as shown in Figure 10, the electrical conductivity of MnO@rGO (0.8:1) /C electrodes at different mass loadings of active material (1.08 mg cm −2 -5.40 mg cm −2 ) is 16.22 S cm −1 , 10.77 S cm −1 , 9.48 S cm −1 , 8.90 S cm −1 , respectively. The decreasing conductivity of MnO@rGO/C is due to the semiconductor properties of manganese oxide, resulting in poor electrical transport in electrodes. We then analyzed the effect of the mass loading of active material (MnO@rGO(0.8:1)) on the electrochemical performance of MnO@rGO(0.8:1)/C electrodes (1.08 mg cm −2 -5.40 mg cm −2 , Figure 11). From the near-symmetrical GCD curves (Figure 11a), it indicates that all MnO@rGO(0.8:1)/C electrodes possess electrochemical reversibility as well as excellent pseudo-capacitive characteristics.
More importantly, as shown in Figure 11b, the areal specific capacitances of the MnO@rGO(0.8:1)/C (1.08 mg cm −2 -5.40 mg cm −2 ) are 180.75 mF cm −2 , 571.75 mF cm −2 , 518.75 mF cm −2 , and 831.25 mF cm −2 , respectively, suggesting that MnO@rGO(0.8:1)/C (5.40 mg We then analyzed the effect of the mass loading of active material (MnO@rGO (0.8:1) ) on the electrochemical performance of MnO@rGO (0.8:1) /C electrodes (1.08 mg cm −2 -5.40 mg cm −2 , Figure 11). From the near-symmetrical GCD curves (Figure 11a), it indicates that all MnO@rGO (0.8:1) /C electrodes possess electrochemical reversibility as well as excellent pseudo-capacitive characteristics.  Additionally, cycling stability is also a vital performance index for supercapacitors. Figure 12 shows that the MnO@rGO(0.8:1)/C (5.40 mg cm −2 ) electrode has a long-term cycling stability of 97.2% capacitance retention after 10,000 charge-discharge cycles at 0.4 mA cm −2 current density. More importantly, as shown in Figure 11b, the areal specific capacitances of the MnO@rGO (0.8:1) /C (1.08 mg cm −2 -5.40 mg cm −2 ) are 180.75 mF cm −2 , 571.75 mF cm −2 , 518.75 mF cm −2 , and 831.25 mF cm −2 , respectively, suggesting that MnO@rGO (0.8:1) /C (5.40 mg cm −2 ) has the highest C A among all samples. It is believed to be caused by the high loading of MnO@rGO nanohybrids on the carbon fiber, forming a continuous nanostructure that increases the specific surface area. Therefore, it can enhance the electrode's ability to store more charges and electrons, thus increasing the area-specific capacitance. Manganese oxide has poor conductivity, which can lead to a low coulombic effect. The coulombic efficiency of the MnO@rGO (0.8:1) /C at 5.40 mg cm −2 mass loadings of active material was 0.44. This may be attributed to rGO covered by MnO, which hinders the penetration of electrolyte ions into the electrodes.
Additionally, cycling stability is also a vital performance index for supercapacitors. Figure 12 shows that the MnO@rGO (0.8:1) /C (5.40 mg cm −2 ) electrode has a long-term cycling stability of 97.2% capacitance retention after 10,000 charge-discharge cycles at 0.4 mA cm −2 current density. Considering flexible supercapacitor applications, we evaluated the mechanica bility and electrode resistance stability during the deformation of the as-pre MnO@rGO(0.8:1)/C electrode with the active mass loading of 5.40 mg cm −2 , which is es for wearable electronics. The MnO@rGO(0.8:1)/C electrode possesses superior mech flexibility, which can be restored to the original condition under 45°, 90°, and 180° be angles, as shown in Figure 13a. Meanwhile, the electrode resistance stability is dis in Figure 13b. We can see that the MnO@rGO(0.8:1)/C electrode shows less obvious d ration in its resistance. To verify the feasibility of our MnO@rGO(0.8:1)/C supercapacit practical applications, we assembled a type of MnO@rGO/C supercapacitor syst electrically connecting four pieces of MnO@rGO(0.8:1)/C supercapacitors in seri shown in Figure 13c, the integrated MnO@rGO(0.8:1)/C supercapacitor can provide e power output to light an electronic watch ( Figure 13a) and a thermo-hygrometer ( 13d). Considering flexible supercapacitor applications, we evaluated the mechanical flexibility and electrode resistance stability during the deformation of the as-prepared MnO@rGO (0.8:1) /C electrode with the active mass loading of 5.40 mg cm −2 , which is essential for wearable electronics. The MnO@rGO (0.8:1) /C electrode possesses superior mechanical flexibility, which can be restored to the original condition under 45 • , 90 • , and 180 • bending angles, as shown in Figure 13a. Meanwhile, the electrode resistance stability is displayed in Figure 13b. We can see that the MnO@rGO (0.8:1) /C electrode shows less obvious deterioration in its resistance. To verify the feasibility of our MnO@rGO (0.8:1)/ C supercapacitors for practical applications, we assembled a type of MnO@rGO/C supercapacitor system by electrically connecting four pieces of MnO@rGO (0.8:1) /C supercapacitors in series. As shown in Figure 13c, the integrated MnO@rGO (0.8:1) /C supercapacitor can provide enough power output to light an electronic watch ( Figure 13a) and a thermohygrometer (Figure 13d).

Conclusions
In summary, we report a simple approach for the fabrication of high-performance flexible carbon fabric combined with MnO and rGO. RGO can provide a path for electron transfer, which improves the surface contact of electrode and electrolyte. In the meantime, the presence of MnO can also reduce the aggregation of graphene. The optimized MnO@rGO(0.8:1)/C electrodes with 5.40 mg cm −2 mass loadings of active material supercapacitor electrode show a high areal specific capacitance of 831.25 mF cm −2 and good cyclic stability of 97.2% capacitance retention after 10,000 charge-discharge cycles. The direct coating of MnO@rGO on carbon fabrics method, with excellent electrochemical properties, provides a promising electrode material in the fields of wearable energy storage supercapacitors.
Author Contributions: Methodology, writing-original draft, Q.K.; investigation, Z.L., Y.F. and F.W.; software, C.Y. and Y.Z.; formal analysis, Q.K.; funding acquisition, Z.L. and Y.Z.; conceptualzation and supervision, Y.W.; writing-review and editing, K.Z. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The raw data presented in this study are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In summary, we report a simple approach for the fabrication of high-performance flexible carbon fabric combined with MnO and rGO. RGO can provide a path for electron transfer, which improves the surface contact of electrode and electrolyte. In the meantime, the presence of MnO can also reduce the aggregation of graphene. The optimized MnO@rGO (0.8:1) /C electrodes with 5.40 mg cm −2 mass loadings of active material supercapacitor electrode show a high areal specific capacitance of 831.25 mF cm −2 and good cyclic stability of 97.2% capacitance retention after 10,000 charge-discharge cycles. The direct coating of MnO@rGO on carbon fabrics method, with excellent electrochemical properties, provides a promising electrode material in the fields of wearable energy storage supercapacitors.

Data Availability Statement:
The raw data presented in this study are available upon request from the corresponding author.