Fabrication of g-C3N4 Quantum Dots/MnCO3 Nanocomposite on Carbon Cloth for Flexible Supercapacitor Electrode

In this study, the nanocomposite of g-C3N4 quantum dots/MnCO3 on carbon cloth (q-MC//CC) is prepared via a simple hydrothermal method. The obtained q-MC//CC composite is employed for a flexible supercapacitor electrode. The g-C3N4 quantum dots could effectively improve the interface electrical conductivity and ion transportation of the MnCO3 electrode, which results in superior electrochemical performance. The q-MC//CC electrode delivers a high specific capacity of 1001 F·g−1 at a current density of 1 A·g−1 and a good cycling performance of 96% capacity retention after 5000 cycles. Moreover, an asymmetric flexible supercapacitor (ASC) is assembled with q-MC//CC and carbon cloth as a positive and negative electrode, respectively, which exhibits a high energy density of 27.1 Wh·kg−1 at a power density of 500 W·kg−1. In addition, the fabricated ASC device demonstrates the ability to power the light-emitting diode effectively under mechanical bending.


Introduction
Supercapacitors have attracted considerable attention for the application of advanced energy storage devices of electronic vehicles and portable electronic devices due to their beneficial properties such as excellent power density, rapid charging time, and durability [1]. Electric double layer capacitors (EDLC) mainly use various carbon-based materials, such as activated carbon, carbon nanotubes (CNTs), and graphene. In EDLC, an accumulation of ionic charges occurs between the electrode and electrolyte of carbon-based electrodes, and carbon-based electrodes with large surface areas deliver high power density and long cycle stability. On the other hand, pseudocapacitors can exhibit substantially high specific capacitance and high energy density compared to EDLC, owing to the fast and reversible surface Faradaic redox reaction. In addition, transition metal oxide and carbonate materials have been explored as electrode materials of pseudocapacitors due to their high theoretical storage capacity and superior reversibility. Among them, Mn-based active materials have attracted a great deal of interest owing to the high theoretical capacitance, low cost, and natural abundance [2][3][4][5]. However, most research has been done with Mn-based oxides, and manganese carbonates (MnCO 3 ) are relatively new and popular active materials for pseudocapacitors. Recently, MnCO 3 has been extensively studied as a potential electrode of supercapacitors due to the low manganese valence state of +2 and the redox process to high manganese valence states (+3 or +4), which could show high electrochemical performance. The low manganese valence (+2) in MnCO 3 can exhibit a better hydroxyl ions insertion of alkaline electrolytes for the formation of MnOHCO 3 compared to high manganese valence states (+3 or +4) [5][6][7]. However, low electrical conductivity and poor cycle stability restrict the application of MnCO 3 as an active material in energy storage applications. To overcome these

Preparation of g-C 3 N 4 Quantum Dots
First, 5 g of melamine were calcinated in a muffle furnace at 550 • C for 4 h to obtain bulk g-C 3 N 4 . Then, 2 g of bulk g-C 3 N 4 were treated in a mixture solution of sulfuric acid (40 mL) and nitric acid (40 mL) for about 2 h at room temperature. Then, the mixture was rinsed several times with deionized (DI) water, and the as-obtained white product was dispersed in the concentrated NH 3 ·H 2 O. The mixture solution was transferred into a Teflon-lined stainless-steel autoclave with heating at 180 • C for 12 h. After the hydrothermal process, the precipitate was rinsed with DI water to remove residual NH 3 molecules. Subsequently, the treated g-C 3 N 4 nanosheets were dispersed and sonicated in 100 mL water for 6 h, and the as-obtained aqueous suspension was centrifuged at 7000 rpm and dialyzed in a dialysis bag (3000 Da, Spectrum Lab. Inc) for 48 h to remove large particles. Finally, the CNQDs solution was obtained.

Synthesis of q-MC//CC Nanocomposites
First, carbon cloth was treated by immersing in a mixture acid solution (H 2 SO 4 :HNO 3 = 1:3) at room temperature for 24 h and then washed with DI water. For the typical synthesis of q-MC//CC nanocomposites (Figure 1), MnSO 4 ·H 2 O (3.38 g) was dissolved in DI water (134 mL), and NH 4 F (0.6 g) was added to the above solution with stirring to form a transparent solution. After several minutes, urea (2.4 g) and 5 mg of CNQDs was added to the above homogeneous solution with 30 min stirring at room temperature. Then, the treated carbon cloth (3 × 1 cm 2 ) was immersed into the mixture solution and treated with the hydrothermal process, maintaining at 120 • C for 4 h. The obtained q-MC//CC nanocomposites were washed with DI water to remove the residual reactants and dried at 60 • C. As a control sample, the MnCO 3 on carbon cloth sample was also synthesized by following the same procedure without adding CNQDs (denoted as MC//CC).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 10 60 °C. As a control sample, the MnCO3 on carbon cloth sample was also synthesized by following the same procedure without adding CNQDs (denoted as MC//CC).

Electrochemical Measurements
Electrochemical performance was investigated by a cyclic voltammetry (CV) test, galvanostatic charge/discharge (GCD) test, and electrochemical impedance spectroscopy (EIS) test. All electrochemical characterizations were carried out using a BioLogic VSP electrochemical workstation at room temperature. A three-electrode system was used to test the electrochemical performance of the prepared electrodes with platinum wire as a counter electrode and Ag/AgCl electrodes as a reference electrode. The obtained products such as MC//CC and q-MC//CC are used as a binder-free working electrode. The mass loading of active materials was about 2.5 mg in this work. All electrochemical characterizations were measured in an aqueous electrolyte of 1 M Na2SO4 at room temperature. EIS was performed at open circuit potential in a frequency range from 10 to 0.01 Hz with an AC amplitude of 5 mV. The solid-state flexible ASC was prepared using the q-MC//CC as a positive electrode and carbon cloth as a negative electrode, which were assembled with polyvinyl alcohol (PVA)/Na2SO4 gel electrolyte and a separator. The prepared two electrodes were wetted with the PVA/Na2SO4 gel electrolyte for 10 min and then assembled face to face with a separator. The specific capacitance (Cs) was calculated from the CV curves and galvanostatic charge-discharge curves using the following equations, respectively: is the area enclosed by the CV curves, ΔV (V) is the potential window, m (g) is the mass of electrode, ν (mV s −1 ) is the potential scan rate, and Δt (s) is the discharge time.

Characterization
A transmission electron microscope (TEM, H-8100, Hitachi, Tokyo, Japan) and a field-emission scanning electron microscope (FE-SEM, JEOL-JSM820, JEOL Ltd., Akishima-shi, Japan) were used to investigate the morphology of the obtained samples. X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Fisher, Waltham, MA, USA) measurements were carried out using monochromatic AlKα radiation (hν = 1486.6 eV). Contact angle measurements with electrolyte (1.0 M Na 2 SO 4 ) droplets were performed to investigate the wettability of the q-MC//CC electrode using an Attension Theta Optical Tensiometer (Biolin Scientific AB, Stockholm, Sweden). Fourier transform infrared (FTIR) spectra were collected using a Nicolet IR 200 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA), and Raman spectroscopy (DXR Raman spectrometer, Thermo Scientific, Madison, WI, USA) was performed with a 532 nm excitation source.

Electrochemical Measurements
Electrochemical performance was investigated by a cyclic voltammetry (CV) test, galvanostatic charge/discharge (GCD) test, and electrochemical impedance spectroscopy (EIS) test. All electrochemical characterizations were carried out using a BioLogic VSP electrochemical workstation at room temperature. A three-electrode system was used to test the electrochemical performance of the prepared electrodes with platinum wire as a counter electrode and Ag/AgCl electrodes as a reference electrode. The obtained products such as MC//CC and q-MC//CC are used as a binder-free working electrode. The mass loading of active materials was about 2.5 mg in this work. All electrochemical characterizations were measured in an aqueous electrolyte of 1 M Na 2 SO 4 at room temperature. EIS was performed at open circuit potential in a frequency range from 10 to 0.01 Hz with an AC amplitude of 5 mV. The solid-state flexible ASC was prepared using the q-MC//CC as a positive electrode and carbon cloth as a negative electrode, which were assembled with polyvinyl alcohol (PVA)/Na 2 SO 4 gel electrolyte and a separator. The prepared two electrodes were wetted with the PVA/Na 2 SO 4 gel electrolyte for 10 min and then assembled face to face with a separator. The specific capacitance (Cs) was calculated from the CV curves and galvanostatic charge-discharge curves using the following equations, respectively: where I(V) dV is the area enclosed by the CV curves, ∆V (V) is the potential window, m (g) is the mass of electrode, ν (mV s −1 ) is the potential scan rate, and ∆t (s) is the discharge time. Figure 2a,b show FE-SEM images of as-prepared q-MC//CC nanocomposites. The pristine carbon cloth shows a smooth surface consisting of aligned carbon fiber bundles. After the hydrothermal reaction of the synthesis of q-MC//CC, it is observed that clusters of spindle-shaped petals of MnCO 3 were densely grown on the surface of carbon fibers. In addition, the prepared q-MC//CC sample shows good flexibility, as shown in the inset of Figure 2a. The morphology of MC//CC ( Figure S1) has a similar nanostructure to that of q-MC//CC, showing that the addition of CNQDs has no effect on the morphological formation of MnCO 3 . The TEM image (Figure 2c) of the q-MC//CC confirms that the CNQDs are uniformly embedded in MnCO 3 , and the lattice fringe of CNQDs is found in the high resolution TEM (HRTEM) image (Figure 2d), where a lattice distance is 0.336 nm, corresponding to the (002) planes of g-C 3 N 4 . The SEM image of q-MC//CC and its elemental mapping from energy-dispersive X-ray spectroscopy (EDS) reveal the existence of the expected atomic elements (Figure 2e), which clearly confirms that Mn, C, and N elements are uniformly distributed on the MnCO 3 surface and the successful formation of q-MC//CC nanocomposite. In addition, the obtained q-MC//CC has a larger surface area (182 m 2 /g) than the pristine carbon cloth (69 m 2 /g) due to its unique nanostructure ( Figure S2).

Results and Discussion
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 10 Figure 2a and b show FE-SEM images of as-prepared q-MC//CC nanocomposites. The pristine carbon cloth shows a smooth surface consisting of aligned carbon fiber bundles. After the hydrothermal reaction of the synthesis of q-MC//CC, it is observed that clusters of spindle-shaped petals of MnCO3 were densely grown on the surface of carbon fibers. In addition, the prepared q-MC//CC sample shows good flexibility, as shown in the inset of Figure 2a. The morphology of MC//CC ( Figure S1) has a similar nanostructure to that of q-MC//CC, showing that the addition of CNQDs has no effect on the morphological formation of MnCO3. The TEM image (Figure 2c) of the q-MC//CC confirms that the CNQDs are uniformly embedded in MnCO3, and the lattice fringe of CNQDs is found in the high resolution TEM (HRTEM) image (Figure 2d), where a lattice distance is 0.336 nm, corresponding to the (002) planes of g-C3N4. The SEM image of q-MC//CC and its elemental mapping from energy-dispersive X-ray spectroscopy (EDS) reveal the existence of the expected atomic elements (Figure 2e), which clearly confirms that Mn, C, and N elements are uniformly distributed on the MnCO3 surface and the successful formation of q-MC//CC nanocomposite. In addition, the obtained q-MC//CC has a larger surface area (182 m 2 /g) than the pristine carbon cloth (69 m 2 /g) due to its unique nanostructure ( Figure S2).  Figure 3a show that both q-MC//CC and MC//CC samples contain the Mn, O, and C atoms, while the N peak is only found in the q-MC//CC nanocomposites due to CNQDs. The deconvolution of the Mn 2p spectrum of q-MC//CC (Figure 3b) displays two peaks located at 655.8 eV and 643.8 eV respectively, assigning to Mn 2p1/2 and Mn 2p3/2 with an energy separation of 12.0 eV, which coincides with Mn 2+ [18,19]. In the high-resolution C1s spectrum of q-MC//CC ( Figure 3c) and MC//CC ( Figure S3), two typical peaks of the C-C bond at 284.6 eV and C=O bond at 288.5 eV are observed, relating to the surface carbon of the carbon cloth and carbon element in carbonate ions [20]. In addition, two additional peaks at 286.0 and 292.7 eV are only observed for q-MC//CC, which correspond to the C-N bond and carboxylate bond of CNQDs, respectively. In addition, the highresolution N 1s spectrum of q-MC//CC can be fitted into three peaks, as shown in Figure 3d Figure 3a show that both q-MC//CC and MC//CC samples contain the Mn, O, and C atoms, while the N peak is only found in the q-MC//CC nanocomposites due to CNQDs. The deconvolution of the Mn 2p spectrum of q-MC//CC (Figure 3b) displays two peaks located at 655.8 eV and 643.8 eV respectively, assigning to Mn 2p 1/2 and Mn 2p 3/2 with an energy separation of 12.0 eV, which coincides with Mn 2+ [18,19]. In the high-resolution C1s spectrum of q-MC//CC ( Figure 3c) and MC//CC ( Figure S3), two typical peaks of the C-C bond at 284.6 eV and C=O bond at 288.5 eV are observed, relating to the surface carbon of the carbon cloth and carbon element in carbonate ions [20]. In addition, two additional peaks at 286.0 and 292.7 eV are only observed for q-MC//CC, which correspond to the C-N bond and carboxylate bond of CNQDs, respectively.

Results and Discussion
In addition, the high-resolution N 1s spectrum of q-MC//CC can be fitted into three peaks, as shown in Figure 3d, corresponding to =N-(398.4 eV), C=N-C (399.1 eV), and N-(C) 3 (400.4 eV) [21], indicating the presence of CNQDs in the q-MC//CC nanocomposite. The contact angle measurements with 1 M Na 2 SO 4 electrolyte were performed to study the effect of CNQDs on the surface wettability of the prepared q-MC//CC composite. Figure S4 illustrates the images of electrolyte droplets on the prepared MC//CC and q-MC//CC composites, respectively. The q-MC//CC composite shows a lower contact angle with Na 2 SO 4 electrolyte than MC//CC, which is due to the more ion-accessible surfaces by CNQDs. This enhanced surface wettability of the q-MC//CC composite facilitates the accessibility of electrolyte, leading to better electrochemical performance compared to the MC//CC composite. In addition, the FTIR spectra of the as-prepared q-MC//CC and MC//CC are shown in Figure S5. The characteristic peak of the Mn-O-Mn bond is observed at 727 cm −1 for both samples, while that of the characteristic peaks corresponding to the C-N (1033 cm −1 ) and C=N bonds (1780cm −1 ) are only found for q-MC//CC due to the existence of CNQDs.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 10 electrolyte were performed to study the effect of CNQDs on the surface wettability of the prepared q-MC//CC composite. Figure S4 illustrates the images of electrolyte droplets on the prepared MC//CC and q-MC//CC composites, respectively. The q-MC//CC composite shows a lower contact angle with Na2SO4 electrolyte than MC//CC, which is due to the more ion-accessible surfaces by CNQDs. This enhanced surface wettability of the q-MC//CC composite facilitates the accessibility of electrolyte, leading to better electrochemical performance compared to the MC//CC composite. In addition, the FTIR spectra of the as-prepared q-MC//CC and MC//CC are shown in Figure S5. The characteristic peak of the Mn-O-Mn bond is observed at 727 cm −1 for both samples, while that of the characteristic peaks corresponding to the C-N (1033 cm −1 ) and C=N bonds (1780cm −1 ) are only found for q-MC//CC due to the existence of CNQDs. The electrochemical performances of q-MC//CC composites as binder-free electrodes of supercapacitors are studied using CV and GCD characterization in 1 M Na2SO4 aqueous electrolyte using a three-electrode system. Figure 4a shows the CV curves of q-MC//CC and MC//CC measured at a scan rate of 5 mV·s −1 with potentials ranging from −0.2 to 0.8 V. CV curves of both electrodes show rectangular shapes in a potential window, indicating excellent capacitive behavior. The larger integral area of the CV curve for q-MC//CC electrode suggests higher specific capacitance than the MC//CC electrode, which represents its superior electrochemical property due to the synergistic effects of CNQDs and MnCO3. The CV curves of q-MC//CC measured at different scan rates from 5 mV·s −1 to 100 mV·s −1 are shown in Figure 4b. The CV curve shapes are maintained upon increasing the scan rate, revealing fast ionic and electronic transportation. The current response also simultaneously increases along with the scan rate, showing a diffusion-controlled phenomenon. The GCD measurement of the q-MC//CC electrode was performed at different current density (Figure 4c). All GCD curves show some linear and symmetric behavior, indicating capacitive characteristics and good reversibility of the q-MC//CC electrode. In addition, a noticeable potential drop at the discharge plot is not observed, suggesting the low internal resistance of the q-MC//CC electrode. The specific The electrochemical performances of q-MC//CC composites as binder-free electrodes of supercapacitors are studied using CV and GCD characterization in 1 M Na 2 SO 4 aqueous electrolyte using a three-electrode system. Figure 4a shows the CV curves of q-MC//CC and MC//CC measured at a scan rate of 5 mV·s −1 with potentials ranging from −0.2 to 0.8 V. CV curves of both electrodes show rectangular shapes in a potential window, indicating excellent capacitive behavior. The larger integral area of the CV curve for q-MC//CC electrode suggests higher specific capacitance than the MC//CC electrode, which represents its superior electrochemical property due to the synergistic effects of CNQDs and MnCO 3 . The CV curves of q-MC//CC measured at different scan rates from 5 mV·s −1 to 100 mV·s −1 are shown in Figure 4b. The CV curve shapes are maintained upon increasing the scan rate, revealing fast ionic and electronic transportation. The current response also simultaneously increases along with the scan rate, showing a diffusion-controlled phenomenon. The GCD measurement of the q-MC//CC electrode was performed at different current density (Figure 4c). All GCD curves show some linear and symmetric behavior, indicating capacitive characteristics and good reversibility of the q-MC//CC electrode. In addition, a noticeable potential drop at the discharge plot is not observed, suggesting the low internal resistance of the q-MC//CC electrode. The specific capacitance decreases with increasing current densities, due to slow ionic diffusion at high current. Figure 4d compares the calculated specific capacitance of q-MC//CC and MC//CC electrodes at current densities. It demonstrates the superior capacitive performance of q-MC//CC compared to the MC//CC electrode by the incorporation of CNQDs. The calculated specific capacitances of the q-MC//CC electrode are 1001.4, 609.3, 469.8, and 322.6 F·g −1 at 1, 2, 3, and 5 A·g −1 , respectively. The charge storage mechanism of MnCO 3 involves the intercalation/de-intercalation of Na + with the redox reactions of the Mn ions. In addition, a surface process of the adsorption/desorption of Na + ions occurs. The redox reaction of Mn (II) ↔ Mn (I) occurs in Na 2 SO 4 electrolyte: MnCO 3 + Na + + e − ↔ NaMnCO 3 . CNQDs on MnCO 3 can facilitate the intercalation/de-intercalation of Na+ ions and the redox reaction of Mn ions. This excellent electrochemical performance of the q-MC//CC electrode is highly comparable with previously reported MnCO 3 -based electrodes (Table S1).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 10 capacitance decreases with increasing current densities, due to slow ionic diffusion at high current.  (Table S1). To further investigate the kinetics of ion and charge transfer, an EIS test was performed, and the Nyquist plots of q-MC//CC and MC//CC are presented in Figure 5a. The plots consist of a depressed circle at the high-frequency region and a straight line at the low-frequency region. The depressed is attributed to the charge transfer process of the Faradaic reaction at the interface of the electrode and electrolyte, and it belongs to the kinetic control, while the low-frequency region belongs to the mass transfer control. The formation of a depressed circle due to the roughness of the electrode/electrolyte surface results in the change of capacitance in double layer. In addition, the high slope of observed straight lines at the low-frequency region represents a small Warburg resistance. At the lowfrequency region, the linear shape suggests the fast ion diffusion in electrolyte and the adsorption process at the electrode surface. The solution resistances (Rs) of q-MC//CC and MC//CC are 2.1 Ω and 2.5 Ω; the as-prepared q-MC//CC enhances electron transport from electrode to current collector, implying the decrease in equivalent series resistance by the strong synergistic effect between MnCO3 and CNQDs. The diameter of the depressed circle denotes the charge transfer resistance (Rct). The q- To further investigate the kinetics of ion and charge transfer, an EIS test was performed, and the Nyquist plots of q-MC//CC and MC//CC are presented in Figure 5a. The plots consist of a depressed circle at the high-frequency region and a straight line at the low-frequency region. The depressed is attributed to the charge transfer process of the Faradaic reaction at the interface of the electrode and electrolyte, and it belongs to the kinetic control, while the low-frequency region belongs to the mass transfer control. The formation of a depressed circle due to the roughness of the electrode/electrolyte surface results in the change of capacitance in double layer. In addition, the high slope of observed straight lines at the low-frequency region represents a small Warburg resistance. At the low-frequency region, the linear shape suggests the fast ion diffusion in electrolyte and the adsorption process at the electrode surface. The solution resistances (Rs) of q-MC//CC and MC//CC are 2.1 Ω and 2.5 Ω; the as-prepared q-MC//CC enhances electron transport from electrode to current collector, implying the decrease in equivalent series resistance by the strong synergistic effect between MnCO 3 and CNQDs. The diameter of the depressed circle denotes the charge transfer resistance (R ct ). The q-MC//CC electrode exhibits a much lower R ct value (10.8 Ω) than the MC//CC electrode (18.0 Ω), clearly revealing that the highly conductive interface between CNQDs and MnCO 3 significantly improves the charge transfer kinetics. The long-term cycle stability of q-MC//CC and MC//CC electrodes was examined by CV measurement for 5000 cycles at a scan rate of 50 mV·s −1 . As shown in Figure 5b, the q-MC//CC electrode exhibits superior cycle performance with a 96% retention of its initial specific capacitance compared to the MC//CC electrode (71%). These results may be attributed to the distribution of CNQDs on MnCO 3 , which can suppress the deformation and damage of the structure. As shown in the insert images in Figure 5b, the q-MC//CC electrode retains its initial microstructure after cycling, while the MC//CC electrode shows a swollen structure, revealing the excellent cycle stability of the q-MC//CC electrode.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 10 revealing that the highly conductive interface between CNQDs and MnCO3 significantly improves the charge transfer kinetics. The long-term cycle stability of q-MC//CC and MC//CC electrodes was examined by CV measurement for 5000 cycles at a scan rate of 50 mV·s −1 . As shown in Figure 5b, the q-MC//CC electrode exhibits superior cycle performance with a 96% retention of its initial specific capacitance compared to the MC//CC electrode (71%). These results may be attributed to the distribution of CNQDs on MnCO3, which can suppress the deformation and damage of the structure. As shown in the insert images in Figure 5b, the q-MC//CC electrode retains its initial microstructure after cycling, while the MC//CC electrode shows a swollen structure, revealing the excellent cycle stability of the q-MC//CC electrode. To study the practical application of q-MC//CC as a binder-free electrode, an all-solid-state asymmetric supercapacitor (ASC) device was constructed. The q-MC//CC electrode was the positive electrode and a carbon cloth was the negative electrode, which were then assembled with a polyvinyl alcohol (PVA)/Na2SO4 gel electrolyte and separator (Figure 6a). The CV curves measured at different scan rates are shown in Figure 6b. All CV curves present typical quasi-rectangular shapes without any obvious redox peaks, indicating the ideal capacitive behavior of the prepared ASC with the q-MC//CC electrode. Figure 6c presents the GCD curves at different current densities from 1 to 5 A·g −1 . The nearly symmetric charge-discharge curves imply the high reversibility of the q-MC//CC electrode. The specific capacitances of ASC using the q-MC//CC electrode obtained from the curves are 195, 168, 147, and 130 F·g −1 at current densities of 1, 2, 3, and 5 A·g −1 , respectively. The energy density of the assembled ASC is calculated and compared with other previous Mn-based ASC devices in the Ragone plot, as shown in Figure 6d. The prepared q-MC//CC electrode ASC delivers an energy density of 27.1 Wh·kg −1 at a power density of 500 W·kg −1 and retains 18.1 Wh·kg −1 even at a high power density of 2500 W·kg −1 , which is superior to other systems reported [22][23][24][25][26][27]. To explore mechanical stability, the CV curves of the all-solid-state ASC using q-MC//CC are measured under different bending angles at a scan rate of 50 mV·s −1 . The results in Figure 6e reveal that the CV curve shapes are well maintained with no remarkable changes, even at a bending angle of 180 o , which reveals the good mechanical stability of the a-MC//CC electrode during bending. In addition, the application potential of the assembled ASC is further explored. Figure 6f shows that the ASC device under a flat and folded state lights up the green light-emitting diode (LED). To study the practical application of q-MC//CC as a binder-free electrode, an all-solid-state asymmetric supercapacitor (ASC) device was constructed. The q-MC//CC electrode was the positive electrode and a carbon cloth was the negative electrode, which were then assembled with a polyvinyl alcohol (PVA)/Na 2 SO 4 gel electrolyte and separator (Figure 6a). The CV curves measured at different scan rates are shown in Figure 6b. All CV curves present typical quasi-rectangular shapes without any obvious redox peaks, indicating the ideal capacitive behavior of the prepared ASC with the q-MC//CC electrode. Figure 6c presents the GCD curves at different current densities from 1 to 5 A·g −1 . The nearly symmetric charge-discharge curves imply the high reversibility of the q-MC//CC electrode. The specific capacitances of ASC using the q-MC//CC electrode obtained from the curves are 195, 168, 147, and 130 F·g −1 at current densities of 1, 2, 3, and 5 A·g −1 , respectively. The energy density of the assembled ASC is calculated and compared with other previous Mn-based ASC devices in the Ragone plot, as shown in Figure 6d. The prepared q-MC//CC electrode ASC delivers an energy density of 27.1 Wh·kg −1 at a power density of 500 W·kg −1 and retains 18.1 Wh·kg −1 even at a high power density of 2500 W·kg −1 , which is superior to other systems reported [22][23][24][25][26][27]. To explore mechanical stability, the CV curves of the all-solid-state ASC using q-MC//CC are measured under different bending angles at a scan rate of 50 mV·s −1 . The results in Figure 6e reveal that the CV curve shapes are well maintained with no remarkable changes, even at a bending angle of 180 o , which reveals the good mechanical stability of the a-MC//CC electrode during bending. In addition, the application potential of the assembled ASC is further explored. Figure 6f shows that the ASC device under a flat and folded state lights up the green light-emitting diode (LED).

Conclusions
In summary, q-MC//CC for a binder-free electrode of supercapacitor was successfully prepared via a simple hydrothermal method. The synergistic effects of CNQDs embedded on MnCO3 can improve the interface electrical conductivity and shorten the ion diffusion paths for the utilization of MnCO3. The binder-free q-MC//CC electrode shows enhanced supercapacitive performance, including a high specific capacitance of 1001.4 F·g −1 at a current density 1 A·g −1 and a superior cycling performance of 96% retention after 5000 cycles. In addition, the q-MC//CC electrode-based ASC was assembled with a gel electrolyte, which exhibited a high energy density of 27.1 Wh·kg −1 with a power density of 500 W·kg −1 with good mechanical stability. These results provide a promising alternative method to prepare a binder-free electrode for high-performance flexible supercapacitors.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: SEM images of as-synthesized MC//CC, Figure S2. N2 adsorption/desorption isotherms for CC and q-MC//CC composite, Figure S3. High resolution C 1s XPS spectrum of MC//CC, Figure S4. Image of the contact angle measurement of (a) MC//CC and (b) q-MC//CC with 1M Na2SO4 electrolyte, Figure S5. FT-IR spectrum for q-MC//CC and MC//CC, Table S1. Comparison of the electrochemical performance of q-MC//CC with previous reports.

Conclusions
In summary, q-MC//CC for a binder-free electrode of supercapacitor was successfully prepared via a simple hydrothermal method. The synergistic effects of CNQDs embedded on MnCO 3 can improve the interface electrical conductivity and shorten the ion diffusion paths for the utilization of MnCO 3 . The binder-free q-MC//CC electrode shows enhanced supercapacitive performance, including a high specific capacitance of 1001.4 F·g −1 at a current density 1 A·g −1 and a superior cycling performance of 96% retention after 5000 cycles. In addition, the q-MC//CC electrode-based ASC was assembled with a gel electrolyte, which exhibited a high energy density of 27.1 Wh·kg −1 with a power density of 500 W·kg −1 with good mechanical stability. These results provide a promising alternative method to prepare a binder-free electrode for high-performance flexible supercapacitors.