## 3 Results and Discussion
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\(\cdot\)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\(\cdot\)s\({}^{-1}\) to 100 mV\(\cdot\)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  
Figure 3: **(a)** XPS full survey scan of q-MC//CC and MC//CC. **(b–d)** High-resolution XPS of Mn 2p, C 1s, and N 1s of q-MC//CC, respectively. 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\(\cdot\)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) \(\leftrightarrow\) Mn (I) occurs in Na\({}_{2}\)SO\({}_{4}\) electrolyte: MnCO\({}_{3}\)+ Na\({}^{+}\)+ e\({}^{-}\)\(\leftrightarrow\) 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). 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  
Figure 4: (a) Cyclic voltammetry (CV) curves of q-MC//CC and MC//CC. (b) CV curves of q-MC//CC at different scan rates. (c) Galvanostatic charge–discharge curves of q-MC//CC at different current densities. (d) Gavimetric specific capacitance of q-MC//CC at different current densities.