## 3 Results and Discussion
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 \(\Omega\) and 2.5 \(\Omega\); 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\({}_{\rm ct}\)). The q-MC//CC electrode exhibits a much lower R\({}_{\rm ct}\) value (10.8 \(\Omega\)) than the MC//CC electrode (18.0 \(\Omega\)), 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\(\cdot\)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. 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\(\cdot\)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\(\cdot\)g\({}^{-1}\) at current densities of 1, 2, 3, and 5 A\(\cdot\)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\(\cdot\)kg\({}^{-1}\) at a power density of 500 W\(\cdot\)kg\({}^{-1}\) and retains 18.1 Wh\(\cdot\)kg\({}^{-1}\) even at a high power density of 2500 W\(\cdot\)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\(\cdot\)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\({}^{\rm o}\), 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). Figure 5: (**a**) Nyquist plots and (**b**) Cycling test of q-MC//CC and MC//CC along 5000 cycles at the scan rate of 50 mV\(\cdot\)s\({}^{-1}\) (insets show their SEM images after cycle test).