###### Abstract  
In this study, the nanocomposite of g-C\({}_{3}\)N\({}_{4}\) quantum dots/MnCO\({}_{3}\) 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-C\({}_{3}\)N\({}_{4}\) quantum dots could effectively improve the interface electrical conductivity and ion transportation of the MnCO\({}_{3}\) 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. MnCO\({}_{3}\); carbon cloth; g-C\({}_{3}\)N\({}_{4}\) quantum dots; supercapacitors +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_
drawbacks, an effective approach is to hybridize MnCO\({}_{3}\) using conductive carbon-based additives such as carbon [8], CNTs [9; 10], and graphene [11; 12; 13; 14], which have proved to be effective to improve electrochemical performance. Graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\)) recently attracted a lot of interest as a promising electrode candidate for electrochemical devices due to its high nitrogen content, good structural stability, and low interfacial impedance. Thus, g-C\({}_{3}\)N\({}_{4}\) was employed in the transition metal oxides or hydroxide-based electrodes for supercapacitors. [PERSON] et al. [15] synthesized novel sandwich-like MnO\({}_{2}\)/g-C\({}_{3}\)N\({}_{4}\) composites for supercapacitor electrodes and showed the enhanced electrochemical performance, which was ascribed to various active sites and the existence of N of g-C\({}_{3}\)N\({}_{4}\) for better wettability with electrolytes. [PERSON] et al. [16] reported the endurable supercapacitor using MnO\({}_{2}\)/protonated g-C\({}_{3}\)N\({}_{4}\). It showed that the strong hydrogen bonding between MnO\({}_{2}\) and the g-C\({}_{3}\)N\({}_{4}\) can restrain particle agglomeration in the cycling test, resulting in stable energy storage performance. [PERSON] et al. [17] fabricated the honeycomb nanostructured g-C\({}_{3}\)N\({}_{4}\)@Ni(OH)\({}_{2}\) for an asymmetric supercapacitor, where the g-C\({}_{3}\)N\({}_{4}\) showed a good charge transport with good chemical stability. However, there are limited reports studying the application of g-C\({}_{3}\)N\({}_{4}\) quantum dots (CNQDs) in energy storage fields. Herein, we report a simple hydrothermal process to prepare the nanostructured MnCO\({}_{3}\) and CNQDs nanocomposites on carbon cloth (denoted as q-MC/CC). The active material of MnCO\({}_{3}\) and CNQDs nanocomposites is synthesized directly on the carbon cloth as a flexible conductive substrate, which is then also used for an electrode of a solid-state flexible asymmetric supercapacitor (ASC). MnCO\({}_{3}\) and CNQDs nanocomposites exhibit excellent electrochemical performance, including good specific capacitance and rate capability, which is attributed to the enhanced electrical conductivity and shorter ions pathway. Moreover, CNQDs have generally large surface areas and accessible edges that can function as active sites of electrochemical reaction, which would improve the electrochemical performance. The q-MC/CC electrode delivers a high specific capacitance of 1001.4 F-g\({}^{-1}\) under a current density of 1 A-g\({}^{-1}\) as well as good cycling performance (96% capacity retention). Furthermore, a solid-state binder-free flexible ASC using q-MC/CC electrode is demonstrated, showing a high energy density of 27.1 Wh\(\cdot\)kg\({}^{-1}\) at a power density of 500 W\(\cdot\)kg\({}^{-1}\) and good mechanical stability.