Application of Carbon-/Graphene Quantum Dots for Supercapacitors

Supercapacitors have attracted considerable attention as new-generation energy storage devices because of their high charge-discharge rate, ultralong lifetime, and high power density. However, the performance of supercapacitors is severely restricted by either the low intrinsic capacitance of porous carbons or the poor conductivity and sluggish electrochemical kinetics of pseudocapacitive components. Therefore, high-performance electrode materials integrated with high gravimetric and volumetric capacitances, high rate capability, and superb cycling stability are urgently needed. As emerging carbon nanomaterials, carbon-/graphene quantum dots (CDs/GQDs) have uniquely small particle sizes, abundant edge sites, and various functional groups, thus endowing them with great potential for developing high-performance electrode materials for supercapacitors. With the purpose of identifying the application advantages and critical problems of CDs/GQDs for supercapacitor electrodes, this review summarizes the development of CDs/GQDs, quantum dots (QDs)/conductive carbon, QDs/metal oxides, QDs/conductive polymer composites, and QD-derived carbon materials in recent years. In each section of this paper, we introduce the typical and updated studies from corresponding fields in terms of novel preparation routes, the crucial roles of CDs/GQDs in composite materials, and the electrochemical performance of electrode materials and assembled devices. Finally, the advantages and limitations of CD/GQD electrode materials are described, and the future development of QD-based materials is discussed. In general, the previous studies have shown that when directly used as electrode materials for micro-supercapacitors, CDs/GQDs performed at an ultrahigh charge-discharge rate of up to 1000 V.s(-1). However, because of the discontinuous conductive network and good dispersibility in electrolytes, they are not suitable for use in common devices because of poor cycle stability. One of the most promising means for fully realizing active ion storage sites for portable devices is to strongly anchor CDs/GQDs onto conductive carbon scaffolds such as activated carbon, graphene nanosheets, and carbon nanofibers. QDs simultaneously improve the capacitive and rate performance owing to the active sites and improved surface wettability of the composite materials. To further improve the capacitance and cycle stability of electrode materials, different types of QDs/pseudocapacitive material composites (metal oxides or conductive polymers) have been developed. QDs have been shown to increase entire conductivity, accelerate ion transport, and depress the volume expansion of metal oxides and conductive polymers. Benefiting from their small-sized structure and outstanding reactivity, CDs/GQDs can also be used as emerging precursors for constructing advanced carbon electrode materials. Several forward-looking studies have shown that QDs can be converted to carbon nanosheets, heteroatom-doped carbon, and dense and porous carbon blocks with high gravimetric/volumetric capacitances and remarkable rate performance for fast and compact energy storage. Although related fields have rapidly developed in recent years, several critical problems including those related to green and effective methods to prepare CDs/GQDs, construction of entire conductive networks without active sites being sacrificed, the synergistic effect, and the underlying mechanism of QDs in composite materials still must be solved. We hope that this review can provide inspiration and references for further investigation in this promising field.