Carbon quantum dot-based composites for energy storage and electrocatalysis: Mechanism, applications and future prospects

Zero-dimensional (OD) carbon nanomaterials such as carbon (CQDs) and graphene quantum dots (GQDs) have been attracting attention due to their outstanding properties of biocompatibility, nontoxicity, chemical inertness, tunable photoluminescence, low cost and facile surface functionalization. Their potential applications range from biomedical, drug delivery, environmental, photocatalytic to energy storage sectors. Among these, investigations have largely focused on their behavior in environmental sensing, biosensing, and optoelectronics, yet energy storage and conversion systems are progressing rapidly as new promising methods are emerging to solve some of the outstanding challenges with energy at low cost and environmental footprint. By virtue of their rapid electron transfer and high surface area, CQD/GQDs are desirable in these electrochemical applications. Further, functional groups with rich heteroatoms (oxygen, nitrogen, sulfur, phosphorus, boron) on OD carbon nanomaterials offer desirable active sites for enhanced electrochemical properties. Our review presents recent advances in the fabrication of CQD/GQD based composites for electrochemical systems, their mechanism of action, applications in energy storage (electrochemical capacitors, lithium/sodium ion batteries) and electrocatalysis (oxygen reduction reaction, oxygen/hydrogen evolution reactions, CO2 electroreduction, biofuel cells and electrochemical biosensors) with an analysis of their potential prospects.