Green Carbon Quantum Dots: State‐of‐the‐Art Strategies on Performance Enhancement of Supercapacitor Applications—A Review

ABSTRACT Carbon quantum dots (CQDs) synthesized from green biomass and biowaste have swiftly found their place as sustainable materials for high‐performance supercapacitors due to their unique structural, optical, and electronic properties. In this regard, this review appraises cutting‐edge green synthesis techniques such as hydrothermal, microwave‐assisted, and sand bath reflux treatments, taken on from the viewpoint of scalability concerning energy consumption and yield and also that regarding CQD physicochemical properties. Specific capacitance values up to 808 F/g and energy densities approaching 78.6 Wh/kg have been recorded for an optimized synthesis and heteroatom doping approach, with cycling stability greater than 98% retention with 10,000 cycles. The synergistic integration of CQDs with a hybrid architecture of metal oxides, MXenes, and conductive polymers brings about an enhancement of electrical double‐layer capacitance and pseudocapacitance. The electrolyte compatibility is discussed throughout aqueous, nonaqueous, and ionic liquid systems, highlighting the common trade‐offs of ionic conductivity, voltage window, and long‐term stability. The comparative tables highlight some benchmarks related to recent literature for a more objective orientation regarding green CQD‐based electrode materials. Some trends that appear include growing use of artificial intelligence and machine learning to accelerate optimization of precursor‐dopant pairs, scaling‐up methods, and structural engineering toward tailoring electrochemical properties. In summary, this review gives the forefront challenges and opportunities with green CQD as materials of the next generation for green and sustainable energy storage devices.