Flexible Electrodes Coated with Carbon Quantum Dots Derived from Spent Coffee Grounds for Electric Double-Layer Capacitors

Spent coffee grounds from coffee beverages are typically discarded, contributing to landfill pollution and caffeine contamination. This study explores the upcycling of coffee-ground waste to produce carbon quantum dots (CQDs) for application in energy storage devices. An environmentally friendly and low-cost hydrothermal process is employed to fabricate CQDs from locally produced spent coffee grounds. The physical and optical properties of the synthesized CQDs are characterized and discussed. The feasibility of applying the synthesized CQDs in electrochemical energy storage devices is investigated. Flexible carbon electrodes are prepared by combining CQDs with carbon materials. The flexible electrodes are then used to fabricate electric double-layer capacitors (EDLCs) with poly(vinyl alcohol) (PVA)-based gel electrolytes. The performance of these EDLCs is analyzed in linear sweep voltammetry (LSV), cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD). Notably, optimal results are achieved by doping the polymer gel electrolyte with an optimum ionic liquid concentration (25 wt%).