Synthesis and comprehensive evaluation of waste biomass-derived carbon dots as sustainable electrolytes for Vis-NIR modulation of electrochromic devices

Carbon dots (CDs) have demonstrated outstanding capability in enhancing redox reactions as both electrode and electrolyte modifiers in electrochromic devices (ECDs). However, although biomass-derived carbon dots provide low toxicity, low cost, intrinsic doping, and high stability, they remain insufficiently explored despite the existence of various synthesis methods and sources. Herein, we report the excellent electrolytic activity of carbon dots from coffee waste (CWCD), orange peel (ORCD), spent hibiscus flower (HICD), and red onion peel (ROCD) prepared via a simple bottom-up method. Rich surface functional sites and well-balanced sp²-carbon domains enabled excellent ionic conductivity in the ECDs, resulting in impressive color switching performance. The carbon dot derived from orange peels (ORCD) exhibited the highest performance among all samples. A high optical contrast (ΔT) of 94.9 % and 92.6 % was obtained under 6 s in the visible and near infrared (NIR) region, respectively. In addition, it exhibits a high coloration efficiency (CE) of 150 cm²C^-1, which is comparable to and even higher than other reported electrolyte materials. Cyclic voltammetry confirmed complete and reversible redox activity of the viologen species, while electrochemical impedance spectroscopy revealed that ORCD had the lowest charge transfer resistance. Additionally, all devices demonstrated robust cyclic stability over 500 cycles. These results highlight the potential of biomass-derived CDs, particularly ORCD, as efficient electrolyte materials for high-performance and sustainable electrochromic applications.