Iodine doped composite with biomass carbon dots and reduced graphene oxide: a versatile bifunctional electrode for energy storage and oxygen reduction reaction

We prepared carbon dots (CDs) via a sustainable, low-cost synthetic route employing eggplant biomass. A three-dimensionally porous composite consisting of eggplant-CDs and reduced graphene oxide (RGO) was first synthesized via one-pot hydrothermal process. The as-synthesized RGO/CD hybrid was further deoxygenated with HI acid to enhance ionic conductivity and introduce iodine dopants in the structure. The I-doped graphene/CD composite exhibited specific capacitances of 432 and 460 F g(-1) at 2 mV s(-1) and 1 A g(-1) with excellent capacitance retention of 68.4% and 76.5%, as the scan rate and current density increased from 2 to 1000 mV s(-1) and from 1 to 100 A g(-1), respectively. Superior cycling stability of 94.6% capacitance retention over 10 000 cycles at 10 A g(-1) was measured. As an electrocatalyst for oxygen reduction reaction (ORR) in 0.1 M KOH, the composite demonstrated an onset potential of 0.93 V vs. RHE and a kinetic limiting current density of 19.8 mA cm(-2) at 0.1 V vs. RHE with an average electron transfer number of 3.57. The I-doped graphene/CD composite showed excellent ORR stability with current density retention of 97.7% under electrochemical stress (0.65 V vs. RHE) in comparison with 43.4% of the benchmark Pt/C under similar conditions. Thus, a sustainable process was developed to synthesize a multifunctional electrode based on agricultural wastes for energy storage and ORR.