▎ 摘　　要

Zucchini-derived carbon dots (CDs) were synthesized by a green and low-cost synthetic route using waste zucchini biomass as an inexpensive and naturally available carbon precursor. Apart from a cost neutral resource, the rationale for this choice are the porous nature and the presence of natural carbon and oxygen in the precursor. The as-prepared CDs were hydrothermally treated with graphene oxide by a two-step procedure: hydrothermal process to assemble three dimensionally porous RGO/CDs composites, followed by facile reduction by HI vapor, where CDs serve as spacers to hinder restacking of graphene nanosheets and HI treatment deoxygenates the composites to enhance electrical conductivity and dope heteroatoms in graphene. The electrochemical performance of the composites with variable mass loadings of CDs (0-50 wt.%) and reduction time (0-30 min) was investigated. For the optimal CD mass loading of 33 wt.% at 10 min reduction time, the resulting graphene/CD composite had 6.1 wt.% I, 185 m(2) g(-1) surface area and 0.58 cc g(-1) total pore volume. The composite specific capacitance was 374 F g(-1) at 2 mV s(-1) with excellent capacitance retention of 71.7% as the scan rate increased from 2 to 1000 mV s(-1). The composite exhibited superior cycling stability over 10,000 cycles at 10 A g(-1) with 93.8% capacitance retention. The composite results based on the physicochemical properties, and the ionic conductivity and diffusivity demonstrate the development of a sustainable supercapacitor product from agricultural wastes. (C) 2019 Elsevier Ltd. All rights reserved.