Porous biochars with nitrogen defects prepared from hydrogel template-modified food waste for high-performance supercapacitors

Abstract

The complex composition and heterogeneous nature of food waste limit the application of biochar materials derived from it in supercapacitors. In this study, biochar was prepared from simulated food waste using a hydrogel template and employed to fabricate supercapacitor electrodes. Food waste was first transformed into a soluble state through an advanced oxidation process (potassium persulfate/heat). Small-molecule polymerization was then performed to generate a structurally uniform food waste hydrogel (FWH). Urea was added during the synthesis of FWH to achieve the uniform incorporation of nitrogen into its structure. FWH biochar (FWHB) was obtained from FWH via pyrolysis at different temperatures (500, 700, and 900 °C). Then, we evaluated the physicochemical properties and electrochemical performances of the obtained FWHB samples. The FWHB pyrolyzed at 700 °C (FWHB700) exhibited a unique sponge-like microstructure with a high specific surface area of 693 m2∙g−1, which was 20 times that of untreated food waste biochar. FWHB700 also showed excellent energy storage performance, with a specific capacitance of up to 461 F∙g−1 at a current density of 1 A∙g−1. Based on physicochemical analysis and density functional theory calculations, the energy storage capacity of FWHB700 was found to be related to its high specific surface area, developed pore structure, as well as abundant nitrogen defects and heteroatom active sites. The symmetric FWHB700//FWHB700 supercapacitor delivered a high energy density of 9.99 Wh∙kg−1 at a power density of 125 W∙kg−1, with 88.2 % capacitance retention after 10,000 charge–discharge cycles. In summary, this study introduces a promising electrode material for energy storage and provides a new approach for the efficient utilization of food waste resources.