A Simple Route to Porous Graphene from Carbon Nanodots for Supercapacitor Applications

Abstract

A facile method to convert biomolecule-based carbon nanodots (CNDs) into high-surface-area 3D-graphene networks with excellent electrochemical properties is presented. Initially, CNDs are synthesized by microwave-assisted thermolysis of citric acid and urea according to previously published protocols. Next, the CNDs are annealed up to 400 °C in a tube furnace in an oxygen-free environment. Finally, films of the thermolyzed CNDs are converted into open porous 3D turbostratic graphene (3D-ts-graphene) networks by irradiation with an infrared laser. Based upon characterizations using scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and Raman spectroscopy, a feasible reaction mechanism for both the thermolysis of the CNDs and the subsequent laser conversion into 3D-ts-graphene is presented. The 3D-ts-graphene networks show excellent morphological properties, such as a hierarchical porous structure and a high surface area, as well as promising electrochemical properties. For example, nearly ideal capacitive behavior with a volumetric capacitance of 27.5 mF L⁻¹ is achieved at a current density of 560 A L⁻¹, which corresponds to an energy density of 24.1 mWh L⁻¹ at a power density of 711 W L⁻¹. Remarkable is the extremely fast charge–discharge cycling rate with a time constant of 3.44 ms.

Small-molecule-based carbon nanodots serve as precursors for 3D turbostratic graphene in a simple laser-assisted conversion process. Very high conductivity, high capacitance, and extremely fast charging rates render 3D-ts-graphene an interesting biomass-derived material for supercapacitor applications.

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