Rice husk waste-derived carbon aerogels: a sustainable approach for advanced supercapacitor electrodes

Carbon aerogels (CAs) are porous solids made of a network of interconnected carbon structures offering outstanding properties, i.e. low density, electrically conductive frameworks, chemical stability and high surface to volume ratio. Due to their abovementioned features, CAs have been successfully used in many fields like hydrogen adsorption, catalysis or energy storage, and many different synthesis strategies have been studied. However, regardless of the chosen starting material, CAs production involves three fundamental steps, namely gelation, drying and carbonization. [1,2] The aim of this work is to study structure, morphology, and electrochemical performances of cellulose based CAs obtained by a facile synthetic route, including preparation, purification and gelification, of Rice Husk (RH). This new synthetic pathway represents a green and economical alternative that starts from a nontoxic and biodegradable source of carbon. [3,4] The process hereby presented involves a two-step pretreatment with NaClO2 in acetic acid and NaOH in order to remove lignin, hemicellulose, and silica. After purification, the cellulose is dissolved in sodium hydroxide and urea, aged, regenerated with pure water, freeze dried and carbonized in inert atmosphere at 800°C. The thus obtained CAs have been characterized using different techniques to fully understand its structure and best potential applications. Composition, morphology, and dimensions have been evaluated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, while X-Ray Diffraction and Raman spectroscopy have been used to investigate structures and graphitization extent. Finally, the material has been exploited for fabrication of electrodes, whose electrochemical performances have been tested in symmetric T cells by means of cyclic voltammetry measurements and galvanostatic cyclations. This study represents a first step to fully understand the potential of rice husk as a green source of cellulose derived CAs and its most promising applications, and it paves the way to the enhancement of a low value agricultural waste.