Synthesis and Electrochemical Applications of Carbon Nano-onions

Carbon nano-onions, a family of carbon nanomaterials, consist of multiple concentric fullerene- like carbon shells which are highly defective and disordered. Due to their unique physicochemical properties, such as high conductivity, high surface area, biocompatibility, thermal stability, and others, they are promising nanomaterials for different electrochemical applications. In this sense, this review outlines the synthetic methods available to afford carbon nano-onions in their pristine, functionalized (covalent and non covalent) and doped forms and their use in energy storage, electrocatalysis and sensing. Particularly, we review the performance and properties of carbon nano-onions as electrode materials for supercapacitors, electrocatalysts in different reactions for fuel cells, and electrode materials for sensors. In the last decade, as we will discuss, scientists have found that functionalized and doped carbon nano-onions have better electrochemical properties than pristine carbon nanoonions, such as specific capacitance, surface wettability, energy power, adsorption on an electrode surface, and charge delocalization, among others.