A comparative study of different carbon materials as metal-free catalysts for oxygen reduction and hydrogen evolution reactions in alkaline media

Herein we investigate four different carbon materials, pristine Carbon Nano-Onions (p-CNOs), oxidized Carbon Nano-Onions (o-CNOs), Graphene Material (GM), and Multi-Walled Carbon Nanotubes (MWCNTs), for the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). The physicochemical properties of the carbon materials were investigated by X-ray Diffraction analysis (XRD), High-Resolution Transmission Electron Microscopy (HRTEM), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Cyclic Voltammetry (CV) including the use of a rotating disk electrode (RDE). CV was used to determine capacitance values in the studied materials. The CV and RDE studies for ORR indicate that o-CNOs exhibit better catalytic performance than p-CNOs and MWCNTs. HER studies showed that o-CNOs catalyze the reaction with higher current density values (5 mA.cm(-2) at 0.6 V) and the lowest onset potential (0.43 V) than those observed for the other studied materials. Impedance studies correlated with Tafel analysis, where p-CNOs and o-CNOs are the most active materials for this reaction.