Functional molecule guided evolution of MnOx nanostructure patterns on N-graphene and their oxygen reduction activity

Abstract

In this work, we systematically followed the growth of MnO_x nanostructures on trimesic acid (TMA)/benzoic acid (BA) functionalised nitrogen doped graphene (NG) and studied their electrocatalytic activity towards oxygen reduction reaction (ORR). In these hybrid materials the MnO_x phase, their morphology and Mn surface valency were guided by the functional molecules, their concentration and the duration of reaction, which in turn significantly affected the ORR activity. During the growth in the presence of TMA, agglomerated nanostructures were formed at 2 h reaction, which transformed to well dispersed 4–7 nm particles at 6 h over a large area of NG. However, in the presence of BA, MnOOH nano-flecks were formed at 2 h and transformed to MnOOH nanowires and oval shaped Mn₃O₄ particles at 8 h of reaction. The valency of surface Mn on the different MnO_x nanostructures was ascertained by X-ray photoelectron spectroscopy (XPS). The ORR activity of samples were studied by cyclic voltammetry (CV) and rotating disc electrode (RDE) in alkaline medium. Among all the studied samples, the highest ORR activity with most efficient 4e⁻ transfer process is observed for TMA modified NG-MnO_X obtained at 6 h of reaction, which is due to its well dispersed nanostructure morphology.