Elucidating the effects of oxygen- and nitrogen-containing functional groups in graphene nanomaterials for applied electrochemistry by density functional theory

Abstract

Graphene nanomaterials functionalized with oxygen groups [graphene oxide (GO) and reduced GO (rGO)] are either doped with element nitrogen or nitrogen-containing aromatic moieties followed by the investigation of electrochemical properties that generally show enhanced electroanalytical performance. We studied structural, morphological, and physical–chemical properties using correlative techniques. While we attribute their improved properties promoted simultaneously by topologically interconnected mesoporous network morphology, the presence of heteroatom species, and lattice vibrational structure, the complex interpretation requires the need to supplement the experimental observations with theoretical calculations for further insights. The complex interplay of pore size and redox properties revealing distinctive supercapacitive (ion-adsorption controlled) and pseudocapacitive (diffusion-controlled) energy storage mechanistic contributions arises from the combined effects of oxygen and nitrogen functional groups, most likely located on the basal plane and at the pore edge plane sites. The density functional theory calculations provided band structure and electron transfer from Mulliken and Hirshfeld population analyses helping discern the nature of various functional groups in diverse graphene. Interestingly, while quaternary (N—Q) and pyridinic-N-oxides (N—O) on the basal planes show enhanced capacitance due to positive charge and thus an improved electron transfer at higher current loads identified in nitrogen-doped aerogel (AG/nitrogenated) and GO-derived rGO by chemical and electrochemical properties, the other important functional groups affecting the energy storage are pyridinic (N-6) and pyrrolic (N-5) nitrogen groups on the edge of the rGO nanosheet in association with carboxylic (—COOH) and quinone (C=O) functional groups in nitrogenated functional graphene/graphene aerogel and rGO coated polyaniline, contributing to a pseudocapacitive character.