Activation of Graphenic Carbon due to Substitutional Doping by Nitrogen: Mechanistic Understanding from First-principles

Abstract

Nitrogen doped graphene and carbon nanotubes are popularly in focus as metal-free electro-catalysts for oxygen reduction reactions (ORR) central to fuel-cells. N doped CNTs have been also reported to chemisorb mutually, promising a route to their robust pre-determined assembly into devices and mechanical reinforcements. We propose from first-principles a common mechanistic understanding of these two aspects pointing further to a generic chemical activation of carbon atoms due to substitution by nitrogen in experimentally observed configurations. Wannier-function based orbital resolved study of mechanisms suggests increase in C-N bond-orders in attempt to retain $\pi$-conjugation among carbon atoms, causing mechanical stress and loss of charge neutrality of nitrogen and carbon atoms, which remedially facilitate chemical activation of N coordinated C atoms, enhancing sharply with increasing coordination to N and proximity to zigzag edges. Activated C atoms facilitate covalent adsorption of radicals in general, diradicals like O$_2$ relevant to ORR, and also other similarly activated C atoms leading to self-assembly of graphenic nano-structures, while remaining inert to ordinary graphenic C atoms.