An In Silico Study of Carbon Active Sites for Oxygen Electroreduction on the Nitrogen and Weak-Binding Transition Metals (Cu, Zn) Doped Carbon Nanotubes

Abstract

The relative catalytic activity of the vinylic C₂ center adjacent to CuN₄ and ZnN₄ fragments in the copper- and zinc-doped nitrogen anchored (6,6)-armchair carbon nanotubes have been studied theoretically at the ωB97XD/DGDZVP level. The C₂ center is shown to be capable of competing with the metal center at the initial step of molecular oxygen adsorption, demonstrating a lowered overpotential. A detailed analysis of the mechanism and comparison of activation barriers of the reaction on the two competing centers revealed the possibility for the ORR process to occur on the C₂ center of the model CuN₄-doped CNT catalyst both via the associative and dissociative four-electron pathways. The replacement of Сu for Zn increases the activity of the C₂ center at the initial step. However, the Zn center is slowly but irreversibly oxidized to the stable hydroxide HO*. As a result, the endoergicity of O₂ adsorption and, hence, the probability of 4e associative ORR is increased, but the overpotential for ORR on the C₂ center is slightly decreased from ~1.0 on the non-poisoned to 0.63 V on the Zn-poisoned catalyst.