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

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.