Exploring of catalytic oxygen reduction reaction activity of lattice carbons of vanadium and niobium doped nitrogen codoped carbon nanotubes by density functional theory

The oxygen electroreduction mechanism on the V- and Nb-doped nitrogen-codoped (6,6)armchair carbon nanotube with incorporated MN4 fragment has been studied using the omega B97XD and PBE density functional theory approaches. The metal center in MN4 fragment and the adjacent NC(sic)CN double bond (C-2 site) of the support have been revealed as active centers. The metal active centers turned out to be irreversibly oxidized at the first step of ORR affording stable O*, 2O*, or O*HO* adsorbates depending on the applied electrode potential U, that makes them no longer active in ORR. Therefore, the C-2 site comes at the forefront in ORR catalysis. Among the metal oxidized forms M(O)N-4-, M(O)(O)N-4-, and M(O)(OH)N-4-CNT, the C-2 site of the latter turned out to be most active for 4e dissociative ORR. For both metals the last protonation/electron transfer step, HO* + H* -> H2O, is the rate-limiting step. The alternative hydrogen peroxide formation is not only thermodynamically less favorable but also kinetically slower than the 4e dissociative ORR route on the C-2 site of model M(O)(OH)N-4-CNT catalyst.