Unavoidable Silicon Oxidation—the Key to Understanding the High Catalytic Activity of Si,N-Doped Carbon Nanotubes in Oxygen Electroreduction Reaction

The possibility for the model tetravalent silicon-doped nitrogen-anchored divacancy carbon nanotube, Si–N4/CNT, to catalyze the oxygen reduction reaction (ORR) was studied theoretically. Thermodynamically stable oxo O* and hydroxy HO* adsorbates are formed in the first ORR catalytic cycle on heteroatom leading to irreversible poisoning of the silicon atom. The two adsorbates are in equilibrium at the electrode potential U = 0.7 V. The new active catalytic center was found to be the dicarbon site adjacent to the Si(O/OH)N4 motif, on which the ORR follows thermodynamically favorable 4e dissociative pathway. The most active form of the Si-poisoned catalyst is Si(O)–N4/CNT, for which the calculated overpotential η is as low as 0.31 V. The second Si-poisoned form, Si(OH)–N4/CNT, is less active but possesses enhanced dioxygen adsorption activity.