Mechanistic Discussion of the Oxygen Reduction Reaction at Nitrogen-Doped Carbon Nanotubes

Abstract

The oxygen reduction reaction (ORR) at undoped and nitrogen-doped carbon nanotubes (CNTs and N-CNTs, respectively) was studied by cyclic voltammety, rotating disk electrode voltammetry, and gasometric analysis in neutral and alkaline aqueous solutions. At undoped CNTs, the ORR proceeds by two successive two-electron processes with hydroperoxide (HO 2 – ) as the intermediate. At N-CNTs, the ORR occurs through a “pseudo”-four-electron pathway involving a catalytic regenerative process in which hydroperoxide is chemically disproportionated to form hydroxide (OH – ) and molecular oxygen (O 2 ). The ORR mechanism at both undoped and N-doped varieties is supported by steady state polarization and gasometric measurements of hydroperoxide disproportionation rates. An enhancement of over 1000-fold for hydroperoxide disproportionation is observed for N-CNTs, with rates comparable to the best known peroxide decomposition catalysts. A positive correlation between nitrogen content and ORR activities is observed where the ORR potential shifts by up to 11.6 mV per at. % N incorporated into the N-CNTs and exhibits an oxygen reduction potential, E p, of −0.23 V vs Hg/Hg 2 SO 4 (+0.640 V vs NHE) in 1 M Na 2 HPO 4 for N-CNTs containing 7.4 at. % N. A detailed mechanism is proposed that involves a dual site reduction in which O 2 is initially reduced at a N–C type site in a 2-electron process to form HO 2 –, which then can undergo either further electrochemical reduction to form OH – species or chemical disproportionation to form OH – species and molecular O 2 at a decorating Fe x O y /Fe surface phase.