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

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 mVper 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.