Mechanistic discussion of the oxygen reduction reaction at nitrogen-doped carbon nanotubes

Jaclyn D. Wiggins-Camacho, Keith J. Stevenson

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192 Citations (Scopus)

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 (HO2-) 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 (O2). 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, Ep, of -0.23 V vs Hg/Hg2SO4 (+0.640 V vs NHE) in 1 M Na2HPO 4 for N-CNTs containing 7.4 at. % N. A detailed mechanism is proposed that involves a dual site reduction in which O2 is initially reduced at a N-C type site in a 2-electron process to form HO2-, which then can undergo either further electrochemical reduction to form OH - species or chemical disproportionation to form OH- species and molecular O2 at a decorating FexO y/Fe surface phase.

Original languageEnglish
Pages (from-to)20002-20010
Number of pages9
JournalJournal of Physical Chemistry C
Volume115
Issue number40
DOIs
Publication statusPublished - 13 Oct 2011
Externally publishedYes

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