Skip to main content Skip to secondary navigation
Journal Article

Mechanisms of Two-Electron and Four-Electron Electrochemical Oxygen Reduction Reactions at Nitrogen-Doped Reduced Graphene Oxide

Doped carbon-based systems have been extensively studied over the past decade as active electrocatalysts for both the two-electron (2e) and four-electron (4e) oxygen reduction reactions (ORRs). However, the mechanisms for ORR are generally poorly understood. Here, we report an extensive experimental and first-principles theoretical study of the ORR at nitrogen-doped reduced graphene oxide (NrGO). We synthesize three distinct NrGO catalysts and investigate their chemical and structural properties in detail via X-ray photoelectron spectroscopy, infrared and Raman spectroscopies, high-resolution transmission electron microscopy, and thin-film electrical conductivity. ORR experiments include the pH dependences of 2e versus 4e ORR selectivity, ORR onset potentials, Tafel slopes, and H/D kinetic isotope effects. These experiments show very different ORR behavior for the three catalysts, in terms of both selectivity and the underlying mechanism, which proceeds either via coupled proton–electron transfers (CPETs) or non-CPETs. Reasonable structural models developed from density functional theory rationalize this behavior. The key determinant between CPET vs non-CPET mechanisms is the electron density at the Fermi level under operating ORR conditions. Regardless of the reaction mechanism or electrolyte pH, however, we identify the ORR active sites as sp2 carbons that are located next to oxide regions. This assignment highlights the importance of oxygen functional groups, while details of (modest) N-doping may still affect the overall catalytic activity, and likely also the selectivity, by modifying the general chemical environment around the active site.

Link to article

Author(s)
Hyo Won Kim
Vanessa J. Bukas
Hun Park
Sojung Park
Kyle M. Diederichsen
Jinkyu Lim
Young Hoon Cho
Juyoung Kim
Wooyul Kim
Tae Hee Han
Johannes Voss
Alan C. Luntz
Bryan. D. McCluskey
Journal Name
ACS Catal.
Publication Date
2020
DOI
10.1021/acscatal.9b04106