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

preprint
revised on 07.12.2019 and posted on 17.12.2019 by Hyo Won Kim, Vanessa Jane 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. McCloskey
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 reaction (ORR). 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 oxides (NrGO). We synthesize three distinct NrGO catalysts and investigate their chemical and structural properties in detail via X-ray photoelectron spectroscopy, infrared and Raman spectroscopy, 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, both in terms of selectivity and the underlying mechanism which proceeds either via coupled proton-electron transfers (CPETs) or non-CPETs. Reasonable structural models developed from DFT 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.

Funding

BDM and HWK gratefully acknowledge support from the National Science Foundation under grant number CBET-1604927.

HWK also acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (2016R1A6A3A03012382).

This work was supported in part by the U.S. Department of Energy, Chemical Sciences, Geosciences, and Biosciences (CSGB) Division of the Office of Basic Energy Sciences, via Grant DE-AC02-76SF00515 to the SUNCAT Center for Interface Science and Catalysis.

This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

THH and HP gratefully acknowledge support from the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (2016R1A6A1A03013422).

Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

History

Email Address of Submitting Author

vbukas@fysik.dtu.dk

Institution

Technical University of Denmark

Country

Denmark

ORCID For Submitting Author

0000-0002-0105-863X

Declaration of Conflict of Interest

The authors declare no competing financial interest.

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