The Oxygen Reduction Reaction on Nitrogen-Doped Graphene
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The Oxygen Reduction Reaction on Nitrogen-Doped Graphene Felix Studt
Received: 9 August 2012 / Accepted: 26 September 2012 / Published online: 9 October 2012 Ó Springer Science+Business Media New York 2012
Abstract The oxygen reduction reaction on a graphene sheet with 6.3 % doping of nitrogen is analyzed using density functional theory calculations. It is found that all intermediates involved in the oxygen reduction reaction bind on the carbon atom next to the nitrogen dopant. The first reduction step to produce the OOH* intermediate is only moderately downhill in free energy while further reduction to O* and OH* are more exothermic. The reduction from step from O* to OH* is found responsible for the experimentally observed overpotential. Keywords Electro-catalysis Oxygen reduction reaction Nitrogen-doped carbon Density functional theory calculations Reaction mechanism
1 Main Text The development of catalytic materials for the oxygen reduction reaction (ORR) that are efficient, cost-effective and stable is one of the greatest challenges in a sustainable society [1]. Up to now, the most efficient catalysts are based on platinum, often alloyed with one or more non-precious metals, like Ni, Fe or Co [2–6]. Yet, even for the best materials, the overpotential necessary to drive the reaction is still substantial and they still incorporate significant amounts
Electronic supplementary material The online version of this article (doi:10.1007/s10562-012-0918-x) contains supplementary material, which is available to authorized users. F. Studt (&) SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA e-mail: [email protected]
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of platinum [7]. Recently, a new class of materials, nitrogendoped carbon has been discovered to act as metal-free catalysts for the ORR [2, 8–12]. These materials reduce oxygen to water at an overpotential similar to that of platinum. So far, there is still a lot of discussion about the active site of the material and the function of the nitrogen [12], and there have only been few theoretical studies on this subject [13–15]. Herein, density functional theory (DFT) calculations on the mechanism of the ORR on nitrogen-doped graphene are presented. The computational hydrogen electrode model is used in order to analyze the electro-chemical reduction of oxygen to water. Within this model, zero voltage is defined based on the reversible hydrogen electrode where protons and electrons are in equilibrium with gas phase hydrogen. The chemical potential of a proton–electron pair is therefore equal to half of the potential of H2 gas and can hence be calculated using DFT. The influence of the applied potential is adjusted by the standard relation between chemical and electrical potential, DG = -eU. This model has proven successful in describing the ORR on transitionmetals [16] and transition-metal oxides [17] as well as the electro-chemical reduction of N2 and CO2 [18, 19]. A 16 atom graphene slab is used for the
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