Oxygen Activation by N-doped Graphitic Carbon Nanostructures
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Oxygen Activation by N-doped Graphitic Carbon Nanostructures
Benjamin W. Noffke, Qiqi Li, Liang-shi Li, and Krishnan Raghavachari Department of Chemistry, Indiana University, Bloomington, IN 47405
ABSTRACT Fundamental understanding of the oxygen reduction reaction (ORR) electrocatalyzed by nitrogen-doped carbon requires a well-defined structure to correlate structure to function. Wellcharacterized N-doped graphitic nanostructures derived from benzene derivatives have been synthesized in our group, and shown to catalyze a four-electron ORR under alkaline conditions. Density functional theory calculations have been performed on a model N-doped graphitic nanostructure, C50N2H20, to determine an oxygen activation mechanism. With guidance through an experimentally determined Pourbaix diagram, DFT calculations clearly indicate that the catalyst must undergo a 2e−,1H+ reduction to generate a reactive carbanionic intermediate that activates oxygen with a spin inversion. INTRODUCTION The standard catalyst for the oxygen reduction reaction (ORR) is Pt on carbon supports [1], but several factors limit its commercial use. The most glaring obstacle of commercial Pt-based catalysts is the high cost and low abundance of Pt. While there is a broad effort to improve nonprecious metal catalysts for commercial viability [2, 3], nitrogen-doped carbon based materials are also an extensively studied alternative to precious metals [2, 4]. N-doped graphitic carbon nanostructures have been synthesized in our group and shown to catalyze the four-electron pathway of the ORR under alkaline conditions [5]. These nanostructures are thoroughly characterized, providing a well-defined structure that allows mechanistic studies to be carried out on the elementary steps of the ORR. The nanostructures and their linear sweep voltammograms are shown in Figure 1. Each nanostructure has the same configuration with two pyridinic nitrogen atoms, but differs in the increasing size of the graphitic domain. As the size of the nanostructure increases, there is an observed positive shift in the onset potential for the ORR, and the current density increases. The pH dependence of the ORR for these nanostructures shows that the catalyst loses most of its electrocatalytic activity below a pH of 11. Experiments measured the change in current density within a pH range of 11-14 and found that the reciprocal of the current density increased linearly with proton concentration, suggesting competition between protonation of a key intermediate and some elementary step in the ORR. To help explain the experimental observations, theoretical investigations can be conducted to determine the possible mechanistic steps of the ORR of these N-doped graphitic nanostructures. The first step to be addressed by these computations is oxygen activation.
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i i Figure 1. Structures of synthesized N-doped graphitic nanostructures and their electrochemical behavior with respect to ORR. Oxygen activation pertains to two vital aspects of catalyzing the ORR: the selecti
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