Structure of Fe−Nx−C Defects in Oxygen Reduction Reaction Catalysts from First-Principles Modeling

The structure of active sites in Fe-based nonprecious metal oxygen reduction reaction catalysts remains unknown, limiting the ability to follow a rational design paradigm for catalyst improvement. Previous studies indicate that N-coordinated Fe defects at graphene edges are the most stable such sites. Density functional theory is used for determination of stable potential oxygen reduction reaction active sites. Clusters of Fe−Nx defects are found to have N-coordination-dependent stability. Previously reported interedge structures are found to be significantly less stable than in-edge defect structures under relevant synthesis conditions. Clusters that include Fe−N3 defects are found to spontaneously cleave the O−O bond. S development and possible implementation of nonprecious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) promise to significantly decrease the materials cost associated with proton exchange fuel cells but require detailed characterization of the chemical and structural composition of the ORR active site. A scientific understanding of active site structure will provide synthesis targets that optimize activity, stability, and selectivity, with the ultimate goal of increasing active site density, durability, and efficiency. Graphene nanoribbon (GNR)a two-dimensional carbon sheet with edgeshas previously been employed as a model system for NPMCs since it can host a variety of potential ORR active sites and is consistent with the observation of high-levels of sp (graphitic) carbon in synthesized NPMCs with high ORR activity. It has been shown that nitrogen, metal, and vacancy defects (which, in some combination, must constitute the active site or sites) are more stable at edges than in bulk graphene. The edge nature of these defects is an important requirement for product and reactant accessibility, facilitating mass-transport efficiency. Nanoribbon edge sites also have unique electronic and magnetic properties that differentiate them from their counterparts in bulk graphene. To maintain an optimal configuration under operating conditions, it is likely that the active sites are covalently embedded within the planar structure of the graphitic matrix. Nitrogen has been shown to coordinate the nonprecious metal atoms directly, but the nature of the active MxNy complexes is actively debated in the literature. In fact, the stability of any particular MxNy complex will be dependent upon the Mand N-chemical potentials realized during synthesis conditions, a fact which has not been addressed previously in the literature. It has been suggested that the active site is composed of multiple metal atoms in close association and that this facilitates the multielectron reduction steps in ORR. In this paper, we use density functional theory (DFT) and ab initio molecular dynamics (MD) to characterize the properties of the active sites in NPMCs. We focus on their molecular configuration, surface accessibility, sensitivitiy to N and Fe chemical potentials, and response to an aqueous environment. Particular attention is paid to the clustering tendencies of different N-coordinated structures to illuminate the structure of the MxNy centers. We then explore the role that chemical potentials of Fe and N play in stabilizing/destabilizing such sites. Finally, we consider the adsorption of O2 to various MxNy centers embedded within graphene edge defect sites and the implications for ORR. ■ METHODOLOGY To study the formation of FexNy active sites, three geometries are considered using DFT: 2N-coordinated interedge, 3Ncoordinated intraedge, and 4N-coordinated intraedge. These geometries are composed of Fe atoms between the edges of Nterminated zigzag edges, Fe atoms situated above a monovacancy coordinated by 3 N atoms (two of which are at the zigzag edge termination), and Fe atoms in divacancy positions coordinated by 4 N atoms (two of which are at the zigzag edge termination) (see Figure 1). A graphene nanoribbon eight-C-pairs-long and five-C-pairs-wide, with two FeNx defects of a given geometry, is used for the 3N and 4N cases, allowing for possible defect clustering distances of 1, 2, 3, and 4 Received: April 2, 2014 Revised: June 5, 2014 Published: June 6, 2014 Article