Density-Functional-Theory Calculation Analysis of Active Sites for Four-Electron Reduction of O2 on Fe/N-Doped Graphene

Abstract

Nanocarbons doped with nitrogen (N) and/or metal-N coordination structures hold great promise in replacing Pt for catalyzing the oxygen reduction reaction (ORR) in fuel cells. The lack of clear views on the natures of ORR active sites in these materials has hindered the progress in reducing their activity gap to Pt through a rational desire of doping structures. Using 14 types of N and Fe–N doping structures in graphene as model systems, systematic density-functional-theory (DFT) calculations are performed within a unified electrochemical thermodynamic framework and the same reaction mechanism to gain insights into ORR active sites in doped nanocarbons. Scaling relations are obtained between the calculated adsorption free energy of key ORR intermediates at surface sites associated with various graphene doping structures. Reaction free energy analysis indicates that the proton–electron transfer coupled O 2 adsorption and/or reduction of adsorbed hydroxyl group (*OH) are the activity-determining steps in the ORR on most doped graphenes and that the ORR activity of various graphene doping structures can be described with a single thermodynamic descriptor, namely, the adsorption free energy of *OH (Δ G *OH ). A model volcano plot of ORR activity as a function of Δ G *OH is established for active sites in doped graphenes, which indicates that the surface sites associated with a few edge N-doping structures, such as armchair graphitic N, zigzag pyridinic N, and zigzag pyridinic N oxide, offer optimized binding strength of oxygenated species for catalyzing the ORR. Some other structures, such as in-plane graphitic N and the Fe–N 4 complex and hydrogenated zigzag pyridinic N, are also expected to form ORR activity sites. The possible electronic structure origin of the differing binding strength of oxygenated species on various graphene doping structures is analyzed in terms of the density of p z states near the Fermi level of active carbon atoms. These results may serve as guidance for designing ORR electrocatalysts of doped nanocarbons. Especially, it is revealed that merely N doping indeed can produce highly active electrocatalytic sites for the ORR in nanocarbons.