Computational Modeling of Atomically Dispersed Iron/Nitrogen Carbon Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) can covert chemical energy stored in hydrogen fuels to electricity and produce environmentally benign product water. However, the emergent application of PEMFCs is hindered by the requirement of expensive Pt group metals as their electrocatalysts. To advance PEMFC technology, it is of great interest to develop earth-abundant, non-precious metal-based catalysts in replacement of Pt, especially for oxygen reduction reaction (ORR) occurring at the cathode of PEMFCs. In this regard, atomically dispersed iron/nitrogen carbon catalysts (denoted as Fe-N-C) have drawn wide attention since they exhibited promising ORR activity close to Pt and improved stability than macrocyclic molecules. However, the structure of the active sites in these Fe-N-C catalysts and their catalytic mechanism for ORR have not been fully understood. Recent study reveals that there exist two distinct types of FeN₄ sites, namely porphyrin-like S1 FeN₄ sites and S2 FeN₄ sites containing four pyridinic nitrogen, in Fe-N-C catalysts active for ORR. In this study, we have performed density functional theory (DFT) calculations to predict the reaction energetics of ORR on both S1 and S2 FeN₄ sites. We calculated the adsorption energies of all the possible chemical species and the activation energies for O-O bond dissociation reactions involved in ORR on the FeN₄sites. Our DFT calculations predicted that the ORR could happen through 4e^- associative pathway on the FeN₄ sites and with higher kinetic rate on S1 FeN₄ sites than S2 FeN₄ sites. The theoretical results are in agreement with experimental observations. Furthermore, we predicted the Gibbs free energy change for demetallation of Fe from both S1 and S2 FeN₄ sites. We assume that the demetallation of Fe from FeN₄ sites consists of two steps. At the first step, the Fe atom moves away from the N₄-coordinated state to an inactive N₂-coordinated state, with two N atoms passivated by H from the acidic environment. At the second step, the Fe atom with ORR adsorbates will be desorbed from the catalytic surface. Our computational results show that the tendency of demetallation is much higher for S1 FeN₄ sites than that for S2 FeN₄ sites. Consequently, computational modeling provides in-depth atomistic understanding of the chemical nature of active sites in Fe-N-C catalysts for PEMFCs.