Design and Preparation of Fe–N₅ Catalytic Sites in Single-Atom Catalysts for Enhancing the Oxygen Reduction Reaction in Fuel Cells

Abstract

There is an urgent need for developing nonprecious metal catalysts to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Atomically dispersed M-N_<i>x</i>/C catalysts have shown promising ORR activity; however, enhancing their performance through modulating their active site structure is still a challenge. In this study, a simple approach was proposed for preparing atomically dispersed iron catalysts embedded in nitrogen- and fluorine-doped porous carbon materials with five-coordinated Fe-N₅ sites. The C@PVI-(DFTPP)Fe-800 catalyst, obtained through pyrolysis of a bio-inspired iron porphyrin precursor coordinated with an axial imidazole from the surface of polyvinylimidazole-grafted carbon black at 800 °C under an Ar atmosphere, exhibited a high electrocatalytic activity with a half-wave potential of 0.88 V versus the reversible hydrogen electrode for ORR through a four-electron reduction pathway in alkaline media. In addition, an anion-exchange membrane electrode assembly (MEA) with C@PVI-(DFTPP)Fe-800 as the cathode electrocatalyst generated a maximum power density of 0.104 W cm^-2 and a current density of 0.317 mA cm^-2. X-ray absorption spectroscopy demonstrated that a single-atom catalyst (Fe-N_<i>x</i>/C) with an Fe-N₅ active site can selectively be obtained; furthermore, the catalyst ORR activity can be tuned using fluorine atom doping through appropriate pre-assembling of the molecular catalyst on a carbon support followed by pyrolysis. This provides an effective strategy to prepare structure-performance-correlated electrocatalysts at the molecular level with a large number of M-N_<i>x</i> active sites for ORR. This method can also be utilized for designing other catalysts.