Boosting active site accessibility and alleviating mass transfer resistance for high-performance fuel cells

Precise engineering of single-atom catalysts (SACs) with hierarchical porous structures and optimized mass/charge transfer properties is crucial for advancing oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Herein, we present a novel molten salt-assisted pyrolysis strategy that employs a “dimensional reduction and pore creation” approach to exfoliate three-dimensional (3D)-metal–organic frameworks (MOFs) into three-dimensional porous carbon nanosheets doped with single-atom Fe, resulting in Fe SACs supported on hierarchical porous nitrogen-doped carbon (Fe SA@HPNC). The molten salt treatment simultaneously induces exfoliation and etching, resulting in a hierarchical porous structure with both micropores and mesopores, and a remarkably high specific surface area of 919.5 m²·g⁻¹. The two-dimensional nanosheet structure enhances the anchoring of Fe by exposing more surface micropores, which reduces Fe being deeply buried in internal micropores and improves oxygen accessibility and mass/charge transfer efficiency. The Fe SA@HPNC demonstrates excellent ORR performance with a half-wave potential of 0.90 V and a kinetic current density of 19.9 mA·cm⁻². When applied as the cathode in PEMFCs, the Fe SA@HPNC-based cell achieves a remarkable maximum power density of 900 mW·cm⁻². Distribution of relaxation times analysis further reveals that the exfoliated catalyst exhibits enhanced ORR kinetics and reduced oxygen transport resistance.