Influence of the Pore Structure on the Activity and Stability of Non-precious Metal Catalysts for Proton Exchange Membrane Fuel Cells

Abstract

Abstract The activity/performance of non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells is critically dependent on the pore structure of the catalyst layer, but a systematic understanding of how micropores and mesopores affect activity and stability remains limited. In this work, we employ a sacrificial template method to prepare a series of FeNC catalysts with variable weight fractions (from 0.125 to 0.75) of SiO2 template, followed by pyrolysis, SiO2 removal, and re-pyrolysis. The results confirm that the ORR activity/performance of the catalysts is mainly correlated with the surface area of the micropores rather than that of the mesopores, whereas no correlation (or even a negative one) is found for the macropores. Similarly, the activity/performance is also primarily correlated with the volume of the micropores, while mesopore and macropore volumes do not display any positive effect. In contrast, achieving good stability requires simultaneously minimizing both the mesopore surface area and the micropore volume of the catalysts. Within this catalyst series, this creates an inherent trade-off since high activity demands a large micropore volume, whereas high stability demands the opposite. Density functional theory (DFT) calculations on cylindrical pore models prove that the rate-determining step (RDS) for ORR has a lower energy reaction barrier in the micropores compared to the reaction barrier in the mesopores. The calculation of the orbital occupancy shows that a lower occupation of the dx2–y2 + dz2 orbitals will facilitate the 4e ORR pathway in the micropores compared with that in the mesopores. DFT also indicates that Fe2+ ions exhibit a lower dissolution barrier in the micropores, making FeN4 sites located in the micropores less stable than those located in the mesopores. Molecular dynamics (MD) simulations show that the increased interaction of protons and oxygen molecules with the micropore walls leads to a severe limitation of their diffusion. Furthermore, slow diffusion in small micropores allows H3O+ to adsorb on the N atoms of the FeN4 sites, due to the electronegativity difference between Fe and N. This leads to a weakening of the Fe–N bond, accelerating demetallation and explaining why demetallation is more severe in micropores, especially ultramicropores, than in mesopores. Overall, this work clarifies the distinct roles of micro and mesopores in the ORR activity and stability of the FeNC catalysts, highlighting the trade-off between them.