Challenging the Activity-Durability Tradeoff of Fe-N-C Fuel Cell Catalysts via Controlling thermal Activation Atmosphere

Abstract

Fe-N-C catalysts, the most promising platinum group metal (PGM)-free oxygen-reduction catalysts, often simultaneously contain pyrrolic N- (S1) and pyridinic N (S2) -coordinated FeN4 sites. These two types of active sites show significantly different intrinsic activity and stability. S1 sites are more active but less stable compared to S2 sites. Designing a Fe-N-C catalyst, which exclusively contains active S1 sites with enhanced intrinsic stability, is highly desirable to break the activity-stability trade-off. Herein, we report a Fe-N-C model catalyst that solely comprises S1 sites prepared by adding H2 in the pyrolysis atmosphere (i.e., 10% H2/Ar). A membrane electrode assembly (MEA) with the Fe-N-C cathode demonstrated compelling activity and generated a current density of 50.8 mA cm−2 at 0.9 ViR-free (H2-O2) and 211 mA cm−2 at 0.8 V (H2-air), which have significantly exceeded the U.S. DOE 2025 targets. The highly active Fe-N-C catalyst also demonstrated improved stability during life tests and accelerated stability tests (ASTs). The knowledge obtained from experimental and theoretical results elucidates that the FeN4 site formation process can be controlled by thermal activation atmospheres, which is essential to breaking activity-stability trade-off and design viable Fe-N-C catalysts with adequate activity and stability for proton exchange membrane fuel cells.

Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods

“…A 0.5Fe-N-C-UCI catalyst (UCI = University of California, Irvine) was synthesized by the impregnation of an Fe/nicarbazin mixture into a sacrificial nanoporous silica template via pyrolysis and acid etching. , A commercial fuel cell catalyst was obtained from Pajarito Powder, denoted 0.5Fe-N-C-PAJ . Two x Fe-N-C-UB catalysts were prepared by the pyrolysis of an Fe 2 O 3 @ZIF-8 composite under different gas compositions, namely, 10% H 2 /Ar (0.9Fe-N-C-UB; UB = University at Buffalo) and Ar (0.8Fe-N-C-UB) . A 0.8Fe-N-C-CNRS catalyst was synthesized by the pyrolysis of a ball-milled mixture of ZIF-8, Fe­(OAc) 2 , and 1,10-phenanthroline under Ar at 1323 K (CNRS = Centre National de la Recherche Scientifique) …”