Elucidating Activity-Stability Trade-Off to Design Highly Durable Fe-N-C Catalysts

Abstract

In this work, we developed a Fe2O3@ZIF-8 composite precursors and combined with a novel thermal activation treatment under an Ar/H2 mixture atmosphere (i.e., forming gas), which successfully prepares a highly active Fe-N-C catalyst. 57Fe Mossbauer spectroscopy analysis experimentally verified that the catalyst only contains S1 sites. The highly active Fe-N-C catalyst achieved exceptional ORR initial activity in acids, exceeding that of a Pt/C baseline catalyst (60 µgPt cm−2) in rotating disk electrode (RDE) tests. The MEA tests further verified that the Fe-N-C catalyst's activity is competitive with Pt/C cathode (0.1 mgPt cm−2) in the kinetic range. Notably, the catalyst demonstrated remarkable activity of 50.8 mA cm−2 (@0.9 ViR-free) under H2-O2 conditions, exceed the U.S. DOE target. As expected, the catalyst degraded significantly during the stability ASTs.To increase S1 sites and address the stability issues, we further developed in-situ CVD methods to treat Fe2O3@ZIF-8 precursors under forming gas flow but with additional N/C precursors that was put at the upstream sides of the tube furnace. Therefore, additional nitrogen and carbon sources are added to the Fe2O3@ZIF-8 precursors during the thermal activation of catalysts. As a results, we can populate the intrinsically stable S2 sites to design highly durable Fe-N-C catalyst. The resulting catalyst presented outstanding stability in both RDE and fuel cell tests. The E 1/2 gained 21 mV after 100,000 cycles of accelerated degradation test (ADT), achieving 0.869 VRHE at the end of the test, which is close to Pt/C (60 µgPt cm−2) in acid electrolyte. The current density of MEA at 0.8 V retained unchanged, and the peak power density increased from 454.0 to 512.0 mW cm−2 in H2-fuel cell after 30,000 cycles of AST.