No abstract

Pyrolysis is indispensable for synthesizing highly active Fe-N-C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and output products, clouding the pathway toward Fe-N-C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single atom Fe1(II)-N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 ℃, and then to tetrahedral Fe1(II)-O4 via a crystal-to-melt-like transformation below 600 ℃. The Fe1(II)-O4 releases a single Fe atom that flows into the N-doped carbon defect forming Fe1(II)-N4 above 600 ℃. This vapor phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)-N4 sites via "non-contact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) has largely been impeded by the low activity of M-N-C, in turn limited by low site density and low site utilization. Herein, we overcome these limits by implementing chemical vapor deposition (CVD) to synthesize Fe-N-C, an approach fundamentally different from previous routes. The Fe-N-C catalyst, prepared by flowing iron chloride vapor above a N-C substrate at 750 ℃, has a record Fe-N₄ site density of 2×10²⁰ sites·gram^-1 with 100% site utilization. A combination of characterizations shows that the Fe-N₄ sites formed via CVD are located exclusively on the outer-surface, accessible by air, and electrochemically active. This catalyst delivers an unprecedented current density of 33 mA·cm^-2 at 0.90 V<i>_i</i>_R-free (<i>iR</i>-corrected) in an H₂-O₂ PEMFC at 1.0 bar and 80 ℃.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) has largely been impeded by the low activity of M-N-C, in turn limited by low site density and low site utilization. Herein, we overcome these limits by implementing chemical vapor deposition (CVD) to synthesize Fe-N-C, an approach fundamentally different from previous routes. The Fe-N-C catalyst, prepared by flowing iron chloride vapor above a N-C substrate at 750 ℃, has a record Fe-N₄ site density of 2×10²⁰ sites·gram^-1 with 100% site utilization. A combination of characterizations shows that the Fe-N₄ sites formed via CVD are located exclusively on the outer-surface, accessible by air, and electrochemically active. This catalyst delivers an unprecedented current density of 33 mA·cm^-2 at 0.90 V<i>_i</i>_R-free (<i>iR</i>-corrected) in an H₂-O₂ PEMFC at 1.0 bar and 80 ℃.