Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeN_x Moieties in Fe-N-C Catalysts

Two Fe-N-C catalysts comprising only atomically-dispersed FeN x moieties were prepared, differing only in the fact that the second catalyst (Fe 0.5 -NH 3 ) was obtained by subjecting the first one (Fe 0.5 -Ar) to a short pyrolysis in ammonia. While the initial ORR activity in acid medium in rotating disk electrode is similar for both catalysts, the activity in alkaline medium is significantly higher for Fe 0.5 -NH 3 . Time-resolved Fe dissolution reveals a circa 10 times enhanced Fe leaching rate in acidic electrolyte for Fe 0.5 -NH 3 relative to Fe 0.5 -Ar. Furthermore, for the former, the leaching rate is strongly enhanced when the electrochemical potential is in the range 0.75–0.3 V vs. RHE. This may explain the reduced stability of ammonia-pyrolyzed Fe-N-C catalysts in operating PEMFCs. In alkaline medium in contrast, Fe 0.5 -NH 3 is more active and more stable, with minimized Fe leaching during electrochemical operation in load-cycling mode. Operando X-ray absorption spectroscopy measurements in alkaline electrolyte reveals similar trends of the XANES and EXAFS spectra as a function of the electrochemical potential for both catalysts, but the magnitude of change is much less for Fe 0.5 -NH 3 , as evidenced by a Δμ analysis. This is interpreted as a lower average oxidation state of FeN x moieties in Fe 0.5 -NH 3 at open circuit potential.