Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O₂ reduction in PEM fuel cells

The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts prepared through a pyrolysis in NH₃ is mostly imparted by acid-resistant FeN₄-sites whose turnover frequency for the O₂ reduction can be regulated by fine chemical changes of the catalyst surface. We show that surface N-groups protonate at pH 1 and subsequently bind anions. This results in decreased activity for the O₂ reduction. The anions can be removed chemically or thermally, which restores the activity of acid-resistant FeN₄-sites. These results are interpreted as an increased turnover frequency of FeN₄-sites when specific surface N-groups protonate. These unprecedented findings provide new perspective for stabilizing the most active Fe/N/C-catalysts known to date.