• Herranz, J.; Jaouen, F.; Lefèvre, M.; Kramm, U.I.; Proietti, E.; Dodelet, J.-P.; Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Bertrand, P.; Arruda, T.M.; Mukerjee, S.: Unveiling N-protonation and anion-binding effects on Fe/N/C catalysts for O2 reduction in Proton-Exchange-Membrane fuel cells. The Journal of Physical Chemistry C 115 (2011), p. 16087-16097

10.1021/jp2042526

Abstract:
The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinum-based 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 a new perspective for stabilizing the most active Fe/N/C catalysts known to date.