Catalytic activity and stability of heat-treated iron phthalocyanines for the electroreduction of oxygen in polymer electrolyte fuel cells

Iron phthalocyanine (FePc) and tetracarboxylic iron phthalocyanine (FePcTc) have been adsorbed on carbon black (C). The resulting FePc/C and FePcTc/C have been heat-treated in Ar at various temperatures ranging from 100 to 1100 °C to obtain catalysts for the electroreduction of oxygen. The electrochemical properties of these materials have been measured by rotating disk electrode and in polymer electrolyte fuel cells. These properties have been correlated with the bulk and surface characterizations of the catalysts. The most active catalyst is unpyrolyzed FePcTc/C but it is also the least stable one. The only catalysts which are active and stable are those obtained at high pyrolysis temperatures (≥ 900 °C). At those temperatures there is no Fe---N bond anymore, and Fe is mainly observed as a metal surrounded by a graphitic envelope. After 10 h in a fuel cell at 50 °C, 0.5 V versus reversible hydrogen electrode (RHE), FePcTc/C and FePc/C pyrolyzed at 1000 °C yielded currents 37 and 40% that of a commercial Pt catalyst containing the same metal loading (2 wt.%), respectively.