Physical, chemical and electrochemical characterization of heat-treated tetracarboxylic cobalt phthalocyanine adsorbed on carbon black as electrocatalyst for oxygen reduction in polymer electrolyte fuel cells

Tetracarboxylic cobalt phthalocyanine (CoPcTc) has been adsorbed on carbon black (C). The resulting CoPcTc/C has been heat-treated in Ar at various temperatures ranging from 100 to 1100 °C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. Heattreated CoPcTc/C materials have been characterized by TGA, DSC, bulk elemental analyses, XRD, XPS and ToF SIMS. Their electrocatalytic properties have been evaluated by rde and gde measurements. The highest activity is found for CoPcTc/C heat-treated between 500 and 700 °C. In this temperature range, the catalytic site can be traced back either to the intact polymer (<600 °C) or to phthalocyanine fragments still containing Co, even as CoN₄ chelates. However, short term life tests on the initially most active catalysts indicate that these catalysts are not stable compared to those obtained after pyrolysis of CoPcTc/C at 900 °C. The active site of the latter catalysts is related to inorganic cobalt present as metal and oxides. TEM reveals that inorganic cobalt is surrounded by a protecting graphite shell rendering it chemically stable in acidic media.