Graphitization and particle size analysis of pyrolyzed cobalt phthalocyanine/carbon catalysts for oxygen reduction in fuel cells

Abstract

Cobalt phthalocyanine (CoPc) adsorbed on a carbon black support (Vulcan XC-72) and pyrolyzed at various temperatures is a potential catalyst for the reduction of oxygen in solid polymer electrolyte fuel cells. This paper reports the results of the microstructural characterization of β-Co particles that are formed after pyrolysis at temperatures of 700, 900, and 1050 °C. Transmission electron microscopy (TEM) indicated that (i) for a pyrolysis temperature of 700 °C, the size distribution of the Co particles is bell-shaped with an average value of 4 nm and mean deviation of 1 nm; (ii) for a pyrolysis temperature of 900 °C, the Co particle size distribution skews toward larger particle sizes. The most probable particle size is about 6 nm, and the average particle size is 13 nm. By comparison with the TEM results, the particle size estimated from a spectroscopic method like x-ray absorption is underestimated, while from x-ray diffraction is overestimated. The TEM images show that Co particles act as heterogeneous nucleation sites for the graphitization of amorphous carbon. It is shown that (i), at least for pyrolysis temperature of 900 °C and above, most of the β-Co particles are surrounded by a shell of graphitic carbon layers that appears to protect the particles from corrosion in acidic media; (ii) for pyrolysis temperature of 1050 °C, graphite strings also appear throughout the amorphous carbon support in areas where Co particles are not detected. This behavior was not observed after pyrolysis of as-received carbon support at 1050 °C. These results allow for a better understanding of the behavior of the pyrolyzed catalysts immersed in an acidic solution or in a solid polymer fuel cell.