Graphitization and particle size analysis of pyrolyzed cobalt phthalocyanine/carbon catalysts for oxygen reduction in fu
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M. L. Tradeau, A. My, and R. Schulz Technologie des Materiaux, Institut de Recherche d'Hydro-Quebec, 1800 Montee Ste-Julie, Varennes, Quebec, Canada, J3X 1S1
G. Lalande, D. Guay, and J. P. Dodelet 1NRS Energie et Materiaux, 1650 Montee Ste-Julie, C.P. 1020, Varennes, Quebec, Canada, J3X1S2 (Received 22 December 1993; accepted 18 August 1994)
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 /?-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 ft - C o 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.
I. INTRODUCTION Low temperature fuel cells, especially solid polymer fuel cells, appear to be the most promising candidates for powering electrical vehicles.1"3 In these fuel cells, as in phosphoric acid fuel cells, platinum and its alloys are used as catalysts for the electroreduction of oxygen.4"6 However, since platinum is expensive and of limited supply, research in electrocatalysis has evolved in two directions: the first aims to minimize the amount of noble metal in the electrodes7"10; while the second aims to replace Pt with a non-noble metal.11"20 Among the numerous attempts made in replacing Pt, there has been considerable effort spent in evaluating Co and Fe-based catalysts prepared by pyrolysis of N4 metal macrocyclic complexes adsorbed on a carbon support.11"20 The effect of the pyrolysis has been reported to improve the activity and stability of the catalysts; the J. Mater. Res., Vol. 9, No. 12, Dec 1994
exact nature of the catalytic species, however, is still controversial.
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