CoPc- and CoPcF₁₆-Modified Ag Nanoparticles as Novel Catalysts with Tunable Oxygen Reduction Activity in Alkaline Media

A new class of non-Pt “hybrid” catalysts is reported that is intended as a lower cost alternative that meets the performance and durability requirements of the oxygen cathode in fuel cells with alkaline electrolytes. These novel hybrid catalysts are cobalt (Co) phthalocyanine on silver (Ag) nanoparticles on a carbon (C) support. For O 2 reduction, a catalyst with cobalt phthalocyanine (CoPc) on Ag nanoparticles on carbon (CoPc@Ag/C) or the fully fluorinated cobalt phthalocyanine (CoPcF 16 ) on Ag nanoparticles on carbon (CoPcF 16 @Ag/C) showed lower overpotentials and higher rate constants at a given potential than did CoPc and CoPcF 16 on carbon (CoPc/C and CoPcF 16 /C) and Ag nanoparticles on carbon (Ag/C). The oxygen reduction reaction (ORR) activity of the cobalt-phthalocyanine-modified Ag nanocatalysts is tuned by adjusting the composition of the metalated phthalocyanine molecules adsorbed on Ag. X-ray photoelectron spectroscopy (XPS) and electrochemical characterizations indicate that the ORR on these new catalysts occurs almost entirely by a 4-electron pathway and shows 50 mV lower overpotential in the half-wave potential for CoPc@Ag/C and 82 mV for CoPcF 16 @Ag/C as compared to that found when using Ag/C as the catalyst. The current densities are higher and Tafel slopes are lower for ORR on CoPc@Ag/C and CoPcF 16 @Ag/C than on Ag/C catalysts in the high overpotential region. This indicates higher rate constants at any given potential are obtained with CoPc@Ag/C and CoPcF 16 @Ag/C than with Ag/C as the catalyst.