Pyrolyzed Cobalt Phthalocyanine as Electrocatalyst for Oxygen Reduction

Abstract

Cobalt phthalocyanine (CoPc) adsorbed on carbon black (Vulcan XC-72) and heat-treated at temperatures ranging from 300 to 1150~ display catalytic activity toward the electroreduction of oxygen in acidic medium (H2SO4, pH 0.5). The best catalysts are obtained for pyrolysis temperatures ranging from 700 to 950~ X-ray diffraction performed on CoPc/XC-72 pyrolyzed between 700 and 1150~ reveals the presence of B-Co particles whose average size varies from 9 nm at 700~ to 44 nm at 1150~ Co and N bulk elemental analyses have been performed on CoPc/XC-72 heat-treated from 20 to 1150~ These show that: (i) there is no loss of Co even after pyrolysis at 1150~ when the loading is at 2 weight percent (w/o) Co;(it) the bulk N content decreases as the pyrolysis temperatures are increased and the N content reaches the detection limit (0.5 w/o) at pyrolysis temperatures -> 1000~ Our x-ray photoelectron spectroscopy (XPS) study shows that at 600~ there is a sudden three-fold increase in the surface concentration of Co and N at the surface of the carbon black support. A sublimation-redistribution of the CoPc is proposed. The effect appears to limit the Co loading to approximately 2 w/o (At loadings of 4 and 8 w/o Co, most of the Co is lost due to the sublimation.) The XPS study also shows that metallic Co particles begin to be formed at 600~ and that the formation and growth of Co particles occurs as the pyrolysis temperature increases to 1050~ The chemical stability of the pyrolyzed catalysts was evaluated in concentrated H2SO4, HC1, and HNO3 for time periods ranging from 1 to 30 min. Bulk Co analysis, after immersion in acid, indicate that up to 40% of the Co can be lost in the process, and that this induces a decrease in the catalyst activity. All of the Co leaching was found to occur during the first minute of immersion. As both Co and Co oxides readily dissolve in these acidic conditions, it is proposed that the metallic Co particles detected by x-ray diffraction and XPS are protected by a corrosion resistant material whose structure and composition still remains to be established.Solid polymer electrolyte fuel cells (PEFC) are particularly attractive for transportation and industrial applications.

Carbon-supported Co(III) dimer for oxygen reduction reaction in alkaline medium

“…These catalysts suffered from low ORR activity and stability in the ORR regime and are, thus, far from satisfying the requirements for their application in commercial fuel cells. Subsequently, in the 1970s, pyrolysis of metal-N 4 macrocycles adsorbed on carbon black in inert atmosphere was discovered to significantly improve the ORR activity as well as the stability [12][13][14][15][16][17] of the catalyst. It is proposed in the literature that, upon pyrolysis, the metal-N4 macrocycle molecules decompose, resulting in the catalysts containing the M-N 4 type of active sites to be embedded on the carbon support [18].…”

Carbon Alloy Catalysts for Polymer Electrolyte Fuel Cells: Exploration of Materials and Understanding of Mechanisms

“…The difference in the characteristics between NSCC and N 4 -M catalysts are as follows: (1) The ORR activity of N 4 -M catalysts diminishes by acid leaching; however, the ORR activity of NSCC is maintained or increased by the same treatment. 29,30 (2) Both N and metal species are required for the preparation of N 4 -M; however, they are not required for the preparation of NSCC. 31,32 (3) The formation of an NS structure inhibits ORR activity in the case of N 4 -M catalysts; however, ORR activity is not inhibited in the case of NSCC.…”

The use of macrocyclic compounds as electrocatalysts in fuel cells

“…The most active CoPc/C catalyst for ORR was obtained after pyrolysis at 600°C under an inert atmosphere for 2 h [94]. Detailed studies [95,96] showed that the SIMS intensities of Co ? and all other Co-containing organic fragments decreased with increasing pyrolysis temperature.…”

“…The formation of Co metal and/or Co-containing fragments was responsible for catalytic activity enhancement in the temperature range of 600-700°C. Ladouceur et al [95] investigated the influence of pyrolysis temperature on the electrocatalysis of CoPc from 300 to 1,150°C. It was found that the best catalysts for ORR can be obtained when CoPc was pyrolyzed at a temperature ranging from 700 to 950°C.…”