Surface characterization by time-of-flight SIMS of a catalyst for oxygen electroreduction: pyrolyzed cobalt phthalocyanine-on-carbon black

Cobalt phthalocyanine (CoPc) loaded on carbon black (Vulcan XC-72) has been heat-treated at various temperatures ranging from 400 to 1100°C in order to obtain catalysts (CoPc/XC-72) for the electroreduction of O2 in solid polymer electrolyte fuel cells. The CoPc precursor and catalysts have been analyzed by time-of-flight secondary ion mass spectrometry (ToF SIMS) in positive and negative ion modes. The electroactivity of the catalysts has also been measured and its evolution with the heat-treatment temperature has been compared with the chemical analysis of the extreme surface of the catalysts. SIMS results show that CoPc loaded on carbon black pyrolyzes at temperatures above 400°C. All heat-treated CoPc/XC-72 spectra display peaks that are either present in the spectrum of CoPc powder or in the spectrum of XC-72 carbon black support. There is no additional peak corresponding to other molecular fragments. By combining SIMS results with XPS and TEM results already obtained on the same catalysts, one concludes that: (i) Co metal surrounded by graphite layers is the only species responsible for the catalytic activity of CoPc/XC-72 pyrolyzed above 700°C; (ii) either Co metal and/or CoPc fragments containing Co are responsible for the catalytic activity of CoPc/XC-72 pyrolyzed at 600 or 700°C where the maximum of the electrocatalytic activity occurs. SIMS results indicate that a modified carbon surface — Co ion complex, where the carbon surface would be modified either by Nx or by other functional groups, cannot be a model for the active site of CoPc/XC-72 pyrolyzed at 600 or 700°C.