On the Reduction of O₂ on Cathode Surfaces of Co–Corrin and Co–Porphyrin: A Computational and Experimental Study on Their Relative Efficiencies in H₂O/H₂O₂ Formation

The mechanisms for O₂ reduction and H₂O/H₂O₂ formation on Co–corrin and Co–porphyrin cathode surfaces of the proton exchange membrane fuel cell (PEMFC) systems have been studied by hybrid Hartree–Fock/density functional theory (B3LYP) calculations with the LANL2DZ basis set. The calculations show that the reduced Co–corrin with a single negative charge (Co–corrin–) is more reactive than the neutral Co–corrin and the doubly charged Co–corrin²–. Both O₂ and O adsorptions are most stable on Co–corrin–, rather than Co–corrin or Co–corrin²–. The potential energy profiles show that the decomposition of O₂ on both Co–corrin and Co–corrin– can take place energetically favorably without thermal activation. The formation of H₂O and H₂O₂ are predicted to occur by two separate reaction paths: the HO path and the HOO path. The HO path with H₂O as the predominant product on the reduced Co–corrin– surface, the energetically favored surface, under operational cathodic conditions, which is consistent with recent experimental findings, wherein the PEMFCs with pyrolyzed vitamin B12 containing Co–corrin as catalysts loaded at the cathode, can deliver up to 14.5 A cm–³ at 0.8 V with IR compensation. A similar calculation performed for a Co–porphyrin system shows a significantly less efficient O₂ reduction, consistent with the experiment results of the PEMFC power output studies.