On the Reduction of O-2 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-corrin^2-. Both O₂ and O adsorptions are most stable on Co-corrin^-, rather than Co-corrin or Co-corrin^2-. 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.