Modifications of Metal and Ligand to Modulate the Oxygen Reduction Reaction Activity of Two-Dimensional MOF Catalysts

Metal-organic framework catalysts have attracted particular interest in recent years due to their good oxygen reduction reaction (ORR) activity in fuel cells. Herein, four Ni-3(HITP)(2) analogs (Co-THT, Ni-THT, Co-CAT, and Ni-CAT) as ORR catalysts were systematically studied by density functional theory calculation. Possible adsorption sites of O-containing species on the four catalysts were explored, and top of the metal atomic site (T-M) and top of the sulfur/oxygen atomic site (T-s/o) are found. Adsorption strength of O-containing species on T M is approximately decreased in the order of Co-THT > Co-CAT > Ni-THT > Ni-CAT. Namely, Co-THT and Co-CAT show relatively strong adsorption to O-containing species, which is consistent with the prediction results of IP (ionization potential) and E-gap (energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) evaluation. For Co-THT and Ni-THT, both T-co and T-s sites can spontaneously catalyze 2e(-) and 4e(-) ORR processes, but the 4e(-) process is more energetically beneficial. For Co-CAT and Ni-CAT, only T-M can effectively catalyze the entire ORR, and the favorable ORR pathway of the former is the 4e(-) process and the latter is the 2e(-) process. Among the four studied catalysts, Co-CAT has the highest ORR catalytic performance, and the reaction energy of the rate-determining step is -0.86 eV, which is competitive with that on Pt(111).