DFT study of the two dimensional metal–organic frameworks X3(HITP)2 as the cathode electrocatalysts for fuel cell

Abstract

The search for cheap, stable, and more active oxygen reduction reaction (ORR) catalysts is of great significance to proton exchange membrane fuel cells, and two-dimensional metal–organic frameworks with metal–nitrogen active sites are attracting increasing attention. The ORR mechanisms on X3(HITP)2 (X = Cr, Mn, Fe, Co, Rh, Os, Ir) catalysts are studied by density functional theory. All possible adsorption sites are explored, and the central metals rather than hydrogen or carbon atoms are demonstrated as the most favorable catalytic sites, completely different from the case of the well-known Ni3(HITP)2. The predicted ORR activities of X3(HITP)2 follow the order of Ir3(HITP)2, Rh3(HITP)2, Co3(HITP)2, Fe3(HITP)2, Os3(HITP)2, Cr3(HITP)2, and Mn3(HITP)2, during which the first two have the highest onset potential of 0.92 and 0.86 V, respectively. This result uncovers that the ORR behavior of X3(HITP)2 can be engineered by substituting the central metals. The linear relationships between the adsorption energies of O2, OOH, O and that of OH species illustrate that the adsorption strength of OH can be used as the activity descriptor for the current X3(HITP)2. Ir3(HITP)2 and Rh3(HITP)2 not only have good ORR activities and stabilities, but also have high anti-poison abilities to some impurity gases, as well as CH3OH molecule.