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

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 X-3(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 Ni-3(HITP)(2). The predicted ORR activities of X-3(HITP)(2) follow the order of Ir-3(HITP)(2), Rh-3(HITP)(2), Co-3(HITP)(2), Fe-3(HITP)(2), Os-3(HITP)(2), Cr-3(HITP)(2), and Mn-3(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 X-3(HITP)(2) can be engineered by substituting the central metals. The linear relationships between the adsorption energies of O-2, OOH, O and that of OH species illustrate that the adsorption strength of OH can be used as the activity descriptor for the current X-3(HITP)(2). Ir-3(HITP)(2) and Rh-3(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.