Two-Dimensional Metal-Organic Frameworks as Ultrahigh-Performance Electrocatalysts for the Fuel Cell Cathode: A First-Principles Study.

Abstract

Except for metal-organic frameworks (MOFs) with traditional metal-nitrogen sites, MOFs with metal-oxygen sites may also possess good oxygen reduction reaction (ORR) catalytic activity due to their unique electronic structures. Herein, using density functional theory methods, the ORR performances of a series of M3(HHTT)2 (where M is a 3d, 4d, or 5d transition metal and HHTT is 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene)) catalysts are explored. The binding energy (ΔEspecies) results suggest that the binding energy of *OH (ΔE*OH) shows a good linear relationship with the binding energies of *O and *OOH (ΔE*O and ΔE*OOH, respectively), indicating that ΔE*OH can serve as a descriptor to reflect the catalytic activity of M3(HHTT)2. In addition, the volcano plot suggests that M3(HHTT)2 catalysts with a moderate binding strength of the intermediate *OH (0.6 eV < ΔE*OH < 0.9 eV) show relatively high ORR activity. Therefore, four highly active ORR catalysts are screened out, namely, Fe3(HHTT)2, Co3(HHTT)2, Rh3(HHTT)2, and Ir3(HHTT)2, which possess very small overpotentials of 0.35, 0.24, 0.31, and 0.29 V, respectively. Their potential-determining step is the reduction of O2 to the intermediate *OOH. It is encouraging that the theoretically lowest overpotential of this kind of catalyst is 0.21 V, which is superior to that on Pt(111). Moreover, Co3(HHTT)2 has excellent poisoning-tolerance ability for impurity gases (CO, NO, and SO2) as well as fuel molecules (CH3OH and HCOOH).