Theoretical study of covalent organic frameworks and heterojunctions for the oxygen reduction reaction

In this study, the catalytic potential of two representative covalent organic frameworks (COFs), COF-366 and Pc-PBBA COF, was investigated in the oxygen reduction reaction (ORR). Transition metal atoms (Fe or Co) were incorporated into the center of porphyrin or phthalocyanine rings to form Fe(Co)-N-C single-atom catalysts, which can catalyze the ORR with overpotentials between 0.49 V and 0.69 V. In order to improve the catalytic activity, we introduced graphene or C₃N₄ layers to build heterojunctions with the COFs. Charge density difference and Bader charge analysis demonstrated that the introduction of a graphene (or C₃N₄) layer increases the population of electrons on the Fe and Co ions, which facilitates the flow of electrons to the intermediates and promotes the ORR catalytic reaction. For COFs@graphene heterojunctions, the overpotential decreases by 10.9-20.3% for Fe-based active sites and 30.3-36.4% for Co-based active sites. For COFs@C₃N₄ heterojunctions, the overpotential decreases by 7.3-39.1% for Fe-based active sites and 15.116.3% for Co-based active sites. The analysis of the volcano plot reveals that the COF-366_Co@graphene heterojunction is positioned at the apex of the volcano plot with an overpotential of 0.31 V.