Single active-site catalysts constructed with nonpolar-bond linked covalent organic frameworks for oxygen reduction reaction

Abstract

Covalent organic frameworks (COFs) with polar linkages have been employed as metal-free catalysts for the oxygen reduction reaction (ORR). However, it is still a big challenge to precisely design or locate the catalytic sites for such kinds of COFs because their polar linkages always make some catalytic activity. In addition, the polar linkages are facile to bind with O₂ and oxygen-contained intermediates in the catalytic process, severely weakening the long-term stability of these COFs. In this work, we demonstrated the single metalfree catalytic sites based on the pyridine-cored COFs with nonpolar linkages (C=C bonds) to catalyze the ORR. The nonpolar linkages excluded their potential roles as catalytic sites and also circumvented the possible decomposition in the process of catalysis. By modulating the pyridine N with positive charges, the catalytic performance can be previously improved, because of the enhanced Lewis acidity of the carbon atoms next to the pyridine N, and thus favorable for the electrons transfer to the catalytic sites. The newly-synthesized charged COF showed high activity of a half-wave potential of 0.74 V with a mass activity of 4.34 A g⁻¹, which was 50 mV more positive and 1.63 times higher than those of the neutral COF. And the nonpolar linkages made the COFs display better long-term stability than other metal-free COFs. The theoretical calculation revealed that the ionization of pyridine promoted the formation of the intermediate OOH*, and thus improved the catalytic activity. This work gives us a new insight into designing single sites based on COFs.