Solvothermal Synthesis of Uniform Covalent Organic Framework Microspheres Enabling High-Loading Palladium for Oxygen Reduction Reaction

Although the solvothermal synthesis has become a prevailingstrategyfor the preparation of diverse spherical covalent organic frameworks(COFs), synthesis of uniform COF microspheres has still been a grandchallenge due to the uncontrollable kinetics. Herein, we introduced2,4,6-trimethylbenzaldehyde (TBA) as a molecular regulator in thereaction system, where the size and uniformity of spherical COFs werecontrolled by dynamic reversibility of imine exchange between TBAand building units and depended on the addition amount, molecularstructure, and synthesis temperature. The applicability of the syntheticstrategy was demonstrated by varying different structural units. Moreover,imine linkages of COF microspheres function as active sites suitablefor coordinating with Pd single atoms and atomic clusters to promoteelectron transfer and synergistically enhance electrocatalytic oxygenreduction reaction. This work addressed the problems of poor uniformityand untunable size of COF spheres previously faced by the solvothermalmethod, thereby providing guidance for the construction of sphericalCOF-based catalyst supports.