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

Although the solvothermal synthesis has become a prevailing strategy for the preparation of diverse spherical covalent organic frameworks (COFs), synthesis of uniform COF microspheres has still been a grand challenge due to the uncontrollable kinetics. Herein, we introduced 2,4,6-trimethylbenzaldehyde (TBA) as a molecular regulator in the reaction system, where the size and uniformity of spherical COFs were controlled by dynamic reversibility of imine exchange between TBA and building units and depended on the addition amount, molecular structure, and synthesis temperature. The applicability of the synthetic strategy was demonstrated by varying different structural units. Moreover, imine linkages of COF microspheres function as active sites suitable for coordinating with Pd single atoms and atomic clusters to promote electron transfer and synergistically enhance electrocatalytic oxygen reduction reaction. This work addressed the problems of poor uniformity and untunable size of COF spheres previously faced by the solvothermal method, thereby providing guidance for the construction of spherical COF-based catalyst supports.