Downsizing Porphyrin Covalent Organic Framework Particles Using Protected Precursors for Electrocatalytic CO2 Reduction

Covalent organic frameworks (COFs) are promising electrocatalyst platforms owing to their designability, porosity, and stability. Recently, COFs with various chemical structures were developed as efficient electrochemical CO2 reduction catalysts. However, controlling the morphology of COF catalysts remains a challenge, which can limit their electrocatalytic performance even if the chemical structure is optimally designed. Especially, while metalated porphyrinoids show great promise as catalytically active COF building blocks, their intermolecular stacking and coordination interactions make it difficult to conduct solution-based COF synthesis which can control the particle size dominated by the aggregation of crystallites. In this work, we report a new synthetic methodology for rationally downsized COF catalyst particles, where a tritylated amine is employed as a novel protected precursor for COF synthesis. Trityl protection provides high solubility to a representative cobalt porphyrin precursor, while its deprotection proceeds in situ under typical solvothermal COF synthesis conditions. This colloidal deprotection–polycondensation process yields smaller COF particles with less crystallite aggregation than a conventional synthesis, maintaining crystallinity and porosity. The downsized COF particles exhibit superior catalytic performance in electrochemical CO2 reduction, with higher CO production rate and faradaic efficiency with similar stability compared to conventional COF particles. The improved performance of downsized COF particles is attributed to the higher contact area with a conductive agent. This study provides a strategy for the preparation of COF electrocatalysts with controlled morphology and enhanced performance and also reveals an important factor in the evaluation of COF electrocatalysts.