Ultrathin Cage‐based Covalent Organic Framework Nanosheets as Precursor for Pyrolysis‐Free Oxygen Evolution Reaction Electrocatalyst

Abstract Ultrathin covalent organic framework (COF) nanosheets are very appealing 2D materials, but the mass production of ultrathin COF nanosheets remains a great challenge. Here, by using cage‐like bicyclocalix[2]arene[2]triazines tri‐aldehyde (BCTAL) as the building block, 2D COF Cage‐COF‐1 was estimated to have a very weak interaction between adjacent layers (around 1/50 compared to that of graphite). As a result, 1.2 nm thick trilayer COF nanosheets were facilely exfoliated from the pristine COFs with large lateral size and high thickness homogeneity. The Cage‐COF‐1 nanosheet is featured by imine linkage, but it is catalytic inactive in oxygen evolution reaction (OER). After post‐metalation with Co 2+ under ambient conditions, remarkable catalysis activity and stability was observed for Cage‐COF‐1‐Ns/Co, which has lower overpotential (330 mV) and Tafel slope (56 mV/dec) in catalytic OER compared to many other Co catalysts. This work has confirmed that weakening the interlaminar interaction is an effective strategy for the production of ultra‐thin COFs nanosheets. Due to the fully exposed and accessible imine linkage, catalysis active metal site can be controllably produced via post‐synthesis from nanosheets under very mild conditions.