Covalent Organic Frameworks with Carbon-Centered Radical Sites for Promoting the 4e Oxygen Reduction Reaction.

Radical covalent organic frameworks (RCOFs) have demonstrated significant potential in redox catalysis and energy conversion applications. However, the synthesis of stable RCOFs with well-defined neutral carbon radical centers is challenging due to the inherent radical instability, limited synthetic methods and characterization difficulties. Building upon the understanding of stable carbon radicals and structural modulations for preparing crystalline COFs, herein we report the synthesis of a crystalline carbon-centered RCOF through a facile post-oxidation process. Moreover, the RCOF demonstrated outstanding catalytic activity in the 4e oxygen reduction reaction (ORR) with a half-wave potential of 0.82 V (vs. RHE) and electron transfer number of 3.98, among the highest in reported COF-based electrocatalysts. The promoted 4e and suppressed 2e ORR pathway (99.5 % vs. 0.5 %) can be attributed to facilitated reaction initiation and smoother transition steps at the carbon radical sites, which is practically beneficial for minimizing peroxide formation, thus contributing to safer and more sustainable fuel cell and metal-air battery applications. Overall, our study not only provided a facile strategy for preparing stable RCOFs with well-defined neutral carbon radical centers but also demonstrated their capability to fine-tune the redox catalytic activity of COF materials, which could be potentially useful in electrocatalysis and energy conversion applications.