Polarized 3D Bicontinuous bcu Covalent Organic Frameworks with Accessible Octa-Sites for High-Efficiency Oxygen Electrocatalysis.

Abstract

Three-dimensional (3D) covalent organic frameworks (COFs) are potential metal-free electrocatalysts for the oxygen reduction reaction (ORR) due to their inherent porosity and potential for fully exposed active sites. However, their practical application is severely hindered by inefficient electron transport along the extended framework and the resultant high kinetic barriers. Herein, we report a series of highly crystalline and non-interpenetrated 8-connected body-centered cubic (bcu) COFs featuring eight unstacked catalytic nodes per unit cell for ORR. By integrating redox-active azobenzene (Azo) units into the linkages, we successfully bridge the gap between electronic conductivity and catalytic activity. The incorporated Azo moieties not only narrow the bandgap but also serve as electron reservoirs, facilitating rapid electron shuttling to the imine-based active sites. Consequently, the optimal DZPy-COF achieves a remarkable half-wave potential of 0.77 V under alkaline conditions, coupled with a high turnover frequency (0.03 s-1) and enhanced capacitance (13.7 mF cm-2). Theoretical calculations reveal that the Azo units polarize the framework, lowering the energy barrier for intermediate formation, which is the rate-determining step in conventional 3D COFs. This work underscores the efficacy of leveraging high-valency topologies combined with redox-active linkages to overcome electron transport limitations in crystalline porous catalysts.