Construction of symbiotic one-dimensional ionic channels in a cobalt-based covalent organic framework for high-performance oxygen reduction electrocatalysis

With the advantages of high specific activity and selectivity, molecular catalysts are regarded as burgeoning electrode materials for anion exchange membrane fuel cells (AEMFCs). However, their catalytic performance in the catalyst layer of AEMFCs is often limited by the uneven distribution of triple-phase boundaries due to the agglomeration of the catalyst and the intermittent ionic channels. Herein, we constructed a porphyrin cobalt-based covalent organic framework (TAPPCo COF) composed of symbiotic ordered ionic channels and anchored quaternary ammonium (QA) cationic groups to facilitate the conduction of OH-. Microscopic and spectroscopic analyses confirm that the TAPPCo COF maintains plenty of Co catalytic active sites for the oxygen reduction reaction (ORR) and a reticular porous structure for O-2 reactant permeation. Atomic force microscopy coupled with electrochemical impedance spectroscopy (AFM-EIS) demonstrated nonlinear local variation of double-layer capacitance and ion transportation resistance at the nanoscale on the catalyst surface, as well as a decreased OH- mass transfer barrier with modification of hydrophilic QA groups. Owing to its structural advantages, the TAPPCo-QA COF catalyst exhibited a significantly enhanced ORR turnover frequency and alleviated voltage drop in the high-mass transport region during the single-cell test. This design strategy of "ion channel-in-catalyst" may serve as a platform for the application of versatile molecular catalysts in electrochemical energy conversion applications where both high intrinsic electrocatalytic activity and effective ion transport are required.