Catalytic Linkage Engineering of Covalent Organic Frameworks for the Oxygen Reduction Reaction

Abstract

Metal‐free covalent organic frameworks (COFs) have been employed to catalyze the oxygen reduction reaction (ORR). To achieve high activity and selectivity, various building blocks containing heteroatoms and groups linked by imine bonds were used to create catalytic COFs. However, the roles of linkages of COFs in ORR have not been investigated. In this work, the catalytic linkage engineering has been employed to modulate the catalytic behaviors. To create single catalytic sites while avoiding other possible catalytic sites, we synthesized COFs from benzene units linked by various bonds, such as imine, amide, azine, and oxazole bonds. Among these COFs, the oxazole‐linkage in COFs enables to catalyze the ORR with the highest activity, which achieved a half‐wave potential of 0.75 V and a limited current density of 5.5 mA cm−2. Moreover, the oxazole‐linked COF achieved a conversion frequency (TOF) value of 0.0133 S−1, which were 1.9, 1.3, and 7.4‐times that of azine‐, amide‐ and imine‐COFs, respectively. The theoretical calculation showed that the carbon atoms in oxazole linkages facilitated the formation of OOH* and promoted protonation of O* to form the OH*, thus advancing the catalytic activity. This work guides us on which linkages in COFs are suitable for ORR.

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

“…For example, Jiang et al explored COFs with various analogues, including azine, imine, amide, and oxazole linkages, and found that oxazole had the best ORR performance. 56 Song et al reported a porphyrin−bipyridinebased Co-PorBpy, where Co was incorporated with the porphyrin units to form Co−N 4 and the iminopyridine sites to form unsaturated Co−N 2 coordination. 89 Through this method, Co loading was found to be very high at 10.3 wt %, and the two different metal centers show synergistic effects, exhibiting E 1/2 of 0.84 V vs RHE.…”

“…There are studies focused on modifying COF linkages, metal coordination, or geometry. For example, Jiang et al explored COFs with various analogues, including azine, imine, amide, and oxazole linkages, and found that oxazole had the best ORR performance . Song et al reported a porphyrin–bipyridine-based Co-PorBpy, where Co was incorporated with the porphyrin units to form Co–N 4 and the iminopyridine sites to form unsaturated Co–N 2 coordination .…”

“…However, due to monomer solubility, sometimes COF structures cannot be synthesized directly and require post-treatments such as linkage interconversion. Taking the example of imine linkages, they can be transformed into amide, amine, thioazole, oxazole, quinoline, and protonated imine linkages through oxidation, reduction, molten sulfur, monomer exchange, protonation, etc., respectively. − In summary, exploring other functional monomers, linkers, synthesis chemistries, and postmodification methods would expand the scope of current COFs and their applications. Moreover, COF design and modification rely on precise characterizations and structural analysis.…”