Accurate identification of carbon‐based metal‐free electrocatalyst (CMFE) activity and enhancing their catalytic efficiency for O2 conversion is an urgent and challenging task. This study reports a promising strategy to simultaneously develop a series of covalent organic frameworks (COFs) with well‐defined heterocyclic‐free biphenyl or fluorenyl units. Unlike heteroatom doping, the developed method not only supplies methyl‐induced molecular configuration to promote activity, but also provides a direct opportunity to identify heteroatom‐free carbon active centers. The introduction of methyl groups (MGs) with reversible valence bonds into a pristine biphenyl‐based COF results in an excellent performance with a half‐wave potential of 0.74 V versus the reversible hydrogen electrode (RHE), which is among the highest values for CMFE‐COFs as oxygen reduction reaction (ORR) electrocatalysts. Combined with in situ Raman spectra and theoretical calculations, the MG‐bound skeleton (DAF‐COF) is found to produce ortho activation, confirming the ortho carbon (site‐5) adjacent to MGs as active centers. This may be attributed to the opening and binding of MGs, which effectively regulate the molecular configuration and charge redistribution, as well as improve charge transfer and reduce the energy barrier. This study provides insight into the design of highly efficient metal‐free organic electrocatalysts via the regulation of valence bonds.

Metal‐Free Carbon‐Based Covalent Organic Frameworks with Heteroatom‐Free Units Boost Efficient Oxygen Reduction

A novel type of covalent organic frameworks has been developed by assembling definite cobalt-nitrogen-carbon configurations onto carbon nanotubes using linkers that have varying electronic effects. This innovative approach has resulted in an efficient electrocatalyst for oxygen reduction, which is understood by a combination of in situ spectroelectrochemistry and the bond order theorem. The strong interaction between the electron-donating carbon nanotubes and the electron-accepting linker mitigates the trend of charge loss at cobalt sites, while inducing the generation of high spin state. This enhances the adsorption strength and electron transfer between the cobalt center and reactants/intermediates, leading to an improved oxygen reduction capability. This work not only presents an effective strategy for developing efficient non-noble metal electrocatalysts through reticular chemistry, but also provides valuable insights into regulating the electronic configuration and charge behavior of active sites in designing high-performance electrocatalysts.

Regulated High-Spin State and Constrained Charge Behavior of Active Cobalt Sites in Covalent Organic Frameworks for Promoting Electrocatalytic Oxygen Reduction.

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, 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 form 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.

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

Ring-opening of phenol in wastewater is the pivotal step in photocatalytic degradation. The highly selective generation of catalytical active species (•OH) to facilitate this process presents a significant scientific challenge. Therefore, a novel approach for designing photocatalysts with single-atom containment in COFs (M-COFs) is proposed. The selection of imine-linked COFs containing abundant N and O-chelate sites provides a solid foundation for anchoring metal atom. These dispersed metal atom possess rapid accumulation and transfer capabilities for photogenerated electrons, while the periodic π-conjugated structure in 2D-COFs establishes an effective platform. Additionally, the Lewis acid properties of imine bonds in COFs can enhance the adsorption capacity towards gases with Lewis base properties such as O2 and N2 . It has been demonstrated that the Pd2+ @Tp-TAPT, designed based on this concept, exhibits efficient oxygen adsorption and follows the reaction pathway of O2 →•O2 - →H2 O2 →•OH with high selectivity, thereby achieving completely degradation of refractory phenol through photocatalysis within 10 mins. We anticipate that the selective generation of catalytic active species via advanced material design concepts will serve as a significant reference for achieving precise material catalysis in the future. This article is protected by copyright. All rights reserved.

Structural Regulation of Photocatalyst to Optimize Hydroxyl Radical Production Pathways for Highly Efficient Photocatalytic Oxidation.

The growth of disordered lithium dendrite and the notorious reaction between Li and electrolyte hamper the practical application of Li metal batteries (LMBs). Herein, an artificial solid electrolyte interphase (ASEI) constructed by a nitro‐functionalized covalent organic framework (NO2‐COF) is designed to regulate Li+ deposition and stable Li anodes. Strong electron‐withdrawing nitro groups can gather the surrounding electrons of connected monomer by the donor‐acceptor (D‐A) effect, thus regulating the electron structure of the covalent organic framework (COF) and constructing a specific cation‐oriented channel. The uniform Li+ deposition and inhibition of Li dendrites are achieved under such a high‐selective Li+ transportation channel and regulated surface electric charge. In addition, the nitro can also be reduced to NO2− and further react with Li to produce high ionic‐conductivity Li3N and LiNxOy during the charging/discharging, which contributes to the migration of Li+. As a result, NO2‐COF‐modified symmetrical batteries realize an ultra‐long cycling life of more than 6000 h under a current density of 5 mA cm−2 compared to bare Li and TpBD‐COF/Li (without nitro). The full cells coupled with LiFePO4 stably cycle 1000 times with a capacity retention of 91%. Hence, effectively optimizing electron structure by the donor‐acceptor (D‐A) effect provides a better platform to elevate the performance of LMB.

Regulating the Electron Structure of Covalent Organic Frameworks by Strong Electron‐Withdrawing Nitro to Construct Specific Li+ Oriented Channel

Unsaturated environment is the key to affect catalytic activity of the oxygen reduction reaction (ORR). Unveiling the effect of unsaturated sites toward ORR activity is of importance due to the vague unsaturated states. Reported here is a proof-of-concept strategy on the evaluation of unsaturated bonds (UBs) on adjacent carbon environment by precisely developing two metal-free vinyl-/azo-decorated covalent organic frameworks ( Vinyl-COF and Azo-COF ) as catalysts. The as-prepared UB-COFs exhibit good performance than the control Py-COF and comparable to the most reported carbon catalysts. Supported by theory calculations and in situ Raman spectra-electrochemistry, it is revealed that the UBs in organic catalysts can produce para -activation, identifying the para C=N groups as active centers. Importantly, the intrinsic UBs can induce local charge redistribution, and make the molecular skeleton possess high isosurface map distribution, with an efficient affinity for oxygen intermediates.

An Unsaturated Bond Strategy to Regulate Active Centers of Metal-Free Covalent Organic Frameworks for Efficient Oxygen Reduction.

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An Unsaturated Bond Strategy to Regulate Active Centers of Metal-Free Covalent Organic Frameworks for Efficient Oxygen Reduction.