Construction of dangling and staggered stacking aldehyde in covalent organic frameworks for 2e⁻ oxygen reduction reaction

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Summary:
The authors from CAS Shanghai Advanced Research Institute and Shanghai Jiao Tong University developed [4+3] covalent organic frameworks (COFs) bearing dangling, staggered-stacked aldehyde (–CHO) catalytic sites, achieving 91 % H₂O₂ selectivity and 12.2 A g⁻¹ mass activity for 2e⁻ oxygen reduction reaction (ORR).

Background:
1. To replace the energy-intensive anthraquinone process, metal-free carbons have been explored for electrosynthetic H₂O₂ via 2e⁻ ORR, yet random oxygen functionalities and unclear active sites limit selectivity and activity.
2. The authors proposed an innovative “dangling –CHO in staggered COF layers” strategy to expose well-defined, accessible active centers and modulate intermediate binding.

Research Content:
1. Synthesis:
   The authors synthesized two COFs (B18C6-TAPB-COF and B18C6-PTTA-COF) by Schiff-base condensation of a crown-ether tetraaldehyde (B18C6) with triangular amines TAPB or PTTA under solvothermal conditions.
2. Characterizations:
   1) BET: 549.6 m² g⁻¹ (TAPB) and 779.3 m² g⁻¹ (PTTA); micropores at ~1.3 & 2.3 nm.
   2) SEM/TEM: dendritic (PTTA) vs. ball-cluster (TAPB) morphologies; lattice fringes 2.5–4.8 nm confirm high crystallinity.
   3) PXRD shows c-AB staggered stacking (layer gap 3.37 Å); FT-IR/XPS verify dangling –CHO (1692 cm⁻¹, 288 eV); UV-Vis band gaps 2.40–2.60 eV; excellent chemical & thermal stability.
3. Application:
   RRDE tests in 0.1 M KOH gave onset 0.72 V vs RHE, 91 % H₂O₂ selectivity, electron-transfer number 2.17–2.20, Faradaic efficiency 85.9 % and stable 10 h operation for B18C6-PTTA-COF.
4. Mechanism:
   DFT calculations reveal pyridine-N in PTTA lowers ΔG for OOH* adsorption on dangling –CHO carbon (1.489 eV vs 1.541 eV), accelerating 2e⁻ pathway.

Outlook:
This work demonstrates that engineering exposed, staggered –CHO sites in crystalline COFs enables superior metal-free H₂O₂ electrosynthesis, offering a general blueprint for designing precise molecular catalysts.

Construction of dangling and staggered stacking aldehyde in covalent organic frameworks for 2e⁻ oxygen reduction reaction
Authors: Shuang Zheng, Zhaofeng Ouyang, Minghao Liu, Shuai Bi, Guojuan Liu, Xuewen Li, Qing Xu, Gaofeng Zeng
DOI: 10.1002/cnl2.123
Link: https://onlinelibrary.wiley.com/doi/10.1002/cnl2.123

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