Linker polarization engineering in covalent organic frameworks enabling metal-free electrocatalysts for high-efficiency oxygen reduction

Abstract

The catalytic performances of covalent organic frameworks (COFs) for the oxygen reduction reaction (ORR) are significantly hindered by the limited oxygen adsorption capability. To address this issue, we proposed a polarity modulation strategy on the molecular scale, where two COFs catalysts, TAPP-COF_BDC and TAPP-COF_BPY, were constructed with biphenyl and bipyridine units, respectively. The impact of molecular polarity on the H₂O₂ production rate was systematically investigated. The TAPP-COF_BPY with higher molecular polarity exhibited superior oxygen adsorption capability compared with TAPP-COF_BDC. At a working potential of 0.6 V vs. RHE, TAPP-COF_BPY achieves an impressive H₂O₂ selectivity of 88.0%, a high H₂O₂ production rate of 7.4 mmol g_catalyst⁻¹ h⁻¹, and a remarkable faradaic efficiency of 90%, all surpassing those of TAPP-COF_BDC. Scanning electrochemical microscopy characterization further revealed the three-dimensional spatial distribution of H₂O₂ generation on the catalyst surface, providing direct evidence of the active sites and their electrocatalytic efficiency. Density functional theory calculations showed that enhanced polarity regulated the oxygen molecule adsorption at active sites, which simultaneously improved the kinetics and selectivity of the ORR based on the two-electron mechanism. This work provides a detailed understanding of the structure and performance from a molecular polarity engineering perspective, offering valuable theoretical insights for designing highly efficient metal-free COFs electrocatalysts for H₂O₂ production.