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 in molecular scale, where two COFs catalysts, TAPP-COFBDC and TAPP-COFBPY, were constructed with biphenyl and bipyridine units, respectively. The impact of molecular polarity on the H2O2 producing rate were systematically investigated. The TAPP-COFBPY with higher molecular polarity exhibited superior oxygen adsorption capability compared with TAPP-COFBDC. At a working potential of 0.6 V vs. RHE, TAPP-COFBPY achieves an impressive H2O2 selectivity of 88.0%, a high H₂O₂ production rate of 7.4 mmol gcatalyst-1 h-1, and a remarkable Faradaic efficiency of 90%, all surpassing those of TAPP-COFBDC. 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 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.