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

Accurate identification of carbon-based metal-free electrocatalyst (CMFE) activity and enhancing their catalytic efficiency for O-2 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.

Automated summary

This study presents a promising strategy to develop a series of covalent organic frameworks (COFs) with well-defined heterocyclic-free biphenyl or fluorenyl units. The introduction of methyl groups (MGs) into a pristine biphenyl-based COF results in excellent performance as oxygen reduction reaction (ORR) electrocatalysts. The study provides insights into the design of highly efficient metal-free organic electrocatalysts via the regulation of valence bonds.