Metal-Free Thiophene-Sulfur Covalent Organic Frameworks: Precise and Controllable Synthesis of Catalytic Active Sites for Oxygen Reduction

Abstract

Defective or heteroatoms-doped metal-free carbon materials (MFCMs) are regarded as efficient oxygen reduction reaction (ORR) catalysts in the past decade. However, the active site structures for ORR in MFCMs are hard to be precisely confirmed and controllably synthesized through the common methods such as high temperature pyrolysis or heteroatom doping process. To verify the precise structure acted as active center for ORR, we firstly report two crystalline metal-free thiophene-sulfur covalent organic frame-works (MFTS-COFs) as ORR catalysts. The MFTS-COFs show more positive catalytic capability than that of the thiophene-free COF, indicating pentacyclic thiophene-sulfur building blocks acted as active centers induce ORR catalytic activity. MFTS-COFs with higher numbers of thiophene-sulfur exhibits better ORR performance. The experimental identification is supported by density functional theory calculations. These results thus demonstrate that rational design and precise synthesis of metal-free crystal-line organic materials can promote the development of new ORR catalysts.