2D quasi-ordered nitrogen and sulfur co-doped carbon materials from ionic liquid as metal-free electrocatalysts for ORR

Abstract

This study explores the novel 2D heteroatom-doped carbon materials for high performance oxygen reduction reaction (ORR). Our strategy involves the ionization of a nitrogen-rich small molecule to prevent its evaporation and subsequent carbonization of the nonvolatile ionic liquids to achieve simultaneous porosity formation and heteroatoms doping. In this context, a nitrogen rich adenine-based ionic liquid possessing a low melting point at 70 °C is synthesized by mixing of commercial adenine and bis(trifluoromethane sulfonimide) (HNTf₂). In virtue of protic acid HNTf₂, direct carbonization of an ionic liquid results in enhanced carbon yields compared to that of adenine itself. The as-synthesized NC features a uniform 2D quasi ordered nanosheet structure and high surface areas up to 1027 m² g⁻¹. The nitrogen contents incorporated within the carbon framework can be varied from 37% to 7% as a result of different thermal treatment temperatures from 600 to 1000 °C. Particularly, both nitrogen and sulfur are homogeneously co-doped within the carbon structure as confirmed by EELS maps. The resulting materials show outstanding electrochemical activity towards ORR in alkaline electrolyte, better than most nitrogen doped carbon materials.