Urea with trifunctional effects: an assistant for high exposure of single-atom active sites on 2D nanosheets via structural transformation

Abstract

Two-dimensional (2D) mesoporous carbon nanosheets are ideal supports for maximizing the exposure of active sites and facilitating mass/electron transfer to accelerate the oxygen reduction reaction (ORR) for Zn–air batteries (ZABs). Herein, a novel thermal exfoliation strategy is proposed to construct single-atom Fe dispersed nitrogen-doped multilayered porous carbon nanosheets (SAFe-NMPC) by one-step pyrolysis with the assistance of urea. Urea exhibits a trifunctional effect during the catalyst formation: (1) as molten urea to exfoliate the precursor and transform it into 2D nanosheets, (2) providing nitrogen to anchor metal single-atom sites, (3) synergizing with Zn to form numerous mesopores. The obtained SAFe-NMPC possesses a specific surface area of 1207 m² g⁻¹ and an electrochemically active surface area (ECSA) of 851.3 m² g⁻¹, which realizes the full utilization of active sites and thus boosts the ORR catalytic performance. Consequently, the ZAB assembled with SAFe-NMPC exhibits a high open-circuit voltage of 1.49 V and a superior peak power density of 169.1 mW cm⁻² with favorable stability. This work for the first time reveals the application of molten urea in the thermal exfoliation strategy and explores the trifunctional effects of urea, which provides a new direction in catalyst morphology design.