Infrared-assisted synthesis of highly amidized graphene quantum dots as metal-free electrochemical catalysts

Abstract

Highly amidized graphene quantum dot (GQD) catalysts are successfully synthesized and employed as metal-free electrocatalyst for electrochemical hydrogen adsorption and oxygen reduction reaction (ORR) in acidic and alkaline electrolytes. The GQD catalysts are prepared through an efficient IR-assisted technique involving the pyrolysis of citric acid and urea at 250 °C. The as-prepared GQDs with an average particle size of 30 nm contain a high amidation level (N/C atomic ratio: 23‒46 at.%) with a large amount of O-rich functionalities. The highly amidized GQD catalytic electrodes provide ultra-high electrochemically active surface area toward H-adsorption with superior durability upon extended cycling (> 400 cycles). The GQD electrodes demonstrate an onset potential appearing at ca. -0.17 V with a well-defined reduction peak at ca. -0.35 V vs. Ag/AgCl in KOH electrolyte. The ORR catalytic activity in both acidic and alkaline electrolytes is significantly enhanced via N-doping into the GQDs nanostructure. Two ORR mechanisms are proposed to elaborate the sequential reduction pathways within acid and base electrolytes, including chemsorption of O2, four-electron reduction route, and two-electron charge transfer pathway. Therefore, the design of amidized GQDs paves the pathway towards developing highly efficient metal-free catalysts for electrochemical H-storage and ORR.