Improved Oxygen Reduction Reaction Activity of Graphene via Mechanochemical Activation and Halogen-Doping

Abstract

There has been an increased effort to replace the expensive and rare platinum and platinum group metals to speed up the sluggish oxygen reduction reaction (ORR) kinetics, which limits the efficiency of fuel cells. One class of promising Pt-alternative catalysts for ORR is metal-free halogen-doped carbon materials. Herein, bromine-doped and iodine-doped graphene were synthesized via mechanochemical activation. The synthesized samples exhibited sub-rounded particles. Mechanical activation via ball milling increased the specific surface area of graphene by reducing particle size. Ball milling also enhanced dopant dispersibility and increased surface roughness, though it reduced surface area compared to ball-milled graphene, likely due to the size difference between carbon and halogen atoms. Among the synthesized catalysts, iodine-doped graphene exhibits the highest limiting current density of 1.806 mA cm⁻² with the highest ORR onset potential of 0.74 V vs reversible hydrogen electrode (RHE). The iodine-doped graphene also showed good stability after 1000 cycles of accelerated degradation test. The enhanced ORR performance of iodine-doped graphene was reached using the optimized iodine-to-graphene mass ratio of 4 : 1 after 48 h ball milling time.