Co1xS–Graphene Hybrid: A High-Performance Metal ChalcogenideElectrocatalyst for Oxygen Reduction**Hailiang Wang, Yongye Liang, Yanguang Li, and Hongjie Dai*Owing to their high energy-conversion efficiency, low or evenzero emission, and high energy and power density, fuel cellssuch as proton-exchange membrane fuel cells (PEMFC) anddirect methanol fuel cells (DMFC) have drawn tremendousattention as potential clean and efficient power sources forboth electric vehicles and portable electronics.[1, 2] A majorlimiting factor of energy-conversion efficiency for present fuelcells is the sluggish kinetics of the oxygen reduction reaction(ORR) at the cathode.[3–9] Platinum and its alloys have beenso far the most active ORR catalysts.[3–9] However, theprohibitive cost, scarcity, and declining activity of Pt-basedcatalysts have hindered widespread application and commercializationof fuel cells.[3–9] Consequently, alternative electrocatalystsbased on nonprecious metals have been activelypursued.[3–9]Cobalt sulfides have been investigated as ORR catalystwith the highest activity among all chalcogenides of nonpreciousmetals in acidic solution.[2,10–16] Theoretical studiespredicted electrocatalytic activity of Co9S8 similar to that ofPt via a four-electron ORR pathway.[11] However, currentORR catalysts based on cobalt sulfides and cobalt selenideshave exhibited activities far lower than that of Pt.[12–16]Moreover, it has been shown that two-electron reduction isthe dominant reaction pathway for cobalt sulfide catalysts,especially at potentials higher than 0.5 V versus the reversiblehydrogen electrode (RHE).[2] It is thus highly desirable todesign and synthesize high-performance cobalt chalcogenidebased electrocatalyst materials capable of catalyzing fourelectronORR.Here we describe a novel cobalt sulfide–graphene hybridelectrocatalyst for ORR, obtained by controlled two-stepsynthesis of Co1xS nanoparticles on reduced graphene oxide(RGO).
