Single-phase bimetal sulfide or metal sulfide heterojunction: Which one is better for reversible oxygen electrocatalyst?

Single-phase bimetallic sulfides (CoFeS 2 ) with the uniform structural electron distribution and higher central energy level, affords higher intrinsic activity and faster reaction kinetics than metal sulfide heterojunctions (CoS/FeS). Bimetallic sulfides, integrating the merits of individual components, are ideal structures for efficient electrocatalysis. However, for bimetallic sulfides including metal sulfide heterojunctions (MSH) and single-phase bimetallic sulfides (SBS), it is still unclear about which one has better catalytic activity toward reversible oxygen catalysis and its difference on catalytic mechanism. In this work, we demonstrate a bimetallic sulfide electrocatalyst that could transform from metal sulfide heterojunction (CoS/FeS) to single-phase bimetallic sulfide (CoFeS 2 ) through a facile temperature control strategy. The single-phase bimetallic sulfide (CoFeS 2 ) affords high intrinsic activity, fast reaction kinetics and superior durability toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Density functional theory (DFT) simulations reveal that the (CoFeS 2 ) has homogeneous electron distribution of the CoFeS 2 structure, improves the central energy level of d band, and optimizes the O* and OOH* intermediate and efficiently reduces the energy barrier of the reaction rate-determining step (RDS). The assembled rechargeable zinc-air battery is more stable than the Pt/C and IrO 2 assemblies due to the excellent electrocatalytic activity and stability of CoFeS 2 /NC, suggesting that it has potential for use in practical applications.