Kirkendall effect induced bifunctional hybrid electrocatalyst (Co9S8@MoS2/N-doped hollow carbon) for high performance overall water splitting

Rational design of electrocatalysts with the tuneable structural and electrocatalytic properties are essential to accomplish bifunctional activity of hydrogen and oxygen evolution reaction (HER/OER), and thus efficient water splitting systems. Here, we present a bifunctional hybrid electrocatalyst consisting of Co9S8 (OER-active) and MoS2 (HER-active) with the structural merits of hierarchical morphology, huge electrochemically active sites, and greater charge transfer ability. The structural features of the hybrid electrocatalyst (Co9S8@MoS2/N-doped hollow carbon) that is derived from metal organic framework (MOF) precursor are elaborately controlled by the temperature dependent-Kirkendall diffusion effect. Specifically, the imbalanced diffusion rates between two ionic species (Co2+ and S2-), confined in the MOF-precursor geometry, induces the formation of internal voids and outward growth of Co9S8 while retaining MoS2 layers on the surface. As a result, the hybrid electrocatalyst exhibits greatly enhanced HER (126 mV@-10 mA cm(-2)) and OER (233 mV@10 mA cm(-2)) performance. Furthermore, the bifunctional electrode couple demonstrates efficient overall water splitting performance (1.56 V@10 mA cm(-2)) compared to the benchmark electrode couple of IrO2/C//Pt/C (1.63 V@10 mA cm(-2)) in 1 M KOH solution. The results highlight that inducing "Kirkendall effect" through temperature-tuning approach for structure-properties modulation can be extensively applied to design various bifunctional hybrid electrocatalysts.