Hierarchically Structured Bifunctional Electrocatalysts of Stacked Core–Shell CoS_1−xP_x Heterostructure Nanosheets for Overall Water Splitting

The rational design and strategy of obtaining stable bifunctional electrocatalysts with unique functionalities are prerequisite to achieving robust catalytic activity. In this study, a composition-controlled partial sulfurization/phosphorization strategy to synthesize a doughnut-like 3D heterostructured electrocatalyst for overall water splitting is proposed, wherein core–shell 2D CoS_1−xP_x nanosheets decorated with N-doped carbon are self-assembled to form a hierarchical 3D architecture. The composition and phase structure in core–shell CoS_1−xP_x can be readily modified by controlling the liquid phase sulfurization and subsequent phosphorization, thereby modifying the electronic structure and activating the intrinsic active sites. The resulting CoS_1−xP_x benefits from the unique structural features including high accessible active surface area, adequate amount of reactive sites, intimate interfacial coupling between the components, interconnected electron highway, and accelerated charge/mass transfer ability. Consequently, the optimized CoS_0.46P_0.54 electrocatalyst achieves a catalytic current density of 10 mA cm⁻² at overpotentials as low as 101 and 302 mV for hydrogen evolution reaction and oxygen evolution reaction, respectively, with outstanding long-term operational stability in alkaline solution. The CoS_0.46P_0.54 couple enables an alkaline water electrolysis with a current density of 10 mA cm⁻² at a low cell voltage of 1.62 V, comparable to that of the RuO₂||Pt/C couple (1.6 V).