Nano-Au-decorated hierarchical porous cobalt sulfide derived from ZIF-67 toward optimized oxygen evolution catalysis: Important roles of microstructures and electronic modulation

Enhancing both the number of active sites available and the intrinsic activity of Co-based electrocatalysts simultaneously is a desirable goal. Herein, a ZIF-67-derived hierarchical porous cobalt sulfide decorated by Au nanoparticles (NPs) (denoted as HP-Au@CoxSy@ZIF-67) hybrid is synthesized by low-temperature sulfuration treatment. The well-defined macroporous-mesoporous-microporous structure is obtained based on the combination of polystyrene spheres, as-formed CoxSy nanosheets, and ZIF-67 frameworks. This novel three-dimensional hierarchical structure significantly enlarges the three-phase interfaces, accelerating the mass transfer and exposing the active centers for oxygen evolution reaction. The electronic structure of Co is modulated by Au through charge transfer, and a series of experiments, together with theoretical analysis, is performed to ascertain the electronic modulation of Co by Au. Meanwhile, HP-Au@CoxSy@ZIF-67 catalysts with different amounts of Au were synthesized, wherein Au and NaBH4 reductant result in an interesting "competition effect" to regulate the relative ratio of Co2+/Co3+, and moderate Au assists the electrochemical performance to reach the highest value. Consequently, the optimized HP-Au@CoxSy@ZIF-67 exhibits a low overpotential of 340 mV at 10 mA cm-2 and a Tafel slope of 42 mV dec-1 for OER in 0.1 M aqueous KOH, enabling efficient water splitting and Zn-air battery performance. The work here highlights the pivotal roles of both microstructural and electronic modulation in enhancing electrocatalytic activity and presents a feasible strategy for designing and optimizing advanced electrocatalysts. HP-ZIF-67 with Co2+ appears to be placed in the dark and exhibits low activity. However, after realizing Au decoration and structure evolution, HP-Au@CoxSy@ZIF-67 with plentiful Co3+ is activated as if bathed in sunlight. The obtained catalyst exhibits superb oxygen evolution reaction electrocatalytic performance because the novel hierarchical porous architecture with appropriate Au facilitates the effective mass transfer and optimizes electronic structure.image