Defect-Engineered NiCo-S Composite as a Bifunctional Electrode for High-Performance Supercapacitor and Electrocatalysis.

Defect engineering is a reasonable solution to improve the surface properties and electronic structure of nanomaterials. However, how to introduce dual defects into nanomaterials by a simple way is still facing challenge. Herein, we propose a facile two-step solvothermal method to introduce Fe dopants and S vacancies into metal-organic framework-derived bimetallic nickel cobalt sulfide composites (NiCo-S). The as-prepared Fe-doped NiCo-S (Fe-NiCo-S) possesses improved charge storage kinetics and activities as electrode material for supercapacitors and the oxygen evolution reaction (OER). The obtained Fe-NiCo-S nanosheet has a high specific capacitance (2779.6 F g-1 at 1 A g-1) and excellent rate performance (1627.2 F g-1 at 10 A g-1). A hybrid supercapacitor device made of Fe-NiCo-S as the positive electrode and reduced graphene oxide (rGO) as the negative electrode presents a high energy density of 56.0 Wh kg-1 at a power density of 847.1 W kg-1 and excellent cycling stability (capacity retention of 96.5% after 10,000 cycles at 10 A g-1). Additionally, the Fe-NiCo-S composite modified by Fe doping and S vacancy has an ultralow oxygen evolution overpotential of 247 mV at 10 mA cm-2. Based on the density functional theory (DFT) calculation, defects cause more electrons to appear near the Fermi level, which is conducive to electron transfer in electrochemical processes. Our work provides a rational strategy for facilely introducing dual defects into metal sulfides and may provide a novel idea to prepare electrode materials for energy storage and energy conversion application.