Graphene-Confined NiCo ₂ S ₄ Nanoflowers: A High-Efficiency Electrocatalyst for Oxygen Evolution and Zn-Air Batteries

Transition metal sulfides have an electronic structure that can be tuned to multiple active sites, making them ideal as electrocatalysts for the oxygen evolution reaction (OER). However, their poor conductivity and aggregation tendencies hinder charge transfer and active site availability. In this work, we developed a graphene-encapsulated flower-like NiCo2S4 composite (NiCo2S4@GA/NF) through a sequential water bath and hydrothermal synthesis method. The synthesized catalyst exhibits outstanding performance in the OER, requiring only 194 mV overpotential at a current density of 50 mA cm–2 and 294 mV at 100 mA cm–2, with a Tafel slope of 38.2 mV dec–1. The material exhibits outstanding durability, maintaining structural integrity and catalytic activity after 40 h stability testing. The NiCo2S4@GA/NF dual-electron system achieved current densities of 20 and 50 mA cm–2 at 1.41 and 1.68 V, respectively, demonstrating excellent charge-transfer kinetics. Additionally, zinc-air batteries assembled with NiCo2S4@GA/NF catalysts as air electrodes demonstrated excellent stability and outstanding charge–discharge performance during long-term cycling, enabling the batteries to operate stably at a high current density of 300 mA cm–2 and to deliver a peak power density of 20 mW cm–2. This study develops a design strategy for high-efficiency bifunctional electrocatalysts, demonstrating its utility in renewable energy applications such as water splitting and zinc-air batteries.