Mn-incorporated high-entropy quaternary sulfide CoNiFeMnS for efficient electrocatalytic oxygen evolution reaction

Electrochemical water splitting is promising for sustainable hydrogen production but limited by sluggish oxygen evolution reaction (OER) kinetics. Noble-metal catalysts (e.g., RuO₂ and IrO₂) are benchmarks but suffer from high cost and poor durability, prompting the search for noble-metal-free alternatives. Quaternary high-entropy sulfides (HESs) are attractive due to their structural stability, tunable electronics, and abundant active sites. An Mn-doped CoNiFeMnS HES is fabricated via a facile two-step coprecipitation-solvothermal strategy using a CoNiFeMn high-entropy metal-organic framework (HEMOF) precursor. Characterization studies confirm uniform Mn doping into the face-centered cubic (fcc) structure, forming a micropolygonal morphology that enhances active site exposure. Mn doping modulates the electronic structure via electron redistribution among Co, Fe, and Ni, generating highly active Ni centers. Electrochemical tests show that CoNiFeMnS outperforms Mn-free CoNiFeS, with OER overpotential reduced from 301 mV to 208 mV at 10 mA cm^-2, accompanied by a low Tafel slope (65.19 mV dec^-1), enhanced double-layer capacitance (12.15 mF cm^-2), reduced charge-transfer resistance (141.1 Ω), and excellent 80 h stability. This work provides a scalable route for Mn-doped HESs and clarifies Mn's regulatory role in OER optimization, guiding the design of high-performance noble-metal-free OER catalysts.