Ternary Heterojunction Co ₉ S ₈ –MnO ₂ @MoS ₂ /NF as a High-Performance Bifunctional Electrocatalyst for Seawater Electrolysis

Developing highly efficient and durable bifunctional electrocatalysts for large-scale alkaline water and seawater electrolysis remains a major challenge. In this work, we synthesized core–shell Co 9 S 8 –MnO 2 @MoS 2 /NF with ternary heterojunction interfaces via a two-step hydrothermal method. This ternary structure modulates the electronic structure at the interface, improves the adsorption of HER/OER intermediates, and enhances overall catalytic activity and stability. The MoS 2 overlayer acts as a protective and electronic-modulation layer. Consequently, the Co 9 S 8 –MnO 2 @MoS 2 /NF exhibits outstanding bifunctional performance in alkaline freshwater, simulated seawater, and natural seawater, requiring overpotentials of only 64/87/97 mV for HER and 181/191/215 mV for OER at 10 mA cm –2 . In a two-electrode configuration, the symmetric electrolyzer achieves 10 mA cm –2 at cell voltages of 1.46/1.51/1.52 V in the three electrolytes, outperforming most reported electrocatalysts. Moreover, it maintains stable operation for 210 h under multicurrent step testing in natural seawater. Density functional theory (DFT) calculations confirm that the MoS 2 incorporation increases the density of states near the Fermi level and upshifts the d-band center, which significantly optimizes the Gibbs free energy for hydrogen adsorption (ΔG H* ) and reduces the energy barrier of the OER rate-determining step (ΔG RDS ). This work demonstrates that heterojunction engineering and electronic modulation provide an effective strategy for designing high-performance, corrosion-resistant bifunctional electrocatalysts for practical water-splitting and seawater-splitting applications.