Rational Design and Engineering of Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting

The transition to sustainable energy necessitates the exploration of efficient and economical electrocatalysts for water splitting. Transition metal sulfides (TMSs), benefiting from their tunable electronic configurations, intrinsic conductivity, and abundance in nature, have gained attention as viable substitutes for noble-metal-based catalysts in hydrogen and oxygen evolution reactions. To establish a solid foundation, this thesis begins with a comprehensive review of the electrocatalytic processes involved in water splitting. This includes the core reaction pathways of HER and OER, the associated kinetic and thermodynamic parameters, and performance evaluation metrics. It further summarizes the synthetic methodologies—such as hydrothermal, solvothermal, and microwave-assisted techniques—and examines the structure–property relationships of TMSs. The review also critically assesses the electrocatalytic roles of TMSs in HER, OER, and overall water splitting, with a focus on structural modulation, heterostructure formation, phase engineering, and dopant incorporation for improving catalytic activity and durability. Building on the reviewed literature, the second part of this thesis presents the author’s original research on heterostructured electrocatalysts composed of 1T- and 2H-phase MoS₂ integrated with Ni₃S₂. The design leverages phase-complementary behavior and interfacial effects to achieve dual functionality. Electrochemical evaluations revealed low overpotentials, high catalytic current densities, and excellent long-term durability in both HER and OER. When applied in a two-electrode alkaline electrolyzer, the MoS₂/Ni₃S₂ heterostructure delivered full water splitting at a cell voltage as low as 1.58 V at 20 mA cm⁻², demonstrating performance comparable to commercial noble-metal-based systems. This study highlights the critical role of interface engineering, phase control, and compositional tuning in developing next-generation TMS-based bifunctional catalysts. It also identifies key challenges—such as structural stability under harsh electrochemical conditions, dynamic surface reconstruction, and the need for mechanistic elucidation—and proposes future research directions toward industrial deployment of TMS electrocatalysts for scalable green hydrogen production.