Vertical 2D MoO₂/MoSe₂ Core–Shell Nanosheet Arrays as High-Performance Electrocatalysts for Hydrogen Evolution Reaction

Electrochemical water splitting is very attractive for green fuel energy production, but the development of active, stable, and earth-abundant catalysts for the hydrogen evolution reaction (HER) remains a major challenge. Here, core–shell nanostructured architectures are used to design and fabricate efficient and stable HER catalysts from earth-abundant components. Vertically oriented quasi-2D core–shell MoO₂/MoSe₂ nanosheet arrays are grown onto insulating (SiO₂/Si wafer) or conductive (carbon cloth) substrates. This core–shell nanostructure array architecture exhibits synergistic properties to create superior HER performance, where high density structural defects and disorders on the shell generated by a large crystalline mismatch of MoO₂ and MoSe₂ act as multiple active sites for HER, and the metallic MoO₂ core facilitates charge transport for proton reduction while the vertical nanosheet arrays ensure fully exposed active sites toward electrolytes. As a HER catalyst, this electrode exhibits a low Tafel slope of 49.1 mV dec⁻¹, a small onset potential of 63 mV, and an ultralow charge transfer resistance (R_ct) of 16.6 Ω at an overpotential of 300 mV with a long cycling durability for up to 8 h. This work suggests that a quasi 2D core–shell nanostructure combined with a vertical array microstructure is a promising strategy for efficient water splitting electrocatalysts with scale-up potential.