Atomic-Scale Confined Synthesis of Ultrathin W₂C Nanowires in Single-Wall Carbon Nanotubes for the High-Performance Hydrogen Evolution Reaction

Phase-pure ultrafine W₂C nanostructures are promising electrocatalysts but face synthesis challenges due to unclear formation mechanisms and harsh thermodynamics. Here, we reveal the formation mechanism of ultrathin W₂C nanowires (NWs) confined in the cavity of single-wall carbon nanotubes (SWCNTs) at the atomic scale by combined in situ transmission electron microscopy and density functional theory calculations. It was found that the hollow core of SWCNTs can control the phase, axial orientation, and diameter of W₂C NWs. Leveraging this mechanism, we synthesized SWCNT-encapsulated W₂C NWs, WS₂-W₂C heterostructures, and WS₂ NWs (1D@1D), which assembled into free-standing hybrid films. The integrated W₂C NWs@SWCNT membrane was primarily tested, exhibiting a low overpotential of 44 mV to reach a current density of 10 mA cm^-2 and outstanding durability (500 h at a high current density of 250 mA cm^-2 in acidic conditions).