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 <i>in situ</i> 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).