Sub-Nanometer Cobalt on Tungsten Titanium Carbide MXene (W₂TiC₂T_x): An Electrocatalyst for Highly Efficient and...

Developing non-precious electrocatalysts that simultaneously deliver high activity, long-term durability, and industrial operability remains the critical challenge for the alkaline hydrogen evolution reaction (HER). Herein, a structurally well-defined two-dimensional metal carbide MXene, tungsten titanium carbide (W₂TiC₂T_x), is synthesized for the first time via the W₂TiAlC₂ MAX-phase precursor. Cobalt loading combined with rational modulation of local atomic configurations and metal-support interactions (MSI) enables the construction of a highly active and robust Co/W₂TiC₂ HER catalyst. The optimized Co/W₂TiC₂-700 exhibits small overpotentials of 63 and 191 mV at 10 and 100 mA cm^{- 2}, and outstanding long-term durability of over 1000 h stable hydrogen production at 4000 mA cm^{- 2}. In a flow-cell MEA electrolyzer, Co/W₂TiC₂ delivers near-unity hydrogen Faradaic efficiency across a wide current range (50-400 mA cm^{- 2}) while requiring significantly lower cell voltages than commercial Pt/C. Quasi-in-situ XPS, XANES, and EXAFS analyses reveal that thermal modulation induces the transformation of Co from isolated atoms and large nanoparticles into uniform sub-nanometer particles anchored on the outer tungsten layers. DFT calculations identify Co-W interfacial sites as the primary active centers. This work highlights the critical role of rational design and utilization of MSI in MXene-supported catalysts for electrochemical water splitting.