Synthesis of a 2D tungsten MXene for electrocatalysis

Two-dimensional transition metal carbides, nitrides, and carbonitrides, known as MXenes, are of interest as electrocatalysts. Tungsten- (W) based MXenes are predicted to have low overpotentials in the hydrogen evolution reaction (HER), however, their synthesis has proven difficult due to the calculated instability of its hypothetical MAX precursors. In this study, we present a theory-guided synthesis of a W-based MXene, W2TiC2Tx, derived from a non-MAX nanolaminated ternary carbide (W,Ti)4C4-y precursor by the selective etching of one of the covalently bonded tungsten layers. Our results indicate the importance of W and Ti ordering, the presence of vacancy defects in the metal layers, and lack of oxygen impurities in the carbon layers for the successful selective etching of the precursor. We confirm the atomistic out-of-plane ordering of W and Ti using computational and experimental characterizations. The W-rich basal plane endows W2TiC2Tx MXene with high electrocatalytic HER performance (overpotential ~144 mV at 10 mA/cm2). This study reports a W-based MXene synthesized from a covalently bonded non-MAX precursor, adding to the synthetic strategies for 2D materials.