Controlling The Hydrogen Evolution Activity by Tailoring The Outer Transition Metal Layers in Double Transition Metal MXenes

Two-dimensional transition metal carbides, nitrides, or carbonitrides, known as MXenes, have shown great promise as active materials in catalytic applications such as the hydrogen evolution reaction. In double transition metal (DTM) MXenes two different transition metals occupy the metal sites which can enhance the tunability of MXenes electrocatalytic properties. The transition metal occupying the outer atomic layers controls the hydrogen evolution reaction (HER) performance based on their basal plane catalytic activity. We investigated out-of-plane ordered DTM MXenes, with inner layers of transition metal (M″: Ti) sandwiched by outer layers of different transition metals (M′: Mo, W, Cr) in a layered M'₂M"C₂T<i>_x</i> MXenes. We investigated the role of exposed basal plane transition metals on the HER catalytic activity of these out-of-plane ordered DTM MXenes and compared their behavior to a mono-transition metal MXene, Ti₃C₂T<i>_x</i>. W₂TiC₂T<i>_x</i> MXene shows the lowest HER overpotential of 147 mV at 10 mA/cm² under acidic conditions compared with Mo₂TiC₂T<i>_x</i>, Cr₂TiC₂T<i>_x,</i> and Ti₃C₂T<i>_x</i> MXenes. The improved electrocatalytic performance of W₂TiC₂T<i>_x</i> can be due to the presence of tungsten atoms in the outer M' layers. Additionally, W₂TiC₂T<i>_x</i> MXene showed long-term stability of more than 24 hours. Our findings further demonstrate that MXenes are precious-metal-free 2D flakes with highly HER active basal planes and promising and tunable electrocatalysts for clean energy applications.