Tailoring of the Exterior Transition Metal on the Electrocatalytic Performance of Double Transition Metal MXenes

Two-dimensional transition metal carbides, known as MXenes, have shown great promise as active materials in catalytic applications such as the hydrogen evolution reaction. Double transition metal (DTM) MXenes, in which two different transition metals occupy the metal sites, can enhance the tunability of MXenes electrocatalytic properties. The transition metal occupying the outer atomic layers controls the HER performance based on their basal plane 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> structures. We demonstrate the role of exposed basal plane transition metals on the HER catalytic activity of these out-of-plane ordered DTM MXenes and compare their behaviour to a mono-transition metal MXene, Ti₃C₂T<i>_x</i>. W₂TiC₂T<i>_x</i> MXene shows the lowest HER overpotential under acidic conditions over 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, we assessed W₂TiC₂T<i>_x</i> MXene for its long-term stability of more than 24 hours. Our findings, further demonstrate MXenes are precious-metal-free 2D flakes with highly HER active basal planes as well as promising and tunable electrocatalysts for clean energy applications.