Self-Assembly of Large-Area 2D Polycrystalline Transition Metal Carbides for Hydrogen Electrocatalysis

Low-dimensional (0/1/2 dimension) transition metal carbides (TMCs) possess intriguing electrical, mechanical, and electrochemical properties, and they serve as convenient supports for transition metal catalysts. Large-area single-crystalline 2D TMC sheets are generally prepared by exfoliating MXene sheets from MAX phases. Here, a versatile bottom-up method is reported for preparing ultrathin TMC sheets (≈10 nm in thickness and >100 μm in lateral size) with metal nanoparticle decoration. A gelatin hydrogel is employed as a scaffold to coordinate metal ions (Mo⁵⁺ , W⁶⁺ , Co²⁺ ), resulting in ultrathin-film morphologies of diverse TMC sheets. Carbonization of the scaffold at 600 °C presents a facile route to the corresponding MoC_x , WC_x , CoC_x , and to metal-rich hybrids (Mo_{2- x} W_x C and W/Mo₂ C-Co). Among these materials, the Mo₂ C-Co hybrid provides excellent hydrogen evolution reaction (HER) efficiency (Tafel slope of 39 mV dec^-1 and 48 mVj = 10 mA cm-2 in overpotential in 0.5 m H₂ SO₄ ). Such performance makes Mo₂ C-Co a viable noble-metal-free catalyst for the HER, and is competitive with the standard platinum on carbon support. This template-assisted, self-assembling, scalable, and low-cost manufacturing process presents a new tactic to construct low-dimensional TMCs with applications in various clean-energy-related fields.