Ordered Double-Metal Carbonitride MXenes with Tunable Nitrogen Content for the Hydrogen Evolution Reaction

Ordered double transition-metal (DTM) MXenes are a subfamily of two-dimensional (2D) carbides, nitrides, and carbonitrides, predicted to outperform single-metal MXenes in hydrogen evolution reaction (HER) catalysis due to the synergistic effect of two metals and their nonmetal (X) sublattice tailoring (X = C, N), resulting in tunable electronic structures. However, all synthesized DTM MXenes to date contain only carbon in the X sublattice. Here, we report the synthesis of a series of out-of-plane ordered DTM carbonitride MXenes (o-MXenes), Mo₂Ti(CN)₂T_x and Mo₂Ti₂(CN)₃T_x, to systematically investigate the role of carbon to nitrogen ratio. To determine the optimal nitrogen content, we first evaluated the HER activity of the Mo₂TiC_2-yN_yT_x MXenes with density functional theory calculations and identified that 0.3 to 0.6 mol of nitrogen give enhanced performance compared to the carbide. We next synthesized and characterized 11 carbonitride MXenes with varying their C:N ratios and found nitrogen incorporation enhances HER activity compared to their carbide counterparts. Among them, Mo₂TiC_2-yN_yT_x MXene with 0.6 mol of nitrogen (y = 0.6) achieved the best performance, with an overpotential of ∼155 mV at 10 mA/cm² under acidic conditions, compared with ∼236 mV for Mo₂TiC₂T_x. Our experimental and computational findings indicate that carbonitrides with ∼25-30 atom % nitrogen outperform all other o-MXene counterparts, with the improved performance arising from nitrogen-induced modulation of the electronic structure. This study identifies nonmetal sublattice control as a critical frontier in optimizing MXenes for sustainable energy applications.