Computational Screening of 2D Ordered Double Transition-Metal Carbides (MXenes) as Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) is vital for sustainable energy production and plays a key role in achieving a hydrogen economy. Herein, density functional theory calculations are used to screen for suitable HER catalysts among 24 two-dimensional double transition-metal (TM) carbide MXenes (chemical composition M-2'M '' C2Tx and M-2'M-2 '' C3Tx; M' and M '' are two different metals, M' = Cr, V, Ti, or Nb; M '' = Nb, Ta, Ti, or V; and T = O and/or OH) and determine their thermodynamic stability under HER-relevant conditions. The established surface Pourbaix diagrams, describing the chemistry on the basal planes of the MXenes, reveal the most stable terminations under the standard conditions (U = 0, pH = 0, p = 1 bar, and T = 298 K) for Mo2Mx '' C-y, Ti2Mx '' C-y, and Nb2Ta2C3 to be O-termination, whereas Cr2Mx '' C-y and V2M2 '' C-3 expose a mixed O- and OH-termination (M '' = Nb, Ta, Ti, or V; x = 1, y = 2, or x = 2, y = 3). Eighteen different carbides are predicted to be active HER electrocatalyst candidates, and Mo2NbC2O2 showed the lowest overpotential. The Pourbaix diagrams and free energy diagrams reveal that the stability of the functional groups under HER-relevant conditions and the HER performance of the investigated double metal MXenes are closely related to its outermost TM. In other words, the outermost metal dominates the basal plane chemistry of the double TM carbide MXenes. Bader charge, density of states, and the crystal orbital Hamilton population analyses indicate that the hydrogen binding strength on different functionalized MXenes is related to the initial outer layer metal M'-O bond. A guiding observation is that the weaker the M'-O bond of the MXenes, the stronger the bonding between the terminated O* and the adsorbed H. Overall, this investigation demonstrates that the double TM carbides, as a subfamily of MXenes, provide a plethora of design opportunities, in particular as promising electrocatalysts for HER and other reactions.