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2D transition metal carbides and nitrides (MXenes) are promising hydrogen evolution reaction (HER) catalysts owing to their metallic conductivity, abundant surface active sites, and high specific surface area. The introduction of a single transition metal atom (TM) at the surface is a good way to improve the HER performance of MXenes. However, the effect of TM on MXenes in previous theoretical studies focused on pure functional groups, and ignored the hybrid‐functionalized ones, which are mostly observed in experiments. Herein, the HER performance of four O/F ratios stable hybrids MXenes, Ti2CTx (T = O, F), is explored. Ti2CO1.33F0.67 exhibits superior HER catalytic activity, comparable to that of platinum metals. Further combinatorial screening of ≈200 TMs based on Ti2CTx structures suggests that Rh, Ti, Ir, and Pt are optimal TM candidates that enhance the sensitivity to strain modulation and reduce the activation barrier for hydrogen generation. A descriptor ψ is used to quantify HER performance and reveals the role of the electron filling of TM to the antibonding orbitals. These findings propose feasible candidates with high HER performance through single‐atom modification for hybrid‐functional MXenes, and a useful descriptor to screen for MXenes with desirable catalytic properties. [ABSTRACT FROM AUTHOR]

Density functional theory calculations are employed to systematically investigate the trend of hydrogen evolution reaction (HER) performance of oxygen-terminated MXenes. By studying 30 transition-metal carbides and 30 transition-metal nitrides, Mn+1CnO2 and Mn+1NnO2 (M = Sc, Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr; n = 1, 2, 3), the tendency of oxygen desorption after hydrogen adsorption is elucidated to play a key role in HER performance of oxygen-terminated MXenes. On the basis of these observations, we propose a suitable HER descriptor, oxygen vacancy formation energy (Ef), which scales linearly with the adsorption free energy of hydrogen, ΔGH. In addition, this new descriptor is linearly correlated with the lithium binding strength on oxygen-terminated MXenes. Therefore, Ef is a universal descriptor for identifying the trend of adsorption processes where adsorbed species donate electrons to oxygen-terminated MXenes. This work provides a general guideline for large-scale screening of promising MXene-based materials for energy storage and conversion. [ABSTRACT FROM AUTHOR]