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Two-dimensional (2D) transition metal carbides/nitrides (MXenes) materials exhibit potential as catalysts for the hydrogen evolution reaction (HER) due to their large surface area, excellent conductivity, and notable electrical properties. Despite these advantages, pristine MXenes often display insufficient catalytic activity compared to noble metal electrocatalysts. Enhancing the HER performance of MXenes by introducing nonmetal atoms on the surface has proven effective. However, understanding the precise regulation of the HER performance by nonmetal atoms remains challenging. In this study, we utilized density functional theory calculations to explore how the introduction of fourteen distinct nonmetal atoms into Mo 2 CO 2 catalysts (NM-Mo 2 CO 2) influences the catalytic behavior in HER. The results indicated that nonmetal atom doping significantly activates oxygen atoms, substantially enhancing catalytic activity. Notably, the addition of nonmetal atoms like chlorine (Cl), bromine (Br), and iodine (I) onto the Mo 2 CO 2 surface effectively improved HER activity, showcasing their potential as HER catalysts. Our study proved that nonmetal atom doping not only causes localized structural distortions but also charge redistribution, influencing the HER performance of NM-Mo 2 CO 2. Furthermore, we developed two descriptors to quantify the regulation mechanism of nonmetal atoms, successfully predicting the HER activity of NM-Mo 2 CO 2 and extending these insights to NM-Cr 2 CO 2 and NM-V 2 CO 2. These findings offer a comprehensive understanding of how nonmetal atom doping enhances HER activity, providing valuable insights for innovative catalyst design. [Display omitted] • NM doping activates oxygen atoms, boosting catalytic activity in Mo 2 CO 2 for HER. • Chlorine, bromine, and iodine enhance Mo 2 CO 2 , showcasing potential as HER catalysts. • Study unveils localized structural distortions and charge redistribution in NM-Mo 2 CO 2. • Two descriptors predict and extend insights to NM-Cr 2 CO 2 and NM-V 2 CO 2. [ABSTRACT FROM AUTHOR]

Electrocatalytic Ammonia Synthesis Process on the Mo 2 TiC 2 MXene. • Nineteen different possible pathways (five association pathway and fourteen dissociation pathway) analyzed by DFT calculation and Gibbs free energy calculation. • Valid N 2 -philicity, N≡N triple bond of the N 2 molecule (the optimal distance) is sufficiently activated, from 1.11 Å to 1.268Å • Mo 2 TiC 2 MXene can reduce the overpotential by changing the reaction pathway. • Mo 2 TiC 2 as an ordered, double transition metals carbides is an eligible electrocatalyst for the NRR Electrocatalytic ammonia synthesis provides an energy-efficient alternative to the Haber−Bosch process. The aim is to find promising electrocatalysts which are able to change the reaction pathway and reduce the overpotential. Here, based on density functional theory, a comprehensive mechanism study of the N 2 activation and NH 3 synthesis on the Mo 2 TiC 2 MXenes is presented. For catalytic reaction mechanism, nineteen different possible pathways are screened for the lowest overpotential, where the corresponding potential-determining step are compared by Gibbs free energy calculation. The result reveals Mo 2 TiC 2 MXenes exhibit both valid N 2 -philicity and high catalytic activity for electrocatalytic ammonia synthesis through a dissociation mechanism with a low overpotential of 0.26 V. Further, the competing reaction of H 2 evolution is simultaneously suppressed which shows a relatively high potentials of 0.74 V. This study shows a brand new material for catalyzing NH 3 synthesis under ambient conditions and provides the theory background to reduce the overpotential by changing the reaction pathway. [ABSTRACT FROM AUTHOR]