Two-Dimensional, Ordered, Double Transition Metal Carbides (MXenes): A New Family of Promising Catalysts for the Hydrogen Evolution Reaction

Abstract

Generation of hydrogen by splitting water with the electrocatalytic approach could become a more sustainable way following the discovery of new materials, such as the 2D transition-metal carbides. Developing eco-friendly, low-cost, stable, and highly active nonprecious hydrogen evolution reaction (HER) catalysts is one of key factors for hydrogen energy economy. Two-dimensional metal carbide and nitride (MXenes) materials have shown characteristics of promising HER catalysts. Herein, we explored the conductive and thermal stability and electrocatalyst performance of four 2D ordered double MXenes M2 ′M″C2, Cr2TiC2, Cr2VC2, Mo2TiC2, and Mo2VC2, and their corresponding oxygen (O*)- or hydroxyl (OH*)-terminated MXenes by using density functional calculations. Results indicated that all the above MXenes are conductive, which are favored to charge transfer during HER. Four MXenes are fully terminated by O* under standard conditions [pH = 0, p(H2) = 1 bar, U = 0 V]. The Gibbs free energy for the adsorption of atomic hydrogen (ΔG H*) on the O*-terminated M2 ′M″C2 (e.g., Cr2TiC2O2) is close to 0 eV (the ideal value) at suitable H coverage. The formability of oxygen vacancy in the fully O*-terminated M2 ′M″C2, that is, M2 ′M″C2O2 was studied, and a linear relationship between the formation energy of oxygen vacancy (E f) and ΔG H* was obtained. The electronic structure analysis indicates that the more electrons gained by the terminated O* from M2 ′M″C2, the higher is the occupation of the p orbitals of the terminated O* and thus the weaker is the binding strength between the terminated O* and the adsorbed H. Our results indicated that O*-terminated M2 ′M″C2 are promising HER electrocatalysts for generating hydrogen by water splitting.

Theoretical Study of Transition‐Metal‐Modified Mo2CO2 MXene as a Catalyst for the Hydrogen Evolution Reaction

“…The results demonstrate that the of TM‐modified Mo 2 CO 2 is generally larger in value than that of the pure Mo 2 CO 2 (except for Ni at 12.5 % ML coverage), which implies that the former presents weaker interaction between H and O. It is generally accepted that the | |<0.2 eV represents the promising HER activity [28] . Based on this threshold criteria (highlighted by the grey area in Figure 5d), the surface‐modification of Mo 2 CO 2 with 12.5 %−V, 12.5 %−Cr, 12.5 %−Mn, 12.5 %−Fe, 16.7 %−Fe, 16.7 %−Co, 16.7 %−Ni and 16.7 %−Cu are potentially active electrocatalysts for HER (the corresponding are 0.13, −0.18, 0.08, −0.08, 0.19, 0.13, −0.17, and 0.16 eV, respectively).…”

“…It is generally accepted that the j DG H* j < 0.2 eV represents the promising HER activity. [28] Based on this threshold criteria (highlighted by the grey area in Figure 5d), the surface-modification of Mo 2 CO 2 with 12.5 %À V,12.5 %À Cr,12.5 %À Mn,12.5 %À Fe,16.7 %À Fe,16.7 %À Co,16.7 %À Ni and 16.7 %À Cu are potentially active electrocatalysts for HER (the corresponding DG H* are 0. 13, À 0.18, 0.08, À 0.08, 0.19, 0.13, À 0.17, and 0.16 eV, respectively).…”

Strategies to improve electrocatalytic and photocatalytic performance of two-dimensional materials for hydrogen evolution reaction

“…The electrocatalytic activity of MXenes is closely related to their oxygen-containing groups [187]. energy and hydrogen Gibbs free energy showed that increasing the number of electrons obtained by the end group oxygen enhanced the electrocatalytic performance.…”

Tuning 2D MXenes by Surface Controlling and Interlayer Engineering: Methods, Properties, and Synchrotron Radiation Characterizations

“…[126] It was also proved through DFT calculations that Cr 2 TiC 2 O 2 , Cr 2 VC 2 O 2 , Mo 2 TiC 2 O 2 , and Mo 2 VC 2 O 2 could exhibit excellent HER activity with surface oxygen atoms as the active sites. [127] Moreover, another extensively adopted strategy to improve the electrocatalysis property was heteroatom doping [128] or composite with other materials. [129] In 2018, Wang and co-workers reported the Mo 2 TiC 2 T x nanosheets with abundant surface Mo vacancies (V Mo ) by electrochemical exfoliation (Mo 2 TiC 2 T x -V Mo ), in which the Mo vacancies could act as the anchoring sites for single Pt atoms (Mo 2 TiC 2 T x -Pt SA ).…”