Etching and Exfoliation Properties of Cr2AlC into Cr2CO2 and the Electrocatalytic Performances of 2D Cr2CO2 MXene

In the recent years, two-dimensional (2D) metal carbide and nitride (MXenes) materials have shown characteristics of promising electrocatalytic properties for hydrogen evolution reaction (HER). Here, by using density functional theory calculations, the etching and exfoliating properties of Cr2AlC to Cr2CO2 and electrocatalytic (for HER) properties with and without carbon defect were studied. Results show that etching the pristine Cr2AlC by HF solutions could intend to generate Cr2C MXenes with O* termination, that is, Cr2CO2, and Cr2CF2 and Cr2C(OH)2 will translate into Cr2CO2 even if they were generated first during the etching reactions. The exfoliation energy of multilayer Cr2CO2 into monolayer Cr2CO2 MXene is large (2.78 eV/nm2), and the delamination process requires Li+ (LiF) cation. Carbon vacancy is easily generated during the etching and exfoliation reactions, and the formation energy of carbon vacancy (1.71 and 1.59 eV with and without considering charge correction) of Cr2CO2 is lower than that of common 2D materials, such as graphene (7.4 eV) and MoS2 (2.12 eV for forming S vacancy and 6.2 eV for forming Mo vacancy). HER performances of Cr2CO2 were further studied considering the solvent effect. The studies indicated that the solvent can affect the performance of HER of Cr2CO2 at medium hydrogen coverage. Gibbs free energy of hydrogen adsorption (ΔGH*) increases at low hydrogen coverage, then reduces at medium hydrogen coverage, and finally increases at high hydrogen coverage. These results provide a guideline for experimentally synthesizing the 2D MXene materials and developing new promising HER catalysts for water splitting.