Two-dimensional Vanadium Carbide for Simultaneously Tailoring the Hydrogen Sorption Thermodynamics and Kinetics of Magnesium Hydride

Magnesium hydride (MgH₂) is a potential material for solid-state hydrogen storage. However, the thermodynamic and kinetic properties are far from practical application in the current stage. In this work, two-dimensional vanadium carbide (V₂C) MXene with layer thickness of 50−100 nm was fist synthesized by selectively HF-etching the Al layers from V₂AlC MAX phase and then introduced into MgH2 to improve the hydrogen sorption performances of MgH2. The onset hydrogen desorption temperature of MgH₂ with V2C addition is significantly reduced from 318 °C for pure MgH2 to 190 °C, with a 128 °C reduction of the onset temperature. The MgH₂+ 10 wt% V₂C composite can release 6.4 wt% of H₂ within 10 min at 300 °C and does not loss any capacity for up to 10 cycles. The activation energy for the hydrogen desorption reaction of MgH₂ with V₂C addition was calculated to be 112 kJ mol−1 H₂ by Arrhenius's equation and 87.6 kJ mol−1 H₂ by Kissinger's equation. The hydrogen desorption reaction enthalpy of MgH₂ + 10 wt% V₂C was estimated by van't Hoff equation to be 73.6 kJ mol−1 H₂, which is slightly lower than that of the pure MgH₂ (77.9 kJ mol−1 H₂). Microstructure studies by XPS, TEM, and SEM showed that V₂C acts as an efficient catalyst for the hydrogen desorption reaction of MgH₂. The first-principles density functional theory (DFT) calculations demonstrated that the bond length of Mg−H can be reduced from 1.71 Å for pure MgH₂ to 2.14 Å for MgH₂ with V₂C addition, which contributes to the destabilization of MgH₂. This work provides a method to significantly and simultaneously tailor the hydrogen sorption thermodynamics and kinetics of MgH₂ by two-dimensional MXene materials.