Combinations of V₂C and Ti₃C₂ MXenes for Boosting the Hydrogen Storage Performances of MgH₂

Two-dimensional vanadium carbide (V₂C) and titanium carbide (Ti₃C₂) MXenes were first synthesized by exfoliating V₂AlC or Ti₃AlC₂ and then introduced jointly into magnesium hydride (MgH₂) to tailor the hydrogen desorption/absorption performances of MgH₂. The as-prepared MgH₂-V₂C-Ti₃C₂ composites show much better hydrogen storage performances than pure MgH₂. MgH₂ with addition of 10 wt % of 2V₂C/Ti₃C₂ initiates hydrogen desorption at around 180 °C; 5.1 wt % of hydrogen was desorbed within 60 min at 225 °C, while 5.8 wt % was desorbed within 2 min at 300 °C. Under 6 MPa H₂, the dehydrided MgH₂-2V₂C/Ti₃C₂ can start to recover hydrogen at room temperature, and 5.1 wt % of H₂ is obtained within 20 s at a constant temperature of 40 °C. The reversible capacity (6.3 wt %) does not decline for up to 10 cycles, which shows excellent cycling stability. The addition of 2V₂C/Ti₃C₂ can remarkably lower the activation energy for the hydrogen desorption reaction of MgH₂ by 37% and slightly reduce the hydrogen desorption reaction enthalpy by 2 kJ mol^-1 H₂. It was demonstrated that the combination of V₂C and Ti₃C₂ promotes the hydrogen-releasing process of MgH₂ compared with addition of only V₂C or Ti₃C₂, while Ti₃C₂ impacts MgH₂ more significantly than V₂C in the hydrogen absorption process of MgH₂ at ambient temperatures. A possible mechanism in the hydrogen release and uptake of the MgH₂-V₂C-Ti₃C₂ system was proposed as follows: hydrogen atoms or molecules may preferentially transfer through the MgH₂/V₂C/Ti₃C₂ triple-grain boundaries during the desorption process and through the Mg/Ti₃C₂ interfaces during the absorption process. Microstructure studies indicated that V₂C and Ti₃C₂ mainly act as efficient catalysts for MgH₂. This work provides an insight into the hydrogen storage behaviors and mechanisms of MgH₂ boosted by a combination of two MXenes.