Effects of vacancies on Mg2NiH4 hydrogen desorption and capture: A first-principles study

摘要

The effects of vacancies on Mg2NiH4 hydrogen desorption and capture are investigated by first principles calculations and the crystal structure, enthalpy and electronic structure are discussed in this work. The introduction of vacancies reduces the thermodynamic stability, and the dehydrogenation enthalpies of Mg2NiH4-Vm (m = Mg1, Mg2, Mg3 and Ni) are lower than that of Mg2NiH4. The dangling bonds are generated in Mg2NiH4-VMg2 and Mg2NiH4 -VMg3, corresponding to smaller changes of crystal structures and lower stabilities. The trapping of H atoms in Mg2NiH4-Vm (m = Mg1, Mg2 and Mg3) can proceed spontaneously. The captured hydrogen atom H33 and the nearby NiH44-group forms a Ni-5H group in Mg2NiH4-Vm (m = Mg1, Mg2 and Mg3). The weak hybridization of H33 with Ni d orbital and the generation of free electrons are responsible for the much higher formation enthalpy of Mg16Ni8H33-VMg1 than Mg16Ni8H33-VMg2 and Mg16Ni8H33-VMg3. The strong hybridization between the H33 atom and the free H3 atom in Mg16Ni8H33-VNi indicates that the addition of H33 leads to the transition of the electrons, and this process cannot take place spon-taneously. This work confirms the positive effects of vacancies on Mg2NiH4 and provides a new research perspective to control the hydrogen storage capacities of Mg-based alloys.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.