A First-principles Study Of Hydrogen Storage Of High Entropy Alloy TiZrVMoNb

Hydrogen,with its quite plentiful amount,cost-effective renewability,high energy density and zero emission characteristics,has been considered as a potential substituent for fossil fuels.In the application of hydrogen energy,the safe,efficient and abundant storage of hydrogen has been a challenging task.In the past decades,great efforts have been devoted to develop different methods and techniques for hydrogen storage.High entropy alloys(HEAs),as a class of materials that contain five or more elements in near-equiatomic proportions,have exhibited promising hydrogen storage properties like single-phase transformation reversibility and rapid absorption kinetics.However,the hydrogen storage capacity still needs to be improved and the underlying mechanism for hydrogenation/dehydrogenation of HEAs needs to be unveiled for their application.By employing first-principles methods,we systemically explore the hydrogen absorption properties of HEA TiZrVMoNb and the hydrogen desorption behaviors from HEA TiZrVMoNb hydride surface.The main work of this paper is as follows:Firstly,we have carried out density functional theory calculations to study the hydrogen absorption properties of high entropy alloy(HEA)TiZrVMoNb.It reveals that during hydrogenation process a phase transformation from Body Centered Cubic(BCC)to Face Centered Cubic(FCC)occurs at a hydrogen content of 1.5 wt.%.The site preference for hydrogen occupation before and after phase transformation is octahedral and tetrahedral interstitial sites,respectively.Further binding energy and formation enthalpy analyses show that hydrogen occupation at tetrahedral sites can enhance the thermal stability of FCC HEA hydride,while additional octahedral site occupation would destabilize the hydride.The maximum hydrogen storage capacity for HEA TiZrVMoNb is predicted to be2.65 wt.%,which is comparable to the reported largest value of 2.7 wt.%for HEA Ti Zr VHf Nb and larger than that of other HEA hydrogen storage materials reported in the literature.A detailed comparison of the hydrogen storage property between TiZrVMoNb and Ti Zr Hf Mo Nb shows that the substitution of Hf for V in Ti Zr Hf Mo Nb HEA not only enhances the hydrogen storage capacity,but also decreases the hydrogen desorption temperature.It turns out that the site occupation,lattice distortion,atomic weight,and chemical effect of metal elements in HEAs all affect the hydrogen storage properties of HEAs.Secondly,the desorption behaviors of hydrogen from high entropy alloy TiZrVMoNb hydride surface have been investigated by the density functional theory.The most stable surface for hydrogen desorption from TiZrVMoNb hydrides is determined to be(110)surface.It is found that the desorption behaviors of hydrogen from TiZrVMoNb hydride surface are complex,due to the high lattice distortion and heterogeneous chemical environment in HEA hydrides.A comparison of molecular and atomic hydrogen desorption reveals that hydrogen prefer to desorb in atomic states from TiZrVMoNb hydride(110)surface rather than molecular states during the hydrogen desorption process,suggesting that the hydrogen desorption of HEA TiZrVMoNb is a chemical process.Two steps have been proposed for the hydrogen desorption process on TiZrVMoNb hydride(110)surface:(a)the hydrogen-metal bonds on TiZrVMoNb hydride(110)surface are broken and hydrogen desorb in atomic states near the surface;(b)the hydrogen atoms overcome attractive interaction from TiZrVMoNb hydride(110)surface to combine as H₂ molecules.In summary,the hydrogen absorption properties of HEA TiZrVMoNb and the hydrogen desorption behaviors on HEA TiZrVMoNb hydride surface are systemically explored.It is shown that the HEA TiZrVMoNb is a promising candidate for hydrogen storage.This study unveils the underlying mechanism for hydrogen absorption/desorption of HEA TiZrVMoNb and may be beneficial to promote further experimental and theoretical investigations to design new structural materials for hydrogen storage.