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In the past several years, Mg-based high entropy alloys (HEAs) have attracted great attention in high gravimetric hydrogen storage capacity. In this work, the hydrogen storage properties of Mg 0.10 Ti 0.30 V 0.25 Zr 0.10 Nb 0.25 HEA are investigated by first-principles calculations. With increasing hydrogen concentration, the structural stability of BCC and FCC hydrides decreases and increases, respectively. By comparing the lattice constants, number of hydrogenation-induced abnormal short hydrogen-hydrogen bonds and radial distribution functions of BCC and FCC hydrides, it is found that the hydrogen accommodation ability of BCC phase is weaker than that of FCC phase, and a BCC to FCC phase transformation occurs during hydrogenation process. Further total energy calculations suggest that the threshold hydrogen content of phase transformation for Mg 0.10 Ti 0.30 V 0.25 Zr 0.10 Nb 0.25 is around H/M = 1.16. By phonon spectra calculation, the maximum gravimetric hydrogen storage capacity of Mg 0.10 Ti 0.30 V 0.25 Zr 0.10 Nb 0.25 is predicated to be as high as 3.16 wt%, demonstrating that Mg 0.10 Ti 0.30 V 0.25 Zr 0.10 Nb 0.25 HEA is a potential hydrogen storage material. • Mg 0.10 Ti 0.30 V 0.25 Zr 0.10 Nb 0.25 has a maximum hydrogen storage capacity of 3.16 wt%. • A BCC to FCC phase transition occurs as the hydrogen content reaches H/M = 1.16. • The FCC phase presents stronger ability to accommodate hydrogen than BCC phase. • The structure of HEA hydrides becomes more ordered after phase transition. [ABSTRACT FROM AUTHOR]

Doping can modify the properties of metal hydrogen storage materials significantly. Currently, the metal doping is a frequent strategy, while the non-metal cation doping has not been examined extensively so far. In this study, the effects of NH 4 + doping on the hydrogen storage properties of different metal hydrides, including TiH 2 , Ti 0·25 V 0·25 Nb 0·25 Zr 0·25 H 2 , Ti 0·5 V 0·5 H 2 and VH 2 , are investigated by first-principles calculations. It is found that the NH 4 + presents a good affinity for metal hydrides and the NH 4 + incorporation leads to charge redistribution and formation of dihydrogen bond. Furthermore, the NH 4 + doping in metal hydrides is favorable for enhancing the hydrogen storage capacity and decreasing the thermal stability simultaneously. The possible reason for the NH 4 + doping induced destabilization in metal hydrides is the relatively weak interaction between NH 4 + and hydrogen atoms. [Display omitted] • The NH 4 + is first proposed to dope metal hydrides. • All the investigated metal hydrides are still dynamically stable after NH 4 + doping. • The NH 4 + doping enhances the hydrogen storage capacity of metal hydrides. • The thermal stability of metal hydride decreases with increasing NH 4 + content. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (HEAs) show promise for novel hydrogen storage technology, but addressing their limited gravimetric hydrogen storage capacity necessitates exploration of light alloying elements including Mg and Al. Here, we employ density-functional theory to investigate the hydrogen desorption energetics of Ti 0.325 V 0.275 Zr 0.125 Nb 0.275 alloy and the impact of doping with Mg and Al. Our analysis reveals that Mg and Al addition thermodynamically destabilize the Ti 0.325 V 0.275 Zr 0.125 Nb 0.275 -hydride and lower its storage capacity. The observed destabilization is attributed to reduced chemical contributions to the desorption enthalpy in the Mg and Al-doped hydrides. Detailed examination of the electronic density of states, electron localization function, and crystal orbital Hamilton population analysis unveils fundamental features of chemical bonding in these hydrides. Notably, H-H antibonding states occur for hydrogen atoms located in the nearest-neighbor interstices of Mg and Al atoms. Charge transfer facilitates formation of these antibonding states. This comprehensive analysis enhances our understanding of the intricate interplay between electronic structure, hydrogen desorption energetics, and chemical bonding in HEA hydrides, offering valuable insights for the design and optimization of advanced hydrogen storage materials. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, density functional theory calculations have been carried out to study the hydrogen storage properties of high entropy alloy (HEA) TiZrVMoNb. It reveals that a BCC→FCC phase transformation occurs when the hydrogen content reaches 1.5 wt% during hydrogenation process, and octahedral and tetrahedral interstitial sites are preferable for hydrogen occupation before and after phase transformation, respectively. Further energetic analyses show that different hydrogen occupations in HEAs play an important role in the thermal stability of hydrides. The maximum hydrogen storage capacity for TiZrVMoNb is predicted to be 2.65 wt%, which is comparable to the largest value of 2.7 wt% for TiZrVHfNb and larger than that of other reported HEA hydrogen storage materials reported in the literature. As compared with the previously reported HEA TiZrHfMoNb with the change of only one principal element, the TiZrVMoNb not only has much higher hydrogen storage capacity, but also has more moderate hydrogen desorption temperature. The difference in hydrogen storage properties between these two HEAs is mainly attributed to the atomic weight, site occupation, lattice distortion and chemical effect of metal elements. The present study thus suggests that the TiZrVMoNb HEA has great potential as hydrogen storage materials and proposes a strategy to enhance the hydrogen storage properties of HEAs. [Display omitted] • HEA TiZrVMoNb has a maximum hydrogen storage capacity of 2.65 wt%. • A BCC to FCC phase transition occurs when the hydrogen content reaches to 1.5 wt%. • The thermal stability of HEA hydrides can be influenced by hydrogen occupation. • The hydrogen storage property of HEA TiZrVMoNb is superior to HEA TiZrHfMoNb. [ABSTRACT FROM AUTHOR]

A body-centered cubic equiatomic TiZrTaNbAl multi-principal element alloy (MPEA) with elemental fluctuations was investigated to further understand the relationship between the microstructure and hydrogen distribution. In this study, a composition dependence of the hydrogen distribution was observed in the TiZrTaNbAl MPEA. An inhomogeneous electron density distribution of the MPEA was revealed by advanced differential phase-contrast scanning electron microscopy (DPC-STEM) for the first time. The results showed that the electron density has a significant effect on the hydrogen distribution in TiZrTaNbAl MPEAs. This work provides new insight into the design of materials with high hydrogen storage capacity and high hydrogen embrittlement resistance. [ABSTRACT FROM AUTHOR]