## 1 Introduction  
All of the world's leading countries have been paying great attention to the development of alternative (renewable) energy sources in recent years. Hydrogen is considered as one of the most promising fuels with a high energy capacity. However, for widespread use, it is necessary to solve many problems, such as separation of hydrogen from gas mixtures, compact storage and recycling for energy production. High-entropy alloys (HEAs) formed by five or more metals with close to equiatomic concentrations [1] can be used to overcome these problems. These alloys have a high hardness [2], corrosion resistance [3, 4], wear resistance and thermal stability. Some of them show a high hydrogen sorption capacity [5] because of strong deformation of the HEAs' crystal lattice due to the difference in atomic sizes, which creates additional opportunities for hydride formation. For example, the equiatomic alloy TiVZrNbHf absorbs a large amount of hydrogen (H/M = 2.5, which exceeds the hydrogen absorption capacity of any of the alloy elements) [6]. Such a high H/M ratio is not typical for transition metals, except for very high pressures (TiH\({}_{1.8}+\gamma\) at 5 GPa [7]). In order to predict and adjust HEAs' properties, it is customary to consider parameters such as the VEC (concentration of valence electrons), \(\delta\) (parameter characterizing difference in atomic sizes of elements in an alloy), \(\delta_{\rm x}\) (parameter characterizing the difference in electronegativity of elements in an alloy), \(\Delta\)H\({}_{\rm mix}\) (enthalpy of mixing of alloy components) and \(\Omega\) (parameter relating to the melting point (T\({}_{\rm m}\)), \(\Delta\)H\({}_{\rm mix}\) and \(\Delta\)S\({}_{\rm mix}\)) [8, 9]. Two trends correlating with the VEC were found experimentally: first, the volume expansion per metal unit (\(\alpha\)) increases linearly with increasing the VEC; second, the beginning of the hydrogen desorption temperature decreases linearly with increasing the VEC, and it reaches room temperature at VEC = 6.4 [8]. The calculation of various thermodynamic parameters of alloys makes it possible to predict the formation of single-phase structures, to evaluate the hydrogen absorption properties of alloys. As it has been shown in [10], at \(\Omega<\) 1, large values of \(\Delta\)H\({}_{\rm mix}\) determine the predominant contribution of the enthalpy factor to the free energy change (\(\Delta\)G), which contributes to the formation of highly stable binary and ternary intermetallic compounds. At \(\Omega>\) 1, the entropy contribution prevails and there is a tendency for the formation of disordered solid solutions. It is possible to construct phase stability diagrams based on the calculations, but the limiting factor of the phase formation and stability prediction method is related to its inability to take into account all competing phases [11]. Compared to the empirical approach, thermodynamic models have much greater accuracy and were able to determine the composition areas in which single-phase HEAs exist [11; 12; 13; 14]. The CALPHAD (CALculation of PHAse Diagrams) is another method for calculating the composition of stable phases in studies of wind farms for hydrogen storage [15; 16]. The construction of phase diagrams of equilibrium phases is based on the use of thermodynamic models that analytically estimate the Gibbs-Helmholtz free energy for all expected competing phases [17]. At the moment, several promising HEA systems for hydrogen storage have been discovered using the CALPHAD method [18; 19; 20], for example, the Ti-V-Zr-Nb-Hf alloy. The CALPHAD method has such disadvantages as limiting the prediction possibilities outside the ranges approximated by polynomial functions, errors introduced by the lack of data on competing intermetallic phases and limitations in predicting metastable phases (which may be the most successful). Conventional arc melting is the most common and the simplest method for the synthesis of HEAs.