High-entropy alloys and the broader class of compositionally complex alloys have recently attracted significant attention as promising materials for solid-state hydrogen storage. Their potential arises not only from high configurational entropy but also from the possibility of tailoring phase composition, crystal structure, local chemical environment, and defect states that govern hydrogen sorption thermodynamics and kinetics. This review summarizes current understanding of hydrogen interaction mechanisms in HEAs and discusses the role of body-centered cubic (BCC), face-centered cubic (FCC), and Laves phases in determining hydrogen capacity, reversibility, and cyclic stability. The limitations of commonly used descriptors, including valence electron concentration (VEC), atomic size mismatch δ, enthalpy of mixing ΔH_mix, and Ω parameter, in predicting hydrogen storage behavior are critically analyzed. Particular attention is given to the effects of processing methods, phase transformations during hydrogenation/dehydrogenation, and the energetic heterogeneity of interstitial sites in multicomponent systems. The review highlights that future progress will depend on the transition from empirical alloy discovery toward physically informed multiparametric design integrating CALPHAD, DFT modeling, machine learning, and in situ/operando characterization techniques for the development of efficient and durable hydrogen storage materials.

Hydrogen is widely regarded as a key energy carrier of the future; however, its efficient and safe storage remains one of the central scientific and technological challenges. This article systematizes contemporary approaches to hydrogen storage, with a particular emphasis on solid-state materials as the most promising direction. The main storage technologies—compressed, liquid, and solid hydrogen—are reviewed, and there is an analysis of their advantages and limitations. Special attention is given to the fundamental aspects of solid-state hydrogen storage, including the thermodynamics and kinetics of hydrogen absorption and desorption processes. A comparative analysis of various material classes is presented, including intermetallic metal hydrides based on AB₅-type and AB₂-type compositions, magnesium-based systems, palladium-modified structures, as well as complex hydrides (alanates, borohydrides, and amide–imide systems).The role of nanostructuring and carbon-based nanomaterials in enhancing storage capacity and improving kinetics is also examined. Key challenges are discussed, including high desorption temperatures, slow kinetics, and material degradation. Modern strategies for performance enhancement, such as catalytic activation, alloying, and nanostructuring, are highlighted.

The effects of post-processing heat treatment on the structural and phase state and hydrogen sorption characteristics of an equiatomic TiZrNbVTa high-entropy alloy produced by mechanical activation and spark plasma sintering were investigated. The powder mixture was mechanically activated under argon and consolidated at 1450 °C. Two processing conditions were compared: SPS with a 7 min hold without subsequent treatment, and SPS with a 5 min hold followed by annealing at 1050 °C for 5 h. Phase composition was analyzed by X-ray diffraction. Hydrogenation was performed at 400–600 °C under a hydrogen pressure of 37–40 bar. Hydrogen content was evaluated using the SOAK kinetic method and inert gas fusion analysis with a RHEN-602 analyzer. Both materials were predominantly composed of a Nb-enriched body-centered cubic phase, whereas their secondary-phase compositions differed. After annealing, weak reflections tentatively attributed to ZrO₂ and TaO₂ appeared, whereas reflections previously assigned to Zr, V, and TiO₂ could no longer be resolved. According to the RHEN-602 measurements, the hydrogen content ranged from 1.12 to 1.37 wt.% for the SPS sample and from 0.64 to 2.08 wt.% for the SPS-HT sample between 400 and 600 °C. These results indicate that processing involving high-temperature annealing modifies the phase state and may enhance hydrogen uptake at 500–600 °C.

Hydrogen storage remains a major challenge for the large-scale implementation of hydrogen energy technologies. In this study, the hydrogen sorption behavior of Pd–Ti–Mg alloys synthesized by spark plasma sintering (SPS) was investigated using a combination of experimental characterization and numerical modeling. The phase composition and structural state of the synthesized material were analyzed by X-ray diffraction (XRD), while hydrogen absorption measurements were performed at different temperatures to evaluate its sorption behavior. A finite-difference numerical model was developed in MATLAB to simulate hydrogen diffusion, concentration distribution, thermally stimulated desorption, relaxation-stimulated gas evolution, pressure–composition–temperature (PCT) behavior, and hydrogen absorption–desorption kinetics. XRD analysis indicated a multiphase structure consisting predominantly of Pd₂Mg₅ and Pd₅Ti₃ intermetallic phases, together with residual Ti. Williamson–Hall analysis confirmed the nanocrystalline state of the synthesized material, with an average crystallite size of approximately 20–30 nm and microstrains of (3–5) · 10⁻³. The individual calculated crystallite sizes ranged from 18 to 35 nm, whereas the representative average value was approximately 20–30 nm. Experimental measurements demonstrated a pronounced temperature dependence of hydrogen uptake, with the highest measured hydrogen content reaching approximately 0.40 wt.% at 500 °C. Numerical simulations indicated that hydrogen transport is predominantly diffusion-controlled, resulting in a pronounced concentration gradient across the sample thickness. The thermally stimulated desorption model predicted a maximum hydrogen evolution rate of approximately 1.8 · 10⁻⁸ atoms s⁻¹ at 560–600 °C. The calculated PCT characteristics demonstrated a strong dependence of equilibrium hydrogen content on temperature and hydrogen pressure, with higher hydrogen contents predicted at lower temperatures and elevated pressures. Simulated absorption kinetics showed a rapid approach to hydrogen saturation within 10–30 s, whereas hydrogen desorption approached equilibrium within 80–120 s, depending on the applied pressure. The combined experimental and numerical results demonstrate the complementary roles of Pd-, Ti-, and Mg-containing phases in the hydrogen sorption behavior of the material, with Pd-containing intermetallic phases contributing to hydrogen dissociation and transport, while Ti- and Mg-containing phases contribute to hydrogen retention and storage. These findings demonstrate the potential of Pd–Ti–Mg-based intermetallic materials as candidate materials for solid-state hydrogen storage and related hydrogen-energy applications.

This work examines the structural–phase state and microstructural organization of mechanically activated ternary Pd-Ti-Mg powders as candidate materials for solid-state hydrogen storage. Elemental powders were processed by high-energy planetary milling (Pulverisette 7, 500 rpm; BPR=10:1) in Ar. X-ray diffraction (Cu Kα) reveals fcc-Pd (Fm-3m, a=3.887 Å), α-Ti (hcp, P6₃/mmc, a=2.950 Å, c=4.681 Å), a metastable fcc-Ti fraction (Fm-3m, a=4.060 Å), and minor hcp-Mg (P63/mmc, a=3.209 Å, c=5.211 Å); no distinct intermetallic peaks are resolved at the bulk level. Laser diffraction shows a multimodal size distribution with D10≈5 μm, D50≈20 μm, D90≈50 μm (span≈2.25), a fine sub-5 μm mode, and a coarse 300-600 μm tail attributed to Mg. SEM-EDS mapping evidences a mosaic microstructure: isometric Pd-rich domains (~10-60 μm), platy Ti-rich fragments (~20–80 μm), local Pd-Ti colocalization at interfaces (interpreted as a prereaction state for Pd-Ti intermetallics), and predominantly intergranular, dispersed Mg. The combined data indicate abundant high-energy interfaces and shortened diffusion paths created by mechanical activation, which are favorable for accelerating (de)hydrogenation while avoiding premature formation of bulk intermetallics. The results provide a microstructure-guided route to tune capacity, kinetics, and cycling stability via controlled milling parameters and targeted thermal schedules that promote beneficial Pd↔Ti interfacial reactions while limiting Mg segregation and oxidation.

This study investigates the effect of oxygen pressure during high-velocity oxygen fuel (HVOF) spraying on the microstructure, phase evolution, and tribological performance of Al ₆₅ Cu ₂₀ Fe ₁₅ quasi-crystalline coatings deposited on American Iron and Steel Institute (AISI) 321 stainless steel. Oxygen pressure was varied between 3.0, 3.5, and 4.0 bar while all other processing parameters were kept constant. Microstructural analysis shows that oxygen pressure significantly influences particle deformation, splat formation, and coating densification. An intermediate pressure of 3.5 bar produces a dense and homogeneous microstructure with reduced porosity and improved inter-splat cohesion. X-ray diffraction confirms the coexistence of quasi-crystalline and intermetallic phases, with the highest quasi-crystalline phase fraction obtained at 3.5 bar. Tribological tests under dry sliding conditions reveal a strong correlation between microstructure and frictional behavior. The coating deposited at 3.5 bar exhibits stable tribological performance with a coefficient of friction of 0.55–0.60 and the lowest wear rate. The 3.0 bar coating shows higher friction and wear due to insufficient particle heating and weak cohesion, while the 4.0 bar coating exhibits lower average friction (0.48–0.52) but unstable behavior and increased wear due to oxidation-induced brittleness. All coatings demonstrate significantly enhanced hardness (>750 HV) compared to the substrate, while adhesion strength is maximized at 3.5 bar. Overall, 3.5 bar is identified as the optimal oxygen pressure for producing Al ₆₅ Cu ₂₀ Fe ₁₅ quasi-crystalline coatings with balanced wear resistance, structural integrity, and tribological stability.