Optimizing the microstructure of high-entropy alloys to achieve efficient hydrogen storage at room temperature

Despite the great interest in the safe and compact storage of hydrogen in the form of metal-hydrides, obtaining alloys capable of reversibly and rapidly storing large amounts of hydrogen at ambient conditions represents a challenge. High-entropy alloys (HEAs) have great potential for hydrogen storage (HS) applications because of their broad compositional design space. In this study, we designed and synthesized V₃₅Ti₃₅Cr₁₀Fe_{20-<em>x</em>}Mn<em>_x</em> (<em>x</em> = 6, 8, 10, 12, and 14) alloys based on high entropy engineering for room temperature HS. With an increase in the Mn/Fe ratio, the abundance of the BCC phase gradually increased until the formation of a single-phase BCC-structured solid-solution alloy. The V₃₅Ti₃₅Cr₁₀Fe₆Mn₁₄ alloy reached 3.79 wt% of hydrogen absorption at 298 K, which is the highest capacity reported for HEAs. All alloys were fully activated in one hydrogen ab/desorption cycle and saturated with hydrogenation within 100 s. Quasi-in situ X-ray diffraction characterization of the hydrogenation of HEAs revealed a phase transition from BCC to FCC with an intermediate pseudo-BCC structure. The cycling characteristics of the alloys evidenced that their stability gradually increased with decreasing Mn content. The microstructural analysis revealed that the capacity decay of HEAs during cycling is mainly caused by lattice deformation from repeated expansion and contraction. In addition, the HS properties of HEAs were investigated by a combination of First-principles simulation and experiments. Moreover, the thermal conductivity of the alloys was investigated. This work provides new perspectives for the design of HS alloys that can rapidly absorb large amounts of hydrogen under ambient conditions.