Metal hydrides (MH_x) provide a promising solution for the requirement to store large amounts of hydrogen in a future hydrogen-based energy system. This requires the design of alloys which allow for a very high H/M ratio. Transition metal hydrides typically have a maximum H/M ratio of 2 and higher ratios can only be obtained in alloys based on rare-earth elements. In this study we demonstrate, for the first time to the best of our knowledge, that a high entropy alloy of TiVZrNbHf can absorb much higher amounts of hydrogen than its constituents and reach an H/M ratio of 2.5. We propose that the large hydrogen-storage capacity is due to the lattice strain in the alloy that makes it favourable to absorb hydrogen in both tetrahedral and octahedral interstitial sites. This observation suggests that high entropy alloys have future potential for use as hydrogen storage materials.

Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuIn_xGa_1−xSe₂, are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results.

While the overwhelming number of papers on multi-principal-element alloys (MPEAs) focus on the mechanical and microstructural properties, there has been growing interest in these alloys as solid-state hydrogen stores. We report here the synthesis optimization, the physicochemical and the hydrogen sorption properties of Ti_0.325V_0.275Zr_0.125Nb_0.275. This alloy was prepared by two methods, high temperature arc melting and ball milling under Ar, and crystallizes into a single-phase bcc structure. This MPEA shows a single transition from the initial bcc phase to a final bct dihydride and a maximum uptake of 1.7 H/M (2.5 wt%). Interestingly, the bct dihydride phase can be directly obtained by reactive ball milling under hydrogen pressure. The hydrogen desorption properties of the hydrides obtained by hydrogenation of the alloy prepared by arc melting or ball milling and by reactive ball milling have been compared. The best hydrogen sorption properties are shown by the material prepared by reactive ball milling. Despite a fading of the capacity for the first cycles, the reversible capacity of the latter material stabilizes around 2 wt%. To complement the experimental approach, a theoretical investigation combining a random distribution technique and first principle calculation was done to estimate the stability of the hydride.

Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 ‘Energy Storage and Conversion based on Hydrogen’ of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group ‘Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage’. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage.

Magnetic and magnetocaloric properties of high-purity, giant magnetocaloric polycrystalline and single-crystalline Fe${}_{2}$P are investigated. Fe${}_{2}$P displays a moderate magnetic entropy change, which spans over 70 K and the presence of strong magnetization anisotropy proves this system is not fully itinerant but displays a mix of itinerant and localized magnetism. The properties of pure Fe${}_{2}$P are compared to those of giant magnetocaloric (Fe,Mn)${}_{2}$(P,$A$) (where $A=\text{As}$, Ge, Si) compounds helping understand the exceptional characteristics shown by the latter, which are so promising for heat pump and energy conversion applications.

Al_0.10Ti_0.30V_0.25Zr_0.10Nb_0.25 was prepared to evaluate the effect of 10% aluminum into the previously reported quaternary alloy, Ti_0.325V_0.275Zr_0.125Nb_0.275. The as-cast quinary alloy formed a single-phase body centered cubic solid solution and transformed into a body centered tetragonal after hydrogenation. The alloy had a storage capacity of 1.6 H/M (2.6 wt.%) with fast absorption kinetics at room temperature, reaching full capacity within the first 10 min. The major improvements of Al addition (10%) were related to the desorption and cycling properties of the material. The temperature for hydrogen release was significantly decreased by around 100 °C, and the quinary alloy showed superior cycling stability and higher reversible storage capacity than its quaternary counterpart, 94% and 85% of their respective initial capacity, after 20 hydrogenation cycles without phase decomposition.