Nom de l'entreprise / du laboratoire: Institut Pprime et LSPM

In the context of greenhouse gas reduction, the safe and compact storage of hydrogen (H) becomes of strategic significance. Compared to conventional methods of H storage in the gas or liquid-form, solid-state H storage in the form of metal hydrides (MH) represents the most compact and safest technology, as it avoids the handling at high pressure (up to 700 bar) or at very low temperatures (-253°C). In addition, the solid storage of H offers high volumetric capacity (~60 kg/m3) and is a reversible process with adsorption/desorption cycles occurring at low pressures (Pmax = 30 bar) and temperatures (absorption/release T < 400 °C).

Recently, refractory high entropy alloys (RHEAs) with cubic structure (bcc), have gained interest for their great potential as H storage material since they can absorb and release up to 2.7 wt%. of H . In addition, the vast composition space offered by these new alloys (constituted by ≥4 elements), can be exploited to optimize the performance of H storage, in terms of gravimetric capacity, temperature and sorption/desorption kinetics, reversibility and cyclability.

The proposed PhD project will focus on the deposition and structural characterization of thin films high entropy alloys  (Pprime) with a fixed ternary TiZrHf (elements characterized by high H storability) base system, while investigating the addition of Ta, V or Mo with the aim to explore the hydrogenation process, identifying the effect of the single elemental addition (Fig. 1a). The hydrogenation tests (LSPM) will be conducted under diverse conditions, Pmax = 30 bar and T<400 °C and cyclic behavior. Simultaneously, advanced microscale mechanical characterization (LSPM) will be carried out by optoacoustic techniques (Picosecond Ultrasonics and Brillouin Light Scattering) and in situ scanning electron microscope (SEM) nanoindentation . This integrated approach aims to elucidate the processes of H absorption and the subsequent formation of metal hydrides (MH). Both techniques can be carried out in situ varying the temperature (~800 °C) unveiling the MH dissolution/desorption process, which will be compared with the results of in situ wafer curvature experiments (Pprime).
The collaboration with IMaP (UCL, Be) will enable to exploit high resolution transmission electron microscopes (HRTEM) in order to explore at the nanoscale the hydrogenation and the formation of MH (Fig. 1c). Finally, upon having optimized the composition, novel TF-RHEAs architectures will be explored (i.e. nanoporous, with high lattice distortion) in order to further improve the gravimetric capacity, while ultimately controlling the conditions of H absorption-release.

Overall, the results of the PhD will pave the way to understanding fundamental questions involving the hydrogenation mechanisms in TF-RHEAs, opening new scenarios for the design of new materials with high H storage performances with clear impact for the present green transition.