High Entropy Alloys porous coatings by Glancing Angle Deposition - Magnetron Sputtering technology for solid state Hydrogen Storage

The large-scale use of hydrogen is critical to the energy transition in order to achieve the ambitious decarbonization levels required by the European Union. The development of efficient and durable systems for hydrogen production, storage and transport is therefore essential. H2 can be stored as a gas at pressures up to 700 MPa and as a liquid at 20 K. Aiming at alternative solutions, many research efforts on materials for solid-state storage were carried out to meet the energy plans' targets in terms of H2 capacity, sorption and release T, volumetric and gravimetric density [1]. High Entropy Alloys (HEAs) emerged recently as a potential candidate for solid-state reversible H2 storage [2]. They are brand-new multi-principal element alloys that can form single-phase solid solutions with highly symmetric lattices (BCC, FCC, HCP) [3]. In our research, we have focused on the synthesis of most promising HEA candidates with a novel approach which combines Glancing Angle Deposition and PVD Magnetron Sputtering technology (GLAD-MS) to grow porous films with nanostructured columnar architectures, where H2 should easily flow through the gaps separating the columns and be absorbed. Generally, in a MS process, the angle α between the incoming particle flux and the substrate normal is close to 0. Tilting the substrate to an oblique angle of incidence α>70° leads to the growth of a coating made of separated tilted nanocolumns (oblique angle deposition, OAD). Finally, the combination of OAD and a simultaneous substrate rotation (angle α, angular speed φr) is called GLancing Angle Deposition (GLAD) [4]. GLAD allows depositing isolated slanted columns, vertical pillars, zigzag or helicoidal structures. After the deposition system was set up, some preliminary tests were carried out with Al and V metal targets, to verify the effective growth of porous columnar films. Then, we synthesized MgAlVCrNi and TiZrHfNbV HEA films with almost equimolar composition applying the GLAD-MS technique. The samples were produced varying some process parameters such as pressure and rotation speed and were characterized by XRD and SEM-EDS to verify their crystal structure, morphology and chemical composition. After that, they were exposed to H2 to evaluate their stability and make a first screening