Initial project (completed):

A key enabler for the widespread use of hydrogen is its cheap and easy storage. Promising candidates for solid-state hydrogen storage are metal hydrides; examples of such hydride-forming metals are FeTi, Mg, LaNi_5, and high-entropy alloys. Metal hydrides exhibit high volumetric energy densities, safety, and efficiency. Despite these superior properties, many challenges, including complex activation treatments, sluggish kinetics, insufficient (mechanical and chemical) stability, and unfavorable pressure-temperature conditions for loading-unloading operations, need to be surmounted.

This project aimed to tackle the mentioned problems by prototyping porous metal hydride materials (Fig. 1). The project encompasses all required steps for achieving this - from preparation over characterization to functionalization. Nanoporous material can exhibit excellent properties such as high surface-to-volume ratio, high strength-to-weight ratio, and electrical and thermal conductivity. Heat treatment allows the tailoring of the composites and subsequent foam microstructures.

The pores can be infiltrated with a polymer that provides mechanical stability and selective hydrogen gas permeability. Additionally, the polymer adds another degree of freedom for engineering a high-performance storage material with minimal detrimental effects on the gravimetric storage density.

The investigated material systems include FeTi, as a chemically simple and well-studied model system, and promising high-entropy alloys, e.g., TiZrVNbHf.

As a starting point, composites of immiscible storage and sacrificial phases are synthesized by High-Pressure Torsion (HPT), with the first model system being FeTi-Cu. Then, the sacrificial phase is selectively dissolved by wet-chemical processes, yielding a nanoporous and nanocrystalline metallic foam with high defect densities. Finally, the pores are infiltrated with polymers. The resulting hybrid material, the precursors (nanocomposite, metallic foam), and the involved processes (ball milling, HPT, (electro)chemical etching) are carefully investigated to shed light on the underlying processes and find optimization and tuning possibilities.

The investigations involve detailed structural characterization of the composition, nano- and microstructure, and morphology. In particular, the aim is to understand the impact and interplay of various interfaces (surface, grain boundaries, phase boundaries) on the material's (hydrogen sorption) properties. Primary characterization methods include electron microscopy (SEM, TEM) and (Synchrotron) X-ray diffraction. We complementarily probe the mechanical properties using micro- and nanoindentation. Hydrogen sorption performance is investigated using a Sievert's apparatus, enabling the determination of absorption and hydride formation capacity, kinetics, and thermodynamics.

A first milestone was achieved by preparing a FeTi-Cu nanocomposite with tunable grain size and mechanical alloying.^1,2 These structural features directly translate into the nanoporous metallic foam. Additionally, the investigation of the promising HEA TiVZrNbHf, encompassing in-situ synchrotron experiments, revealed the effect of hydrogen on the microstructural evolution and decomposition during annealing.³