Study and characterisation of innovative materials for hydrogen storage through Solid-State NMR spectroscopy and NMR relaxometry

Advances in hydrogen technology, such as hydrogen generation, storage, and conversion to electricity, are critical to guaranteeing sustainable energy supply. The quest for novel materials for hydrogen production from chemical reservoirs in the frame of the emerging “Hydrogen Economy” is constantly increasing. The low energy density per volume at ambient temperature of hydrogen requires innovative storage solutions to be usable and cost-effective option compared to other common fuels. Safety considerations must also be addressed, particularly the flammability of hydrogen and the risks connected with its storage and transportation under high pressure. To address this issue, intensive research over the last 25 years has been centred on developing novel stable and non-toxic materials which can quickly and efficiently store and release hydrogen on demand. Light-weight hydrides have attracted considerable interest in this regard, as they offer an appropriate balance of volumetric and gravimetric energy densities – both essential components of a practicable energy storage system. Ammonia borane (AB, 19.5 wt.% H) and hydrazine Bisborane (HBB, 16.7 wt.% H) are potentially excellent hydrogen carriers. They are non-flammable and stable crystalline solids, bearing protic (N−H) and hydridic (B−H) hydrogens. When present together, their opposite polarity triggers weak dihydrogen bonding (DHB): N−Hδ+∙∙∙ δ-H−B, which release hydrogen via thermal decomposition (pyrolysis). In principle, from this process, a significant amount of hydrogen should be produced, but the low kinetic of dehydrogenation pathway and the poor reversibility associated with the process make these materials unsuitable for the load and discharge cycles that are essential for a system designed to operate with hydrogen. Metal-Organic Frameworks (MOFs) are a class of crystal highly porous materials characterized by a wide range of combinations of metal centers and organic linkers, which confer exceptional properties including high specific surfaces, modulable pore topology and good stability. Nanoconfinement has proven to be an effective strategy for lowering the hydrogen release temperature. Additionally, within the pores, the hydride finds an isolated environment where the formation of polymers and aromatic products, typically generated by the combustion of the pristine material, is reduced. Here, we report the design of different MOFs specifically tailored to achieve the appropriate porosity and robustness for confining hydrides. We selected three metal cluster, composed of in zirconium, aluminium, and beryllium, chosen for their gravimetric suitability, and as ligands, we opted for bifunctional carboxylic acids featuring aromatic rings or heteroatoms that contribute to the stabilization of the hydrides within the pores and could form DHB. The characterization of these materials was carried out using a combination of common techniques, typically employed for material characterization e.g. X-ray Diffraction, Infrared Spectroscopy, and NMR spectroscopy. Solid-State NMR (SS-NMR) spectroscopy allowed us to study the structure of the MOFs before and after the loading of the hydrides, understanding the fate of the hydrides once loaded within the MOFs. In this work we used the Fast Field Cycling (FFC) Relaxometry to study the diffusion of gaseous hydrogen and solvents within the MOFs, and we tested Dynamic Nuclear Polarization SS-NMR (DNP SS-NMR) spectroscopy. The atomic-level insights provided by NMR allowed us to gain valuable information on the behaviour of hydrides when confined within the porous material, revealing key details about their structural identity, hydrid loading, and reactivity. These findings contribute to a deeper understanding of the interaction between the MOF framework and chemical compound to store hydrogen, providing crucial guidance for the design of more efficient materials for hydrogen storage applications. SS-NMR extends its applications beyond the study of materials, thanks in part to technological advancements that have enabled the use of ultra-high-speed magic angle spinning (MAS). These advances address the problem of dipolar broadening of hydrogen, allowing for direct proton acquisition. SS-NMR spectroscopy is not fundamentally limited by the dimension scale of the systems under investigation, making it ideal for big and complicated structures. Additionally, SS-NMR can also be applied to paramagnetic proteins, as the absence of molecular tumbling in the solid state suppresses the Curie relaxation contribution, a major source of line broadening in solution-state spectra.