Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.

Hydrogen is a key element in the energy transition. Hydrogen-metal systems have been studied for various energy-related applications, e.g., for their use in reversible hydrogen storage, catalysis, hydrogen sensing, and rechargeable batteries. These applications depend strongly on the thermodynamics of the metal-hydrogen system. Therefore, tailoring the thermodynamics of metal-hydrogen interactions is crucial for tuning the properties of metal hydrides. Here we present a case of large metal hydride destabilization by elastic strain. The addition of small amounts of zirconium to yttrium leads to a compression of the yttrium lattice, which is maintained during (de)hydrogenation cycles. As a result, the equilibrium hydrogen pressure of YH₂ → YH₃ can be rationally and precisely tuned up to five orders of magnitude at room temperature. This allows us to realize a hydrogen sensor which indicates the ambient hydrogen pressure over four orders of magnitude by an eye-visible color change.

This work deals with the thermodynamics of hydride formation in 3-D nanoconfined Mg. Two ensembles of nearly monodisperse Mg nanodots (NDs) with different diameters (60 and 320 nm), were grown by the template nanopatterning method, using ultra-thin alumina membranes (UTAMs) with ordered porosity as evaporation masks. Multilayer NDs consisting of 30 nm Mg, 5 nm Ti and 5 nm Pd were deposited on UTAM-coated glass substrates by molecular beam epitaxy. The lateral surface of the NDs is constituted by native MgO. The morphology of the NDs was characterized by field emission scanning electron microscopy and atomic force microscopy. Hydride formation and decomposition was studied at low temperature (363–393 K) by means of optical hydrogenography. Compared to bulk Mg, the plateau pressure for hydrogen absorption in NDs exhibits an upward shift, which is larger for small NDs. Differently, the desorption plateau pressure is almost the same for the two NDs size and is lower than for bulk Mg. These hydrogen sorption features are discussed in the frame of a model that takes into account both interface energy and elastic strain energy in the constrained nanodots. The onset of plastic deformation, marked by a high pressure hysteresis between hydrogen absorption and desorption isotherms, limits the extent of hydride destabilization that can be achieved by elastic strain engineering.