Nanometer-sized Mg hydride clusters may form in a self-organized way by the hydrogenation of a nonequilibrium Mg-Ti alloy. Here the Mg hydride is destabilized by the interface energy between the two metal hydrides. To obtain an even more destabilized Mg hydride, we increased the interface energy by the addition of Cr, which is immiscible with Mg as Ti. Indeed, Mg layers surrounded by Ti-Cr layers show hydrogen plateau pressures higher than when Mg is surrounded by Ti. Destabilization of Mg hydride is also observed in hydrogenated Mg-Ti-Cr thin-film alloys, resulting in hydrogenation plateaus flatter and at higher pressures than in hydrogenated Mg-Ti thin film alloys. Our results suggest that by screening alloys on the basis of their immiscibility with Mg, we can tune the thermodynamics and kinetics of hydrogen absorption and desorption in Mg-H. This concept paves the way for the development of light-weight and cheap Mg-based functional materials in the metal-hydrogen system.

Using the change in the intrinsic optical properties of YMg-based thin films upon exposure to hydrogen, we observe the presence of hydrogen at concentrations as low as 20 ppm just by a change in color. The eye-visible color change circumvents the use of any electronics in this device, thereby making it an inexpensive H₂ detector. The detector shows high selectivity towards H₂ in H₂-O₂-mixtures, and responds within 20 s to 0.25% H₂ in the presence of 18% O2.

For many hydrogen related applications it is preferred to use optical hydrogen sensors above electrical systems. Optical sensors reduce the risk of ignition by spark formation and are less sensitive to electrical interference. Currently palladium and palladium alloys are used for most hydrogen sensors since they are well known for their hydrogen dissociation and absorption properties at relatively low temperatures. The disadvantages of palladium in sensors are the low optical response upon hydrogen loading, the cross sensitivity for oxygen and carbon, the limited detection range and the formation of micro-cracks after some hydrogen absorption/desorption cycles. In contrast to Pd, we find that the use of magnesium or rear earth bases metal-hydrides in optical hydrogen sensors allow tuning of the detection levels over a broad pressure range, while maintaining a high optical response. We demonstrate a stable detection layer for detecting hydrogen below 10% of the lower explosion limit in an oxygen rich environment. This detection layer is deposited at the bare end of a glass fiber as a micro-mirror and is covered with a thin layer of palladium. The palladium layer promotes the hydrogen uptake at room temperature and acts as a hydrogen selective membrane. To protect the sensor for a long time in air a final layer of a hydrophobic fluorine based coating is applied. Such a sensor can be used for example as safety detector in automotive applications. We find that this type of fiber optic hydrogen sensor is also suitable for hydrogen detection in liquids. As example we demonstrate a sensor for detecting a broad range of concentrations in transformer oil. Such a sensor can signal a warning when sparks inside a high voltage power transformer decompose the transformer oil over a long period.