The discovery of a high capacity, energy efficient and safe method of reversibly storing hydrogen for its transportation is imperative in the development of a sustainable hydrogen economy. Mixed metal hydride materials are considered a promising route to achieve such a goal.
Metal hydrides can be optimised to provide high gravimetric and volumetric hydrogen capacities. Favourable thermodynamic and kinetic properties to allow reversible hydrogenation at ambient temperatures and low pressures are also required. Cost, phase stability and resistance to poisoning must be considered, along with good thermal conductivity, to prevent sintering in a dispersed form. Strategies to achieve these goals include the modification of the base metal alloy; this is achieved through the synthesis of ternary and quaternary materials to produce optimal thermodynamic phases. Furthermore catalysing components can be added to improve the kinetics of the dissociative adsorption and associative desorption of molecular hydrogen at the surface. As a result of the number of parameters requiring optimisation, and the increasing number of compositional permutations with increasing alloy components, combinatorial synthetic and screening methods provide an ideal tool in the search for new hydrogen storage materials.
The high throughput physical vapour deposition (HT-PVD) methodology recently developed to control graded compositions of thin film materials using molecular beam epitaxy (MBE) sources is used to directly synthesise mixed metal hydrides. This has been achieved by co-deposition of the metal elements in the presence of molecular hydrogen, or atomic hydrogen supplied by a plasma discharge source. The hydrides are deposited on a screening chip specifically designed to carry out fast sequential temperature programmed desorption and infra-red thermography.
Screening is carried out on masked fields of the hydride deposited on an array of hotplates. The array is silicon micro-fabricated, consisting of 49 MEMS hot-plates in a 7x7 matrix. Each hot-plate comprises a 1 micron thick continuous silicon nitride membrane (2 x 2 mm)2, which has been fabricated through a back etch of the silicon substrate. Embedded in the centre of each membrane is a 160 nm thick platinum serpentine track (1x1mm)2 used for heating. Each of the 49 MEMS heaters are independently controllable, with electrical contact being achieved through contact pads at the edge of the chip. The UHV holder provides electrical contact to the edge contacts on the chip, and 100 UHV electrical feed-throughs provide external control. Screening is carried out in a UHV analysis chamber and high pressure cell, attached to the synthesis chambers via a transport system and masking station which allows the synthesised materials to be transported under UHV conditions for screening without contamination.
