Solid-state Hydrogen Storage, Hydrogen Separation Membranes, Permanent Magnets, Hydrogen Embrittlement.
Hydrogen is widely regarded as the most promising alternative to carbon-based fuels: it can be produced from a variety of renewable resources, and - when coupled with fuel cells - offers near-zero emissions of pollutants and greenhouse gases. However, developing hydrogen as a major energy carrier, will require solutions to many scientific and technological challenges.
Solid-state Hydrogen Storage
One challenge is to how to effectively store hydrogen on vehicles. Conventional storage solutions include liquefaction or compression, however there are energy efficiency and major safety concerns associated with both these options. Therefore, there is a great need to develop viable solid-state storage materials.
Magnesium: With a theoretical reversible hydrogen uptake value of 7.6 weight%, Mg is a candidate for a new storage medium. However, the hydrogen sorption temperature needs to be reduced (from around 300 °C to 100-150 °C), and the kinetics need to be accelerated. It has been shown that the sorption kinetics can be greatly improved by: introducing a nanoscale microstructure to provide a pathway for hydrogen diffusion; and by catalyzing the surface. The thermodynamics now need to be improved by alloying Mg to form a new compound or phase. Our work is investigating nanostructured Mg alloys produced by ball-milling, thin-film multilayers, and by rapid solidification.
Complex Hydrides: Borohydride compounds are promising hydrogen storage materials (e.g. lithium borohydride is able to store up to 18 wt%), but which require elevated temperatures (200 – 300 °C) for hydrogen desorption and suffer poor reversibility (i.e. re-absorption of hydrogen is difficult). We are investigating Transition-metal-based borohydrides, produced by ball-milling and by high-pressure synthesis. We have found that the hydrogen desorption temperature in such compounds can be greatly reduced. We are now using in situ XRD and Raman spectroscopy (with 100 bar hydrogen cells) to study the phases that form during hydrogen desorption and reabsorption, with the aim of producing more reversible materials.
Nanocarbons: nanostructured graphite-based materials may store up to 7 wt% hydrogen, which offers the prospect of an inexpensive, widely available storage medium. However, this material needs to be heated to 800 °C to remove all the hydrogen, and reversibility is poor (limited to a few cycles after mixing with LiH). In order to improve the reabsorption process, we are studying how the hydrogen is stored, the role of carbon ‘dangling bonds’, and the effect of microstructure.
Hydrogen Separation Membranes
Any important challenge is how to provide extremely pure hydrogen, for use with PEM Fuel Cells?
Hydrogen produced from natural gas reformers and from biomass sources, usually contains small amount of impurity gases, such as carbon monoxide, methane, and sulphur. A PEM Fuel Cell converts hydrogen and oxygen gases into electricity; however, even very small amounts of impurities in the hydrogen can reduce the operating life of the Fuel Cell. In addition, there are applications in semiconductor and LED manufacture that require ultra-pure hydrogen.
Metallic diffusion membranes can be used to purify hydrogen: certain Pd-based alloys will allow only hydrogen gas to pass through (the impurity gas molecules are too large), resulting in parts-per-billion level pure hydrogen. However, the conventional membrane alloy used (Pd-Ag) is rather expensive, and cannot be used in the presence of impurities such as CO and S. We are investigating materials with less or no Pd with comparable membranes properties. We have also been studying the fabrication of thin-film and rapidly solidified membranes.
Permanent magnets are now essential components in many fields of technology, and have found applications in a wide range of devices. In 1984, the Nd2Fe14B magnet phase was developed by: powder metallurgy to form anisotropic, fully dense sintered magnets; and melt-spinning to produce isotropic magnetic powders, which can then be compacted to form bonded magnets. Although bonded magnets have poorer magnetic properties, the ability to form complex geometries has lead to bonded magnets becoming the fastest growing sector of the permanent magnet market.
Therefore, there was great interest in 1989, when a new technique – which came to be called Hydrogen Disproportionation Desorption Recombination (HDDR) – was developed, that subsequently allowed the production of anisotropic magnetic powders (anisotropic magnet powders have better magnetic properties than isotropic). The HDDR process involves exposing ingots of Nd-Fe-B to a series of carefully controlled heat treatments under hydrogen and vacuum. However, the mechanism behind the formation of anisotropic material still requires further study.