Development of light materials for hydrogen storage

The most compact technique to store hydrogen is the metal hydride technique. With trus, hydrogen is stored more compact than in liquid form, and it occurs in many instances reversibly at pressures close to the atmospheric pressure. There is though a serious obstacle to many uses of the technique, namely the weight. Most metal hydrides only store 1-2 weight percent hydrogen at moderate temperatures, while there for mobile applications is a demand for 5-10 %. The goal of the present project was to develop new effective materials for hydrogen storage based on light elements. There are many known hydrides with higher weight percentages of hydrogen; however, these have high operation temperatures. In the international research community efforts are made to decrease the operation temperature for some of these compounds, and the goal is preferably to get below 80°C to match the PEM fuel cells. This has proved to be difficult. The goal in this project was to get below 200°C, as this fits with the new high temperature PEM fuel cells that are under development in Denmark. One promising candidate for this is NaAlH4, which is known to store hydrogen reversibly in the temperature range 150-200 °C with a hydrogen weight capacity of 4-7.5 %.The kinetics of the absorption and desorption of hydrogen in this material when doped with titanium have been shown to improve dramatically yielding it viable for practical applications. In the present project the kinetics were examined by neutron scattering and computer simUlations. Here, the kinetic barrier for desorption of hydrogen in NaA1H4 was found to be reduced to 1/3 when titanium is added, thus giving an explanation for the effectiveness of titanium doping of NaAlH4. Experimentally,a result of the project was a dramatic lowering of the desorption temperature by mixing with the compound LiBH4. LiBH4 was also examined in combination with BCC metals, which also proved to lower the desorption temperature. Other, new materials were examined with regard to their use for hydrogen storage, such as the Li-Si-N and Li-AI-N systems. Li5SiN3 was synthesized from Li3N and Si by ball milling under nitrogen pressure, and the compound was shown to take up 4 wt%H2. Most of this could be desorbed under the reaction conditions(below300°C), thus showing partly reversibility. A related compound, Li2SiN2, proved to be able to take up nearly 5 wt% (close to the assumed theoretical amount). A complimentary Li-Si-N system containing hydrogen from the start, 2LiNH2+Si, was examined with regard to hydrogen desorption, and it was found that up to 5 wt% could be desorbed below 200°C when doping with TiCl3. An issue for this system is ammonia release starting around 200°C, and it has yet to show reversibility. Li3AlN2 was also synthesized, and it proved to be able to store 2 wt% below 300 °C of which about 1 wt% was reversible. Li2AlN2 has a problem of desorbing ammonia in its hydrogenated state. As part of the experimental work, several pieces of specialized equipment for characterization and synthesis of hydrogen storage materials were acquired or built. Examples include an improved glovebox at Risø DTU and a high pressure microbalance at DTU.