The development of an on-board hydrogen storage system for automotive applications is a daunting challenge. Although there are many technical targets and design criteria that must be met, four of the most important ones are the system volume and weight, discharging and charging rates, thermal management associated with charging, and dormant system over-pressurization. It is common for many materials to release copious amounts of heat during charging. Unless a complicated heat exchanger system is integrated into the on-board filling operation, off-board refilling has to be used. It is also common for many materials to release hydrogen uncontrollably during dormant heating. To prevent hydrogen from being vented to the environment to circumvent over pressurization of the storage system during dormant heating either a complex on-board "hydrogen on demand" system is integrated into the on-board storage system or a material that only releases hydrogen at temperatures above some minimum level is needed. The PI and his team have discovered and are proposing to study and fully develop such materials.
Intellectual Merit: Based on complex hydrides of Li, Al and/or B and various catalysts and dopants, reversibility of this new class of materials has been fostered in two ways: first, through the use of a novel Physiochemical Pathway Approach (PPA) and second, through the use of a simple Thermal Hydrogenation Approach (THA). These two transformative approaches developed by the PI and his team utilize either a liquid complexing agent or high temperature, in conjunction with one or more catalysts, and a hydrogen atmosphere to foster reversibility in novel Li, Al and/or B complex hydrides. The PI recently demonstrated the PPA with LiAlH4, which can now be rehydrogenated with reasonable rates at ambient temperature and low pressures of 3 to 60 bars. They also applied the THA successfully to the new class of Li, Al and B complex hydride materials that so far exhibit a reversible hydrogen storage capacity in the 6 to 9 wt% range, reasonable discharge and charge rates in the 300 to 400C range, and reasonable charge pressures of around 100 bars. As a key objective, it is proposed herein to elucidate the rich chemistry associated with Li, Al and/or B complex hydrides through both the PPA and THA to understand the structure-property-discharge-charge relationships that will allow for the design and control of material performance.
Broader Impact: It is anticipated that the fundamental insight gained from this project will allow the PI and his team to develop new and better materials. In the end these new materials will constitute a new class of high capacity, high temperature, reversible hydrogen storage materials that have the potential to meet or exceed the design criteria being sought for automotive applications. Unique educational opportunities are also afforded to two PhD students. Through the PI's established interaction with the Separations Research Program at UT-Austin, these students will have an opportunity to interact with representatives from the top chemical and petrochemical companies in the world.
The development of an on-board hydrogen storage system for automotive applications is a daunting challenge. Although there are many technical targets and design criteria that must be met, four of the most important ones are the system volume and weight, discharging and charging rates, thermal management associated with charging, and dormant system over pressurization. It is common for many materials to release copious amounts of heat during charging. It might be impossible to fill an on-board storage system with hydrogen in 3 to 10 min because of inadequate thermal management. This problem can be resolved with off-board refilling of an exchangeable canister. Many materials may also release hydrogen uncontrollably during dormant heating. It might be impossible to prevent hydrogen from being vented to the environment to circumvent over pressurization of the storage system during dormant heating This problem can be resolved with a material that only releases hydrogen at temperatures above some minimum level. A new class of high temperature hydrogen storage materials has been discovered that meets some if not all of the design criteria and potentially solves the above problems. The PI and his team discovered and studied such materials during the course of this three-year project. Based on complex hydrides of Li, Al and/or B and various catalysts and dopants, reversibility of this new class of materials has been fostered in two ways: first, through the use of a novel Physiochemical Pathway Approach (PPA) and second, through the use of a simple Thermal Hydrogenation Approach (THA). These two approaches developed by the PI and his team utilize either a liquid complexing agent or high temperature, in conjunction with one or more catalysts, and a hydrogen atmosphere to foster reversibility in novel Li, Al and/or B complex hydrides. The goal of this project was to elucidate the rich chemistry associated with Li, Al and/or B complex hydrides through both the PPA and THA to understand the structure-property-discharge-charge relationships that will allow for the design and control of material performance. It is anticipated that the fundamental insight gained from this project will allow the PI and his team to develop new and better materials. In the end these new materials will constitute a new class of high capacity, high temperature, reversible, hydrogen storage materials that have the potential to meet or exceed the design criteria being sought for automotive applications. The proposed program was divided into two major tasks. The first task studied the PPA and the second task studied the THA. Included within each task were materials development, materials characterization including significant spectroscopic analyses to understand structure and function, and materials longevity. Outcomes from this work included the following: 1) The Li-Al-B system, whether doped or not, did not cycle very well and lost significant reversible hydrogen storage capacity after the third cycle. Although attempts were made to try to understand why this complex hydride would not cycle, the mechanism was never elucidated. 2) An in-situ Raman spectroscopy study was carried out with a model compound, NaAlH4, which is one of the complex hydrides that are known to be fully reversible. The results suggested that one of the decomposition paths of pure NaAlH4 and Ti-doped NaAlH4 may go through the same path: lattice expansion. Ball milling and the presence of Ti reduced the crystal size of NaAlH4 and produced more defect sites, which are possible reasons for the improvement in dehydrogenation kinetics. 3) Three effective Ti catalysts for NaAlH4 were made by stoichiometrically reacting TiCl3 with LiAlH4 in tetrahydrofuran (THF), NaAlH4 in THF, and LiAlH4 in diethyl ether (Et2O). The solid products produced after drying were named ex situ catalysts. One of the ex situ catalysts exhibited properties commensurate with some of the best NaAlH4 catalysts to date, such as CeCl3, ScCl3 and Ti nanocluster. The good news about this work was that ex situ catalyst is easy to make, readily available and relatively inexpensive. 4) Samples of NaAlH4 co-metal catalyzed with Zr and Fe exhibited significant, synergistic and sustained catalytic activity during hydrogenation (charge) and dehydrogenation (discharge) cycling. For the first time, this synergistic behavior of Zr-Fe bimetallic catalysis was explained in terms of the metal-metal bond polarity concept that relates co-metal catalytic activity to the relative location of early (Zr) and late (Fe) transition metals in the periodic table.
