ABSTRACT
The development of a hydrogen-based economy is contingent on the development of safe and cost-effective hydrogen storage systems. Several studies have been published on the storage of hydrogen by different techniques [1-27]. Hydrogen storage is a key element in the utilization of hydrogen fuel (Figure 11.1) [18], and the energy consumed in each step of the process-production, multiple storages, and transport-should be minimized for a high energy effi ciency. Today, the annual consumption of hydrogen in the United States alone is more than 90 billion m3 [28,29], mostly in refi neries and chemical plants where it is produced. For such an application, storage has not been critical. However, for fuel usage in vehicles, storage has been the main technical challenge. Present storage techniques for hydrogen include compressed gas, cryogenic liquid, and absorption on solids. This chapter deals with the hydrogen storage using solid metal hydrides. For optimum hydrogen storage, the metal should have following desirable properties:
High hydrogen capacity per unit mass and unit volume Low dissociation temperature
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Moderate dissociation pressure Low heat of formation to minimize the energy necessary for hydrogen release and low heat dissipation during the exothermic hydride formation Reversibility for limited energy loss during charge and discharge of hydrogen Fast kinetics High stability against O2 and moisture for a long cycle life Cycleability Low cost of recycling and charging infrastructures High safety
Current metal hydrides, suitable for automotive applications, typically store 0.5-2 wt% hydrogen. Hydrides release heat when charged with pressurized hydrogen, and absorb heat to release hydrogen. More than 50 metallic elements of the periodic table can absorb hydrogen in good quantities; hence, the possible choices of hydride materials are enormous. But, only some of them are suitable for hydrogen storage at moderate temperatures and pressures. Practically, the temperature should not go above 100°C and below 10°C during charging and discharging, respectively. The choice is further narrowed by the need for a moderate charging pressure and a suitable discharging pressure. For example, a hydrogen supply pressure of 2.7 MPa is used, and the discharge pressure >0.2 MPa is required. To tailor metal properties to meet the requirements, alloys of various compositions are being utilized. These alloys are classifi ed as AB5 (e.g., LaNi5), AB (e.g., FeTi), A2B (e.g., Mg2Ni), and AB2 (e.g., ZrV2).
