Hydrogen storage in Mg: A most promising material

Abstract

In the last one decade hydrogen has attracted worldwide interest as an energy carrier. This has generated comprehensive investigations on the technology involved and how to solve the problems of production, storage and applications of hydrogen. The interest in hydrogen as energy of the future is due to it being a clean energy, most abundant element in the universe, the lightest fuel and richest in energy per unit mass. It will provide, Cheap Electricity, Cook Food, Drive Car, Run Factories, Jet Planes, Hydrogen Village and for all our domestic energy requirements. In short hydrogen shows the solution and also allows the progressive and non-traumatic transition of today's energy sources, towards feasible safe reliable and complete sustainable energy chains. The present article deals with the hydrogen storage in metal hydrides with particular interest in Mg as it has potential to become one of the most promising storage materials. Many metals combine chemically with Hydrogen to form a class of compounds known as Hydrides. These hydrides can discharge hydrogen as and when needed by raising their temperature or decreasing the external pressure. An optimum hydrogen-storage material is required to have various properties viz. high hydrogen capacity per unit mass and unit volume which determines the amount of available energy, low dissociation temperature, moderate dissociation pressure, low heat of formation in order to minimize the energy necessary for hydrogen release, low heat dissipation during the exothermic hydride formation, reversibility, limited energy loss during charge and discharge of hydrogen, fast kinetics, high stability against O 2 and moisture for long cycle life, cyclibility, low cost of recycling and charging infrastructures and high safety. So far the most of hydrogen storage alloys such as LaNi 5, TiFe, TiMn 2, have hydrogen storage capacities, not more than 2 wt% which is not satisfactory for practical application as per DOE Goal. A group of Mg based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity upto 7.6 wt% for on board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system as well as by ball milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower desorption temperatures, however, the state of the art materials are still far from meeting the aimed target for their transport applications. Therefore further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides. In the present article the possibility of commercialization of Mg based alloys has been discussed.