A review on the current progress of metal hydrides material for solid-state hydrogen storage applications

Energy is one of the basic requirements in our daily lives. Daily activities such as cooking, cleaning, working on the computer and commuting to work are more or less dependent on energy. The world's energy demand is continuously increasing over the years due to the ever-increasing growth in the human population as well as economic development. At present, approximately 90% of energy demands are fulfilled by fossil fuels. With the rising demands of energy throughout the globe, it can be expected that the availability of fossil fuels is depleting at an alarming rate since fossil fuels are non-renewable sources of energy. In addition, fossil fuels are the main contributor of greenhouse gas emissions and therefore, they have a detrimental impact on human health and environment in the long term. Hence, there is a critical need to develop alternative sources of energy in replacement of fossil fuels. Hydrogen fuels have gained much interest among researchers all over the world since they are clean, non-toxic and renewable, making them suitable for use as substitutes for petroleum-derived fuels in vehicular applications. However, the greatest challenge in using hydrogen fuels lies in the development of hydrogen storage systems, especially for on-board applications. Hydrogen fuels can be stored in gaseous, liquid or solid states, and much effort has been made to develop hydrogen storage systems that are safe, cost-effective, environmental-friendly and more importantly, with high energy densities. Current technologies used for hydrogen storage include high-pressure compression at about 70 MPa, liquefaction at cryogenic temperatures (20 K) and absorption into solid state compounds. Among the three types of hydrogen storage technologies, the storage of hydrogen in solid state compounds appears to be the most feasible solution since it is a safer and more convenient method compared to high-pressure compression and liquefaction technologies. In this regard, metal hydrides are potential chemical compounds for solid-state hydrogen storage, and a large number of studies have been carried out to synthesize low-cost metal hydrides with low absorption/desorption temperatures, high gravimetric and volumetric hydrogen storage densities, good resistance to oxidation, good reversibility and cyclic ability, fast kinetics and reactivity, and moderate thermodynamic stability. In general, these studies have shown that the absorption/desorption properties of hydrogen can be improved by: (1) the addition of catalysts into the metal hydrides, (2) alloying the metal hydrides, or (3) nanostructuring. This review article is focused on the latest developments of metal hydrides for solid-state hydrogen storage applications, which will be of interest to scientists, researchers, and practitioners in this field.