Size effects on the hydrogen storage properties of nanostructured metal hydrides:: A review

Hydrogen is considered a good energy carrier candidate for future automotive applications that could be part of a carbon-free cycle. Metal hydrides are often preferred over pressurized gas and other hydrogen storage methods because of their gravimetric and volumetric storage capacities and safe operating pressures. In addition to the hydrogen storage capacity, other properties that have often been disregarded must now be addressed before hydrogen storage in metal hydrides becomes feasible. The slow hydriding/ dehydriding kinetics, high release temperature, low storage efficiency due to the high enthalpy of formation, and thermal management during the hydriding reaction remain important difficulties in meeting the objectives set by the Department of Energy (DOE) for hydrogen storage systems. Nanotechnology offers new ways of addressing those issues by taking advantage of the distinctive chemical and physical properties observed in nanostructures. Nanostructured materials significantly improve the reaction kinetics, reduce the enthalpy of formation, and lower the hydrogen absorption and release temperatures through destabilization of the metal hydride and multiple catalytic effects in the system. But nanostructures can also lead to poor cyclability, degradation of the sorption properties, and a significant reduction of the thermal conductivity that could make metal hydrides impractical for hydrogen storage. This review summarizes the effects that nanotechnology can have on the main properties of metal hydrides and highlights the main competing behaviours between the system requirements, the necessary trade-offs, and the research priorities necessary to obtain hydride storage materials for practical automotive applications. Copyright (C) 2007 John Wiley & Sons, Ltd.