This brief article describes the content of this issue of MRS Bulletin on Hydrogen Storage. Hydrogen is a powerful, clean, synthetic fuel with the inconvenient property of being an ideal gas under ambient conditions. In order to use hydrogen efficiently as a fuel, compacting it for mobile storage is a key issue. As an introduction to the following seven contributions on different storage techniques and their potential, we start with a description of the technical and socioeconomic aspects of the mobility and transport issues involved and present an overview of volumetric and gravimetric storage densities for hydrogen.

This review outlines the prospects for gaseous hydrogen, stored in high-pressure cylinders, as a fuel for automotive applications. Following an initial description of hydrogen embrittlement problems encountered in the past in steel cylinders, the article explores the use of other types of gas cylinders, including the recent examples of hoop-wound and fully wound composites. Central to the article is the concept of the volume of hydrogen transported for the minimum amount of container weight. Finally, the role of international standards in developing safe and efficient cylinders for this application is emphasized.

This survey focuses on the use of liquid hydrogen as an automotive fuel in comparison with the use of compressed gaseous hydrogen. The energy penalties associated with liquefaction versus gas compression are compared, followed by an examination of the weight of hydrogen relative to carrier weight for the two alternative approaches. The optimum form and design of LH2 tanks are discussed, followed by the important topic of how to achieve quick and easy transfer of LH2 from a storage tank to a vehicle.

A brief survey is given of the various classes of metal alloys and compounds that are suitable for hydrogen-storage and energy-conversion applications. Comparisons are made of relevant properties including hydrogen absorption and desorption pressures, total and reversible hydrogen-storage capacity, reaction-rate kinetics, initial activation requirements, susceptibility to contamination, and durability during long-term thermal cycling. Selected applications are hydrogen storage as a fuel, gas separation and purification, thermal switches, and sorption cryocoolers.

For a century, nickel-cadmium (Ni-Cd) batteries have been widely used as electrochemical energy-storage cells. However, due to the rapid development of portable electronic devices and the increasing search for cleaner electric vehicles, new generations of batteries have been investigated during the last few decades. Among them, nickel metal hydride (Ni-MH) batteries, with their larger capacities and improved environmental compatibility, have shown their ability to replace Ni-Cd cells. The negative electrodes of Ni-MH batteries are made of reversibly hydride-forming intermetallic compounds. In this article, the crystallographic and thermodynamic properties of these compounds will be reviewed. Their hydrogen-absorption properties, their electrochemical performance, and the solutions that have been found to achieve reliable cycle life will be presented. The industrial market for Ni-MH batteries will also be discussed in comparison with other battery systems.

Hydrogen-absorbing alloys with bcc (body-centered-cubic) structures, such as Ti-V-Mn, Ti-V-Cr, Ti-V-Cr-Mn, and Ti-Cr-(Mo, Ru), have been developed since 1993. These alloys have a higher hydrogen capacity (about 3.0 mass%) than conventional intermetallic hydrogen-absorbing alloys. Generally, bcc metals and alloys exhibit two plateaus in pressure–composition isotherms, but the lower plateau is far below atmospheric pressure at room temperature. Many efforts have been made to increase hydrogen capacity and raise the equilibrium pressure of this lower plateau. The crystal structure and morphology of Laves-phase-related bcc solid-solution alloys are reviewed.

Recent developments in hydrogen interaction with carbonaceous materials are reviewed in this article. The interaction is based on van der Waals attractive forces (physisorption), or the overlap of the highest occupied molecular orbitals of carbon with the hydrogen electron, overcoming the activation-energy barrier for hydrogen dissociation (chemisorption). While the physisorption of hydrogen limits the hydrogen-to-carbon ratio to less than one hydrogen atom per two carbon atoms (i.e., 4.2 mass%), in chemisorption, a ratio of two hydrogen atoms per one carbon atom is realized (e.g., in polyethylene). However, the materials with large hydrogen-to-carbon ratios only liberate the hydrogen at elevated temperature. No evidence, apart from theoretical calculations, was found for a new hydrogen-adsorption phenomenon on carbon nanotubes (CNTs), as compared with high-surface-area graphite. The curvature of CNTs and fullerenes increases the reactivity of these materials with hydrogen and leads more easily to the formation of hydrocarbons, as compared with graphite. Nanocrystalline or amorphous carbon exhibits an intermediate state for hydrogen between physisorption and chemisorption and absorbs up to one hydrogen atom per carbon atom. Nanostructured carbonaceous and metallic materials offer a large potential for hydrogen storage and must therefore be investigated in more detail.

Complex hydrides are mixed ionic–covalent compounds that can serve as reversible H2 storage media only when they are catalyzed by a transition metal such as Ti. As the prime example, the phenomenology of Ti-catalyzed sodium alanate (NaAlH4) is reviewed from a historical perspective. Dehydriding yields a theoretical 5.6 wt% H2 during two-step decomposition, NaAlH4 → Na3AlH6 → NaH + Al, although 100% recovery of that H2 is not currently possible. H2 can be discharged and recharged at practical rates at 125°C. More work is needed on the alanates, in particular, as well as the identification and optimization of the catalytic mechanism and a broad extension of the concept to other than Na-based alanates. The possibility of an even further extension of the concept to other complex hydrides (e.g., the borohydrides and transition-metal complexes) is discussed.