The need for new and sustainable energy technologies is particularly urgent in the transport sector, where energy demands keep growing and give rise to significant global and local pollution. Hydrogen is expected to play a key role in this development (Satyapal et al.,2006). Hydrogen storage is regarded as one of the most critical issues that has to be solved before a technically and economically viable hydrogen economy can be established. In fact, without effective storage systems, a hydrogen economy will be difficult to achieve. One of the most challenging applications in this field is hydrogen storage for mobile applications. This chapter addresses the current state of the various on-board hydrogen-storage systems.

Requirements for hydrogen storage

In hydrogen-fuelled passenger cars, 4–5 kg (130–160 kWh) H2 must be stored in a small, preferably lightweight, tank in order to achieve a driving range of 500 km (i.e., 80–125 km/kg H2). However, whereas the gravimetric energy density of hydrogen is extremely high, the volumetric storage density of the lightweight gas is low. At ambient temperature and pressure, 5 kg H2 would fill a ball 5 m in diameter, which is roughly comparable to the volume of an inflated hot-air balloon. Consequently, the most important technical and economic challenges to be overcome in a practical hydrogen-storage system are the storage density related to the system (including tank, heat management, and valves), the costs of the system, its safety, a short refuelling time, and the ability to deliver enough hydrogen during the driving cycle.

With the expected increasing significance of hydrogen as a universal chemical and as an energy vector, its physical and thermodynamic properties are undergoing extensive investigation. To provide a basis of understanding for the themes covered in the remainder of the book, this chapter briefly describes the fundamental properties of hydrogen.

Discovery and occurrence

Named by a French chemist, Lavoisier, hydrogen (H) is the first chemical element of the periodic table of elements with an atomic number of one. At standard temperature and pressure, hydrogen is a colourless, tasteless, odourless and easily flammable gas. With its atomic mass of 1.00797 g/mol, hydrogen is the lightest element. The British scientist, Henry Cavendish, was the first to identify H as a distinct element in 1766, publishing precise values for its specific weight and density (NHA, 2007).

Hydrogen is also one of the most abundant chemical elements in the Universe (70–80 wt.% H2 content); more than 50 wt.% of the Sun consists of hydrogen. However, on Earth it mostly occurrs naturally in the form of chemical compounds, most frequently water and hydrocarbons. As a gas in its free state, hydrogen is very rare (1 ppm by volume in the Earth's atmosphere), owing to its light weight, and it can only be found in natural gas and some volcanic gases, as well as trapped in small quantities in some minerals and rocks (Ullmann, 2003).