General

Human development over the past centuries has resulted – apart from a depletion of natural energy resources – in global warming and climate changes with non-predictable consequences. National and intergovernmental panels urgently recommend mandatory reductions of emission of polluting gases from industrial and transport activities. Even if the concrete scenario is not yet fully clear, it is conceivable that classical energy carriers – carbon hydrides – have to be replaced and the within connected technologies - today’s guarantee for welfare - will wither. That means that apart from the duty of mankind to soften the above mentioned ecological consequences, we will have to develop new scientific and technological knowledge to lay the basis of our future economy.

Most promising future scenario is the hydrogen economy. Hydrogen is the ideal means of energy storage, transportation and conversion in a comprehensive clean-energy concept. It is non-polluting as its combustion only generates water. It is abundant and can be produced from a variety of conventional and renewable energy resources. These are the main reasons, why there is now widespread agreement that hydrogen will play a key role in the European Union’s energy policy towards the middle of the century. However, the storage of hydrogen is still the problematic bottleneck. The very low boiling point of liquid hydrogen around -250 deg C makes storage in liquid form energy inefficient and its low density in the gaseous state requires storage in high-pressure vessels which causes safety problems for its storage in mobile applications and in particular in the future “zero-emission vehicle”.

Hydrogen storage in matter offers a safe alternative for transportation and storage of hydrogen. Several promising systems are under discussion: adsorbed hydrogen on nano-structures (nano tubes, metal organic frameworks), and hydrogen absorbed in metal hydrides (transition metal based hydrides, complex hydrides). For hydrogen fuel tanks to be used in vehicles, at least 6 mass% hydrogen has to be stored (6.5 mass% and 62 kg H₂/m³ are the targets of the US Department of Energy). Because it is unlikely that these targets can be achieved in hydrogen adsorbed on nano-structures as well as absorbed in classical transition metal-based materials, intense interest has developed in light-weight complex hydrides such as alanates and borohydrides.

These light-weight complex hydrides have the required high storage capacity. E.g. the partial decomposition of LiBH₄ to LiH + B + 3/2H₂ yields 13.6 mass% hydrogen. Use of complex hydrides for hydrogen storage is challenging because of both kinetic and thermodynamic limitations. Strong improvements in kinetics are possible with suitable catalysts. The second problem is the heat released during hydride formation. The standard enthalpy for the above given reaction is -67 kJ/(mol of H₂) and thus, if reversible, an equilibrium pressure of 1 bar would require a temperature of >400 °C. The high desorption temperature bears two problems: (i) the energy loss during desorption reduces the efficiency of the total storage process. (ii) The same amount of energy is released as heat during absorption, which causes enormous technical problems during filling of the tank (ca. 1 MW heat has to be dissipated in a typical car tank). The above

mentioned problems are typical for complex hydrides and clearly demonstrate the need to adjust the thermodynamic properties to technical needs. Unfortunately, in contrast to intermetallic hydrides, a simple tuning of thermodynamic properties by alloying additions seems to be generally impossible.

Recapitulating, the gap between the present state of the art in hydrogen storage and the technological demands is despite the recent research efforts still too wide to be bridged by incremental advances. It will take breakthroughs of the kind that only fundamental research can deliver.