Hydrogen can readily be stored in large quantities as a cryogenic liquid for sta­tionary applications, e. g. for use as a rocket fuel, since the size and weight of the storage tanks are not limited in practice. This is, however, not the case for mobile applications, for which the US Department of Energy (DOE) has established some well-known criteria for a working system that would provide a driving range of 300 miles (480 km) for a hydrogen fuelled car. These include both gravimetric and volumetric storage capacities, operating conditions, and several other important factors. The requirements for a hydrogen storage system for mobile applications can be summarized in a qualitative way as follows:

(i) Appropriate thermodynamics (favourable enthalpies of hydrogen absorp­tion and desorption).

(ii) Fast kinetics (quick uptake and release).

J. Eckert (H)

Department of Chemistry, University of South Florida, Tampa, FL, USA e-mail: juergen@usf. edu

W. Lohstroh

Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universitat Mflnchen,

Garching, Germany

e-mail: wiebke. lohstroh@frm2.tum. de

© Springer International Publishing Switzerland 2015 205

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_8

(iii) High storage capacity.

(iv) Effective heat transfer.

(v) High gravimetric and volumetric densities (light in weight and conserva­tive in use of space).

(vi) Long cycle lifetime for hydrogen absorption/desorption.

(vii) High mechanical strength and durability of material and containers.

(viii) Safety under normal use and acceptable risk under abnormal conditions.

Most of the considerable effort to realize such a working system has been focused on increasing the capacities of storage materials in terms of wt.%. The ultimate requirement specified by the US DOE (updated to 7 wt.% for the entire system, not just the storage medium) requires that the hydrogen molecules be packed much more closely than they are in liquid H2. This can, of course, more readily be accomplished if hydrogen is stored in atomic form, which occupies much less space. In these cases hydrogen is rather strongly bound, as, for example, in chemical compounds, which does, however, make desorption kinetics and revers­ibility more difficult if not impossible.

The types of materials that are candidate hydrogen-storage systems (Fig. 8.1) may briefly be summarized as follows:

(1) Hydrogen in metals, such as FeTiHx and LaNi5H6, that were actively inves­tigated in the early 1980 and tested for vehicular use. Here H2 dissociates at the metal surface and forms a solid solution with the metal. The hydrogen gas can be released at elevated temperatures. These materials can have very high volumetric capacities, but their gravimetric capacities are low because of the high densities of the metal hosts.

(2) In so-called complex or light metal hydrides, where hydrogen is essentially covalently bonded to metals such as Al or Li in the form of a chemical compound. Elevated temperatures and/or a catalyst are required for a release of hydrogen, and on-board regeneration is not readily possible.

(3) Chemical hydrides, such as hydrocarbons, ammonia, or amino compounds, which have the highest hydrogen content (both volumetric and gravimetric), but where the release of hydrogen is only by a chemical reaction, and not all of the hydrogen content is available under reasonable conditions. On-board regeneration does not seem feasible.

(4) Molecular hydrogen adsorbed on surfaces, such as carbons of various types, or inside large pore materials, such as zeolites, clathrates, and metal-organic frameworks (MOFs). High volumetric and gravimetric capacities have been achieved in cases where the surface area is very large, but at operating conditions (low temperature and elevated pressures) that are not currently considered to be suitable for vehicular applications, on account of the weak interaction (physisorption) with the host material.

(5) Molecular hydrogen stored as a gas or liquid. These are straightforward, established technologies, and therefore are typically used in cars used to test hydrogen fuel-cells, or simply for combustion in a normal engine.

Targets for the capacities of a hydrogen storage system have been somewhat modified by the US DOE from those of the original ‘Freedom Car’ program. The emphasis has shifted towards system targets that must be met, i. e. the weights of the containers, and all associated plumbing, valves etc. must be included. This can easily be a factor of two, so that a material needs to have an intrinsic capacity of 12 wt.% recoverable H2 to meet the system target (for 2015) of 6 wt.%.

Prospects for a practical hydrogen storage system based on the DOE guidelines basically depend on two critical, but very difficult developments (Fig. 8.1): (1) in the case of sorption-based systems binding energies of hydrogen to the host must be substantially increased so that the necessary capacities can be achieved at room temperature and modest pressures, while (2) for chemical or light metal hydrides desorption conditions must be improved to the point where all available hydrogen is released at or below 100 °C, and, more importantly, on-board regeneration becomes facile. DOE guidelines, for example, also specify a refueling rate of 2 kg H2/min in addition to the better-known targets on gravimetric and volumetric capacities. Pro­gress in these areas ultimately depends on materials synthesis, namely the ability to design and to produce (on a large scale, at low cost) the storage material, which overcomes the current limitations. While much progress in synthesis can be made by applying chemical principles in conjunction with standard thermodynamic mea­surements (i. e. adsorption isotherms), the use of advanced, molecular-level char­acterization methods should provide additional essential details on the interactions of hydrogen with the host material, and this is the type of information which serves
both as input and all-important validation of the extensive computational studies, which are also being employed in the search for suitable hydrogen storage media.

Perhaps the most powerful experimental methods for molecular-level studies of hydrogen in materials involve the use of neutrons because of their outstanding sensitivity to hydrogen. Neutrons can provide both the rotational and vibrational dynamics of the adsorbed hydrogen, be it H2, H, or one of many other forms in the complex or chemical hydrides, as well as diffusive motions through the material. The dynamics of the molecule are affected by its surroundings and can therefore be taken as an indirect measure of its interaction. The microscopic diffusion of hydrogen is critical for the desired rapid loading, and release, but has not been studied in great detail to date.