The hydrogen storage problem is one of the major issues that needs to be resolved if hydrogen is to become a viable energy carrier in the future. An earlier EUR report examined the problem, including its possible solution through the use of a solid state storage material. This option is one of the most attractive for a number of reasons, although no material currently satisfies the practical requirements, in terms of storage capacity, operating temperature and pressure ranges, impurity resistance, long term cycling stability and cost. The search for a material that fulfils the criteria for a practical store is therefore generating a great deal of scientific research interest, and an increasing number of publications on this topic are appearing in the scientific literature.

A crucial part of the search for new storage materials is the accurate determination of the hydrogen sorption or storage characteristics of new or modified materials. This is an issue that has come to prominence in recent years due to the controversy over the potential storage capacity of carbon nanostructures, such as nanotubes and nanofibres, during which widely varying claims of potential storage capacity and hydrogen uptake behaviour were made. A significant contribution to this controversy was made by inaccuracy in the measurement of the potential gas phase hydrogen storage capacities of these nanostructured carbon materials.

Some studies focuses on the gas phase, as opposed to electrochemical, characterisation of the equilibrium hydrogen sorption properties of potential storage materials, and covers the common techniques that can be used to determine the hydrogen uptake behaviour of potential hydrogen storage media. Some concentrate on the accuracy of these measurement techniques and discusses the possible sources of error in these methods with reference to previous work that has appeared in the scientific press, as well as existing measurement standards and guidelines. In addition, checklists of the issues that affect the accuracy of hydrogen sorption measurement, in the case of absorbers and adsorbents, based on the discussion presented here, are tentatively proposed. The Appendix also includes a discussion of the conversion of the experimentally-determined hydrogen adsorption parameter, the excess adsorption, to a total, or absolute, adsorbed quantity. 

The search for potential hydrogen storage materials has recently been receiving a great deal of attention, as the hydrogen storage problem is one of the major issues that needs to be resolved if hydrogen is to become a viable energy carrier in the future. The search for new materials and the various proposed solutions to the problem have both been covered in a number of recent review articles. Aside from the socalled ‘chemical hydrides’, the materials currently being considered can be broadly separated into two categories: one in which the hydrogen is absorbed in the bulk of the material in atomic form and the second in which the hydrogen is weakly bonded in molecular form to the surface of the material. The former is hydrogen absorption and the latter hydrogen adsorption. Both of these processes can be measured or monitored using bulk gas phase characterisation methods that loosely mimic the process by which a practical solid state hydrogen store would be charged or discharged. Volumetric and gravimetric techniques determine the hydrogen sorption properties of a material by exposing it to a pressure of hydrogen at a given temperature, causing a reaction that results either in the sorption or desorption of hydrogen, depending on whether there has been an increase or decrease in the applied pressure from the previous step. The measured hydrogen content at different hydrogen pressures is then plotted to form an isotherm. Temperature-programmed techniques, on the other hand, use temperature changes to sorb and desorb the hydrogen. An experiment therefore consists of taking the sample from a low temperature, at which hydrogen is thermodynamically or kinetically trapped in the material, to a higher temperature, at which the hydrogen will be partially or fully desorbed, and monitoring the quantity of hydrogen released.

These types of sorption measurements have previously been performed for a number of different reasons. In metal hydride research, the measurement of the absorption of hydrogen by elemental metals and intermetallic compounds has been made for reasons ranging from understanding the fundamental interaction of hydrogen with matter to the development of specific technological applications, both gas phase, such as metal hydride heat pumps, and electrochemical, such as nickelmetal hydride batteries. The measurement of gas adsorption on porous materials, meanwhile, is another mature research field in which measurements have been made using similar techniques to those of hydride research but for the purpose of surface area, pore size distribution (PSD) and porosity determination. However, for a number of reasons, hydrogen is not a common probe gas, and so there is a great deal more literature available on the adsorption of the typical probe gases compared to the adsorption of hydrogen. Although there are many similarities between the methods used for these measurements, there are also issues that are specific to hydrogen sorption and, in particular, to the application of gaseous hydrogen storage.

Typical (non-hydrogen) adsorption measurements are performed at sub- or nearcritical temperatures, with the isotherm expressed in terms of relative pressure, P/P0, where P0 is the saturation pressure of the adsorptive (atmospheric pressure in the case of nitrogen at 77 K). An example being routine BET specific surface area (SSA) measurement. There are IUPAC isotherm classifications (types I to VI), that are commonly used to assess adsorption isotherms, but these apply only to condensable vapours and therefore do not apply to hydrogen at the temperatures of practical interest for storage applications (generally 77 K or above). For reasons of practicality, the adsorption behaviour of hydrogen at or below its critical temperature (33 K) is not of great interest for storage applications and we are therefore interested in the measurement of supercritical adsorption. Furthermore, for storage purposes, the gaseous hydrogen will need to be stored and released at above ambient pressures; therefore, generally speaking, we are interested in higher absolute pressure measurements than those routinely performed for surface area, PSD and porosity determination using other gases.

Read More:

• Hodoshima S., Arai H., Saito Y., "Liquid-film-type catalytic decalin dehydrogeno-aromatization for longterm storage and long-distance transportation of hydrogen" International Journal of Hydrogen Energy

• Fukai Y., The metal-hydrogen system: basic bulk properties (2nd Edition) Springer-Verlag

• Alefeld G., Völkl J., (editors) Topics in applied physics Vol. 28: Hydrogen in metals I. Basic properties Springer-Verlag

• Alefeld G., Völkl J., (editors) Topics in applied physics Vol. 29: Hydrogen in metals II. Applicationoriented properties Springer-Verlag

• Schlapbach L., (editor) Topics in applied physics Vol. 63: Hydrogen in intermetallic compounds I. Electronic, thermodynamic and crystallographic properties, preparation Springer-Verlag

• Schlapbach L., (editor) Topics in applied physics Vol. 67: Hydrogen in intermetallic compounds II. Surface and dynamic properties, applications Springer-Verlag

• Wipf H., (editor) Topics in applied physics Vol. 73: Hydrogen in metals III. Properties and applications Springer-Verlag

• Sandrock G., Bowman Jr R. C., "Gas-based hydride applications: recent progress and future needs" Journal of Alloys and Compounds