Hydrogen Sorption Measurements on Potential Storage Materials

Abstract

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 hydrogen storage issue, including the possible solution to the problem through the use of solid state storage materials. 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 to satisfy the above criteria 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. This report 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. The latter part of the report concentrates 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 part of this report dealing with measurement accuracy is based on a review article published recently in the International Journal of Hydrogen Energy.