The general concept of hydrogen storage capacity is straightforward. Unfortunately, behind the conceptual simplicity is the difficult task of making the translation from laboratory measurements of gas sorption to reliable values for the practical amount of hydrogen that can be stored for use on-board a hydrogen fuel cell vehicle. At an initial level, “storage capacity” values depend on a wide range of precise and often confusing definitions being used in the field of hydrogen storage research. For example, the term “Total Capacity” is used in different ways. It can refer to the total amount of hydrogen stored by a storage system as well as the amount stored within a storage material. It is important to be able to distinguish between the various capacities reported in literature and understand which capacities are most informative at different levels of hydrogen storage research. The definitions that are used for capacity can be very different depending on whether the context is; understanding fundamental sorption processes, developing improve materials, or hydrogen delivery at system level. For this reason, we begin this section with, what we hope to be a concise and definitive summary of the different capacity definitions as they relate to different material types and research focus.

Definitions of capacity can be divided into two overlapping categories, those based on the where the hydrogen located (gas, near surface or in the bulk of a material) and those based on the physical considerations that limit the quantity of deliverable hydrogen (pressure, temperature and time).

The two most instructive capacities at the materials development and system performance levels are the total material and system capacities. The total material capacity is the maximum amount of stored hydrogen that is associated with the storage material itself; this includes hydrogen stored by compound formation or absorbed in the material, hydrogen physisorbed to the surface of the material and any gaseous hydrogen within the pores of the material. For chemical hydrides and traditional metal hydrides, the total material capacity is typically equivalent to the atomic hydrogen stored within the bulk of the material because of metal hydrides’ minimal porosity and negligible storage contribution made by adsorption. For porous materials, the hydrogen stored as a gas within the pores and physisorbed to the surface can be significant as the porosity or total surface area increases. The total system capacity is determined by adding the total material capacity and the hydrogen stored as a gas in the free (unoccupied) volume of the total storage system. Being able to determine the total system capacity is critical for practical applications because this represent the total amount of hydrogen available to a fuel cell, combustion engine, or other real world devices.

Another important consideration is that, for the purpose of the research being conducted for which this document has been created, hydrogen is stored to be used as an energy carrier. Because this covers hydrogen storage in a wide range of forms, compressed, liquid, physisorbed, onboard reversible or off-board rechargeable hydrogen it is sometimes difficult to make useful comparisons of storage methods based only on the quantity of hydrogen stored. Ultimately it is the energy storage capacity (or efficiency) that is important. It seems reasonable then, that eventually materials and systems will need to be evaluated on a complete wells-to-wheel energy balance basis. Such an analysis would need to account for all energy inputs in producing the hydrogen to be stored, the transportation and dispensing and perhaps cooling (physisorption) of the hydrogen to the storage system (for on-board regenerable materials), the energy required to release or initiate release of the hydrogen, and for off-board systems, the energy input required to regenerate the fuel from spent fuel, and the transportation and dispensing of fuel and spent fuel to and from the reprocessing plant.

Currently, such a complicated wells-to-wheels analysis is premature, even though certain aspects are being worked on, but will not be covered here. However, hydrogen storage capacity at a materials level and best approximations at a systems level are key parameters in evaluating the potential for materials to meet the goals of a practical hydrogen energy economy. With respect to hydrogen storage capacity, it is critical that the terminology, measurement methods, and relationship to stored energy are well understood and employed.

An important consideration with respect to these measurements is that, while hydrogen capacity is of significant technologically of interest, in many of the materials systems being explored, the potential for generation of volatile gas-phase impurities exists, and the various impacts of these impurities must be assessed as well as the total hydrogen storage capacity. How gas-phase impurities influence the capacity measurement, or how the presence of gas-phase impurities might impact the reversibility of the material, or the loss of material from the system, and the impact of gas-phase impurities on the downstream uses of the hydrogen stream must be considered when developing methodology to accurately and precisely determine the true hydrogen capacity of a system and the quality of the hydrogen stream released.