Material Hydrogen Content: the total quantity of hydrogen that is present in a material.

Reversible capacity: the quantity of stored hydrogen for on-board reversible materials that can actually be taken up and subsequently released by a material under reasonable (Laboratory) pressures and temperatures (-196 to 500oC and 100 mtorr to 300 bar).

Usable capacity: the total quantity of stored hydrogen that can be released by a material within the operating range (high and low) of temperatures and pressures found in a mobile storage application (USDOE operating targets: Temperature of -196 °C [LN2] to 85oC [PEM fuel cell] and a delivery pressure range of 3 to 40 bar).

Onboard Irreversible capacity: the quantity of stored hydrogen that can be released but not reabsorbed by a material under temperature and pressure conditions reasonably achievable in a mobile storage application.

GSE (Gibbs Surface Excess): Gibbs proposed a model to represent the gas-solid adsorption system as a two dimensional interface which is arbitrarily located within the bulk gas phase that creates a new adsorbed phase, called the Gibbsian adsorbed phase.

Excess adsorption amount (sometimes refer to as excess capacity): Extra hydrogen present due to adsorption of the gas to the surface of a material. Excess capacity is the measured surface excess adsorption capacity of a material. It represents excess hydrogen per unit sample over what would be present if the bulk gas density prevailed all the way to the surface, it can be positive, negative or zero. It is the material hydrogen storage capacity that is determined directly using volumetric and gravimetric methods. In Gibbs’ view, the gas-phase properties extend unchanged up to the solid surface. Differences between the actual and the unchanged properties are attributed to a hypothetical surface layer. The surface excess adsorption is the difference between the actual amount of gas adsorbed and the amount of gas that would be present in this adsorbed layer if the density of the adsorbed phase was the same as the equilibrium (bulk gas) phase.

Maximum excess adsorption amount: The peak excess hydrogen adsorption capacity at a specific temperature for a range of pressures.

Absolute adsorption capacity: the total amount of hydrogen present in the space where the attractive potential from the surface is effective (adsorbed phase). This is all of the gas in the adsorption boundary layer including the bulk phase gas and the surface excess gas. Because the boundary layer is a hypothetical construct, the absolute adsorption capacity can not be measured experimentally. It can be a useful conceptual quantity but relies on uncertain assumptions regarding the arbitrary placement of the dividing line and surface shape between adsorbed and bulk gas.

Total material capacity: all hydrogen actually absorbed in the bulk, as well as, all gas adsorbed to the surface of a material and present as a gas within the pores and cracks of individual particles of a material. Thus, total adsorption often includes compressed gas, which is not present in the adsorption boundary layer.

Active material capacity: is a measure of how much hydrogen can actually be delivered from the portion of the material that actively stores the hydrogen. It excludes the mass of material such as catalysts or thermal conductivity additives that do not store the hydrogen. This is a useful quantity when making materials performance comparison for kinetics studies.

Theoretical material capacity: all hydrogen that could theoretically be absorbed in the bulk and adsorbed to the surface of a material and as a gas within the pores and cracks of individual particles of a material.

Total system capacity: all hydrogen actually absorbed in the bulk and adsorbed to the surface of a material and as a gas within the pores and cracks in a material and in the free volume of the storage container.

System excess capacity (also referred to as “engineering” excess capacity): Is the capacity of a storage tank filled with a hydrogen storage material and hydrogen over and above the amount of hydrogen stored in the same tank at the same hydrogen temperature and pressure if the tank were empty (no hydrogen storage material). It answers the question: At what point is it better to have an empty tank?

Gravimetric material capacity: the total quantity of stored hydrogen with respect to the mass of the storage material (may be energy/mass material, mass H2/mass material, wt.% [wt-H/(wt-material + wt-H)], wt-ratio [wt-H/wt-material], etc.).

Gravimetric system capacity: the total quantity of stored hydrogen with respect to the mass of the storage system including the vessel and ancillary equipment to operate the storage system (may be energy/mass material, mass H2/mass material, wt.% with respect to the material mass, etc.).

Volumetric material capacity: the total quantity of stored hydrogen with respect to the apparent volume of the storage material (may be energy/volume material, mass H2/volume material, wt.% with respect to the material volume, etc.).

Volumetric system capacity: the total quantity of stored hydrogen with respect to the total volume of storage system including the vessel and ancillary equipment to operate the storage system (may be energy/volume material, mass H2/volume material, wt.% with respect to the material volume, etc.).