Measurements at all three of the levels (system performance, materials development and fundamental science) investigation probe similar hydrogen storage properties of both systems and materials. The relative importance of each property may depend on the application or the purpose of investigation. The following is a very brief summary of the principal measured properties of hydrogen storage materials and systems.

• Property: Capacity

Capacity is the maximum steady-state hydrogen content of a storage material. Capacity can have several different definitions that reflect the application or material considerations including reversible capacity, usable capacity and excess material capacity. These definitions in turn may depend on the material’s stability, composition, temperature, pressure and number of cycles. Each of these variables has the potential to change the capacity of a material. Additionally, there is a very important distinction between concentration and capacity: capacity is a material property that does not vary at a set state (after an extended period of time). In this text, a material’s capacity will be referred to as its maximum steady-state hydrogen content and concentration as its temporal hydrogen content.

• Property: Kinetics

Kinetics is a measure of the rate of hydrogen sorption or desorption of a material and may not be exclusively dependent on intrinsic material properties. Sample size, heat transfer effects and other parameters that are highly dependent on the experimental method can affect kinetic measurements and conclusions. Minimizing the effects of external influences on kinetics is very difficult and requires a great deal of knowledge and preparation. Thus, extreme caution should be used in ascribing measured kinetics to fundamental sorption mechanisms or an intrinsic property of a given material. Sample preparation, catalyst and additives, particle size, heat transfer capabilities, among other things can strongly influence the kinetics. As one example, degassing at high temperature can remove or diffuse oxide layer from metals and increase subsequent hydrogen absorption kinetics.

• Property: Thermodynamics

The intrinsic thermodynamic properties of hydrogen storage materials influence a number of other parameters, most notably the hydrogen capacity based on temperature and pressure. Unfortunately, the relationship between measured temperature and pressure conditions and intrinsic thermodynamic properties may be complicated by kinetics considerations. In many of today’s materials, the temperature and pressure conditions required for hydrogen uptake and release are dictated by kinetic considerations, not necessarily the intrinsic thermodynamics of hydrogen bonding. The ability to distinguish between the two material properties is especially important because the techniques used to improve one property are often ineffective or not available for the other.

In addition, we refer to thermodynamic properties with respect to thermal equilibrium (the measure equilibrium state of hydrogen concentration in the material, hydrogen pressure and temperature). Thermal equilibrium may be dictated not only by the chemical interaction thermodynamics, but other terms as well such as strain energy, dislocation production and interfacial energy. In many metal-hydrogen systems these terms lead to differences in the absorption versus desorption equilibrium pressures at a given concentration and temperature. This is generally observed as hysteresis in the Pressure Concentration Temperature (PCT) isotherms.

• Property: Cycle-Life

Cycle-life testing is restricted to reversible hydrogen storage materials such as metal and complex hydrides, amides and physisorbing materials. Materials that store hydrogen irreversibly (i.e. not reversible under practical conditions), like chemical slurries of elemental hydrides (e.g. LiH, NaBH4), cannot be cycled. Cycle-life measurements are typically performed to characterize the effect of cycling on capacity that stems from activation effects, grain growth, structural degradation or chemical degradation due to gas impurities. However, kinetics may also be impacted by cycling and some of the observed change in capacity may in fact be due to changing kinetics during cycling. For example, if measurement time intervals during a cycle-life experiment do not reflect the changing kinetic properties of the material, the measured capacity will not be representative of capacity at quasi-equilibrium. Impurities in the hydrogen gas supply may have a profound effect on the cycle-life behavior of a storage material.