For hydrogen storage, kinetics is generally taken to mean the rates of hydrogen sorption and desorption from a storage material occur. A primary difference between capacity and kinetics in reversible systems is that capacity measurements are theoretically taken at thermodynamic equilibrium, independent of the time required to reach equilibrium, while kinetics investigates how the material approaches equilibrium and what influences this approach. The availability of hydrogen in a storage material is dependent on the kinetics of the material under the system operating conditions. While a material might demonstrate high hydrogen storage capacity, the amount of hydrogen practically available may be significantly less depending on the material’s intrinsic kinetics versus what the application requires. A number of different intrinsic properties of a storage material may control kinetics including surface interactions, transport phenomena, hydrogen-substrate storage mechanisms and phase change. External factors such as temperature and pressure (in the case or reversible systems) also have a profound effect on hydrogen sorption and desorption kinetics.

In its most simple form, a kinetics experiment provides a useful measure of the rate of hydrogen uptake or release from a storage material. Unfortunately, comparing kinetics data across materials and experimental setups can be complicated. For example, one way to compare kinetics is to consider average rates. A common practice is to define the average kinetic rate as the time to reach 95% of the full capacity. However, as is demonstrated in Figure 1, it is possible to derive the same average sorption rate for materials that, in fact, exhibit very different kinetic character. Thus, it is important not only to compare average rates but also to compare the shape of the kinetic curves.

Figure 1: hypothetical concentration versus time curves showing three different kinetic behaviors that have the same average rate at t95%.

Kinetics measurements are conducted to quantify the kinetics performance of hydrogen storage materials and identify the potential intrinsic mechanisms controlling hydrogen uptake and release. Identification of the kinetic mechanisms, most specifically the rate-controlling mechanism, is instrumental in developing materials with improved kinetics properties. It is not easy to perform measurements to accurately determine the potential intrinsic rate-controlling mechanism however. Early in hydrogen storage research, intrinsic material properties like surface effects, mass transport and storage mechanisms were generally assumed to be the rate-controlling mechanisms. In reality, heat transfer and other thermal effects dominate rates for nearly all reactions in hydrogen storage systems and measurements.

In taking kinetics measurements, heat transfer (i.e. maintaining as much as possible a constant sample temperature) is the most important effect for which researchers must account. The temperature of sorption/desorption is the most influential variable in hydrogen storage kinetics for both chemisorbing and physisorbing materials. It is imperative that measurements are taken under isothermal conditions in order to minimize the effects of heat transfer and identify the potential intrinsic reaction mechanisms. Isothermal measurements are difficult in systems with poor heat transfer and fast intrinsic kinetics because the heat generated or taken up during hydrogen-substrate interactions can cause local temperature excursions that profoundly affect rates. All too often, rates reported in literature are in fact heat transfer rates because the sorption/desorption process is limited by an experiment’s ability to supply or sorption and desorption in physisorbing materials (at least) is a two-step process consisting of mass and energy transport and surface interactions/binding mechanisms. Hydrogen is only transported through the void volume of the material and physisorption occurs at the surface of the host material with minimal effect on the structure of the material. In this sense, surface interactions/binding mechanisms can be considered equivalent phenomena in physisorbing material. In chemisorbing media, surface interactions and bulk diffusion present distinct steps in the process of hydrogen storage and one or both may play the key role in hydrogen storage kinetics.

Reference List: 

• Dantzer, P., “Metal-Hydride Technology: A Critical Review”, Topics in Applied Physics - Hydrogen in Metals III, 73 (1997) 

• Wang, X.L., and Suda, S., “Reaction Kinetics of Hydrogen-Metal hydride systems”, Int. J. Hydrogen Energy, 15 (1990)

• Rudman, P.S., “Hydriding and Dehydriding Kinetics”, J. Less-Common Metals, 89 (1983)