The typical approach in hydrogen storage research is to identify promising new materials based on high hydrogen capacity and then work to improve their kinetic and thermodynamic properties. In general, the last step has been to evaluate and improve the cyclability of the materials. Ultimately, a material with great hydrogen storage capacity, kinetics and thermodynamics may be of little use if these properties are not maintained over tens, hundreds or even thousands of cycles. For this reason, accurate Cycle-Life measurements are extremely important for the development of practical hydrogen storage materials. The following are some considerations and caveats that should be considered when making Cycle-Life measurements.

1. Pretreatment / Activation

Hydrogen storage materials, in particular, physisorption storage materials almost always require a pretreatment process to prepare the samples for optimal performance in Cycle-Life testing. For physisorption materials, this generally means baking out the sample at a specific temperature for a minimum of several hours to a day or more under moderately high vacuum. The bake-out procedure will depend on the material being studied. For example most carbon-based materials can withstand relatively high temperatures (300°C or more), however, some MOFs will decompose at these temperatures. Preliminary examination (e.g., XRD, IR, microscopy….) of samples before and after a bake out procedure is vital to ensure that the bake-out conditions are sufficient to remove residual contaminants and that the samples are not damaged by the bake-out conditions. Care must always be taken after performing a bake-out to not expose the sample to air or other contaminants, before preliminary examinations or cycling measurements.

Activation is the process of the initial cycling of a material to reach the point of steady-state hydrogen uptake and release behavior. It is important to properly activate samples to be able to study their long-term Cycle-Life performance. Activation is often required in hydride hydrogen storage materials.

For classic hydrides, the activation process involves the initial decrepitation and potentially surface segregation of the alloy and complete hydriding of all portions of the sample. It is sometimes necessary to heat the alloy to elevated temperatures (up to 100°C above the desired Cycle-Life measurement temperature) and expose the sample to hydrogen at high pressure. For example, depending on the preparation and degree of exposure to air LaNi5 samples may will not completely hydride without performing a set of charge (100 bar) and discharge (rough vacuum) at 100°C.

Initial cycling may involve other mechanisms than decrepitation or surface segregation to completely activate hydrogen storage materials. An example of such an activation process in cycling measurements on Sodium Alanate (NaAlH4) is shown in Figure 1. In this interesting case of non-reactive (TiO2) doping, the desorption rates were found to be comparable with direct doping of 1 mol% TiCl3. The advantage of using nanoparticles TiO2 is that there is no capacity loss due to the Na-halide reaction (forming NaCl and excess Al) found when doping with TiCl3. The measurement points out the extreme effect of activation on the measured desorption capacity. Had the measurement been stopped after the first cycle or two, the material may have been disregarded as having very little hydrogen storage capacity. On-the-other-hand, had long-term cycling been done without recording data in the first 20 cycles the activation process would have been overlooked. In this case, it is believed that the activation properties have something to do with increasing the dispersion or reduction of the TiO2. Such knowledge is key in understanding the Ti-enhanced hydriding properties of the alanates and methods to improve introduction of the Ti-dopants into the materials.

Figure 1: Hydrogen cycling of NaAlH4 (NaH + Al starting material) doped with 10 mol% of TiO2 nanoparticles. An activation process is observed in the first 20 cycles. Red: hydrogen desorption capacity, Gray: hydrogen charging pressure at each cycle.

2. Preferred Experimental Procedures

The following is a few suggestions about specific experimental considerations that are recommended to help improve the reliability of Cycle-Life measurements.

• Pressure-Temperature Cycling

1) Before starting cycling one should measure isotherms to define the pressure and temperature range that are optimal for cycling.

2) In addition, from the time data of the isotherms or individual kinetics measurements, kinetic data should be extracted and reviewed particularly near the full capacity region. This will provide an idea of how many hours it takes to absorb or desorbs the hydrogen at a particular temperature. It is suggested that majority of the cycling should be performed at the temperature at which an optimized isotherm is obtained; that is the temperature at which a complete equilibrium isotherm can be collected in a reasonable amount of time for convenience. Such kinetic data should be taken as a reference to determine time and temperature for the cycling experiments.

3) In some case where the times are long for hydriding and dehydriding, it is suggested that one does partial hydriding. In this case, the results may not be indicative of the optimal performance of the material itself. However, when one considers the typical real-world application of hydrogen-powered transportation, a vehicle will be filled with hydrogen at a refilling station where the time to fill is limited. Therefore, such curtailed Cycle-Life results are still meaningful even when kinetics is slow. At issue is that if one attempts Cycle-Life measurements that go to near completion at each cycle the experimental time for a reasonably representative number of cycles will be prohibitively long and may take years to complete. So, one must balance an acceptable level of accuracy with practical measurement times. What is critical is that these issues and experimental conditions are well documented in the presentation of results.

4) The above consideration with respect to kinetic effects is overcome in large part by performing full (near equilibrium) isotherm measurements at several points during the Cycle-Life experiments. These complete isotherms will provide a lot more detailed and accurate performance properties of the materials than simple capacity versus cycle plots.

5) Another factor to take into account is the amount of material used for cycling; it should be small enough to avoid large thermal excursions such that even the materials with slow kinetics can be tailored for the simulation experiment. And large enough to provide accurate hydrogen uptake and release data. Depending on the density of the material; 1 to 10 gram sample sizes are recommended.

• Thermal Aging

1) Thermal aging should be performed near the critical temperature that is obtained from isotherm data at various temperatures. This is perhaps the fastest method to perform aging experiments.

2) As with other types of cycling experiments, it is highly recommended to perform full (near equilibrium) isotherm measurements at several points during the thermal aging experiments (or at a minimum; before and after thermal aging). These complete isotherms will provide a lot more detailed and accurate performance properties of the materials than simple capacity versus cycle plots.

3) While it will be shown below that thermal aging is a good (and fast) substitute for certain metal hydrides, it may not be appropriate for other materials. Therefore, at least one set (but preferably several sets) of comparisons measurements (P-T cycling and thermal aging) should be done on a representative sample to ensure that thermal aging approach is appropriate.

3. Impact of Cycle Testing Parameters

When making Cycle-Life experiments, one has to choose a set of conditions or parameters within which the cycles will be performed. It is vital that the results are interpreted within the context of the conditions of the experiments. For example, in doing automated cycling test, a condition must be met that switches the measurement from absorption to desorption and again back to absorption portions of each cycle. Typically, the parameter that is used is time. That is an absorption will be performed for x number of minutes or hours and the subsequent desorption will be for y minutes or hours (not necessarily the same amount of time). Data should then be considered within the context of this parametric limit. Since the limit parameter is time, then it is important to evaluate time-dependent properties such as the rates of absorption or desorption. This is particularly true for materials that have slow kinetics and may or may not reach true equilibrium conditions within a fixed amount of time.

Reversible Li-amides provide a good example. With respect to amides for hydrogen storage, the initial publication on the reversible release and uptake of hydrogen involving Li-amide, -imide, -hydride, and -nitride (Equation 1) opened up a whole new class of light-weight materials for hydrogen storage.

Equation 1 

PCT absorption and desorption isotherm Measurements on the first portion of the reaction (left part of Equation 1) were performed on these materials over several cycles ranging in temperatures between 120°C and 160°C. These measurements demonstrated that these are still relatively high-temperature hydrides (Figure 2).

Figure 2: PCT measurement of 11th hydrogen desorption cycle of LiNH2 with LiH after partial absorption

Pressure versus time data collected during PCT dosing measurements of these materials also showed that sorption kinetics are slow in these solid-phase reactions (Figure 3).

Figure 3: Approach to equilibrium during PCT measurements of hydrogen desorption from the reaction of LiNH2 with LiH.

The development of amides for hydrogen storage was followed by Li/Mg-based amides that were reversible at lower temperatures. Advances on these materials focused not only on the fundamental processes involved in hydrogen uptake and release but also on practical investigations of the hydrogen storage properties of these materials. In particular, Cycle-Life measurements were carried out to determine the performance of these materials over long-term hydrogen cycling. The starting material was LiNH2 and MgH2 mechanically milled together in a 2:1 ratio, in a tungsten-carbide vessel under argon for 2 hours. Complete hydrogen absorption and desorption measurements were made on an automated volumetric apparatus with the sample held at 200°C (Figure 4). Absorptions were at constant pressures of 83 and 103 bar. Desorptions were made to a calibrated 2 liter vessel at 0.5 bar. Only desorption concentrations were measured. Each individual desorption measurement lasted 15 hours. The absorption measurements were initially set to 30 hours then reduced to 15 hours. However, the capacity dropped off indicating that hydrogen absorption was not complete in the shorter time period, so the absorption times were again increased to 30 hours. The initial tests on Li-Mg-N-H system showed relatively stable cycling properties at 200°C: 0.1% capacity loss per cycle - estimate 500 cycles to 50% capacity loss.

Figure 4: Reversible hydrogen desorption capacity with cycling of Li-Mg-N-H

In Cycle-Life measurements, care should be taken to evaluate all of the measurement conditions to avoid ascribing such an artifact of the experimental parameter settings to one material property (e.g. loss of capacity) when it may be due to another material property (reduction in kinetics).

The ideal situation in such cycling measurements would be to be able to wait until a true (or close to true) equilibrium condition is achieved in each cycle. But with materials such as these amides, that have relatively poor kinetics, such a measurement would take an unreasonably long amount of time.

A good approach would be to make a very long term absorption and desorption measurement near the beginning of cycling (cycle 10 or so) and at the completion of cycling and to compare the capacity and rates of these initial and final measurements.

Even better would be to perform a complete set of absorption and desorption PCT isotherm measurements near the beginning of cycling (cycle 10 or so) and at the completion of cycling. Then to compare plateau capacities (and shape) at the beginning and after many cycles.

Reference List: 

• Gross, K.J., “Hydride Development for Hydrogen Storage”, presentation DOE Hydrogen and Fuel Cells Annual Merit Review, Berkeley

• Gross, K.J., Luo, W., Rönnebro, E., and Spangler, S., “Sorption Properties of Novel Hydrogen Storage Materials”, Presentation, MH2004 Symposium on Metal-Hydrogen Systems, Fundamentals and Applications Krakow, Poland

• Luo, W. and Sickafoose, S., “(LiNH2-MgH2): A Viable Hydrogen Storage System”, J. Alloys and Compounds

• Nakamori, Y. and Orimo., “Li-N based Hydrogen Storage Materials”, S. Mat. Sci. and Eng. B