Cycle-Life performance measurements are critical for evaluating the practical use of hydrogen storage materials in applications such as hydrogen powered automobiles where hundreds to thousands of cycles will be required. It is important that these measurements be performed accurately and with a thorough understanding of: 

a) how measurements should be performed to best represent the conditions under which hydrogen storage materials will be used in life applications, 

b) what impact the experimental setup itself can have on the performance results, 

c) how to best gain insight from the measurements into what causes cycling performance degradation. 

Cycle-Life Measurements is the study of performance of on-board reversible hydrogen storage materials as they may be used in real applications. Typically, the first step is to make basic measurements of the total reversible capacity of the material as hydrogen is charged and released from the material over several cycles. Following this, measurements may then focus on modifying the storage material itself to improve the ability to maintain capacity over many cycles. This may be supported by more fundamental studies of what causes degradation of the material with cycling. An important metric of performance besides capacity is the rate of hydrogen uptake and release which is also likely to degrade with cycling. In the later stages, measurements are aimed at evaluating how storage materials perform when cycled under non-ideal conditions (temperature excursions, exposure to air, impurities in the hydrogen gas supply….). Finally, thorough testing of the materials and storage system at a scaled up level are necessary to be able to evaluate true application level performance. Each level of Cycle-Life testing may require its own unique experimental setup, procedures and special attention to details that may unexpectedly adversely impact the reliability of the measurements.

Cycle-Life measurements have been performed most extensively on reversible metal hydrides and to a much lesser extent on reversible physisorption hydrogen storage materials. While some of these hydrogen-materials interactions (such as decrepitation) are specific to hydrides, many similar issues may affect the cycling properties of physisorption materials. For example, hydrogen uptake is known to change the lattice dimensions in some MOFs. This may ultimately affect the long-term cycling performance of these materials. Another issue with hydrides to consider for physisorption materials is degradation or long-term cycling stability. For example, exposure to excessive temperature is known to cause degradation or complete decomposition of some MOFs. It may be that hydrogen cycling will lead to similar degradation behavior of these materials. Certainly, exposure to impurities in the hydrogen supply that has a strong impact on hydrides, are likely to affect hydrogen uptake of some physisorption materials (in particular those that contain active catalysts e.g. platinum in spill-over materials). However, little is know about the long-term effects on physisorption materials due to the presence of impurities.

Very few Cycle-Life measurements of hydrogen storage properties physisorption materials exist. The lack of data points out the need for more intensive work in this area; in particular with respect to long term cycling performance of physisorption materials in view of automotive applications. For this reason, issues that will certainly be germane to such measurements are outlined below.

1. Even though hydrogen storage in physisorption materials is through weak surface interactions of molecular hydrogen, it can not be assumed that performance degradation does not occur with hydrogen cycling for all physisorption materials.

2. Measurements can be difficult because typical 77K temperatures will have to be consistently maintained throughout hundreds of cycles.

3. However, kinetics are generally fast, therefore, pressure cycling can be done within a reasonable time. Thermal cycling may not be needed.

4. Because physisorption capacity measurements are susceptible to large errors (as described in the Capacity Section of this document), it is critical that any evaluation of long-term cycling performance be done using isotherms done under identical conditions at regular cycling intervals. It is unlikely to be sufficient to simply compare maximum uptake (or release) per cycle.

5. Impurities in the hydrogen supply are likely to impact Cycle-Life performance (e.g. MOFs that decompose with water contact). Yet some impurities (Air…) may condense out of the hydrogen supply at low temperatures before being exposed to the storage materials. Contamination issues should be evaluated with respect to how the materials will be used in an actual application.

6. Studies involving natural gas storage have shown that impurities in the gas accumulate in high surface area carbon adsorbents. However, this principally impacts low pressures capacities (<2 bar) which might not be utilized in hydrogen storage applications. Thus, the impact of contaminates should be investigated as a function of pressure as well as temperature.

7. Pressure cycling may induce some lattice changes in the materials (some MOFs for example). The impact of these changes, such as hydrogen gas trapping, or structural degradation with repeated cycling should be investigated. It is recommended to use simultaneous measurement techniques, when possible, to investigate structural changes with hydrogen content. An example would be in-situ X-ray Diffraction or neutron diffraction while performing hydrogen uptake and release isotherm measurement.