Analyzing Hydrogen Storage Potential of MOFs at High Pressures

In an effort to improve the hydrogen economy it is important to determine the hydrogen storage capabilities of metal-organic frameworks (MOFs) and other microporous materials. An important aspect of hydrogen fuel cell design is the development of an efficient method of hydrogen storage.

When hydrogen gas is stored as a compressed gas, it has a low energy density by volume and a high energy density by mass, thus making it unsuitable for storage. Moreover maintaining hydrogen in a liquid state is not energy efficient.

Hydrogen Storage Potential

Hydrogen can be stored in a solid material by adsorption. This is a suitable alternative as it requires less volume when compared to compressed gaseous hydrogen and uses less energy than that required to liquefy hydrogen. High-pressure hydrogen can be dosed onto MOFs and can be stored as an adsorbed gas. This process is highly desirable, thanks to the high hydrogen energy density acquired and the accessibility of reversible adsorption.

High-Pressure Volumetric Analyzer

Four MOFs were examined using a high-pressure volumetric analyzer (HPVA) from Micromeritics. The aim was to determine the hydrogen storage potential of these MOFs, which include Basolite F300, an iron-based organic framework, and Basolite C300, a copper-based organic framework. In this study, about 500 mg of each MOF was positioned under vacuum and gradually heated up to 200 °C for 12 hours using the HPVA degas port.

All four samples were then examined at liquid nitrogen temperature in a liquid nitrogen bath, using the cryogenic option of the HPVA at up to 100 bar pressure. An isothermal jacket was utilized to maintain the sample cryogenic temperature zone during analysis. At 77 K each MOF revealed varying amounts of hydrogen uptake with F300 adsorbing the least and C300 adsorbing the most. Figure 1 shows a plot of the isotherms produced from the analyses.

Figure 1. An overlay of the excess isotherms generated from the analysis of various MOFs with hydrogen at 77K. The solid circles represent the adsorption isotherms and the hollow circles represent the desorption isotherms.

In Figure 1 the isotherms show a phenomenon where the adsorption reaches a maximum and reduces as the pressure increases. This is attributed to increased hydrogen density in the pores of the material at increased pressures. The density of the adsorbing hydrogen is much higher compared to that of a non-adsorbing gas such as helium (He).

Static Volumetric Method

Given that the measured amount of gas in the sample cell is built on the helium density and its ensuing free-space volume including the volume within the pores, the quantity of free gas in the sample cell is found to be overrated. When a static volumetric technique such as HPVA is used, a maximum in the isotherm can be seen. This is utilized to generate the surplus isotherm (Figure 1).

In order to create an absolute isotherm, pore volume and gas density must be considered in the calculations. Given that the pore size and distribution of these materials are not instantly available to the majority of users, the surplus isotherm will be adequate and is usually reported.

Another technique for obtaining material storage capacity from the surplus isotherm is to observe the quantity of gas adsorbed as a function of sample weight. For storage purposes, the hydrogen uptake in weight percent ranges from 7% to 8%. Figure 2 displays an overlay of the weight percentage plots. These plots are based on the isotherms shown in Figure 1.

Figure 2. Weight percentage plots of various MOFs analyzed with hydrogen at 77 K.

Since a maximum amount of hydrogen was adsorbed by Basolite C300 at 77 K, the MOF was also examined at two more temperatures. In one analysis, the sample was maintained at 0 °C using an ice bath, while in another the sample was maintained at 30 °C by means of a recirculating water vessel. For both these experiments hydrogen was used to dose the sample at up to 200 bar, which is the maximum pressure range obtainable on the HPVA. Figure 3 shows the surplus isotherms and Figure 4 displays the weight percentage plots.

Figure 3. Hydrogen uptake on Basolite C300 at 0 °C (dark blue) and 30 °C (light blu).

Figure 4. Weight percentage plots of hydrogen on Basolite C300 at 0°C (dark blue) and 30 °C (light blue).

Conclusion

The Micromeritics HPVA is a suitable research tool for assessing hydrogen storage potential in high surface area MOFs and other highly microporous materials. With its broad temperature range and its capability to dose up to 200 bar of pressure the HPVA is suitable for studying samples under complex conditions whilst providing precise information to users.

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.