Background

For a molecule travelling through nanospace the likelihood of crashing is great! If not with other molecules, certainly into the walls of the canyon-like nanopores through which nano-flight is taken. And as the canyon narrows and the temperature falls, the chance of escaping the crash site diminishes.

AZoM - the A to Z of Materials - Molecules travelling through nanospace

Figure 1. Molecules travelling through nanospace
Pores Accessible To Hydrogen

There exists a need therefore, for rapid characterization tools, ones that are available now so as not to suffer any further delay in advancing the development of useful nanoscale materials. Thankfully that technology does already exist in the gas sorption arena. Cryogenic gas sorption analyzers have been characterizing microporous and mesoporous materials such as zeolites and activated carbons for generations. Until now however, pore size and pore volume measurements have been almost exclusively limited to the adsorption of nitrogen and argon. But some pores (or parts of pores) accessible to H2 may not be accessible to other molecules because of size restrictions or due to very slow diffusion. Therefore it only seems sensible to use H2 for the PSD analysis of porous materials considered for H2 applications.
Appreciable Adsorption

Current state-of-the-art volumetric adsorption equipment (Autosorb-1-MP) is already being used to measure hydrogen adsorption isotherms, since at cryogenic temperatures (77 and 87 K) appreciable adsorption of hydrogen begins at about 10-4 atm for microporous materials. It is important to note that the critical temperature of hydrogen is much lower, around 33 K. Hence, even though measurements at temperatures of liquid nitrogen (77 K) or liquid argon (87 K) might seem far from forecast/actual storage temperature, both are at supercritical conditions. In fact, lowering the temperature has a similar effect of increasing the amount adsorbed (over room temperature amount) as does increasing pressure (at room temperature or above), the latter being conceivably the practical solution for mobile fuel cell applications.

Therefore, hydrogen adsorption experiments performed even at sub-atmospheric pressures provide important information about the hydrogen storage potential of an adsorbent.
New Models Developed

Adsorption data measured at different temperatures (Fig. 2) can be used to calculate the isosteric heat of adsorption, Qst. Materials showing high Qst values over a wide range of adsorbed amount will have high adsorption capacity at ambient temperatures.

AZoM - The A to Z of Materials - Isosteric heat of adsorption calculated from H2 adsorption isotherms

Figure 2. Isosteric heat of adsorption calculated from H2 adsorption isotherms

Pore Size Distribution calculations can also be done from the sub atmospheric data, but not using classical models. Therefore new models have been developed and analysis using Density Functional Theory (DFT) applied to H2 adsorption isotherms measured for several porous carbons were presented recently.
A Tool For Other Nanoscale Materials

This new development in size analysis and characterization of sub-nano pores (small micropores), though created to meet the need to investigate the hydrogen storage potential of various materials, will undoubtedly be adopted for the characterization of other material structures with nanospace, for other nanoscale applications.