Study of highly porous polymers for H2 fuel storage using positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) was applied to study the porosity of four highly porous polymers: Amberlite XAD 4, Amberlite XAD 16, Haysep S and Hypersol-Macronet MN 200, all of which have been evaluated for H2 fuel storage. PALS revealed two types of pores, with average sizes around 2.0 Å and 6.0 Å. It was also determined that the smaller pores have a larger adsorption potential, while the larger pores constitute most of the fractional free volume. At low temperature (77 K), large pores trap most H2 molecules in the condensed gas state, rather than adsorb H2 molecules on their internal wall, as characterized by the Brunauer–Emmett–Teller (BET) method. In addition, it was found that 1) the fractional free volume (fv) of large pores, 2) the uniformity of large pores, and 3) the relative percentage of small pores are all key factors in storing H2 molecules at a low temperature. Ultimately, the PALS technique, one of the most powerful tools in characterizing micropores at the Å scale, can provide more informational (e.g. pore size, number, distribution) and accurate (e.g. fractional free volume rather than specific surface area) knowledge on the porosity of materials potentially used for H2 fuel storage in the future.