Microporous materials for hydrogen storage

Microporous materials (with pores of less than 2 nm in diameter) have attracted considerable attention due to the variety of applications such as gas storage. Physisorption of molecular hydrogen offers several advantages over chemical absorption, namely, fast kinetics and complete reversibility. The overall aim of this work was to investigate the potential of microporous materials for hydrogen storage, with particular attention given to a relatively new class of material: Polymers of Intrinsic Microporosity (PIMs). Generally the PIMs were seen to adhere to Chahine’s rule, which predicts a linear correlation of hydrogen adsorption capacity, at 77 K, with surface area (1 wt.% per 500 m\(^2\) g\(^{-1}\)). IRMOF 1 and Cu-BTC exhibited the largest gravimetric storage capacities of 4.86 and 4.50 wt.% at 77 K and 15 bar, respectively. The largest for a microporous polymer was 3.26 wt.% at 77 K and 15 bar, for the methyl triptycene-based PIM. Two empirical equations, Sips and Tóth, were used in addition to a multi-parameter Virial type thermal equation to fit hydrogen adsorption curves over a range of temperatures (77 to 137 K), in order to calculate the enthalpy of adsorption for all materials as a function of hydrogen adsorption. The findings in this investigation suggest that there is a trade-off between gas sorption capacity and enthalpy of adsorption where dispersive van der Waals interactions dominate adsorption. It is unlikely that the optimal enthalpy of adsorption (of ca. 20 kJ mol\(^{-1}\)) will be achieved by simply reducing the pore size of the material