# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 5 Conclusions and Prospective Future  
Porous organic polymers have their own advantages in H\({}_{2}\) storage, for instance, the structures and porosity can be controlled by the use of different monomers and synthetic routes; the cost is relatively  
Figure 13: Schematic diagram of positron annihilation lifetime spectroscopy (PALS) technique: (a) setup and (b) mechanism. low and it is suitable for mass production, and normally, there is no heavy-metal incorporated, making the materials and processes friendly to the environment. These types of polymers store H\({}_{2}\) molecules via physical adsorption through van der Waals interaction, and hence can release H\({}_{2}\) as fuel under relatively mild conditions. Four types of highly porous organic polymers, hypercrosslinked polymers (HCPs), polymers with intrinsic microporosity (PIMs), conjugated microporous polymers (CMPs), and porous aromatic frameworks (PAFs) demonstrated high internal surface areas and good H\({}_{2}\) uptake at cryogenic temperature. It is seen that the polymers originated from the star shaped precursors, such as Trip(R)-PIM and PAFs, can retain the highest specific surface area and outperform the other polymers generated from planar shaped precursors, and therefore, induce higher H\({}_{2}\) uptake capacity under higher pressures. With the limited data published, only PIMs have been shown to have H\({}_{2}\) storage capacities approaching 5 wt % at 77 K. However, due to the weak van der Waals interactions, the H\({}_{2}\) adsorption enthalpy is only approximate \(\sim\)6.0 kJ/mol for most carbon based polymers, and hence the H\({}_{2}\) storage ability is very low under ambient temperature. Most of these materials would have an H\({}_{2}\) absorption of \(\sim\)1.0 wt % at room temperature, which is far below the targets specified by the DOE, despite of the fact that many highly porous polymers can store up to 10 wt % H\({}_{2}\) at liquid N\({}_{2}\) temperature (77 K) and high pressures (10 bar or more). To enhance the H\({}_{2}\) adsorption ability at room temperature and ambient pressure, it is imperative to increase H\({}_{2}\) adsorption enthalpy, in addition to high porosity. Theoretical considerations revealed two effective methods to achieve such goals: first, producing as many ultra micropores at several A as possible; second, introducing some light metal ions, such as Li and Na ions, to create some charge-induced dipole interactions between H\({}_{2}\) molecules and charge site. Ultra micropores, in contrast with macropores at nm scale or mesopores at \(\upmu\)m scale, promote interaction between H\({}_{2}\) molecules and pore walls, via the overlap of potentials from opposite walls and the quantum sieving effect that can be maintained up to 300 K, while larger pores can only have van der Waals forces between pore walls and H\({}_{2}\). Unfortunately, most reports give only the content of overall porosity, which is probably composed primarily of macropores rather than ultra micropores, and therefore, the systems studied yield good to excellent adsorption result at 77 K, but do not perform well at room temperature. Nevertheless, there are some reports of highly porous polymers with high ultra micropore content. For instance, [PERSON] et al. (2013) synthesized microporous polymer HTP-B using a hexaphenylbenzene-based triptycene monomer, in an attempt to introduce some ultra micropores under 10 A. Although the exact ultra micropore content is not known, the H\({}_{2}\) uptake is significantly higher than that of its counterpart, HTP-A that has no ultra-micropore content (1.09 wt % vs. 0.55 wt %). Therefore, future efforts should be directed to synthesizing highly porous polymers that have both high BET surface area and high content of ultra micropores. To further enhance the room temperature H\({}_{2}\) absorption activity of these polymers, light metal ions can be doped into the materials. A third approach might be to add some transition metals to create so called spillover of H\({}_{2}\) molecules.