# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 3 Highly Porous Organic Polymers for H\({}_{2}\) Storage
### Polymers of Intrinsic Microporosity (PIMs)
The best performing polymer, TMP-3, exceeded the performance of other PIMS, with a total surface area (\(S_{\text{BET}}\) = 1372 m\({}^{2}\)/g), significant micropore volume (0.163 mL/g), and 4.42 wt % H\({}_{2}\) uptake at 77 K/1 bar [81]. The advantage of these three systems two PIMs is that all three show ultra microporosity with the range of 6-8 A, which is needed to retain H\({}_{2}\) molecules at room temperature. In addition to PIMs synthesized via the dibenzodioxane reaction, other PIMs were made via imidization and amidation, using amine or amide precursors, as shown in schematic reaction diagrams in Figure 8[82; 83]. The synthesis of PIMs and its applications was systematically reviewed by [PERSON] et al. [84]. Generally speaking, polymer chains in PIMs must contain aromatic heterocyclic ladder components in the polymer backbone that restricts the free rotation of the backbone, preventing dense packing, and therefore retaining high porosity. Figure 8: _Cont._  
Figure 7: Basic structure of CTC-PIM, cyclotricatechylene-core polymer of intristic microporosity (**a**) and Trip(R)-PIM, a triptycene-core with variable R groups (**b**). Different monomers are used to generate different PIM polymers. The complete list of monomers for all three types of reactions can be found in [84]. Here we give an overview of the PIM polymers produced by these three synthetic pathways, in terms of the surface area and hydrogen storage capacity. From the general structures of the products in Figure 8, it is seen the first PIM is most rigid due to the restriction of rotation by two -O- bonds in the backbone, while the third is least rigid due to the free rotation of -CO- and -NH- bonds. The corresponding internal surface area of the first type of PIM is 120-1760 m\({}^{2}\)/g, and that of PIM-PIs is 551-1407 m\({}^{2}\)/g and that of PIM-PAs is 50-156 m\({}^{2}\)/g, with surface area decreasing with increasing bond mobility. Therefore, the first type of synthesis is more interesting in terms of hydrogen storage and a variety of different monomers have been used to make PIM polymers of this type for H\({}_{2}\) storage. Since hydrogen storage is more about meeting technical specifications, rather than methodology development, we hereby highlight the PIMs with highest surface area. The PIMs with the largest surface area are called star triptycene-based microporous polymer (STPs), which have structures similar to Trip(R)-PIM as in Figure 7b. STPs can reach as high surface areas as high as 2000 m\({}^{2}\)/g and H\({}_{2}\) storage capacity at 77 K and 1 bar can reach 1.92 wt %. Therefore, PIMs with star-shaped structures possess higher specific surface area (SSA) and then higher H\({}_{2}\) uptake, aided by their three-dimensional networks. As seem with HCPs, the best PIMs still fall significantly short of H\({}_{2}\) adsorption achieved with the best MOFs at elevated pressures; however, PIMs outperform MOFs at 1 bar/77K, with H\({}_{2}\) storage capacities up to 4.5 wt % vs. MOFs of \(>\) 2.5%. Both fall short of DOE targets.