# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 3 Highly Porous Organic Polymers for H\({}_{2}\) Storage
### Porous Aromatic Frameworks (PAFs)  
A major breakthrough in the use of highly porous organic polymers for H\({}_{2}\) storage, was the discovery of a porous aromatic framework (PAF-1), which was synthesized via a cross coupling reaction of the tetrahedral building block tetrakis(4-bromophenyl)methane as shown in Figure 11.  
The PAF-1 retains the highly porous networks that characterize MOFs and COFs, but with an amorphous structure, so it is classified as a polymer. PAF-1 not only has a very high internal surface area (\(\sim\)5600 m\({}^{2}\)/g), comparable to MOFs, but also is very thermally and hydrothermally stable, similar to COFs. Therefore, it combines the advantages of both MOFs and COFs. Due to its high specific surface area, the absolute and excess H\({}_{2}\) uptake ability for PAF-1 can reach as high as 10.7 wt % and 7.0 wt %, respectively, at 77 K and 48 bar [103]. More recently, other larger tetrahedral molecules with different building blocks have been used to synthesize new PAFs, also prepared via Yamamoto homocoupling reactions [104; 105]. For instance, [PERSON] et al. developed a series of PAFs, porous polymer networks (PPNs), using tetrahedral precursors, X(C\({}_{6}\)H\({}_{4}\)Br)\({}_{4}\), where X = C, Ge, Si, etc. [105]. Among them, the PPN-4, which was developed from X = Si, has internal surface as high as 6461 m\({}^{2}\)/g, comparable to that of MOFs and COFs. The excess H\({}_{2}\) adsorption can go as high as 9.1 wt % at 77 K and 55 bar. There is no H\({}_{2}\) adsorption data reported at 298 K. However, H\({}_{2}\) absorption is expected to drop to a low value at ambient temperature, due to the very weak van der Waals interaction between H\({}_{2}\) molecules and hydrocarbon network. Another approach along this line is that organic linkers with different lengths were inserted between the tetrahedral building blocks to obtain different pore sizes. For example, by inserting diphenylacetylene (DPA), 1,4-diphenyl-luta- diyne (DPB), 1,4-bis (phenylethynyl) benzene (BPEB), or 1,4-bis (phe-nylbutadiynyl) benzene (BPBB) linkers, [PERSON] et al. [106] synthesized a series of porous aromatic frameworks (PAF-322, PAF-324, PAF-332, and PAF-334). The size of the building unit in PAFs was significantly expanded and so were the pore size and internal surface area. The total H\({}_{2}\) uptake can go as high as 63.96 wt % at 77 K/100 bar, and excess H\({}_{2}\) uptake can reach 10.69 wt % at 77 K/20 bar. In particular, the total H\({}_{2}\) uptake of PAF-334 at 298 K/100 bar is simulated to be 16.03 wt %. However, it is worthwhile to mention that the above results are the simulation results, not experimental results. Furthermore, due to the large open pores inside the polymer, the volumetric capacity of H\({}_{2}\) storage for these PAFs is only approximate 9.0 g/L at 298 K/120 bar, far below the DOE criteria (40 g/L). Similarly, theoretical simulations on designing up to 115 organic PAF-XXXs show that the weight storage capability of H\({}_{2}\) can reach 5.9 wt % at 298 K/100 bar, while the volumetric capacity of H\({}_{2}\) storage can only reach 7.9 g/L [107]. Therefore, simply increasing the pore size via different synthetic strategies might not be the right direction to make highly porous polymers that meet the DOE standards for H\({}_{2}\) storage. In additional studies, several metals have been added to PAFs to enhance their H\({}_{2}\) adsorption capacity. Similar to that in conjugated microporous polymers (CMPs), doping PAF-1 with 5% of Li (Li@PAF-1) significantly increases ultra micropore content (<10A size) and possible adsorption enthalpy, and hence increases H\({}_{2}\) adsorption ability by 22% [108]. Lithium ion doping in the PAF derivative, PAF-18-OH, also increases the polymer's H\({}_{2}\) adsorption [109]. In a related simulation, lithium-decorated fullerenes (Li\({}_{6}\)C\({}_{60}\)) were impregnated in PAFs and H\({}_{2}\) uptake was predicted to be 5.5 wt % at 77 K/1 bar [110].