# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 3 Highly Porous Organic Polymers for H\({}_{2}\) Storage
### Hypercrosslinked Polymers (HCPs)  
Hypercrosslinked polymers are co-polymers synthesized via the Friedel-Crafts method, e.g., poly(styrene-co-vinylbenzyl chloride) (PS-VBC) [52]. By choosing the right monomers, the polymer will retain networks with a very fine pore structure, and large internal surface area [53]. The surface area of this type of polymer can reach 2000 m\({}^{2}\)/g with pore sizes of 2-4 nm, and hydrogen storage capacity of 5 wt % at 77 K/80 bar, but only 0.2 wt % at 298 K/90 bar [54]. In addition to the high internal surface area, hypercrosslinked polymers also retain a high content of micropores. Large internal surface area and high micropore content make this type of polymer an attractive candidate for H\({}_{2}\) storage. The basic synthetic route for hyper-cross linked polymers is shown in Figure 5. The benzene groups in polystyrene (PS) polymers are crosslinked by the crosslinkers, such as RCICCIR', and the microporosity within depends on the precursors used in the synthesis. When the initial precursor in making hypercrosslinked polymers is linear polystyrene (PS), as shown in Figure 5, the surface area can reach 1000 m\({}^{2}\)/g [55; 56]. In addition to PS, other precursors have been used to create higher internal surface area; for instance, when Poly(vinylbenzyl chloride)-co-DVB (VBC-DVB) was used as the precursor, the internal surface area was extended to 1900 cm\({}^{2}\)/g, and the corresponding H\({}_{2}\) storage ability at 77 K/15 bar could reach 3.0 wt % [53]. Other precursors, such as polyanilines [57], polypyrrole [58], bischloromethyl monomers [59], etc. were also explored. The surface area for hypercrosslinked polymers based on these precursors can reach 632 m\({}^{2}\)/g, and the H\({}_{2}\) storage ability at 77 K/30 bar reached 2.2 wt %. These values are significantly lower that the best MOFs and COFs a 77 K and high pressure, both of which possess much larger internal surface area. This is understandable since at 77 K, the surface area is the key parameter needed to store large amounts of H\({}_{2}\) molecules. However, as the temperature increases to room temperature, both internal surface area and adsorption enthalpy play vital roles in storing and delivering the large amounts of H\({}_{2}\) needed to meet DOE specifications, and hypercrosslinked polymers with more micropores could be competitive. In addition, at 1 bar pressure better-performing HPC's have comparable H\({}_{2}\) storage capacities to MOFs (see Table 3). Some typical hypercrosslinked polymers (HCPs) and their specific surface areas (SSAs) as well as H\({}_{2}\) adsorption capacities are listed in Table 3.  
To enhance the gas adsorption, some metals, or metal complexes such as ferrocene, have been incorporated into hypercrosslinked polymers [60; 61]. However, H\({}_{2}\) storage ability did not improve significantly for these systems at 77K, remaining at about 1.0 wt %, probably due to the continued small internal surface area of about 1000 m\({}^{2}\)/g. In addition, other elements, such as Si, have also been included in the polymer structures to improve the thermal stability. However, there are only small changes in the surface area (~1200 m\({}^{2}\)/g) and H\({}_{2}\) storage capacity (~1.25 wt %) at 77 K/1.12 bar [62; 63]. The intercalated sulfur atoms in hypercrosslinked pitch samples provide incrementally better performance, with internal surfaces areas of 1377 m\({}^{2}\)/g, yielding 1.83 wt % H\({}_{2}\) storage at 77  
Figure 5: Basic synthetic route of hyper crosslinked polymers: polymers with aromatic side chains are linked using a high concentration of methylene dihalides in the presence of a Lewis acid catalyst, to create a high concentration of cross-links. Most hypercrosslinked polymers are synthesized using metal catalysts. In contrast, hydroxy-group -containing porous organic polymers have been synthesized using organic catalysts.