# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 3 Highly Porous Organic Polymers for H\({}_{2}\) Storage
### Conjugated Microporous Polymers (CMPs)
Other polyaromatic-based CMPs, such as hexaphenylbenzene (HPB)-based porous organic polymers (HPOPs) [88], tetraphenylethylene (TPE)-based porous organic polymers (TPOPs) [89; 90], phenolic-resin porous organic polymers (PPOPs) [73], and carbazole-spacer-carbazole type conjugated microporous polymers (P-1 and P-2) [91] have also been prepared and tested for H\({}_{2}\) storage characteristics. The highest H\({}_{2}\) uptake for HPOPs is HPOP-1 (\(S_{\text{BET}}\) = 1148 m\({}^{2}\)/g) with 1 50 wt % at 77 K/1.13 bar, for TPOPs (\(S_{\text{BET}}\) = 810 m\({}^{2}\)/g) is TPOP-5 with 1.07 wt % at 77 K/1.13 bar, for PPOPs is PPOP-3 (\(S_{\text{BET}}\) = 880 m\({}^{2}\)/g) with 1.28 wt % at 77 K/1 bar and for P-1/2 is P-2 (\(S_{\text{BET}}\) = 1222 m\({}^{2}\)/g) with 1.66 wt % at 77 k/1 bar. There is no significant improvement in terms of H\({}_{2}\) adsorption for these different modified CMPs, and all fall far short of DOE specifications. Recently, three novel CMPs were synthesized, based on a 4,4-difluoro-4-bora-3a, 4a-diaz-s-indacene (BODIYP) core coupled with 1,3,5-triethynylbenzene, named BDT1-3. The charged non-metal sites, in addition to high microporosity lead to improved H\({}_{2}\) adsorption, particularly for BDT3, which had a total surface area of 101 m\({}^{2}\)/g, an average pore size of 0.7 nm, and 2.2 wt % uptake at 77K. While these values need still to be improved, presence of charged non-metals on the polymer backbone and small pore size of this system are promising [92]. In addition to varying the structure of the monomers, the microporosity can also be controlled by varying the percentages of building block precursors, as well as experimental conditions. For instance, conjugated carbazole backbone was spaced and linked by different length of alkenes to modify the pore structures. However, the BET surfaces were very similar (-800 m\({}^{2}\)/g) and the highest H\({}_{2}\) uptake is 1.33 wt % at 1.0 bar/77 K [93]. Most pure organic CMPs possess internal surface of \(\sim\) 1000 m\({}^{2}\)/g or less, and the H\({}_{2}\) uptake is around 1.0 wt % at -77 K/1 bar [94; 95; 96]. In contrast, the Li doped CMP, with a relatively low surface area of 795 m\({}^{2}\)/g, has a much higher H\({}_{2}\) uptake (\(\sim\)6.1 wt % at 77K/1 bar) than pure organic CMPs [97], due to the charge-induced dipole interaction between H\({}_{2}\) and Li atoms as described in the Background Considerations section, and hence higher adsorption enthalpy. Unfortunately, Li ions tend to aggregate in the CMPs. As an improvement, another format of Li\({}^{+}\), methyllithium (MeLi) doped in a naphthyl-containing conjugated microporous polymer (N-CMP) can bring excess H\({}_{2}\) uptake to 6.5 wt % at 77 K/80 bar [98]. Halides, specifically Cl and Br have been incorporated into the amine-based CMPs, borazine-linked polymers (BLPs): BLP(Cl) and BLP(Br). BLP-2(Cl) to yield specific surface areas of up to 1174 m\({}^{2}\)/g and H\({}_{2}\) uptake of 1.30 wt % at 77 K/1 bar, while BLP-2(Br) has specific surface area of 849 m\({}^{2}\)/g an H\({}_{2}\) uptake of 0.98 wt % at the same conditions [99]. Other organic heteroatomic systems are tried as well. For instance, nitrogen-rich conjugated microporous polymers (N-CMPs) and nitrogen-rich azo-bridged porphyrin conjugated microporous networks (N-Azos) have been synthesized and yielded \(S_{\text{BET}}\) up to 485 and 675 m\({}^{2}\)/g with H\({}_{2}\) uptake of 1.02 and 1.15 wt % respectively at 77K/1 bar [100; 101]. Thiophene-based conjugated microporous polymers (ThPOPs) have been synthesized and tested for H\({}_{2}\) storage [102]. Among them, ThPOP-5 that retains both high BET surface (\(S_{\text{BET}}\) = 1300 m\({}^{2}\)/g) and micropores (\(V_{\text{micro}}\) = 0.28 cm\({}^{3}\)/g) can uptake H\({}_{2}\) gas as high as 2.17 wt % at 77 K/1 bar. Incorporating organic groups such as azo and thiophene, increases the polarity of adsorption site, in addition to modifying the porosity.