# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 3 Highly Porous Organic Polymers for H\({}_{2}\) Storage  
Several types of highly porous materials for H\({}_{2}\) storage, other than organic polymers have been explored, with the aim of meeting the DOE criteria. They include: (a) Activated carbon and its modifications: a variety of methods were used to generate different types of activated carbon with large internal surface areas [37], as well as doping the carbons with different elements such as Pt, Pd, Rh, Ni, and Cu to enhance the adsorption enthalpy [38]. Surface area of some specially-activated carbons, e.g. open carbon frameworks (OCFs), can reach as high as 3800-6500 m\({}^{2}\)/g and excess \begin{table}
\begin{tabular}{c c c} \hline \hline
**Interaction type (Material – H\({}_{2}\))** & **Energy dependence** & **Typical values (kJ/mol)** \\ \hline Charge – H\({}_{2}\) quadropole & \(\propto\) 1/\(r\)\({}^{3}\) & \(\sim\) 3.5 \\ Charge – induced H\({}_{2}\) dipole & \(\propto\) 1/\(r\)\({}^{4}\) & \(\sim\) 6.8 \\ Dipole – induced H\({}_{2}\) dipole & \(\propto\) 1/\(r\)\({}^{5}\) & \(\sim\) 0.6 \\ van der Waals & \(\propto\) 1/\(r\)\({}^{6}\) & \(\sim\) 5 – 6 \\ Orbital interaction & \(<\)vdW radii & \(\sim 20-160\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Dependence on distance and magnitude of different type of interactions between an H\({}_{2}\) and adsorbent molecules (Modified from Ref. [35]). Figure 3: Schematic diagram of the different types of interactions between an H\({}_{2}\) molecule and absorbent materials. H\({}_{2}\) adsorption ability can reach 8.5 wt % at 77 K and 100 bar [39]; (b) Carbon nanotubes (CNTs): many studies have been performed on H\({}_{2}\) storage in single walled (SWCNTs) or multiwalled carbon nanotubes (MWCNTs). The H\({}_{2}\) storage capacity in CNTs, particularly SWCNTs with some metal doping, is relatively high, around 7.0-10 wt %, at 77 K. Some reports claim 14-20 wt % of storage at the temperature of 298 K or above, but these results have not been widely replicated [40]. There are some excellent reviews on the H\({}_{2}\) storage in CNT materials [17; 41]; (c) Metal organic frameworks (MOFs): MOFs are crystalline solids composed of multidetate organic ligands connecting to metal ions. The surface area of MOFs can be extremely high, e.g., 6200 m\({}^{2}\)/g [18; 42]. A wide range of MOF materials have been synthesized [43; 44]. Here are just a few examples: For instance, Zn\({}_{4}\)O(CO\({}_{2}\))\({}_{6}\) has an adsorption of 1.0 wt % at 298 K and 20 bar and 4.5-7.5% at 78 K and 0.8 bar; MOF-5 with composition Zn\({}_{4}\)O(BDC)\({}_{3}\) (BDC = 1,4-benzenedicarboxylate) gives 4.5 wt % at 77 K and 1.0 wt % at 298 K and 20 bar [21]. MOFs have weak interactions and readily release H\({}_{2}\), but storage capacity is low under ambient conditions; and (d) Covalent organic frameworks (COFs): COFs are similar to MOFs, but with organic covalent bonds linking the frameworks together. The internal surface area of COFs is generally smaller than that of MOFs, however, lower mass organic elements and better stability of the networks make COFs an attractive material for H\({}_{2}\) storage [34; 45]. Similar to all other porous carbon materials, while COFs are good candidates for storing H\({}_{2}\) gas at low temperature, their performance at ambient temperatures is low. Recently, several theoretical simulations suggest that the intercalation of other elements in COFs can enhance the H\({}_{2}\) adsorption ability [46; 47; 48; 49]. Comparison of H\({}_{2}\) storage in the above porous materials has been reviewed by [PERSON] et al., and a summary plot is shown in Figure 4[50]. As seen in [PERSON]'s review, and illustrated in Figure 4, MOFs outperform COFs and activated carbon at 77 K due to their high surface area. However, when at room temperature, none of them can meet the DOE criteria due to their small adsorption enthalpy. In addition, these higher storage capacities are achieved at high pressure, not 1 bar as would be needed for practical applications.