# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 4 Characterization of Porosity in Highly Porous Organic Polymers  
As described above, ultra microporosity and adsorption enthalpy are key parameters for a highly porous organic polymer to adsorb large amounts of H\({}_{2}\). The adsorption enthalpy can be measured by the adsorption dynamics. The porosity is traditionally characterized by the N\({}_{2}\) gas adsorption method owing to the high relatively critical temperature of N\({}_{2}\) gas, and well-defined theory. Normally, N\({}_{2}\) gas is applied at different relative pressures from 10\({}^{-8}\) to 1 to provide an adsorption isotherm, which describes the adsorption of N\({}_{2}\) molecules over a wide range of porosity. To extract the information on porosity from H\({}_{2}\) adsorption isotherms, different models have been proposed based on the mechanism of the micro-filling process in pores. The most widely used model is the Brunauer-Emmett-Teller (BET) model, which takes into consideration the multilayer adsorption of N\({}_{2}\) molecules at liquid N\({}_{2}\) temperature [117; 118], yielding the total surface area. Alternatively, the Horvathe-Kawazoe (HK) method [119; 120] and Dubinin-Radushkevich (DR) analyses [121; 122] are often used to analyze micropore (\(r<20\) A) volume and pore distribution at low pressure with the aid of the density functional theory (DFT) [123; 124; 125]. All three methods are based on the same experimental data: gas adsorption--normally, N\({}_{2}\) gas adsorption at 77 K.  
Unfortunately, N\({}_{2}\) adsorption experiments have some intrinsic limitations when measuring ultra micropores (\(\sim\)A) in highly porous polymers for H\({}_{2}\) storage at ambient conditions. First, N\({}_{2}\) adsorption methods do not effectively measure ultra micropores that are crucial to H\({}_{2}\) storage due to large kinetic diameter of N\({}_{2}\) molecules, 3.64 A [126], which is comparable to the optimum pore sizes (\(\sim\)3.8 A) for maximum H\({}_{2}\) adsorption at room temperature. The adsorption of other gas media, such as CO\({}_{2}\) and H\({}_{2}\) has also been used to study the porosity [127]. However, no significant improvement in measuring micropores has been achieved, due to either the large size of CO\({}_{2}\) molecules (3.3 A), or the low critical temperature of H\({}_{2}\) (32 K). Second, the N\({}_{2}\) gas adsorption method provides isotherms at liquid N\({}_{2}\) temperature (77 K), and is unable to generate the data needed to examine the H\({}_{2}\) storage ability of a material at room temperature. Since pore volume in polymers significantly changes with temperature, the results obtained at 77 K might be very different from those at room temperature. In particular, ultra micropores (\(\sim\)A) are much more sensitive to temperature, in that the expansion coefficient of those pores is almost ten times larger than the bulk value [128]. Therefore, some candidate materials ruled out by the N\({}_{2}\) adsorption method at 77 K may be appropriate for H\({}_{2}\) storage at room temperature, and vice versa. Finally, the adsorption of N\({}_{2}\) at 77 K can cause a swelling effect or warp formation in the pore structure, giving hysteresis in the adsorption-desorption isotherm curve and therefore inaccurate information on the pristine pore sizes. Recently, another technique, positron annihilation lifetime spectroscopy (PALS) has been applied to study ultra micropores in H\({}_{2}\) storage materials, providing a useful alternative to the N\({}_{2}\) gas absorption method [84; 108; 129]. The positron is a particle with a positive charge and the same mass as an electron, and is generated from a positron source, normally a radioactive isotope \({}^{22}\)Na. The schematic diagram for PALS technique and its mechanism is illustrated in Figure 13.  
Figure 13a shows the general outline of the PALS setup. When a positron \(e^{+}\) emits from the radioactive source, an accompany \(\upnu\) ray with the energy of 1274 \(keV\) is given off simultaneously and detected by one detector, marking the birth of a positron.