# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 1 Introduction  
The world is reaching its \"Oil Peak\" and petroleum resources are expected to be exhausted in coming decades. Seeking alternatives to fossil fuels is imperative. H\({}_{2}\) is thought to be a viable alternative fuel due to its large abundance and clean combustion. In addition, H\({}_{2}\) has a much higher energy density (142 kJ/g) than that of petroleum oil (47 kJ/g) [1]. However, the use of H\({}_{2}\) as a fuel in automobiles has a major obstacle: the onboard storage of H\({}_{2}\) gas. Many methods to store hydrogen have been proposed. The most direct way is to storage H\({}_{2}\) as a liquid or a high-pressure gas [2; 3]. This approach requires significant energy for liquefying or pressurizing H\({}_{2}\) gas, which has a very low boiling temperature (20 K) and critical temperature (33 K), and also poses many safety concerns due to extremely low temperature and high pressure. In addition, H\({}_{2}\) can embrittle steel gas tanks after long storage, generating additional risk. As an alternative, H\({}_{2}\) storage media have been extensively explored, under the H\({}_{2}\) program initiated by the U.S. Department of Energy (DOE) in 2003. The DOE set several criteria for materials for onboard H\({}_{2}\) storage: (1) high storage capability, that is, 5.5 wt % and 40 g/L at ambient conditions; (2) rapid H\({}_{2}\) release and recharge under moderate conditions; and (3) long recycling life, that is, more than 1000 recharge and discharge cycles [4; 5]. The first two criteria have proved particular challenging, so that the DOE has postponed its target date several times. For instance, the DOE set its initial target for 2015, extended it to 2017, and then to 2020 [6]. The ultimate goal of the DOE is 6.5 wt % and 60 g/L by 2050, as summarized in Table 1[7]. As seen from Table 1, there are many engineering and economic requirements, as well as scientific requirements for hydrogen fuel to be successfully employed as an onboard energy power for vehicles. In order to be adopted commercially, a hydrogen storage medium must meet all above criteria. Nevertheless, two fundamental scientific criteria, gravimetric capacity, and volumetric capacity, are the primary and most important parameters for scientists to pursue. Therefore, in this review, we will focus on progress made in meeting the first two criteria, particularly the gravimetric capacity, using porous organic polymers for the hydrogen storage. Currently there are two major ways to store H\({}_{2}\) molecules: (a) chemical absorption by forming hydrogen-containing molecules and (b) physical adsorption in highly porous materials. The former approach includes metal hydrides such as NaH, LiH, NaAlH\({}_{4}\)[8; 9; 10], etc. as well as other hydrogen-containing molecules such as H\({}_{3}\)N-BH\({}_{3}\)[11]. These materials store H\({}_{2}\) via chemical bonding. The advantage of these materials is that they have a relatively high storage capability. The disadvantage is that they require high temperatures to break the chemical bonds and release H\({}_{2}\) molecules, so that the recovery of H\({}_{2}\) gas from these chemicals is not energy efficient. Organic hydrogen carriers or organic hydrides are quite reactive, leading to safety concerns, and disposal of metals poses the environmental concerns. In contrast, physical adsorption uses weak van der Waals interactions, and therefore H\({}_{2}\) molecules can be released easily under moderate conditions. However, due to these weak interactions, physical adsorption, which can retain relatively large amount of H\({}_{2}\) at liquid nitrogen temperature, has greatly reduced storage capability at room temperature. Physical adsorption methods can be achieved with materials with high porosity, including metal organic frameworks (MOFs), covalent organic frameworks (COFs), activated carbons, carbon nanotubes, carbides, and highly porous polymers [12; 13; 14]. Currently, the general approach for using physical adsorption in H\({}_{2}\) storage is to increase the internal surface area in materials.