# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 1 Introduction
For instance, activated carbons can have surface areas of up to 2000 m\({}^{2}\)/g and store H\({}_{2}\) up to 5 wt % at 77 K/40 bar [15], but storage capacity decreases to 1.0 wt % at 298 K/200 bar [16]. Single walled and multiwalled nanotubes can store 6 wt % at 77 K [17]. MOFs, which are crystalline solids consisting of multidentate organic ligands connecting to a metal ion, afford a very large internal surface area, e.g., 5000 m\({}^{2}\)/g [18]. A wide range of MOF materials have been synthesized. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Target** & **Year** & **2020** & **2025** & **Ultimate** \\ \hline Gravimetric capacity (wt \%) & 4.5\% & 5.5\% & 6.5\% \\ Volumetric capacity (g/L) & 30 & 40 & 50 \\ Cost (\$/kg H\({}_{2}\)) & 333 & 300 & 266 \\ Durability/Operability: & & & \\ \(\bullet\) & Operating temperature (\({}^{\circ}\)C) & \(-\)40/60 & \(-\)40/60 & \(-\)40/60 \\ \(\bullet\) & Min/max delivery temperature (\({}^{\circ}\)C) & \(-\)40/85 & \(-\)40/85 & \(-\)40/85 \\ \(\bullet\) & Operational cycles & 1500 & 1500 & 1500 \\ \(\bullet\) & Operational cycles & 5/12 & 5/12 & 5/12 \\ \(\bullet\) & Min/max delivery pressure (bar) & 90\% & 90\% & 90\% \\ \(\bullet\) & Onboard efficiency & & & \\ Charging/Discharging rate: & & & & \\ \(\bullet\) & System fill time (min) & 3–5 & 3–5 & 3–5 \\ \(\bullet\) & Min full flow rate (\(\text{(g/s)/}k\text{W}\)) & 0.02 & 0.02 & 0.02 \\ \(\bullet\) & Start time to full flow @ 20 \({}^{\circ}\)C (s) & 5 & 5 & 5 \\ \(\bullet\) & Start time to full flow @ -20 \({}^{\circ}\)C (s) & 15 & 15 & 15 \\ \(\bullet\) & Transient response at operating & 0.75 & 0.75 & 0.75 \\ temperature 10\%–90\% and 90\%–0\% & & & & \\ (based on full flow rate) (s) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Major U.S. Department of Energy (DOE) Targets of Onboard Hydrogen Storage for Light Duty Vehicle. Porous organic polymers have great advantages over than other materials; for instance, highly porous polymers are more stable at the ambient conditions under which the metal ions in MOFs are sensitive the moisture. The polymeric structures can be well controlled via different organic synthetic routes and starting materials. Four groups of highly porous polymers have been studied intensively for hydrogen storage: (a) hypercrosslinked polymers (HCPs), (b) polymers of intrinsic microporosity (PIMs), (c) conjugated microporous polymers (CMPs), and (d) porous aromatic frameworks (PAFs). There are many excellent studies in this field that are making gradual progress towards the DOE hydrogen storage targets. [PERSON] et al. provide an overview on porous materials for hydrogen storage, including practical considerations of the technology [19]. This review is not a summary of all of these works, rather, it starts from basic theory for H\({}_{2}\) storage, and an overview of each storage medium, highlighting the most effective results, and then addresses the challenges and directions for developing highly porous polymers needed to meet DOE standards. The paper is arranged in the following structure: first, basic theoretical considerations for storing the maximum amount of H\({}_{2}\) at ambient conditions are introduced; then the different types of highly porous polymers are summarized in terms their structure, surface area, porosity characterization, and hydrogen storage ability. Finally, future directions for investigation and possible effective solutions are proposed.