Introduction

With the increase in population and the development in economy, human society has a growing demand for energy resources. Exploitation of fossil fuels has incurred various environmental issues including climate change, which propels people to seek new energy carriers to minimize carbon footprint. Hydrogen has been regarded as one promising alternative because it has the highest energy per mass of any fuel and emits zero carbon dioxide. Therefore, there is an urgent need for the investigation of hydrogen storage.

The boiling point and critical temperature of hydrogen are only 20 K and 38 K, making it notoriously difficult to liquefy or compress. Thus, at present, two exploratory techniques to store hydrogen involve the utilization of cryogenic liquid hydrogen tanks and high-pressure tanks, respectively. For the former method, large quantities of energy are consumed to liquefy hydrogen and keep the tanks cool. While for the latter, high-pressure techniques entail the use of extremely heavy apparatus to prevent hydrogen leakage, which severely reduces the “real gravimetric capacity” of the tanks. MOFs and COFs as adsorbent can overcome the above problems, because their large surface areas and tunable structures can provide high densities of relatively strong interaction sites to adsorb hydrogen.

Hydrogen Storage by Using MOFs

MOFs can be used in hydrogen storage in two directions: cryo-temperature and room temperature hydrogen storage.

• Cryo-temperature hydrogen storage:Cryo-temperature hydrogen storage refers to retaining hydrogen in a tank filled with MOFs at usually 77 K and relatively low pressure (less than 100 bar). Compared to the traditional method using cryogenic liquid hydrogen tanks (~20 K), much less energy is required to “liquefy” hydrogen and keep the tanks cool in this scenario due to the interaction between MOFs and hydrogen. Some examples used MOFs to store hydrogen are shown in below:

Table 1. Hydrogen storage of selected MOFs

Compound	Hydrogen uptake (wt%)	Temperature (K)	Pressure (bar)
MOF-5	10	77	100
MOF-177	7.5	77	70
IRMOF-6	4.63	77	45
Mn-BTT	6.9	77	90
Ni-MOF-74	2.95	77	10
SNU-6	10.0	77	70
PCN-10	6.84	30	3.5

• Room-temperature hydrogen storage: The ultimate goal for hydrogen storage is to compact hydrogen at room temperature and under reasonable pressure. The U.S. Department of Energy (DOE) set a series of targets to guide the development of hydrogen storage systems: 9.0 wt% and 81 g/L by 2015^[1]. Although no MOFs has met the DOE's targets to date, some methods have been explored to improve the density and strength of hydrogen interaction sites for high hydrogen uptakes at ambient temperature. For example, Kapelewski et al. evaluated the hydrogen storage capacities of Ni₂(m-dobdc) which has a high density of coordinatively unsaturated metal sites that can interact with hydrogen strongly at room temperature, providing a possibility way to achieve improved adsorbents for mobile hydrogen storage applications^[2].

Hydrogen Storage by Using COFs

COFs, with their low density and high surface area, show a potential for hydrogen capture in cryo-temperature with capacities of 0.3 7.2 wt % at 77 K, which are compatible to those of MOFs (the best one around 7.6 wt %). In 2012, Yaghi and his co-workers reported that a new COF-301-PdCl₂ reaches the hydrogen storage capacities of 60 g/L (100 bar)^[3]. Owing to a larger surface area and pore volume, the hydrogen uptake capacities of 3D COFs are predicted to be 2.5 to 3 times higher than those of 2D COFs. For example, 3D COF-102 with the BET surface area of 3620 m²/g and pore size of 1.2 nm exhibits the highest hydrogen uptake capacity of 72.4 mg/g (7.24 wt %) at 77 K and 25 bar (saturation adsorption). In contrast, 2D COF-10 (SBET=1760 m²/g, pore size=3.2 nm) exhibits a hydrogen adsorption capacity of 39.2 mg/g (3.92 wt %) under otherwise identical conditions^[4].

How Does They Work

MOFs and COFs have high surface areas which provide higher gas uptake place. The positions of surface cages and channels of MOFs and COFs provide Van der Waals interactions with hydrogen. The two point makes MOFs and COFs to store hydrogen very well and a simultaneous high gravimetric and volumetric hydrogen densities can be achieved in MOFs and COFs. What is noteworthy is that the open metal sites of MOFs can enhance the interactions between MOF and H₂, further improving the interactions between MOF and H₂.

What Can Alfa Chemistry Do

Alfa Chemistry provides high-quality MOFs and COFs for using hydrogen storage and our professional technology team that can also provide customers with specialized MOFs and COFs design and customization services in application of hydrogen storage. No matter what design ideas you have, we will implement them together with you. In addition, Alfa Chemistry is committed to supporting customers a series of solutions in hydrogen storage by using MOFs and COFs. Please contact us immediately to order or cooperate in research and development with high quality and reasonable price.