Hydrogen Storage Using Metal-Organic Frameworks (MOFs): A Computational Fluid Dynamic Study
Authors: Pourrahmani Hossein , Mohammadi Mohammad Hadi , Pourhasani Bahar , Van herle Jan 
Published: September 2023

The existing obstacles toward the usage of fossil fuels as the prime mover of the cars have improved the commercialization of the fuel cells and batteries to replace the internal combustion engines (ICEs). Although the required infrastructure is already established for the ICE cars, the low number of hydrogen refueling stations, low range of batteries, high charging time of the batteries, and the size/weight of the hydrogen tanks are the main concerns toward the transition from ICE cars to environmentally friendly alternatives.
Fuel cells can be directly used in the vehicles as the prime mover (mobility applications), or they can be considered as the energy provider of the electric vehicle charging stations (stationary applications). Although the hydrogen storage is not being considered as an obstacle for the stationary applications, the required weight and size of the hydrogen tanks is a barrier to facilitate the usage of hydrogen in the automotive sector. Based on the given standards [1], it is possible to pressurize hydrogen up to 700 bars, hence reducing the size of the hydrogen tanks. This solution has been already used in the development of the Toyota Mirai, which has 114kW/155hp power and 500km range with the fuel consumption of 0.76 kg H2/100km [2]. Similarly, Honda Clarity could reach the range of 650km with 5kg of hydrogen tank capacity at the rated power of 130kW/ 176hp [3]. Although pressurizing the hydrogen is a feasible solution, it will demand further costs and safety procedures to reach the 700 bars. In other words, the best solution would be reaching the same driving range without pressurizing the hydrogen.
In this regard, Metal-Organic Framework (MOF) can be used to increase the hydrogen adsorption in the hydrogen tank due to higher gravimetric storage density. Among different types of MOFs, the MOF-5 has shown promising results to increase the hydrogen storage up to wt. 10% absolute at 70 bar and 77K. It is believed that the low thermal conductivity of the MOF-5 can reduce the performance of the system when rapid gas uptake and release is required [4]. Although there have been studies to evaluate the overall possibilities of using MOF-5 to improve the adsorption of hydrogen in the hydrogen tanks, there is not a comprehensive study to simulate and characterize the changes in the hydrogen adsorption once the hydrogen tanks are filled with different types of MOFs.
The goal of this study is to use the computational fluid dynamic methodologies to model a hydrogen tank filled with MOFs and to analyze the hydrogen adsorption by the changes in the time. This study can a step toward improving the design of the hydrogen tanks, which will facilitate the hydrogen storage at low pressures close to the ambient temperatures. This study can be also a good start to find the right type of MOF to be used in the hydrogen tanks to have the highest possible hydrogen adsorption. The developed model is based on mass, momentum, and energy conservation equations of the adsorbent-adsorbate system composed of gaseous and adsorbed hydrogen, adsorbent bed and tank wall. It is noteworthy to mention that the adsorption process is based on the modified Dubinin-Astakov (D-A) adsorption isotherm model.

Keywords
: Hydrogen Storage; Metal Organic Frameworks; Adsorption; Fuel cells.

Figure 1