Hydrogen energy has a great potential to become one of the clean energies of the future. The current use of hydrogen gas as an energy source still has problems, namely in the distribution and storage system. One solution to overcome these problems is to use the adsorption method. Zeolite material is considered to be a good material to be used as a storage medium for hydrogen gas. Experimental research generally still requires a fairly high cost. Therefore, we need another method that can support it. In this research, the author used the Molecular Dynamics Simulation method. The variation of temperature used in this simulation is 233, 253, 273 and 293 K with variation of pressure at each temperature is 1, 2, 4, 6, 8, and 10 bar. The results of this simulation successfully visualize the phenomenon of hydrogen adsorption on ZSM-5 zeolites and indicate that the material has good capabilities in adsorbing hydrogen. The best results of hydrogen storage in ZSM-5 zeolites are 1.9712% at a temperature of 77 K and a pressure of 10 bar.
Hydrogen is an environmentally friendly alternative energy source (Dutta, 2014). One of the most promising uses of hydrogen is as a fuel cell for applications such as electricity generation and transportation. Although the use of hydrogen energy has many advantages, the use of hydrogen energy still poses several problems, including safety and lack of infrastructures such as production, distribution, fuel filling, and hydrogen storage (Song, 2012). One of the biggest obstacles in realizing the commercialization of hydrogen energy at this time is not the production or utilization of hydrogen but is a more effective and safer way to store hydrogen (Thomas, 2007).
There are several methods of storing hydrogen gas that is known at this time, namely liquefaction of hydrogen, pressurized hydrogen, metal hydride and adsorption on porous material (Durbin, 2013). From these various methods, the adsorption method on porous material is a better method than other methods. That is because the adsorption on porous material is safer because of the lower pressure used, simple design of the storage system, and lower operational costs (F. Zhang, 2016).
Research on hydrogen gas adsorption on porous materials has been developed using zeolite and carbon materials (Darkim, 2002 & Nijkamp, 2011). Material used as a hydrogen storage must have a high surface area, high porosity, and strong interactions to obtain a relatively high hydrogen storage capacity (Froudakis, 2011). Zeolite is a material with aluminosilicate crystals that is good to be used as a hydrogen storage material because of good thermal and chemical stability, straight structure, large pore volume, and high porosity but zeolite has a relatively small surface area, so the hydrogen adsorption capacity produced is relatively low (Broom, 2011). Zeolite has a strong electrostatic force on its structure originating from cation exchange so that it can adsorb (Hirscher, 2009).
Zeolites can store hydrogen gas between 0.08-4.5% by weight under varying temperature and pressure conditions (Chung, 2010).
To conduct experimental research on gas adsorption on porous materials has constraints on the equipment and materials to be used. Besides, safety and cost issues are also considered in conducting experimental research. Conducting research by making modeling using molecular dynamics simulation methods is one of the solutions to the problem.
Molecular dynamics simulation is a method that can be used to see the movements of interacting molecules. Things that influence the movement of these molecules is the potential formed by the forces between particles. There are several potentials used in this molecular dynamics method, namely the potential of Lennard Jones, Morse, Born-Mayer, and Buckingham. The potential used in this simulation is the Lennard Jones potential. The advantage possessed by this potential is its deterministic characteristic. By using this potential, the potential cutting feature can be used which will limit the area to be calculated in a system by limiting the distances between atoms so that the atoms that are far away can be ignored. By ignoring distant atoms, the simulation process becomes faster.
Materials and Methods
The software used in running this simulation is LAMMPS. First, we form the atomic structure of ZSM-5 Zeolite and hydrogen. The ZSM-5 zeolite material atomic coordinate database was obtained from http://www.iza-structure.org/. Hydrogen is placed near the surface of the material. Each atom is given a different color so that it is easy to identify its type. The simulation is based on Lennard Jones potential to define interactions between atoms. Equation 1 is the Lennard Jones equation.
Results and Discussions
This simulation was conducted under conditions of temperature 77, 100, 150, 200, and 298 K with variations in pressure of 1, 2, 4, 6, 8, and 10 bar which are fixed at each temperature. The number of hydrogen molecules is determined at initial conditions to make the pressure constant at each temperature by using equation 4. The number of hydrogen molecules at each temperature and pressure can be seen in table 2. In this simulation, hydrogen adsorption is calculated every 10,000 time step with the number of running times is 800,000.
The initial conditions and final conditions of the phenomenon of hydrogen adsorption at 77 K temperature and 10 bar pressure. The red atom located above is a hydrogen atom. Blue and yellow atoms are the atoms of ZSM-5 zeolite material where the blue atom is silicon atom and the yellow atom is oxygen atom. The initial condition is a condition where no hydrogen has been adsorbed yet. The final condition is a condition after reaching the equilibrium state where no more hydrogen atoms will be adsorbed.
Based on the simulation results, it can be concluded that this molecular dynamics simulation has successfully visualized the mechanism of hydrogen adsorption on ZSM-5 zeolite. The amount of hydrogen adsorbed on the material is affected by temperature and pressure. The simulation results show that the higher the temperature, the lower the amount of hydrogen adsorbed and the higher the pressure, the higher the amount of hydrogen adsorbed. The concentration of hydrogen adsorbed on the material has a higher value at low temperatures such as 77 K than at higher temperatures (100, 150, 200, 173, and 298 K). The optimum operating conditions on the adsorption of hydrogen on ZSM-5 zeolite through this simulation are at a temperature of 77 K and a pressure of 10 bar with the amount of hydrogen adsorbed is 1.9712 wt%. Based on these results, it can be concluded that the ZSM-5 zeolite can be a promising candidate for use as a hydrogen storage media.
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