Hydrogen diffusion

The advantages of the CIG system to physisorb the guest molecule are based on the fast loading and unloading of the gas. Comparison with other systems is shown in Table 4.2.

1.0

The demonstrated better storage capacities for some of the materials rely on either intercalation of the gas molecules in metal frameworks or on chemisorption. However, the H2 diffusion in the metal frameworks is known to be rather slow, while the utilisation of chemically bound hydrogen is energy demanding process and need high operating temperatures.

The abundance of molecular hydrogen in sandwiched structures suggests the presence of a two-dimensional H2 gas (see 3.2). This should facilitate diffusion inside the storage material and hence define the loading and unloading capabilities of the system. Since the introduction of spacers suggests breaking of the delocalised lateral modes 3.3, it is of great importance to know if the hydrogen molecule will diffuse in materials such as CIG.

The calculations of the molecular hydrogen's diffusion coefficient has been done using Born$-$Oppenheimer molecular dynamics simulations (DC$-$DFTB using a NVE ensemble at 300K for 1 ns). The calculated H2 diffusion constant is $\sim$8.5x10$^3$ cm$^2$/s, while the values for C60 molecules (1.7x10$^4$ cm$^2$/s) are more than one order of magnitude lower. The value shows that hydrogen molecule can still easily penetrate the bulk of the material. Comparison with diffusion coefficients of other molecules like N2, O2, CO2, represents another advantages of materials such as CIG. Since CIG bulk consists of altered narrow channels and larger cavities the size of the channels is going to control the permeability for different molecules. The narrower is the channel $-$ the smaller must be the molecule to be able to pass through. It has been found that diffusion coefficient of molecular nitrogen (1.7x10$^4$ cm$^2$/s) is comparable to that of C60. In other words, nitrogen molecules are trapped in the space between the bulky balls and move together with the cavities. In the light of other findings [72] this effect is very important, since the presence of other molecules with higher physisorption energies (as N2) than hydrogen may substitute it completely from the bulk. As a consequence, the possibility of modifying the channel width would give an opportunity to create different types of molecular sieving materials based on the physisorption interaction.