(490f) A Computational Study on H2 Absorption in a Porous Framework Structure or: The Curse of the Exponential Function

In the context of hydrogen storage, we have proposed a covalent organic framework based on adamantane units that can be decorated with Sc atoms, which in return act as anchor sites for adsorbing H₂ molecules.[1] We will first briefly describe the idea behind the design of the framework. Afterwards, we will illustrate our simple thermodynamics model used to determine macroscopic observables like the H₂ coverage rate from adsorption energies computed at DFT-level. The model approximates all nuclear degrees of freedom of the framework and the adsorbed H₂ as harmonic vibrations. We will demonstrate the importance of the thermodynamic model for the actual evaluation of the performance of a H₂ storage material, as the amount of adsorbed hydrogen is highly dependent on the operating conditions.

In a second part, we will discuss the error of the calculated results. We will show that a small deviation in the Gibbs free energy of H₂ adsorption can lead to a large error in the hydrogen adsorption rate. The reason for this is the exponential dependence of the adsorption constant on the adsorption energy. In addition, we will also discuss potential error sources of the Gibbs free energy of adsorption, among others the treatment of (hindered) rotation or translation modes as harmonic vibrations.

References

1. A. Deshmukh, T.N.M. Le, C.-c. Chiu, J.-L. Kuo,. submitted.