Thermodynamic Properties of Hydrogen Adsorbed on Graphite Surfaces at Temperatures Above 100 K: A Molecular Dynamics and Classical Density Functional Theory Study

Improved technological solutions for the transport and storage of hydrogen are crucial for the widespread adoption of hydrogen as a clean energy carrier. Graphite-based materials have been identified as potential candidates due to their high surface area and ability to adsorb hydrogen molecules. In this study, we investigate the adsorption and thermodynamic properties of hydrogen adsorbed on a graphite surface using molecular dynamics (MD) simulation and classical density functional theory (cDFT). We demonstrate how to use the MD parameters for graphite to derive an effective wall potential for hydrogen-graphite interactions that can be used in the cDFT calculations. The methodology results in good agreement between cDFT and MD, with the enthalpy and entropy of adsorption differing by 3.5% and 7%, respectively. We determine the enthalpy and entropy of adsorption at 298K to be in the ranges of -6.37 kJ mol^-1 to -6.16 kJ mol^-1 and -75.42 J mol^-1 K^-1 to -79.95 J mol^-1 K^-1, respectively. We find that the adsorbed hydrogen has a 12.4 J mol^-1 K^-1 to 11.4 J mol^-1 K^-1 lower heat capacity than the bulk hydrogen in the temperature range from 150 K to 400 K. This suggests that the adsorbed molecules are bound to adsorption sites that arrest nearly all the translational degrees of freedom.