Molecular Density Fluctuations Control Solubility and Diffusion for Confined Aqueous Hydrogen.

Area of Science:

• Geochemistry
• Materials Science
• Energy Storage

Background:

• Underground hydrogen storage (UHS) is vital for a sustainable energy transformation, necessitating accurate modeling of hydrogen behavior in subsurface environments.
• Understanding hydrogen's thermodynamic and transport properties within geological formations is critical for designing efficient and safe UHS sites.

Purpose of the Study:

• To quantify the thermodynamic and transport properties of aqueous hydrogen (H2) confined within kaolinite slit pores using atomistic molecular dynamics (MD) simulations.
• To investigate the influence of pore geometry and mineral surface interactions on hydrogen solubility and diffusion.

Main Methods:

• Atomistic molecular dynamics (MD) simulations were employed to model aqueous H2 within slit-shaped kaolinite pores of 10 and 20 Å widths.
• Analysis focused on hydration layer formation, H2 distribution, solubility, and diffusion coefficients.

Main Results:

• Confined water forms distinct hydration layers, significantly increasing H2 solubility (up to ~25x bulk) near siloxane surfaces due to water density fluctuations.
• A dense hydration layer on the gibbsite surface largely excluded H2.
• Despite reduced water mobility, H2 diffusion increased with decreasing pore width, linked to water density fluctuations.

Conclusions:

• Confinement effects in kaolinite pores dramatically alter hydrogen's solubility and transport properties compared to bulk conditions.
• These findings provide crucial insights into H2 permeability relevant for the design and optimization of underground hydrogen storage facilities.