Hydrogen diffusion in water-saturated Illite: From molecular simulations to a simple model

Understanding hydrogen diffusion in water-saturated clays is crucial for evaluating the viability and safety of underground hydrogen storage (UHS). This study investigates hydrogen self-diffusion at infinite dilution in water-saturated Illite, a clay commonly found in caprocks, under two representative subsurface conditions: ( T = 298.15 K, P = 10 MPa) and ( T = 353.15 K, P = 30 MPa). Both stable interlayer spaces and nanopores with widths ranging from 6 Å to 100 Å were considered. Grand canonical Monte Carlo simulations were used to determine stable interlayer configurations and to estimate the water content in nanopores. Hydrogen diffusion coefficients were then calculated using both equilibrium (EMD) and non-equilibrium (NEMD) molecular dynamics simulations, leveraging TIP4P/2005, CLAYFF, and Marx force fields for water, Illite, and hydrogen, respectively. The unlike fluid–solid interactions were validated through excess chemical potential and Henry's law constant evaluations. Results reveal a strong dependence of hydrogen diffusivity on pore size, with values increasing from 0.10 × 10 −9 m 2 /s to 4.27 × 10 −9 m 2 /s at 298.15 K and from 0.24 × 10 −9 m 2 /s to 9.11 × 10 −9 m 2 /s at 353.15 K. NEMD proved particularly effective, yielding smaller uncertainties even with shorter sampling durations. Analysis of density profiles showed no significant hydrogen adsorption on Illite surfaces, in contrast to structured water adsorption layers. Based on these findings, a simple arithmetic-type mixing model based on surface diffusion was proposed, which successfully predicts hydrogen effective diffusion across all pore sizes without the need for fitting parameters. • Hydrogen diffusion in Illite modeled using MD under geological conditions. • TIP4P/2005 water model improves accuracy in hydrogen diffusivity simulations. • Diffusivity increases with pore size from interlayers to 10 nm-wide nanopores. • No significant hydrogen adsorption observed on Illite surfaces. • A simple model predicts effective hydrogen diffusion without fitting parameters.