Atomistic simulation of dilute hydrogen in water-saturated kaolinite nanopores: Implications for underground hydrogen storage

Abstract

Hydrogen is gaining traction as a viable option for sustainable energy solutions, with underground hydrogen storage (UHS) in saline aquifers presenting considerable promise for extensive storage. Nonetheless, hydrogen leakage through clay-rich cap-rocks poses a notable risk, which is predominantly determined by the hydrogen transport properties, especially its diffusion coefficient within clay minerals. This study addresses a critical gap in understanding the properties of dilute hydrogen within water-saturated kaolinite nanopores, a key sealing component in cap rocks, by investigating the effects of pore size, salinity level, ion type, and temperature on hydrogen diffusion and structural behavior. Molecular Dynamics (MD) simulations are employed to evaluate these variables under the thermodynamic conditions of the UHS. The findings reveal that the mobility of dilute hydrogen is significantly influenced by the degree of confinement, with smaller pore sizes resulting in denser hydrogen and water layers near the kaolinite surfaces, which further hinder hydrogen diffusion. Beyond the impact of confinement, salinity level and ion type play a significant role in influencing hydrogen properties, with divalent ions such as Mg²⁺ reducing hydrogen diffusion more significantly than monovalent ions due to their larger hydration shells and stronger electrostatic interactions. Elevated salinities further restrict hydrogen movement, highlighting the importance of ionic composition in determining diffusion behavior. Furthermore, the temperature enhanced hydrogen diffusion by increasing molecular mobility, while also weakening hydrogen-water interactions. This study contributes to a better understanding of hydrogen leakage mechanisms through cap-rocks, helping mitigate risks and optimize the design of storage systems.