Molecular diffusion of hydrogen in porous media: A comprehensive review with insights for underground storage

Hydrogen (H₂) is a key enabler of energy transition and decarbonization across sectors, but large-scale deployment requires efficient and secure storage solutions. Underground hydrogen storage (UHS) in geological formations is promising due to its scalability and cost-effectiveness. However, H₂'s high diffusivity raises concerns about containment and loss under subsurface conditions. This study provides a diffusion-focused synthesis of H₂ molecular diffusion in porous media relevant to UHS. The fundamentals of molecular diffusion are outlined, and key factors influencing diffusion—including pressure, temperature, confinement, fluid saturation and salinity, tortuosity, and pore-surface interactions—are critically discussed. Experimental and computational methods for evaluating H₂ diffusivity are also reviewed. The review highlights diffusion-driven risks, including H₂ leakage through sealing units, mixing with formation fluids, and impacts on long-term containment, while identifying key knowledge gaps to guide future research. Diffusion-informed site-screening criteria are further summarized, showing that H₂ diffusion is most effectively suppressed in tight, clay-rich, water-saturated caprocks characterized by effective pore sizes of ≤ 2–5 nm, high tortuosity, and ultralow permeability, whereas wider or drier pores and elevated temperatures tend to enhance H₂ diffusivity and leakage risk. These insights support improved understanding of diffusion behavior, guide site selection, and inform the safe and efficient design of UHS systems.