Nanopore Confinement Alters Hydronium and Hydroxide Ion Diffusion Dynamics

Understanding the transport of hydronium (H₃O⁺) and hydroxide (OH⁻) ions under extreme confinement is crucial for a wide range of applications, from ion transport across cell membranes to proton transport in fuel cells. Our study employs molecular simulations accelerated by ab initio based machine learning potentials to investigate the behavior of these ions in sub-nanometer pores. Remarkably, we find that the relative diffusion coefficients of H₃O⁺ and OH⁻ are reversed in sub-nm pores compared to bulk solution. Moreover, OH⁻ ions exhibit significantly slower diffusion in pores with diameters ranging from 1 to 3 nm, while H₃O⁺ ions diffuse faster within these confined spaces relative to their behavior in bulk solution. These findings provide new insights into the fundamental mechanisms of ion transport under extreme confinement and have important implications for the design of advanced materials and devices in bioengineering and energy applications.