Hydrogen Isotope (¹H/²H/³H) Separation in Solution Using Graphene Transferred on PTFE-Reinforced Nafion

The separation and enrichment of tritium from radioactive wastewater remain critical challenges in the nuclear power and fusion industries. While hydrogen isotope separation from H₂/D₂ gas mixtures has been demonstrated using electrochemical pumping systems with Nafion|graphene composite membranes, their application in liquid water remains limited, despite tritium and deuterium predominantly existing as water molecules. This limitation primarily arises from the swelling behavior of Nafion, which absorbs water and induces defects in the graphene layer. In this study, we propose a modified Nafion|graphene composite membrane structure, where graphene is transferred onto polytetrafluoroethylene-reinforced Nafion, to enable efficient hydrogen isotope (¹H, ²H, and ³H) separation in liquid water. The monolayer graphene exhibited six-fold higher conductivity for H⁺ over D⁺ under electrically driven conditions, while diffusion-driven transport showed lower isotope selectivity due to the contribution of the vehicular mechanism. Notably, graphene imposed a 1.7-fold higher diffusion energy barrier for tritium than for deuterium, indicating that the separation is governed by zero-point energy differences in O-H(D, T)···O bonding. These results highlight the role of transport mechanism and graphene integrity in isotope separation performance and suggest a practical pathway toward implementing graphene-based membranes in polymer electrolyte membrane water electrolysis systems for tritium concentration and removal.