Electronic-Level Insight into Interfacial Effects and Their Induced Anisotropic Ion Diffusion and Ion Selectivity in Nanochannels

Osmotic energy conversion features directional ion migration in selective nanochannels, dominated by interfacial effects, temperature, and concentration. Current efforts emphasize membrane modification for superior reliability and durability, whereas the origin and implication of interfacial effects are unclear. This work performs ab initio molecular dynamics simulations for hydrated ion-graphene oxide interfaces by regulating the temperature and concentration. The interfacial effects associated with their induced anisotropic ion diffusion and ion selectivity are revealed. The scientific essence of the interfacial effects is an electron transfer triggered by hydrated ion-functional group interactions. The interfacial effects are clarified to include dynamic solvation structures, interfacial H-bonds, and chemical reactions. Ions possess incomplete hydration shells, and their arrangements vary from ordered to disordered to overlapped. Interfacial H-bonds restrict hydrated ions by constraining water molecules, whereas continuous reactions provide lateral pathways to generate anisotropy. Cation selectivity is further clarified by negative surface charges from hydroxyl deprotonation. Besides, temperature rise induces disordered hydrated ions as well as frequent and violent reactions, enhancing ion diffusion, selectivity, and anisotropy; excessive concentrations produce overlapped hydrated ions, more H-bonds, and inferior reactions, weakening ion diffusion, selectivity, and anisotropy. Finally, the bottom-up concept for osmotic energy conversion is summarized, and elevated temperature combined with low concentration is found to boost ion diffusion and ion selectivity synergistically. This work provides an in-depth understanding of interfacial phenomena and ion behaviors in nanochannels.