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.

Nanochannel‐based ion transport is an important field of study in various disciplines, including physics, chemistry, energy, materials science, biology, and earth science. The unique features of natural nanochannels have inspired numerous innovative designs that seek to achieve high ion permeability, selectivity, and rectification. A notable example is osmotic energy conversion, which harvests sustainable salinity‐gradient energy to generate electricity without moving parts, noise, or carbon emissions and thus serves as a promising alternative to traditional fossil fuel utilization. This review focuses on the fundamental principles, regulatory methods, and practical applications of nanochannel‐based ion transport for osmotic energy conversion. The physical mechanisms of ion transport and the intriguing phenomena of ion behaviors in nanoconfined spaces are discussed first, followed by a thorough examination of the overall process of osmotic power generation from mathematical, numerical, parametric, first‐principles, molecular simulation, and modeling perspectives. Strategies for enhancing the osmotic performance are then discussed to overcome the trade‐off between ion selectivity and ion flux, including the theoretical design of nanochannel geometry and electrification, experimental optimization of nanoporous membranes, and electrolyte thermal enhancement. The existing challenges and opportunities for the future development of nanochannel‐based ion transport and osmotic power generation are addressed at the end.