Interfacial binding rope theory of ion transport in sub-nanochannels and its application for osmotic energy conversion

Osmotic energy conversion is a promising method of sustainable electricity generation using seawater and river water. Artificial pure NaCl solutions are commonly used, whereas the effects of multivalent-ion ingredients present in natural seawater on osmotic power generation remain unclear. In this study, the impacts of coexisting ions in multicomponent seawater (MS) are experimentally assessed using graphene oxide membrane (GOM). Divalent cation in MS shows inferior transport to monovalent cation, thereby presenting a lower power density than when using pure NaCl solutions. Inspired by experimental results, an interfacial binding rope theory is proposed to clarify ion-wall interactions and cation transport in sub-nanochannels with hydrophilic functional groups. The binding rope networks include direct cation-wall connections mediated by composition-changed hydration shells, indirect connections by H-bonds linking solvated water molecules and functional groups, and electrostatic attractions between cations and negatively charged surfaces. The theory feasibility is confirmed by first-principles calculations focusing on the charge distribution, binding site and length, surface electrification, and cation diffusivity and selectivity in GOM sub-nanochannels. Divalent cation diffuses slower than monovalent cation in both pure solutions and confined channels. For the mixed solution in channels, both monovalent and divalent cations show worse diffusivities due to additional monovalent-divalent electrostatic repulsions, as well as inferior cation selectivity due to weakened deprotonation reactions. Bridged by environment-controlled interfacial potential distribution, ion diffusivity, and ion selectivity, this theory guides the improvement of MS-based osmotic performance under regulations of concentration gradient, pH, and temperature. A power density of 8.52 W m–2 and stable output within 27 days are achieved. The interfacial binding rope theory could be popularized to refine the transport mechanisms of various cations in diverse hydrophilic sub-nanochannel materials, promoting the application of osmotic power generation and other nanoscale technologies.