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

Abstract

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 and Its Application in Osmotic Energy Conversion: A Critical Review

“…In contrast, rough interfaces, such as those between graphene oxide and an aqueous solution (Figure 5e), exhibit clear contours of the electron density difference. [ 16 ] This electron transfer can be observed on the aqueous surfaces of functionalized MXene, metal–organic framework (MOF), and COF.…”

“…As a quantitative presentation, Figure a shows the Na + ion self‐diffusion coefficient (diffusivity) on the graphene oxide surface under the temperature and concentration regulations. [ 16 ] The interface features a rough plate‐like configuration, where the molecular‐scale roughness is a result of the extended surface functional groups. All diffusivities are lower than those in bulk dilute solutions, indicating the restriction imposed by the solid–liquid interface.…”

“…a) Reproduced with permission. [ 16 ] Copyright 2022, American Chemical Society. b) Reproduced with permission.…”

“…In particular, the concentration gradient can drive the ionic Fick diffusion, and the pressure gradient caused by the concentration gradient can generate a diffusio-osmosis of the overall fluid. These transport modes play crucial roles in nanochannel-based applications, such as [16] Copyright 2022, American Chemical Society. b) Reproduced with permission.…”

“…[ 15 ] In this case, ion transport laws in the bulk solution are no longer applicable, because of interfacial phenomena such as solvation structure rearrangement, interfacial H‐bonds, and chemical reactions. [ 16 ] When the channel size exceeds that of typical inorganic ions, exit‐entry effects and steric exclusion occur. As the channel size decreases to the sub‐nanometer range and approaches the size of the ions and water molecules, the ion size effect is amplified, leading to energy‐consuming hydration shell deformation.…”