Interfacial Super-Assembly of Vacancy Engineered Ultrathin-Nanosheets toward Nanochannels for Smart Ion Transport and Salinity Gradient Power Conversion

Ion-selective nanochannel membranes assembled from two-dimensional (2D) nanosheets hold immense promise for power conversion using salinity gradient. However, they face challenges stemming from insufficient surface charge density, which impairs both permselectivity and durability. Herein, we present a novel vacancy-engineered, oxygen-deficient NiCo layered double hydroxide (NiCoLDH)/cellulose nanofibers-wrapped carbon nanotubes (VOLDH/CNF-CNT) composite membrane. This membrane, featuring abundant angstrom-scale, cation-selective nanochannels, is designed and fabricated through a synergistic combination of vacancy engineering and interfacial super-assembly. The membrane shows interlayer free-spacing of ~3.62 Å, which validates the membrane size exclusion selectivity.This strategy, validated by DFT calculations and experimental data, improves hydrophilicity and surface charge density, leading to the strong interaction with K+ ions to benefit the low ion transport resistance and exceptional charge selectivity. When employed in an artificial river water|seawater salinity gradient power generator, it delivers a high-power density of 5.35 W/m2 with long-term durability (20,000s), which is almost 400% higher than that of the pristine NiCoLDH membrane. Furthermore, it displays both pH- and temperature-sensitive ion transport behavior, offering additional opportunities for optimization. This work establishes a basis for high-performance salinity gradient power conversion and underscores the potential of vacancy engineering and super-assembly in customizing 2D nanomaterials for diverse advanced nanofluidic energy devices.

Interfacial Super-Assembly of Intertwined Nanofibers toward Hybrid Nanochannels for Synergistic Salinity Gradient Power Conversion

Capturing the abundant salinity gradient power into electric power by nanofluidic systems has attracted increasing attention and has shown huge potential to alleviate the energy crisis and environmental pollution problems. However, not only the imbalance between permeability and selectivity but also the poor stability and high cost of traditional membranes limit their scale-up realistic applications. Here, intertwined "soft-hard" nanofibers/tubes are densely super-assembled on the surface of anodic aluminum oxide (AAO) to construct a heterogeneous nanochannel membrane, which exhibits smart ion transport and improved salinity gradient power conversion. In this process, one-dimensional (1D) "soft" TEMPO-oxidized cellulose nanofibers (CNFs) are wrapped around "hard" carbon nanotubes (CNTs) to form three-dimensional (3D) dense nanochannel networks, subsequently forming a CNF-CNT/AAO hybrid membrane. The 3D nanochannel networks constructed by this intertwined "soft-hard" nanofiber/tube method can significantly enhance the membrane stability while maintaining the ion selectivity and permeability. Furthermore, benefiting from the asymmetric structure and charge polarity, the hybrid nanofluidic membrane displays a low membrane inner resistance, directional ionic rectification characteristics, outstanding cation selectivity, and excellent salinity gradient power conversion performance with an output power density of 3.3 W/m². Besides, a pH sensitive property of the hybrid membrane is exhibited, and a higher power density of 4.2 W/m² can be achieved at a pH of 11, which is approximately 2 times more compared to that of pure 1D nanomaterial based homogeneous membranes. These results indicate that this interfacial super-assembly strategy can provide a way for large-scale production of nanofluidic devices for various fields including salinity gradient energy harvesting.

Interfacial Super-Assembly of T-Mode Janus Porous Heterochannels from Layered Graphene and Aluminum Oxide Array for Smart Oriented Ion Transportation

Salinity gradient energy existing in seawater and river water is a sustainable and environmentally energy resource that has drawn significant attention of researchers in the background of energy crisis. Nanochannel membrane with a unique nano-confinement effect has been widely applied to harvest the salinity gradient energy. Here, Janus porous heterochannels constructed from 2D graphene oxide modified with polyamide (PA-GO) and oxide array (anodic aluminum oxide, AAO) are prepared through an interfacial super-assembly method, which can achieve oriented ion transportation. Compared with traditional nanochannels, the PA-GO/AAO heterochannels with asymmetric charge distribution and T-mode geometrical nanochannel structure shows directional ionic rectification features and outstanding cation selectivity. The resulting heterochannel membrane can achieve a high-power density of up to 3.73 W m^-2 between artificial seawater and river water. Furthermore, high energy conversion efficiency of 30.3% even in high salinity gradient can be obtained. These achievable results indicate that the PA-GO/AAO heterochannels has significant potential application in salinity gradient energy harvesting.

Interfacial Super-Assembly of Ordered Mesoporous Silica-Alumina Heterostructure Membranes with pH-Sensitive Properties for Osmotic Energy Harvesting

Osmotic energy existing between seawater and freshwater is a potential blue energy source that can mitigate the energy crisis and environmental pollution problems. Nanofluidic devices are widely utilized to capture this blue energy owing to their unique ionic transport properties in the nanometer scale. However, with respect to nanofluidic membrane devices, high membrane inner resistance and a low power density induced by disordered pores and thick coating as well as difficulty in manufacturing still impede their real-world applications. Here, we demonstrate an interfacial super-assembly strategy that is capable of fabricating ordered mesoporous silica/macroporous alumina (MS/AAO) framework-based nanofluidic heterostructure membranes with a thin and ordered mesoporous silica layer. The presence of a mesoporous silica layer with abundant silanol and a high specific surface area endows the heterostructure membrane with a low membrane inner resistance of about 7 KΩ, excellent ion selectivity, and osmotic energy conversion ability. The power density can reach up to 4.50 W/m² by mixing artificial seawater and river water through the membrane, which is 20 times higher than that of the conventional 2D nanofluidic membrane, and outperforms about 30% compared to other 3D porous membranes. More intriguingly, the interesting pH-sensitive osmotic energy conversion property of the MS/AAO membrane is subsequently recognized, which can realize a higher power density even in acidic or alkaline wastewater, expanding the application range, especially in practical applications. This work presents a valuable paradigm for the use of mesoporous materials in nanofluidic devices and provides a way for large-scale production of nanofluidic devices.