Cation-selective two-dimensional polyimine membranes for high-performance osmotic energy conversion

Scalable Fabrication of Ionic-Conductive Covalent Organic Framework Fibers for Capturing of Sustainable Osmotic Energy

High-performance covalent organic framework (COF) fibers are demanded for an efficient capturing of blue osmotic power because of their excellent durability, simple integration, and large scalability. However, the scalable production of COF fibers is still very challenging due to the poor solubility and fragile structure of COFs. Herein, for the first time, it is reported that COF dispersions can be continuously processed into macroscopic, meter-long, and pure COF fibers using a wet spinning approach. The two presented COF fibers can be directly used for capturing of osmotic energy, avoiding the production of composite materials that require other additives and face challenges such as phase separation and environmental issues induced by the additives. A COF fiber exhibits power densities of 70.2 and 185.3 W m-2 at 50-fold and 500-fold salt gradients, respectively. These values outperform those of most reported systems, which indicate the high potential of COF fibers for capturing of blue osmotic energy.

Ion-Fluid Transport-Control Feedback along Nanopore Networks

Biological signaling correlates with the interrelation between ion and nanofluidic transportation pathways. However, artificial embodies with reconfigurable ion-fluid transport interaction aspects remain largely elusive. Herein, we unveiled an intimate interplay between nanopore-driven advancing flow and ion carriage for the spontaneous imbibition of aqueous solutions at the nanoporous thin film level. Ionic factors dominate transport phenomena processing and integration (ions influence fluid motion, which in turn governs the self-regulated ion traveling). We show an ion-induced translation effect that finely converts a chemical input, the nature of ions, into a related fluidic output: modulation of the extent of imbibition. We further find complex imbibition dynamics induced by the ion type and population. We peculiarly pinpoint a stop-and-go effective transport process with a programmable delay time triggered by selective guest-host interactions. The ion-fluid transport interplay is captured by a simple model that considers the counterbalance between the capillary infiltration and solution concentration, owing to water loss at the nanoporous film-air interface. Our results demonstrate that nanopore networks present fresh scenarios for understanding and controlling autonomous macroscopic liquid locomotion and offer a distinctive working principle for smart ion operation.

Nanoarchitectonics in Advanced Membranes for Enhanced Osmotic Energy Harvesting

Osmotic energy, often referred to as "blue energy", is the energy generated from the mixing of solutions with different salt concentrations, offering a vast, renewable, and environmentally friendly energy resource. The efficacy of osmotic power production considerably relies on the performance of the transmembrane process, which depends on ionic conductivity and the capability to differentiate between positive and negative ions. Recent advancements have led to the development of membrane materials featuring precisely tailored ion transport nanochannels, enabling high-efficiency osmotic energy harvesting. In this review, ion diffusion in confined nanochannels and the rational design and optimization of membrane architecture are explored. Furthermore, structural optimization of the membrane to mitigate transport resistance and the concentration polarization effect for enhancing osmotic energy harvesting is highlighted. Finally, an outlook on the challenges that lie ahead is provided, and the potential applications of osmotic energy conversion are outlined. This review offers a comprehensive viewpoint on the evolving prospects of osmotic energy conversion.

TRPM4-Inspired Polymeric Nanochannels with Preferential Cation Transport for High-Efficiency Salinity-Gradient Energy Conversion

Biological ion channels exhibit switchable cation transport with ultrahigh selectivity for efficient energy conversion, such as Ca2+-activated TRPM4 channels tuned by cation-π interactions, but achieving an analogous highly selective function is challenging in artificial nanochannels. Here, we design a TRPM4-inspired cation-selective nanochannel (CN) assembled by two poly(ether sulfone)s, respectively, with sulfonate acid and indole moieties, which act as cation-selective activators to manage Na+/Cl- selectivity via ionic and cation-π interactions. The cation selectivity of CNs can be activated by Na+, and thereby the Na+ transference number significantly improves from 0.720 to 0.982 (Na+/Cl- selectivity ratio from 2.6 to 54.6) under a 50-fold salinity gradient, surpassing the K+ transference number (0.886) and Li+ transference number (0.900). The TRPM4-inspired nanochannel membrane enabled a maximum output power density of 5.7 W m-2 for salinity-gradient power harvesting. Moreover, a record energy conversion efficiency of up to 46.5% is provided, superior to most nanochannel membranes (below 30%). This work proposes a novel strategy to biomimetic nanochannels for highly selective cation transport and high-efficiency salinity-gradient energy conversion.

Enhancing Sustainable Energy Conversion Efficiency by Incorporating Photoelectric Responsiveness into Multiporous Ionic Membrane

The evolution of porous membranes has revitalized their potential application in sustainable osmotic-energy conversion. However, the performance of multiporous membranes deviates significantly from the linear extrapolation of single-pore membranes, primarily due to the occurrence of ion-concentration polarization (ICP). This study proposes a robust strategy to overcome this challenge by incorporating photoelectric responsiveness into permselective membranes. By introducing light-induced electric fields within the membrane, the transport of ions is accelerated, leading to a reduction in the diffusion boundary layer and effectively mitigating the detrimental effects of ICP. The developed photoelectric-responsive covalent-organic-framework membranes exhibit an impressive output power density of 69.6 W m-2 under illumination, surpassing the commercial viability threshold by ≈14-fold. This research uncovers a previously unexplored benefit of integrating optical electric conversion with reverse electrodialysis, thereby enhancing energy conversion efficiency.

Polyoxometalate-based plasmonic electron sponge membrane for nanofluidic osmotic energy conversion

Nanofluidic membranes have demonstrated great potential in harvesting osmotic energy. However, the output power densities are usually hampered by insufficient membrane permselectivity. Herein, we design a polyoxometalates (POMs)-based nanofluidic plasmonic electron sponge membrane (PESM) for highly efficient osmotic energy conversion. Under light irradiation, hot electrons are generated on Au NPs surface and then transferred and stored in POMs electron sponges, while hot holes are consumed by water. The stored hot electrons in POMs increase the charge density and hydrophilicity of PESM, resulting in significantly improved permselectivity for high-performance osmotic energy conversion. In addition, the unique ionic current rectification (ICR) property of the prepared nanofluidic PESM inhibits ion concentration polarization effectively, which could further improve its permselectivity. Under light with 500-fold NaCl gradient, the maximum output power density of the prepared PESM reaches 70.4 W m-2, which is further enhanced even to 102.1 W m-2 by changing the ligand to P5W30. This work highlights the crucial roles of plasmonic electron sponge for tailoring the surface charge, modulating ion transport dynamics, and improving the performance of nanofluidic osmotic energy conversion.

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 composite 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.

Phase Engineering of Zirconium MOFs Enables Efficient Osmotic Energy Conversion: Structural Evolution Unveiled by Direct Imaging

Creating structural defects in a controlled manner within metal-organic frameworks (MOFs) poses a significant challenge for synthesis, and concurrently, identifying the types and distributions of these defects is also a formidable task for characterization. In this study, we demonstrate that by employing 2-sulfonylterephthalic acid as the ligand for synthesizing Zr (or Hf)-based MOFs, a crystal phase transformation from the common fcu topology to the rare jmt topology can be easily facilitated using a straightforward mixed-solvent strategy. The jmt phase, characterized by an extensively open framework, can be considered a derivative of the fcu phase, generated through the introduction of missing-cluster defects. We have explicitly identified both MOF phases, their intermediate states, and the novel core-shell structures they form using ultralow-dose high-resolution transmission electron microscopy. In addition to facilitating phase engineering, the incorporation of sulfonic groups in MOFs imparts ionic selectivity, making them applicable for osmotic energy harvesting through mixed matrix membrane fabrication. The membrane containing the jmt-phase MOF exhibits an exceptionally high peak power density of 10.08 W m-2 under a 50-fold salinity gradient (NaCl: 0.5 M|0.01 M), which surpasses the threshold of 5 W m-2 for commercial applications and can be attributed to the combination of large pore size, extensive porosity, and abundant sulfonic groups in this novel MOF material.

Giant Blue Energy Harvesting in Two-Dimensional Polymer Membranes with Spatially Aligned Charges

Blue energy between seawater and river water is attracting increasing interest, as one of the sustainable and renewable energy resources that can be harvested from water. Within the reverse electrodialysis applied in blue energy conversion, novel membranes with nanoscale confinement that function as selective ion transport mediums are currently in high demand for realizing higher power density. The primary challenge lies in constructing well-defined nanochannels that allow for low-energy barrier transport. This work proposes a concept for nanofluidic channels with a simultaneous dual electrostatic effect that can enhance both ion selectivity and flux. To actualize this, this work has synthesized propidium iodide-based two-dimensional polymer (PI-2DP) membranes possessing both skeleton charge and intrinsic space charge, which are spatially aligned along the ion transport pathway. The dual charge design of PI-2DP significantly enhances the electrostatic interaction between the translocating anions and the cationic polymer framework, and a high anion selectivity coefficient (≈0.8) is reached. When mixing standard artificial seawater and river water, this work achieves a considerable power density of 48.4 W m-2, outperforming most state-of-the-art nanofluidic membranes. Moreover, when applied between the Mediterranean Sea and the Elbe River, an output power density of 42.2 W m-2 is achieved by the PI-2DP. This nanofluidic membrane design with dual-layer charges will inspire more innovative development of ion-selective channels for blue energy conversion that will contribute to global energy consumption.

All-natural 2D nanofluidics as highly-efficient osmotic energy generators

Two-dimensional nanofluidics based on naturally abundant clay are good candidates for harvesting osmotic energy between the sea and river from the perspective of commercialization and environmental sustainability. However, clay-based nanofluidics outputting long-term considerable osmotic power remains extremely challenging to achieve due to the lack of surface charge and mechanical strength. Here, a two-dimensional all-natural nanofluidic (2D-NNF) is developed as a robust and highly efficient osmotic energy generator based on an interlocking configuration of stacked montmorillonite nanosheets (from natural clay) and their intercalated cellulose nanofibers (from natural wood). The generated nano-confined interlamellar channels with abundant surface and space negative charges facilitate selective and fast hopping transport of cations in the 2D-NNF. This contributes to an osmotic power output of ~8.61 W m-2 by mixing artificial seawater and river water, higher than other reported state-of-the-art 2D nanofluidics. According to detailed life cycle assessments (LCA), the 2D-NNF demonstrates great advantages in resource consumption (1/14), greenhouse gas emissions (1/9), and production costs (1/13) compared with the mainstream 2D nanofluidics, promising good sustainability for large-scale and highly-efficient osmotic power generation.

Addressing Challenges in Ion-Selectivity Characterization in Nanopores

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors. Quantitative characterization of ion selectivities in nanopores often employs the Nernst or Goldman-Hodgkin-Katz (GHK) equation to interpret transmembrane potentials. However, the direction of the measured transmembrane potential drop is not specified in these equations, and selectivity values calculated using absolute values of transmembrane potentials do not directly reveal the ion for which the membrane is selective. Moreover, researchers arbitrarily choose whether to use the Nernst or GHK equation and overlook the significant differences between them, leading to ineffective quantitative comparisons between studies. This work addresses these challenges through (a) specifying the transmembrane potential (sign) and salt concentrations in terms of working and reference electrodes and the solutions in which they reside when using the Nernst and GHK equations, (b) reporting of both Nernst-selectivity and GHK-selectivity along with solution compositions and transmembrane potentials when comparing different nanopores/channels, and (c) performing simulations to define an ideal selectivity for nanochannels. Experimental and modeling studies provide significant insight into these fundamental equations and guidelines for the development of nanopore/channel-based devices.

Single-Layer Hexagonal Boron Nitride Nanopores as High-Performance Ionic Gradient Power Generators

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for efficient energy harvesting from ionic gradients. However, the exploration of robust 2D atomically thin nanopore membranes, which hold sufficient ionic selectivity and high ion permeability, remains challenging. Here, the single-layer hexagonal boron nitride (hBN) nanopores are demonstrated as various high-performance ion-gradient nanopower harvesters. Benefiting from the ultrathin atomic thickness and large surface charge (also a large Dukhin number), the hBN nanopore can realize fast proton transport while maintaining excellent cation selectivity even in highly acidic environments. Therefore, a single hBN nanopore achieves the pure osmosis-driven proton-gradient power up to ≈3 nW under 1000-fold ionic gradient. In addition, the robustness of hBN membranes in extreme pH conditions allows the ionic gradient power generation from acid-base neutralization. Utilizing 1 m HCl/KOH, the generated power can be promoted to an extraordinarily high level of ≈4.5 nW, over one magnitude higher than all existing ionic gradient power generators. The synergistic effects of ultrathin thickness, large surface charge, and excellent chemical inertness of 2D single-layer hBN render it a promising membrane candidate for harvesting ionic gradient powers, even under extreme pH conditions.

2D Graphene Oxide Membrane Nanoreactors for Rapid Directional Flow Ring-Opening Reactions with Dominant Same-Configuration Products

Nanoconfinement within enzymes can increase reaction rate and improve selectivity under mild conditions. However, it remains a great challenge to achieve chemical reactions imitating enzymes with directional molecular motion, short reaction time, ≈100% conversion, and chiral conversion in artificial nanoconfined systems. Here, directional flow ring-opening reactions of styrene oxide and alcohols are demonstrated with ≈100% conversion in <120 s at 22 °C using graphene oxide membrane nanoreactors. Dominant products have the same configuration as chiral styrene oxide in confined reactions, which is dramatically opposed to bulk reactions. The unique chiral conversion mechanism is caused by spatial confinement, limiting the inversion of benzylic chiral carbon. Moreover, the enantiomeric excess of same-configuration products increased with higher alkyl charge in confined reactions. This work provides a new route to achieve rapid flow ring-opening reactions with specific chiral conversion within 2D nanoconfined channels, and insights into the impact of nanoconfinement on ring-opening reaction mechanisms.

Proton-selective coating enables fast-kinetics high-mass-loading cathodes for sustainable zinc batteries

The pressing demand for sustainable energy storage solutions has spurred the burgeoning development of aqueous zinc batteries. However, kinetics-sluggish Zn2+ as the dominant charge carriers in cathodes leads to suboptimal charge-storage capacity and durability of aqueous zinc batteries. Here, we discover that an ultrathin two-dimensional polyimine membrane, featured by dual ion-transport nanochannels and rich proton-conduction groups, facilitates rapid and selective proton passing. Subsequently, a distinctive electrochemistry transition shifting from sluggish Zn2+-dominated to fast-kinetics H+-dominated Faradic reactions is achieved for high-mass-loading cathodes by using the polyimine membrane as an interfacial coating. Notably, the NaV3O8·1.5H2O cathode (10 mg cm-2) with this interfacial coating exhibits an ultrahigh areal capacity of 4.5 mAh cm-2 and a state-of-the-art energy density of 33.8 Wh m-2, along with apparently enhanced cycling stability. Additionally, we showcase the applicability of the interfacial proton-selective coating to different cathodes and aqueous electrolytes, validating its universality for developing reliable aqueous batteries.

Bioinspired 2D nanofluidic membranes for energy applications

Bioinspired two-dimensional (2D) nanofluidic membranes have been explored for the creation of high-performance ion transport systems that can mimic the delicate transport functions of living organisms. Advanced energy devices made from these membranes show excellent energy storage and conversion capabilities. Further research and development in this area are essential to unlock the full potential of energy devices and facilitate the development of high-performance equipment toward real-world applications and a sustainable future. However, there has been minimal review and summarization of 2D nanofluidic membranes in recent years. Thus, it is necessary to carry out an extensive review to provide a survey library for researchers in related fields. In this review, the classification and the raw materials that are used to construct 2D nanofluidic membranes are first presented. Second, the top-down and bottom-up methods for constructing 2D membranes are introduced. Next, the applications of bioinspired 2D membranes in osmotic energy, hydraulic energy, mechanical energy, photoelectric conversion, lithium batteries, and flow batteries are discussed in detail. Finally, the opportunities and challenges that 2D nanofluidic membranes are likely to face in the future are envisioned. This review aims to provide a broad knowledge base for constructing high-performance bioinspired 2D nanofluidic membranes for advanced energy applications.

Interfacial Assembly of 2D Graphene-Derived Ion Channels for Water-Based Green Energy Conversion

The utilization of sustained and green energy is believed to alleviate increasing menace of global environmental concerns and energy dilemma. Interfacial assembly of 2D graphene-derived ion channels (2D-GDICs) with tunable ion/fluid transport behavior enables efficient harvesting of renewable green energy from ubiquitous water, especially for osmotic energy harvesting. In this review, various interfacial assembly strategies for fabricating diverse 2D-GDICs are summarized and their ion transport properties are discussed. This review analyzes how particular structure and charge density/distribution of 2D-GDIC can be modulated to minimize internal resistance of ion/fluid transport and enhance energy conversion efficiency, and highlights stimuli-responsive functions and stability of 2D-GDIC and further examines the possibility of integrating 2D-GDIC with other energy conversion systems. Notably, the presented preparation and applications of 2D-GDIC also inspire and guide other 2D materials to fabricate sophisticated ion channels for targeted applications. Finally, potential challenges in this field is analyzed and a prospect to future developments toward high-performance or large-scale real-word applications is offered.

Advancing Ion Selective Membranes with Micropore Ion Channels in the Interaction Confinement Regime

Ion exchange membranes allowing the passage of charge-carrying ions have established their critical role in water, environmental, and energy-relevant applications. The design strategies for high-performance ion exchange membranes have evolved beyond creating microphase-separated membrane morphologies, which include advanced ion exchange membranes to ion-selective membranes. The properties and functions of ion-selective membranes have been repeatedly updated by the emergence of materials with subnanometer-sized pores and the understanding of ion movement under confined micropore ion channels. These research progresses have motivated researchers to consider even greater aims in the field, i.e., replicating the functions of ion channels in living cells with exotic materials or at least targeting fast and ion-specific transmembrane conduction. To help realize such goals, we briefly outline and comment on the fundamentals of rationally designing membrane pore channels for ultrafast and specific ion conduction, pore architecture/chemistry, and membrane materials. Challenges are discussed, and perspectives and outlooks are given.

Enhancing ion selectivity by tuning solvation abilities of covalent-organic-framework membranes

Understanding the molecular-level mechanisms involved in transmembrane ion selectivity is essential for optimizing membrane separation performance. In this study, we reveal our observations regarding the transmembrane behavior of Li+ and Mg2+ ions as a response to the changing pore solvation abilities of the covalent-organic-framework (COF) membranes. These abilities were manipulated by adjusting the lengths of the oligoether segments attached to the pore channels. Through comparative experiments, we were able to unravel the relationships between pore solvation ability and various ion transport properties, such as partitioning, conduction, and selectivity. We also emphasize the significance of the competition between Li+ and Mg2+ with the solvating segments in modulating selectivity. We found that increasing the length of the oligoether chain facilitated ion transport; however, it was the COF membrane with oligoether chains containing two ethylene oxide units that exhibited the most pronounced discrepancy in transmembrane energy barrier between Li+ and Mg2+, resulting in the highest separation factor among all the evaluated membranes. Remarkably, under electro-driven binary-salt conditions, this specific COF membrane achieved an exceptional Li+/Mg2+ selectivity of up to 1352, making it one of the most effective membranes available for Li+/Mg2+ separation. The insights gained from this study significantly contribute to advancing our understanding of selective ion transport within confined nanospaces and provide valuable design principles for developing highly selective COF membranes.

Ultrathin H-MXM as An "Ion Freeway" for High-Performance Osmotic Energy Conversion

Nanofluidic membranes are currently being explored as potential candidates for osmotic energy harvesting. However, the development of high-performance nanofluidic membranes remains a challenge. In this study, the ultrathin MXene membrane (H-MXM) is prepared by ultrathin slicing and realize the ion horizontal transportation. The H-MXM membrane, with a thickness of only 3 µm and straight subnanometer channels, exhibits ultrafast ion transport capabilities resembling an "ion freeway". By mixing artificial seawater and river water, a power output of 93.6 W m-2 is obtained. Just as cell membranes have an ultrathin thickness that allows for excellent penetration, this straight nanofluidic membrane also possesses an ultrathin structure. This unique feature helps to shorten the ion transport path, leading to an increased ion transport rate and improveS performance in osmotic energy conversion.

Enhanced fluidity of water in superhydrophobic nanotubes: estimating viscosity using jump-corrected confined Stokes-Einstein approach

Accurately predicting the viscosity of water confined within nanotubes is vital for various technological applications. Traditional methods have failed in this regard, necessitating a novel approach. We introduced the jump-corrected confined Stokes-Einstein (JCSE) method and now employ the same to estimate the viscosity and diffusion in superhydrophobic nanotubes. Our study covers a temperature range of 230-300 K and considers three nanotube diameters. Results show that water inside superhydrophobic nanotubes exhibits a significantly lower viscosity and higher diffusion than those inside hydrophobic nanotubes. Narrower nanotubes and lower temperatures accentuate these effects. Furthermore, water inside superhydrophobic nanotubes display a lower viscosity than bulk water, with the difference increasing at lower temperatures. This reduction is attributed to weaker water-water interactions caused by a lower water density in the interfacial region. These findings highlight the importance of interfacial water density and its influence on nanotube viscosity, shedding light on nanoscale fluid dynamics and opening avenues for diverse applications.

Bio-inspired Double Angstrom-Scale Confinement in Ti-deficient Ti0.87 O2 Nanosheet Membranes for Ultrahigh-performance Osmotic Power Generation

Osmotic power, a clean energy source, can be harvested from the salinity difference between seawater and river water. However, the output power densities are hampered by the trade-off between ion selectivity and ion permeability. Here we propose an effective strategy of double angstrom-scale confinement (DAC) to design ion-permselective channels with enhanced ion selectivity and permeability simultaneously. The fabricated DAC-Ti0.87 O2 membranes possess both Ti atomic vacancies and an interlayer free spacing of ≈2.2 Å, which not only generates a profitable confinement effect for Na+ ions to enable high ion selectivity but also induces a strong interaction with Na+ ions to benefit high ion permeability. Consequently, when applied to osmotic power generation, the DAC-Ti0.87 O2 membranes achieved an ultrahigh power density of 17.8 W m-2 by mixing 0.5/0.01 M NaCl solution and up to 114.2 W m-2 with a 500-fold salinity gradient, far exceeding all the reported macroscopic-scale membranes. This work highlights the potential of the construction of DAC ion-permselective channels for two-dimensional materials in high-performance nanofluidic energy systems.

Preanchoring Enabled Directional Modification of Atomically Thin Membrane for High-Performance Osmotic Energy Generation

Salinity gradient energy is an environmentally friendly energy source that possesses potential to meet the growing global energy demand. Although covalently modified nanoporous graphene membranes are prospective candidates to break the trade-off between ion selectivity and permeability, the random reaction sites and inevitable defects during modification reduce the reaction efficiency and energy conversion performance. Here, we developed a preanchoring method to achieve directional modification near the graphene nanopores periphery. Numerical simulation revealed that the improved surface charge density around nanopores results in exceptional K+/Cl- selectivity and osmotic energy conversion performance, which agreed well with experimental results. Ionic transport measurements showed that the directionally modified graphene membranes achieved an outstanding power density of 81.6 W m-2 with an energy conversion efficiency of 35.4% under a 100-fold salinity gradient, outperforming state-of-the-art graphene-based nanoporous membranes. This work provided a facile approach for precise modification of nanoporous graphene membranes and opened up new ways for osmotic power harvesting.

Robust Cellulose Nanocrystal-Based Self-Assembled Composite Membranes Doped with Polyvinyl Alcohol and Graphene Oxide for Osmotic Energy Harvesting

Osmotic energy from the salinity gradients represents a promising energy resource with stable and sustainable characteristics. Nanofluidic membranes can be considered as powerful alternatives to the traditional low-performance ion exchange membrane to achieve high-efficiency osmotic energy harvesting. However, the development of a highly efficient and easily scalable core membrane component from low-cost raw materials remains challenging. Here, a composite membrane based on the self-assembly of cellulose nanocrystals (CNCs) with polyvinyl alcohol (PVA) and graphene oxide (GO) nanoflakes as additives is developed to provide a solution. The introduction of soft PVA polymer significantly improves the mechanical strength and water stability of the composite membrane by forming a nacre-like structure. Benefiting from the abundant negative charges of CNC nanorods and GO nanoflakes and the generated network nanochannels, the composite membrane demonstrates a good cation-selective transport capacity, thus contributing to an optimal osmotic energy conversion of 6.5 W m-2 under a 100-fold salinity gradient and an exemplary stability throughout 25 consecutive days of operation. This work provides an option for the development of nanofluidic membranes that can be easily produced on a large scale from well-resourced and sustainable biomass materials for high-efficiency osmotic energy conversion.

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

Anomalous Mechanical and Electrical Interplay in a Covalent Organic Framework Monolayer Membrane

Ion transport through nanoconfinement, driven by both electrical and mechanical forces, has drawn ever-increasing attention, due to its high similarity to stress-sensitive ion channels in biological systems. Previous studies have reported only pressure-induced enhancement in ion conductance in low-permeable systems such as nanotubes, nanoslits, or single nanopores. This enhancement is generally explained by the ion accumulation caused by the capacitive effect in low-permeable systems. Here, we fabricate a highly permeable COF monolayer membrane to investigate ion transport behavior driven by both electrical and mechanical forces. Our results show an anomalous conductance reduction activated by external mechanical force, which is contrary to the capacitive effect-dominated conductance enhancement observed in low-permeable nanopores or channels. Through simulations, we uncovered a distinct electrical-mechanical interplay mechanism that depends on the relative rate between the ion diffusion from the boundary layer to the membrane surface and the ion transport through the membrane. The high pore density of the COF monolayer membrane reduces the charge accumulation caused by the capacitive effect, resulting in fewer accumulated ions near the membrane surface. Additionally, the high membrane permeability greatly accelerates the dissipation of the accumulated ions under mechanical pressure, weakening the effect of the capacitive layer on the streaming current. As a result, the ions accumulated on the electrodes, rather than in the capacitive layer, dominating the streaming current and giving rise to a distinct electrical-mechanical interplay mechanism compared to that in low-permeable nanopores or channels. Our study provides new insights into the interplay between electrical and mechanical forces in ultra-permeable systems.

Synthesis of sp2 Carbon-Conjugated Covalent Organic Framework Thin-Films via Copper-Surface-Mediated Knoevenagel Polycondensation

sp2 carbon-conjugated covalent organic framework (sp2 c-COF) featured with high π-conjugation, high chemical stabilities, and designable chemical structures, are thus promising for applications including adsorption and separation, optoelectronic devices, and catalysis. For the most of these applications, large-area and continuous films are required. However, due to the needs of harsh conditions in the formation of CC bonds, classical interfacial methodologies are challenged in the synthesis of sp2 c-COFs films. Herein, a novel and robust interfacial method namely copper-surface-mediated Knoevenagel polycondensation (Cu-SMKP), is shown for scalable synthesis of sp2 c-COF films on various Cu substrates. Using this approach, large-area and continuous sp2 c-COF films could be prepared on various complicated Cu surfaces with thickness from tens to hundreds of nanometers. The resultant sp2 c-COF films on Cu substrate could be used directly as functional electrode for extraction of uranium from spiked seawater, which gives an exceptionally uptake capacity of 2475 mg g-1 . These results delineate significant synthetic advances in sp2 c-COF films and implemented them as functional electrodes for uranyl capture.

Superassembly of 4-Aminothiophenol-Modified Mesoporous Titania-Alumina Oxide Heterochannels for Smart Ion Transport Based on the Photo-Induced Electron-Transfer Process

Light-responsive nanochannels have attracted extensive attention due to their noninvasive external field control and intelligent ion regulation. However, the limited photoresponsive current and the low photoelectric conversion efficiency still restrict their development. Here, a light-controlled nanochannel composed of 4-aminothiophenol and gold nanoparticles-modified mesoporous titania nanopillar arrays and alumina oxide (4-ATP-Au-MTI/AAO) is fabricated by the interfacial super-assembly strategy. Inspired by the process of electron transfer between photosystem I and photosystem II, the efficient electron transfer between TiO₂, AuNPs, and 4-ATP under light is achieved by coupling the photoresponsive materials and functional molecules. Under illumination, 4-ATP is oxidized to p-nitrothiophenol (PNTP), which brings about changes in the wettability of the nanochannel, resulting in significant improvement (252.8%) of photoresponsive current. In addition, under the action of the reductant, the nanochannels can be restored to the initial dark state, enabling multiple reversible cycles. This work opens a new route for the fabrication of high-performance light-controlled nanochannels by coupling light-responsive materials and light-responsive molecules, which may guide the development of photoelectric conversion nanochannel systems.

Green and Scalable Synthesis of Atomic-Thin Crystalline Two-Dimensional Triazine Polymers with Ultrahigh Photocatalytic Properties

Scalable and eco-friendly synthesis of crystalline two-dimensional (2D) polymers with proper band gap and single-layer thickness is highly desired for the fundamental research and practical applications of 2D polymers; however, it remains a considerable and unresolved challenge. Herein, we report a convenient and robust method to synthesize a series of crystalline covalent triazine framework nanosheets (CTF NSs) with a thickness of ∼80 nm via a new solvent-free salt-catalyzed nitrile cyclotrimerization process, which enables the cost-effective large-scale preparation of crystalline CTF NSs at the hundred-gram level. Theoretical calculations and detailed experiments revealed for the first time that the conventional salts such as KCl can not only act as physical templates as traditionally believed but also more importantly can efficiently catalyze the cyclotrimerization reaction of carbonitrile monomers as a new kind of green solid catalysts to achieve crystalline CTF NSs. Upon simple liquid-phase sonication, these CTF NSs can be easily further exfoliated into abundant single-layer crystalline 2D triazine polymers (2D-TPs) in high yields. The obtained atomically thin crystalline 2D-TPs with a band gap of 2.36 eV and rich triazine active groups exhibited a remarkable photocatalytic hydrogen evolution rate of 1321 μmol h^-1 under visible light irradiation with an apparent quantum yield up to 29.5% at 420 nm and excellent photocatalytic overall water splitting activity with a solar-to-hydrogen efficiency up to 0.35%, which exceed all molecular framework materials and are among the best metal-free photocatalysts ever reported. Moreover, the processable 2D-TPs could be readily assembled on a support as a photocatalytic film device, which demonstrated superior photocatalytic performance (135.2 mmol h^-1 m^-2 for hydrogen evolution).

Janus Metal-Organic Framework Membranes Boosting the Osmotic Energy Harvesting

The unique ion-transport properties in nanoconfined pores enable nanofluidic devices with great potential in harvesting osmotic energy. The energy conversion performance could be significantly improved by the precise regulation of the "permeability-selectivity" trade-off and the ion concentration polarization effect. Here, we take the advantage of electrodeposition technique to fabricate a Janus metal-organic framework (J-MOF) membrane that possesses rapid ion-transport capability and impeccable ion selectivity. The asymmetric structure and asymmetric surface charge distribution of the J-MOF device can suppress the ion concentration polarization effect and enhance the ion charge separation, exhibiting an improved energy harvesting performance. An output power density of 3.44 W/m² has been achieved with the J-MOF membrane at a 1000-fold concentration gradient. This work provides a new strategy for fabricating high-performance energy-harvesting devices.

Monolayer-Assisted Surface-Initiated Schiff-Base-Mediated Aldol Polycondensation for the Synthesis of Crystalline sp2 Carbon-Conjugated Covalent Organic Framework Thin Films

sp² carbon-conjugated covalent organic frameworks (sp²c-COFs) with superb in-plane π-conjugations, high chemical stability, and robust framework structure are expected to be ideal films/membranes for a wide range of applications including energy-related devices and optoelectronics. However, so far, sp²c-COFs have been mainly limited to microcrystalline powders, and this consequently hampered their performances in devices. Herein, we report a simple and robust methodology to fabricate large-area, free-standing, and crystalline sp²c-COF films (TFPT-TMT and TB-TMT) on various solid substrates (e.g., fluorine-doped tin oxide, aluminum sheet, polyacrylonitrile membrane) by self-assembly monolayer-assisted surface-initiated Schiff-base-mediated aldol polycondensation (namely, SI-SBMAP). The resultant sp²c-COF films show lateral sizes up to 120 cm² and tunable thickness from tens of nanometers to a few micrometers. Owing to the robust framework and highly ordered quasi-1D channels, the sp²c-COF membrane-based osmotic power generator presents an output power density of 14.1 W m^-2 under harsh conditions, outperforming most reported COF membranes as well as commercialized benchmark devices (5 W m^-2). This work demonstrates a simple and robust interfacial methodology for the fabrication of sp²c-COF films/membranes for green energy applications and potential optoelectronics.

Giant Gateable Osmotic Power Generation from a Goldilocks Two-Dimensional Polymer

Generating electricity from a salinity gradient, known as osmotic power, provides a sustainable energy source, but it requires precise nanoscale control of membranes for maximum performance. Here, we report an ultrathin membrane, where molecule-specific short-range interactions enable giant gateable osmotic power with a record high power density (2 kW/m² for 1 M∥1 mM KCl). Our membranes are charge-neutral two-dimensional polymers synthesized from molecular building blocks and operate in a Goldilocks regime that simultaneously maintains high ionic conductivity and permselectivity. Molecular dynamics simulations quantitatively confirm that the functionalized nanopores are small enough for high selectivity through short-range ion-membrane interactions and large enough for fast cross-membrane transport. The short-range mechanism further enables reversible gateable operation, as demonstrated by polarity switching of osmotic power with additional gating ions.

Vacancy Engineering for High-Efficiency Nanofluidic Osmotic Energy Generation

Two-dimensional (2D) nanofluidic membranes have shown great promise in harvesting osmotic energy from the salinity difference between seawater and fresh water. However, the output power densities are strongly hampered by insufficient membrane permselectivity. Herein, we demonstrate that vacancy engineering is an effective strategy to enhance the permselectivity of 2D nanofluidic membranes to achieve high-efficiency osmotic energy generation. Phosphorus vacancies were facilely created on NbOPO₄ (NbP) nanosheets, which remarkably enlarged their negative surface charge. As verified by both experimental and theoretical investigations, the vacancy-introduced NbP (V-NbP) exhibited fast transmembrane ion migration and high ionic selectivity originating from the improved electrostatic affinity of cations. When applied in a natural river water|seawater osmotic power generator, the macroscopic-scale V-NbP membrane delivered a record-high power density of 10.7 W m^-2, far exceeding the commercial benchmark of 5.0 W m^-2. This work endows the remarkable potential of vacancy engineering for 2D materials in nanofluidic energy devices.

Switchable Na+ and K+ selectivity in an amino acid functionalized 2D covalent organic framework membrane

Biological cell membranes can efficiently switch Na⁺/K⁺ selectivity in response to external stimuli, but achieving analogous functions in a single artificial membrane is challenging. Here, we report highly crystalline covalent organic framework (COF) membranes with well-defined nanochannels and coordinative sites (i. e., amino acid) that act as ion-selective switches to manipulate Na⁺ and K⁺ transport. The ion selectivity of the COF membrane is dynamic and can be switched between K⁺-selective and Na⁺-selective in a single membrane by applying a pH stimulus. The experimental results combined with molecular dynamics simulations reveal that the switchable Na⁺/K⁺ selectivity originates from the differentiated coordination interactions between ions and amino acids. Benefiting from the switchable Na⁺/K⁺ selectivity, we further demonstrate the membrane potential switches by varying electrolyte pH, miming the membrane polarity reversal during neural signal transduction in vivo, suggesting the great potential of these membranes for in vitro biomimetic applications.

Self-Healable, Malleable, Ecofriendly Recyclable and Robust Polyimine Thermosets Derived from Trifluoromethyl Diphenoxybenzene Backbones

Dynamic covalent chemistry opens up great opportunities for a sustainable society by producing reprocessable networks of polymers and even thermosets. However, achieving the closed-loop recycling of polymers with high performance remains a grand challenge. The introduction of aromatic monomers and fluorine into covalent adaptable networks is an attractive method to tackle this challenge. Therefore, we present a facile and universal strategy to focus on the design and applications of polyimine vitrimers containing trifluoromethyl diphenoxybenzene backbones in applications of dynamic covalent polymers. In this study, fluorine-containing polyimine vitrimer networks (FPIVs) were fabricated, and the results revealed that the FPIVs not only exhibited good self-healability, malleability and processability without the aid of any catalyst, but also possessed decent mechanical strength, superior toughness and thermal stability. We hope that this work could provide a novel pathway for the design of high-performance polyimine vitrimers by recycling of plastic wastes.

An Ionic Diode Covalent Organic Framework Membrane for Efficient Osmotic Energy Conversion

Heterogeneous membranes that exhibit an ionic diode effect are promising candidates for osmotic energy conversion. However, existing heterogeneous membranes lack molecular-level designed ion channels, thereby limiting their power densities. Here, we demonstrate ionic diode covalent organic framework (COF) membranes with well-defined ion channels, asymmetric geometry and surface charge polarity as high-performance osmotic power generators. The COF diode membranes are comprised of heterojunctions combining a positively charged ultrathin COF layer and a negatively charged COF layer supported by a porous COF nanofiber scaffold, exhibiting an ionic diode effect that effectuates fast unidirectional ion diffusion and anion selectivity. Density functional theory calculations reveal that the differentiated interactions between anions and COF channels contributed to superior I^- transport over other anions. Consequently, the COF diode membranes achieved high output power densities of 19.2 and 210.1 W m^-2 under a 50-fold NaCl and NaI gradient, respectively, outperforming state-of-the-art heterogeneous membranes. This work suggests the great potential of COF diode membranes for anion transport and energy-related applications.

Engineering Polymeric Nanofluidic Membranes for Efficient Ionic Transport: Biomimetic Design, Material Construction, and Advanced Functionalities

Design elements extracted from biological ion channels guide the engineering of artificial nanofluidic membranes for efficient ionic transport and spawn biomimetic devices with great potential in many cutting-edge areas. In this context, polymeric nanofluidic membranes can be especially attractive because of their inherent flexibility and benign processability, which facilitate massive fabrication and facile device integration for large-scale applications. Herein, the state-of-the-art achievements of polymeric nanofluidic membranes are systematically summarized. Theoretical fundamentals underlying both biological and synthetic ion channels are introduced. The advances of engineering polymeric nanofluidic membranes are then detailed from aspects of structural design, material construction, and chemical functionalization, emphasizing their broad chemical and reticular/topological variety as well as considerable property tunability. After that, this Review expands on examples of evolving these polymeric membranes into macroscopic devices and their potentials in addressing compelling issues in energy conversion and storage systems where efficient ion transport is highly desirable. Finally, a brief outlook on possible future developments in this field is provided.