Large-scale, robust mushroom-shaped nanochannel array membrane for ultrahigh osmotic energy conversion

Potassium-Selective Covalent Organic Framework Membranes Enable Dynamic Monitoring of Microbial K+ Metabolism

Ultraselective and rapid transport of potassium ion (K+) is crucial for maintaining life activities such as osmotic pressure equilibrium, protein synthesis regulation, microbial growth, and communication. However, it is challenging to achieve high efficiency and precise K+ transport due to the existence of competitive cations with similar size and valence. Here, a biomimetic K+ nanochannel based on sulfonated covalent organic frameworks (COF) is reported with high K+ screening selectivity to achieve dynamic microbial K+ metabolism monitoring. Similar to the structure and function of biological KcsA channels, sulfonated COF feature ordered nanochannels and abundant surface charges, facilitating effective sieving of K+ and sodium ions (Na+) through size screening and electrostatic interactions, achieving a K+/Na+ selectivity ratio of 17.3. Molecular dynamic simulations indicate that the K+/Na+ selectivity of the COF nanochannels arises from the interaction of K+ with the sulfonate functional groups on the nanochannels, resulting in a decreased energy barrier for K+. Given the excellent K+ screening selectivity and efficiency, the designed COF nanochannels enable real-time monitoring of K+ in complex microbial systems and provide guidance for the synthesis of high value-added products. These findings suggest approaches for developing efficient and selective nanochannels for ion separation, nanofluidic, and complex microbial metabolism systems.

Clay-Based Nanofluidic Membrane with Enhanced Space Charge for Robust Osmotic Energy Harvesting

Converting the salinity gradient energy into electric energy through permselective membranes has great potential to alleviate the energy crisis. However, the competition between selectivity and permeability, along with the instability of traditional permselective membranes, limits their realistic applications. Herein, a robust clay-based nanofluidic membrane of aramid nanofiber@palygorskite/anodic alumina oxide (ANF@PAL/AAO) with a 3D interworking network has been fabricated for efficient osmotic energy harvesting. The 3D interconnected nanochannels stacked by needle-like PAL provide more and shorter paths for ion transport, thereby increasing the permeability. Moreover, the collaboration between the surface charge of PAL and the space charge brought by ANFs improves ion selectivity, further enhancing the energy conversion performance. Results show that the as-prepared ANF@PAL/AAO membrane displays a power output of 45 W m-2 at 500-fold NaCl gradient and can withstand acidity/alkalinity and high salinity environments. The present work paves a facile way for the application of clay-based nanofluidic devices in practical energy conversion.

Two-sided asymmetric nanofluidic membrane for enhanced ion transport and osmotic energy harvesting

Nanofluidic membranes hold great potential for osmotic energy conversion. Creating high-efficiency ion-permselective membranes with well-fit channel structures continues to pose a persistent challenge. In this work, we design a novel dual asymmetric nanofluidic membrane with MXene and Nafion separately on the two sides of anodic aluminum oxide (AAO) for enhanced ion selective transport. Driven by osmosis, cations are initially separated by the Nafion layer with abundant negative charges, then followed by accelerated transport due to the interface potential abruptness between AAO channels and the MXene layer. Following that, the MXene layer acts as the second cation selective layer to further achieve ion charge separation. Benefiting from the dual ion selectivity and accelerated ion transfer, a high cation transfer number of 0.95 can be realized using the present membrane. In addition, the photothermal property of MXene could generate an additional thermal gradient under light irradiation, further promoting ion transfer. Taking advantage of the present two-sided asymmetric nanofluidic membrane, the output power could be up to 65.6 W m-2 at 500-fold NaCl salinity gradient, which is much higher than that of the majority of previously reported reverse electrodialysis membranes (3.0-35.0 W m-2). The present work opens up a new strategy for constructing novel asymmetric nanofluidic devices for enhanced ion transport and osmotic energy harvesting.

Recent advances in the design principles of lithium selective membranes

The growing demand for lithium in energy storage applications has intensified the need for efficient lithium extraction technologies, with membrane processes emerging as a promising approach. Among various membrane technologies, nanostructured membranes with precisely engineered channels have shown exceptional potential for selective lithium extraction due to their ability to control ion transport at the molecular level. This review provides a comprehensive analysis of the fundamental design principles governing lithium-selective membranes, with a specific focus on nanochannel-based systems. We examine the critical parameters that influence lithium selectivity, including surface charge distribution, nanochannel dimensions, morphology, and wettability, while exploring how these factors interact with external driving forces to enable selective ion transport. This work extensively analyzes recent developments in nanochannel engineering and ion transport mechanisms, providing crucial insights into optimizing membrane selectivity and performance. By critically analyzing current challenges in scaling up these technologies and identifying promising research directions, this work provides a roadmap for developing next-generation lithium-selective membranes with enhanced efficiency and selectivity.

Construction of Biomimetic Nanochannel, Property Regulation, and Biomarker Detection

The significance of biomimetic nanochannel in the field of biosensors is gaining increasing recognition. The controllable construction of biomimetic nanochannels and their performance modulation have demonstrated great importance and obtained wide interest. The nanochannels offer high sensitivity, enabling sensors to swiftly identify target biomarkers in complex biological samples, with detection limits reaching the picomolar level. Furthermore, they demonstrate exceptional selectivity and reproducibility, making them ideal tools for biomarker detection. In recent years, biosensors utilizing biomimetic nanochannel have shown remarkable performance in detecting a wide range of biomarkers. This review aims to explore the opportunities and challenges associated with biomimetic nanochannel technology in biosensor applications, focusing on the construction and performance modulation of these nanochannels, as well as their applications in detecting nucleic acids, proteins, organisms, and small molecules. Providing forward-looking insights into this cutting-edge field is aspired, with particular emphasis on technological advancements, addressing current challenges, and discussing future trends.

Engineering Bipolar Covalent Organic Framework Membranes for Selective Acid Extraction

Nitric acid (HNO3) is a vital industrial chemical, and its recovery from complex waste streams is essential for sustainability and resource optimization. This study demonstrates the effectiveness of bipolar covalent organic framework (COF) membranes with tunable ionic site distributions as a solution for this challenge. The membranes are fabricated by layering anionic COF nanosheets on cationic COF layers, supported by a porous substrate. The resulting membranes exhibit significant rectifying behavior, driven by the asymmetric charge polarity and the intrinsic electric field, which enhances HNO3 transport. The transmembrane diffusion coefficient of 2.74 × 10-5 cm2 s-1 exceeds the self-diffusion rate of NO3 -, leading to increased HNO3 flux and selectivity compared to the individual anionic and cationic COF membranes. The optimized bipolar membrane configuration achieves remarkable separation factors, ranging from 22 to 242,000 for HNO₃, in comparison to other solutes such as HCl, H2SO4, H3PO4, and various metal salts in an eight-component mixed waste stream. This results in a substantial increase in HNO₃ purity, from 12.5% to 94.1% after a single membrane separation. With the broad range of COF materials and the versatility of the proposed membrane design, this work represents a significant advancement in chemical separation technologies.

Ice-Confined Synthesis of Stacked Polymer Nanospheres as Osmotic Power Generation Membranes

Osmotic power extracts electricity from salinity gradients and provides a viable route toward clean energy. To improve the energy conversion efficiency, common strategies rely on fabricating precisely controlled nanopores to meet the requirements of high ionic conductivity and selectivity. We report ion transport through the free-volume networks in stacked polymer nanospheres for osmotic power harvesting. Such nanospheres, composed of coiled poly(acrylic acid) molecules, are synthesized at an ice-liquid interface where they self-assemble into continuous membranes with controlled thicknesses and morphologies. We achieve a rival power density of a few thousand watts per square meter, attributed to the fast and selective ion transport through the nanostructured membranes. The selectivity is further found to originate from the membranes' tunable charging states determined by the association/dissociation equilibrium of the residual groups and the presence of translocation ions. Our work suggests polymer membranes absent of straight-through pores as a new platform for efficient osmotic energy generation.

Renewable, All-Natural Flute Membrane with Excellent Mechanical Properties for Osmotic Energy Harvesting

Ion-selective membranes serve as key materials for reverse electrodialysis (RED) technology in osmotic energy harvesting, and the search for a class of membranes that are economical, highly robust, and sustainable has been a relentless goal for researchers. In this work, all-natural biomass membranes (reed membranes) are often used as a flute diaphragm, which makes the flute produce a brighter and crisper sound, presenting high strength and elasticity. Ultrathin natural reed membranes (thickness of ≈4.06 μm) were selected as representative materials due to their impressive mechanical properties with a top-level combination of yield strength (≈63.5 MPa) and strain (∼2%) among all reported natural materials. More importantly, there are numerous nanoscale pores and negatively charged -OH groups on the reed surface, providing tiny nanofluidic channels for efficient cation transmembrane transport, which endow the flute membrane with excellent selectivity for caution and stable salinity-gradient energy conversion performance. The reed membrane delivers excellent osmotic energy conversion performance with a power output density of 22.2 W m-2 in 500-fold NaCl concentration as well as high stability (power density maintained at 98.53% for more than 6000 s). This work provides a strategy for all-natural ion-selective membranes in terms of economy, fabrication simplicity, and stability, which has potential utility in various applications such as osmotic energy harvesting.

Characterization of Diffusioosmotic Ion Transport for Enhanced Concentration-Driven Power Generation via Charge Heterogeneity in Nanoporous Membranes

Nanoscopic mass/ion transport through heterogeneous nanostructures with various physicochemical environments occurs in both natural and artificial systems. Concentration gradient-driven mass/ion transport mechanisms, such as diffusioosmosis (DO), are primarily governed by the structural and electrical features of the nanostructures. However, these phenomena under various electrical and chemical conditions have not been adequately investigated. In this study, we fabricated a pervaporation-based particle-assembled membrane (PAM)-integrated micro-/nanofluidic device that facilitates easy tuning of the surface charge heterogeneity in nanopores/nanochannels. The nanochannels in the device consisted of two heterogeneous and in-series PAMs. The device was used to quantitatively measure electric signals generated by DO within the nanochannels with a single electrolyte or a combination of two electrolytes. Then, we characterized ion transport by changing surface charge heterogeneity and applying various electrolytic conditions, characterizing the concentration-driven power generation under these conditions. We found that not only does the charge heterogeneity provide additional resistance to ion transport but also the manipulation of the heterogeneity enables the effective modulation of ion transport and optimization of concentration-driven power generators regarding ion selectivity. In conjunction with the surface charge heterogeneity, the electrolytic conditions significantly affected the net flux of ion transport by enhancing or even negating the ion selectivity. Hence, we anticipate that both the platform and results will provide a deeper understanding of ion transport in nanostructures within complex environments by optimizing and improving practical concentration-driven applications, such as energy conversion/harvesting, molecular focusing/separation, and ionic diodes and memristors.

Anti-Swelling Polyelectrolyte Hydrogel with Submillimeter Lateral Confinement for Osmotic Energy Conversion

Harvesting the immense and renewable osmotic energy with reverse electrodialysis (RED) technology shows great promise in dealing with the ever-growing energy crisis. One key challenge is to improve the output power density with improved trade-off between membrane permeability and selectivity. Herein, polyelectrolyte hydrogels (channel width, 2.2 nm) with inherent high ion conductivity have been demonstrated to enable excellent selective ion transfer when confined in cylindrical anodized aluminum pore with lateral size even up to the submillimeter scale (radius, 0.1 mm). The membrane permeability of the anti-swelling hydrogel can also be further increased with cellulose nanofibers. With real seawater and river water, the output power density of a three-chamber cell on behalf of repeat unit of RED system can reach up to 8.99 W m-2 (per unit total membrane area), much better than state-of-the-art membranes. This work provides a new strategy for the preparation of polyelectrolyte hydrogel-based ion-selective membranes, owning broad application prospects in the fields of osmotic energy collection, electrodialysis, flow battery and so on.

Nanofiltration Membrane with Enhanced Ion Selectivity Based on a Precision-Engineered Ultrathin Polyethylene Supporting Layer

Nanofiltration (NF) technology is increasingly used in the water treatment and separation fields. However, most research has focused on refining the selective layer while overlooking the potential role of the supporting layer. With expertise in ultrathin polymer films, particularly in the production of polyethylene (PE) membranes, we explore the possibility of improving NF membrane performance by precisely controlling the structure and surface properties of the ultrathin supporting layer in this work. Here, we introduced an innovative NF membrane that used a submicrometer ultrathin PE membrane produced through a biaxial stretching process, which is significantly thinner than commercial PE membranes available on the market. The core innovations are as follows: first, we focused on precise control of the supporting layer rather than just the selective layer, achieving significant enhancements in overall NF membrane performance; second, the ultrathin PE supporting layer served as a tunable interface for interfacial polymerization, offering possibilities for structural control of the selective layer and advancing membrane performance innovations. The resulting NF membrane boasts an overall thickness of ∼630 nm, which represents the thinnest NF membrane documented to date. This ultrathin NF membrane showed an ultrahigh Cl-/SO42- selectivity of 338.03, placing it at the forefront of existing literature. This study sheds light on the important role of the supporting layer in the preparation of selective layers. We believe that this approach has the potential to contribute to the development of ultrathin, high-performance NF membranes.

Nanoarchitecture via Synchronic Stacking of Metallic and Nonmetallic Two-Dimensional Nanosheets for Optimal Light-Driven Ion Transport

The exceptional selectivity and responsive ion transport in biological channels inspire technology breakthrough in energy, environmental, and resource sectors. However, existing nanofluidic systems with a high photothermal conversion efficiency often exhibit excessive thermal conductivity, which impedes the creation of effective temperature gradients and results in a low ion transport efficiency. In this study, a strategy based on the synchronic stacking of metallic and nonmetallic two-dimensional (2D) nanosheets was presented to construct heterogeneous nanofluidic channels. This specific nanoconfined architecture sustained high temperatures in the illuminated area while maintaining low temperatures in the nonilluminated area, thus obtaining a robust driving force from sunlight for directional ion transport. As a result, our light-responsive ion transport system demonstrated significant potential in solar energy conversion and osmotic energy harvesting, surpassing those of all previously reported nanofluidic systems. Additionally, although it is still at the proof-of-concept stage, it shows great promise in light signal monitoring. This work provides an effective strategy for developing advanced light-responsive ion transport systems and their important applications.

Modular Customized Biomimetic Nanofluidic Diode for Tunable Asymmetric Ion Transport

Artificial ion diodes, inspired by biological ion channels, have made significant contributions to the fields of physics, chemistry, and biology. However, constructing asymmetric sub-nanofluidic membranes that simultaneously meet the requirements of easy fabrication, high ion transport efficiency, and tunable ion transport remains a challenge. Here, a direct and flexible in situ staged host-guest self-assembly strategy is employed to fabricate ion diode membranes capable of achieving zonal regulation. Coupling the interfacial polymerization process with a host-guest assembly strategy, it is possible to easily manipulate the type, order, thickness, and charge density of each module by introducing two oppositely charged modules in stages. This method enables the tuning of ion transport behavior over a wide range salinity, as well as responsive to varying pH levels. To verify the potential of controllable diode membranes for application, two ion diode membranes with different ion selectivity and high charge density are coupled in a reverse electrodialysis device. This resulted in an output power density of 63.7 W m-2 at 50-fold NaCl concentration gradient, which is 12 times higher than commercial standards. This approach shows potential for expanding the variety of materials that are appropriate for microelectronic power generation devices, desalination, and biosensing.

Angstrom-Scale 2D Channels Designed For Osmotic Energy Harvesting

Confronting the impending exhaustion of traditional energy, it is urgent to devise and deploy sustainable clean energy alternatives. Osmotic energy contained in the salinity gradient of the sea-river interface is an innovative, abundant, clean, and renewable osmotic energy that has garnered considerable attention in recent years. Inspired by the impressively intelligent ion channels in nature, the developed angstrom-scale 2D channels with simple fabrication process, outstanding design flexibility, and substantial charge density exhibit excellent energy conversion performance, opening up a new era for osmotic energy harvesting. However, this attractive research field remains fraught with numerous challenges, particularly due to the complexities associated with the regulation at angstrom scale. In this review, the latest advancements in the design of angstrom-scale 2D channels are primarily outlined for harvesting osmotic energy. Drawing upon the analytical framework of osmotic power generation mechanisms and the insights gleaned from the biomimetic intelligent devices, the design strategies are highlighted for high-performance angstrom channels in terms of structure, functionalization, and application, with a particular emphasis on ion selectivity and ion transport resistance. Finally, current challenges and future prospects are discussed to anticipate the emergence of more anomalous properties and disruptive technologies that can promote large-scale power generation.

Self-Assembly of Hierarchical Silicon-Containing Block Copolymers with Cross-Linkable 3 nm Smectic Motifs for Nanopatterning and Osmotic Energy Conversion Membranes

Highly-dense small-feature-size nanopatterns and nanoporous membranes are important in advanced microelectronics, nanofiltration, and biomimic device manufacturing. Here, we report the synthesis and self-assembly of a series of high-interaction-parameter (high-χ) silicon-containing hierarchical block copolymers (BCPs) with cross-linkable subordering chalcone motifs, which possess both an intrinsic native etching contrast for nanofabrication and cross-linkability under ultraviolet light for generating free-standing membranes. BCPs with a volume fraction of chalcone block of 55-74% form ordered primary nanostructures with period 15-22 nm including lamellae, double gyroid, hexagonally packed cylinders, and body-centered cubic spheres of the minority Si-containing block. The majority PChMA block self-assembles into a highly ordered 3 nm smectic sublattice, and cross-linking after self-assembly enables the formation of free-standing isoporous membranes. Both silicon oxide nanopatterns and free-standing nanoporous osmotic energy conversion membranes are generated by etching films of these BCPs. This work demonstrates that the combination of hierarchical ordering and cross-linkable motifs in a high-interaction parameter BCP enables applications in both nanofabrication and free-standing functional porous membranes.

Lithium extraction from brine through a decoupled and membrane-free electrochemical cell design

The sustainability of lithium-based energy storage or conversion systems, e.g., lithium-ion batteries, can be enhanced by establishing methods of efficient lithium extraction from harsh brines. In this work, we describe a decoupled membrane-free electrochemical cell that cycles lithium ions between iron-phosphate electrodes and features cathode (brine) and anode (fresh water) compartments that are isolated from each other yet electrochemically connected through a pair of silver/silver-halide redox electrodes. This design is compatible with harsh brines having magnesium/lithium molar ratios of up to 3258 and lithium concentrations down to 0.15 millimolar, enabling the production of battery-grade (>99.95% pure) lithium carbonate. Energy savings of up to ~21.5% were realized by efficiently harvesting the osmotic energy of the brines. A pilot-scale cell with an electrode surface area of 33.75 square meters was used to realize lithium extraction from Dead Sea brine with a recovery rate of 84.0%.

Diurnal humidity cycle driven selective ion transport across clustered polycation membrane

The ability to manipulate the flux of ions across membranes is a key aspect of diverse sectors including water desalination, blood ion monitoring, purification, electrochemical energy conversion and storage. Here we illustrate the potential of using daily changes in environmental humidity as a continuous driving force for generating selective ion flux. Specifically, self-assembled membranes featuring channels composed of polycation clusters are sandwiched between two layers of ionic liquids. One ionic liquid layer is kept isolated from the ambient air, whereas the other is exposed directly to the environment. When in contact with ambient air, the device showcases its capacity to spontaneously produce ion current, with promising power density. This result stems from the moisture content difference of ionic liquid layers across the membrane caused by the ongoing process of moisture absorption/desorption, which instigates selective transmembrane ion flux. Cation flux across the polycation clusters is greatly inhibited because of intensified charge repulsion. However, anions transport across polycation clusters is amplified. Our research underscores the potential of daily cycling humidity as a reliable energy source to trigger ion current and convert it into electrical current.

Performance of Single Nanopore and Multi-Pore Membranes for Blue Energy

The salinity gradient power extracted from the mixing of electrolyte solutions at different concentrations through selective nanoporous membranes is a promising route to renewable energy. However, several challenges need to be addressed to make this technology profitable, one of the most relevant being the increase of the extractable power per membrane area. Here, the performance of asymmetric conical and bullet-shaped nanopores in a 50 nm thick membrane are studied via electrohydrodynamic simulations, varying the pore radius, curvature, and surface charge. The output power reaches ~60 pW per pore for positively charged membranes (surface charge σw=160 mC/m2) and ~30 pW for negatively charges ones, σw=-160 mC/m2 and it is robust to minor variations of nanopore shape and radius. A theoretical argument that takes into account the interaction among neighbour pores allows to extrapolate the single-pore performance to multi-pore membranes showing that power densities from tens to hundreds of W/m2 can be reached by proper tuning of the nanopore number density and the boundary layer thickness. Our model for scaling single-pore performance to multi-pore membrane can be applied also to experimental data providing a simple tool to effectively compare different nanopore membranes in blue energy applications.

Memristive Characteristics in an Asymmetrically Charged Nanochannel

The emergent nanofluidic memristor provides a promising way of emulating neuromorphic functions in the brain. The conical-shaped nanopore showed promising features for a nanofluidic memristor, inspiring us to investigate the memory effects in asymmetrically charged nanochannels due to their high current rectification, which may result in good memory effects. Here, the memory effects of an asymmetrically charged nanofluidic channel were numerically simulated by Poisson-Nernst-Planck equations. Our results showed that the I-V curves represented a diode in low scanning frequency and then became a memristor and finally a resistor as frequency increased. We successfully replicated the learning behavior in our system with history-dependent ion redistribution in the nanochannel. Some critical factors were quantitatively analyzed for the memory effects including voltage amplitude, optimal frequency, and Dukhin number. Experimental characterizations were also carried out. Our findings are useful for the design of nanofluidic memristors by the principle of enrichment and depletion as well as the determination of the best memory settings.

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.

Nano-Confined Effect and Heterojunction Promoted Exciton Separation for Light-Boosted Osmotic Energy Conversion

The osmotic energy conversion properties of biomimetic light-stimulated nanochannels have aroused great interest. However, the power output performance is limited by the low light-induced current and energy conversion efficiency. Here, nanochannel arrays with simultaneous modification of ZnO and di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium (II) (N719) onto anodic aluminum oxide (AAO) to combine the nano-confined effect and heterojunction is designed, which demonstrate rectified ion transport behavior due to the asymmetric composition, structure and charge. High cation selectivity and ion flux contribute to the high power density of ≈7.33 W m-2 by mixing artificial seawater and river water. Under light irradiation, heterojunction promoted the production and separation of exciton, enhanced cation selectivity, and improved the utilization efficiency of osmotic energy, providing a remarkable power density of ≈18.49 W m-2 with an increase of 252% and total energy conversion efficiency of 30.43%. The work opens new insights into the biomimetic nanochannels for high-performance energy conversion.

Large-Scale, Vertically Aligned 2D Subnanochannel Arrays by a Smectic Liquid Crystal Network for High-Performance Osmotic Energy Conversion

The osmotic energy, an abundant renewable energy source, can be directly converted to electricity by nanofluidic devices with ion-selective membranes. 2D nanochannels constructed by nanosheets possess abundant lateral interfacial ion-exchange sites and exhibit great superiority in nanofluidic devices. However, the most accessible orientation of the 2D nanochannels is parallel to the membrane surface, undoubtedly resulting in the conductivity loss. Herein, first vertically aligned 2D subnanochannel arrays self-assembled by a smectic liquid crystal (LC) network that exhibit high-performance osmotic energy conversion are demonstrated. The 2D subnanochannel arrays are fabricated by in situ photopolymerization of monomers in the LC phase. The as-prepared membrane exhibits excellent water-resistance and mechanical strength. The 2D subnanochannels with excellent cation selectivity and conductivity show high-performance osmotic energy conversion. The power density reaches up to about 22.5 W m-2 with NaCl solution under a 50-fold concentration gradient, which is among with ultrahigh power density. This membrane design concept provides promising applications in osmotic energy conversion.

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.

Anti-Entropy Aggregation of Minority Groups in Polymers: Design and Applications

Minority groups are non-repeating units with very low content that inevitably exist in polymers. Typically, these minority groups are easily surrounded by the majority of repeating units and randomly dispersed, maximizing the entropy of minority groups. In the concept, anti-entropy aggregation (AEA) of minority groups is described, and different pathways are outlined. They are polymer crystallization-driven AEA, supramolecular interaction-induced AEA, phase separation-confined AEA, and hierarchical interactions-driven AEA. Typical applications of AEA materials are also presented, including fluorescence probes, self-healing materials, ion transporting regulation, and osmotic energy conversion. The concept of AEA is expected to inspire the fabrication of novel functional systems.

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.

Encounter between Gyroid and Lamellae in Janus Colloidal Particles Self-Assembled by a Rod-Coil Block Copolymer

Controlling the internal structure of block copolymer (BCP) particles has a significant influence on its functionalities. Here, a structure-controlling method is proposed to regulate the internal structure of BCP Janus colloidal particles using different surfactants. Different microphase separation processes take place in two connected halves of the Janus particles. An order-order transition between gyroid and lamellar phases is observed in polymeric colloids. The epitaxial growth during the structural transformation from gyroid to lamellar phase undergoes a two-layered rearrangement to accommodate the interdomain spacing mismatch between these two phases. This self-assembly behavior can be ascribed to the preferential wetting of BCP chains at the interface, which can change the chain conformation of different blocks. The Janus colloidal particles can further experience a reversible phase transition by restructuring the polymer particles under solvent vapor. It is anticipated that the new phase behavior found in Janus particles can not only enrich the self-assembly study of BCPs but also provide opportunities for various applications based on Janus particles with ordered structures.

Bacterial nanocellulose membrane with opposite surface charges for large-scale and large-area osmotic energy harvesting and ion transport

How to optimize ion-exchange membrane materials has been the key for researchers recently working on the use of reverse electrodialysis to harvest osmotic energy. Based on the considerations of improving membrane performance and conversion to large-area industrial production, this work first proposes an easy-industrialized strategy to treat bacterial cellulose membranes by hot pressing and hot pressing with etherification modification, and then to obtain anion-selective and cation-selective membrane pairs (PBC-M and NBC-M) with opposite charges. The PBC-M obtained by multi-step treatment has excellent hydrophobicity, good surface charge density, and more favorable nanochannel size for the functioning of double layer. The maximum output power density of 44.1 mW m-2 was obtained in artificial river water and seawater simulated salinity gradient power generation. Applied to a larger test area, the power output of the system where a single membrane is located can reach 2.2 × 10-3 mW, which is ahead of similar experimental products. The two membranes prepared can also be used in combination, which provides a new idea for full cell design. It's important to open up a new route for optimizing nanofluidic channel design, regulating ion flux transport, and advancing the large-scale industrialization of biomass nanofluidic membrane RED system.

Paradigm shift in Nutrient-Energy-Water centered sustainable wastewater treatment system through synergy of bioelectrochemical system and anaerobic digestion

With advancements in research and the necessity of improving the performance of bioelectrochemical system (BES), coupling anaerobic digestion (AD) with BES is crucial for energy gain from wastewater and bioremediation. Hybridization of BES-AD concept opens new avenues for pollutant degradation, carbon capture and nutrient-resource recovery from wastewater. The strength of merging BES-AD lies in synergy, and this approach was employed to differentiate fads from strategies with the potential for full-scale implementation and making it an energy-positive system. The integration of BES and AD system increases the overall performance and complexity of combined system and the cost of operation. From a technical standpoint, the primary determinants of BES-AD feasibility for field applications are the scalability and economic viability. High potential market for such integrated system attract industrial partners for more industrial trials and investment before commercialization. However, BES-AD with high energy efficacy and negative economics demands performance boost.

Bioinspired light-driven chloride pump with helical porphyrin channels

Halorhodopsin, a light-driven chloride pump, utilizes photonic energy to drive chloride ions across biological membranes, regulating the ion balance and conveying biological information. In the light-driven chloride pump process, the chloride-binding chromophore (protonated Schiff base) is crucial, able to form the active center by absorbing light and triggering the transport cycle. Inspired by halorhodopsin, we demonstrate an artificial light-driven chloride pump using a helical porphyrin channel array with excellent photoactivity and specific chloride selectivity. The helical porphyrin channels are formed by a porphyrin-core star block copolymer, and the defects along the channels can be effectively repaired by doping a small number of porphyrins. The well-repaired porphyrin channel exhibits the light-driven Cl- migration against a 3-fold concentration gradient, showing the ion pumping behavior. The bio-inspired artificial light-driven chloride pump provides a prospect for designing bioinspired responsive ion channel systems and high-performance optogenetics.

A facile strategy for the preparation of carbon nanotubes/polybutadiene crosslinked composite membrane and its application in osmotic energy harvesting

The osmotic energy between riverine water and seawater can be converted into electricity by reverse electrodialysis (RED). However, the facile fabrication of advanced RED membranes with high energy conversion efficiencies, large areas, and excellent mechanical properties remains a challenge. Carbon nanotubes (CNTs) exhibit excellent conductivity and provide suitable channels for ion transport but cannot form membranes independently, which limits the related applications in osmotic energy conversion. Herein, a new organic-inorganic composite membrane is prepared by combining hydroxyl-terminated polybutadiene as a matrix and carbon nanotubes as transport nanochannels. The nanotubes are pre-subjected to plasma treatment to increase the surface charge density and transport capacity of the nanochannels, improving the ion selectivity and energy conversion efficiency. Under actual seawater/river water conditions, the developed membrane delivers a power density of ∼5.1 W/m2 and shows good mechanical strength (219 MPa). Our work provides a facile solution to the problem posed by the inability of ideal nanochannels to form membranes independently and paves the way for the application of RED membranes in osmotic energy conversion.

Confined amphipathic ionic-liquid regulated anodic aluminum oxide membranes with adjustable ion selectivity for improved osmotic energy conversion

To attain carbon neutrality and carbon peaking, there is an urgent need to convert the vast amount of blue energy present between seawater and river water into usable electricity. Reverse electrodialysis based on ion-exchange membranes is a promising way to efficiently achieve osmotic energy conversion. Anodic aluminum oxide (AAO) membranes are frequently used for osmotic energy harvesting because of their uniform nanopore channels, high flux, and excellent stability. However, the existing surface modification methods are complex and inefficient. In this study, an amphiphilic ionic liquid was selected to modify a porous anodic alumina membrane via simple capillary insertion. Due to the abundance of pH-dependent amphiphilic OH groups on the surface of AAO pore channels, the ionic liquids not only provide abundant surface charge but can also intelligently adjust its surface charge to different environments. In addition, it fills the AAO nanochannels to provide a continuous ion transport network. The modified hybrid membrane achieves efficient and stable osmotic energy conversion performance. This simple and feasible strategy paves the way for further improvements in commercial membranes.

Covalently Functionalized Nanopores for Highly Selective Separation of Monovalent Ions

Biological ion channels possess prominent ion transport performances attributed to their critical chemical groups across the continuous nanoscale filters. However, it is still a challenge to imitate these sophisticated performances in artificial nanoscale systems. Herein, this work develops the strategy to fabricate functionalized graphene nanopores in pioneer based on the synergistic regulation of the pore size and chemical properties of atomically thin confined structure through decoupling etching combined with in situ covalent modification. The modified graphene nanopores possess asymmetric ion transport behaviors and efficient monovalent metal ions sieving (K+ /Li+ selectivity ≈48.6). Meanwhile, it also allows preferential transport for cations, the resulting membranes exhibit a K+ /Cl- selectivity of 76 and a H+ /Cl- selectivity of 59.3. The synergistic effects of steric hindrance and electrostatic interactions imposing a higher energy barrier for Cl- or Li+ across nanopores lead to ultra-selective H+ or K+ transport. Further, the functionalized graphene nanopores generate a power density of 25.3 W m-2 and a conversion efficiency of 33.9%, showing potential application prospects in energy conversion. The theoretical studies quantitatively match well with the experimental results. The feasible preparation of functionalized graphene nanopores paves the way toward direct investigation on ion transport mechanism and advanced design in devices.

Artificial Sodium Channels for Enhanced Osmotic Energy Harvesting

Inspired by the ion channels of electric eels, we can use biomimetic nanofluidic materials to harvest the osmotic power released by mixing seawater and river water. While biological ion channels have both cation/anion and inter-cation selectivity, previous nanofluidic materials neglected the latter. As a result, NaCl solutions were generally used to simulate river water, ignoring the fact that the dominating cation in river water is typically Ca2+. In this work, we show that the different ionic compositions of seawater and river water can be exploited to improve osmotic power density by employing biomimetic sodium selective materials. Inspired by a range of properties of biological sodium channels, we constructed artificial sodium channels with zeolitic imidazolate framework-65 crystals, which selectively transport Na+ but almost completely block Ca2+. Resultantly, the effective concentration gradient of seawater/river water is dramatically increased by preventing the major cations in the river water from participating in the ion diffusion. As a result, the osmotic power density can be increased by more than 1 order of magnitude. These results should open new avenues to develop high-performance osmotic generators and may advance other applications based on biomimetic ion channels such as neuromorphic information processing.

High Strength MXene/PBONF Heterogeneous Membrane with Excellent Ion Selectivity for Efficient Osmotic Energy Conversion

Layered MXene nanofluidic membranes still face the problems of low mechanical property, poor ion selectivity, and low output power density. In this work, we successfully constructed heterostructured membranes with the combination of the layered channels of the MXene layer on the top and the nanoscale poly(p-phenylene-benzodioxazole) nanofiber (PBONF) layer on the bottom through a stepwise filtration method. The as-prepared MXene/PBONF-50 heterogeneous membrane exhibits high mechanical properties (strength of 221.6 MPa, strain of 3.2%), high ion selectivity of 0.87, and an excellent output power density of 15.7 W/m2 at 50-fold concentration gradient. Excitingly, the heterogeneous membrane presents a high power density of 6.8 W/m2 at a larger testing area of 0.79 mm2 and long-term stability. This heterogeneous membrane construction provides a viable strategy for the enhancement of mechanical properties and osmotic energy conversion of 2D materials.

Room-Temperature Synthesis of a COFs Membrane Via LBL Self-Assembly Strategy for Energy Harvesting

The covalent organic frameworks (COFs) membrane with ordered and confined one-dimensional channel has been considered as a promising material to harvest the salinity gradient energy from the seawater and river water. However, the application of the COFs in the field of energy conversion still faces the challenges in membrane preparation. Herein, energy harvesting is achieved by taking advantage of a COFs membrane where TpDB-HPAN is synthesized via layer-by-layer self-assembly strategy at room temperature. The carboxy-rich TpDB COFs can be expediently assembled onto the substrate with an environmental-friendly method. The increased open-circuit voltage (Voc ) endows TpDB-HPAN membrane with a remarkable energy harvesting performance. More importantly, the application perspective is also illuminated by the cascade system. With the advantages of green synthesis, the TpDB-HPAN membrane can be considered as a low-cost and promising candidate for energy conversion.

Meta-Aerogel Ion Motor for Nanofluid Osmotic Energy Harvesting

Osmotic power, also known as "blue energy", is a vast, sustainable, and clean energy source that can be directly converted into electricity by nanofluidic membranes. However, the key technological bottleneck for large-scale osmotic electricity is that macroscopic-scale bulky membrane cannot synergistically satisfy the demands of high power density and low resistance without sacrificing scalability and mechanical robustness. Here, inspired by the anatomy and working principle of electric eels, which harness osmotic energy through embedded neuron-mediated fibril nanochannels with nanoconfined transport dynamics. Fibrous nanofluidic meta-aerogel ion motors, 3D-assembled from nanofluidic cable fibers with actuatable stimulation/transport "ion highways" are engineered. The meta-aerogel exhibits the integrated coupling effect of boosted ion propulsion and surface-charge-dominated selective ion transport. Driven by osmosis, the meta-aerogel ion motor can produce an unprecedented output power density of up to 30.7 W m-2 under a 50-fold salinity gradient. Advancing ultra-selective ion transport in nanofluidic meta-aerogels may provide a promising roadmap for blue energy harvesting.

Ultra-mechanosensitive Chloride Ion Transport through Bioinspired High-Density Elastomeric Nanochannels

Mechanosensitive ion channels play crucial roles in physiological activities, where small mechanical stimuli induce the membrane tension, trigger the ion channels' deformation, and are further transformed into significant electrochemical signals. Artificial ion channels with stiff moduli have been developed to mimic mechanosensory behaviors, exhibiting an electrochemical response by the high-pressure-induced flow. However, fabricating flexible mechanosensitive channels capable of regulating specific ion transporting upon dramatic deformation has remained a challenge. Here, we demonstrate bioinspired high-density elastomeric channels self-assembled by polyisoprene-b-poly4-vinylpyridine, which exhibit ultra-mechanosensitive chloride ion transport resulting from nanochannel deformation. The PI-formed continuous elastic matrix can transmit external forces into internal tensions, while P4VP forms transmembrane chloride channels that undergo dramatic deformation and respond to mechanical stimuli. The integrated and flexible chloride channels present a dramatic and stable electrochemical signal toward a low pressure of 0.2 mbar. This research first demonstrates the artificial mechanosensory chloride channels, which could provide a promising avenue for designing flexible and responsive channel systems.

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.

In-depth understanding of boosting salinity gradient power generation by ionic diode

Ionic diodes constructed with asymmetric channel geometry and/or charge layout have shown outstanding performance in ion transport manipulation and reverse electrodialysis (RED) energy collection, but the working mechanism is still indistinct. Herein, we systematically investigated RED energy conversion of straight nanochannel-based bipolar ionic diode by coupling the Poisson-Nernst-Planck and Navier-Strokes equations. The effects of nanochannel structure, charging polarity, and symmetricity as well as properties of working fluids on the output voltage and output power were investigated. The results show that as high-concentration feeding solution is applied, the bipolar ionic diode-based RED system gives higher output voltage and output power compared to the unipolar channel RED system. Under optimal conditions, the voltage output of the bipolar channel is increased by ∼100% and the power output is increased by ∼260%. This work opens a new route for the design and optimization of high-performance salinity energy harvester as well as for water desalination.

Electrodeposited MOFs Membrane with In Situ Incorporation of Charged Molecules for Osmotic Energy Harvesting

Ion-selective membranes are considered as the promising candidates for osmotic energy harvesting. However, the fabrication of highly perm-selective membrane is the major challenge. Metal-organic frameworks (MOFs) with well-defined nanochannels along functional charged groups show great importance to tackle this problem. Here, a series of dense sodium polystyrene sulfonate (PSS) incorporated MOFs composite membranes (PSS@MOFs) on a porous anodic aluminum oxide (AAO) membrane via in situ anodic electrodeposition process are developed. Benefiting to the novel structural design of the confined Ag layer, PSS@MOFs dense composite membrane with less defects formed. The sulfonated nanochannels of the PSS@MOFs composite membrane provided rapid and selective transport of cations due to the enhanced electrostatic interaction between the permeating ions and MOFs. While osmotic energy conversion, 860 nm thick negatively charged PSS@MOFs composite membrane achieves an ultrahigh cation transfer number of 0.993 and energy conversion efficiency of 48.8% at a 100-fold salinity gradient. Moreover, a large output power of 2.90 µW has been achieved with an ultra-low internal resistance of 999 Ω, employing an effective area of 12.56 mm² . This work presents a promising strategy to construct a high-performance MOFs-based osmotic energy harvesting system for practical applications.

Fluidics for energy harvesting: from nano to milli scales

A large amount of untapped energy sources surrounds us. In this review, we summarize recent works of water-based energy harvesting systems with operation scales ranging from miniature systems to large scale attempts. We focus particularly on the triboelectric energy, which is produced when a liquid and a solid come into contact, and on the osmotic energy, which is released when salt water and fresh water are mixed. For both techniques we display the state of the art understanding (including electrical charge separation, electro-osmotic currents and induced currents) and the developed devices. A critical discussion of present works confirms the significant progress of these water-based energy harvesting systems in all scales. However, further efforts in efficiency and performance amelioration are expected for these technologies to accelerate the industrialization and commercialization procedure.

Enhancing the osmotic energy conversion of a nanoporous membrane: influence of pore density, pH, and temperature

Salinity gradient power, which converts Gibbs free energy of mixing to electric energy through an ion-selective pore, has great potential. Towards practical use, developing membrane-scaled nanoporous materials is desirable and necessary. Unfortunately, the presence of a significant ion concentration polarization (ICP) lowers appreciably the power harvested, especially at a high pore density. To alleviate this problem, we suggest applying an extra pressure difference ΔP across a membrane containing multiple nanopores, taking account of the associated power consumption. The results gathered reveal that the application of a negative pressure difference can improve the power harvested due to the enhanced selectivity. In addition, if the pore density of a membrane is high, raising its pore length is necessary to make the energy harvested economic. For example, if the pore length is 2000 nm and the pore density is 2.5 × 10⁹ pores per cm², an increment in the power density of 213 mW m^-2 can be obtained by applying ΔP = -1 bar at pH 11 and 323 K, where a net positive power density can be retrieved. The performance of the system considered under various conditions is examined in detail, along with associated mechanisms.

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.

Well-Defined Nanostructures by Block Copolymers and Mass Transport Applications in Energy Conversion

With the speedy progress in the research of nanomaterials, self-assembly technology has captured the high-profile interest of researchers because of its simplicity and ease of spontaneous formation of a stable ordered aggregation system. The self-assembly of block copolymers can be precisely regulated at the nanoscale to overcome the physical limits of conventional processing techniques. This bottom-up assembly strategy is simple, easy to control, and associated with high density and high order, which is of great significance for mass transportation through membrane materials. In this review, to investigate the regulation of block copolymer self-assembly structures, we systematically explored the factors that affect the self-assembly nanostructure. After discussing the formation of nanostructures of diverse block copolymers, this review highlights block copolymer-based mass transport membranes, which play the role of “energy enhancers” in concentration cells, fuel cells, and rechargeable batteries. We firmly believe that the introduction of block copolymers can facilitate the novel energy conversion to an entirely new plateau, and the research can inform a new generation of block copolymers for more promotion and improvement in new energy applications.

Essence of the Enhanced Osmotic Energy Conversion in a Covalent Organic Framework Monolayer

Low membrane conductivity originated from a high membrane thickness has long been the "Achilles heel" of the conventional polymeric membrane, greatly hampering the improvement of the output power density in osmotic power generation. Herein, we demonstrate a molecularly-thin two-dimensional (2D) covalent organic framework (COF) monolayer membrane, featured with ultimate thickness, high pore density, and tight pore size distribution, which performs as a highly efficient osmotic power generator. Despite the large pore size up to 3.8 nm and relatively low surface charge density of 2.2 mC m^-2, the monolayer COF membrane exhibits a high osmotic current density of 16.7 kA m^-2 and an output power density of 102 W m^-2 under 50 times the NaCl salinity gradient (0.5 M/0.01 M). This superior power density could be further improved to 170 W m^-2 in the real seawater/river water gradient system. When the large pore size and low surface charge density are considered, this superior performance is not expected. Computational studies further reveal that the ultimate membrane permeability originated from the high membrane porosity, rather than ion selectivity, plays a dominant role in the production of high current density, especially under high salinity. This work provides an alternative strategy to realize improved output power density in ultrapermeable membranes.