Metallic Two-Dimensional MoS2 Composites as High-Performance Osmotic Energy Conversion Membranes

Interfacial Mesochannels as Cation Pump for Enhanced Osmotic Energy Harvesting

Membranes integrating 1D materials are rapidly emerging as highly promising platforms for osmotic energy harvesting. However, their power output is often constrained by insufficient ion selectivity. Herein, we demonstrate a cation pumping strategy by designing mesoporous silica coated multiwalled carbon nanotubes/aramid nanofiber (MCNTs@mSiO2/ANF) composite membranes as osmotic power generators. Cations can be initially enriched in the negatively charged and small-pore-sized (∼ 3 nm) interfacial mesopore channels, establishing a strong cation concentration gradient toward the interfiber nanochannels. The gradient continuously drives cations into the interfiber pores, facilitating charge separation, and improving ion selectivity. Additionally, the hydrophilic nature of the mesoporous silica shells promotes ion transport and contributes to high ion flux. Consequently, the fabricated MCNTs@mSiO2/ANF composite nanochannel membranes can deliver a notable power density of 8.24 W m-2 with an excellent ion selectivity of 0.91 under a 50-fold NaCl salinity gradient. Importantly, the membranes demonstrate long-term stability for osmotic energy capturing. When placed between natural seawater and river water, the composite membranes yield an impressive power density of 9.93 W m-2, surpassing that of the state-of-the-art 1D material-based membranes. This work paves the way for the practical applications of nanofiber-based membranes in sustainable osmotic energy conversion.

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.

Super-Assembled Lamellar Conductive Heterochannels with Optical-Electrical Coupling Sensitivity for Smart Ion Transport

Artificial nanofluidic devices inspired bylight-driven ion transport in biological systems, leveraging the photoelectric effect, have attracted extensive attention for their potential in signal transduction and smart ion transport applications. However, effective separation of photogenerated carriers in traditional p-n junction interface can be hindered by energy band structure of different semiconductor materials. Here, we present a novel approach using conductive polypyrrole (PPy) to modify graphene oxide (GO), creating polypyrrole-graphene oxide (PyGO) functional lamellar conductive nanochannels with tailored channel-sized gradients and inherent optical-electrical coupling sensitivity via a facile super-assembly strategy. This design facilitates the PyGO own conductive lamellar channels and efficient separation of photogenerated carriers, resulting in significantly enhanced selective ion transport behavior. Coupling the conductivity and photosensitivity of PPy contributes to a peak power density of 14.1 W m-2 under a salinity differential of 0.5/0.01 M NaCl, which is 35.6 % higher than that under dark conditions. Additionally, combing the salinity gradients with optical-electrical coupling sensitivity of the nanofludic devices, we demonstrate the application of PyGO in a real-time detection device for monitoring ion concentrations in nutrient solutions, paving the way for smart irrigation systems in agriculture. This work presents a novel and effective strategy for light-driven ion transport with potential applications in energy conversion and beyond.

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.

One-Step Mechanochemical Synthesis of Bulk-Sized Electroactive MoS2/Oxalic Acid Dihydrate Composite for Catalytic Nitrophenol Reductions

Nitrophenols are used in fabricating explosives and reported to be carcinogenic in nature, dictating a necessity for their efficient and eco-friendly reduction. The majority of such reductions involve multistep protocols that use expensive reagents. Herein, a one-step greener approach has been developed to mechanochemically prepare a cost-effective molybdenum disulfide (MoS2) and oxalic acid dihydrate-based composite to catalytically reduce nitrophenols into their corresponding amine in aqueous medium. The catalyst works in situ by virtue of the electroactivation of the morphologically rough MoS2 surface, which generates nascent active sites. The active sites are composed of paramagnetic Mo5+ centers generated during the mechanical grinding process. The peritectoidically transformed oxalic acid dihydrate, which is physisorbed on the MoS2 surface during the catalyst's preparation process, gets released in aqueous medium, lowering the pH and accelerating the hydrolysis of BH4 -. The BH4 - quickly interacts with the nascent active sites, propagating the reduction at a faster rate. We establish an Eley-Rideal mechanism that is responsible for a remarkably high rate constant. These findings are based on a thorough analysis using UV-visible spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, Raman and electron paramagnetic resonance spectroscopies, high-resolution mass spectrometry, along with first-principles quantum mechanical solid-state calculations.

Quantitative Skeletal-Charge Engineering of Anion-Selective COF Membrane for Ultrahigh Osmotic Power Output

Osmotic energy contained in water bodies can generate abundant renewable electricity through reverse electrodialysis (RED) that relies on ion permselective membranes. Anion-permselective membrane RED offers a stable and sustainable energy output potential by maintaining a consistent driving force across the membrane, providing advantages in sustainability and versatility over cation-selective membranes. But significant challenges persist in developing anion-selective membranes that feature high selectivity and low impedance. Herein, this study presents the development of an anion-permselective osmotic power generation system using a free-standing chloride-selective covalent organic framework (COF) membrane. Inspired by biological chloride channels, the membrane is engineered with smooth, straight, and highly charged nanochannels for rapid chloride-anion transport. Its inner structure is stoichiometrically controlled to atomically distribute positive charges on the COF intraplane rings without introducing heterometal atoms or branch groups, enabling selective and efficient single-directional movement of anions. The RED device with this ionic-COF membrane achieves a remarkable output power density of 239.6 W m-2, outperforming commercial benchmarks by 2 orders of magnitude, with low intermediate resistance under demanding gradients. Theoretical simulations corroborate that anion transport within ionic-COF membranes is governed by electrostatic interactions with the charged skeletons, thereby enhancing the anion selectivity and permeability. The findings highlight the potential of ionic-COF membranes for high-efficiency osmotic energy capture, demonstrating a substantial step toward sustainable and stable energy output from salinity gradients.

Light-Boosted Osmotic Energy Conversion and Ion Pumping through a Graphdiyne Oxide-Based Membrane

Two-dimensional (2D) membranes with engineered light-responsive ion transport dynamics have been explored to construct efficient nanofluidic platforms that show great potential in osmotic and solar-osmotic energy conversion. However, the power density is still limited by poor responsivity and the inevitable trade-off effect between ion selectivity and flux. Here, we observed light-pumping ion transport behavior in graphdiyne oxide (GDYO) with a unique carbon hybrid skeleton that provides sensitive photoelectric responsivity and high-speed cation pathways. Molecular dynamics simulations verify that the coexistent interaction effects between cations and the negatively charged sites in GDYO (i.e., oxygen-containing groups and electron-rich acetylenic bonds) could significantly promote cation transmembrane transport via an absorption-acceleration mechanism. Furthermore, the GDYO-based system, possessing a coupled photon-electron-ion transport behavior due to its inherent semiconductor properties, could subtly realize unidirectional ion movement, consuming luminous energy either from low concentration to high concentration or vice versa, flexibly promoting the osmotic power density by ∼195% to 11.91 W m-2.

Plasmonic Ion Diode Membrane (PIDM) for Enhanced Nanofluidic Ion Transport

Efficient applications of nanofluidic devices are often limited by the insufficient ion permselectivity and inherent ion concentration polarization (ICP) phenomenon. In this work, a bio-inspired plasmonic ion diode membrane (PIDM) was designed and fabricated for enhanced ion transport and osmotic energy harvesting by integrating covalent organic frameworks (COFs) and three-dimensional Au nanoparticles (3D AuNPs) into anode aluminum oxide (AAO). Under light irradiation, localized surface plasmon resonance (LSPR) excitation of 3D AuNPs can release huge plasmonic heat and produce abundant hot charge carriers (hot electrons and holes) simultaneously. The former heats the solution and generates a thermal gradient for boosting ion flux, while the latter transfers to the COFs layer, increasing charge density for promoting ion permselectivity. Importantly, it has been found that different COFs with varied pore sizes and charges have an obvious influence on energy harvesting efficiency. Under the optimum condition, a high output power density of 65.7 W m-2 in a 500-fold concentration gradient could be achieved. This work provides a practical and efficient way to boost ion transport and enhance osmotic energy conversion by utilizing the synergistic effect of plasmonics and ion diode (ID) property.

Vacancy engineering in tungsten oxide nanofluidic membranes for high-efficiency light-driven ion transport

Bioinspired light-driven ion transport has shown great potential in solar energy harvesting. To achieve efficiencies comparable to biological counterparts, effective coregulation of permselectivity and photoresponsivity is crucial. Herein, vacancy engineering has been proven to be a powerful strategy for considerably increasing the efficiency of light-driven ion transport in tungsten oxide (WO3-x) nanofluidic membranes by enhancing the negative surface charges and narrowing bandgaps. The enhancement in light-driven ion transport can be attributed to the efficient redistribution of surface charges due to the effective separation of photogenerated carriers. At an optimized vacancy concentration, WO2.66 membrane (WO2.66M) delivers an ionic photocurrent of 0.8 μA cm-2 in a 10-4 M KCl electrolyte, which is four times higher than that generated by the original WO2.85 membrane (WO2.85M). Following this strategy, uphill ion transport and photoenhanced osmotic energy conversion are successfully achieved in the WO3-x nanofluidic membrane system. This study shows that atomic vacancy engineering is an efficient approach to increase the light-driven ion transport dynamics of nanofluidics, providing an efficient strategy to enhance light-driven ion transport for potential applications in power harvesting and ion separation.

Engineered sulfonated porous carbon/cellulose nanofiber hybrid membrane for high-efficiency osmotic energy conversion applications

Harnessing ionic gradients to generate electricity has inspired the development of nanofluidic membranes with charged nanochannels for osmotic energy conversion. However, achieving high-performance osmotic energy output remains elusive due to the trade-off between ion selectivity and nanochannel membrane permeability. In this study, we report a homogeneous nanofluidic membrane, composed of sulfonated nanoporous carbon (SPC) and TEMPO-oxidized cellulose nanofibers (T-CNF), engineered to overcome these limitations. Incorporation of carboxyl groups on the T-CNF surface and the sulfonic acid groups within SPC confers high cation selectivity, reaching up to 0.88, and enhances high energy conversion efficiency to 38.3 %. Furthermore, the SPC component forms three-dimensional interconnected nanopore channels that serve as extensive ion transport pathways, allowing the hybrid membranes to exhibit high transmembrane ion flux. This structural design enhances ion conductivity, reaching up to 0.8 S/cm at low KCl concentrations (≤0.01 M). With their high ion selectivity and rapid ion transport capabilities, SPC/T-CNF hybrid membranes achieve high-performance osmotic energy conversion, delivering an output power density of 1.08 W/m2 under conditions simulating the interface of seawater and river water, while maintaining stability over 25 days. This economical and easily fabricated porous nanofluidic hybrid membrane featuring nano-porous carbon paves the way for advanced energy-harvesting devices.

Linear Enhanced 3D Nanofluid Force-Electric Conversion Device

The inherent trade-off between permeability and selectivity has constrained further improvement of passive linear force-electric conversion performance in nanofluidic pressure sensors. To overcome this limitation, a 3D nanofluidic membrane with high mechanical strength utilizing aramid nanofibers/carbon nanofiber (ANF/CNF) dual crosslinking is developed. Due to the abundant surface functional groups of CNF and the high mechanical strength of ANF, this large-scale integrated 3D nanofluidic membrane exhibits advantages of high flux, high porosity, and short ion transport path, demonstrating superior force-electric response compared to conventional 1D and 2D configurations. The enhancement mechanism of the ANF/CNF membrane is systematically investigated through experimental results and theoretical calculations. The optimized device has a sensitivity of 111 nA cm-2 kPa-1, a response/recovery time of 63/68 ms, and a stability of 45 000 cycles. This study successfully overcomes the inherent performance limitations of traditional nanofluidic membranes, offering promising potential for applications across artificial intelligence, the Internet of Things, and smart wearable devices.

Bioinspired solid-state nanochannels for molecular analysis

The acute sensory behaviour in living organisms relies on the highly efficient transport behaviour of ions in biological nanochannels, which has inspired the design and applications of artificial solid-state nanochannels in the field of sensitive analysis. The application of nanochannels for analysis is now widely investigated, and a variety of sensors have been developed. By coupling reliable nanochannel fabrication techniques with a multitude of surface modification strategies, novel sensors with customized sensing capabilities are generated by the integration of recognition elements and nanochannels. The altered physicochemical properties of these solid-state nanochannel sensors will be manifested by steady-state currents when they are affected by the target analyte. In this mini-review, we focus on emerging solid-state nanochannels based on different fabrication processes such as electron-beam etching, anodic oxidation, ion track etching, and self-assembly. Also, modifications of recognition elements are discussed, including nucleic acids, proteins, small molecules, and responsive materials. The key factors of ion transport behaviour during detection are also reviewed, including surface charge, channel size, and wettability. The applications of these bioinspired nanochannels in the exploration of analysis of small molecules (gas molecules, drug molecules and biological molecules) are concisely presented. Furthermore, we discuss the future developments and challenges.

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.

Scalable integration of photoresponsive highly aligned nanochannels for self-powered ionic devices

Artificial ionic nanochannels with light perception capabilities hold promise for creating ionic devices. Nevertheless, most research primarily focuses on regulating single nanochannels, leaving the cumulative effect of numerous nanochannels and their integration underexplored. We herein develop a biomimetic photoreceptor based on photoresponsive highly aligned nanochannels (pHANCs), which exhibit uniform channel heights, phototunable surface properties, and excellent compatibility with microfabrication techniques, enabling the scalable fabrication and integration into functional ionic devices. These pHANCs demonstrate exceptional ion selectivity and permeability due to the high surface charges and well-ordered conduits, resulting in outstanding energy harvesting from concentration gradients. Large-scale fabrication of pHANCs has been successfully realized, wherein hundreds of biomimetic photoreceptors produce an ultrahigh voltage over 76 volts, which has not been achieved previously. In addition, we demonstrate that the biomimetic photoreceptor can be further upscaled to be a self-powered ionic image sensor, capable of sensing and decoding incident light information.

Chlorophyll-Inspired Magnesium Porphyrin Array Membrane for Vis-Light-Enhanced Osmotic Energy Conversion

Osmotic energy, a renewable clean energy, can be directly converted into electricity through ion-selective membranes. Inspired by the magnesium porphyrin (MgP) in plant chlorophyll, which absorbs vis-light and promotes photoelectric conversion, we demonstrate a MgP array membrane, realizing vis-light-enhanced ion transport regulation ability and osmotic energy conversion. The MgP arrays are self-assembled by a MgP-cored block copolymer under the coordination effect of block copolymer self-assembly and MgP π-π stacking, providing chloride-selective transport channels. Due to the unique photochemical properties of MgP, the chloride ion transport conductance and selectivity can be simultaneously increased under visible-light irradiation, benefiting the osmotic energy conversion. Specifically, the maximum power density increases from 26.7 to 34.5 W·m-2 after visible-light illumination, representing approximately a 30% increase. The construction of MgP arrays realizes photofacilitated osmotic energy conversion, providing an idea for designing an efficient photoelectric conversion system.

Fabrication of Porous MXene/Cellulose Nanofibers Composite Membrane for Maximum Osmotic Energy Harvesting

Two-dimensional (2D) nanofluidic channels are emerging as potential candidates for harnessing osmotic energy from salinity gradients. However, conventional 2D nanofluidic membranes suffer from high transport resistance and low ion selectivity, leading to inefficient transport dynamics and limiting energy conversion performance. In this study, we present a novel composite membrane consisting of porous MXene (PMXene) nanosheets featuring etched nanopores, in conjunction with cellulose nanofibers (CNF), yielding enhancement in ion flux and ion selectivity. A mild H2O2 oxidant is employed to etch and perforate the MXene sheets to create a robust network of cation transportation nanochannels that effectively reduces the energy barrier for cation transport. Additionally, CNF with a unique nanosize and high charge density further enhances the charge density and mechanical stability of the nanofluidic system. Under neutral pH and room temperature, the PMXene/CNF membrane demonstrates a maximum output power density of 0.95 W·m-2 at a 50-fold KCl gradient. Notably, this represents a 43% improvement over the performance of the pristine MXene/CNF membrane. Moreover, 36 nanofluidic devices connected in series are demonstrated to achieve a stable voltage output of 5.27 V and power a calculator successfully. This work holds great promise for achieving sustainable energy harvesting with efficient osmotic energy conversion utilization.

Molybdenum Disulfide-Based Catalysts in Organic Synthesis: State of the Art, Open Issues, and Future Perspectives

In the field of heterogeneous organic catalysis, molybdenum disulfide (MoS2) is gaining increasing attention as a catalytically active material due to its low toxicity, earth abundance, and affordability. Interestingly, the catalytic properties of this metal-based material can be improved by several strategies. In this Perspective, through the analysis of some explicative examples, the main approaches used to prepare highly efficient MoS2-based catalysts in relevant organic reactions are summarized and critically discussed, namely: i) increment of the specific surface area, ii) generation of the metallic 1T phase, iii) introduction of vacancies, iv) preparation of nanostructured hybrids/composites, v) doping with transition metal ions, and vi) partial oxidation of MoS2. Finally, emerging trends in MoS2-based materials catalysis leading to a richer organic synthesis are presented.

Turing-type nanochannel membranes with extrinsic ion transport pathways for high-efficiency osmotic energy harvesting

Two-dimensional (2D) nanofluidic channels with confined transport pathways and abundant surface functional groups have been extensively investigated to achieve osmotic energy harvesting. However, solely relying on intrinsic interlayer channels results in insufficient permeability, thereby limiting the output power densities, which poses a significant challenge to the widespread application of these materials. Herein, we present a nanoconfined sacrificial template (NST) strategy to create a crafted channel structure, termed as Turing-type nanochannels, within the membrane. Extrinsic interlaced channels are formed between the lamellae using copper hydroxide nanowires as sacrificial templates. These Turing-type nanochannels significantly increase transport pathways and functional areas, resulting in a 23% enhancement in ionic current while maintaining a cation selectivity of 0.91. The output power density of the Turing-type nanochannel membrane increases from 3.9 to 5.9 W m-2 and remains stable for at least 120 hours. This membrane exhibits enhanced applicability in real saltwater environments across China, achieving output power densities of 7.7 W m-2 in natural seawater and 9.8 W m-2 in salt-lake brine. This work demonstrates the promising potential of the Turing-channel design for nanoconfined ionic transport in the energy conversion field.

Bioinspired Homonuclear Diatomic Iron Active Site Regulation for Efficient Antifouling Osmotic Energy Conversion

Membrane-based reverse electrodialysis is globally recognized as a promising technology for harnessing osmotic energy. However, its practical application is greatly restricted by the poor anti-fouling ability of existing membrane materials. Inspired by the structural and functional models of natural cytochrome c oxidases (CcO), the first use of atomically precise homonuclear diatomic iron composites as high-performance osmotic energy conversion membranes with excellent anti-fouling ability is demonstrated. Through rational tuning of the atomic configuration of the diatomic iron sites, the oxidase-like activity can be precisely tailored, leading to the augmentation of ion throughput and anti-fouling capacity. Composite membranes featuring direct Fe-Fe motif configurations embedded within cellulose nanofibers (CNF/Fe-DACs-P) surpass state-of-the-art CNF-based membranes with power densities of ca. 6.7 W m-2 and a 44.5-fold enhancement in antimicrobial performance. Combined, experimental characterization and density functional theory simulations reveal that homonuclear diatomic iron sites with metal-metal interactions can achieve ideally balanced adsorption and desorption of intermediates, thus realizing superior oxidase-like activity, enhanced ionic flux, and excellent antibacterial activity.

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.

High-performance osmotic energy harvesting enabled by the synergism of space and surface charge in two-dimensional nanofluidic membranes

As promising prospects for renewable power harvesting, two-dimensional (2D) nanochannels for osmotic energy capture in a reverse electrodialysis arrangement have garnered significant attention. However, existing 2D nanochannel membranes have shown limited power generation capabilities due to challenges in balancing ion flux and selectivity. Here, we construct montmorillonite (MMT)/TEMPO-mediated oxidation cellulose nanofibers (TOCNFs) nanocomposite membranes for enhanced ion transmembrane transport. The intercalation of TOCNFs not only enlarges the interlayer distance, but also provides abundant space charge inside the nanochannels. Benefiting from the strong ion selectivity and high ion flux, the composite membrane achieves a remarkable power output of ∼16.57 W/m2 in the gradient of artificial seawater and river water, exceeding that of the state-of-the-art heterogeneous membrane-based osmotic energy conversion systems. Both experimental and theoretical findings confirm that the synergism of space and surface charge plays a crucial role in promoting osmotic energy conversion. This research contributes valuable insights into the optimization of 2D membranes for efficient clean energy harvesting purposes.

Large-Area Layered Membranes with Precisely Controlled Nano-Confined Channels

Two-dimensional (2D) nanosheets-based membranes, which have controlled 2D nano-confined channels, are highly desirable for molecular/ionic sieving and confined reactions. However, it is still difficult to develop an efficient method to prepare large-area membranes with high stability, high orientation, and accurately adjustable interlayer spacing. Here, we present a strategy to produce metal ion cross-linked membranes with precisely controlled 2D nano-confined channels and high stability in different solutions using superspreading shear-flow-induced assembly strategy. For example, membranes based on graphene oxide (GO) exhibit interlayer spacing ranging from 8.0±0.1 Å to 10.3±0.2 Å, with a precision of down to 1 Å. At the same time, the value of the orientation order parameter (f) of GO membranes is up to 0.95 and GO membranes exhibit superb stability in different solutions. The strategy we present, which can be generalized to the preparation of 2D nano-confined channels based on a variety of 2D materials, will expand the application scope and provide better performances of 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%.

Massively Enhanced Charge Selectivity, Ion Transport, and Osmotic Energy Conversion by Antiswelling Nanoconfined Hydrogels

Developing a nanofluidic membrane with simultaneously enhanced ion selectivity and permeability for high-performance osmotic energy conversion has largely been unexplored. Here, we tackle this issue by the confinement of highly space-charged hydrogels within an orderedly aligned nanochannel array membrane. The nanoconfinement effect endows the hydrogel-based membrane with excellent antiswelling property. Furthermore, experimental and simulation results demonstrate that such a nanoconfined hydrogel membrane exhibits massively enhanced cation selectivity and ion transport properties. Consequently, an amazingly high power density up to ∼52.1 W/m2 with an unprecedented energy conversion efficiency of 37.5% can be reached by mixing simulated salt-lake water (5 M NaCl) and river water (0.01 M NaCl). Both efficiency indexes surpass those of most of the state-of-the-art nanofluidic membranes. This work offers insights into the design of highly ion-selective membranes to achieve ultrafast ion transport and high-performance osmotic energy harvesting.

Impact of Atomic Defects on Water Contact Angle of 2D Molybdenum Disulfide Surfaces

Interfacial dynamics within nanofluidic systems are crucial for applications like water desalination and osmotic energy harvesting. Understanding these dynamics can inform the rational optimization of two-dimensional (2D) materials and devices for such applications. This study explores the wetting behavior of realistic 2D MoS2 surfaces incorporating vacancies and atomic steps, known as atomic defects. We employ a combined density functional theory (DFT) and molecular dynamics (MD) computational approach to elucidate the influence of atomic defects on the MoS2-water interface. DFT calculations are utilized to determine the charge distribution within MoS2. Subsequently, free energy calculations are obtained through MD simulations of the MoS2-water interface. Our findings underscore the importance of incorporating atomic defects into MoS2 surfaces for accurate water contact angle (WCA) predictions in nanofluidic simulations, particularly when using Abal et al. force field parameters. However, the force field developed by Liu et al. yielded more accurate results for pristine MoS2 surfaces. While these parameters provide reliable outcomes for pristine MoS2 surfaces, their application to surfaces with defects may lead to underestimation of WCA. This highlights the critical need for realistic surface representations in nanofluidic modeling to accurately capture the complex interactions between water and MoS2 materials.

On-Water Surface Synthesis of Two-Dimensional Polymer Membranes for Sustainable Energy Devices

ConspectusIon-selective membranes are key components for sustainable energy devices, including osmotic power generators, electrolyzers, fuel cells, and batteries. These membranes facilitate the flow of desired ions (permeability) while efficiently blocking unwanted ions (selectivity), which forms the basis for energy conversion and storage technologies. To improve the performance of energy devices, the pursuit of high-quality membranes has garnered substantial interest, which has led to the exploration of numerous candidates, such as polymeric membranes (e.g., polyamide and polyelectrolyte), laminar membranes (e.g., transition metal carbide (MXene) and graphene oxide (GO)) and nanoporous 2D membranes (e.g., single-layer MoS2 and porous graphene). Despite impressive progress, the trade-off effect between ion permeability and selectivity remains a major scientific and technological challenge for these membranes, impeding the efficiency and stability of the resulting energy devices.Two-dimensional polymers (2DPs), which represent monolayer to few-layer covalent organic frameworks (COFs) with periodicity in two directions, have emerged as a new candidate for ion-selective membranes. The crystalline 2DP membranes (2DPMs) are typically fabricated either by bulk crystal exfoliation followed by filtration or by direct interfacial synthesis. Recently, the development of surfactant-monolayer-assisted interfacial synthesis (SMAIS) method by our group has been pivotal, enabling the synthesis of various highly crystalline and large-area 2DPMs with tunable thicknesses (1 to 100 nm) and large crystalline domain sizes (up to 120 μm2). Compared to other membranes, 2DPMs exhibit well-defined one-dimensional (1D) channels, customizable surface charge, ultrahigh porosity, and ultrathin thickness, enabling them to overcome the permeability-selectivity trade-off challenge. Leveraging these attributes, 2DPMs have established their critical roles in diverse energy devices, including osmotic power generators and metal ion batteries, opening the door for next-generation technology aimed at sustainability with a low carbon footprint.In this Account, we review our achievements in synthesizing 2DPMs through the SMAIS method and highlight their selective-ion-transport properties and applications in sustainable energy devices. We initially provide an overview of the SMAIS method for producing highly crystalline 2DPMs by utilizing the programmable assembly and enhanced reactivity/selectivity on the water surface. Subsequently, we discuss the critical structural parameters of 2DPMs, including pore sizes, charged sites, crystallinity, and thickness, to elucidate their roles in selective ion transport. Furthermore, we present the burgeoning landscape of energy device applications for 2DPMs, including their use in osmotic power generators and as electrode coating in metal ion batteries. Finally, we conclude persistent challenges and future prospects encountered in synthetic chemistry, material science, and energy device applications within this rapidly evolving field.

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.

Optimizing Nanofluidic Energy Harvesting in Synthetic Clay-based Membranes by Annealing Treatment

Nanofluidic energy harvesting from salinity gradients is studied in 2D nanomaterials-based membranes with promising performance as high ion selectivity and fast ion transport. In addition, moving forward to scalable, feasible systems requires environmentally friendly materials to make the application sustainable. Clay-based membranes are attractive for being environmentally friendly, non-hazardous, and easy to manipulate materials. However, achieving underwater stability for clay-based membranes remains challenging. In this work, the synthetic clay Laponite is used to prepare clay-based membranes with high stability and excellent performance for osmotic energy harvesting. The Laponite membranes (Lap-membranes) are stabilized by low-temperature annealing treatment to effectively reduce the interlayer space, achieving a continuous operation under salinity gradients. Furthermore, the Lap-membranes conserve integrity while soaking in water for more than one month. The output power density improves from ≈4.97 W m-2 on the pristine membrane to ≈9.89 W m-2 in the membrane treated 12 h at 300 °C from a 30-fold concentration gradient. Especially, It is found that the presence of interlayer water to be favorable for ion transport. Different mechanisms are proposed in the Lap-membranes involved for efficient ion selectivity and the states found with varying annealing temperatures. This work demonstrates the potential application of Laponite based nanomaterials for nanofluidic energy harvesting.

Tunable Surface Charge of Layered Double Hydroxide Membranes Enabling Osmotic Energy Harvesting from Anion Transport

Membrane-based osmotic energy harvesting is a promising technology with zero carbon footprint. High-performance ion-selective membranes (ISMs) are the core components in such applications. Recent advancement in 2D nanomaterials opens new avenues for building highly efficient ISMs. However, the majority of the explored 2D nanomaterials have a negative surface charge, which selectively enhances cation transport, resulting in the underutilization of half of the available ions. In this study, ISMs based on layered double hydroxide (LDH) with tunable positive surface charge are studied. The membranes preferentially facilitate anion transport with high selectivity. Osmotic energy harvesting device based on these membranes reached a power density of 2.31 W m-2 under simulated river/sea water, about eight times versus that of a commercial membrane tested under the same conditions, and up to 7.05 W m-2 under elevated temperature and simulated brine/sea water, and long-term stability with consistent performance over a 40-day period. A prototype reverse electrodialysis energy harvesting device, comprising a pair of LDH membranes and commercial cation-selective membranes, is able to simultaneously harvest energy from both cations and anions achieving a power density of 6.38 W m-2 in simulated river/sea water, demonstrating its potential as building blocks for future energy harvesting systems.

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.

Functionalized 2D membranes for separations at the 1-nm scale

The ongoing evolution of two-dimensional (2D) material-based membranes has prompted the realization of mass separations at the 1-nm scale due to their well-defined selective nano- and subnanochannels. Strategic membrane functionalization is further found to be key to augmenting channel accuracy and efficiency in distinguishing ions, gases and molecules within this range and is thus trending as a research focus in energy-, resource-, environment- and pharmaceutical-related applications. In this review, we present the fundamentals underpinning functionalized 2D membranes in various separations, elucidating the critical "method-interaction-property" relationship. Starting with an introduction to various functionalization strategies, we focus our discussion on functionalization-induced channel-species interactions and reveal how they shape the transport- and operation-related features of the membrane in different scenarios. We also highlight the limitations and challenges of current functionalized 2D membranes and outline the necessary breakthroughs needed to apply them as reliable and high-performance separation units across industries in the future.

Wood Ion Pumps Enabled by Light-Responsive MoS2-Decorated Nanocellulosic Channels

Light-driven active ion transport discovered in nanomaterials (e.g., graphene, metal-organic framework, and MXene) implicates crucial applications in membrane-based technology and energy conversion systems. However, it remains a challenge to achieve bulk assembly. Herein, we employ the scalable wood as a framework for in situ growth of MoS2 nanosheets to facilitate light-responsive ion transport. Owing to the aligned and negatively charged wood nanochannels, the MoS2-decorated wood exhibits an excellent nanofluidic conductivity of 8.3 × 10-5 S cm-1 in 1 × 10-6 M KCl. Asymmetric light illumination creates the separation of electrons and holes in MoS2 nanosheets, enabling ions to move uphill against a wide range of concentration gradients. As a result, the MoS2-decorated wood can pump ions uphill against a 20-fold concentration gradient at a light intensity of 300 mW cm-2. When the illumination is applied to the opposite side, the osmotic current along the 20-fold concentration gradient can be enhanced to 75.1 nA, and the corresponding osmotic energy conversion power density increases to more than 12.6 times that of the nonilluminated state. Based on the light-responsive behaviors, we are extending the use of MoS2-decorated wood as the ionic elements for nanofluidic circuits, such as ion switches, ion diodes, and ion transistors. This work provides a facile and scalable strategy for fabricating light-controlled nanofluidic devices from biomass materials.

Unipolar Ionic Diode Nanofluidic Membranes Enabled by Stepped Mesochannels for Enhanced Salinity Gradient Energy Harvesting

Developing ionic diode membranes featuring asymmetric structures is in high demand for salinity gradient energy harvesting. These membranes offer benefits in mitigating ion concentration polarization, thereby promoting ion permeability. However, most reported works focus on the role of heterogeneous charge-based bipolar ionic diode membranes for ion concentration polarization suppression, with comparatively less attention given to maintaining ion selectivity. Herein, unipolar ionic diode nanofluidic mesoporous silica membranes featuring stepped mesochannels were developed via a micellar sequential oriented interfacial self-assembly strategy as a salinity gradient energy harvester. Due to the asymmetric mesochannels and unipolar structure (both sides carry negative charge), the ionic diode membranes exhibit a strong rectification ratio of ∼15.91 to facilitate unidirectional ion transport while maintaining excellent cation selectivity (cation transfer number of ∼0.85). Besides, the vertically aligned mesochannels significantly reduce ion transport resistance, generating a high ionic flux. Consequently, the unipolar ionic diode nanofluidic membranes demonstrate a power output of 5.88 W/m2 between artificial sea and river water. The unipolar feature gives notable enhancements of 296% and 144% in power output compared to the symmetric membrane and bipolar ionic diode membrane, respectively. This work opens up new routes for designing ionic diode membranes for salinity gradient energy harvesting.

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.

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.

Advances in Two-Dimensional Ion-Selective Membranes: Bridging Nanoscale Insights to Industrial-Scale Salinity Gradient Energy Harvesting

Salinity gradient energy, often referred to as the Gibbs free energy difference between saltwater and freshwater, is recognized as "blue energy" due to its inherent cleanliness, renewability, and continuous availability. Reverse electrodialysis (RED), relying on ion-selective membranes, stands as one of the most prevalent and promising methods for harnessing salinity gradient energy to generate electricity. Nevertheless, conventional RED membranes face challenges such as insufficient ion selectivity and transport rates and the difficulty of achieving the minimum commercial energy density threshold of 5 W/m2. In contrast, two-dimensional nanostructured materials, featuring nanoscale channels and abundant functional groups, offer a breakthrough by facilitating rapid ion transport and heightened selectivity. This comprehensive review delves into the mechanisms of osmotic power generation within a single nanopore and nanochannel, exploring optimal nanopore dimensions and nanochannel lengths. We subsequently examine the current landscape of power generation using two-dimensional nanostructured materials in laboratory-scale settings across various test areas. Furthermore, we address the notable decline in power density observed as test areas expand and propose essential criteria for the industrialization of two-dimensional ion-selective membranes. The review concludes with a forward-looking perspective, outlining future research directions, including scalable membrane fabrication, enhanced environmental adaptability, and integration into multiple industries. This review aims to bridge the gap between previous laboratory-scale investigations of two-dimensional ion-selective membranes in salinity gradient energy conversion and their potential large-scale industrial applications.

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.

Energy harvesting from acid mine drainage using a highly proton/ion-selective thin polyamide film

A huge chemical potential difference exists between the acid mine drainage (AMD) and the alkaline neutralization solution, which is wasted in the traditional AMD neutralization process. This study reports, for the first time, the harvest of this chemical potential energy through a controlled neutralization of AMD using H+-conductive films. Polyamide films with controllable thickness achieved much higher H+ conductance than a commercially available cation exchange membrane (CEM). Meanwhile, the optimal polyamide film had an excellent H+/Ca2+ selectivity of 63.7, over two orders of magnitude higher than that of the CEM (0.3). The combined advantages of fast proton transport and high proton/ion selectivity greatly enhanced the power generation of the AMD battery. The power density was 3.1 W m-2, which is over one order of magnitude higher than that of the commercial CEM (0.2 W m-2). Our study provides a new sustainable solution to address the environmental issues of AMD while simultaneously enabling clean energy production.

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.

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.

Ion transport in nanofluidics under external fields

Nanofluidic channels with tailored ion transport dynamics are usually used as channels for ion transport, to enable high-performance ion regulation behaviors. The rational construction of nanofluidics and the introduction of external fields are of vital significance to the advancement and development of these ion transport properties. Focusing on the recent advances of nanofluidics, in this review, various dimensional nanomaterials and their derived homogeneous/heterogeneous nanofluidics are first briefly introduced. Then we discuss the basic principles and properties of ion transport in nanofluidics. As the major part of this review, we focus on recent progress in ion transport in nanofluidics regulated by external physical fields (electric field, light, heat, pressure, etc.) and chemical fields (pH, concentration gradient, chemical reaction, etc.), and reveal the advantages and ion regulation mechanisms of each type. Moreover, the representative applications of these nanofluidic channels in sensing, ionic devices, energy conversion, and other areas are summarized. Finally, the major challenges that need to be addressed in this research field and the future perspective of nanofluidics development and practical applications are briefly illustrated.

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.

2D Ordered Mesoporous Lamellar Hetero-Nanochannels with Asymmetric Wettability for Controllable Ion Transport

Heterogeneous membranes play a crucial role in osmotic energy conversion by effectively reducing concentration polarization. However, most heterogeneous membranes mitigate concentration polarization through an asymmetric charge distribution, resulting in compromised ion selectivity. Herein, hetero-nanochannels with asymmetric wettability composed of 2D mesoporous carbon and graphene oxide are constructed. The asymmetric wettability of the membrane endows it with the ability to suppress the concentration polarization without degrading the ion selectivity, as well as achieving a diode-like ion transport feature. As a result, enhanced osmotic energy harvesting is achieved with a power density of 6.41 W m-2 . This represents a substantial enhancement of 102.80-137.85% when compared to homogeneous 2D membranes, surpassing the performance of the majority of reported 2D membranes. Importantly, the membrane can be further used for high-performance ionic power harvesting by regulating ion transport, exceeding previously reported data by 89.1%.

Superassembled MXene-carboxymethyl chitosan nanochannels for the highly sensitive recognition and detection of copper ions

Copper ions (Cu2+), as a crucial trace element, play a vital role in living organisms. Thus, the detection of Cu2+ is of great significance for disease prevention and diagnosis. Nanochannel devices with an excellent nanoconfinement effect show great potential in recognizing and detecting Cu2+ ions. However, these devices often require complicated modification and treatment, which not only damages the membrane structure, but also induces nonspecific, low-sensitivity and non-repeatable detection. Herein, a 2D MXene-carboxymethyl chitosan (MXene/CMC) freestanding membrane with ordered lamellar channels was developed by a super-assembly strategy. The introduction of CMC provides abundant space charges, improving the nanoconfinement effect of the nanochannel. Importantly, the CMC can chelate with Cu2+ ions, endowing the MXene/CMC with the ability to detect Cu2+. The formation of CMC-Cu2+ complexes decreases the space charges, leading to a discernible variation in the current signal. Therefore, MXene/CMC can achieve highly sensitive and stable Cu2+ detection based on the characteristics of nanochannel composition. The linear response range for Cu2+ detection is 10-9 to 10-5 M with a low detection limit of 0.095 nM. Notably, MXene/CMC was successfully applied for Cu2+ detection in real water and fetal bovine serum samples. This work provides a simple, highly sensitive and stable detection platform based on the properties of the nanochannel composition.

Enhanced Selective Ion Transport in Highly Charged Bacterial Cellulose/Boron Nitride Composite Membranes for Thermo-Osmotic Energy Harvesting

Significant untapped energy exists within low-grade heat sources and salinity gradients. Traditional nanofluidic membranes exhibit inherent limitations, including low ion selectivity, high internal resistance, reliance on nonrenewable resources, and instability in aqueous solutions, invariably constraining their practical application. Here, an innovative composite membrane-based nanofluidic system is reported, involving the strategy of integrating tailor-modified bacterial nanofibers with boron nitride nanosheets, enabling high surface charge densities while maintaining a delicate balance between ion selectivity and permeability, ultimately facilitating effective thermo-osmotic energy harvesting. The device exhibits an impressive output power density of 10 W m-2 with artificial seawater and river water at a 50 K temperature gradient. Furthermore, it demonstrates robust power density stability under prolonged exposure to salinity gradients or even at elevated temperatures. This work opens new avenues for the development of nanofluidic systems utilizing composite materials and presents promising solutions for low-grade heat recovery and osmotic energy harvesting.

Construction of metal-organic framework/cellulose nanofibers-based hybrid membranes and their ion transport property for efficient osmotic energy conversion

The development of advanced nanofluidic membranes with better ion selectivity, efficient energy conversion and high output power density remains challenging. Herein, we prepared nanofluidic hybrid membranes based on TEMPO oxidized cellulose nanofibers (T-CNF) and manganese-based metal organic framework (MOF) using a simple in situ synthesis method. Incorporated T-CNF endows the MOF/T-CNF hybrid membrane with a high cation selectivity up to 0.93. Nanoporous MOF in three-dimensional interconnected nanochannels provides massive ion transport pathways. High transmembrane ion flux and low ion permeation energy barrier are correlated with a superior energy conversion efficiency (36 %) in MOF/T-CNF hybrid membrane. When operating under 50-fold salinity gradient by mixing simulated seawater and river water, the MOF/T-CNF hybrid membrane achieves a maximum power density value of 1.87 W m-2. About 5-fold increase in output power density was achieved compared to pure T-CNF membrane. The integration of natural nanofibers with high charge density and nanoporous MOF materials is demonstrated an effective and novel strategy for the enhancement of output power density of nanofluidic membranes, showing the great potential of MOF/T-CNF hybrid membranes as efficient nanofluidic osmotic energy generators.

Mono-component bacterial cellulose heterogeneous membrane mediated by ionic liquids for osmotic energy harvesting

The massive reserves of osmotic energy existing in estuary will be highly desired as promising energy source that avails to solve the problem of energy shortage and environment deterioration. The ion transport membrane is core component optimized through composite membrane heterostructure to maximize the osmotic energy harvesting but suffer from gaps and resistance increase, which limit their practical applications. Here we demonstrate mono-component heterogeneous regenerated bacterial cellulose (RBC) membranes fabricated by subtle regenerated technique through Ionic Liquids (ILs). Such membranes obtain heterogeneous nature by the difference in fiber intertwining states due to the different treatment conditions on both sides. It achieves osmotic energy conversion with maximum power density of 0.70 W·m-2at 100-fold, which provides ingenious strategy for excellent performance and low-cost osmotic energy harvesting. By minimizing pores and maximizing the surface charges, energy barriers can be lowered, ion permeable and selective transport channels for energy harvesting device can be increased, as supported by the numerical simulation. This is the first time the construction strategy for mono-component heterogeneous membrane mediated by ILs for osmotic energy harvesting is proposed, which averts gaps between the layers of different materials effectively and provides theoretical guidance for subsequent in-depth research on mono-component ion-selective heterogeneous membrane.

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.

Asymmetric Nanoporous Alumina Membranes for Nanofluidic Osmotic Energy Conversion

The potential of harnessing osmotic energy from the interaction between seawater and river water has been recognized as a promising, eco-friendly, renewable, and sustainable source of power. The reverse electrodialysis (RED) technology has gained significant interest for its ability to generate electricity by combining concentrated and diluted streams with different levels of salinity. Nanofluidic membranes with tailored ion transport dynamics enable efficient harvesting of renewable osmotic energy. In this regard, anodic aluminum oxide (AAO) membranes with abundant nanochannels provide a cost-effective nanofluidic platform to obtain structures with a high density of ordered pores. AAO can be utilized in constructing asymmetric composite membranes with enhanced ion flux and selectivity to improve output power generation. In this review, we first present the fundamental structure and properties of AAO, followed by summarizing the fabrication techniques for asymmetric membranes using AAO and other nanostructured materials. Subsequently, we discuss the materials employed in constructing asymmetric structures incorporating AAO while emphasizing how material selection and design can resist and promote efficient energy conversion. Finally, we provide an outlook on future applications and address the challenges that need to be overcome for successful osmotic energy conversion.