Two-Dimensional Ti3C2Tx MXene Membranes as Nanofluidic Osmotic Power Generators

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.

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.

Nanofluidic sensing inspired by the anomalous water dynamics in electrical angstrom-scale channels

Manipulation of confined water dynamics by voltage keeps great importance for diverse applications. However, limitations on the membrane functions, voltage-control range, and unclear dynamics need to be addressed. Herein, we report an anomalous electrically controlled gating phenomenon on cation-intercalated multi-layer Ti3C2 membranes and reveal the confined water dynamics. The water permeation rate was improved rapidly following the application and rise of voltage and finally reached a maximum rate at 0.9 V. The permeation rate starts to decrease from 0.9 V. Below 0.9 V, the electric field affects the charge and polarity of water molecules and then leads to ordered and denser rearrangement in the two-dimensional (2D) channel to accelerate the permeation rate. Above 0.9 V, with the assistance of metal cations, the surge in current induced aggregation of water molecules into clusters, thereby limiting the water mobility. Based on these findings, a high-performance humidity sensor was developed by simultaneously optimizing the response and recovery speeds through electric manipulation. This work provides flexible strategies in intelligent membrane design and nanofluidic sensing.

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.

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.

MXene and AgNW based flexible transparent conductive films with sandwich structure for high-performance EMI shielding and electrical heaters

With the popularization of 5G technology and the development of science and technology, flexible and transparent conductive films (TCF) are increasingly used in the preparation of optoelectronic devices such as electromagnetic shielding devices, transparent flexible heaters, and solar cells. Silver nanowires (AgNW) are considered the best material for replacing indium tin oxide to prepare TCFs due to their excellent comprehensive properties. However, the loose overlap between AgNWs is a significant reason for the high resistance. This article investigates a sandwich structured conductive network composed of AgNW and Ti3C2Tx MXene for high-performance EMI shielding and transparent electrical heaters. Polyethylene pyrrolidone (PVP) solution was used to hydrophilic modify PET substrate, and then MXene, AgNW, and MXene were assembled layer by layer using spin coating method to form a TCF with a sandwich structure. One-dimensional AgNW is used to provide electron transfer channels and improve light penetration, while two-dimensional MXene nanosheets are used for welding AgNWs and adding additional conductive channels. The flexible TCF has excellent transmittance (85.1 % at 550 nm) and EMI shielding efficiency (27.1 dB). At the voltage of 5 V, the TCF used as a heater can reach 85.6 °C. This work offers an innovative approach to creating TCFs for the future generation.

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.

Ultrasonication-assisted synthesis of transition metal carbide of MXene: an efficient and promising material for photocatalytic organic dyes degradation of rhodamine B and methylene blue in wastewater

Water pollutants of non-biodegradable toxic aromatic dye including Methylene blue (MB) and Rhodamine (RhB) are extremely carcinogenic thiazines used in various industries such as leather industry, paper industry, and the dyeing industry. The presence of dyes in wastewater causes severe threats to human health that are responsible for various harmful chronic or acute diseases and also shows an adverse impact on the environment as it reduces transparency and is harmful to water microorganisms. To overcome severe issues, many traditional techniques have been used to remove toxic pollutants, but these methods are insufficient to remove chemically stable dyes that remain in the treated wastewater. However, the photocatalytic degradation process is an efficient approach to degrade the dye up to the maximum extent with improved efficiency. Therefore, in this work, a new class of two-dimensional (2D) transition metal carbide of Titanium Carbide (Ti3C2Tx) MXene material was used for the organic dyes degradation such as MB and RhB using a photocatalytic process. A layered structure of hexagonal lattice symmetry of Ti3C2Tx MXene was successfully synthesized from the Titanium Aluminum Carbide of Ti3AlC2 bulk phase using an exfoliation process. Further, the XRD spectrum confirms the transformation of bulk MAX phase having (002) plane at 9.2° to Ti3C2Tx MXene of (002) plane at 8.88° confirms the successful removal of Al layer from MAX phase. A smooth, transparent, thin sheet-like morphology of Ti3C2Tx nanosheet size were found to be in the range of 70 to 150 nm evaluated from TEM images. Also, no holes or damages in the thin sheets were found after the treatment with strong hydrofluoric acid confirms the formation Ti3C2Tx layered sheets. The synthesized Ti3C2Tx MXene possesses excellent photocatalytic activity for the degradation of dyes MB, RhB, and mixtures of MB and RhB dyes. MB dye degraded with a degradation percentage efficiency of 99.32% in 30 min, while RhB dye was degraded upto 98.9% in 30 min. Also, experiments were conducted for degradation of mixture of MB and RhB dyes by UV light, and the degradation percentage efficiency were found to be 98.9% and 99.75% for mixture of MB and RhB dye in 45 min, respectively. Moreover, reaction rate constant (k) was determined for each dye of MB, RhB, and mixtures of MB and RhB and was found to be 0.0215 min-1 and 0.0058 min-1, and for mixtures, it was 0.0020 min-1 and 0.009 min-1, respectively.

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

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.

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.

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.

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.

Solar-driven surface-heating membrane distillation using Ti3C2Tx MXene-coated spacers

The emergence of two-dimensional (2D) MXenes as efficient light-to-heat conversion materials offers significant potential for solar-based desalination, particularly in photothermal interfacial evaporation, enabling cost-effective solar-powered membrane distillation (MD). This study investigates solar-powered MD afforded by a photothermally functionalized spacer, which is built by spray-coating Ti3C2Tx MXene sheets on metallic spacers. 2D Ti3C2Tx MXene gives an ultrahigh photothermal conversion efficiency; thereby, by Ti3C2Tx MXene-coated metallic spacer, this rationally designed spacer allows for a localized photothermal conversion and interfacial feed heating effect on the membrane surface, especially for MD operation. As a feed spacer and a photothermal element, Ti3C2Tx MXene-coated metallic spacer exhibited stable enhanced water flux of up to 0.36 kg·m-2h-1 under one sun illumination for a feed salinity of 35 g·L-1, corresponding energy conversion efficiency of 28.3 %. Overall, the developed photothermal Ti3C2Tx MXene-coated spacers displayed great potential in enhancing the performance, scalability, and feasibility of solar-driven MD process, paving the way for further development of photothermal elements that can be implemented in solar MD applications.

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.

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

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

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.

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.

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.

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.

Light-Augmented Multi-ion Interaction in MXene Membrane for Simultaneous Water Treatment and Osmotic Power Generation

The mixing of wastewater and natural water releases abundant osmotic energy. Harvesting this energy could significantly reduce the energy and economic cost of water treatment, leading to sustainable wastewater treatment technology. Yet, such energy harvesting is highly challenging because it requires a material that is highly permeable to nontoxic ions while rejecting toxic ions in wastewater to reach high power density and prevent environmental pollution. In this work, we demonstrate that a light-augmented biomimetic multi-ion interaction in an MXene membrane can simultaneously realize high permeability of Na+ ions for enhanced osmotic power generation and high selectivity to heavy metal ions up to a ratio of 2050 for wastewater treatment. The Na+ permeability is enhanced by the photothermal effect of the MXene membrane. The transport of heavy metal ions, however, is suppressed because, under angstrom-confinement, heavy metal ions are strongly electrostatically repelled by the increased number of permeating Na+ ions. As a result, the membrane can stably generate osmotic power from simulated industrial wastewater, and the power density can be enhanced by 4 times under light illumination of approximate 1 sun intensity. This work highlights the importance of multi-ion interaction for the transport properties of ionic materials, which remains rarely investigated and poorly understood in previous studies.

MXene-based Membranes for Drinking Water Production

The soaring development of industry exacerbates the shortage of fresh water, making drinking water production an urgent demand. Membrane techniques feature the merits of high efficiency, low energy consumption, and easy operation, deemed as the most potential technology to purify water. Recently, a new type of two-dimensional materials, MXenes as the transition metal carbides or nitrides in the shape of nanosheets, have attracted enormous interest in water purification due to their extraordinary properties such as adjustable hydrophilicity, easy processibility, antifouling resistance, mechanical strength, and light-to-heat transformation capability. In pioneering studies, MXene-based membranes have been evaluated in the past decade for drinking water production including the separation of bacteria, dyes, salts, and heavy metals. This review focuses on the recent advancement of MXene-based membranes for drinking water production. A brief introduction of MXenes is given first, followed by descriptions of their unique properties. Then, the preparation methods of MXene membranes are summarized. The various applications of MXene membranes in water treatment and the corresponding separation mechanisms are discussed in detail. Finally, the challenges and prospects of MXene membranes are presented with the hope to provide insightful guidance on the future design and fabrication of high-performance MXene membranes.

Extreme Ion-Transport Inorganic 2D Membranes for Nanofluidic Applications

Inorganic 2D materials offer a new approach to controlling mass diffusion at the nanoscale. Controlling ion transport in nanofluidics is key to energy conversion, energy storage, water purification, and numerous other applications wherein persistent challenges for efficient separation must be addressed. The recent development of 2D membranes in the emerging field of energy harvesting, water desalination, and proton/Li-ion production in the context of green energy and environmental technology is herein discussed. The fundamental mechanisms, 2D membrane fabrication, and challenges toward practical applications are highlighted. Finally, the fundamental issues of thermodynamics and kinetics are outlined along with potential membrane designs that must be resolved to bridge the gap between lab-scale experiments and production levels.

Essence of the Giant Reduction of Power Density in Osmotic Energy Conversion in Porous Membranes: Importance of Testing Area

Harvesting osmotic energy through nanofluidic devices with diverse materials has received considerable attention in recent years. Often, a small testing area on a membrane was chosen to assess its power performance by calculating power density as output power per effective area. Since the choice of this testing area is arbitrary, and it is usually quite small, the result obtained can be too optimistic. There is a need to come up with a common standard so that the performance of a device/membrane can be assessed reasonably. In this study, we systematically investigate the power density as a function of testing area in nanoporous anodic-aluminum-oxide membranes. Through changing the aperture size of substrates, we clearly show that the obtained power density decreases drastically with increasing testing area. For instance, the power density acquired from the testing area of μm2-scale can be five orders of magnitude larger than that from the pristine membrane of cm2-scale. We also advance simulations by building a 3D model to simulate osmotic-driven ion transport in the multichannel system. The result of modeling agrees with our experimental observation that the power density decreases with increasing number of channels, and the ionic concentration profile reveals that the concentration polarization becomes serious as the number of channels increases. Our result highlights the importance of effective area on testing the power performance in nanofluidic devices.

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.

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

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

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

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

Superhigh and Robust Ion Selectivity in Membranes Assembled with Monolayer Clay Nanosheets

It is crucial to control the ion transport in membranes for various technological applications such as energy storage and conversion. The emerging functional two-dimensional (2D) nanosheets such as graphene oxide and MXenes show great potential for constructing ordered nanochannels, but the assembled membranes suffer from low ion selectivity and stability. Here a class of robust charge-selective membranes with superhigh cation/anion selectivity, which are assembled with monolayer nanosheets of cationic/anionic clays that inherently have permanent and uniform charges on each layer is reported. The transport number of cations/anions of cationic vermiculite nanosheet membranes (VNMs)/anionic Co-Al layered double hydroxide (CoAl-LDH) nanosheet membranes is over 0.90 in different NaCl concentration gradients, outperforming all the reported ion-selective membranes. Importantly, this excellent ion selectivity can persist at high-concentration salt solutions, under acidic and alkaline conditions, and for a wide range of ions of different sizes and charges. By coupling a pair of cation-selective vermiculite membrane and anion-selective CoAl-LDH membrane, a reverse electrodialysis device which shows an output power density of 0.7 W m-2 and energy conversion efficiency of 45.5% is constructed. This work provides a new strategy to rationally design high-performance ion-selective membranes by using 2D nanosheets with inherent surface charges for controllable ion-transport applications.

Biocompatible and Oxidation-Resistant Ti3 C2 Tx MXene with Halogen-Free Surface Terminations

Surface chemistry influences not only physicochemical properties but also safety and applications of MXene nanomaterials. Fluorinated Ti3 C2 Tx MXene, synthesized using conventional HF-based etchants, raises concerns regarding harmful effects on electronics and toxicity to living organisms. In this study, well-delaminated halogen-free Ti3 C2 Tx flakes are synthesized using NaOH-based etching solution. The transversal surface plasmon mode of halogen-free Ti3 C2 Tx MXene (833 nm) confirmed red-shift compared to conventional Ti3 C2 Tx (752 nm), and the halogen-free Ti3 C2 Tx MXene has a different density of state by the high proportion of -O and -OH terminations. The synthesized halogen-free Ti3 C2 Tx exhibits a lower water contact angle (34.5°) and work function (3.6 eV) than those of fluorinated Ti3 C2 Tx (49.8° and 4.14 eV, respectively). The synthesized halogen-free Ti3 C2 Tx exhibits high biocompatibility with the living cells, as evidenced by no noticeable cytotoxicity, even at very high concentrations (2000 µg mL⁻1 ), at which fluorinated Ti3 C2 Tx caused ≈50% reduction in cell viability upon its oxidation. Additionally, the oxidation stability of halogen-free Ti3 C2 Tx is enhanced unexpectedly, which cumulatively provides a good rationale for pursuing the halogen-free routes for synthesizing MXene materials for their uses in biomedical and therapeutic applications.

Horizontally Asymmetric Nanochannels of Graphene Oxide Membranes for Efficient Osmotic Energy Harvesting

Reverse electrodialysis (RED) directly harvests renewable energy from salinity gradients, and the achievable potential power heavily relies on the ion exchange membranes. Graphene oxides (GOs) are considered a solid candidate for the RED membrane because the laminated GO nanochannels with charged functional groups provide an excellent ionic selectivity and conductivity. Yet, a high internal resistance and poor stability in aqueous solutions limit the RED performance. Here, we develop a RED membrane that concurrently achieves high ion permeability and stable operation based on epoxy-confined GO nanochannels with asymmetric structures. The membrane is fabricated by reacting epoxy-wrapped GO membranes with ethylene diamine via vapor diffusion, overcoming the swelling properties in aqueous solutions. More importantly, the resultant membrane exhibits asymmetric GO nanochannels in terms of both channel geometry and electrostatic surface charges, leading to the rectified ion transport behavior. The demonstrated GO membrane exhibits the RED performance up to 5.32 W·m^-2 with >40% energy conversion efficiency across a 50-fold salinity gradient and 20.3 W·m^-2 across a 500-fold salinity gradient. Planck-Nernst continuum models coupled to molecular dynamics simulations rationalize the improved RED performance in terms of the asymmetric ionic concentration gradient within the GO nanochannel and the ionic resistance. The multiscale model also provides the design guidelines for ionic diode-type membranes configuring the optimum surface charge density and ionic diffusivity for efficient osmotic energy harvesting. The synthesized asymmetric nanochannels and their RED performance demonstrate the nanoscale tailoring of the membrane properties, establishing the potentials for 2D material-based asymmetric membranes.

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.

Photothermal Surface Heating Membrane Distillation Using 3D-Printed Ti3C2Tx MXene-Based Nanocomposite Spacers

To address the growing global need for freshwater, it has become essential to use nonpotable saline water. Solar membrane distillation is a potential desalination method that does not need conventional electricity and may cut water production costs. In this study, we develop a photothermal surface heating membrane distillation using a new class of photothermal spacers constructed with Ti₃C₂T_x MXene-based nanocomposites. In contrast to traditional membrane distillation, which utilizes energy-intensive bulk feed heating, solar-powered surface heating membrane distillation removes the external thermal energy input requirements, hence reducing operating costs significantly. In particular, three-dimensional (3D)-printing technology was used to fabricate the functional spacer, which allowed the design and materials to be fine-tuned per the needs of the process. Under solar illumination, the printed spacer can exhibit a localized photothermal conversion-driven heating effect near the surface of distillation membranes, which generates vapor pressure strong enough to operate distillation across membranes. Importantly, a two-dimensional Ti₃C₂T_x MXene with outstanding photothermal conversion efficiency and stability in hypersaline ionic solutions was incorporated into the 3D-printed spacers as the crucial nanofiller for imparting a local heating effect of feed. The fabricated nanocomposite spacers showed superior photothermal heating response under sunlight with an average permeate flux and energy conversion efficiency of 0.49 kg·m^-2·h^-1 and 30.6%, respectively. An enhancement in both photothermal efficiency and permeate flux was noticed as the amount of MXene nanosheets increased in the 3D-printed spacers. This study demonstrates the feasibility of using 3D-printed photothermal spacers for high-performance and sustainable surface heating membrane distillation, providing a promising avenue for further improvement with other photothermal nanofillers or spacer modifications.

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

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

Engineering 2D Aligned Nanowires Assembled Porous Hetero-Membrane for Smart Ion Transport

Engineering 2D nanosheets with well-defined porous structures and their assembled heterostructure membrane is a promising method to improve osmotic energy conversion. However, it is still a great challenge to directly fabricate 2D nanosheets with regular parallel nanochannels in aqueous media. Here, the desired functional nanosheets and heterostructure membrane device are successfully prepared through a simple interfacial assembly strategy. In this method, monolayer cylindrical monomicelles closely arrange and assemble on the surfaces of graphene oxide, and the resulting nanosheets with monolayered aligned nanowire polymer arrays parallel to the substrate surfaces are then obtained. Subsequently, a heterostructured membrane is constructed by assembling these 2D nanosheets on macroporous alumina. The nanofluidic membrane device with asymmetric geometry and charge polarity exhibits smart ion transport properties, and the output osmotic power density is ≈1.22 and 1.63 times over the reported pure 2D graphene oxide and biomass-derived membranes, respectively. In addition, theoretical calculations are carried out to reveal the mechanisms for ion selectivity and salinity gradient energy conversion. This monolayered interfacial assembly approach can open up new avenues for the synthesis of functional porous low-dimensional nanomaterials and membrane devices, and expand the palette of materials selection for many applications.

Selective Ion Transport in Two-Dimensional Lamellar Nanochannel Membranes

Precise and ultrafast ion sieving is highly desirable for many applications in environment-, energy-, and resource-related fields. The development of a permselective lamellar membrane constructed from parallel stacked two-dimensional (2D) nanosheets opened a new avenue for the development of next-generation separation technology because of the unprecedented diversity of the designable interior nanochannels. In this Review, we first discuss the construction of homo- and heterolaminar nanoarchitectures from the starting materials to the emerging preparation strategies. We then explore the property-performance relationships, with a particular emphasis on the effects of physical structural features, chemical properties, and external environment stimuli on ion transport behavior under nanoconfinement. We also present existing and potential applications of 2D membranes in desalination, ion recovery, and energy conversion. Finally, we discuss the challenges and outline research directions in this promising field.

Membranes for Osmotic Power Generation by Reverse Electrodialysis

In recent years, the utilization of the selective ion transport through porous membranes for osmotic power generation (blue energy) has received a lot of attention. The principal of power generation using the porous membranes is same as that of conventional reverse electrodialysis (RED), but nonporous ion exchange membranes are conventionally used for RED. The ion transport mechanisms through the porous and nonporous membranes are considerably different. Unlike the conventional nonporous membranes, the ion transport through the porous membranes is largely dictated by the principles of nanofluidics. This owes to the fact that the osmotic power generation via selective ion transport through porous membranes is often referred to as nanofluidic reverse electrodialysis (NRED) or nanopore-based power generation (NPG). While RED using nonporous membranes has already been implemented on a pilot-plant scale, the progress of NRED/NPG has so far been limited in the development of small-scale, novel, porous membrane materials. The aim of this review is to provide an overview of the membrane design concepts of nanofluidic porous membranes for NPG/NRED. A brief description of material design concepts of conventional nonporous membranes for RED is provided as well.

Two-Dimensional Nanofluidic Membranes with Intercalated In-Plane Shortcuts for High-Performance Blue Energy Harvesting

Two-dimensional nanofluidic membranes offer great opportunities for developing efficient and robust devices for ionic/water-nexus energy harvesting. However, low counterion concentration and long pathway through limited ionic flux restrict their output performance. Herein, it is demonstrated that rapid diffusion kinetics can be realized in two-dimensional nanofluidic membranes by introducing in-plane holes across nanosheets, which not only increase counterion concentration but also shorten pathway length through the membranes. Thus, the holey membranes exhibited an enhanced performance relative to the pristine ones in terms of osmotic energy conversion. In particular, a biomimetic multilayered membrane sequentially assembled from pristine and holey sections offers an optimized combination of selectivity and permeability, therefore generating a power density up to 6.78 W m^-2 by mixing seawater and river water, superior to the majority of the state-of-the-art lamellar nanofluidic membranes. This work highlights the importance of channel morphologies and presents a general strategy for effectively improving ion transport through lamellar membranes for high-performance nanofluidic devices.

Laser-Assisted Scalable Pore Fabrication in Graphene Membranes for Blue-Energy Generation

The osmotic energy from a salinity gradient (i. e. blue energy) is identified as a promising non-intermittent renewable energy source for a sustainable technology. However, this membrane-based technology is facing major limitations for large-scale viability, primarily due to the poor membrane performance. An atomically thin 2D nanoporous material with high surface charge density resolves the bottleneck and leads to a new class of membrane material the salinity gradient energy. Although 2D nanoporous membranes show extremely high performance in terms of energy generation through the single pore, the fabrication and technical challenges such as ion concentration polarization make the nanoporous membrane a non-viable solution. On the other hand, the mesoporous and micro porous structures in the 2D membrane result in improved energy generation with very low fabrication complexity. In the present work, we report femtosecond (fs) laser-assisted scalable fabrication of μm to mm size pores on Graphene membrane for blue energy generation for the first time. A remarkable osmotic power in the order of μW has been achieved using mm size pores, which is about six orders of magnitudes higher compared to nanoporous membranes, which is mainly due to the diffusion-osmosis driven large ionic flux. Our work paves the way towards fs laser-assisted scalable pore creation in the 2D membrane for large-scale osmotic power generation.

Fast and High-Yield Anhydrous Synthesis of Ti3 C2 Tx MXene with High Electrical Conductivity and Exceptional Mechanical Strength

2D transition metal carbides or nitrides (MXenes) have attracted considerable attention from materials scientists and engineers owing to their physicochemical properties. Currently, MXenes are synthesized from MAX-phase precursors using aqueous HF. Here, in order to enhance the production of MXenes, an anhydrous etching solution is proposed, consisting of dimethylsulfoxide as solvent with its high boiling point, NH₄ HF₂ as an etchant, CH₃ SO₃ H as an acid, and NH₄ PF₆ as an intercalant. The reaction temperature can be increased up to 100 °C to accelerate the etching and delamination of Ti₃ AlC₂ MAX crystals; in addition, the destructive side reaction of the produced Ti₃ C₂ T_x MXene is suppressed in the etchant. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high-quality monolayer Ti₃ C₂ T_x with an electrical conductivity of 8200 S cm^-1 and yield of over 70%. The Ti₃ C₂ T_x MXene fabricated via this modified synthesis exhibits different surface structures and properties arising from more F-terminations than those of Ti₃ C₂ T_x synthesized in aqueous HF₂ T. The atypical surface structure of Ti₃ C₂ T_x MXene results in an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is five times larger than those of Ti₃ C₂ T_x MXenes synthesized in aqueous HF solution (31.7 ± 7.8 MPa).

Electrical Conductance of Charged Nanopores

A nanopore's response to an electrical potential drop is characterized by its electrical conductance, . For the last two decades, it has been thought that at low electrolyte concentrations, , the conductance is concentration-independent such that . It has been recently demonstrated that surface charge regulation changes the dependency to , whereby the slope typically takes the values α = 1/3 or 1/2. However, experiments have observed slopes of 2/3 and 1 suggesting that additional mechanisms, such as convection and slip-lengths, appear. Here, we elucidate the interplay between three mechanisms: surface charge regulation, convection, and slip lengths. We show that the inclusion of convection does not change the slope, and when the effects of hydrodynamic slip are included, the slope is doubled. We show that when all effects are accounted for, α can take any value between 0 and 1 where the exact value of the slope depends on the material properties. This result is of utmost importance in designing any electro-kinetically driven nanofluidic system characterized by its conductance.

Nanofluidic proton channels based on a 2D layered glass membrane with improved aqueous and acid stability

Layered two-dimensional (2D) membranes hold great promise in the study of confined ion transport and nanofluidic applications. However, 2D layered membranes suffer from poor stability in water and harsh chemical conditions. Here we use amorphous silica-based nanosheets obtained by vermiculite to assemble a layered glass membrane (LGM) with a 2D nanofluidic channel network in the interlayer space. We find that the water stability and corrosion resistance of the LGM are improved compared to that of the layered vermiculite membrane. Moreover, the surface charge-governed proton conductivity of LGM remains stable at about 4 × 10^-2 S cm^-1 when the HCl solution concentration changes by orders of magnitude. The enhanced stability of LGM is of great significance for the study of confined ion transport and is expected to be applied to more nanofluidic applications, such as water treatment, molecular sieving, and osmotic energy conversion.

Flexible Two-Dimensional Vanadium Carbide MXene-Based Membranes with Ultra-Rapid Molecular Enrichment for Surface-Enhanced Raman Scattering

Two-dimensional (2D) MXene materials have attracted broad interest in surface-enhanced Raman scattering (SERS) applications by virtue of their abundant surface terminations and excellent photoelectric properties. Herein, we propose to design highly sensitive MXene-based SERS membranes by integrating a 2D downsizing strategy with molecular enrichment approaches. Two types of 2D vanadium carbide (V₄C₃ and V₂C) MXenes are demonstrated for ultrasensitive SERS sensing, and corresponding SERS mechanisms including the effect of 2D vanadium carbide thickness on their electron density states and interfacial photoinduced charge transfer resonance were discussed. A 2D downsizing strategy authorizes nonplasmonic SERS detection with a sensitivity of 1 × 10^-7 M. Moreover, the performance can be further upgraded by vacuum-assisted filtration, which enables an ultrarapid molecular enrichment (within 2 min), ultrahigh molecular removal rate (over 95%), and improved sensitivity (5 × 10^-9 M). This work may shed light on the MXene-based materials as an innovative platform for nonplasmonic SERS detection.

Ultrathin and Ultrastrong Kevlar Aramid Nanofiber Membranes for Highly Stable Osmotic Energy Conversion

An ion-selective membrane can directly convert the osmotic energy to electricity through reverse electrodialysis. However, developing an advanced membrane that simultaneously possesses high power density, excellent mechanical strength, and convenient large-scale production for practical osmotic energy conversion, remains challenging. Here, the fabrication of ultrathin and ultrastrong Kevlar aramid nanofiber (KANF) membranes with interconnected three-dimensional (3D) nanofluidic channels via a simple blade coating method is reported. The negatively charged 3D nanochannels show typical surface-charge-governed nanofluidic ion transport and exhibit excellent cation selectivity. When applied to osmotic energy conversion, the power density of the KANF membrane-based generator reaches 4.8 W m^-2 (seawater/river water) and can be further increased to 13.8 W m^-2 at 328 K, which are higher than most of the state-of-the-art membranes. Importantly, a 4-µm-thickness KANF membrane shows ultrahigh tensile strength (565 MPa) and Young's modulus (25 GPa). This generator also exhibits ultralong stability over 120 days, showing great potential in practical energy conversions.