Osmotic Power Generation with Positively and Negatively Charged 2D Nanofluidic Membrane Pairs

In nature, hierarchically assembled nanoscale ionic conductors, such as ion channels and ion pumps, become the structural and functional basis of bioelectric phenomena. Recently, ion‐channel‐mimetic nanofluidic systems have been built into reconstructed 2D nanomaterials for energy conversion and storage as effective as the electrogenic cells. Here, a 2D‐material‐based nanofluidic reverse electrodialysis system, containing cascading lamellar nanochannels in oppositely charged graphene oxide membrane (GOM) pairs, is reported for efficient osmotic energy conversion. Through preassembly modification, the surface charge polarity of the 2D nanochannels can be efficiently tuned from negative (?123 mC m^?2) to positive (+147 mC m^?2), yielding strongly cation‐ or anion‐selective GOMs. The complementary two‐way ion diffusion leads to an efficient charge separation process, creating superposed electrochemical potential difference and ionic flux. An output power density of 0.77 W m^?2 is achieved by controlled mixing concentrated (0.5 m ) and diluted ionic solutions (0.01 m ), which is about 54% higher than using commercial ion exchange membranes. Tandem alternating GOM pairs produce high voltage up to 2.7 V to power electronic devices. Besides simple salt solutions, various complex electrolyte solutions can be used as energy sources. These findings provide insights to construct cascading nanofluidic circuits for energy, environmental, and healthcare applications.     

Organic/Inorganic Hybrid Nanochannels Based on Polypyrrole‐Embedded Alumina Nanopore Arrays: pH‐ and Light‐Modulated Ion Transport

Inspired by the asymmetric structure and responsive ion transport in biological ion channels, organic/inorganic hybrid artificial nanochannels exhibiting pH‐modulated ion rectification and light‐regulated ion flux have been constructed by introducing conductive polymer into porous nanochannels. The hybrid nanochannels are achieved by partially modifying alumina (Al₂O₃) nanopore arrays with polypyrrole (PPy) layer using electrochemical polymerization, which results in an asymmetric component distribution. The protonation and deprotonation of Al₂O₃ and PPy upon pH variation break the surface charge continuity, which contributes to the pH‐tunable ion rectification. The ionic current rectification ratio is affected substantially by the pH value of electrolyte and the pore size of nanochannels. Furthermore, the holes (positive charges) in PPy layer induced by the cooperative effect of light and protons are used to regulate the ionic flux through the nanochannels, which results in a light‐responsive ion current. The magnitude of responsive ionic current could be amplified by optimizing this cooperation. This new type of stimuli‐responsive PPy/Al₂O₃ hybrid nanochannels features advantages of unique optical and electric properties from conducting PPy and high mechanical performance from porous Al₂O₃ membrane, which provide a platform for creating smart nanochannels system.     

Anomalous Channel‐Length Dependence in Nanofluidic Osmotic Energy Conversion

Recent advances in materials science and nanotechnology have lead to considerable interest in constructing ion‐channel‐mimetic nanofluidic systems for energy conversion and storage. The conventional viewpoint suggests that to gain high electrical energy, the longitudinal dimension of the nanochannels (L) should be reduced so as to bring down the resistance for ion transport and provide high ionic flux. Here, counterintuitive channel‐length dependence is described in nanofluidic osmotic power generation. For short nanochannels (with length L < 400 nm), the converted electric power persistently decreases with the decreasing channel length, showing an anomalous, non‐Ohmic response. The combined thermodynamic analysis and numerical simulation prove that the excessively short channel length impairs the charge selectivity of the nanofluidic channels and induces strong ion concentration polarization. These two factors eventually undermine the osmotic power generation and its energy conversion efficiency. Therefore, the optimal channel length should be between 400 and 1000 nm in order to maximize the electric power, while balancing the efficiency. These findings reveal the importance of a long‐overlooked element, the channel length, in nanofluidic energy conversion and provide guidance to the design of high‐performance nanofluidic energy devices.     

Heterogeneous MXene/PS-b-P2VP Nanofluidic Membranes with Controllable Ion Transport for Osmotic Energy Conversion

Membrane-based osmotic power harvesting is a strategy for sustainable power generation. 2D nanofluids with high ion conductivity and selectivity are emerging candidates for osmotic energy conversion. However, the ion diffusion under nanoconfinement is hindered by homogeneous 2D membranes with monotonic charge regulation and severe concentration polarization, which results in an undesirable power conversion performance. Here, an asymmetric nanochannel membrane with a two-layered structure is reported, in which the angstrom-scale channels of 2D transition metal carbides/nitrides (MXenes) act as a screening layer for controlling ion transport, and the nanoscale pores of the block copolymer (BCP) are the pH-responsive arrays with an ordered nanovoid structure. The heterogeneous nanofluidic device exhibits an asymmetric charge distribution and enlarged 1D BCP porosity under acidic and alkaline conditions, respectively; this improves the gradient-driven ion diffusion, allowing a high-performance osmotic energy conversion with a power density of up to 6.74 W m⁻² by mixing artificial river water and seawater. Experiments and theoretical simulations indicate that the tunable asymmetric heterostructure contributes to impairing the concentration polarization and enhancing the ion flux. This efficient osmotic energy generator can advance the fundamental understanding of the MXene-based heterogeneous nanofluidic devices as a paradigm for membrane-based energy conversion technologies.     

Synthetic Asymmetric‐Shaped Nanodevices with Symmetric pH‐Gating Characteristics

Synthetic stimuli‐gated nanodevices displaying intelligent ion transport properties similar to those observed in biological ion channels have attracted increasing interests for their wide potential applications in biosensors, nanofluidics, and energy conversions. Here, bioinspired asymmetric shaped nanodevices are reported that can exhibit symmetric and linear pH‐gating ion transport features based on polyelectrolyte‐asymmetric‐functionalized asymmetric hourglass‐shaped nanochannels. The pH‐responsive polymer brushes grafted on the inner channel surface are acted as a gate that open and close in response to external pH changing to linearly and symmetrically regulate transmembrane ionic currents of the channel. A complete experimental characterization of the pH‐dependent ion transport behaviors of the nanodevice and a comprehensive discussion of the experimental results in terms of theoretical simulation are also presented. Both experimental and theoretical data shown in this work demonstrate the feasibility of using the asymmetric chemical modification method to achieve symmetric pH gating behaviors inside the asymmetric nanochannels, and lay the foundation to build diverse stimuli‐gated artificial asymmetric shaped ion channels with symmetric gating ion transport features.     

Bioinspired Graphene Oxide Membranes with Dual Transport Mechanisms for Precise Molecular Separation

The implementation of membrane technology to replace or combine with energy‐intensive cryogenic distillation for precise separation of ethylene/ethane mixture proves an extremely important yet highly challenging task. Inspired by the hierarchical structure and facilitated gas transport of biological membranes, a highly selective ethylene/ethane separation membrane is explored through the fixation of a silver ion carrier and the impregnation of ionic liquid within 2D nanochannels of graphene oxide laminate, where plenty of ethylene‐permeating in‐plane nano‐wrinkles and ethylene‐facilitated plane‐to‐plane nanochannels are constructed. By virtue of synergistic effects of molecular sieving and carrier‐facilitated transport, an unprecedented combination of high ethylene permeance (72.5 GPU) and superhigh ethylene/ethane selectivity (215) is achieved, out‐performing currently reported advanced membranes. Moreover, molecular dynamics simulations verify a favorable membrane nanostructure for fast and selective transport of ethylene molecules. This bioinspired approach with dual transport mechanisms may open novel avenues to the design of high‐performance membranes for precise molecular separation.     

On the Origin of Ion Selectivity in Ultrathin Nanopores: Insights for Membrane‐Scale Osmotic Energy Conversion

Nanopores in ultrathin or atomically thin membranes attract broad interest because the infinitesimal pore depth allows selective transport of ions and molecules with ultimate permeability. Toward large‐scale osmotic energy conversion, great challenges remain in extrapolating the promising single‐pore demonstration to really powerful macroscopic applications. Herein, the origin of the selective ion transport in ultrathin nanopores is systematically investigated. Based on a precise Poisson and Nernst–Planck model calculation, it is found that the generation of net diffusion current and membrane potential stems from the charge separation within the electric double layer on the outer membrane surface, rather than that on the inner pore wall. To keep the charge selectivity of the entire membrane, a critical surface charged area surrounding each pore orifice is therefore highly demanded. Otherwise, at high pore density, the membrane selectivity and the overall power density would fall down instead, which explains the giant gap between the actual experimental achievements and the single‐pore estimation. To maximize the power generation, smaller nanopores (pore diameter ≈1–2 nm) are appropriate for large‐scale osmotic energy conversion. With a porosity of ≈10%, the total power density approaches more than 200 W m^‐2, anticipating a substantial advance toward high‐performance large‐scale nanofluidic power sources.     

Ionotronics Based on Horizontally Aligned Carbon Nanotubes

Controlled ion transport through ion channels of cell membranes regulates signal transduction processes in biological systems and has also inspired the thriving development of ionic electronics (ionotronics or iontronics) and biocomputing. However, for constructing highly integrated ionic electronic circuits, the integration of natural membrane‐spanning ion channel proteins or artificial nanomembrane‐based ionic diodes into planar chips is still challenging due to the vertically arranged architecture of conventional nanomembrane‐based artificial ionic diodes. Here, a new design of ionic diode is reported, which allows chip‐scale integration of ionotronics, based on horizontally aligned nanochannels made from multiwalled carbon nanotubes (MWCNTs). The rectification of ion transport through the MWCNT nanochannels is enabled by decoration of oppositely charged polyelectrolytes on the channel entrances. Advanced ionic electronic circuits including ionic logic gates, ionic current rectifiers, and ionic bipolar junction transistors (IBJT) are demonstrated on planar nanofluidic chips by stacking a series of ionic diodes fabricated from the same bundles of MWCNTs. The horizontal arrangement and facile chip‐scale fabrication of the MWCNT ionic diodes may enable new designs of complex but monolithic ionotronic systems. The MWCNT ionic diode may also prove to be an excellent platform for investigation of electrokinetic ion transport in 1D carbon materials.     

High-Strength,Thin PBO Nanofiber Membrane with Long-Term Stability for Osmotic Energy Conversion

Ion-selective membrane embedded in a reverse electrodialysis system can achieve the conversion of osmotic energy into electricity. However, the ingenious design and development of pure polymer membranes that simultaneously satisfy excellent mechanical strength, long-term stability, high power density, and increased testing area is a crucial challenge. Here, high-strength, thin PBO nanofiber membranes (PBONM) with 3D nanofluidic channels and a thickness of 0.81 µm are prepared via a simple vacuum-assisted filtration technology. The thin PBONM exhibits excellent mechanical properties: stress of 235.8 MPa and modulus of 16.96 GPa, outperforming the state-of-the-art nanofluidic membranes. The obtained PBONM reveals surface-charge-governed ion transport behavior and high ion selectivity of 0.88 at a 50-fold concentration gradient. The PBO membrane-based generator delivers a power density of 7.7 and 40.2 W m⁻² at 50-fold and 500-fold concentration gradient. Importantly, this PBONM presents excellent stability in response to different external environments including various saline solutions, pH, and temperature. In addition, the maximum power density of PBONM reaches up to 5.9 W m⁻² under an increased testing area of 0.79 mm², exceeding other membrane-based generators with comparable testing areas. This work paves the way for constructing high-strength fiber nanofluidic membranes for highly efficient osmotic energy conversion.     

Paper-Mill Waste Reinforced Nanofluidic Membrane as High-Performance Osmotic Energy Generators

Nanofluidic membranes consisting of 2D materials and polymers are considered promising candidates for harvesting osmotic energy from river estuaries owing to their unique ion channels. However, micron-scale polymer chains agglomerate in the nanochannels, resulting in steric hindrance and affection ion transport. Herein, a nanofluidic membrane is designed from MXene and xylan nanoparticles that are derived from paper-mill waste. The demonstrated membrane reinforced by paper-mill waste has the characteristics of green, low-cost, and outstanding performance in mechanical properties and surface-charge-governed ionic transport. The MXene/carboxmethyl xylan (CMX) membrane demonstrates a high surface charge (ζ-potential of −44.3 mV) and 12 times higher strength (284.96 MPa) than the pristine MXene membrane. The resulting membrane shows intriguing features of high surface charge, high ion selectivity, and reduced steric hindrance, enabling it high osmotic energy generation performance. A potential of the nanofluidic membrane is ≈109 mV, the corresponding current of up to 2.73 µA, and the output power density of 14.52 mW m⁻² are obtained under a 1000-fold salt concentration gradient. As the electrolyte pH increases, the power density reaches 56.54 mW m⁻². This works demonstrate that CMX nanoparticles can effectively enhance the properties of the nanofluidic membrane and provide a promising strategy to design high-performance nanofluidic devices.     

Multi-Control of Ion Transport in a Field-Effect Iontronic Device based on Sandwich-Structured Nanochannels

Biomimetic smart nanochannels can regulate ion transport behavior responsive to the external stimuli, having huge potential in nanofluidic devices, sensors and energy conversion. Field-effect nanofluidic diodes or transistors based on electric-responsive nanochannels are emerging owing to their advantages such as non-invasiveness, in situ, real time, and high efficiency. However, simultaneously realizing the voltage-control of the ion conductance and ion current rectification (ICR) properties is still a big challenge. Here, a field-effect iontronic device is developed based on ionomer/anodic aluminum oxide/conducting polymer sandwich-structured nanochannel to realize the multi-control of ion transport behaviors including ion conductance, ICR magnitude, and ICR direction by modulating the surface charge, wettability, and morphology of the nanochannel. The electroactive conducting polymer carries tunable surface charges responsive to the electric stimuli, leading to the regulation of ICR values. The complex three-segment structures lead to the reverse of ICR direction by reconfiguring the charge distribution along with the whole channel. The switching wettability between hydrophilic and hydrophobic results in the regulation of ion conductance. Furthermore, the field-effect iontronic device functions in a wide salinity range especially in hypersaline environment, due to the salinity-adaptive properties of the membrane. A new route is provided for designing more functional field-effect nanofluidic devices.     

Solution‐pH‐Modulated Rectification of Ionic Current in Highly Ordered Nanochannel Arrays Patterned with Chemical Functional Groups at Designed Positions

A new ionic current rectification device responsive to a broad range of pH stimuli is established using highly ordered nanochannels of porous anodic alumina membrane with abrupt surface charge discontinuity. The asymmetric surface charge distribution is achieved by patterning the nanochannels with surface amine functional groups at designed positions using a two‐step anodization process. Due to the protonation/deprotonation of the patterned amine and the remaining intrinsic hydroxyl groups upon solution pH variation, the nanochannel‐array‐based device is able to regulate ion transport selectivity and has ionic current rectification properties. The rectification ratio of the device is mainly determined by the nanochannel size, and the rectification ratio is less sensitive to the patterned length of the amine groups when the nanochannels size is defined. Thus, the isoelectric point of nanochannels can be easily estimated to be the pH value with a unit rectification ratio. The present ionic device is promising for biosensing, molecular transport and separation, and drug delivery in confined environments.     

Bioinspired Self‐Gating Nanofluidic Devices for Autonomous and Periodic Ion Transport and Cargo Release

The biochemical oscillatory reaction induced self‐gating process of biological ion channels is essential to life processes, characterized as autonomous, continuous, and periodic. However, few synthetic nanochannel systems can achieve such excellent self‐gating property. Their gating properties work greatly depending on the frequent addition of reactants or the supply of external stimuli. Herein, a novel bioinspired self‐gating nanofluidic device that can transport mass in a continuous and periodic manner is reported. This self‐gating device is constructed by using a fully closed‐system pH oscillator to control the gating processes of the artificial proton‐gated nanochannels. With cyclic oscillation of protons inside the nanochannel induced by the oscillatory chemical reactions of the pH oscillator, surface charge density and polarity of the nanochannels can be self‐regulated, resulting in an autonomous and periodic switching of the nanochannel conductance between high and low states as well as the selectivity between cation selective and anion selective states. Moreover, by using Rhodamine B and Ruthenium(II) compound as the cationic cargoes, periodic release of these charged molecules is also observed. Therefore, this work opens up a new avenue to build self‐gating nanofluidic devices, which may not only act as ion oscillators, but potentially find applications in controlled‐release fields as well.     

Self-Assembled Nanoporous Metal–Organic Framework Monolayer Film for Osmotic Energy Harvesting

Osmotic energy represents a promising energy resource because it is sustainable and environmentally benign. Subnanoscale channels are considered as a competitive platform for generating this blue energy due to their highly selective and ultrafast ion transport. However, fabricating functional subnanochannels capable of high energy output remains challenging. Here, a heterogeneous subnanochannel membrane formed by coating a functionalized self-assembled metal−organic framework (MOF) monolayer (SAMM) film on a porous anodic aluminum oxide membrane, is reported. The SAMM film, with a thickness of ≈160 nm, is fabricated by self-assembly of poly(methyl methacrylate-co-vinylimidazole)-modified UiO-66-NH₂ nanoparticles at the water−air interface. In the SAMM, imidazole and NH₂ groups provide abundant positive charges, while the angstrom-scale windows act as ionic filters for selective screening of anions with different hydration diameters. As a result, the heterogeneous membrane exhibits excellent capacity for anion-selective transport, which contributes to an optimal osmotic power of 6.76 W m⁻² under a 100-fold NaCl gradient, as well as a high Cl⁻/SO₄²⁻ selectivity of ≈42.2. Further, the output power is increased to 10.5 W m⁻² by methylating imidazole moieties on the MOF surface. This work provides a facile and modular approach to fabricate subnanochannels for enabling highly selective and efficient osmotic energy conversion.     

The Role of Nanowrinkles in Mass Transport across Graphene‐Based Membranes

Laminar membranes stacked by 2D materials are an emerging selective unit in separating processes across disciplines for their controllable mass transport properties. In general, parallel nanochannels formed between neighboring layers, owing to their adjustable size and surface chemistry, are considered the dominant transport regulator. Besides these flat interlayer channels, wrinkled morphology has also existed in 2D membranes, but the structure and potential transporting role of such curved channel remain largely unexplored. This study demonstrates that nanowrinkles are intrinsically formed in graphene‐based membranes, featuring an arc‐like shape with around 2.5 nm high center and two narrow wedge corners. By a facile “solvent‐treatment” during assembly, the membranes are tuned to possess different wrinkle density. In transport tests involving water and ions, the appearance of more wrinkles yields higher water permeation yet has limited effect on ion passage. These findings suggest that nanowrinkles by themselves serve as fast transporting ways while their connection with narrow interlayer channels can form a selective network. Results here are expected to deepen the understanding of mass transport mechanisms in current laminar membranes (e.g., graphene‐based) and provide strategies for designing future 2D membranes via wrinkle engineering.     

A High‐Energy Lithium‐Ion Capacitor by Integration of a 3D Interconnected Titanium Carbide Nanoparticle Chain Anode with a Pyridine‐Derived Porous Nitrogen‐Doped Carbon Cathode

Lithium‐ion capacitors (LICs) are hybrid energy storage devices that have the potential to bridge the gap between conventional high‐energy lithium‐ion batteries and high‐power capacitors by combining their complementary features. The challenge for LICs has been to improve the energy storage at high charge?discharge rates by circumventing the discrepancy in kinetics between the intercalation anode and capacitive cathode. In this article, the rational design of new nanostructured LIC electrodes that both exhibit a dominating capacitive mechanism (both double layer and pseudocapacitive) with a diminished intercalation process, is reported. Specifically, the electrodes are a 3D interconnected TiC nanoparticle chain anode, synthesized by carbothermal conversion of graphene/TiO₂ hybrid aerogels, and a pyridine‐derived hierarchical porous nitrogen‐doped carbon (PHPNC) cathode. Electrochemical properties of both electrodes are thoroughly characterized which demonstrate their outstanding high‐rate capabilities. The fully assembled PHPNC//TiC LIC device delivers an energy density of 101.5 Wh kg^?1 and a power density of 67.5 kW kg^?1 (achieved at 23.4 Wh kg^?1), and a reasonably good cycle stability (≈82% retention after 5000 cycles) within the voltage range of 0.0?4.5 V.     

Light‐Gating Titania/Alumina Heterogeneous Nanochannels with Regulatable Ion Rectification Characteristic

Bioinspired artificial nanochannels exhibiting ion transport properties similar to biological ion channels have been attracting some attention for biosensors, separation technologies, and nanofluidic diodes. Herein, an easily available artificial heterogeneous nanochannel shows both ion gating and ion rectification characteristics when irradiated by ultraviolet light. The fabrication of heterogeneous nanochannels includes the coating of an anatase TiO₂ porous layer on an alumina porous supporter, followed by a chemical modification with octadecyltrimethoxysilane (OTS) molecules. The irreversible decomposition of OTS molecules by TiO₂ photocatalysis under ultraviolet light results in a change of surface wettability and an asymmetric distribution of surface negative charges simultaneously, which contributes to the ion gating and ion rectification. The asymmetric distribution of negative charges in the TiO₂ porous layer can be controlled by the irradiation time of ultraviolet light, which regulates the ion rectification characteristic.     

Photoinduced Directional Proton Transport through Printed Asymmetric Graphene Oxide Superstructures: A New Driving Mechanism under Full‐Area Light Illumination

2D‐material‐based membranes with densely packed sub‐nanometer‐height fluidic channels show exceptional transport properties, and have attracted broad research interest for energy‐, environment‐, and healthcare‐related applications. Recently, light‐controlled active transport of ionic species in abiotic materials have received renewed attention. However, its dependence on inhomogeneous or site‐specific illumination is a challenge for scalable application. Here, directional proton transport through printed asymmetric graphene oxide superstructures (GOSs) is demonstrated under full‐area illumination. The GOSs are composed of partially stacked graphene oxide multilayers formed by a two‐step direct ink writing process. The direction of the photoinduced proton current is determined by the position of top graphene oxide multilayers, which functions as a photogate to modulate the horizontal ion transport through the beneath lamellar nanochannels. This transport phenomenon unveils a new driving mechanism that, in asymmetric nanofluidic structures, the decay of local light intensity in depth direction breaks the balance of electric potential distribution in horizontal direction, and thus generates a photoelectric driving force for ion transport. Following this mechanism, the GOSs are developed into photonic ion transistors with three different gating modes. The asymmetrically printed photonic‐ionic devices provide fundamental elements for light‐harvesting nanofluidic circuits, and may find applications for artificial photosynthesis and artificial electric organs.     

Bamboo-Membrane Inspired Multilevel Ultrafast Interlayer Ion Transport for Superior Volumetric Energy Storage

Interlayer transport of charges and carriers of 2D nanomaterials is a critical parameter that governs the material and device performance in energy storage applications. Inspired by multilevel natural bamboo-membrane with ultrafast water and electrolyte transport properties to support its super-rapid growth rate, 2D–2D multilevel heterostructured graphene-based membranes with tailored gradient interlayer channels are rationally designed for achieving ultrafast interlayer ion transport. The bioinspired heterostructured membranes possess multilevel interlayer spacing distributions, where the closely packed layers with sub-nanosized interlayer space provide ultrafast confined interlayer ion transport, while the loosely stacked outer layers consisting of open channels with large distances up to few micrometres are favorable for rapid wetting and penetration of liquid electrolytes. The combination of advantages of large-size open channels and nanosized confined channels offers ultrafast electrolyte wetting and permeation and interlayer ion transport and provide the devices with superior volumetric capacity as free-standing electrodes for rechargeable batteries.     

Functional Separators Regulating Ion Transport Enabled by Metal-Organic Frameworks for Dendrite-Free Lithium Metal Anodes

Developing high energy density lithium secondary batteries is pivotal for satisfying the increasing demand in advanced energy storage systems. Lithium metal batteries (LMBs) have attracted growing attention due to their high theoretical capacity, but the lithium dendrites issue severely fetter their real-world applications. It is found that reducing anion migration near lithium metal prolongs the nucleation time of dendrites, meanwhile, promoting homogeneous lithium deposition suppresses the dendritic growth. Thus, regulating ion transport in LMBs is a feasible and effective strategy for addressing the issues. Based on this, a functional separator is developed to regulate ion transport by utilizing a well-designed metal-organic frameworks (MOFs) coating to functionalize polypropylene (PP) separator. The well-defined intrinsic nanochannels in MOFs and the negatively charged gap channels both restricts the free migration of anions, contributing to a high Li⁺ transference number of 0.68. Meanwhile, the MOFs coating with uniform porous structure promotes homogeneous lithium deposition. Consequently, a highly-stable Li plating/stripping cycling for over 150 h is achieved. Furthermore, implementation of the separator enables LMBs with high discharge capacity, prominent rate performance and good capacity retention. This work is anticipated to aid developement of dendrite-free LMBs by utilizing advanced separators with ion transport management.