New avenues for the large-scale harvesting of blue energy

High Efficiency Shear-Driven Nanofluidic System for Energy Conversion/Harvesting

In this work, we propose a shear-driven nanofluidic system for energy harvesting/conversion. The system consists of a nanochannel formed by two parallel walls, where the lower wall is negatively charged, while the upper wall is neutral. The motion of the upper wall caused by a shear force drives the solution in the fluidic system to move, which generates an ionic current due to the migration of excess cations in the system. Molecular dynamics simulations demonstrate that the efficiency of the system is affected by the wall charge density, shearing stress, channel height, and binding energy of the walls. The effects of these factors on the efficiency are studied. In particular, it is shown that a high binding energy for the upper wall (e.g., hydrophilic wall) can reduce the flow slip at the upper wall and effectively transfer energy from the wall to the fluid. For the lower wall, a low binding energy, which corresponds to a hydrophobic wall, can reduce the friction at the wall, enhance the flow velocity, and improve the energy conversion efficiency. By varying these parameters, it is found that the maximum energy conversion efficiency of the system reaches 65.8%, which is the highest compared with previous systems. The underlying mechanisms are explained using the slip length at the walls, wall velocity, and charge density profiles. The system proposed in this work provides insights into the design of nanofluidic systems for energy harvesting/conversion.

Angstrom-Scale 2D Channels Designed For Osmotic Energy Harvesting

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

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

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

A nanofluidic exchanger for harvesting saline gradient energy

The energy of saline gradients is a very promising source of non-intermittent renewable energy, the exploitation of which is hampered by the lack of viable technology. The most investigated harvesting methods rely on selective transport of ions or water molecules through semi-permeable or ion-selective membranes, which demonstrate limited power densities of the order of a few W m-2. While in the last decade, single nanofluidic objects such as nanopores of nanotubes have opened up very promising prospects with power density capabilities in the order of kW or even MW m-2, scale-up efforts face serious issues, as concentration polarization phenomena result in a massive loss of performance. We propose here a concept of a nanofluidic exchanger for power generation from saline gradients, focused on designing a nanoscale flow able to harvest the power at the output of the nanopores. We study analytically and numerically a simple exchanger made of a selective nanoslit fed by a nanofluidic assembly. One specific feature of such an exchanger relies on the non-linear ion fluxes through the nanoslit analytically expressed from the integration of the Poisson-Nernst-Planck equations. Such an elemental brick could be massively parallelized in stackable electricity-generating layers using standard technologies of the semi-conductor industry. We demonstrate here a scheme for rationalizing the choice of the exchanger parameters, taking into account the transport properties at all scales. The full numerical resolution of the three-dimensional device shows that net power densities of 300 W m-2 and more can be achieved.

Hydroelectric energy conversion of waste flows through hydroelectronic drag

Hydraulic energy is a key component of the global energy mix, yet there exists no practical way of harvesting it at small scales, from flows with low Reynolds number. This has triggered a search for alternative hydroelectric conversion methodologies, leading to unconventional proposals based on droplet triboelectricity, water evaporation, osmotic energy, or flow-induced ionic Coulomb drag. Yet, these approaches systematically rely on ions as intermediate charge carriers, limiting the achievable power density. Here, we predict that the kinetic energy of small-scale "waste" flows can be directly and efficiently converted into electricity thanks to the hydroelectronic drag effect, by which an ion-free liquid induces an electronic current in the solid wall along which it flows. This effect originates in the fluctuation-induced coupling between fluid motion and electron transport. We develop a nonequilibrium thermodynamic formalism to assess the efficiency of such hydroelectric energy conversion, dubbed hydronic energy. We find that hydronic energy conversion is analogous to thermoelectricity, with the efficiency being controlled by a dimensionless figure of merit. However, in contrast to its thermoelectric analogue, this figure of merit combines independently tunable parameters of the solid and the liquid, and can thus significantly exceed unity. Our findings suggest strategies for blue energy harvesting without electrochemistry, and for waste flow mitigation in membrane-based filtration processes.

Modulation of Ionic Current Rectification in Short Bipolar Nanopores

Bipolar nanopores, with asymmetric charge distributions, can induce significant ionic current rectification (ICR) at ultrashort lengths, finding potential applications in nanofluidic devices, energy conversion, and other related fields. Here, with simulations, we investigated the characteristics of ion transport and modulation of the ICR inside bipolar nanopores. With bipolar nanopores of half-positive and half-negative surfaces, the most significant ICR phenomenon appears at various concentrations. In these cases, the ICR ratios are independent of electrolyte types. In other cases where nanopores have oppositely charged surfaces of different lengths, ICR ratios are related to the mobility of anions and cations. The pore length and surface charge density can enhance ICR. As the pore length increases, ICR ratios first increase and then approach their saturation, which is determined by the surface charge density. External surface charges of nanopores can promote the ICR phenomenon mainly due to the enhancement of ion enrichment inside the nanopores by external surface conductance. The effective width of exterior charged surfaces under various conditions is also explored, which is inversely proportional to the pore length and salt concentration and linearly related to the pore diameter, surface charge density, and applied voltage. Our results may provide guidance for the design of bipolar porous membranes.

Highly Anion-Conductive Viologen-Based Two-Dimensional Polymer Membranes as Nanopower Generators

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) membranes hold great potential for harvesting sustainable osmotic energy. The nascent research has yet to simultaneously achieve high ionic flux and selectivity, primarily due to inefficient ion transport dynamics. This is directly related to ultrasmall pore size (<3 nm), much smaller than the duple Debye length in the diluted electrolyte (6-20 nm), as well as low charge density (<4.5 mC m-2). Here, we introduce a π-conjugated viologen-based 2DP (V2DP) membrane possessing a large pore size of 4.5 nm, strategically enhancing the overlapping of the electric double layer, coupled with an exceptional positive surface charge density (~6 mC m-2). These characteristics enable the membrane to facilitate high anion flux while maintaining ideal selectivity. Notably, V2DP membranes realize an impressive current density of 5.5×103 A m-2, surpassing benchmarks set by previously reported nanofluidic membranes. In the practical application scenario involving the mixing of artificial seawater and river water, the V2DP membranes exhibit a considerable ion transference number of 0.70 towards Cl-, contributing to an outstanding power density of ~55 W m-2. Theoretical calculations reveal the important role of the large quantity of anion transport sites, which act as binding sites evenly located in the positively charged N-containing pyridine rings. These binding sites enable kinematic coupling and decoupling between anions and the V2DP skeleton, establishing a continuous Cl- ion transport pathway. This work demonstrates the great promise of large-area ultrathin 2DP membranes featuring highly organized charged ion transport networks when applied for osmotic energy conversion.

Competition between ion-ion electrostatic correlations and hydrodynamic slip radically changes diffusioosmosis

Existing theories can predict separately the effects of hydrodynamic slip and ion-ion electrostatic correlations on diffusioosmosis. However, a predictive model for the coupled dynamics of hydrodynamic slip and electrostatic correlations in diffusioosmosis is lacking. In this work, we develop a mathematical model to compute the diffusioosmotic mobilities of valence-symmetric electrolytes in a charged parallel-plate channel. We employ the Navier slip condition to model the hydrodynamic slip at the channel walls and the modified Poisson equation to model ion-ion electrostatic correlations. We report two key findings arising from the competition between electrostatic correlations and hydrodynamic slip, which radically change diffusioosmosis. First, in a divalent electrolyte, a minute hydrodynamic slip defers the reversal in the direction of diffusioosmosis caused by electrostatic correlations to a higher concentration. Hydrodynamic slip can even eliminate the diffusioosmosis reversal in a monovalent electrolyte. Second, electrostatic correlations limit the change in the mobility due to hydrodynamic slip, by hindering the slip-enhanced ionic transport via surface charge overscreening. Electrostatic correlations can reduce the change in the mobility by ≈60% in a monovalent electrolyte, whereas the stronger electrostatic correlations in a divalent electrolyte can even reduce the change by an order of magnitude. The model developed from this work can be used to understand and predict diffusioosmosis in natural settings such as metamorphic transformation, in addition to that in applications such as colloidal species separation, nanoparticle drug delivery, and enhanced oil recovery.

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

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

The brine of the times

New processes directly extract lithium from dilute sources.

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

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

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

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

Modular Customized Biomimetic Nanofluidic Diode for Tunable Asymmetric Ion Transport

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

Ion transport and ultra-efficient osmotic power generation in boron nitride nanotube porins

Nanotube porins form transmembrane nanomaterial-derived scaffolds that mimic the geometry and functionality of biological membrane channels. We report synthesis, transport properties, and osmotic energy harvesting performance of another member of the nanotube porin family: boron nitride nanotube porins (BNNTPs). Cryo-transmission electron microscopy imaging, liposome transport assays, and DNA translocation experiments show that BNNTPs reconstitute into lipid membranes to form functional channels of ~2-nm diameter. Ion transport studies reveal ion conductance characteristics of individual BNNTPs, which show an unusual C1/4 scaling with ion concentration and pronounced pH sensitivity. Reversal potential measurements indicate that BNNTPs have strong cation selectivity at neutral pH, attributable to the high negative charge on the channel. BNNTPs also deliver very large power density up to 12 kW/m2 in the osmotic gradient transport experiments at neutral pH, surpassing that of other BNNT-based devices by two orders of magnitude under similar conditions. Our results suggest that BNNTPs are a promising platform for mass transport and osmotic power generation.

Slip Opacity and Fast Osmotic Transport of Hydrophobes at Aqueous Interfaces with Two-Dimensional Materials

We investigate the interfacial transport of water and hydrophobic solutes on van der Waals bilayers and heterostructures formed by stacking graphene, hBN, and MoS2 using extensive ab initio molecular dynamics simulations. We compute water slippage and the diffusio-osmotic transport coefficient of hydrophobic particles at the interface by combining hydrodynamics and the theory of the hydrophobic effect. We find that slippage is dominated by the layer that is in direct contact with water and only marginally altered by the second layer, leading to a so-called "slip opacity". The screening of the lateral forces, where the liquid does not feel the forces coming from the second nearest layer, is one of the factors leading to the "slip opacity" in our systems. The diffusio-osmotic transport of small hydrophobes (with a radius below 2.5 Å) is also affected by the slip opacity, being dramatically enhanced by slippage. Furthermore, the direction of diffusio-osmotic flow is controlled by the solute size, with the flow in the opposite direction of the concentration gradient for smaller hydrophobes, and vice versa for larger ones. We connect our findings to the wetting properties of two-dimensional materials, and we propose that slippage and wetting can be controlled separately: whereas the slippage is mostly determined by the layer in closer proximity to water, wetting can be finely tuned by stacking different two-dimensional materials. Our study advances the computational design of two-dimensional materials and van der Waals heterostructures, enabling precise control over wetting and slippage properties for applications in coatings and water purification membranes.

Enhancing Ionic Selectivity and Osmotic Energy by Using an Ultrathin Zr-MOF-Based Heterogeneous Membrane with Trilayered Continuous Porous Structure

Designing a nanofluidic membrane with high selectivity and fast ion transport property is the key towards high-performance osmotic energy conversion. However, most of reported membranes can produce power density less than commercial benchmark (5 W/m2), due to the imbalance between ion selectivity and permeability. Here, we report a novel nanoarchitectured design of a heterogeneous membrane with an ultrathin and dense zirconium-based UiO-66-NH2 metal-organic framework (MOF) layer and a highly aligned and interconnected branched alumina nanochannel membrane. The design leads to a continuous trilayered pore structure of large geometry gradient in the sequence from angstrom-scale to nano-scale to sub-microscale, which enables the enhanced directional ion transport, and the angstrom-sized (~6.6-7 Å) UiO-66-NH2 windows render the membrane with high ion selectivity. Consequently, the novel heterogeneous membrane can achieve a high-performance power of ~8 W/m2 by mixing synthetic seawater and river water. The power density can be largely upgraded to an ultrahigh ~17.1 W/m2 along with ~48.5 % conversion efficiency at a 50-fold KCl gradient. This work not only presents a new membrane design approach but also showcases the great potential of employing the zirconium-based MOF channels as ion-channel-mimetic membranes for highly efficient blue energy harvesting.

Donnan equilibrium in charged slit-pores from a hybrid nonequilibrium molecular dynamics/Monte Carlo method with ions and solvent exchange

Ion partitioning between different compartments (e.g., a porous material and a bulk solution reservoir), known as Donnan equilibrium, plays a fundamental role in various contexts such as energy, environment, or water treatment. The linearized Poisson-Boltzmann (PB) equation, capturing the thermal motion of the ions with mean-field electrostatic interactions, is practically useful to understand and predict ion partitioning, despite its limited applicability to conditions of low salt concentrations and surface charge densities. Here, we investigate the Donnan equilibrium of coarse-grained dilute electrolytes confined in charged slit-pores in equilibrium with a reservoir of ions and solvent. We introduce and use an extension to confined systems of a recently developed hybrid nonequilibrium molecular dynamics/grand canonical Monte Carlo simulation method ("H4D"), which enhances the efficiency of solvent and ion-pair exchange via a fourth spatial dimension. We show that the validity range of linearized PB theory to predict the Donnan equilibrium of dilute electrolytes can be extended to highly charged pores by simply considering renormalized surface charge densities. We compare with simulations of implicit solvent models of electrolytes and show that in the low salt concentrations and thin electric double layer limit considered here, an explicit solvent has a limited effect on the Donnan equilibrium and that the main limitations of the analytical predictions are not due to the breakdown of the mean-field description but rather to the charge renormalization approximation, because it only focuses on the behavior far from the surfaces.

Interfacial Super-Assembly of Vacancy Engineered Ultrathin-Nanosheets Toward Nanochannels for Smart Ion Transport and Salinity Gradient Power Conversion

Ion-selective nanochannel membranes assembled from two-dimensional (2D) nanosheets hold immense promise for power conversion using salinity gradient. However, they face challenges stemming from insufficient surface charge density, which impairs both permselectivity and durability. Herein, we present a novel vacancy-engineered, oxygen-deficient NiCo layered double hydroxide (NiCoLDH)/cellulose nanofibers-wrapped carbon nanotubes (VOLDH/CNF-CNT) composite membrane. This membrane, featuring abundant angstrom-scale, cation-selective nanochannels, is designed and fabricated through a synergistic combination of vacancy engineering and interfacial super-assembly. The composite membrane shows interlayer free-spacing of ~3.62 Å, which validates the membrane size exclusion selectivity. This strategy, validated by DFT calculations and experimental data, improves hydrophilicity and surface charge density, leading to the strong interaction with K+ ions to benefit the low ion transport resistance and exceptional charge selectivity. When employed in an artificial river water|seawater salinity gradient power generator, it delivers a high-power density of 5.35 W/m2 with long-term durability (20,000s), which is almost 400 % higher than that of the pristine NiCoLDH membrane. Furthermore, it displays both pH- and temperature-sensitive ion transport behavior, offering additional opportunities for optimization. This work establishes a basis for high-performance salinity gradient power conversion and underscores the potential of vacancy engineering and super-assembly in customizing 2D nanomaterials for diverse advanced nanofluidic energy devices.

A Flexible Hybrid Generator for Efficient Dual Energy Conversion from Raindrops to Electricity

Electromagnetic generators are conventionally used to harvest energy from large water bodies, but they are ineffective for harvesting low hydro-energy, such as raindrops or fogs, due to their bulky, heavy and immovable. Unfortunately, developing new strategies that are lightweight, small, and have high conversion efficiency to convert such low hydro-energy into electricity remains a challenge. Herein, a flexible droplet-based hybrid electricity generator (DHEG) consisting of a droplet-based electricity generator (DEG) and an electromagnetic generator (EMG) is proposed to convert the dual energy of water droplets into electricity simultaneously. The DHEG is assembled by facilely merging DEG and EMG using conductive elastic multi-walled carbon nanotubes/polydimethylsiloxane (MWCNTs/PDMS) film. The MWCNTs/PDMS film can not only serve as a bottom electrode for switching on the DEG, but also as an elastic component for the EMG to vibrate the coil when impacted by water droplets. Activated by a single 58.2 µL droplet falling from a height of 50 cm, the peak voltage, current and power generated by the DHEG are ≈84.6 V, ≈19.85 mA, and ≈595.8 µW, respectively. The energy conversion efficiency of the DHEG is up to ≈13.8%. This flexible hybrid generator may provide a promising strategy for effectively harvesting energy from raindrops.

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

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

Understanding the K+/Na+-Selectivity-Enabled Osmotic Power Generation: High Selectivity May Not Be Indispensable

By mixing ionic solutions, considerable energy can be harvested from entropy change. Recently, we proposed a concept of potassium-permselectivity enabled osmotic power generation (PoPee-OPG) by mixing equimolar KCl and NaCl solutions via artificial potassium ion channels (APICs, Natl. Sci. Rev. 2023, 10, nwad260). However, a fundamental understanding of the relationship between the K+/Na+ selectivity and optimal performance remains unexplored. Herein, we establish a primitive molecular thermodynamic model to investigate the energy extraction process. We find PoPee-OPG differs from previous charge-selectivity-based techniques, such as the salinity gradient power generation, in two distinct ways. First, the extractable energy density and efficiency positively depend on concentration. More surprisingly, a very high potassium selectivity is not indispensable for satisfactory efficiency and energy density. An optimal K+/Na+ selectivity region of 3 to 10 is found. This somewhat counterintuitive discovery provides a renewed understanding of the emerging PoPee-OPG, and it predicts a broad applicability among existing APICs.

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.

Estimating water transport in carbon nanotubes: a critical review and inclusion of scale effects

The quasi-frictionless water flow across graphitic surfaces offers vast opportunities for a wide range of applications from biomedical science to energy. However, the conflicting experimental results impede a clear understanding of the transport mechanism and desired flow control. Existing literature proposes numerous modifications and updated boundary conditions to extend classical hydrodynamic theories for nanoflows, yet a consensus or definitive conclusion remains elusive. This study presents a critical review of the proposed modifications of the pressure driven flow or the Hagen-Poiseuille (HP) equations to estimate the flow enhancement through carbon nanotubes (CNTs). For such a case, we performed (semi-)classical molecular dynamics simulations of water flow in various sizes of CNTs, applied the different forms of boundary definitions from the literature, and derived HP equation models by implementing these modifications. By aggregating seven distinct experimental datasets, we tested various flow enhancement models against our measurements. Our findings indicate that including the interfacial layering-based dynamic slip-definition in the proposed HP equations yields accurate estimations. While considering interfacial viscosity predicts the individual CNT experiments well, using the experimental viscosity yields better estimations of measurements for the water flow enhancement through membranes of CNTs. This critical review testing existing literature demonstrates how to refine continuum fluid mechanics to predict water flow enhancement at the nanoscale providing holistic multiscale modeling.

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

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

Holey Sheets Enhance the Packing and Osmotic Energy Harvesting of Graphene Oxide Membranes

Layered membranes assembled from two-dimensional (2D) building blocks such as graphene oxide (GO) are of significant interest in desalination and osmotic power generation because of their ability to selectively transport ions through interconnected 2D nanochannels between stacked layers. However, architectural defects in the final assembled membranes (e.g., wrinkles, voids, and folded layers), which are hard to avoid due to mechanical compliant issues of the sheets during the membrane assembly, disrupt the ionic channel pathways and degrade the stacking geometry of the sheets. This leads to degraded ionic transport performance and the overall structural integrity. In this study, we demonstrate that introducing in-plane nanopores on GO sheets is an effective way to suppress the formation of such architectural imperfections, leading to a more homogeneous membrane. Stacking of porous GO sheets becomes significantly more compact, as the presence of nanopores makes the sheets mechanically softer and more compliant. The resulting membranes exhibit ideal lamellar microstructures with well-aligned and uniform nanochannel pathways. The well-defined nanochannels afford excellent ionic conductivity with an effective transport pathway, resulting in fast, selective ion transport. When applied as a nanofluidic membrane in an osmotic power generation system, the holey GO membrane exhibits higher osmotic power density (13.15 W m-2) and conversion efficiency (46.6%) than the pristine GO membrane under a KCl concentration gradient of 1000-fold.

Potentiometric Studies on Ion-Transport Selectivity in Charged Gold Nanotubes

Under ideal conditions, nanotubes with a fixed negative tube-wall charge will reject anions and transport-only cations. Because many proposed nanofluidic devices are optimized in this ideally cation-permselective state, it is important to know the experimental conditions that produce ideal responses. A parameter called Ccrit, the highest salt concentration in a contacting solution that still produces ideal cation permselectivity, is of particular importance. Pioneering potentiometric studies on gold nanotubes were interpreted using an electrostatic model that states that Ccrit should occur when the Debye length in the contacting salt solution becomes equivalent to the tube radius. Since this "double-layer overlap model" (DLOM), treats all same-charge ions as identical point charges, it predicts that all same-charged cations should produce the same Ccrit. However, the effect of cation on Ccrit in gold nanotubes was never investigated. This knowledge gap has become important because recent studies with a polymeric cation-permselective nanopore membrane showed that DLOM failed for every cation studied. To resolve this issue, we conducted potentiometric studies on the effect of salt cation on Ccrit for a 10 nm diameter gold nanotube membrane. Ccrit for all cations studied were, within experimental error, the same and identical, with values predicted by DLOM. The reason DLOM prevailed for the gold nanotubes but failed for the polymeric nanopores stems from the chemical difference between the fixed negative charges of these two membranes.

Heterogeneous CNF/MoO3 nanofluidic membranes with tunable surface plasmon resonances for solar-osmotic energy conversion

Two-dimensional (2D) nanofluidic membranes are competitive candidates for osmotic energy harvesting and have been greatly developed. However, the use of diverse inherent characteristics of 2D nanosheets, such as electronic or optoelectronic properties, to achieve intelligent ion transport, still lacks sufficient exploration. Here, a cellulose nanofiber/molybdenum oxide (CNF/MoO3) heterogeneous nanofluidic membrane with high performance solar-osmotic energy conversion is reported, and how surface plasmon resonances (SPR) regulate selective cation transport is revealed. The SPR of amorphous MoO3 endows the heterogeneous nanofluidic membranes with tunable surface charge and good photothermal conversion. Through DFT calculations and finite element modeling, the regulation of electronic and optoelectronic properties on the surface of materials by SPR and the influence of surface charge density and temperature gradient on ion transport in nanofluidic membranes are demonstrated. By mixing 0.01/0.5 M NaCl solutions using SPR and photothermal effects, the power density can achieve a remarkable value of ≈13.24 W m-2, outperforming state-of-the-art 2D-based nanofluidic membranes. This work first reveals the regulation and mechanism of SPR on ion transport in nanofluidic membranes and systematically studies photon-electron-ion interactions in nanofluidic membranes, which could also provide a new viewpoint for promoting osmotic energy conversion.

To Pair or not to Pair? Machine-Learned Explicitly-Correlated Electronic Structure for NaCl in Water

The extent of ion pairing in solution is an important phenomenon to rationalize transport and thermodynamic properties of electrolytes. A fundamental measure of this pairing is the potential of mean force (PMF) between solvated ions. The relative stabilities of the paired and solvent shared states in the PMF and the barrier between them are highly sensitive to the underlying potential energy surface. However, direct application of accurate electronic structure methods is challenging, since long simulations are required. We develop wave function based machine learning potentials with the random phase approximation (RPA) and second order Møller-Plesset (MP2) perturbation theory for the prototypical system of Na and Cl ions in water. We show both methods in agreement, predicting the paired and solvent shared states to have similar energies (within 0.2 kcal/mol). We also provide the same benchmarks for different DFT functionals as well as insight into the PMF based on simple analyses of the interactions in the system.

Dynamic Response of Ionic Current in Conical Nanopores

Ionic current rectification (ICR) of charged conical nanopores has various applications in fields including nanofluidics, biosensing, and energy conversion, whose function is closely related to the dynamic response of nanopores. The occurrence of ICR originates from the ion enrichment and depletion in conical pores, whose formation is found to be affected by the scanning rate of voltages. Here, through time-dependent simulations, we investigate the variation of ion current under electric fields and the dynamic formation of ion enrichment and depletion, which can reflect the response time of conical nanopores. The response time of nanopores when ion enrichment forms, i.e., at the "on" state is significantly longer than that with the formation of ion depletion, i.e., at the "off" state. Our simulation results reveal the regulation of response time by different nanopore parameters including the surface charge density, pore length, tip, and base radius, as well as the applied conditions such as the voltage and bulk concentration. The response time of nanopores is closely related to the surface charge density, pore length, voltage, and bulk concentration. Our uncovered dynamic response mechanism of the ionic current can guide the design of nanofluidic devices with conical nanopores, including memristors, ionic switches, and rectifiers.

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

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

Unleashing the Power of Osmotic Energy: Metal Hydroxide-Organic Framework Membranes for Efficient Conversion

Osmotic energy, as a renewable clean energy with huge energy density and stable yield, has received widespread attention over the past decades. Reverse electrodialysis (RED) based on ion-exchange membranes is an important method of obtaining osmotic energy from salinity gradients. The preparation of ion-exchange membranes with both high ion selectivity and ion permeability is in constant exploration. In this work, metal hydroxide-organic framework (MHOF) membranes are successfully prepared onto porous anodic aluminum oxide (AAO) membranes by a facile hydrothermal method to form Ni2(OH)2@AAO composite membranes, used for osmotic energy conversion. The surface is negatively charged with cation selectivity, and the asymmetric structure and extreme hydrophilicity enhance the ionic flux for effective capture of osmotic energy. The maximum output power density of 5.65 W m-2 at a 50-fold KCl concentration gradient is achieved, which exceeds the commercial benchmark of 5 W m-2. Meanwhile, the composite membrane can also show good performance in different electrolyte solutions and acid-base environments. This work provides a new avenue for the construction and application of MHOF membranes in efficient osmotic energy conversion.

Electric Eel Biomimetics for Energy Storage and Conversion

The electric eel is known as the most powerful creature to generate electricity with a discharge voltage up to 860 V and peak current up to 1 A. These surprising properties are the results of billions of years of evolution on the electrical biological structure and bulk, and now have triggered great research interest in electric eel biomimetics for designing innovated configurations and components of energy storage and conversion devices. In this review, first, the bioelectrical behavior of electric eels is surveyed, followed by the physiological structure to reveal the discharge characteristics and principles of electric organs and electrocytes. Additionally, underlying electrochemical mechanisms and models for calculating the potential and current of electrocytes are presented. Central to this review is the recent progress of electric-eel-inspired innovations and applications for energy storage and conversion, particularly including novel power sources, triboelectric nanogenerators, and nanochannel ion-selective membranes for salinity gradient energy harvesting. Finally, insights on the challenges at the moment and the perspectives on the future research prospects are critically compiled. It is suggested that energy-related electric eel biomimetics will greatly boost the development of next-generation high performance, green, and functional electronics.

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.

Effect of Organic Cation Adsorption on Ion-Transport Selectivity in a Cation-Permselective Nanopore Membrane

A key knowledge gap in the emerging field of nanofluidics concerns how the ionic composition and ion-transport properties of a nanoconfined solution differ from those of a contacting bulk solution. We and others have been using potentiometric concentration cells, where a nanopore or nanotube membrane separates salt solutions of differing concentrations to explore this issue. The membranes studied contained a fixed pore/tube wall anionic charge, which ideally would prohibit anions and salt from entering the pore/tube-confined solution. We have been investigating experimental conditions that allow for this ideally permselective cation state to be achieved. Results of potentiometric investigations of a polymeric nanopore membrane (10 ± 2 nm-diameter pores) with anionic charge due to carbonate are presented here. While studies of this type have been reported using alkaline metal and alkaline earth cations, there have been no analogous studies using organic cations. This paper uses a homologous series of tetraalkylammonium ions to address this knowledge gap. The key result is that, in contrast to the inorganic cations, the ideal cation-permselective state could not be obtained under any experimental conditions for the organic cations. We propose that this is because these hydrophobic cations adsorb onto the polymeric pore walls. This makes ideality impossible because each adsorbed alkylammonium must bring a charge-balancing anion, Cl-, with it into the nanopore solution. The alkylammonium adsorption that occurred was confirmed and quantified by using surface contact angle measurements.

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

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

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

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

Diffusiophoretic Fast Swelling of Chemically Responsive Hydrogels

Acid-induced release of stored ions from polyacrylic acid hydrogels (with a free surface fully permeable to the ion and acid) was observed to increase the gel osmotic pressure that leads to rapid swelling faster than the characteristic solvent absorption rate of the gel. The subsequent equilibration of the diffusing ion concentration across the gel surface diminishes the osmotic pressure. Then, the swollen gel contracts, thereby completing one actuation cycle. We develop a continuum poroelastic theory that explains the experiments by introducing a "gel diffusiophoresis" mechanism: Steric repulsion between the gel polymers and released ions can induce a diffusio-osmotic solvent intake counteracted by the diffusiophoretic expansion of the gel network that ceases when the ion gradient vanishes. For applications ranging from drug delivery to soft robotics, engineering the gel diffusiophoresis may enable stimuli-responsive hydrogels with amplified strain rates and power output.

Commercial Nafion Membranes for Harvesting Osmotic Energy from Proton Gradients that Exceed the Commercial Goal of 5.0 W/m2

Osmotic energy from proton gradients in industrial acidic wastewater can be harvested and converted to electricity through membranes, making it a renewable and sustainable power source. However, the currently designed membranes for harvesting proton gradient energy in acidic wastewater cannot simultaneously achieve excellent chemical/mechanical stability and high power density under a large-scale area and require high cost and complex operations. Here, we demonstrate that commercial Nafion membranes with high chemical/mechanical stability and proton transport selectivity can generate a power density of 5.1 W/m2 for harvesting osmotic energy from proton gradients under a test area of 0.2 mm2, which exceeds the commercial goal of 5.0 W/m2. Even under a test area of 12.5 mm2, a power density of 2.1 W/m2 can be achieved under a strong acid condition. In addition, the heat can greatly promote proton transport, and the power density is increased, i.e., 8.1 W/m2 at 333 K (5.1 W/m2 at 293 K) under a test area of 0.2 mm2. By matching membranes with ion selectivity, our work demonstrates the potential of Nafion membranes for harvesting proton gradient energy in acidic wastewater and provides an approach for large-scale conversion of osmotic energy.

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.

Predicting Quantum-Mechanical Partial Charges in Arbitrarily Long Boron Nitride Nanotubes to Accurately Simulate Nanoscale Water Transport

Single-walled boron nitride nanotubes (BNNTs) have been explored for various applications, ranging from water desalination to osmotic power harvesting. However, no simulation work so far has modeled the changes in the partial charge distribution when a flat sheet is rolled into a tube, hindering the ability to perform accurate molecular dynamics (MD) simulations of water flow through BNNTs. To address this knowledge gap, we employ electronic density functional theory (DFT) calculations to precisely estimate quantum-mechanically derived partial charges on boron (B) and nitrogen (N) atoms in BNNTs of varying lengths and diameters. We observe a spatially varying charge distribution inside both armchair and zigzag nanotubes of finite lengths. Performing DFT calculations for longer BNNTs is computationally intractable, even with state-of-the-art computing resources. To solve this issue, we devise a charge assignment scheme to predict partial charges for longer BNNTs using DFT data for shorter nanotubes, thus overcoming the need to perform more expensive DFT calculations. We show that these charges reproduce the electrostatic potential predicted from first-principles simulations. Subsequently, we carried out MD simulations to predict the effect of the charge distribution inside BNNTs on water flow enhancement via them. We find that using uniform charges leads to an underprediction in flow enhancement, as compared to using quantum-mechanical charges for both armchair and zigzag BNNTs. We also incorporate atomic vibrations into our simulations and show that these vibrations lead to a reduction in the water flow through aperiodic BNNTs. Our work demonstrates the requirement of a quantum-mechanical charge assignment scheme for BNNTs and evolves a framework to assign charges to nanotubes of arbitrary length, thus allowing realistic MD simulations of long BNNTs using accurate partial charges.

Ion Transport in Multi-Nanochannels Regulated by pH and Ion Concentration

Nanochannels are a powerful technique for detecting a wide range of biomolecules without labeling. The ion transport phenomena in nanochannel arrays differ from those in single nanochannels and are caused by interchannel communication. This study uses a fully coupled Poisson-Nernst-Planck (PNP) and Navier-Stokes model to investigate ion transport in nanochannel arrays. Instead of being set at a constant value, the surface charge density used in this study is established by the protonation and deprotonation of the silanol groups that are present on the walls of the silicon-based nanochannels. The surface charge density of the nanochannel walls varies with the number of nanochannels, the channel lateral distance, and the background solution properties, which consequently influence the ionic concentration distribution, flow velocity, and electric field strength. For example, in different numbers of nanochannel systems, the ion concentration in nanochannels is not much different, but it is different in reservoirs, especially near the openings of nanochannels. The number of nanochannels and the distance between nanochannels can also affect the formation of electro-convective vortex zones under certain conditions. These findings can aid in optimizing the nanochannel array design by regulating the number and distance of nanochannels and facilitating the construction of solid-state nanochannel arrays with any desired nanochannel dimensions.

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.

The surface force balance: direct measurement of interactions in fluids and soft matter

Over the last half-century, direct measurements of surface forces have been instrumental in the exploration of a multitude of phenomena in liquid, soft, and biological matter. Measurements of van der Waals interactions, electrostatic interactions, hydrophobic interactions, structural forces, depletion forces, and many other effects have checked and challenged theoretical predictions and motivated new models and understanding. The gold-standard instrument for these measurements is thesurface force balance(SFB), orsurface forces apparatus, where interferometry is used to detect the interaction force and distance between two atomically smooth planes, with 0.1 nm resolution, over separations from about 1 µm down to contact. The measured interaction forcevs.distance gives access to the free energy of interaction across the fluid film; a fundamental quantity whose general form and subtle features reveal the underlying molecular and surface interactions and their variation. Motivated by new challenges in emerging fields of research, such as energy storage, biomaterials, non-equilibrium and driven systems, innovations to the apparatus are now clearing the way for new discoveries. It is now possible to measure interaction forces (and free energies) with control of electric field, surface potential, surface chemistry; to measure time-dependent effects; and to determine structurein situ. Here, we provide an overview the operating principles and capabilities of the SFB with particular focus on the recent developments and future possibilities of this remarkable technique.

Ion transport in nanofluidics under external fields

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

Bioinspired 2D nanofluidic membranes for energy applications

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

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

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

Molecular self-assembled cellulose enabling durable, scalable, high-power osmotic energy harvesting

In recent years, renewable cellulose-based ion exchange membranes have emerged as promising candidates for capturing green, abundant osmotic energy. However, the low power density and structural/performance instability are challenging for such cellulose membranes. Herein, cellulose-molecule self-assembly engineering (CMA) is developed to construct environmentally friendly, durable, scalable cellulose membranes (CMA membranes). Such a strategy enables CMA membranes with ideal nanochannels (∼7 nm) and tailored channel lengths, which support excellent ion selectivity and ion fluxes toward high-performance osmotic energy harvesting. Finite element simulations also verified the function of tailored nanochannel length on osmotic energy conversion. Correspondingly, our CMA membrane shows a high-power density of 2.27 W/m2 at a 50-fold KCl gradient and super high voltage of 1.32 V with 30-pair CMA membranes (testing area of 22.2 cm2). In addition, the CMA membrane demonstrates long-term structural and dimensional integrity in saline solution, due to their high wet strength (4.2 MPa for N-CMA membrane and 0.5 MPa for P-CMA membrane), and correspondingly generates ultrastable yet high power density more than 100 days. The self-assembly engineering of cellulose molecules constructs high-performance ion-selective membranes with environmentally friendly, scalable, high wet strength and stability advantages, which guide sustainable nanofluidic applications beyond the blue energy.

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

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

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

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