Nanopore Functionalized by Highly Charged Hydrogels for Osmotic Energy Harvesting

Modulating nanopore size and ion transport using (Anti)-Polyelectrolyte effects inspired by the nuclear pore complex

This study explores the modulation of nanopore size and ion transport through (anti)-polyelectrolyte effects, which is inspired by the nuclear pore complex. We aimed to control ionic selectivity and rectification by applying these effects to synthetic nanopores. Single bicylindrical nanopores were fabricated on the PET membranes and functionalized with PEI/HA or PLL/PAA polyelectrolyte layers. Varying the structural and charge characteristics under different pH levels and ionic strengths revealed that at low salt concentrations, charge density and surface charge polarity significantly impacted ion selectivity and transport. At higher concentrations, conformational changes in the polyelectrolytes influenced the conductance via volume expansion or compaction. Our findings highlight the distinct roles of charge inversion and molecular expansion in nanopore transport, which can be modulated by pH and ionic environment. This work provides insights for developing highly selective ion channels with potential applications in filtration, biosensing, and nanofluidics, where precise ion transport and selective rectification are essential.

Nanochannel functionalization using POFs: Progress and prospects

Biomimetic nanochannels, inspired by natural ion channels found in living organisms, are synthetic systems designed to replicate the highly selective and efficient ion/molecule transport processes essential for various biological functions. These artificial channels mimic the structural and functional properties of their biological counterparts, offering precise control over ion and molecular transport. Porous organic framework materials (POFs), including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have emerged as promising materials for functionalizing nanochannels due to their unique structures and exceptional properties. This functionalization strategy not only enhances the performance of synthetic nanochannels but also broadens their application potential across various fields. This review comprehensively examines the recent progress in the preparation and application of POFs stereoscopic-functionalized solid nanochannels. Special emphasis is placed on their practical applications, including proton conduction, ion-selective membranes, photo-responsive materials, sensing and detection, chiral separation, and catalysis. Finally, the future development prospects and challenges in this research area are discussed, highlighting opportunities for advancing the design and application of biomimetic nanochannels.

Engineered Ionic Rectifier with Steep Channel Gradient from Angstrom-Scale to Mesoscale Based on Ultrathin MXene-Capped Single Conical Mesochannel: A Promising Platform for Efficient Osmotic Energy Generation

Ionic rectifier that mimics the directional ion transport in biological ion channels has been shown with potential toward boosting osmotic energy conversion performance. However, the achieved power by existing rectifying devices is still limited, because they are constructed based on tiny nanoscale channels, which experience high resistance. Here, a novel high-performance ionic rectifier (abbreviated as MXene@MC) with steep channel gradient from angstrom-scale to mesoscale is reported by capping an ultrathin 2D Ti3C2Tx MXene laminate on an asymmetric conical mesochannel (MC). The device can strongly rectify ionic current (with a high ratio of 7.3-fold) even in high 0.5 m electrolyte solution, and thus a single channel can achieve an ultra-large osmotic conductance of 0.596 µS. These features enable MXene@MC as an ultrahigh performance osmotic energy generator, achieving an unprecedented osmotic power of 343 pW under a 1000-fold salinity gradient at neutral pH. Notably, simulations are also provided to demonstrate the findings of the proposed ionic rectifier and efficient osmotic energy conversion. This study unravels the underlying physics of ion transport induced by the apparent structural asymmetry of ion-selective channels, thereby providing a promising platform for further development of high-performance osmotic energy generators.

Shape Optimization of Nanopores by Dissolutive Flow

Nanopores are ubiquitous across disciplines, ranging from bioscience to energy science. Enhancing their transport properties and energy storage capability is crucial to improving functionality. Dissolutive flow, which involves the removal of geometrical obstructions, can transform solid structures into shapes that are less obstructive. In this study, we utilized molecular dynamics simulations to optimize the shape of nanopores through a dissolution-based approach, resulting in optimized inlet and outlet configurations. Furthermore, we examined the distribution of pressure and stress to elucidate the mechanisms underlying shape evolution and edge effects. Conventional inlets impede liquid flow due to high pressure at the apex, a phenomenon we refer to as the reservoir pattern. As dissolution progresses, the apex pressure is significantly reduced over time, allowing a transition to the sliding pattern. By manipulating the dissolubility, wall velocity, and nanopore length, the optimized shape can be fine-tuned. Experiments conducted at the microscale confirmed the emergence of analogous optimized shapes. Additionally, we investigated the transport properties and energy storage capability of various nanopores using molecular dynamics simulations, demonstrating that the optimized nanopore can significantly reduce the hydrodynamic resistance and accelerate the charging rate. This research offers theoretical insights into nanopore dissolution and presents a viable strategy for the practical fabrication of optimized nanopores.

Maximizing Blue Energy via Densely Grafted Soft Layers in Nanopores

We investigate energy generation from salinity gradients inside a nanopore that is connected to reservoirs at both ends. We consider that the inner wall surfaces are grafted with a densely grafted polyelectrolyte layer (PEL). We developed the PEL grafting density-dependent correlation of dielectric permittivity, molecular diffusivity, and dynamic viscosity in this endeavor. Using these correlations, we employ the finite element framework to solve the equations describing the ionic and fluidic transport. We use a partially hydrolyzed polyacrylamide polymer solution, which exhibits a shear-thinning fluid, in combination with the KCl electrolyte for energy-harvesting analysis. To describe the shear-rate-dependent apparent viscosity of non-Newtonian liquid, we have employed the Carreau model. For a window of right-side reservoir concentration, we investigate the effects of ion-partitioning in conjugation with the change in PEL grafting density on the ionic field, ionic selectivity, pore current, osmotic power, energy conversion efficiency, and flow field. The findings of this endeavor demonstrate how the ion-partitioning effect lowers the screening effect and raises the electrical double layer (EDL) potential by reducing the counterions in PEL. We show that the unique distribution of the ionic field leads to a higher prediction of generated osmotic power and power density due to the ion-parting effect. Additionally, we establish that the augmentation in PEL space charge density leads to improvement in average flow velocity, osmotic power, and consequently energy conversion efficiency. We establish that the generated osmotic power density and the energy conversion efficiency become very high at the higher grafting density. In summary, inferences of this analysis are deemed pertinent in designing the nanoscale device intended for high and efficient osmotic energy generation.

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.

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

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

Reproducibility and stability of silane layers in nanoconfined electrochemical systems

Organosilanes are commonly utilized to attach bioreceptors to oxide surfaces. The deposition of such silane layers is especially challenging in nanoscale or nanoconfined devices, such as in nanopipettes, since rinsing off loosely bound silanes may not be possible due to geometric constrictions and because the thickness of multilayered silanes can cover or block nanoscale features. Furthermore, in electrochemical devices, the silane layers experience additional perturbations, such as electric migration and electroosmotic force. Despite its importance, there appears to be no consensus in the current literature on the optimal methodology for nanopipette silanization, with significant variations in reported conditions. Herein, we systematically investigate the reproducibility and stability of liquid- and vapor-phase deposited silane layers inside nanopipettes. Electrochemical monitoring of the changing internal silanized surface reveals that vapor-deposited APTES generates surface modifications with the highest reproducibility, while vapor-deposited APTMS generates surface modifications of the highest stability over a 24-hour time period. Practical issues of silanizing nanoconfined systems are highlighted, and the importance of carefully chosen silanization conditions to yield stable and reproducible monolayers is emphasized as an underappreciated aspect in the development of novel nanoscale systems.

Highly Efficient Conversion of Salinity Difference to Electricity in Nanofluidic Channels Boosted by Variable Thickness Polyelectrolyte Coating

The inherent limits of the current produced by imposing salinity gradients along a nanofluidic channel having "hard" boundary walls heavily constrain the resulting energy harvesting efficacy, acting as major hindrances against the practicability of harnessing high power density from the mixing of water having different salinities. In this work, the infusion of variable-thickness polyelectrolyte layer of a conical shape is projected to augment salinity gradient power generation in nanochannels. Such a progressive thickening of a charged interfacial layer on account of axially declining ion concentration facilitates the shedding of enhanced numbers of mobile ions, bearing a net charge of equal and opposite to the surface-bound ions, into the mainstream current flow. We show that the proposed design can convert energy at a higher efficiency as compared to both solid-state and available polyelectrolyte layer (PEL)-covered nanochannels. The same is true for the maximum power density at moderate and high concentration ratios including natural salt gradient conditions for which more than 50% increase is achievable. The maximum values achieved for efficiency and power density read 50.3% and 6.6 kW/m2, respectively. Our results provide fundamental insights on strategizing variable-thickness polyelectrolyte layer grafting on the nanochannel interfaces, toward realizing high-performance osmotic power generators by altering the local ionic clouds alongside the grafted layers and enhancing the ionic mobility by inducing a driving potential gradient concomitantly. These findings open up a new strategy of efficient conversion of the power of the salinity difference of seawater and river water into electricity in a nanofluidic framework, surpassing the previously established limits of blue energy harvesting technologies.

Experimental and Theoretical Insights into Interfacial Properties of 2D Materials for Selective Water Transport Membranes: A Critical Review

Interfacial properties, such as wettability and friction, play critical roles in nanofluidics and desalination. Understanding the interfacial properties of two-dimensional (2D) materials is crucial in these applications due to the close interaction between liquids and the solid surface. The most important interfacial properties of a solid surface include the water contact angle, which quantifies the extent of interactions between the surface and water, and the water slip length, which determines how much faster water can flow on the surface beyond the predictions of continuum fluid mechanics. This Review seeks to elucidate the mechanism that governs the interfacial properties of diverse 2D materials, including transition metal dichalcogenides (e.g., MoS2), graphene, and hexagonal boron nitride (hBN). Our work consolidates existing experimental and computational insights into 2D material synthesis and modeling and explores their interfacial properties for desalination. We investigated the capabilities of density functional theory and molecular dynamics simulations in analyzing the interfacial properties of 2D materials. Specifically, we highlight how MD simulations have revolutionized our understanding of these properties, paving the way for their effective application in desalination. This Review of the synthesis and interfacial properties of 2D materials unlocks opportunities for further advancement and optimization in desalination.

Addressing Challenges in Ion-Selectivity Characterization in Nanopores

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors. Quantitative characterization of ion selectivities in nanopores often employs the Nernst or Goldman-Hodgkin-Katz (GHK) equation to interpret transmembrane potentials. However, the direction of the measured transmembrane potential drop is not specified in these equations, and selectivity values calculated using absolute values of transmembrane potentials do not directly reveal the ion for which the membrane is selective. Moreover, researchers arbitrarily choose whether to use the Nernst or GHK equation and overlook the significant differences between them, leading to ineffective quantitative comparisons between studies. This work addresses these challenges through (a) specifying the transmembrane potential (sign) and salt concentrations in terms of working and reference electrodes and the solutions in which they reside when using the Nernst and GHK equations, (b) reporting of both Nernst-selectivity and GHK-selectivity along with solution compositions and transmembrane potentials when comparing different nanopores/channels, and (c) performing simulations to define an ideal selectivity for nanochannels. Experimental and modeling studies provide significant insight into these fundamental equations and guidelines for the development of nanopore/channel-based devices.

Advances in Membrane Separation for Biomaterial Dewatering

Biomaterials often contain large quantities of water (50-98%), and with the current transition to a more biobased economy, drying these materials will become increasingly important. Contrary to the standard, thermodynamically inefficient chemical and thermal drying methods, dewatering by membrane separation will provide a sustainable and efficient alternative. However, biomaterials can easily foul membrane surfaces, which is detrimental to the performance of current membrane separations. Improving the antifouling properties of such membranes is a key challenge. Other recent research has been dedicated to enhancing the permeate flux and selectivity. In this review, we present a comprehensive overview of the design requirements for and recent advances in dewatering of biomaterials using membranes. These recent developments offer a viable solution to the challenges of fouling and suboptimal performances. We focus on two emerging development strategies, which are the use of electric-field-assisted dewatering and surface functionalizations, in particular with hydrogels. Our overview concludes with a critical mention of the remaining challenges and possible research directions within these subfields.

Engineering Electrode Rinse Solution Fluidics for Carbon-Based Reverse Electrodialysis Devices

Natural salinity gradients are a promising source of so-called "blue energy", a renewable energy source that utilizes the free energy of mixing for power generation. One promising blue energy technology that converts these salinity gradients directly into electricity is reverse electrodialysis (RED). Used at its full potential, it could provide a substantial portion of the world's electricity consumption. Previous theoretical and experimental works have been done on optimizing RED devices, with the latter often focusing on precious and expensive metal electrodes. However, in order to rationally design and apply RED devices, we need to investigate all related transport phenomena─especially the fluidics of salinity gradient mixing and the redox electrolyte at various concentrations, which can have complex intertwined effects─in a fully functioning and scalable system. Here, guided by fundamental electrochemical and fluid dynamics theories, we work with an iron-based redox electrolyte with carbon electrodes in a RED device with tunable microfluidic environments and study the fundamental effects of electrolyte concentration and flow rate on the potential-driven redox activity and power output. We focus on optimizing the net power output, which is the difference between the gross power output generated by the RED device and the pumping power input, needed for salinity gradient mixing and redox electrolyte reactions. We find through this holistic approach that the electrolyte concentration in the electrode rinse solution is crucial for increasing the electrical current, while the pumping power input depends nonlinearly on the membrane separation distance. Finally, from this understanding, we designed a five cell-pair (CP) RED device that achieved a net power density of 224 mW m-2 CP-1, a 60% improvement compared to the nonoptimized case. This study highlights the importance of the electrode rinse solution fluidics and composition when rationally designing RED devices based on scalable carbon-based electrodes.

Nanofluidic diodes based on asymmetric bio-inspired surface coatings in straight glass nanochannels

In this study, we present nanofluidic diodes fabricated from straight glass nanochannels and functionalized using bio-inspired polydopamine (PDA) and poly-L-lysine (PLL) coatings. The resulting PDA coatings are shown to be asymmetric due to a combination of transport considerations which can be leveraged to provide a measure of control over the effective channel geometry. By subsequently introducing a layer of amine-bearing PLL chains covalently bound to the PDA, we enhance heterogeneities in the charge and ion distributions within the channel and enable significant current rectification between forward-bias and reverse-bias modes; our PDA-PLL-coated channels yielded a rectification ratio greater than 1000 in a 100 nm channel filled with 0.01× phosphate-buffered saline solution (PBS). We further demonstrated that at higher ionic strength conditions, reducing the solution pH increased the number of protonated amines within the PLL layer, amplifying the charge disparities along the channel and leading to greater rectification. As nanofluidic diodes with bipolar surface charge distributions tend to provide superior performance compared to those with a single wall charge polarity, we imposed a more bipolar charge distribution in our devices by partially coating our PDA-PLL-coated channels with negatively charged polyacrylic acid (PAA). These enhanced bipolar channels exhibited greater current rectification than the PDA-PLL-coated channels, reaching rectification ratios in excess of 100 even in more physiologically-relevant 1× PBS solutions. Our fabrication approach and the results herein provide a promising platform from which the scientific community can build upon in the relentless endeavor for improved sensitivity in biosensors and other analytical devices.

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

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

Thermo-Osmosis in Charged Nanochannels: Effects of Surface Charge and Ionic Strength

Thermo-osmosis refers to fluid migration due to the temperature gradient. The mechanistic understanding of thermo-osmosis in charged nano-porous media is still incomplete, while it is important for several environmental and energy applications, such as low-grade waste heat recovery, wastewater recovery, fuel cells, and nuclear waste storage. This paper presents results from a series of molecular dynamics simulations of thermo-osmosis in charged silica nanochannels that advance the understanding of the phenomenon. Simulations with pure water and water with dissolved NaCl are considered. First, the effect of surface charge on the sign and magnitude of the thermo-osmotic coefficient is quantified. This effect was found to be mainly linked to the structural modifications of an aqueous electrical double layer (EDL) caused by the nanoconfinement and surface charges. In addition, the results illustrate that the surface charges reduce the self-diffusivity and thermo-osmosis of interfacial liquid. The thermo-osmosis was found to change direction when the surface charge density exceeds -0.03C · m^-2. It was found that the thermo-osmotic flow and self-diffusivity increase with the concentration of NaCl. The fluxes of solvent and solute are decoupled by considering the Ludwig-Soret effect of NaCl ions to identify the main mechanisms controlling the behavior. In addition to the advance in microscopic quantification and mechanistic understanding of thermo-osmosis, the work provides approaches to investigate a broader category of coupled heat and mass transfer problems in nanoscale space.

Interfacial Super-Assembly of Intertwined Nanofibers toward Hybrid Nanochannels for Synergistic Salinity Gradient Power Conversion

Capturing the abundant salinity gradient power into electric power by nanofluidic systems has attracted increasing attention and has shown huge potential to alleviate the energy crisis and environmental pollution problems. However, not only the imbalance between permeability and selectivity but also the poor stability and high cost of traditional membranes limit their scale-up realistic applications. Here, intertwined "soft-hard" nanofibers/tubes are densely super-assembled on the surface of anodic aluminum oxide (AAO) to construct a heterogeneous nanochannel membrane, which exhibits smart ion transport and improved salinity gradient power conversion. In this process, one-dimensional (1D) "soft" TEMPO-oxidized cellulose nanofibers (CNFs) are wrapped around "hard" carbon nanotubes (CNTs) to form three-dimensional (3D) dense nanochannel networks, subsequently forming a CNF-CNT/AAO hybrid membrane. The 3D nanochannel networks constructed by this intertwined "soft-hard" nanofiber/tube method can significantly enhance the membrane stability while maintaining the ion selectivity and permeability. Furthermore, benefiting from the asymmetric structure and charge polarity, the hybrid nanofluidic membrane displays a low membrane inner resistance, directional ionic rectification characteristics, outstanding cation selectivity, and excellent salinity gradient power conversion performance with an output power density of 3.3 W/m². Besides, a pH sensitive property of the hybrid membrane is exhibited, and a higher power density of 4.2 W/m² can be achieved at a pH of 11, which is approximately 2 times more compared to that of pure 1D nanomaterial based homogeneous membranes. These results indicate that this interfacial super-assembly strategy can provide a way for large-scale production of nanofluidic devices for various fields including salinity gradient energy harvesting.

Hydrogel-Based Flexible Electronics

Flexible electronics is an emerging field of research involving multiple disciplines, which include but not limited to physics, chemistry, materials science, electronic engineering, and biology. However, the broad applications of flexible electronics are still restricted due to several limitations, including high Young's modulus, poor biocompatibility, and poor responsiveness. Innovative materials aiming for overcoming these drawbacks and boost its practical application is highly desirable. Hydrogel is a class of 3D crosslinked hydrated polymer networks, and its exceptional material properties render it as a promising candidate for the next generation of flexible electronics. Here, the latest methods of synthesizing advanced functional hydrogels and the state-of-art applications of hydrogel-based flexible electronics in various fields are reviewed. More importantly, the correlation between properties of the hydrogel and device performance is discussed here, to have better understanding of the development of flexible electronics by using environmentally responsive hydrogels. Last, perspectives on the current challenges and future directions in the development of hydrogel-based multifunctional flexible electronics are provided.

Membranes for Osmotic Power Generation by Reverse Electrodialysis

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

Thermal diffusion of ionic species in charged nanochannels

Diffusion of ions due to temperature gradients (known as thermal diffusion) in charged nanochannels is of interest in several engineering fields, including energy recovery and environmental protection. This paper presents a fundamental investigation of the thermal diffusion of sodium chloride in charged silica nanochannels performed by molecular dynamics (MD). The results reveal the effects of nanoconfinement and surface charges on the sign and magnitude of the Soret coefficient. It is shown that the sign and magnitude of the Soret coefficient are controlled by the structural modifications of the interfacial solutions. These modifications include the ionic solvation and hydrogen bond structure induced by the nanoconfinement and surface charges. The results show that both nanoconfinement and surface charges can make the solutions more thermophilic. Furthermore, the thermal diffusion of solutions in boundary layers is significantly different from that of solutions in bulk fluid, contributing to the overall difference between the thermal diffusivity of pore fluid and that associated with bulk fluid. The findings provide further understanding of thermal diffusion in nano-porous systems. The proposed MD simulation methodology is applicable to a wider category of coupled heat and mass transfer problems in nanoscale spaces.

High-Performance Osmotic Power Generators Based on the 1D/2D Hybrid Nanochannel System

Extracting clean energy by converting the salinity gradient between river and sea into energy is an effective way to reduce the global pollution and carbon emissions. Reverse electrodialysis (RED) is of great importance to realize the energy conversion assisting the ion-selective membrane. However, its higher ion resistance and lower conversion efficiency results in the undesirable power conversion performance. Here, we demonstrate a 1D/2D hybrid nanochannel system to achieve high osmotic energy conversion and output power. This heterogeneous structure is composed of two structures, in which the subnanometer nanochannels in graphene oxide membrane (GOM) can serve as a selective layer and reduce the ion diffusion energy barrier, while the nanochannel in the polymer can introduce asymmetry to enhance ionic rectification and conversion efficiency. This heterogeneous membrane exhibits excellent cation selectivity and enhanced ionic current rectification (ICR) performance. The application of the GOM/PET hybrid nanochannel system in osmotic energy harvesting is evaluated, and the output power can reach up to 118.2 pW with the energy conversion efficiency of 40.3%. Theoretical calculation indicates that the 1D/2D hybrid system can effectively take the advantage of excellent cation selectivity of 2D lamellar nanochannels to improve its RED performance significantly.

Enhanced nanofluidic transport in activated carbon nanoconduits

Carbon has emerged as a unique material in nanofluidics, with reports of fast water transport, molecular ion separation and efficient osmotic energy conversion. Many of these phenomena still await proper rationalization due to the lack of fundamental understanding of nanoscale ionic transport, which can only be achieved in controlled environments. Here we develop the fabrication of 'activated' two-dimensional carbon nanochannels. Compared with nanoconduits with 'pristine' graphite walls, this enables the investigation of nanoscale ionic transport in great detail. We show that activated carbon nanochannels outperform pristine channels by orders of magnitude in terms of surface electrification, ionic conductance, streaming current and (epi-)osmotic currents. A detailed theoretical framework enables us to attribute the enhanced ionic transport across activated carbon nanochannels to an optimal combination of high surface charge and low friction. Furthermore, this demonstrates the unique potential of activated carbon for energy harvesting from salinity gradients with single-pore power density across activated carbon nanochannels, reaching hundreds of kilowatts per square metre, surpassing alternative nanomaterials.

Bioinspired Ultrastable MXene/PEDOT:PSS Layered Membrane for Effective Salinity Gradient Energy Harvesting from Organic Solvents

The waste organic solvents containing inorganic salts have been considered sustainable resources, which can effectively capture salinity gradient energy using ion-selective membranes. However, it still remains a great challenge to fabricate the ion-selective membranes with high conversion efficiency and stability in an organic system. Here, the bioinspired nacre-like layered MXene/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (MP) composite membranes for capturing salinity gradient energy from an organic solvent are fabricated via filtration method, in which PEDOT:PSS molecules are introduced into MXene interlayers. Accordingly, the MP membrane exhibits high mechanical property and wonderful stability in common organic solvents. As expected, the power generation of the MP membrane reaches up to 3216 ± 603 nW in a 2/0.001 M methanol (Met)-LiCl solution and a record high power generation of 6926 ± 959 nW after adding NaOH into the Met-LiCl solution, which is superior to the previous report. Both experimental and theoretical studies confirm that the MP membrane has excellent cation selectivity and fast ion transport performance. The results are attributable to an increased interlayer spacing between MXene layers and an improved cation selectivity due to the insertion of PEDOT:PSS chains and the enhanced dissociation of negative charges by NaOH. The ultrastable two-dimensional (2D) nanochannel membrane provides practical application for harvesting energy from waste organic solvents.

Design of Multifunctional Nanopore Using Polyampholyte Brush with Composition Gradient

Molecular organizations and charge patterns inside biological nanopores are optimized by evolution to enhance ionic and molecular transport. Inspired by the nuclear pore complex that employs asymmetrically arranged disordered proteins for its gating, we here design an artificial nanopore coated by an asymmetric polyampholyte brush as a model system to study the asymmetric mass transport under nanoconfinement. A nonequilibrium steady-state molecular theory is developed to account for the intricate charge regulation effect of the weak polyampholyte and to address the coupling between the polymer conformation and the external electric field. On the basis of this state-of-the-art theoretical method, we present a comprehensive theoretical description of the stimuli-responsive structural behaviors and transport properties inside the nanopore with all molecular details considered. Our model demonstrates that by incorporating a gradient of pH sensitivity into the polymer coatings of the nanopore, a variety of asymmetric charge patterns and functional structures can be achieved, in a pH-responsive manner that allows for multiple functions to be implemented into the designed system. The asymmetric charge pattern inside the nanopore leads to an electrostatic trap for major current carriers, which turns the nanopore into an ionic rectifier with a rectification factor above 1000 at optimized pH and salt concentration. Our theory further predicts that the nanopore design behaves like a double-gated nanofluidic device with pH-triggered opening of the gates, which can serve as an ion pump and pH-responsive molecular filter. These results deepen our understanding of asymmetric transport in nanoconfined systems and provide guidelines for designing polymer-coated smart nanopores.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Nanofluidic osmotic power generators - advanced nanoporous membranes and nanochannels for blue energy harvesting

The increase of energy demand added to the concern for environmental pollution linked to energy generation based on the combustion of fossil fuels has motivated the study and development of new sustainable ways for energy harvesting. Among the different alternatives, the opportunity to generate energy by exploiting the osmotic pressure difference between water sources of different salinities has attracted considerable attention. It is well-known that this objective can be accomplished by employing ion-selective dense membranes. However, so far, the current state of this technology has shown limited performance which hinders its real application. In this context, advanced nanostructured membranes (nanoporous membranes) with high ion flux and selectivity enabling the enhancement of the output power are perceived as a promising strategy to overcome the existing barriers in this technology. While the utilization of nanoporous membranes for osmotic power generation is a relatively new field and therefore, its application for large-scale production is still uncertain, there have been major developments at the laboratory scale in recent years that demonstrate its huge potential. In this review, we introduce a comprehensive analysis of the main fundamental concepts behind osmotic energy generation and how the utilization of nanoporous membranes with tailored ion transport can be a key to the development of high-efficiency blue energy harvesting systems. Also, the document discusses experimental issues related to the different ways to fabricate this new generation of membranes and the different experimental set-ups for the energy-conversion measurements. We highlight the importance of optimizing the experimental variables through the detailed analysis of the influence on the energy capability of geometrical features related to the nanoporous membranes, surface charge density, concentration gradient, temperature, building block integration, and others. Finally, we summarize some representative studies in up-scaled membranes and discuss the main challenges and perspectives of this emerging field.

Pressure-driven power generation and ion separation using a non-uniformly charged nanopore

Due to its versatile potential applications, nanofluidic devices have drawn much attention of researches in various fields. Among these, pressure-driven power generation is considered as a candidate for the next generation alternative green energy source, and pressure-driven ion separation (nanofiltration) for desalination. Aiming to achieve a better performance in these two representative cases, a cylindrical nanopore having different types of non-uniform surface charge profile is adopted, and its performance under various conditions assessed. We show that lower the surface charge density near the nanopore inlet region can suppress the effect of ion concentration polarization (ICP) and improve the selectivity, thereby enhancing appreciably its power generation performance. For a fixed averaged surface charge density, if the bulk salt concentration is low, the higher the surface charge density near the nanopore openings, the better its performance. The degree of ICP can be alleviated by applying a sufficiently large pressure difference. Although previous studies showed that salt rejection is influenced significantly by the profile of the electric field inside a nanopore, we find that the electric field at nanopore openings also plays a role. Through choosing appropriately the surface charge profile, it is possible to solve the trade-off between rejection and flow rate.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Power Harvesting

Salinity gradient power (SGP) has been identified as a promising renewable energy source. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two membrane-based technologies for SGP harvesting. Developing nanopores and nanofluidic membranes with excellent water and/or ion transport properties for applications in those two membrane-based technologies is considered viable for improving power generation performance. Despite recent efforts to advance power generation by designing a variety of nanopores and nanofluidic membranes to enhance power density, the valid pathways toward large-scale power generation remain uncertain. In this review, we introduce the features of ion and water transport in nanofluidics that are potentially beneficial to power generation. Subsequently, we survey previous efforts on nanofluidic membrane synthesis to obtain high power density. We also discuss how the various membrane properties influence the power density in RED and PRO before moving on to other important aspects of the technologies, i.e., system energy efficiency and membrane fouling. We analyze the importance of system energy efficiency and illustrate how the delicately designed nanofluidic membranes can potentially enhance energy efficiency. Previous studies are reviewed on fabricating antifouling and antimicrobial membrane for power generation, and opportunities are presented that can lead to the design of nanofluidic membranes with superior antifouling properties using various materials. Finally, future research directions are presented on advancing membrane performance and scaling-up the system. We conclude this review by emphasizing the fact that SGP has the potential to become an important renewable energy source and that high-performance nanofluidic membranes can transform SGP harvesting from conceptual to large-scale applications.

Interfacial Super-Assembly of T-Mode Janus Porous Heterochannels from Layered Graphene and Aluminum Oxide Array for Smart Oriented Ion Transportation

Salinity gradient energy existing in seawater and river water is a sustainable and environmentally energy resource that has drawn significant attention of researchers in the background of energy crisis. Nanochannel membrane with a unique nano-confinement effect has been widely applied to harvest the salinity gradient energy. Here, Janus porous heterochannels constructed from 2D graphene oxide modified with polyamide (PA-GO) and oxide array (anodic aluminum oxide, AAO) are prepared through an interfacial super-assembly method, which can achieve oriented ion transportation. Compared with traditional nanochannels, the PA-GO/AAO heterochannels with asymmetric charge distribution and T-mode geometrical nanochannel structure shows directional ionic rectification features and outstanding cation selectivity. The resulting heterochannel membrane can achieve a high-power density of up to 3.73 W m^-2 between artificial seawater and river water. Furthermore, high energy conversion efficiency of 30.3% even in high salinity gradient can be obtained. These achievable results indicate that the PA-GO/AAO heterochannels has significant potential application in salinity gradient energy harvesting.

Highly selective and high-performance osmotic power generators in subnanochannel membranes enabled by metal-organic frameworks

The electric organs of electric eels are able to convert ionic gradients into high-efficiency electricity because their electrocytes contain numerous "subnanoscale" protein ion channels that can achieve highly selective and ultrafast ion transport. Despite increasing awareness of blue energy production through nanochannel membranes, achieving high-performance energy output remains considerably unexplored. Here, we report on a heterogeneous subnanochannel membrane, consisting of a continuous UiO-66-NH₂ metal-organic framework (MOF) and a highly ordered alumina nanochannel membrane. In the positively charged membrane, the angstrom-scale windows function as ionic filters for screening anions with different hydrated sizes. Driven by osmosis, the subnanochannel membrane can produce an exceptionally high Br^-/NO₃ ^- selectivity of ~1240, hence yielding an unprecedented power of up to 26.8 W/m² under a 100-fold KBr gradient. Achieving ultrahigh selective and ultrafast osmotic transport in ion channel-mimetic MOF-based membranes opens previously unexplored avenues toward advanced separation technologies and energy-harvesting devices.

Bioinspired hydrogel-based nanofluidic ionic diodes: nano-confined network tuning and ion transport regulation

Biological ion channel-based mass transport and signal transduction play a crucial role in physiological activities, and biomimetic nanochannels in aqueous solutions for ion transport regulation have been extensively studied. Few studies on non-aqueous systems, gel-based nanochannels, mainly focus on the charged gel network or embedded electrolytes. However, the basic issue of how a nanoscale gel network affects the ion transport in nanochannels has been neglected. Here, we demonstrate a non-aqueous biomimetic nanochannel system by employing the agarose hydrogel in conical nanochannels. To tune the hydrogel network by adjusting the gel concentration, the ion transport behavior in gel-based nanochannels is systemically investigated. The experimental results show that the ion transport behaviors in gel-nanochannels with 2% gel present similar ion selectivity and rectification performance to the aqueous system, indicating fast investigation of gel-based systems with the knowledge of the extensively studied aqueous systems. Furthermore, a gel-based solid-state diode and logic circuits were fabricated.

Single Mesopores with High Surface Charges as Ultrahigh Performance Osmotic Power Generators

Numerous studies on osmotic power generators with nanoscale pores are conducted. However, their performance output is limited because of the finite osmotic current and conductance from such tiny pores. Here, a proof-of-concept study demonstrating that the rectified mesopore (sub-micrometer-scale pore) with high surface charges can be applied in osmotic energy conversion is reported. A single conical mesopore of ≈405 nm in tip diameter, which can reach an osmotic conductance as high as 0.284 μS (corresponding to a current of 27.5 nA and voltage of 97 mV), enables a record-high power of 667 pW under a 1000-fold salinity gradient, more than doubling all of the state-of-the-art single-pore osmotic power generators reported. This work extends the knowledge of osmotic energy with solid-state pores from nanoscale to mesoscale and opens up a promising avenue toward ultrahigh performance osmotic power.

Zwitterionic Hydrogel-Impregnated Membranes with Polyamide Skin Achieving Superior Water/Salt Separation Properties

Support-free nonporous membranes have emerged as a new material platform for osmotic pressure-driven processes due to its insusceptibility to internal concentration polarization (ICP). Herein, we demonstrate high-performance membranes of zwitterionic hydrogels impregnated in porous membranes with a skin layer of highly cross-linked polyamides on both sides prepared by gel-liquid interfacial polymerization (GLIP). Such a configuration eliminates the pores and thus ICP, while the thin polyamide layer provides high salt rejection but negligible resistance to the water transport compared with the hydrogels. The polyamide skin layers are characterized using scanning electron microscopy and atomic force microscopy. The effect of the hydrogel compositions and polyamide formation conditions on the water/salt separation properties is thoroughly investigated. Example membranes show water permeance and salt rejection comparable to state-of-the-art commercial forward osmosis membranes and essentially no ICP.

Improved Rectification and Osmotic Power in Polyelectrolyte-Filled Mesopores

Ample studies have shown the use of nanofluidics in the ionic diode and osmotic power generation, but similar ionic devices performed with large-sized mesopores are still poorly understood. In this study, we model and realize the mesoscale ionic diode and osmotic power generator, composed of an asymmetric cone-shaped mesopore with its narrow opening filled with a polyelectrolyte (PE) layer with high space charges. We show that, only when the space charge density of a PE layer is sufficiently large (>1×106 C/m3), the considered mesopore system is able to create an asymmetric ionic distributions in the pore and then rectify ionic current. As a result, the output osmotic power performance can be improved when the filled PE carries sufficiently high space charges. For example, the considered PE-filled mesopore system can show an amplification of the osmotic power of up to 35.1-fold, compared to the bare solid-state mesopore. The findings provide necessary information for the development of large-sized ionic diode and osmotic power harvesting device.

Robust sulfonated poly (ether ether ketone) nanochannels for high-performance osmotic energy conversion

The membrane-based reverse electrodialysis (RED) technique has a fundamental role in harvesting clean and sustainable osmotic energy existing in the salinity gradient. However, the current designs of membranes cannot cope with the high output power density and robustness. Here, we construct a sulfonated poly (ether ether ketone) (SPEEK) nanochannel membrane with numerous nanochannels for a membrane-based osmotic power generator. The parallel nanochannels with high space charges show excellent cation-selectivity, which could further be improved by adjusting the length and charge density of nanochannels. Based on numerical simulation, the system with space charge shows better conductivity and selectivity than those of a surface-charged nanochannel. The output power density of our proposed membrane-based device reaches up to 5.8 W/m2 by mixing artificial seawater and river water. Additionally, the SPEEK membranes exhibit good mechanical properties, endowing the possibility of creating a high-endurance scale-up membrane-based generator system. We believe that this work provides useful insights into material design and fluid transport for the power generator in osmotic energy conversion.

Effect of Anion Species on Ion Current Rectification Properties of Positively Charged Nanochannels

Biological ion channels can realize delicate mass transport under complicated physiological conditions. Artificial nanochannels can achieve biomimetic ion current rectification (ICR), gating, and selectivity that are mostly performed in pure salt solutions. Synthetic nanochannels that can function under mixed ion systems are highly desirable, yet their performances are hard to be compared to those under pure systems. Seeking out the potential reasons by investigating the effect of mixed-system components on the ion-transport properties of the constructed nanochannels seems necessary and important. Herein, we report the effect of anions with different charges and sizes on the ICR properties of positively charged nanochannels. Among the investigated anions, the low-valent anions showed no impact on the ICR direction, while the high-valent component ferrocyanide [Fe(CN)₆^4-] caused significant ICR inversion. The ICR inversion mechanism is evidenced to result from the adsorption of Fe(CN)₆^4--induced surface charge reversal, which relates to solution concentration, pH conditions, and nanochannel sizes and applies to both aminated and quaternized nanochannels that are positively charged. Noticeably, Fe(CN)₆^4- is found to interfere with the transport of protein molecules in the nanochannel. This work points out that the ion species from mixed systems would potentially impact the intrinsic ICR properties of the nanochannels. Replacing highly charged counterions with organic components would be promising in building up future nanochannel-based mass transport systems running under mixed systems.

Polyaniline for Improved Blue Energy Harvesting: Highly Rectifying Nanofluidic Diodes Operating in Hypersaline Conditions via One-Step Functionalization

Solid-state nanochannels have attracted substantial attention of the scientific community due to their remarkable control of ionic transport and the feasibility to regulate the iontronic output by different stimuli. Most of the developed nanodevices are subjected to complex modification methods or show functional responsiveness only in moderate-ionic-strength solutions. Within this project, we present a nanofluidic device with enhanced ionic current rectification properties attained by a simple one-step functionalization of single bullet-shaped polyethylene terephthalate (PET) nanochannels with polyaniline (PANI) that can work in high-ionic-strength solutions. The integration of PANI also introduces a broad pH sensitivity, which makes it possible to modulate the ionic transport behavior between anion-selective and cation-selective regimes depending on the pH range. Since PANI is an electrochemically active polymer, ionic transport also becomes dependent on the presence of redox stimuli in solution. We demonstrate that PANI-functionalized single-nanochannel membranes function as an efficient salinity gradient-based energy conversion device even in acidic concentrated salt solutions, opening the door to applications under a variety of novel operating conditions.

Pressure-driven energy conversion of conical nanochannels: Anomalous dependence of power generated and efficiency on pH

Pressure-driven power generation is one of a simple, green, and promising energy sources. Owing to the overlapping of the electric double layer inside, nanochannel is capable of providing a platform for this power generation approach. Unfortunately, relevant studies, either experimental or theoretical, are very limited in the literature. Here, we present for the first time a comprehensively theoretical study on the pressure-driven energy conversion in a conical nanochannel having carboxyl functional groups, focusing on the influence of its tip size and the solution pH. An anomalous dependence of both the power generated and the efficiency on the latter are observed. Although the charge density on the nanochannel surface increases monotonically with increasing pH, both the power generated and the efficiency exhibit a local maximum as pH varies. This is because the streaming potential has a local maximum as pH varies. Power density (power generated/tip end cross sectional area) also shows a local maximum as the tip radius varies, and the radius at which the local maximum occurs decreases with increasing bulk salt concentration. In addition to explain successfully the behavior reported in the literature, our study also provides desirable and necessary information for designing relevant devices.

Tunable Current Rectification and Selectivity Demonstrated in Nanofluidic Diodes through Kinetic Functionalization

The possibility of tuning the current rectification and selectivity in nanofluidic diodes is demonstrated both experimentally and theoretically through dynamically functionalizing a conical nanopore with poly-l-lysine. We identified an optimum functionalization time equivalent to optimum modification depth that assures the highest rectification degrees. Results showed that the functionalization time-dependent rectification behavior of nanofluidic diodes is dominated by the properties of current at positive voltages that in our electrode configuration indicate the "on" state of the diode and accumulation of ions in the nanopore. The functionalization time also tunes the ion selectivity of the diode. If the functionalization time is sufficiently short, an unusual depletion of counterions near the bipolar interface results in a cation-selective nanopore. However, a further increase in the duration of functionalization renders a nanopore that is an anion-selective nanopore. The dynamic functionalization presented in this Letter enables tuning ion selectivity of nanopores.

Unraveling the Anomalous Surface-Charge-Dependent Osmotic Power Using a Single Funnel-Shaped Nanochannel

Nanofluidic osmotic power, which converts a difference in salinity between brine and fresh water into electricity with nanoscale channels, has received more and more attention in recent years. It is long believed that to gain high-performance osmotic power, highly charged channel materials should be exploited so as to enhance the ion selectivity. In this paper, we report counterintuitive surface-charge-density-dependent osmotic power in a single funnel-shaped nanochannel (FSN), violating the previous viewpoint. For the highly charged nanochannel, the performance of osmotic power decreases with a further increase in its surface charge density. With increasing pH (surface charge density), the FSN enables a local maximum power density as high as ∼3.5 kW/m² in a 500 mM/1 mM KCl gradient. This observation is strongly supported by our rigorous model where the equilibrium chemical reaction between functional carboxylate ion groups on the channel wall and protons is taken into account. The modeling reveals that for a highly charged nanochannel, a significant increase in the surface charge density amplifies the ion concentration polarization effect, thus weakening the effective salinity ratio across the channel and undermining the osmotic power generated.