Thermal dependence of nanofluidic energy conversion by reverse electrodialysis

How Does Temperature Affect the Charge Transfer Process in Flow Electrode Capacitive Deionization?

In this study, we investigate how temperature variations, a key environmental factor, affect the charge transfer process in FCDI systems across seasonal variation and geographical distributions, which is crucial for optimizing FCDI performance but has not received adequate attention. Therefore, thermal-assisted FCDI systems were proposed by controlling the temperatures of the flow electrode and saline water to simulate the environmental conditions, and the temperature effects on the charge transport and desalting ability of FCDI were investigated. First, the isothermal mode was performed, where the flow electrode and saline water were controlled at the same temperatures (0-50 °C) to simulate the natural atmospheric temperature fluctuations and industrial circulating cooling water system. Experimental results showed a strong positive correlation between temperature and electrosorption dynamics. Elevated temperatures significantly improved ion electromigration and diffusion, thereby enhancing the electrosorption capacity of the FCDI device. On this basis, the nonisothermal mode was designed via maintaining the temperature of the flow electrode at 50 °C to improve the desalination performance of FCDI for saline water at different temperatures (0-50 °C). Finally, the East China seawater and industrial circulating cooling water were both desalted successfully to confirm the feasibility of the temperature field in the practical application of FCDI.

Maximizing blue energy: the role of ion partitioning in nanochannel systems

This study describes a numerical analysis on blue energy generation using a charged nanochannel with an integrated pH-sensitive polyelectrolyte layer (PEL), considering ion partitioning effects due to permittivity differences. The mathematical model for ionic and fluidic transport is solved using the finite element method, and the model validation is performed against existing theoretical and experimental results. The study investigates the influence of electrolyte concentration, permittivity ratio, and salt types (KCl, BeCl2, AlCl3) on the energy conversion process. The findings illustrate the substantial role of ion partitioning in modulating ionic concentration and potential fields, thereby affecting current profiles and energy conversion efficiencies. Remarkably, overlooking ion partitioning leads to significant overestimations of power density, highlighting the necessity of this consideration for accurate device performance predictions. This work introduces a promising configuration that achieves higher power densities, paving the way for the next generation of efficient energy-harvesting devices. The findings offer valuable insights into the development of state-of-the-art blue energy harvesting nanofluidic devices, advancing sustainable energy production.

All-natural 2D nanofluidics as highly-efficient osmotic energy generators

Two-dimensional nanofluidics based on naturally abundant clay are good candidates for harvesting osmotic energy between the sea and river from the perspective of commercialization and environmental sustainability. However, clay-based nanofluidics outputting long-term considerable osmotic power remains extremely challenging to achieve due to the lack of surface charge and mechanical strength. Here, a two-dimensional all-natural nanofluidic (2D-NNF) is developed as a robust and highly efficient osmotic energy generator based on an interlocking configuration of stacked montmorillonite nanosheets (from natural clay) and their intercalated cellulose nanofibers (from natural wood). The generated nano-confined interlamellar channels with abundant surface and space negative charges facilitate selective and fast hopping transport of cations in the 2D-NNF. This contributes to an osmotic power output of ~8.61 W m-2 by mixing artificial seawater and river water, higher than other reported state-of-the-art 2D nanofluidics. According to detailed life cycle assessments (LCA), the 2D-NNF demonstrates great advantages in resource consumption (1/14), greenhouse gas emissions (1/9), and production costs (1/13) compared with the mainstream 2D nanofluidics, promising good sustainability for large-scale and highly-efficient osmotic power generation.

Ion transport in nanofluidics under external fields

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

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.

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

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

Modulation mechanism of ionic transport through short nanopores by charged exterior surfaces

Short nanopores have various applications in biosensing, desalination, and energy conversion. Here, the modulation of ionic transport by charged exterior surfaces is investigated through simulations with sub-200 nm long nanopores under applied voltages. Detailed analysis of the ionic current, electric field strength, and fluid flow inside and outside nanopores reveals that charged exterior surfaces can increase ionic conductance by increasing both the concentration and migration speed of charge carriers. The electric double layers near charged exterior surfaces provide an ion pool and an additional passageway for counterions, which lead to enhanced exterior surface conductance and ionic concentrations at pore entrances and inside the nanopores. We also report that charges on the membrane surfaces increase the electric field strength inside nanopores. The effective width of a ring with surface charges placed at pore entrances (Lcs) is considered as well by studying the dependence of the current on Lcs. We find a linear relationship between the effective Lcs and the surface charge density and voltage, and an inverse relationship between the geometrical pore length and salt concentration. Our results elucidate the modulation mechanism of ionic transport through short nanopores by charged exterior surfaces, which is important for the design and fabrication of porous membranes.

Photoelectric responsive ionic channel for sustainable energy harvesting

Access to sustainable energy is paramount in today's world, with a significant emphasis on solar and water-based energy sources. Herein, we develop photo-responsive ionic dye-sensitized covalent organic framework membranes. These innovative membranes are designed to significantly enhance selective ion transport by exploiting the intricate interplay between photons, electrons, and ions. The nanofluidic devices engineered in our study showcase exceptional cation conductivity. Additionally, they can adeptly convert light into electrical signals due to photoexcitation-triggered ion movement. Combining the effects of salinity gradients with photo-induced ion movement, the efficiency of these devices is notably amplified. Specifically, under a salinity differential of 0.5/0.01 M NaCl and light exposure, the device reaches a peak power density of 129 W m-2, outperforming the current market standard by approximately 26-fold. Beyond introducing the idea of photoelectric activity in ionic membranes, our research highlights a potential pathway to cater to the escalating global energy needs.

Enhancing the osmotic energy conversion of a nanoporous membrane: influence of pore density, pH, and temperature

Salinity gradient power, which converts Gibbs free energy of mixing to electric energy through an ion-selective pore, has great potential. Towards practical use, developing membrane-scaled nanoporous materials is desirable and necessary. Unfortunately, the presence of a significant ion concentration polarization (ICP) lowers appreciably the power harvested, especially at a high pore density. To alleviate this problem, we suggest applying an extra pressure difference ΔP across a membrane containing multiple nanopores, taking account of the associated power consumption. The results gathered reveal that the application of a negative pressure difference can improve the power harvested due to the enhanced selectivity. In addition, if the pore density of a membrane is high, raising its pore length is necessary to make the energy harvested economic. For example, if the pore length is 2000 nm and the pore density is 2.5 × 10⁹ pores per cm², an increment in the power density of 213 mW m^-2 can be obtained by applying ΔP = -1 bar at pH 11 and 323 K, where a net positive power density can be retrieved. The performance of the system considered under various conditions is examined in detail, along with associated mechanisms.

Chemiosomotic flow in a soft conical nanopore: harvesting enhanced blue energy

The salinity gradient energy or the 'blue energy' is one of the most promising inexpensive and abundant sources of clean energy, having immense capabilities to serve modern-day society. In this article, we overlay an extensive analysis of reverse electrodialysis (RED) for harvesting salinity gradient energy in a single conical nanochannel, grafted with a pH-tunable polyelectrolyte layer (PEL) on the inner surfaces. We primarily focus on the distinctiveness of the solution pH of the connecting reservoirs. In spite of acquiring a maximum power density of ∼1.2 kW m^-2 in the chosen configuration, we notice a counter-intuitive patterning of the ion transport for a certain span of pH, leading to diminishing power. To this end, we discuss the possible strategic avenues essentially to achieve a higher amount of power density. In order to achieve a desirable outcome within that pH zone, we employ two separate approaches intending to counter the underlying physics. Results reveal a great enhancement in the power density as well as in the efficiency even under the framework of both strategies proposed herein. Moreover, as shown, the window of solution pH has increased by three times, implicating the maximum power density mentioned above. We expect that the strategic procedure of augmented energy harvesting as discussed in this analysis can be of importance from the perspective of fabricating state-of-the-art nanodevices aimed at blue energy harvesting.

Analyses of Pore-Size-Dependent Ionic Transport in Nanopores in the Presence of Concentration and Temperature Gradients

Mass transport through nanopores occurs in various natural systems, including the human body. For example, ion transport across nerve cell membranes plays a significant role in neural signal transmission, which can be significantly affected by the electrolyte and temperature conditions. To better understand and control the underlying nanoscopic transport, it is necessary to develop multiphysical transport models as well as validate them using enhanced experimental methods for facile nanopore fabrication and precise nanoscale transport characterization. Here, we report a nanopore-integrated microfluidic platform to characterize ion transport in the presence of electrolyte and temperature gradients; we employ our previous self-assembled particle membrane (SAPM)-integrated microfluidic platform to produce various nanopores with different pore sizes. Subsequently, we quantify pore-size-dependent ionic transport by measuring the short circuit current (SCC) and open circuit voltage (OCV) across various nanopores by manipulating the electrolyte and temperature gradients. We establish three simple theoretical models that heavily depend on pore size, electrolyte concentration, and temperature and subsequently validate them with the experimental results. Finally, we anticipate that the results of this study would help clarify ion transport phenomena at low-temperature conditions, not only providing a fundamental understanding but also enabling practical applications of cryo-anesthesia in the near future.

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.

Increased ion transport and high-efficient osmotic energy conversion through aqueous stable graphitic carbon nitride/cellulose nanofiber composite membrane

Increased attention has evoked on the utilization of renewable energy, particularly osmotic power as a potential solution to the energy crisis and environmental pollution. Herein, we fabricate graphitic carbon nitride (g-C₃N₄)/cellulose nanofiber (CNF) composite membranes with tailored lamellar nanochannels for capturing osmotic energy from salinity gradients. Composite membranes exhibiting charge-governed ion conductivity were prepared via co-homogenization of g-C₃N₄ with CNF and vacuum filtration. Ion conductivity was efficiently modulated by fine-tuning the charge density through controlling the weight content of CNF in the composite membranes. Higher ion conductivity of 0.014 S cm^-1 at low concentrations (<10^-2 M KCl) was achieved due to the increased charge density of the lamellar nanochannels and the excellent aqueous stability of the membranes. We demonstrate the potential of the composite membranes in nanofluidic osmotic energy conversion, displaying thermo-enhanced power output performance. This work could inspire new designs of cellulose-based nanofluidic devices for improved osmotic energy conversion.

Porous Ti3C2Tx MXene Membranes for Highly Efficient Salinity Gradient Energy Harvesting

Extracting osmotic energy through nanoporous membranes is an efficient way to harvest renewable and sustainable energy using the salinity gradient between seawater and river water. Despite recent advances of nanopore-based membranes, which have revitalized the prospect of blue energy, their energy conversion is hampered by nanomembrane issues such as high internal resistance or low selectivity. Herein, we report a lamellar-structured membrane made of nanoporous Ti3C2Tx MXene sheets, exhibiting simultaneous enhancement in permeability and ion selectivity beyond their inherent trade-off. The perforated nanopores formed by facile H2SO4 oxidation of the sheets act as a network of cation channels that interconnects interplanar nanocapillaries throughout the lamellar membrane. The constructed internal nanopores lower the energy barrier for cation passage, thereby boosting the preferential ion diffusion across the membrane. A maximum output power density of the nanoporous Ti3C2Tx MXene membranes reaches up to 17.5 W·m-2 under a 100-fold KCl gradient at neutral pH and room temperature, which is as high as by 38% compared to that of the pristine membrane. The membrane design strategy employing the nanoporous two-dimensional sheets provides a promising approach for ion exchange, osmotic energy extraction, and other nanofluidic applications.

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.

Miniaturized Salinity Gradient Energy Harvesting Devices

Harvesting salinity gradient energy, also known as "osmotic energy" or "blue energy", generated from the free energy mixing of seawater and fresh river water provides a renewable and sustainable alternative for circumventing the recent upsurge in global energy consumption. The osmotic pressure resulting from mixing water streams with different salinities can be converted into electrical energy driven by a potential difference or ionic gradients. Reversed-electrodialysis (RED) has become more prominent among the conventional membrane-based separation methodologies due to its higher energy efficiency and lesser susceptibility to membrane fouling than pressure-retarded osmosis (PRO). However, the ion-exchange membranes used for RED systems often encounter limitations while adapting to a real-world system due to their limited pore sizes and internal resistance. The worldwide demand for clean energy production has reinvigorated the interest in salinity gradient energy conversion. In addition to the large energy conversion devices, the miniaturized devices used for powering a portable or wearable micro-device have attracted much attention. This review provides insights into developing miniaturized salinity gradient energy harvesting devices and recent advances in the membranes designed for optimized osmotic power extraction. Furthermore, we present various applications utilizing the salinity gradient energy conversion.

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.

Stable Ti3C2Tx MXene-Boron Nitride Membranes with Low Internal Resistance for Enhanced Salinity Gradient Energy Harvesting

Extracting salinity gradient energy through a nanomembrane is an efficient way to obtain clean and renewable energy. However, the membranes with undesirable properties, such as low stability, high internal resistance, and low selectivity, would limit the output performance. Herein, we report two-dimensional (2D) laminar nanochannels in the hybrid Ti₃C₂T_x MXene/boron nitride (MXBN) membrane with excellent stability and reduced internal resistance for enhanced salinity gradient energy harvesting. The internal resistance of the MXBN membrane is significantly reduced after adding BN in a pristine MXene membrane, due to the small size and high surface charge density of BN nanosheets. The output power density of the MXBN membrane with 44 wt % BN nanosheets reaches 2.3 W/m², almost twice that of a pristine MXene membrane. Besides, the output power density can be further increased to 6.2 W/m² at 336 K and stabilizes for 10 h at 321 K, revealing excellent structure stability of the membrane in long-term aqueous conditions. This work presents a feasible method for improving the channel properties, which provides 2D layered composite membranes in ion transport, energy extraction, and other nanofluidic applications.

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.

Tripling the reverse electrodialysis power generation in conical nanochannels utilizing soft surfaces

We theoretically investigate the feasibility of enhancing the reverse electrodialysis power generation in nanochannels by covering the surface with a polyelectrolyte layer (PEL). Along these lines, two conical nanochannels are considered that differ in the extent of the covering. Each nanochannel connects two large reservoirs filled with KCl electrolytes of different ionic concentrations. Considering the Poisson-Nernst-Planck and Navier-Brinkman equations, finite-element-based numerical simulations are performed under a steady-state. The influences of the PEL properties and the salinity gradient on the reverse electrodialysis characteristics are examined in detail via a thorough parametric study. It is shown that the maximum power generated is an increasing function of the charge density and the thickness of the PEL. This means that the maximum power generated may be theoretically increased to any desired degree by covering the nanochannel surface with a sufficiently dense and thick PEL. Considering a typical PEL with a charge density of 100 mol m-3 and a thickness of 8 nm along with a high-to-low concentration ratio of 1000, we demonstrate that it is possible to extract a power density of 51.5 W m-2, which is nearly three times the maximum achievable value employing bare conical nanochannels at the same salinity gradient.

Photothermoelectric Response of Ti3C2Tx MXene Confined Ion Channels

With recent growing interest in biomimetic smart nanochannels, a biological sensory transduction in response to external stimuli has been of particular interest in the development of biomimetic nanofluidic systems. Here we demonstrate the MXene-based subnanometer ion channels that convert external temperature changes to electric signals via preferential diffusion of cations under a thermal gradient. In particular, coupled with a photothermal conversion feature of MXenes, an array of the nanoconfined Ti3C2Tx ion channels can capture trans-nanochannel diffusion potentials under a light-driven axial temperature gradient. The nonisothermal open-circuit potential across channels is enhanced with increasing cationic permselectivity of confined channels, associated with the ionic concentration or pH of permeant fluids. The photothermoelectric ionic response (evaluated from the ionic Seebeck coefficient) reached up to 1 mV·K-1, which is comparable to biological thermosensory channels, and demonstrated stability and reproducibility in the absence and presence of an ionic concentration gradient. With advantages of physicochemical tunability and easy fabrication process, the lamellar ion conductors may be an important nanofluidic thermosensation platform possibly for biomimetic sensory systems.

Temperature effects on electrical double layer at solid-aqueous solution interface

Despite the significant influence of solution temperature on the structure of electrical double layer, the lack of theoretical model intercepts us to explain and predict the interesting experimental observations. In this work, we study the structure of electrical double layer as a function of thermochemical properties of the solution by proposing a phenomenological temperature dependent surface complexation model. We found that by introducing a buffer layer between the diffuse layer and stern layer, one can explain the sensitivity of zeta potential to temperature for different bulk ion concentrations. Calculation of the electrical conductance as function of thermochemical properties of solution reveals the electrical conductance not only is a function of bulk ion concentration and channel height but also the solution temperature. The present work model can provide deep understanding of micro- and nanofluidic devices functionality at different temperatures.

Thermal Dependence of the Mesoscale Ionic Diode: Modeling and Experimental Verification

Mesoscale ionic diodes, which can rectify ionic current at conditions at which their pore size is larger than 100 nm and thus over 100 times larger than the Debye length, have been recently discovered with potential applications in ionic circuits as well as osmotic power generation. Compared with the conventional nanoscale ionic diodes, the mesoscale ionic diodes can offer much higher conductance, ionic current resolution, and power generated. However, the thermal response, which has been proven playing a crucial role in nanofluidic devices, of the mesoscale ionic diode remains significantly unexplored. Here, we report the thermal dependence of the mesoscale ionic diode comprising a conical pore with a tip opening diameter of ∼400 nm. To capture its underlying physics more accurately, our model takes into account the practical equilibrium chemistry reaction of functional carboxyl groups on the pore surface. Modeling results predict that in the mesoscale ionic diode prepared currents increase but the performance decreases with the increase of temperature, which is consistent with our experimental data and indicates that the ion transport properties apparently depend on the presence of highly mobile hydroxide ions. The results gathered can provide important guidance for the design of new mesoscale ionic diodes, enriching their applications in thermoelectric power and thermoresponsive chemical sensors.

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.

Ionic thermal up-diffusion in nanofluidic salinity-gradient energy harvesting

Advances in nanofabrication and materials science give a boost to the research in nanofluidic energy harvesting. Contrary to previous efforts on isothermal conditions, here a study on asymmetric temperature dependence in nanofluidic power generation is conducted. Results are somewhat counterintuitive. A negative temperature difference can significantly improve the membrane potential due to the impact of ionic thermal up-diffusion that promotes the selectivity and suppresses the ion-concentration polarization, especially at the low-concentration side, which results in dramatically enhanced electric power. A positive temperature difference lowers the membrane potential due to the impact of ionic thermal down-diffusion, although it promotes the diffusion current induced by decreased electrical resistance. Originating from the compromise of the temperature-impacted membrane potential and diffusion current, a positive temperature difference enhances the power at low transmembrane-concentration intensities and hinders the power for high transmembrane-concentration intensities. Based on the system's temperature response, we have proposed a simple and efficient way to fabricate tunable ionic voltage sources and enhance salinity-gradient energy conversion based on small nanoscale biochannels and mimetic nanochannels. These findings reveal the importance of a long-overlooked element—temperature—in nanofluidic energy harvesting and provide insights for the optimization and fabrication of high-performance nanofluidic power devices.

Two-Dimensional Ti3C2Tx MXene Membranes as Nanofluidic Osmotic Power Generators

Salinity-gradient is emerging as one of the promising renewable energy sources but its energy conversion is severely limited by unsatisfactory performance of available semipermeable membranes. Recently, nanoconfined channels, as osmotic conduits, have shown superior energy conversion performance to conventional technologies. Here, ion selective nanochannels in lamellar Ti₃C₂T_x MXene membranes are reported for efficient osmotic power harvesting. These subnanometer channels in the Ti₃C₂T_x membranes enable cation-selective passage, assisted with tailored surface terminal groups, under salinity gradient. A record-high output power density of 21 W·m^-2 at room temperature with an energy conversion efficiency of up to 40.6% is achieved by controlled surface charges at a 1000-fold salinity gradient. In addition, due to thermal regulation of surface charges and ionic mobility, the MXene membrane produces a large thermal enhancement at 331 K, yielding a power density of up to 54 W·m^-2. The MXene lamellar structure, coupled with its scalability and chemical tunability, may be an important platform for high-performance osmotic power generators.

Hybrid membrane distillation reverse electrodialysis configuration for water and energy recovery from human urine: An opportunity for off-grid decentralised sanitation

The integration of membrane distillation with reverse electrodialysis has been investigated as a sustainable sanitation solution to provide clean water and electrical power from urine and waste heat. Reverse electrodialysis was integrated to provide the partial remixing of the concentrate (urine) and diluate (permeate) produced from the membrane distillation of urine. Broadly comparable power densities to those of a model salt solution (sodium chloride) were determined during evaluation of the individual and combined contribution of the various monovalent and multivalent inorganic and organic salt constituents in urine. Power densities were improved through raising feed-side temperature and increasing concentration in the concentrate, without observation of limiting behaviour imposed by non-ideal salt and water transport. A further unique contribution of this application is the limited volume of salt concentrate available, which demanded brine recycling to maximise energy recovery analogous to a battery, operating in a 'state of charge'. During recycle, around 47% of the Gibbs free energy was recoverable with up to 80% of the energy extractable before the concentration difference between the two solutions was halfway towards equilibrium which implies that energy recovery can be optimised with limited effect on permeate quality. This study has provided the first successful demonstration of an integrated MD-RED system for energy recovery from a limited resource, and evidences that the recovered power is sufficient to operate a range of low current fluid pumping technologies that could help deliver off-grid sanitation and clean water recovery at single household scale.

Theory of Transport-Induced-Charge Electroosmotic Pumping toward Alternating Current Resistive Pulse Sensing

In this work, we study transport-induced-charge electroosmosis toward alternating current resistive pulse sensing for the next generation of biomedical applications. Transport-induced-charge electroosmosis, being a new class of electrokinetic phenomenon, occurs as a salt concentration gradient works in synergy with an electric field in ultrathin nanopores. Apart from the conventional electric double layer-governed electroosmotic flow in which the flow behavior is subject to the surface charge, it is found that the transport-induced-charge electroosmotic flow behaves independently of surface charge magnitude but can be linearly regulated by the bulk salt concentration bias. The reversal of the electric field simultaneously inverses the induced charge allowing the establishment of a unidirectional flow under the application of a periodic alternating current field. This unique phenomenon permits continuous water and nanoparticles pumping through a two-dimensional material nanopore in spite of the reversal of the electric field. Built upon this mechanism, we propose a theoretical prototype of alternating current resistive pulse sensing in a two-dimensional nanopore system.

Engineered Artificial Nanochannels for Nitrite Ion Harmless Conversion

Inspired by the delicate functions of living organisms to transport or transform nitrite ions (NO₂^-), a bioinspired smart nanochannel that can realize harmless conversion of NO₂^- into N₂ is developed by immobilizing a NO₂^--responsive functional molecule, p-phenylenediamine, onto a single conical polyethylene terephthalate nanochannel. Subsequently, the aromatic primary amine groups could be triggered to transform into a phenyldiazonium molecule based on the acid-activated NO₂^--binding process. The nanochannel exhibits specific selectivity and highly ultratrace recognition of NO₂^-. Fascinatingly, the transformed phenyldiazonium molecules could be triggered to generate phenol groups and release N₂ by ultraviolet light activation, achieving NO₂^- harmless conversion. This system could provide inspiration to construct artificial nanofluidic devices for ion-sensing and nitrogen cycle fields.

Reverse electrodialysis in bilayer nanochannels: salinity gradient-driven power generation

To evaluate the possibility of nano-fluidic reverse electrodialysis (RED) for salinity gradient energy harvesting, we consider the behavior of ion transportation in a bilayer cylindrical nanochannel consisting of different sized nanopores connecting two large reservoirs at different NaCl concentrations. Numerical simulations to illustrate the electrokinetic behavior at asymmetric sub-pore length and surface charge density are conducted, the impacts of which on transference number, osmotic current, diffusive voltage, maximum power and maximum power efficiency are systematically investigated. The results reveal that the transference number in Config. I (where high NaCl concentration is applied at the larger nanopore) is always larger than that in the opposite configuration (Config. II). At low concentration ratios, the osmotic current and maximum power have maximum values, while the maximum power efficiency decreases consistently. For Config. II, the ion transportation is impacted by the surface charge density at both sub-nanopores, while for Config. I, it is determined by the surface charge density at the downstream small nanopore. When large surface charge density is applied at the downstream small nanopore in contact with a very low concentration reservoir, there exists an interesting phenomenon: the larger surface charge density at the large nanopore induces a slight performance drop due to the impact of upstream EDL overlap.

Electrokinetic ion transport in nanofluidics and membranes with applications in bioanalysis and beyond

Electrokinetic transport of ions between electrolyte solutions and ion permselective solid media governs a variety of applications, such as molecular separation, biological detection, and bioelectronics. These applications rely on a unique class of materials and devices to interface the ionic and electronic systems. The devices built on ion permselective materials or micro-/nanofluidic channels are arranged to work with aqueous environments capable of either manipulating charged species through applied electric fields or transducing biological responses into electronic signals. In this review, we focus on recent advances in the application of electrokinetic ion transport using nanofluidic and membrane technologies. We start with an introduction into the theoretical basis of ion transport kinetics and their analogy to the charge transport in electronic systems. We continue with discussions of the materials and nanofabrication technologies developed to create ion permselective membranes and nanofluidic devices. Accomplishments from various applications are highlighted, including biosensing, molecular separation, energy conversion, and bio-electronic interfaces. We also briefly outline potential applications and challenges in this field.