Osmotic Power Generation Research Articles

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.

Density and confinement effects on fluid velocity slip

Understanding the velocity slip of fluids in channels of molecular length scales is essential for accurately manipulating their flow in engineering applications, such as in water desalination or, by reversing the process, osmotic power generation. This study addresses two major open questions, namely (a) the range of validity of the Navier-Stokes equations with slip boundary conditions in describing fluid flows within channels of molecular confinement, and (b) the effect of fluid density and confinement as well as surface properties, such as microscopic roughness and curvature, on fluid slippage along walls. Using a hard-sphere fluid model and describing the fluid-wall interactions by the Maxwell scattering kernel, an event-driven molecular dynamics simulation study was performed in planar and cylindrical channels of different sizes and accommodation coefficients with varying fluid densities. The results show that the well-known criterion for the slip solution, which is valid in rarefied conditions, also holds for a dense gas, i.e., the predictive accuracy improves with decreasing Knudsen numbers, as long as the channel size is larger than about ten molecular diameters. We also find that the key quantity influencing the friction coefficient of a fluid is the peak density at the walls, rather than the nominal density, the fluid confinement, or the channel curvature, as found in previous studies. In particular, higher nominal densities lead to higher density peaks, and therefore a decrease in slip, while tighter confinements lead to lower density peaks, and therefore an increase in slip, regardless of channel geometry. Furthermore, the velocity slip depends on the microscopic roughness via the Smoluchowski factor. These findings are a stepping stone towards a deeper understanding of the molecular mechanisms underlying fluid velocity slip in molecular-scale channels. Published by the American Physical Society 2024

Unlocking osmotic energy harvesting potential in challenging real-world hypersaline environments through vermiculite-based hetero-nanochannels

Nanochannel membranes have demonstrated remarkable potential for osmotic energy harvesting; however, their efficiency in practical high-salinity systems is hindered by reduced ion selectivity. Here, we propose a dual-separation transport strategy by constructing a two-dimensional (2D) vermiculite (VMT)-based heterogeneous nanofluidic system via an eco-friendly and scalable method. The cations are initially separated and enriched in micropores of substrates during the transmembrane diffusion, followed by secondary precise sieving in ultra-thin VMT laminates with high ion flux. Resultantly, our nanofluidic system demonstrates efficient osmotic energy harvesting performance, especially in hypersaline environment. Notably, we achieve a maximum power density of 33.76 W m−2, a 6.2-fold improvement with a ten-fold increase in salinity gradient, surpassing state-of-the-art nanochannel membranes under challenging conditions. Additionally, we confirm practical hypersaline osmotic power generation using various natural salt-lake brines, achieving a power density of 25.9 W m−2. This work triggers the hopes for practical blue energy conversion using advanced nanoarchitecture.

Single Idiosyncratic Ionic Generator Working in Iso‐Osmotic Solutions Via Ligand Confined Assembled in Gaps Between Nanosheets

AbstractNumerous reported bioinspired osmotic energy conversion systems employing cation‐/anion‐selective membranes and solutions with different salinity are actually far from the biological counterpart. The iso‐osmotic power generator with the specific ionic permselective channels (e.g., K+ or Na+ channels) which just allow specific ions to get across and iso‐osmotic solutions still remain challenges. Inspired by nature, we report a bioinspired K+‐channel by employing a K+ selective ligand, 1,1,1‐tris{[(2′‐benzylaminoformyl)phenoxy]methyl}ethane (BMP) and graphene oxide membrane. Specifically, the K+ and Na+ selectivity of the prepared system could reach up to ≈17.8, and the molecular dynamics simulation revealed that the excellent permselectivity of K+ mainly stemmed from the formed suitable channel size. Thus, we assembled the K+‐selective iso‐osmotic power generator (KSIPG) with the power density up to ≈15.1 mW/m2 between equal concentration solutions, which is higher than traditional charge‐selective osmotic power generator (CSOPG). The proposed strategy has well shown the realizable approach to construct single‐ion selective channels‐based highly efficient iso‐osmotic energy conversion systems and would surely inspire new applications in other fields, including self‐powered systems and medical materials, etc.

Single Idiosyncratic Ionic Generator Working in Iso-Osmotic Solutions Via Ligand Confined Assembled in Gaps Between Nanosheets.

Numerous reported bioinspired osmotic energy conversion systems employing cation-/anion-selective membranes and solutions with different salinity are actually far from the biological counterpart. The iso-osmotic power generator with the specific ionic permselective channels (e.g., K+ or Na+ channels) which just allow specific ions to get across and iso-osmotic solutions still remain challenges. Inspired by nature, we report a bioinspired K+ -channel by employing a K+ selective ligand, 1,1,1-tris{[(2'-benzylaminoformyl)phenoxy]methyl}ethane (BMP) and graphene oxide membrane. Specifically, the K+ and Na+ selectivity of the prepared system could reach up to ≈17.8, and the molecular dynamics simulation revealed that the excellent permselectivity of K+ mainly stemmed from the formed suitable channel size. Thus, we assembled the K+ -selective iso-osmotic power generator (KSIPG) with the power density up to ≈15.1 mW/m2 between equal concentration solutions, which is higher than traditional charge-selective osmotic power generator (CSOPG). The proposed strategy has well shown the realizable approach to construct single-ion selective channels-based highly efficient iso-osmotic energy conversion systems and would surely inspire new applications in other fields, including self-powered systems and medical materials, etc.

Electrochemical Biosensor for Protein Concentration Monitoring Using Natural Wood Evaporation for Power Generation.

A high-sensitivity, low-cost, self-powered biomass electrochemical biosensor based on the "evaporating potential" theory is developed for protein detection. The feasibility of experimental evaluation methods was verified with a probe protein of bovine serum albumin. The sensor was then used to detect lung cancer marker CYFRA21-1, and the potential of our sensor for clinical diagnosis was demonstrated by serum analysis. This work innovatively exploits the osmotic power generation capability of natural wood to construct a promising electrochemical biosensor that was driven by kinetics during testing. The detection methods used for this sensor, chronoamperometry and AC impedance, showed potential for quantitative analysis and specific detection, respectively. Furthermore, the sensor could facilitate new insights into the development of high-sensitivity, low-cost, and easy-to-use electrochemical biosensors.

Self-assembled two-dimensional metal–organic framework membranes as nanofluidic osmotic power generators

2D ZnTCPP membranes with nanopores of 1.2 nm and interlayer spacing of 0.9 nm exhibit excellent osmotic power generation.

Metal-organic frameworks' latent potential as High-Efficiency osmotic power generators in Thin-Film nanocomposite membranes

This study investigates enhanced osmotic power generation by incorporating metal–organic framework (MOF) nanorods into the thin-film nanocomposite (TFN) membranes for pressure retarded osmosis (PRO) applications. The spotlight is on an innovative membrane, NH2-Ce-BTC-500, which is fabricated utilizing amine-functionalized MOFs (NH2-Ce-BTC), showcasing the synergistic effects of structure and chemistry in optimizing membrane properties for power generation. The NH2-Ce-BTC-500 membrane exhibits a remarkable increase in hydrophilicity, setting a new power density benchmark of 6.9 W/m2. NH2-Ce-BTC-500 outperforms Ce-BTC-500 and pristine TFC membranes by 15 % and 109.1 %, demonstrating the superior performance of amine-functionalized membranes. Furthermore, through physical and chemical characterizations, it is revealed that the MOF-based membranes exhibit increased surface roughness and porosity, indicating the formation of additional water pathways. This enhancement leads to a substantial increase in the water flux, reaching 31.0 LMH in PRO mode, which is a 106 % improvement compared to pristine TFC membranes. This remarkable increase in water flux directly translates to enhanced power generation capacity. In essence, the NH2-Ce-BTC-500 membrane exhibits promising advancements in PRO performance, offering the potential to lower downstream energy consumption and increase power generation efficiency. This research opens up a new paradigm for developing next-generation osmotic power generation. This renewable and energy-efficient solution can address the global energy crisis by leveraging the potential of MOF nanorods in TFN membranes.

Bio‐inspired Double Angstrom‐Scale Confinement in Ti‐deficient Ti0.87O2 Nanosheet Membranes for Ultrahigh‐performance Osmotic Power Generation

AbstractOsmotic power, a clean energy source, can be harvested from the salinity difference between seawater and river water. However, the output power densities are hampered by the trade‐off between ion selectivity and ion permeability. Here we propose an effective strategy of double angstrom‐scale confinement (DAC) to design ion‐permselective channels with enhanced ion selectivity and permeability simultaneously. The fabricated DAC‐Ti0.87O2 membranes possess both Ti atomic vacancies and an interlayer free spacing of ≈2.2 Å, which not only generates a profitable confinement effect for Na+ ions to enable high ion selectivity but also induces a strong interaction with Na+ ions to benefit high ion permeability. Consequently, when applied to osmotic power generation, the DAC‐Ti0.87O2 membranes achieved an ultrahigh power density of 17.8 W m−2 by mixing 0.5/0.01 M NaCl solution and up to 114.2 W m−2 with a 500‐fold salinity gradient, far exceeding all the reported macroscopic‐scale membranes. This work highlights the potential of the construction of DAC ion‐permselective channels for two‐dimensional materials in high‐performance nanofluidic energy systems.

Bio-inspired Double Angstrom-Scale Confinement in Ti-deficient Ti0.87 O2 Nanosheet Membranes for Ultrahigh-performance Osmotic Power Generation.

Osmotic power, a clean energy source, can be harvested from the salinity difference between seawater and river water. However, the output power densities are hampered by the trade-off between ion selectivity and ion permeability. Here we propose an effective strategy of double angstrom-scale confinement (DAC) to design ion-permselective channels with enhanced ion selectivity and permeability simultaneously. The fabricated DAC-Ti0.87 O2 membranes possess both Ti atomic vacancies and an interlayer free spacing of ≈2.2 Å, which not only generates a profitable confinement effect for Na+ ions to enable high ion selectivity but also induces a strong interaction with Na+ ions to benefit high ion permeability. Consequently, when applied to osmotic power generation, the DAC-Ti0.87 O2 membranes achieved an ultrahigh power density of 17.8 W m-2 by mixing 0.5/0.01 M NaCl solution and up to 114.2 W m-2 with a 500-fold salinity gradient, far exceeding all the reported macroscopic-scale membranes. This work highlights the potential of the construction of DAC ion-permselective channels for two-dimensional materials in high-performance nanofluidic energy systems.

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.

Designing artificial ion channels with strict K+/Na+ selectivity toward next-generation electric-eel-mimetic ionic power generation.

A biological potassium channel is >1000 times more permeable to K+ than to Na+ and exhibits a giant permeation rate of ∼108 ions/s. It is a great challenge to construct artificial potassium channels with such high selectivity and ion conduction rate. Herein, we unveil a long-overlooked structural feature that underpins the ultra-high K+/Na+ selectivity. By carrying out massive molecular dynamics simulation for ion transport through carbonyl-oxygen-modified bi-layer graphene nanopores, we find that the twisted carbonyl rings enable strict potassium selectivity with a dynamic K+/Na+ selectivity ratio of 1295 and a K+ conduction rate of 3.5×107 ions/s, approaching those of the biological counterparts. Intriguingly, atomic trajectories of K+ permeation events suggest a dual-ion transport mode, i.e. two like-charged potassium ions are successively captured by the nanopores in the graphene bi-layer and are interconnected by sharing one or two interlayer water molecules. The dual-ion behavior allows rapid release of the exiting potassium ion via a soft knock-on mechanism, which has previously been found only in biological ion channels. As a proof-of-concept utilization of this discovery, we propose a novel way for ionic power generation by mixing KCl and NaCl solutions through the bi-layer graphene nanopores, termed potassium-permselectivity enabled osmotic power generation (PoPee-OPG). Theoretically, the biomimetic device achieves a very high power density of >1000W/m2 with graphene sheets of <1% porosity. This study provides a blueprint for artificial potassium channels and thus paves the way toward next-generation electric-eel-mimetic ionic power generation.

Two-Dimensional Sodium Channels with High Selectivity and Conductivity for Osmotic Power Generation from Wastewater.

Conducting target ions rapidly while rejecting rival ions efficiently is challenging yet highly demanded for ion separation related applications. Two-dimensional (2D) channels are widely used for ion separation, but highly selective 2D channels generally suffer from a relatively low ionic conductivity. Here we report that the 2D vermiculite channels have a Na+ conductivity higher than bulk and at the same time reject heavy metal ions with a selectivity of a few hundreds. Such performance is attributed to the highly electronegative crystal surface and the extremely narrow channel (0.2 nm high), as also supported by the ab initio molecular dynamics simulation. We demonstrate that the highly selective and conductive sodium channels can be utilized to harvest osmotic power from industrial wastewater, achieving a power density of more than 20 W m-2 while preventing pollution from waste heavy metal ions. This work provides a strategy for wastewater utilization as well as treatment. Moreover, the investigation suggests the possibility to break the ionic permeability-selectivity trade-off by combining Ångstrom-scale confinement with proper surface engineering, which could lead to applications that are challenging for previous materials.

Nanofluidic osmotic power generation from CO2 with cellulose membranes

The diffusion of chemical species down concentration gradient is a ubiquitous phenomenon that releases Gibbs free energy. Nanofluidic materials have shown great promise in harvesting the energy from ionic diffusion via the reverse electrodialysis process. In principle, any chemicals that can be converted to ions can be used for nanofluidic power generation. In this work, we demonstrate the power generation from the diffusion of CO2 into air using nanofluidic cellulose membranes. By dissolving CO2 in water, a power density of 87 mW/m2 can be achieved. Using monoethanolamine solutions to dissolve CO2, the power density can be increased to 2.6 W/m2. We further demonstrate that the waste heat released in industrial and carbon capture processes, can be simultaneously harvested with our nanofluidic membranes, increasing the power density up to 16 W/m2 under a temperature difference of 30 °C. Therefore, our work should expand the application scope of nanofluidic osmotic power generation and contribute to carbon utilization and capture technologies.

Advancing osmotic power generation using bioinspired MXene-based membrane via maze breaking

The osmotic energy generated by the salinity gradient between seawater and river water is a potential blue energy that can be harnessed to alleviate the current energy crisis but without causing environmental pollution. Two dimensional (2D) MXene nano sheets can be horizontally assembled into laminar membranes to create nanochannels enabling the ionic selective transport and thus to capture the osmotic energy. However, the ions have to transport through the maze-like tortuous laminar channels sourced from the interspaces between two neighboring nanosheets, which lead to super-high transport resistance and low power density, hindering their wide applications. Enlightened by the water and mineral ions transport in trees, here, a bioinspired MXene-based membrane with aligned and ordered vertical channels is developed for efficient osmotic power generation from salinity gradient. The peak power density of the electrical devices composed of such maze-breaking membranes could be achieved up to 372 W m−2, which is at least one order of magnitude higher than conventionally horizontal membranes reported in the literature. The high-power generation efficiency was attributed to the shortened ionic transport path (by breaking the maze). Moreover, the membrane possesses a high antifouling performance as well as long-term stability, possibly due to the strong combination of MXene and GO via the hydrogen bonds and cross-linking in preparation. This work reports a new and high-performance MXene-based membrane not only for osmotic energy conversion but also for other practical separations.

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

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

Bioinspired Angstrom-Scale Heterogeneous MOF-on-MOF Membrane for Osmotic Energy Harvesting.

Membrane-based salinity gradient energy generation from the osmotic potential at the interface of a river and seawater through reverse electrodialysis is a promising route for realizing clean, abundant, and sustainable energy. Membrane permeability and selective ion transport are crucial for efficient osmotic energy harvesting. However, balancing these two parameters in the membrane design and synthesis remains challenging. Herein, a hybridized bilayer metal-organic frameworks (MOF-on-MOF) membrane is fabricated for efficient transmembrane conductance for enhanced osmotic power generation. The heterogeneous membrane is constructed from imidazolate framework-8 (ZIF-8) deposited on a UiO-66-NH2 membrane intercalated with poly(sodium-4-styrenesulfonate) (PSS). The angstrom-scale cavities in the ZIF-8 layer promote ion selectivity by size exclusion, and the PSS-intercalated UiO-66-NH2 film ensures cation permeability. The synergistic effect is a simultaneous improvement in ion transport and selectivity from an overlapped electric double layer generating 40.01 W/m2 and 665 A/m2 permeability from a 500-fold concentration gradient interface at 3 KΩ and 9.20 W/m2 from mixing of real sea-river water. This work demonstrates a rational design strategy for hybrid membranes with improved ion selectivity and permeability for the water-energy nexus.

Interfacial binding rope theory of ion transport in sub-nanochannels and its application for osmotic energy conversion

Osmotic energy conversion is a promising method of sustainable electricity generation using seawater and river water. Artificial pure NaCl solutions are commonly used, whereas the effects of multivalent-ion ingredients present in natural seawater on osmotic power generation remain unclear. In this study, the impacts of coexisting ions in multicomponent seawater (MS) are experimentally assessed using graphene oxide membrane (GOM). Divalent cation in MS shows inferior transport to monovalent cation, thereby presenting a lower power density than when using pure NaCl solutions. Inspired by experimental results, an interfacial binding rope theory is proposed to clarify ion-wall interactions and cation transport in sub-nanochannels with hydrophilic functional groups. The binding rope networks include direct cation-wall connections mediated by composition-changed hydration shells, indirect connections by H-bonds linking solvated water molecules and functional groups, and electrostatic attractions between cations and negatively charged surfaces. The theory feasibility is confirmed by first-principles calculations focusing on the charge distribution, binding site and length, surface electrification, and cation diffusivity and selectivity in GOM sub-nanochannels. Divalent cation diffuses slower than monovalent cation in both pure solutions and confined channels. For the mixed solution in channels, both monovalent and divalent cations show worse diffusivities due to additional monovalent-divalent electrostatic repulsions, as well as inferior cation selectivity due to weakened deprotonation reactions. Bridged by environment-controlled interfacial potential distribution, ion diffusivity, and ion selectivity, this theory guides the improvement of MS-based osmotic performance under regulations of concentration gradient, pH, and temperature. A power density of 8.52 W m–2 and stable output within 27 days are achieved. The interfacial binding rope theory could be popularized to refine the transport mechanisms of various cations in diverse hydrophilic sub-nanochannel materials, promoting the application of osmotic power generation and other nanoscale technologies.

Nanochannel‐Based Ion Transport and Its Application in Osmotic Energy Conversion: A Critical Review

AbstractNanochannel‐based ion transport is an important field of study in various disciplines, including physics, chemistry, energy, materials science, biology, and earth science. The unique features of natural nanochannels have inspired numerous innovative designs that seek to achieve high ion permeability, selectivity, and rectification. A notable example is osmotic energy conversion, which harvests sustainable salinity‐gradient energy to generate electricity without moving parts, noise, or carbon emissions and thus serves as a promising alternative to traditional fossil fuel utilization. This review focuses on the fundamental principles, regulatory methods, and practical applications of nanochannel‐based ion transport for osmotic energy conversion. The physical mechanisms of ion transport and the intriguing phenomena of ion behaviors in nanoconfined spaces are discussed first, followed by a thorough examination of the overall process of osmotic power generation from mathematical, numerical, parametric, first‐principles, molecular simulation, and modeling perspectives. Strategies for enhancing the osmotic performance are then discussed to overcome the trade‐off between ion selectivity and ion flux, including the theoretical design of nanochannel geometry and electrification, experimental optimization of nanoporous membranes, and electrolyte thermal enhancement. The existing challenges and opportunities for the future development of nanochannel‐based ion transport and osmotic power generation are addressed at the end.

Optimizing Membranes for Osmotic Power Generation.

The design of ion-selective membranes is the key towards efficient reverse electrodialysis-based osmotic power conversion. The tradeoff between ion selectivity (output voltage) and ion permeability (output current) in existing porous membranes, however, limits the upgradation of power generation efficiency for practical applications. Thus, we provide the simple guidelines based on fundamentals of ion transport in nanofluidics for promoting osmotic power conversion. In addition, we discuss strategies for optimizing membrane performance through analysis of various material parameters in membrane design, such as pore size, surface charge, pore density, membrane thickness, ion pathway, pore order, and ionic diode effect. Lastly, a perspective on the future directions of membrane design to further maximize the efficiency of osmotic power conversion is outlined.