Osmotic Power Generation Research Articles

Editor’s choice: Advancements in nanoconfined materials for osmotic power generation

Editor’s choice: Advancements in nanoconfined materials for osmotic power generation

Rearrangement of cellulose molecular chains in situ for stabilizing two-dimensional nanochannels

Two-dimensional (2D) nanochannels originating from ordered stacking of nanosheets exhibit great potential in osmotic energy harvest, which provides a promising way to address the increasing energy crisis. Although polymer nanorods have unique advantages in constructing and adjusting 2D nanochannels, the stability of these nanochannels still faces great challenges. Herein, cellulose nanocrystal (CNC) nanorods are intercalated between graphene oxide (GO) nanosheets to construct 2D sub-nanochannels. Ordered sub-nanochannel structures are first formed through a vacuum assisted self-assembly of CNC and GO. A facile strategy involving the rearrangement of cellulose molecular chains in situ is subsequently employed to stabilize the dimensions and arrangement of the sub-nanochannels. After the rearrangement, the mechanical properties and water stability of the sample are greatly enhanced due to the increased dynamic hydrogen bond network, and tensile strength and strain at break are increased by 88.7% and 472.2%, respectively. Additionally, the prepared membrane exhibits a power density of 0.49 W m-2 under a 50-fold salinity gradient with a long-term stability and could be further improved to 0.75 W m-2 by reducing the ion transport distance. The photo-thermal conversion property of the membrane could also benefit the osmotic power generation. Our work opens new insights into stabilizing the dimensions and arrangement of 2D nanochannels constructed by nanorods and nanosheets for commercial osmotic power generation.

Segmental-porosity regulation of nanochannel membranes with anomalously improved ion selectivity for enhanced osmotic power generation

Nanochannel-based osmotic energy conversion can directly convert salinity gradient energy between solutions with different ion concentrations into electric energy. In the energy conversion process, the high-concentration side of the nanochannel membrane has high ion permeability but low ion selectivity, whereas the low-concentration side exhibits opposite properties, which is the inherent limitation of osmotic power. However, the relationship between ion selectivity and permeability is unclear, which hinders the improvement of the output power density. This paper reports a segmental-porosity regulation method to break through the inherent constraints of the ion selectivity–permeability trade-off. The nanochannel membrane was divided into three regions along the ion transport direction as upstream, midstream, and downstream. The porosities of these three regions were separately regulated to optimize the structures for ion selectivity–permeability matching. When a high porosity was adopted for the downstream region and a small and mediate porosity combination was used for the upstream and midstream regions, the highest output power density was achieved, which was 78.44% higher than that of a straight nanochannel with homogeneous porosity under a 100-fold concentration ratio. The outstanding osmotic power generation performance of the optimized segmental structure was attributed to the alleviated ion concentration polarization and unexpectedly improved ion selectivity. Finally, the process and underlying mechanisms of segmental-porosity regulation are also proposed. This paper reports a novel methodology for designing the structure and porosity of nanochannel membranes to enhance osmotic power generation.

Horizontal Transport in Ti3C2Tx MXene for Highly Efficient Osmotic Energy Conversion from Saline‐Alkali Environments

AbstractOsmotic energy from the ocean has been thoroughly studied, but that from saline‐alkali lakes is constrained by the ion‐exchange membranes due to the trade‐off between permeability and selectivity, stemming from the unfavorable structure of nanoconfined channels, pH tolerance, and chemical stability of the membranes. Inspired by the rapid water transport in xylem conduit structures, we propose a horizontal transport MXene (H‐MXene) with ionic sequential transport nanochannels, designed to endure extreme saline‐alkali conditions while enhancing ion selectivity and permeability. The H‐MXene demonstrates superior ion conductivity of 20.67 S m−1 in 1 M NaCl solution and a diffusion current density of 308 A m−2 at a 10‐fold salinity gradient of NaCl solution, significantly outperforming the conventional vertical transport MXene (V‐MXene). Both experimental and simulation studies have confirmed that H‐MXene represents a novel approach to circumventing the permeability‐selectivity trade‐off. Moreover, it exhibits efficient ion transport capabilities, addressing the gap in saline‐alkali osmotic power generation.

Horizontal Transport in Ti3C2Tx MXene for Highly Efficient Osmotic Energy Conversion from Saline-Alkali Environments.

Osmotic energy from the ocean has been thoroughly studied, but that from saline-alkali lakes is constrained by the ion-exchange membranes due to the trade-off between permeability and selectivity, stemming from the unfavorable structure of nanoconfined channels, pH tolerance, and chemical stability of the membranes. Inspired by the rapid water transport in xylem conduit structures, we propose a horizontal transport MXene (H-MXene) with ionic sequential transport nanochannels, designed to endure extreme saline-alkali conditions while enhancing ion selectivity and permeability. The H-MXene demonstrates superior ion conductivity of 20.67 S m-1 in 1 M NaCl solution and a diffusion current density of 308 A m-2 at a 10-fold salinity gradient of NaCl solution, significantly outperforming the conventional vertical transport MXene (V-MXene). Both experimental and simulation studies have confirmed that H-MXene represents a novel approach to circumventing the permeability-selectivity trade-off. Moreover, it exhibits efficient ion transport capabilities, addressing the gap in saline-alkali osmotic power generation.

High-performance hydrogel membranes with superior environmental stability for harvesting osmotic energy

A collection of innovative nanofluidic systems has been developed to harness Gibbs free energy and convert it into osmotic energy. Nevertheless, these systems frequently encounter challenges such as low output power density, inadequate mechanical stability, and diminished performance in extreme conditions. In this work, we utilized a simple thermal polymerization method to fabricate a charged phytic acid (PA)–acrylic acid (AA)–3-sulfopropyl acrylate potassium salt (SPAK) hydrogel (PASH) membrane with anti-drying and anti-freezing properties, alongside mechanical stability. Using a silicon window with a testing area of 3 × 10−8 m2, a high power density of 30.94 W/m2 was achieved in artificial seawater and river water environments (0.5/0.01 M NaCl). Notably, an even higher power density of 103.9 W/m2 was attained in artificial saline lake and river water environments (5/0.01 M NaCl). Meanwhile, the PASH membrane demonstrated outstanding performance in low temperatures and various acidic and alkaline environments, offering the potential for osmotic power generation in extreme conditions. This work provides essential theoretical foundations and experimental evidence for the development of sustainable osmotic energy conversion technologies, contributing to the advancement and innovation in the field of blue osmotic energy.

Osmotic energy conversion in serpentinite-hosted deep-sea hydrothermal vents

Cells harvest energy from ionic gradients by selective ion transport across membranes, and the same principle is recently being used for osmotic power generation from salinity gradients at ocean-river interfaces. Common to these ionic gradient conversions is that they require intricate nanoscale structures. Here, we show that natural submarine serpentinite-hosted hydrothermal vent (HV) precipitates are capable of converting ionic gradients into electrochemical energy by selective transport of Na+, K+, H+, and Cl-. Layered hydroxide nanocrystals are aligned radially outwards from the HV fluid channels, constituting confined nanopores that span millimeters in the HV wall. The nanopores change the surface charge depending on adsorbed ions, allowing the mineral to function as a cation- and anion-selective ion transport membrane. Our findings indicate that chemical disequilibria originating from flow and concentration gradients in geologic environments generate confined nanospaces which enable the spontaneous establishment of osmotic energy conversion.

Healable Liquid-Free Osmotic Power Generator Using Nanofluidic Membranes of 2D Vanadium Pentoxide Nanosheets.

Here, we report an osmotic power generator (OPG) using a restacked membrane of vanadium pentoxide (V2O5) nanosheets and an ion-infused gelatin hydrogel. This gel osmotic power generator (GOPG) has several advantages over traditional liquid-based OPG in portability, power output, and spillage safety. Under a 1000-fold salinity gradient, the GOPG achieved an output power density of 0.13 W m-2. We have studied the effect of different ions, the concentration gradient, and the membrane thickness to achieve maximum power output. Notably, physical damage to the V2O5 membrane can be healed by adding a drop of water. Cracks or damage to the gelatin gel can be healed by reheating and cooling. The GOPGs can be easily connected in series and/or parallel to add up the voltage and/or current values generated by individual devices for practical powering applications. We have demonstrated lighting up an LED and powering a humidity meter using a series of combinations with the GOPG.

Osmotic power generation based on nanoconfined materials

Osmotic power generation based on nanoconfined materials

On-Water Surface Synthesis of Two-Dimensional Polymer Membranes for Sustainable Energy Devices.

ConspectusIon-selective membranes are key components for sustainable energy devices, including osmotic power generators, electrolyzers, fuel cells, and batteries. These membranes facilitate the flow of desired ions (permeability) while efficiently blocking unwanted ions (selectivity), which forms the basis for energy conversion and storage technologies. To improve the performance of energy devices, the pursuit of high-quality membranes has garnered substantial interest, which has led to the exploration of numerous candidates, such as polymeric membranes (e.g., polyamide and polyelectrolyte), laminar membranes (e.g., transition metal carbide (MXene) and graphene oxide (GO)) and nanoporous 2D membranes (e.g., single-layer MoS2 and porous graphene). Despite impressive progress, the trade-off effect between ion permeability and selectivity remains a major scientific and technological challenge for these membranes, impeding the efficiency and stability of the resulting energy devices.Two-dimensional polymers (2DPs), which represent monolayer to few-layer covalent organic frameworks (COFs) with periodicity in two directions, have emerged as a new candidate for ion-selective membranes. The crystalline 2DP membranes (2DPMs) are typically fabricated either by bulk crystal exfoliation followed by filtration or by direct interfacial synthesis. Recently, the development of surfactant-monolayer-assisted interfacial synthesis (SMAIS) method by our group has been pivotal, enabling the synthesis of various highly crystalline and large-area 2DPMs with tunable thicknesses (1 to 100 nm) and large crystalline domain sizes (up to 120 μm2). Compared to other membranes, 2DPMs exhibit well-defined one-dimensional (1D) channels, customizable surface charge, ultrahigh porosity, and ultrathin thickness, enabling them to overcome the permeability-selectivity trade-off challenge. Leveraging these attributes, 2DPMs have established their critical roles in diverse energy devices, including osmotic power generators and metal ion batteries, opening the door for next-generation technology aimed at sustainability with a low carbon footprint.In this Account, we review our achievements in synthesizing 2DPMs through the SMAIS method and highlight their selective-ion-transport properties and applications in sustainable energy devices. We initially provide an overview of the SMAIS method for producing highly crystalline 2DPMs by utilizing the programmable assembly and enhanced reactivity/selectivity on the water surface. Subsequently, we discuss the critical structural parameters of 2DPMs, including pore sizes, charged sites, crystallinity, and thickness, to elucidate their roles in selective ion transport. Furthermore, we present the burgeoning landscape of energy device applications for 2DPMs, including their use in osmotic power generators and as electrode coating in metal ion batteries. Finally, we conclude persistent challenges and future prospects encountered in synthetic chemistry, material science, and energy device applications within this rapidly evolving field.

Biomimetic MOFs/GO membranes with enhanced charge density for osmotic power generation

Osmotic power generation is a renewable energy technology that harnesses energy from the natural mixing of freshwater and saltwater, offering a promising solution for sustainable energy. However, it is still a challenge to achieve high energy output without compromising scalability and stability. Herein, inspired by the nano-constrained dynamics of neuronal cell transmembrane transport, an anodic oxidation electrodeposition method is employed to fabricate metal organic frameworks (MOFs) within 2D graphene oxide (GO) layers. The in-situ integration of high-density charges within nanometer pore with 0.34 nm entrance of MOFs facilitates rapid and selective cation transport, which significantly increases the diffusion voltage (363.8 mV, 106 fold concentration gradient). Our device demonstrates a maximum output power of 6.82 μW under a 10 fold concentration gradient at a load resistance of 5000 Ω, possessing an energy conversion efficiency of 48.8% at an effective area of 12.56 mm2, which substantially surpasses the performance reported before. More importantly, the device performs exceptionally well, showing robust and stable performance. This is proven by its ability to power various electronic devices. These findings underscore the potential of biomimetic ion channel membranes in sustainable energy applications, offering a promising avenue for the broader field of energy harvesting technologies.

MOFs/MXene nano-hierarchical porous structures for efficient ion dynamics

Nature's various hierarchical structures exhibit exceptional performance in enhancing mass transport. However, how to bionic nano-hierarchical structures to enhance ion dynamics and achieve charge regulation is a significant challenge. Herein, a nano-hierarchical porous hybrid membrane inspired by nature was designed by integrating 1D metal-organic frameworks (MOFs) on the 2D MXene substrate utilizing MOF nanoparticles with tailorable positive and negative surface charges. The synergistic effects of the bioinspired nanohierarchical porous structure and surface charge interactions significantly enhance the performance and stability of the nanoconfined membrane, as corroborated by experimental characterizations and theoretical simulations. As a result, the optimized nanohierarchical porous membrane exhibits high a cation selectivity of 0.95, an outstanding power density of 35.04 W m−2, and excellent stability over one month. Synergizing enhanced mass transport and ion dynamics in nano-hierarchically structured materials with tailored charge improves osmotic power generation efficiency and benefits logic circuits with controlled charge transport.

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

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

Enhanced osmotic power generation through anodic electrodeposited MOFs@MXene heterostructured nanochannels

Nanochannel membranes exhibit significant potential in osmotic energy generation, however, achieving an optimal balance between ion selectivity and flux remains challenging. Herein, heterostructured MOFs@MXene nanochannels were prepared via an optimized anodic electrodeposition method, which demonstrated a significant advancement in osmotic energy generation. The unique oriented porous MOFs expanded the interlayer spacing of MXene. The inherent negative charged surface of MXene fostered a synergistic effect that substantially enhanced ion selectivity and flux, achieving a significant diffusion potential of 340.2 mV. These improvements enable the membranes to achieve an impressive power density output of 25.3 W/m2 at a concentration gradient of 106 in KCl and 17.2 W/m2 at a gradient of 500 in NaCl. Furthermore, the membranes demonstrate an energy conversion efficiency of up to 45.1 %. Such structural and functional advancements underscore the potential of these heterostructured nanochannels in robust and efficient osmotic power generation, paving the way for applications in sustainable energy technologies.

A theoretical characterization of osmotic power generation in nanofluidic systems

Water desalination and fluid-based energy harvesting systems utilize ion-selective nanoporous materials that allow preferential transport of ions that are oppositely charged to the surface charge, resulting in the creation of an electrical current. The resultant current forms due to a potential drop or a concentration gradient (or both) applied across the system. These systems are electrically characterized by their current-voltage, I−V\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$I-V$$\\end{document}, response. In particular, there are three primary characteristics: the ohmic conductance, GOhmic=I/V\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${G}_{{{{{\\rm{Ohmic}}}}}}=I/V$$\\end{document}, the zero-voltage current, IV=0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${I}_{V=0}$$\\end{document}, and the zero-current voltage, VI=0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${V}_{I=0}$$\\end{document}. To date, there is no known self-consistent theory for these characteristics. Here, we present simple self-consistent expressions for each of these characteristics that provide remarkable insights into the underlying physics of water desalination and energy harvesting systems. These insights can be used to interpret (and reinterpret) the numerical and experimental measurements of any nanofluidic system subject to an arbitrary concentration gradient as well as improve their design.

Novel ultrastable 2D MOF/MXene nanofluidic membrane with ultralow resistance for highly efficient osmotic power harvesting

Two-dimensional (2D) materials have shown great potential in harvesting osmotic power due to their high membrane selectivity, but the high resistance from tortuous pathways of 2D nanofluidic membranes still impedes the further improvement in output performance. Here, we report an innovative 2D MXCT (MXene/Cu-TCPP) lamellar membrane with ultralow resistance for highly efficient osmotic power generation. The incorporation of 2D Ti3C2Tx MXene with rich functional groups not only resolves the water-stability issue of 2D metal-organic framework (MOF) Cu-TCPP, but provides large surface charges for selective ion transport. The orderly sub-2 nm framework channels of Cu-TCPP provide much shorter permeation pathways for fast ion transport, thus endowing the MXCT membrane with ultralow resistance. Consequently, the MXCT membrane reaches an ultrahigh power output of ∼8.29 W/m2 by mixing seawater and river water, which is ∼275 % higher than that of the pristine MXene membrane. Additionally, it outperforms all the reported single-layer 2D nanosheet-based osmotic power generators under the same experimental conditions in terms of output power and internal resistance (9 kΩ). This work presents a reliable strategy for stabilizing 2D Cu-TCPP MOF in electrolytes, opening new avenues for designing promising 2D nanofluidic membranes for efficient blue energy harvesting and ionic devices.

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

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

Gate-All-Around Nanopore Osmotic Power Generators.

Nanofluidic channels in a membrane represent a promising avenue for harnessing blue energy from salinity gradients, relying on permselectivity as a pivotal characteristic crucial for inducing electricity through diffusive ion transport. Surface charge emerges as a central player in the osmotic energy conversion process, emphasizing the critical significance of a judicious selection of membrane materials to achieve optimal ion permeability and selectivity within specific channel dimensions. Alternatively, here we report a field-effect approach for in situ manipulation of the ion selectivity in a nanopore. Application of voltage to a surround-gate electrode allows precise adjustment of the surface charge density at the pore wall. Leveraging the gating control, we demonstrate permselectivity turnover to enhanced cation selective transport in multipore membranes, resulting in a 6-fold increase in the energy conversion efficiency with a power density of 15 W/m2 under a salinity gradient. These findings not only advance our fundamental understanding of ion transport in nanochannels but also provide a scalable and efficient strategy for nanoporous membrane osmotic power generation.

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

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

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.