CMC / PVA nano-fluid hydrogel film as well as preparation method and application thereof

Abstract

The invention discloses a CMC (Carboxymethyl Cellulose) / PVA (Polyvinyl Alcohol) nanofluid hydrogel membrane. The CMC / PVA nanofluid hydrogel membrane is prepared by soaking citric acid cross-linked CMC / PVA hydrogel in a structure promoting ionic salt solution. The invention also provides a method for preparing the CMC / PVA nanofluid hydrogel membrane and application of the CMC / PVA nanofluid hydrogel membrane in osmotic energy conversion. The high-performance nanofluid hydrogel membrane forms a compact dual-crosslinked network, presents a high-charge nanopore structure, has significantly improved power density when applied to osmotic energy conversion, has excellent tensile strength, realizes rapid ion transmission and excellent cation selectivity, and can be used in the field of osmotic energy conversion. Meanwhile, the energy can be effectively dissipated by remarkably improving the mechanical strength.

Description

Technical Field

[0001] This invention belongs to the field of permeation energy conversion, specifically relating to a CMC / PVA nanofluid hydrogel membrane, its preparation method, and its application. Background Technology

[0002] The surge in global energy demand has spurred widespread attention to the development of clean energy. Infiltration energy between seawater and river water is considered a highly promising sustainable energy source. The mainstream technology for capturing this energy is reverse electrodialysis (RED), which directly converts infiltration energy into electricity through selective ion exchange membranes. An ideal ion exchange membrane must possess three key characteristics: high ion selectivity to generate strong voltage, low resistance to ensure high ion flux, and excellent mechanical strength to withstand long-term operation. However, conventional ion exchange membranes used in current RED technology (such as Nafion membranes and microphase separation membranes) generally suffer from the difficulty of simultaneously achieving both selectivity and permeability.

[0003] To date, various nanochannel membranes with one-dimensional, two-dimensional, and three-dimensional pores have been developed. While one-dimensional nanochannels provide an important model for studying confined ion transport, their practical application in energy conversion is limited by low power density and scalability issues. Two-dimensional materials have attracted much attention in the field of nanofluidic ion transport, but their ion transport paths are typically long and tortuous, resulting in relatively low conductivity. Furthermore, scalable fabrication and mechanical stability remain major challenges. In contrast, materials with three-dimensional interconnected network structures are less costly to fabricate, and their pore density, ion channels, and surface charge are highly tunable, providing an ideal platform for efficient permeation energy conversion.

[0004] Hydrogels, as typical 3D materials, have become ideal materials for osmotic energy harvesting due to their unique three-dimensional hydrophilic network and excellent ion conductivity. These flexible membranes possess tunable pore structures and abundant charged groups, enabling efficient ion transport while offering significant cost advantages and scalability. However, their practical application faces two major challenges: insufficient mechanical strength and excessive swelling in aqueous environments. Significant swelling upon contact with seawater leads to pore size expansion and reduced charge density, severely impairing ion selectivity and dimensional stability. These defects greatly limit the practical performance and lifespan of hydrogel membranes in osmotic energy conversion. To address these challenges, the academic community has explored various strategies to enhance mechanical properties and swelling resistance, including constructing dual-network structures, adding nanofillers, high-density covalent crosslinking, spatial confinement effects, and physically crosslinked hydrophobic frameworks. Although these methods improve the strength and swelling resistance of materials to some extent, they often have significant limitations, such as filler leaching risk, increased brittleness, limited ion transport, or complex processing techniques. Crucially, existing methods cannot achieve simultaneous and precise control of pore structure and surface charge. Most of these strategies have failed to resolve the inherent contradiction between mechanical strength and ion transport efficiency. Therefore, developing strong and tough hydrogels that combine high ion selectivity and high conductivity remains a major challenge in this field.

[0005] The Hofmeister effect provides a promising strategy for precisely controlling the structure of hydrogel networks through specific interactions between ions and polymer chains. Based on hydration properties, ions are classified as strongly hydrating structure-promoting ions or weakly hydrating structure-destructing ions. They can dominate the structure of polymers through salting out or salting out processes, thereby regulating the microstructure and properties of hydrogels. Inspired by the Hofmeister effect, a simple salt solution soaking technique has been established, which can significantly enhance the mechanical properties of hydrogels. Mechanistic studies show that the Hofmeister effect originates from the specific perturbation of the hydration shell around the hydrophilic functional groups on the polymer backbone by ions. Specifically, the salting out effect promotes hydrophobic bonding and drives inter-chain entanglement, while the salting out effect destroys the hydration layer and promotes chain extension. It is worth noting that the structural changes induced by the Hofmeister effect indicate that various salt ions can promote the construction of hydrogel nanochannels with both high mechanical strength and efficient ion transport. This is mainly based on two mechanisms: (1) Unlike traditional methods such as increasing covalent crosslinks or adding nanofillers, the Hofmeister effect promotes the formation of dynamic hydrophilic / hydrophobic nanophase separation structures, achieving synergistic improvement of multiple properties. Hydrophobic microdomains, as reversible physical cross-linking points, endow the material with excellent anti-swelling ability and mechanical robustness, while continuous hydrophilic channels ensure efficient ion conduction. (2) The structure in the Hofmeister sequence promotes ion-induced dehydration of hydrogel polymer chains and promotes hydrophobic aggregation, leading to network contraction and the formation of stable nanochannels comparable to Debye lengths, thereby enriching fixed charge groups in a smaller volume and increasing the effective charge density. This "intelligent" strategy of achieving dynamic structural reconstruction by simply switching ion types (without complex chemical modification) provides an innovative way to overcome the traditional trade-off between conductivity, selectivity and mechanical stability of ion exchange membranes. However, research reports on this phenomenon are still very limited. Summary of the Invention

[0006] One object of the present invention is to provide a tough hydrogel membrane for permeation energy conversion that combines high ion selectivity and high electrical conductivity, in order to solve the above-mentioned technical problems.

[0007] Another object of the present invention is to provide a method for preparing the hydrogel membrane.

[0008] Another object of the present invention is to provide applications of the hydrogel membrane.

[0009] To achieve the above-mentioned objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a CMC / PVA nanofluid hydrogel membrane, which is prepared by immersing a citric acid-crosslinked CMC / PVA hydrogel in a structure-promoting ion salt solution.

[0010] Preferably, in the CMC / PVA hydrogel, the mass ratio of polyvinyl alcohol to carboxymethyl cellulose is 0.5.

[0011] Preferably, the degree of substitution of the carboxymethyl cellulose is 1.34.

[0012] Preferably, the carboxymethyl cellulose is prepared by cellulose extraction and a multi-step etherification reaction.

[0013] Preferably, the cellulose is extracted from grapefruit peel.

[0014] Preferably, the structure promotes the presence of Citrate anions in the ionic salt solution. 3- SO4 2- One or more of the above, more preferably SO4 2- .

[0015] Preferably, the structure promotes the presence of Na+ cations in the ionic salt solution. + .

[0016] Preferably, the structure-promoting ion salt solution is a sodium citrate solution or a sodium sulfate solution, and most preferably a sodium sulfate solution.

[0017] Preferably, the structure promotes an anionic salt concentration of 1.5 M in the ionic salt solution.

[0018] Preferably, the average pore size of the CMC / PVA nanofluid hydrogel membrane is 27.7 nm to 33 nm.

[0019] Secondly, the present invention provides a method for preparing the CMC / PVA nanofluidic hydrogel membrane, comprising the following steps: S1. Mix carboxymethyl cellulose solution and polyvinyl alcohol solution, add citric acid, sodium hypophosphite and glycerol, stir for 12 hours, concentrate the homogenized precursor solution by rotary evaporation, let stand overnight to remove bubbles, and pour into a mold; incubate at 50°C for 12 hours to form a pregel layer, then perform four freeze-thaw cycles, each cycle including freezing at -80°C for 24 hours and thawing at room temperature for 6 hours; heat treat at 80°C for 2 hours to obtain a double-network CMC-PVA hydrogel membrane; S2. The dual-network CMC-PVA hydrogel membrane is immersed in a structure-promoting ion salt solution for 24 hours, rinsed thoroughly with deionized water to remove residual ions on the surface, and dried at 50°C to obtain the CMC / PVA nanofluid hydrogel membrane.

[0020] Preferably, the carboxymethyl cellulose solution has a mass concentration of 2%, the polyvinyl alcohol solution has a mass concentration of 10%, citric acid accounts for 25% of the total mass of carboxymethyl cellulose and polyvinyl alcohol, sodium hypophosphite accounts for 10% of the mass of citric acid, and glycerol accounts for 5% of the total mass of carboxymethyl cellulose and polyvinyl alcohol.

[0021] Preferably, the carboxymethyl cellulose is prepared by the following steps: (1) Place the dried grapefruit peel powder in a 12wt% NaOH solution and treat it at 100℃ for 3 hours; the resulting product is further oxidized at 80℃ for 30 minutes in a 2wt% NaClO2 solution with pH 4.6, and then thoroughly washed with a 1:1 volume ratio of ethanol / deionized water mixture to obtain purified grapefruit peel cellulose. (2) The obtained cellulose was alkalized in 90 mL DMSO with 30% w / v NaOH aqueous solution at 35 °C for 60 minutes, and 3.24 mol NaOH was added for each glucose structural unit; then 5 g chloroacetic acid dissolved in 100 mL ethanol was added; after reacting at 70 °C for 30 minutes, 25 mL of ethanol solution containing 2 g NaOH was added, and magnetic stirring was continued at the same temperature for 2 hours; after cooling to room temperature, the pH of the system was neutralized, the precipitate was collected, and washed three times each with 75% v / v and 95% v / v ethanol aqueous solution; the product was purified with 80% v / v ethanol aqueous solution. (3) Repeat step (2) twice, and dry the product to obtain carboxymethyl cellulose.

[0022] Thirdly, the present invention also provides the application of the CMC / PVA nanofluid hydrogel membrane in permeation energy conversion.

[0023] This invention provides a high-performance nanofluidic hydrogel through Hofmeister effect-mediated nanophase separation. The salt-precipitation-induced CMC / PVA nanofluidic hydrogel is prepared by immersing a citric acid-crosslinked CMC / PVA hydrogel in a structure-promoting ion salt solution. The highly substituted CMC provides a strong charge backbone, while PVA not only enhances the crosslinking efficiency of CA but also modulates specific ion interactions. Upon exposure to structure-promoting ions, the hydrogel undergoes controlled dehydration and chain densification, forming a dense, double-crosslinked network, exhibiting a hydrophilic / hydrophobic nanophase separation structure with highly charged nanopores. When applied to osmotic energy conversion, under a 50-fold salinity gradient, the SCPNH membrane achieves a significant 368% increase in power density compared to conventional CMC hydrogels, and possesses an excellent tensile strength of 17.7 MPa, representing a 3401% improvement. This increase in power density stems from the formation of hydrophilic nanochannels with high surface charge density and stability, enabling rapid ion transport and excellent cation selectivity. Meanwhile, the significant improvement in mechanical strength is attributed to the hydrophobic water surface acting as a dynamic physical cross-linking point, effectively dissipating energy. This unique structure allows SCPNH to overcome the traditional contradiction between mechanical robustness and ionic conductivity. Notably, by comparing hydrogels treated with different salt ions, the Hofmeister effect mechanism was further elucidated, and the optimal SCPNH membrane achieved 38.4 W / m² at a 500-fold salinity gradient. 2 The power density is high. This strategy demonstrates broad applicability in various hydrogel systems, opening a general and scalable pathway for designing novel nanofluidic hydrogels for high-performance permeable energy harvesting. Attached Figure Description

[0024] Figure 1 The appearance and microstructure of CMC / PVA pregel are shown.

[0025] Figure 2 This shows the appearance and microstructure of SCPNH.

[0026] Figure 3 Display ATR-FTIR spectra of PVA, CMC, CPH, and SCPNH.

[0027] Figure 4 This shows the tensile stress of PVAH, CMCH, CPH, and SCPNH after swelling in a humid environment for approximately 24 hours.

[0028] Figure 5 Shows the strain of PVAH, CMCH, CPH, and SCPNH after swelling in a humid environment for approximately 24 hours.

[0029] Figure 6Showing the Zeta potentials of CMCH, CPH, and SCPNH.

[0030] Figure 7 This shows the open-circuit voltage generated when CMCH, CPH, and SCPNH are applied to permeation energy conversion. V oc ).

[0031] Figure 8 This shows the short-circuit current generated when CMCH, CPH, and SCPNH are applied to permeation energy conversion. I sc ).

[0032] Figure 9 Showing the maximum power density (Pc) of CMCH, CPH, and SCPNH. max )contrast.

[0033] Figure 10 A schematic diagram of an electrochemical cell for eliminating electrode potentials is shown. Detailed Implementation

[0034] To facilitate understanding of the present invention, a more complete description will be given below with reference to specific embodiments. Preferred embodiments of the invention are shown in the accompanying drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0036] Unless otherwise specified, the experimental methods used in the following examples and comparative examples are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0037] 1. Experiment 1.1 Materials Sodium hydroxide (NaOH, ACS grade, 97%), sodium chlorite (NaClO) 22The following were purchased from Sigma-Aldrich: 95% (analytical grade), 98% (analytical grade), and 37 wt% hydrochloric acid (HCl). Polyvinyl alcohol (PVA, model 1799), glycerol (ACS grade, 99.5%), citric acid (CA, analytical grade, 99.5%), sodium hypophosphite (analytical grade, 98-101%), acrylamide (AM, analytical grade, 99%), ammonium persulfate (APS, analytical grade, 98%), N,N'-methylenebisacrylamide (MBA, analytical grade, 99%), trisodium citrate dihydrate (analytical grade, 99.5%), anhydrous sodium sulfate (analytical grade, 99%), sodium acetate (analytical grade, 99%), sodium chloride (analytical grade, 99.5%), and sodium thiocyanate (analytical grade, 99%). Organic solvents, including ethanol (analytical grade, 99.5%) and dimethyl sulfoxide (DMSO, analytical grade, 99.8%), were also purchased from Maclean's Biotech Ltd. (Shanghai, China). Deionized water was used in all experiments.

[0038] 1.2 Preparation of CMC with high degree of substitution Highly substituted carboxymethyl cellulose (CMC) was prepared from grapefruit peel through cellulose extraction and multi-step etherification reaction.

[0039] First, dried grapefruit peel powder was placed in a 12wt% NaOH solution and treated at 100℃ for 3 hours. The resulting product was then further oxidized in a 2wt% NaClO2 solution (pH=4.6) at 80℃ for 30 minutes, followed by thorough washing with a 1:1 (v / v) ethanol / deionized water mixture to obtain purified grapefruit peel cellulose.

[0040] The obtained cellulose was alkalized in DMSO (90 mL) with an aqueous NaOH solution (3.24 mol NaOH per glucose unit (AGU); 30% w / v) at 35 °C for 60 min, followed by the addition of 5 g chloroacetic acid dissolved in 100 mL ethanol. After reacting at 70 °C for 30 min, 25 mL of an ethanol solution containing 2 g NaOH was added, and the mixture was magnetically stirred for another 2 hours at the same temperature. After cooling to room temperature, the pH of the system was neutralized by dropwise addition of dilute hydrochloric acid (0.1 mol / L to 6 mol / L). The precipitate was collected and washed three times each with 75% v / v and 95% v / v aqueous ethanol solutions.

[0041] Single-step carboxymethylation typically yields only products with low degree of substitution (DS). To achieve high DS, a multi-step carboxymethylation strategy is employed: in each subsequent reaction, the CMC product obtained in the previous step is first purified with an 80% v / v aqueous ethanol solution, and then reacted with freshly prepared NaOH and chloroacetic acid. Finally, the crude CMC products with different DS values ​​are purified with 80% v / v ethanol and dried at 60°C for 12 hours to obtain the final sample.

[0042] 1.3 Preparation of SCPNH membrane The SCPNH membrane is prepared according to the following procedure: First, 2 wt% CMC solution and 10 wt% PVA solution were mixed at a PVA / CMC mass ratio of 0.5. Then, crosslinking modifiers—citric acid (CA) (25% of the total mass of CMC and PVA), sodium hypophosphite (10% of the CA mass), and glycerol (5% of the total mass of CMC and PVA)—were added sequentially. After stirring for 12 hours, the homogenized precursor solution was concentrated by rotary evaporation, allowed to stand overnight to remove bubbles, and then poured into a polytetrafluoroethylene (PTFE) mold. Incubation at 50°C for 12 hours formed a pregel layer, followed by four freeze-thaw cycles (each cycle consisting of 24 hours of freezing at -80°C and 6 hours of thawing at room temperature). To enhance the ester crosslinking reaction, the sample was heat-treated at 80°C for 2 hours to obtain a double-network CMC-PVA hydrogel (CPH) membrane physically-chemically crosslinked via hydrogen and ester bonds. Finally, the CPH membrane was soaked in a 1.5 M sodium sulfate solution for 24 hours, rinsed thoroughly with deionized water to remove residual ions on the surface, and dried at 50°C to obtain the SCPNH membrane.

[0043] The effects of various factors on the hydrogel properties were systematically studied by individually adjusting parameters such as the PVA / CMC mass ratio, salt type (sodium citrate, sodium sulfate, sodium acetate, sodium chloride, sodium thiocyanate) and salt concentration (0-2 M).

[0044] 1.4 Preparation of CMCH and PVAH As mentioned above, both CMCH and PVAH are prepared using an esterification crosslinking mechanism similar to that of CPH.

[0045] The sample without added PVA is CMCH. The sample without added CMC is PVAH.

[0046] 1.5 Characterization The chemical structure of the samples was analyzed using an attenuated total reflectance Fourier transform infrared spectrometer (ATR-FTIR, Bruker Vector 33, Germany). XRD patterns were collected using an X-ray diffractometer (Bruker D8, Advance, Germany) at a scan rate of 2° / min. The pore structure of the samples was observed using a cryogenic scanning electron microscope system, employing supercooled liquid nitrogen (<-196°C) as the freezing medium, achieving an ultra-high freezing rate (typically 10). 4 -10 5This rapid freezing process, by transforming water into a glassy state within microseconds, maximally suppresses ice crystal nucleation and growth, thus preventing the formation of large crystal structures. Zeta potential data were obtained using a Zetasizer nano-ZS potentiometer (Malvern, UK). The cation selectivity of the hydrogel was further confirmed by comparing the potassium (K) and chlorine (Cl) content characterized by energy-dispersive X-ray spectroscopy (EDX) (SuperXG2).

[0047] 1.6 Electrical Performance Measurement The transmembrane ion transport behavior and permeate energy conversion performance of the hydrogel membrane were characterized using an IV curve recorded with a Keithley 2450 source meter (Keithley Instruments, Cleveland, Ohio) and a custom-made apparatus. The voltage scan range was set to -0.2 V to +0.2 V with a step voltage of 0.05 V. Specifically, the hydrogel sample was cut to a size of 10 mm × 5 mm and sandwiched between two polyimide membranes to define an effective test area of ​​approximately 0.03 mm². The assembly was fixed in a self-made electrochemical cell filled with KCl solutions of different concentrations (pH ≈ 5.7): one side was fixed at 0.01 M, and the other side varied the concentration between 0.05 M and 5 M. The transmembrane voltage was applied through two Ag / AgCl electrodes placed in the two chambers on either side, and a pair of salt bridges were used to eliminate the unbalanced redox potential at the electrode / electrolyte interface.

[0048] 2. Results and Discussion 2.1 Design Principle of Nanofluidic Hydrogels Based on the Hofmeister Effect The Hofmeister effect provides a fundamental principle for understanding the specific regulation of the microstructure and macroscopic properties of hydrogels by anions. Based on their hydration properties and interactions with the polymer network, anions can be classified as liquid-leaching agents (such as SCN). - ) and hydrophilic agents (such as SO4) 2-The liquefying anion induces salt-dissolving effect by directly binding to the polymer chain, thereby adding extra charge to the polymer chain, enhancing its hydrophilicity and water solubility, and ultimately leading to network expansion and a loose structure. Conversely, the liquefying anion promotes salting-out behavior through two different mechanisms: firstly, by polarizing hydration water to disrupt the hydrogen bonds between the polymer and water; and secondly, by interfering with hydrophobic hydration to reduce polymer solubility. Both pathways facilitate water expulsion from the polymer matrix, strengthen interchain hydrogen bonding, and promote the formation of densely aggregated crystalline regions, jointly contributing to the construction of a reinforced network structure. Utilizing this mechanism, a strong and tough nanofluidic hydrogel with high mechanical strength, excellent anti-swelling properties, and superior ion transport performance was successfully designed to achieve efficient permeation energy conversion. Compared to traditional CMC hydrogels, the CMC / PVA nanofluidic hydrogel of this invention significantly increases power density by 368% and tensile strength by 3402%.

[0049] To further systematically study the influence of the Hofmeister effect, five anions with different positions in the ion sequence were selected: citrate (Citrate ions) 3- ), sulfate (SO4 2- Acetate (Ac) - ), chloride ions (Cl) - ) and thiocyanate (SCN) - These ions represent typical behaviors ranging from strong salting out to strong salting out, thus enabling a comprehensive study of the specific regulation of hydrogel properties by anions. Using CMC- and PVA-based hydrogel systems as models, the open-circuit voltage, short-circuit current, power density, mechanical properties, and operational stability of hydrogels treated with different anions were compared. The results show that the key performance indicators of nanofluidic hydrogels, including open-circuit voltage, mechanical properties, and operational stability, all significantly improve with the enhancement of anion salting-out ability. Strong salting-out ions induce the formation of a denser and more stable network structure, which not only enhances the mechanical strength and operational stability of the hydrogel but also improves its ion selectivity, thereby significantly improving the permeation energy conversion performance during long-term operation.

[0050] Notably, sulfate-treated hydrogels exhibited the best overall performance, combining excellent mechanical properties, high power density, and superior operational stability. To demonstrate the broad applicability of this Hofmeister effect-based strategy, the study was extended to other hydrogel systems: CMC hydrogels, PVA hydrogels, polyacrylamide (PAM) hydrogels, and CMC / PVA hydrogels. Each hydrogel was treated with a sulfate solution to obtain CMCH-SO4. 2- PVAH-SO4 2- and CPH-SO4 2-(SCPNH). When used as ion exchange membranes, the mechanical properties and power density of all sulfate-treated hydrogels are significantly enhanced. Among them, CPH-SO4 2- The most significant performance improvement was observed, with power density and mechanical strength increasing to 258% and 885% of their original values, respectively, reaching 12.6 W / m². 2 And 17.7 MPa. These results confirm that utilizing the Hofmeister effect to customize hydrogel networks provides a general and efficient approach for constructing high-performance osmotic energy harvesting devices.

[0051] 2.2. Preparation and Characterization of SCPNH To develop hydrogels that combine mechanical robustness and ion transport capabilities, and possess high surface charge and pore sizes smaller than Debye length nanopores, sustainable raw materials including CMC, PVA, and CA were employed. The specific design principles are as follows: (1) Highly substituted CMC (substitution degree 1.34) serves as the main matrix of the hydrogel, and its abundant carboxyl groups are crucial for establishing high ion selectivity within the system. (2) The introduction of PVA not only helps to improve ionic conductivity but also plays a key role in triggering the Hofmeister effect to finely regulate the hydrogel network structure. (3) CA, as a multifunctional crosslinking agent, can stabilize the hydrogel framework through ester bonds and introduce additional carboxyl groups, thereby further increasing the surface charge density and enhancing the ion selectivity of the hydrogel.

[0052] Determination of degree of substitution The degree of substitution of CMC was determined as follows: 1 g of CMC sample was immersed in 40 mL of a nitric acid-methanol mixture (108 mL of 65% nitric acid was diluted to 1 L with methanol) for 3 hours. Afterward, the sample was filtered, washed with 70% methanol, and dried at 55°C for 1 hour. Subsequently, 1 g of the dried CMC was dissolved in 100 mL of distilled water and 15 mL of 1 M NaOH solution, and then titrated with 1 M HCl. The degree of substitution of CMC was calculated using the following formula: Formula (1):

[0053] Formula (2):

[0054] Where A is the equivalent weight (g) of alkali required to dry each gram of CMC, B is the volume (mL) of NaOH solution used, C is the concentration (M) of NaOH solution, D is the volume (mL) of HCl solution used, E is the concentration (M) of HCl solution, and F is the weight (g) of dried CMC.

[0055] Detailed reaction mechanism: First, CMC and PVA were combined using CA as a crosslinking agent, and a pregel was prepared through heating and cyclic freeze-thaw treatment. At this stage, only a limited number of ester bonds and hydrogen bonds were formed, resulting in a loose polymer network with low mechanical strength. To enhance its structural integrity, a salting-out process was subsequently performed. The strengthening mechanism relies on the ability of salting-out ions to redistribute water molecules around the polymer chains. This process disrupts the hydration layer, weakens polymer-water interactions, and simultaneously promotes the bonding between polymer chains, including hydrophobic interactions and hydrogen bonds between PVA-PVA, PVA-CMC, and CMC-CMC chains. As a result, the CMC / PVA chains aggregated more tightly, forming a denser crosslinked network.

[0056] To confirm this structural change, the appearance and microstructure of the hydrogel membrane before and after salting-out treatment were compared. Figure 1 As shown, the untreated pregel membrane is highly transparent, allowing clear visibility of objects below. It exhibits a continuous microporous structure with an average pore size of approximately 1.65 micrometers. In contrast, the salt-precipitated SCPNH membrane becomes slightly whitish and less transparent. This is attributed to the salt-precipitation process inducing nanophase separation with a nanoporous structure, resulting in a refinement of the hydrophilic channel pore size to approximately 33 nanometers. Figure 2 These observations indicate that salting out can effectively regulate the assembly of hydrogel networks and reduce pore size, thereby forming more compact nanostructures.

[0057] The chemical structure of SCPNH was investigated using ATR-FTIR spectroscopy, with CMC, PVA, and CPH used as references. Figure 3 As shown, the original CMC spectrum is at 1609 cm⁻¹. -1 The characteristic stretching peak of -COOH is shown at 3256 cm⁻¹, while the PVA spectrum shows a peak at 3256 cm⁻¹. -1 A relatively wide OH stretching band is observed nearby. In the CPH spectrum, 1729 cm⁻¹... -1 A new peak appeared nearby, corresponding to the C=O stretching vibration of the ester group, confirming the successful covalent cross-linking between CA and the hydroxyl groups of CMC / PVA. Notably, in the sulfate-treated SCPNH sample, the OH stretching peak shifted to a lower wavenumber compared to CPH, indicating enhanced hydrogen bonding. This is attributed to the strong salting-out effect of sulfate ions, which disrupts the hydration layer around the polymer chain, weakens the polymer-water hydrogen bonding, and promotes interchain hydrogen bonding and physical entanglement between CMC and PVA.

[0058] XRD analysis further elucidated the structural evolution of SCPNH. Pure PVA exhibited characteristic diffraction peaks at 2θ ≈ 19.5° and 22.8°, corresponding to the (101) and (200) crystal planes, respectively, indicating its semi-crystalline nature. In contrast, CMC showed a broad peak near 20°, consistent with its amorphous structure formed by carboxymethyl substitution. The CPH sample showed significantly reduced crystallinity due to the disruption of PVA chain order by covalent crosslinking. Notably, SCPNH exhibited a sharp and intense diffraction peak at 19.7°, with significantly higher crystallinity than CPH. This is attributed to PVA chain dehydration induced by the salting-out effect, a process that promotes inter-chain aggregation and facilitates the formation of nanocrystalline domains as physical crosslinking points, thereby enhancing the hydrogel matrix. These results indicate that SCPNH forms a dual-network hydrogel, synergistically enhanced by covalent crosslinking of CA and salting-out-induced PVA crystalline network.

[0059] Furthermore, tensile testing further revealed the significant improvement in the mechanical properties of the hydrogel caused by the Hofmeister effect. For example... Figure 4 and Figure 5 As shown, PVAH and CMCH exhibited relatively low tensile strengths of 1.11 MPa and 0.52 MPa, respectively. The CPH membrane showed higher strength (1.99 MPa), but remained within the range of conventional hydrogels. Surprisingly, sulfate-treated SCPNH achieved a significant tensile stress of 17.69 MPa, nearly 1000% higher than CPH, while also exhibiting a higher elongation at break (81.7%, compared to 73.8% for CPH), achieving a simultaneous improvement in both strength and toughness. As demonstrated by XRD analysis, this significant improvement in mechanical properties stems from the strong salting-out behavior of sulfate ions, which promotes dehydration and aggregation of PVA chains by disrupting their hydration layer, thereby facilitating the formation of PVA nanocrystalline domains. These nanocrystalline regions, acting as strong physical crosslinking points, synergistically work with the covalent ester network formed by CA to construct a dense and stable dual-network structure, thus significantly enhancing the mechanical properties of the hydrogel.

[0060] 2.3. Salting-out effect-induced enhancement of SCPNH ion transmembrane transport performance and its mechanism A systematic study was conducted on the ion transport behavior and mechanism of hydrogel membranes. Traditional hydrogel membranes, due to their loose network and low charge density, typically suffer from poor ion selectivity and low interfacial transport efficiency. In contrast, SCPNH membranes possess a denser and highly charged nanoporous network. This structure endows them with excellent ion selectivity and transport efficiency, thereby significantly improving permeation energy conversion performance.

[0061] The selective ion transport characteristics of SCPNH were evaluated using IV curves. Voltage and current correspond to the ion selectivity and ion transport efficiency of the material, respectively. Figure 7 and Figure 8 As shown, the salt-treated SCPNH membrane exhibited significantly higher open-circuit voltage and short-circuit current than the untreated CMCH and CPH membranes, indicating that the nanopores with higher surface charge density indeed enhance cation selection behavior and reduce ion transport resistance.

[0062] According to the following formula, the output power density of a membrane-based generator decreases sharply as the test area increases.

[0063]

[0064] Among them, P m Where S is the maximum power density, and U is the test area. O R is the open-circuit voltage, α is the loss coefficient caused by polarization potential, and R T K is the internal resistance of the test device. m K P-I and K P-II These represent the ion transport resistance factors of the gradient membrane itself, the high-concentration side polyimide membrane, and the low-concentration side polyimide membrane, respectively.

[0065] The maximum power density calculated using the above formula further reveals that SCPNH treated with salting out has nearly four times the power density of traditional hydrogels. Figure 9 This enhancement is mainly attributed to the salting-out effect's regulation of membrane pore structure and surface charge density.

[0066] The salting-out process regulates the formation of highly charged nanopores within the CA-crosslinked CMC / PVA hydrogel through a synergistic mechanism. It first induces dehydration and densification of the CMC and PVA polymer chains, driving their rearrangement to form a dense nanoporous network with significantly reduced pore size. Simultaneously, this chain densification process exposes more hydroxyl groups on the PVA chains, thereby significantly increasing the available sites for CA crosslinking. The increased crosslinking density not only strengthens the nanostructure but also introduces more anionic carboxylate groups (-COO-). - The introduction of a network significantly increases the fixed surface charge density, thereby enhancing ion selectivity. Therefore, the synergistic mechanism of structural densification and charge enhancement is considered the core reason for the improved performance of SCPNH after salting out.

[0067] To verify these structural changes, the pore size and surface charge density of the hydrogel before and after salting-out treatment were compared. The surface charge density of SCPNH decreased from 3.9 mC / m² after salting-out treatment. -2 Increased to 5.9 mC m -2 The average pore size decreased from 2.22 μm to approximately 33 nm.

[0068] Based on the Zeta potential test results, the surface charge density of the hydrogel film was calculated using the following Grahame equation.

[0069]

[0070] Where σ is the surface charge density and ε0 is the vacuum permittivity (8.8519 × 10⁻⁶). -12 C•m -1 •V -1 ), ε is the dielectric constant of water at 25°C (78.36), ζ is the Zeta potential (mV), λ d The Debye length (nm) can be calculated using the following formula (assuming a KCl concentration of 10). -3 mol•L 1 ).

[0071]

[0072] Where C0 is the concentration of the KCl solution (mol•L) 1 ).

[0073] Because the pore size is comparable to the Debye length, the overlapping electrical double layer (EDL) amplifies the effect of surface charge, further enhancing ion selectivity. The high density of fixed charges effectively excludes like ions and preferentially adsorbs counterions, enabling SCPNH to achieve excellent selectivity while maintaining high ionic conductivity. Figure 6 The stronger adsorption capacity of SCPNH for potassium ions further confirms its superior cation selectivity.

[0074] To further investigate ion transport behavior, the ionic conductivity of the SCPNH membrane at different KCl concentrations was analyzed. The results show that ion transport in SCPNH is dominated by surface charge and occurs within a confined space. Above a critical concentration of approximately 0.1 M, the ionic conductivity increases linearly with concentration, consistent with bulk transport behavior caused by double-layer thinning. However, below this critical concentration, a strong nonlinear relationship is observed, with the conductivity significantly deviating from the bulk value and reaching a plateau. This phenomenon is caused by the overlapping double layers within the nanochannels, leading to ion enrichment within the confined space. Therefore, transport behavior is dominated by the surface and space charges within SCPNH, rather than bulk concentration. Based on this unique ion transport characteristic of nanochannels, SCPNH exhibits superior ionic conductivity and selectivity compared to traditional hydrogels. At a KCl concentration of 0.01 mM, the calculated ionic conductivity of SCPNH is approximately 4 times and 10 times that of CMCH and CPH, respectively.

[0075] Traditional hydrogels typically suffer from severe swelling in aqueous environments, leading to poor network stability and low space charge density, hindering their application in osmotic energy harvesting. In contrast, the salt-out treatment of SCPNH membranes effectively overcomes these limitations. To evaluate the effect of salt-out treatment on the swelling behavior of hydrogels, the swelling rates of SCPNH membranes and CPH membranes after immersion in deionized water were compared. The swelling rate (SR) was defined as Ws / Wd (Ws and Wd being the weights of the swollen and dried membranes, respectively). The results showed a significant difference: the SR of CPH reached 310% within 30 minutes, while the SR of SCPNH was only 30%. Even after 5 hours of immersion, the SR of SCPNH remained low at 49%, confirming the effectiveness of this salt-out strategy in improving the anti-swelling properties of hydrogels.

[0076] Furthermore, the dimensional stability of the nanopores in SCPNH under humid conditions was investigated. After 24 hours of immersion, CPH exhibited a significant swelling rate of up to 452%, with its average pore size increasing from 161.8 nm to 1.65 μm. In contrast, the swelling of SCPNH was significantly suppressed, with the average pore size increasing only slightly from 31.5 nm to 33.0 nm. This enhanced stability stems from the salting-out effect-induced nanophase separation, which promotes the aggregation of polymer chains to form a denser, more robust network with stronger intermolecular forces. This compacted structure helps maintain the structural integrity of the nanopores, preventing them from collapsing or over-expanding. The excellent pore size stability ensures continuous and highly selective ion transport, which is crucial for achieving long-term reliable permeation energy conversion.

[0077] 2.4. The Influence of the Hofmeister Effect on the Structure and Properties of Hydrogels To elucidate the Hofmeister effect-mediated ion transport mechanism, this study used a series of sodium salts to systematically evaluate the transport properties of different anions (citrate, etc.). 3- ), sulfate (SO4 2- Acetate (AC) - ), chloride ions (Cl) - ), thiocyanate (SCN) - The effects of CPH pregels on the structure, mechanical properties, and ion transport behavior of hydrogel membranes were investigated. A series of samples (named CPH-X, where X represents the inorganic salt type) were obtained by immersing CPH pregels in different salt ion solutions (1.5 M) at room temperature. These hydrogel membranes exhibited significant differences in appearance under hydration. Specifically, the transparency and size of the membranes decreased with increasing anion salting-out ability, in the following order: Cit 3- <SO4 2- <AC - <Cl - <SCN -This phenomenon can be attributed to the enhanced nanophase separation induced by a stronger ion salting-out effect.

[0078] To investigate the effect of ion concentration, the influence of different concentrations of sodium sulfate (0-2.0 M) on hydrogel formation was further studied. With increasing sulfate ion concentration, both the transparency and size of the membrane gradually decreased, clearly indicating a positive correlation between the degree of salting out and ion concentration. Higher concentrations of salting-out ions promoted the formation of additional hydrogen bonds, thereby increasing the density of the polymer crystalline regions. This suggests that salting-out ions, by dehydrating the polymer chains, not only enhance interchain hydrogen bonding but also promote the ordered arrangement and crystallization of molecular chains, ultimately leading to the densification of the polymer network.

[0079] To reveal the intrinsic structural changes, the microstructure of these hydrogels in the wet state was precisely characterized. It is well known that characterizing the pore structure of hydrogels in the wet state is particularly difficult. Traditional methods rely on scanning electron microscopy to observe the pore morphology of freeze-dried hydrogels. However, the freezing and sublimation processes introduce mechanical stress, which may lead to pore collapse or deformation, thus failing to accurately reflect the microstructure of the hydrogel in the wet state.

[0080] To reliably assess the effectiveness of the Hofmeister effect, cryo-scanning electron microscopy was employed. This technique utilizes ultra-rapid freezing to prevent ice crystal formation, thereby allowing for precise observation of the true pore structure of the wetted CPH-X membrane. The pore size of the hydrogel gradually decreases with increasing anionic salting-out intensity. Specifically, the pore size of the strong salting-out ion (Cit) increases with increasing anionic salting-out intensity. 3- SO4 2- This induces the formation of dense nanochannels in the hydrogel, with average pore sizes of 27.7 nm and 33.0 nm, respectively. In contrast, the AC-induced... - Cl - and SCN - The treated hydrogels exhibited significantly larger pore sizes, measuring 1.13 μm, 1.70 μm, and 2.60 μm, respectively. Notably, the pore sizes followed a sequence: Cit 3- <SO4 2- <AC - <Cl - <SCN - The microstructure exhibits a systematic change from dense to sparse. This continuous microstructural change is consistent with macroscopic observations, demonstrating the regular influence of different salt ions on the hydrogel pore structure. Strong salting-out ions promote polymer chain aggregation and densification, thereby constructing ideal and stable nanopores. This process is driven by salting-out-induced polymer recrystallization, where abundant grains generate numerous nucleation sites, leading to the formation of denser nanopores.

[0081] To reveal the underlying mechanisms of these macroscopic and microscopic structural changes, FTIR and XRD were used to further evaluate the influence of salting-out effect on the interactions between intrachain chains in the hydrogel. The FTIR spectra of hydrogels with different salting-out induced crosslinking were compared. In hydrogels treated with salting-out ions, the hydroxyl stretching vibration peaks all red-shifted to lower wavenumbers, and the shift amplitude was directly related to the salting-out intensity of the ions. Simultaneously, the intensity of the -OH absorption band also increased in the following order: Cit 3- SO4 2- AC - >Cl - SCN - The peak shift and increased intensity both indicate a significant enhancement of hydrogen bonding between polymer chains. This effect is attributed to the dehydration of polymer chains by salting-out ions, which removes their hydration layer and promotes tighter interchain bonding, thereby facilitating the formation of more hydrogen bonds. These strong interactions are crucial for improving the mechanical strength, network stability, and water retention capacity of hydrogels, and are of great significance for improving their permeation energy conversion properties.

[0082] Furthermore, the crystallization behavior of hydrogels induced by different salt ions was investigated using XRD. All samples exhibited a significant diffraction peak near 2θ ≈ 20°, which is attributed to the (101) crystal plane of the polymer. Notably, the diffraction peak position of the untreated hydrogel showed a significant forward shift. After different anion salting-out treatments, the degree of this forward shift changed in the following order: Cit 3- <SO4 2- <AC - <Cl - <SCN - .

[0083] The relationship between the diffraction angle (θ) and the interplanar spacing (d) is defined by the following equation.

[0084] Formula: 2d sinθ = nλ Where n is the diffraction order and λ is the X-ray wavelength. When the interplanar spacing (d) increases, the diffraction angle (θ) must decrease to maintain the equilibrium of the equation, which causes the diffraction peak to shift towards a smaller angle.

[0085] According to Bragg's law, a decrease in diffraction angle corresponds to an increase in interplanar spacing (d), indicating a looser crystal structure. Conversely, a smaller d value implies a tighter chain packing. Among the tested ions, citrate ions induced the smallest lattice spacing, which can be attributed to their strong salting-out ability promoting dense polymer aggregation. Quantitative analysis of crystallinity further revealed that the order of crystallinity from highest to lowest is: Citrate... 3 - SO4 2- >AC - >Cl - SCN -The increased crystallinity induced by stronger salting-out ions is attributed to the promotion of polymer chain dehydration and interchain hydrogen bonding, which corroborates the FTIR results. These interactions promote the formation of ordered crystalline domains, ultimately improving the structural integrity and properties of the material.

[0086] The mechanical properties and ion transport characteristics of hydrogels depend on their chemical and microstructural features. Mechanical testing results show that with enhanced salting-out effect, the tensile strength of hydrogels significantly increases, while the elongation at break decreases. This is because a denser network structure and higher crystallinity enhance the material's rigidity, but simultaneously restrict the mobility and ductility of molecular chains. These structural changes improve the mechanical durability and dimensional stability of hydrogels under osmotic pressure, which is crucial for their long-term operation in osmotic energy conversion systems.

[0087] Furthermore, this paper also explores the influence of salt ion type on ion transport. Open-circuit voltage ( V oc () Follows an obvious order consistent with the Hofmeister sequence: Cit 3- SO4 2- >AC - >Cl - SCN - This phenomenon can be attributed to the enhanced ion selectivity resulting from narrower nanochannels and higher surface charge density in strong salting-out systems. In contrast, the short-circuit current ( I sc The relationship between power density and salting-out intensity is non-monotonic. Under moderate salting-out conditions, I sc There was some improvement, but when using excessively strong salting-out ions such as citrate, the value decreased significantly due to the overly dense polymer network severely limiting ion transport. (The text abruptly ends here, likely due to an incomplete sentence or missing information.) 2- The treated hydrogel has the best performance. I sc The current is 9.5 μA, and the power density is 12.6 W / m².

[0088] Further investigation using Na2SO4 solutions of different concentrations revealed a similar pattern. V oc and I scThe concentration of Na₂SO₄ increases with increasing concentration (up to 1.5 M), then begins to decrease above this concentration. This behavior stems from the evolution of the hydrogel's microstructure: moderate salting-out promotes the formation of appropriately sized nanopores, which is beneficial for ion selectivity without significantly impairing ion flux. However, at excessively high concentrations (e.g., 2.0 M), excessive aggregation of polymer chains leads to overly dense or even collapsed pore structures, which significantly hinders ion transport by increasing resistance and reducing ion permeability. These results indicate that the concentration of Na₂SO₄ increases with increasing Na₂SO₄ concentration (up to 1.5 M), but then decreases beyond this concentration. 2- The optimal salting-out conditions corresponding to the concentration can achieve the best balance between ion selectivity and conductivity, thereby obtaining the maximum power generation efficiency.

[0089] 2.5. Highly Efficient Osmotic Energy Conversion of SCPNH The osmotic energy harvesting performance of SCPNH was evaluated using an electrochemical cell under different KCl salinity gradients. The SCPNH membrane was immobilized with a 0.03 mm... 2 The polyimide films with openings were interposed to ensure consistent test areas, matching the device areas reported in most studies. Two similar IV curves representing the reverse and forward ion diffusion directions were tested at a 50-fold concentration gradient (0.5 M / 0.01 M KCl). The values ​​were obtained from the absolute values ​​of the X / Y axis intercepts. V oc and I sc The values ​​for the two diffusion directions are approximately 160 mV and 9.5 µA, respectively. This indicates that no preferred ion diffusion direction was observed, confirming the symmetry of the film structure.

[0090] The effect of PVA content on transmembrane ion diffusion was further investigated. V oc and I sc All showed a trend of first increasing and then decreasing with the increase of the PVA / CMC mass ratio.

[0091]

[0092] Among them, P m Where S is the maximum power density, and U is the test area. O R is the open-circuit voltage, α is the loss coefficient caused by polarization potential, and R T K is the internal resistance of the test device. m K P-I and K P-II These represent the ion transport resistance factors of the gradient membrane itself, the high-concentration side polyimide membrane, and the low-concentration side polyimide membrane, respectively.

[0093] The maximum power density calculated using the above formula also follows the same trend, reaching an optimal value of 12.6 W / m² at a PVA / CMC mass ratio of 0.5. This optimal performance is attributed to the dual effect of PVA: it not only improves ionic conductivity but also introduces abundant hydroxyl groups, thereby increasing the crosslinking efficiency of CA and subsequently introducing more carboxyl groups. Combined with the PVA-induced Hofmeister effect, these combined effects lead to a higher surface charge density, thus improving… V oc and I sc However, excessively high PVA content can negatively impact ionic conductivity due to a reduced CMC ratio, and decrease overall carboxyl density, leading to performance degradation. Therefore, SCPNH prepared with an optimal PVA / CMC mass ratio of 0.5 was selected for all subsequent tests.

[0094] To determine the optimized salinity gradient power conversion characteristics of SCPNH, an SCPNH-based reverse electrodialysis unit was connected to an external circuit with a variable load resistance. The output power density was calculated using the formula P = (I² × R). L ) / S, where I and R L S and S represent the generated current, load resistance, and test area of ​​the composite film, respectively. I sc The power density gradually decreases as the load resistance increases, while it first increases and then decreases, approaching R0. L It reaches a maximum power density of approximately 12.6 W / m² at a resistance of 6 kΩ. Furthermore, thanks to its excellent water stability, the membrane exhibits outstanding long-term operational stability. After immersion in artificial seawater and river water for 30 days, its power output remains stable, retaining over 95% of its initial maximum power density (12.1 W / m²).

[0095] Further energy conversion assessments included IV testing at concentration gradients ranging from 10 to 500 times. Low concentration side (C L The concentration was fixed at 0.01 M KCl, while the high concentration side (C) H The concentration of KCl varies between 0.01 M and 5 M. I sc and V oc Both increased systematically with increasing gradient, reaching peak values ​​of 20.3 μA and 227.1 mV, respectively. It should be noted that the measured values... V oc It includes contributions from both diffusion potential and redox potential. To obtain the actual diffusion potential and diffusion current, an agar salt bridge is used to eliminate the redox potential. Figure 10 Subsequently, the cation transport number and maximum energy conversion efficiency are calculated using the following formulas.

[0096] Ion selectivity (t) + The energy conversion efficiency (η) can be calculated using the following formula.

[0097]

[0098] in, E diff λ represents the diffusion potential; R, T, F, and Z represent the universal gas constant, absolute temperature, Faraday constant, and ion valence state (z=1), respectively; λ and c represent the ion activity and concentration, respectively.

[0099] Cation transport number is a key indicator of membrane selectivity, and its value approaches 1 under ideal cation selectivity. Under different gradients, the cation transport number of SCPNH remains close to 1. Although ion selectivity typically decreases in concentrated electrolytes due to reduced electrostatic interaction range and shielding effect, SCPNH maintains a high cation transport number of approximately 0.92 even under high salt conditions, demonstrating excellent cation selectivity. As the gradient increases from 10-fold to 500-fold, the maximum power density increases from 3.7 W / m² to 38.4 W / m², while the energy conversion efficiency decreases from 49% to 34% due to the inevitable shortening of the Debye length. Open-circuit voltage under different gradients (… V oc ), diffusion potential ( E diff ), redox potential ( E redox ), cation transference number (t) + ) and maximum efficiency (η) max Key parameters such as ) are summarized in Table 2.

[0100] Table 2. Performance comparison of cellulose-based nanochannel membranes

[0101] The literature sources for each membrane material are listed in Table 2: 1. Ji, J., et al. Osmotic Power Generation with Positively and Negatively Charged 2D Nanofluidic Membrane Pairs. Adv. Funct. Mater. 27,1603623 (2016). 2. Ding, L., et al. Oppositely Charged Ti3C2Tx MXene Membranes with2D Nanofluidic Channels for Osmotic Energy Harvesting. Angew. Chem. Int. Ed. 59, 8720-8726 (2020). 3. Qian, Y., et al. Boosting Osmotic Energy Conversion of GrapheneOxide Membranes via Self-Exfoliation Behavior in Nano-Confinement Spaces. J. Am. Chem. Soc. 144, 13764-13772 (2022). 4. Wang, J., et al. Unlocking osmotic energy harvesting potential inchallenging real-world hypersaline environments through vermiculite-basedhetero-nanochannels. Nat.com. 15, 608 (2024). 5. Ding, Z., et al. Plasma‐oxidized 2D MXenes subnanochannel membranefor high‐performance osmotic energy conversion. Carbon Energy 6, e509 (2024). 6. Hu, Y., et al. Confined Ionic-Liquid-Mediated Cation Diffusionthrough Layered Membranes for High-Performance Osmotic Energy Conversion. Adv. Mater. 35, e2301285 (2023). 7. Zhu, C., et al. Metallic Two-Dimensional MoS(2) Composites asHigh-Performance Osmotic Energy Conversion Membranes. J. Am. Chem. Soc. 143,1932-1940 (2021). 8. Rao, J., et al. Nacre‐Inspired Mechanically Robust Films forOsmotic Energy Conversion. Adv. Funct. Mater. 34,(2023). 9. Wang, Q., et al. Efficient Solar-osmotic Power Generation fromBioinspired Anti-fouling 2D WS(2) Composite Membranes. J. Am. Chem. Soc. 62,e202302938 (2023). 10. Jia, X., et al. Enhanced Selective Ion Transport in HighlyCharged Bacterial Cellulose / Boron Nitride Composite Membranes for Thermo-Osmotic Energy Harvesting. Nano Lett. 24, 2218-2225 (2024). 11. Chen, W., et al. Ionic Crosslinking-Induced Nanochannels:Nanophase Separation for Ion Transport Promotion. Adv. Mater. 34, e2108410(2022). 12. Sun, Y., et al. Tailoring A Poly(ether sulfone) Bipolar Membrane:Osmotic-Energy Generator with High Power Density. Angew. Chem. Int. Ed. 59,17423-17428 (2020). 13. Zhao, Y., et al. Robust sulfonated poly (ether ether ketone)nanochannels for high-performance osmotic energy conversion. Natl. Sci. Rev. 7,1349-1359 (2020). 14. Zhang, Z., et al. Ultrathin and Ion-Selective Janus Membranes forHigh-Performance Osmotic Energy Conversion. J. Am. Chem. Soc. 139, 8905-8914(2017). 15. Xuanbo Zhu, J. H., Bin Bao, Yahong Zhou, Haibo Zhang, JinhuiPang, Zhenhua Jiang, Lei Jiang. Unique ion rectification in hypersalineenvironment: A high-performance and sustainable power generator system. Sci. Adv. 4, eaau1665 (2018). 16. Li, Z. Q., et al. Light-Enhanced Osmotic Energy Harvester UsingPhotoactive Porphyrin Metal-Organic Framework Membranes. Angew. Chem. Int. Ed. 61, e202202698 (2022). 17. Xin, W., et al. High-performance silk-based hybrid membranesemployed for osmotic energy conversion. Nat.com. 10, 3876 (2019). 18. Xie, L., et al. Sequential Superassembly of Nanofiber Arrays toCarbonaceous Ordered Mesoporous Nanowires and Their Heterostructure Membranesfor Osmotic Energy Conversion. J. Am. Chem. Soc. 143, 6922-6932 (2021). 19. Zhou, S., et al. Interfacial Super-Assembly of Ordered MesoporousCarbon-Silica / AAO Hybrid Membrane with Enhanced Permselectivity forTemperature- and pH-Sensitive Smart Ion Transport. Angew. Chem. Int. Ed. 60,26167-26176 (2021). 20. Pan, S., et al. Toward Scalable Nanofluidic Osmotic PowerGeneration from Hypersaline Water Sources with a Metal-Organic FrameworkMembrane. Angew. Chem. Int. Ed. 62, e202218129 (2023). 21. Huang, K. T., et al. Zwitterionic Gradient Double‐NetworkHydrogel Membranes with Superior Biofouling Resistance for SustainableOsmotic Energy Harvesting. Adv. Funct. Mater. 33, 2211316 (2023). 22. Chen, J., et al. Wood vessel-confined anti-swelling hydrogel forefficient osmotic energy conversion. Nano Energy 104, 107981 (2022). 23. Cao, L.&Wu, H. Dual-network fiber-hydrogel membrane for osmoticenergy harvesting. Front. Chem. 12, 1401854 (2024). 24. Hao, J., et al. Improved ion transfer and osmotic energyconversion via nanofibers / polymer composite membrane with hierarchical 3Dporous. J. Power Sources 604, 234498 (2024). 25. Lin, Y. C., Chen, H. H., Chu, C. W.&Yeh, L. H. Massively EnhancedCharge Selectivity, Ion Transport, and Osmotic Energy Conversion byAntiswelling Nanoconfined Hydrogels. Nano Lett. 24, 11756-11762 (2024). 26. Zhu, R., et al. Negative space charge modulated ion transportthrough PEDOT:PSS hydrogels integrating nanofluidic channels for highlyefficient osmotic energy harvesting. J. Mater. Chem. A 12, 14559-14568 (2024). 27. Sun, Z., et al. Enhanced osmotic energy conversion throughbacterial cellulose based double-network hydrogel with 3D interconnectednanochannels. Carbohydr. Polym. 305, 120556 (2023). 28. Chen, W., et al. Improved Ion Transport in Hydrogel-BasedNanofluidics for Osmotic Energy Conversion. ACS Cent. Sci. 6, 2097-2104 (2020). 29. Bian, G., et al. Anti-Swelling Gradient Polyelectrolyte HydrogelMembranes as High-Performance Osmotic Energy Generators. Angew. Chem. Int. Ed. 60, 20294-20300 (2021). 30. Chen, W., et al. Improved Ion Transport and High EnergyConversion through Hydrogel Membrane with 3D Interconnected Nanopores. Nano Lett. 20, 5705-5713 (2020). 31. Zhang, Z., et al. Improved osmotic energy conversion inheterogeneous membrane boosted by three-dimensional hydrogel interface. Nat. Com. 11,875 (2020). Given that ion exchange membranes in reverse electrodialysis systems often operate under complex salinity conditions, the output power density of SCPNH was tested over a wide concentration gradient range (5 to 500 times). Even at an extreme gradient of 5.0 M / 0.01 M KCl (500 times), the SCPNH membrane still delivered a significant power density of 38.1 W / m². This excellent salt tolerance highlights its potential for extracting permeate energy under harsh salinity conditions, including industrial wastewater. Compared to most existing nanofluidic membranes, SCPNH exhibits superior power output at a 50-fold gradient, outperforming many layered membranes, polymer membranes, Janus membranes, heterogeneous membranes, and hydrogel-based membranes.

[0102] 3. Conclusion In summary, this invention, based on the Hofmeister effect, successfully constructs a novel nanofluidic hydrogel membrane, effectively solving the long-standing problem of balancing mechanical strength and ion transport performance in osmotic energy conversion. This strategy utilizes hydrophilic anions to induce controllable polymer dehydration and chain densification, forming a dense, doubly cross-linked CMC / PVA hydrogel with highly charged nanopores (33 nm) and a significantly increased surface charge density (5.9 mCm). -2 Based on the Hofmeister effect, this membrane contains abundant highly charged nanopores, forming efficient ion channels that significantly enhance ion transmembrane transport performance, including ionic conductivity and cation selectivity. It also improves the membrane's mechanical strength. This unique structure endows the hydrogel with superior performance: at a 50-fold salinity gradient, its power density (12.6 W / m³) is [missing value]. 2 Compared to traditional CMC hydrogels, this method achieves a 368% improvement, with an astonishing 3401% increase in tensile strength (17.7 MPa). Particularly noteworthy is the elucidation of the Hofmeister effect modulation mechanism through comparative studies of hydrogels treated with different salts, which guided the optimization of the hydrogel membrane. This strategy provides a general and scalable approach for designing high-performance nanofluidic devices for sustainable permeable energy harvesting and other applications.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A CMC / PVA nanofluidic hydrogel membrane, characterized in that, It was prepared by immersing a citric acid-crosslinked CMC / PVA hydrogel in a structure-promoting ion salt solution.

2. The CMC / PVA nanofluidic hydrogel membrane according to claim 1, characterized in that, In the CMC / PVA hydrogel, the mass ratio of polyvinyl alcohol to carboxymethyl cellulose is 0.

5.

3. The CMC / PVA nanofluidic hydrogel membrane according to claim 1, characterized in that, The degree of substitution of the carboxymethyl cellulose is 1.

34.

4. The CMC / PVA nanofluidic hydrogel membrane according to claim 1, characterized in that, The structure promotes the presence of Citrate anions in the ion salt solution. 3- SO4 2- One or more of them.

5. The CMC / PVA nanofluidic hydrogel membrane according to claim 1, characterized in that, The structure promotes the presence of Na+ cations in the ionic salt solution. + .

6. The CMC / PVA nanofluidic hydrogel membrane according to claim 1, characterized in that, The structure promotes the concentration of anionic salts of 1.5 M in ionic salt solutions.

7. The CMC / PVA nanofluidic hydrogel membrane according to claim 1, characterized in that, The average pore size of the CMC / PVA nanofluid hydrogel membrane is 27.7 nm to 33 nm.

8. A method for preparing a CMC / PVA nanofluidic hydrogel membrane as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Mix carboxymethyl cellulose solution and polyvinyl alcohol solution, add citric acid, sodium hypophosphite and glycerol, stir for 12 hours, concentrate the homogenized precursor solution by rotary evaporation, let stand overnight to remove bubbles, and pour into a mold; incubate at 50°C for 12 hours to form a pregel layer, then perform four freeze-thaw cycles, each cycle including freezing at -80°C for 24 hours and thawing at room temperature for 6 hours; heat treat at 80°C for 2 hours to obtain a double-network CMC-PVA hydrogel membrane; S2. The dual-network CMC-PVA hydrogel membrane is immersed in a structure-promoting ion salt solution for 24 hours, rinsed thoroughly with deionized water to remove residual ions on the surface, and dried at 50°C to obtain the CMC / PVA nanofluid hydrogel membrane.

9. The method according to claim 8, characterized in that, The carboxymethyl cellulose solution has a mass concentration of 2%, the polyvinyl alcohol solution has a mass concentration of 10%, citric acid accounts for 25% of the total mass of carboxymethyl cellulose and polyvinyl alcohol, sodium hypophosphite accounts for 10% of the mass of citric acid, and glycerol accounts for 5% of the total mass of carboxymethyl cellulose and polyvinyl alcohol.

10. The application of the CMC / PVA nanofluid hydrogel membrane as described in any one of claims 1 to 7 in permeation energy conversion.

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