Selective-membrane-free mixing system for solar seawater desalination and all-weather salinity difference energy collection

Abstract

The invention discloses a non-selective membrane mixing system for solar seawater desalination and all-weather salinity difference energy collection. The non-selective membrane mixing system comprises a chamber for accommodating seawater; the salinity gradient energy power generation module is arranged at the bottom of the chamber and comprises a cation electrode and an anion electrode which are connected through an external circuit, and the cation electrode and the anion electrode are immersed in the seawater in the chamber; the salinity gradient energy generation module comprises an evaporator and a condensation cover, the evaporator floats in the seawater in the cavity, and the condensation cover is located at the top of the cavity; wherein no ion selective membrane is arranged between the cation electrode and the anion electrode. According to the system, photo-thermal evaporation fresh water production and salinity gradient energy power generation / energy storage processes are coupled, and membrane-free all-weather salinity gradient energy collection is realized.

Description

Technical Field

[0001] This invention relates to the intersection of new energy technology and water treatment technology, specifically to a non-selective membrane hybrid system for solar seawater desalination and all-weather salinity gradient energy collection. Background Technology

[0002] Global water scarcity and energy crises have spurred research into the utilization of solar energy for seawater desalination and the development of ocean salinity gradient energy. However, existing technologies still face significant bottlenecks in terms of comprehensive energy utilization and system cost. Interfacial evaporation technology based on the photothermal effect, through localized heating, significantly increases the evaporation rate to 1.4 kg·m³. -2 ·h -1 The above achieves efficient solar-thermal-phase change conversion. However, its core output is only freshwater, and the chemical potential energy (salinity gradient energy) inherent in the salinity gradient inevitably generated during the process is not utilized. This not only results in energy waste, but the scaling problem caused by salt accumulation also affects the long-term stability of the system.

[0003] Salinity gradient power generation technology, exemplified by reverse electrodialysis (RED), relies on cation-anion selective membranes to separate concentrated and dilute solutions and drive the directional migration of ions to generate current. High-performance ion exchange membranes are expensive and susceptible to fouling, scaling, and biofouling in real aquatic environments, leading to decreased system efficiency, complex maintenance, and poor overall economics (J. Long, J. Yin, F. Yang, G. Zhou, H. Cheng, W. Guo and L. Qiu, Adv. Energy Mater. (2025, 15, 2303476–2303486). Although new technologies such as capacitive hybridization attempt to reduce the requirements for membranes, they have not completely solved the inherent challenges posed by membrane materials.

[0004] Recent research attempts to couple solar evaporation with salinity gradient power generation, aiming to achieve "cogeneration" (H. He, X.-M. Song, M. Huang, X. Hou, Z. Song and Y. Zhang, Green Chem. (2023, 25, 9343–9350). However, most solutions employ a configuration where the evaporation unit and power generation unit are physically separated and connected via a fluid loop, resulting in a complex system structure and increased energy consumption. Most critically, their power generation units largely still utilize traditional RED or similar principles, making it impossible to eliminate the use of ion-selective membranes. Some integrated design attempts often suffer from low power generation efficiency or unstable operation.

[0005] To address membrane-dependent issues, materials science has proposed charge storage mechanisms based on ion intercalation / deintercalation reactions or Faraday ion trapping. For example, certain layered metal oxides (such as manganese oxides and Prussian blue analogues) can reversibly intercalate / deintercalate Na in aqueous solutions. + K + Metal ions (B. Yang, J. Yu and T. Ma, J. Mater. Chem.a , 2023, 11, 3388–3398); while some specific materials (such as bismuth oxychloride, silver-based materials) can react with Cl - These materials can undergo reversible chemisorption or reaction. They can generate current driven by a salinity gradient solely through selectivity, or store and release ions via an applied current, theoretically eliminating the need for ion-selective membranes. However, current research primarily focuses on the electrochemical behavior of these materials under static salinity gradients (such as concentrated and dilute solutions of fixed concentrations), and there is still no mature approach to efficiently, stably, and structurally simplify their integration with dynamic, continuous salinity gradients generated in situ by solar evaporation.

[0006] Existing technologies have the following key limitations: on the one hand, efficient solar interfacial evaporation technology generates untapped salinity gradient energy; on the other hand, the two have not yet been organically combined in a streamlined, self-sustaining integrated system. Existing coupling schemes suffer from high costs and maintenance difficulties due to reliance on expensive and fragile ion-selective membranes. Therefore, a membrane-free, compact, all-weather synergistic system that enables simultaneous conversion / storage of solar-driven seawater desalination and salinity gradient energy has significant scientific value and application potential. Summary of the Invention

[0007] The purpose of this invention is to provide a non-selective membrane hybrid system for solar seawater desalination and all-weather salinity gradient energy harvesting. This system couples the process of photothermal evaporation to produce fresh water with salinity gradient energy power generation / storage, and achieves membrane-free all-weather salinity gradient energy harvesting.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution: A non-selective membrane mixing system for solar-powered seawater desalination and all-weather salinity gradient energy harvesting includes: A chamber that holds seawater; A salinity gradient energy generation module is located at the bottom of the chamber and includes a cation electrode and an anion electrode connected by an external circuit. The cation electrode and anion electrode are immersed in seawater in the chamber. The salinity gradient energy generation module includes an evaporator and a condenser shroud, wherein the evaporator floats in the seawater in the chamber and the condenser shroud is located at the top of the chamber; In this embodiment, no ion-selective membrane is provided between the cation electrode and the anion electrode.

[0009] In the system provided by this invention: a salinity gradient energy generation module is used to convert salinity gradient energy into electrical energy, including a cation electrode for reversibly capturing / releasing anions, and an anion electrode for reversibly capturing / releasing anions; the salinity gradient energy generation module, also known as a seawater desalination module, includes an evaporator for absorbing solar energy and driving the evaporation of water within the cavity, thereby generating a salinity gradient within the cavity, and a condenser hood for water vapor condensation and freshwater collection. In the system provided by this invention, photothermal evaporation, ion migration, and electrode reactions are coupled, enabling the system to generate current or store salinity gradient energy in an external circuit while producing freshwater.

[0010] Furthermore, the cation electrode is made of a sodium ion intercalation material, mainly used for the capture and release of sodium ions; the anion electrode is made of a chloride ion capturing material, mainly used for the capture and release of chloride ions.

[0011] This invention captures and releases sodium and chloride ions through a pair of anion and cation electrodes, forming directional migration of ions, thereby generating a stable current in the external circuit and realizing the collection of salinity gradient energy without a membrane.

[0012] Furthermore, the cation electrode is made of sodium manganese oxide, and the anion electrode is made of bismuth oxychloride.

[0013] Furthermore, the evaporator is made of biomass carbonized material. When sunlight shines on the black, porous surface of the evaporator, the evaporator heats up rapidly, causing seawater to evaporate. The seawater concentration inside the chamber increases, and freshwater is collected through the condenser, thus converting solar energy into salinity gradient energy and thermal energy.

[0014] Furthermore, the evaporator is a biomass carbonized rattan.

[0015] Furthermore, a freshwater collection tank is provided on the side of the system. After the seawater evaporates in the evaporator, the water vapor is condensed in the condenser hood to form liquid freshwater, which then flows into the freshwater collection tank.

[0016] The present invention also provides an application of the above system in solar seawater desalination and salinity gradient energy conversion.

[0017] Furthermore, the method of application is as follows: seawater is filled into the chamber, the cation electrode and the anion electrode are immersed in the seawater in the chamber, and the evaporator floats on the seawater; During the daytime, the evaporator's temperature rises after receiving sunlight, and seawater begins to evaporate, increasing the seawater concentration in the chamber and creating a positive salinity gradient. Driven by this positive salinity gradient, cations migrate to and embed into the cation embedding electrode, while anions migrate to and are captured by the anion capturing electrode. Simultaneously, current is generated in the external circuit, and water vapor is continuously produced and condensed into fresh water. During nighttime, when light is absent, low-concentration seawater is re-injected into the chamber, creating a reverse salinity gradient. Driven by this reverse salinity gradient, cations migrate from the cation electrode into the seawater and are released, while anions migrate from the anion electrode into the seawater and are released. Simultaneously, an opposite current is generated in the external circuit compared to the first stage.

[0018] Compared with the prior art, the beneficial effects of the present invention include: 1) Membrane-free: Completely avoids the use of ion-selective membranes, significantly reducing system costs and maintenance difficulty, and improving long-term operational stability.

[0019] 2) High efficiency and synergy: Solar-driven evaporation is not only a desalination process, but also the driving force for establishing salinity gradient (concentration gradient), and the energy flow is fully utilized around the clock.

[0020] 3) Multi-functional: It can simultaneously produce fresh water and electricity, and store salinity gradient energy in an electrochemical form to achieve on-demand power generation.

[0021] 4) Simple structure: The system has a high degree of integration and is easy to scale up and deploy in practice.

[0022] This invention innovatively proposes a hybrid system for salinity gradient energy harvesting and solar thermal seawater desalination. Through the synergistic effect of the salinity gradient energy generation module (seawater desalination module) and the salinity gradient energy power generation module, the processes of "solar-driven evaporation - concentration gradient establishment - ion selective insertion / extraction - power generation / energy storage" form a self-sustaining, all-weather closed loop. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0024] Figure 1 A schematic diagram illustrating the working principle of the hybrid system for seawater desalination and all-weather salinity gradient energy collection provided by this invention; Figure 2 A schematic diagram of the hybrid system structure for seawater desalination and all-weather salinity gradient energy collection provided by the present invention; Figure 3 These are the evaporation efficiency test chart and the condensate ion concentration test chart of the evaporator in this invention; Figure 4The image shows the XRD test results of the synthesized electrode material; Figure 5 Na4Mn9O 18 Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of two electrode materials, BiOCl; Figure 6 These are the charge-discharge curves and cycle stability test graphs of the system provided by this invention at different current densities. Figure 7 This is a test graph showing the salinity gradient energy collection performance of the system provided by this invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0026] like Figure 1 As shown, the system's operating principle can be described as a four-step cyclic process based on ion insertion / deintercalation. Under sunlight, solar energy drives photothermal evaporation, concentrating seawater and creating a positive salt concentration gradient between the liquid phase near the electrode and the electrode surface. This drives ions to migrate from the seawater into the electrode and undergo insertion reactions, generating a positive current. Simultaneously, the water vapor produced by evaporation is recovered as freshwater through a condensation unit. During the non-sunlight phase, low-concentration seawater is reintroduced into the system, decreasing the salt concentration around the electrode. This causes previously inserted ions to deintercalate from the electrode and return to the seawater medium, generating a reverse current. Through this diurnal alternation of ion cycling and concentration gradient reconstruction, the system can achieve continuous salinity gradient energy collection and freshwater production around the clock.

[0027] like Figure 2 The diagram illustrates the basic structure of the hybrid system in this invention.

[0028] The device of this invention adopts a modular, stacked structure design, mainly consisting of a lower salinity gradient energy conversion and storage unit, a middle solar-driven evaporation unit, and an upper steam condensation and collection unit. These units work together to achieve an integrated function of seawater desalination and salinity gradient energy recovery. Details are as follows: 1) Salinity gradient energy conversion and storage unit Located at the bottom of the device, this unit includes a chamber for holding seawater and an electrode system housed within it. The lower part of the chamber contains the seawater to be treated; the electrode system consists of a pair of discrete anion and cation selective electrodes, responsible for the reversible capture and release of chloride and sodium ions, respectively, realizing the direct conversion or storage of salinity gradient energy into electrical energy. The electrodes are positioned and fixed by two acrylic plates with custom-designed square holes, ensuring their stability and effective reaction area in the working medium.

[0029] 2) Solar-driven evaporation unit Located above the seawater surface and covering the electrode units, it mainly consists of a porous photothermal evaporation layer made of biomass-derived carbon materials. Under illumination, this layer can efficiently absorb solar energy and locally heat the interfacial moisture, significantly accelerating the seawater evaporation process. At the same time, it forms and maintains a salinity gradient at the evaporation interface, continuously providing driving force for the lower electrode units.

[0030] 3) Steam condensation and freshwater collection unit An acrylic condenser is installed on the top of the device. Water vapor generated by the evaporation unit rises to the surface of the condenser, condenses into liquid fresh water, and collects in a special collection tank on the side of the device by gravity and flow guidance design, thus realizing the continuous production of fresh water.

[0031] Through the close spatial and functional coupling of the above three units, the present invention provides a way to simultaneously achieve efficient solar-powered seawater desalination while also capturing and converting the salinity gradient energy generated during the evaporation process, demonstrating a high degree of system integration and energy synergy efficiency.

[0032] Example 1 The preparation method of the non-selective membrane mixing system for solar seawater desalination and all-weather salinity gradient energy harvesting provided in this embodiment includes: 1) Preparation of anion exchange electrodes: BiOCl, conductive carbon black, and polytetrafluoroethylene (PVDF) were weighed in a mass ratio of 8:1:1 and ground in an agate mortar for 15 minutes until fully mixed. The fully mixed active powder was then placed in a quartz ball mill jar and ball-milled at 400 rpm for 4 hours using a planetary ball mill. The fully ground powder was then mixed with dimethylformamide and sonicated for 40 minutes to prepare an electrode slurry. The electrode slurry was evenly coated on both sides of a carbon cloth (3cm*1cm) using a 100µl pipette and finally dried in a vacuum oven at 60℃ for 12 hours.

[0033] 2) Preparation of the cation electrode: Weigh Na₂CO₃ and Mn₂O₃ at a molar ratio of 0.55:1 and grind them in an agate mortar for 15 min until fully mixed. Then, place the fully mixed active powder into an agate ball mill jar and ball mill it at 400 rpm for 4 h using a planetary ball mill. Place the mixed powder into an alumina dry pot and heat it to 750 °C in a muffle furnace at a heating rate of 5 °C / min, and hold it at 750 °C for 8 h. Finally, allow it to cool naturally to room temperature. Remove the sample and store it for later use. The electrode sheet is prepared by loading it onto carbon cloth using the same method as the anion electrode.

[0034] 3) Electrode plate fabrication: Two acrylic plates serve as the external support structure, and titanium foil acts as the current collector. Anion and cation electrode plates, based on carbon cloth, are fixed in the two square holes on the left and right sides of the acrylic plates, respectively, to collect salinity gradient energy while absorbing solar energy for seawater desalination. Screws secure the four corners, fixing the electrode plates between the two acrylic plates.

[0035] 4) Evaporator Preparation: In this embodiment, the photothermal evaporator is prepared using biomass-derived carbon material. The specific steps are as follows: First, Indonesian rattan stems are selected, cut into suitable lengths, and soaked in 60°C hot water for 1 hour to correct deformation and improve morphological stability. Then, they are dried in an oven for 2 hours. The dried rattan is then carbonized at 1000°C under a nitrogen atmosphere to obtain a carbonized material with a porous structure. This carbon material is cut into 3 cm long units and assembled into a cubic evaporator main structure using epoxy resin.

[0036] The system provided in this embodiment consists of a square chamber made of acrylic material. Seawater is injected into the bottom of the chamber. The cation intercalation electrode (cation electrode) is sodium manganese oxide (Na4Mn9O4) with carbon cloth as the current collector. 18 The electrodes, namely the anion capturing electrode (anion electrode) and the titanium mesh current collector, are BiOCl electrodes placed parallel to each other on both sides of the chamber. A separate piece of porous biomass carbonized material floats on the water surface between the two electrodes as a photothermal evaporator.

[0037] In the first stage: Sunlight illuminates the evaporator, heating and evaporating the water below. The resulting water vapor condenses and is collected as fresh water in a condenser at the top of the chamber. As evaporation continues, the seawater concentration inside the chamber increases, forming a positive salinity gradient with the inside of the electrodes. Driven by this gradient, Na+... + From seawater to sodium manganese oxide (Na4Mn9O) 18 Electrode migration and embedding, Cl - The seawater migrates and embeds itself into the BiOCl electrode. This process generates a stable current in the external circuit connecting the two electrodes.

[0038] In the second stage: without sunlight, seawater is re-injected into the chamber, reducing the seawater concentration and creating an inverse salinity gradient with the inside of the electrodes. + From sodium manganese oxide (Na4Mn9O) 18 The electrode migrates and embeds itself into the seawater, Cl - The material migrates and embeds itself into the seawater from the BiOCl electrode. This process generates a stable current in the external circuit connecting the two electrodes, opposite to that in the first stage.

[0039] Figure 3The evaporation efficiency test results and the ion concentration change test results before and after evaporation of the evaporator provided in Example 1 are shown. Figure 3 In section a), the system simulates a solar irradiance of 1 kW·m². -2 Seawater evaporation performance was tested under [a certain condition]. The results showed that the evaporation rate gradually increased with increasing sunlight exposure time, and then stabilized after about 1 hour, reaching a stable evaporation rate of approximately 1.5 kg·m³. -2 ·h -1 Based on energy efficiency calculations, the system's photothermal conversion efficiency is approximately 90%, demonstrating highly efficient and stable solar-driven evaporation performance. Figure 3 In section b), the seawater desalination performance of this system was tested, and key ion concentrations were analyzed for both the initial seawater (feed water) and the collected condensed freshwater. The tests targeted the four main cations present in seawater (Na+, Na ... + K + Mg 2+ Ca 2+ (Expand). The results show that the concentrations of the above ions in the desalinated water are Na... + 0.62 mg·L -1 K + 0.13 mg·L -1 Mg 2+ 0.62 mg·L -1 Ca 2+ 0.86 mg·L -1 These values ​​are all far below the recommended limits for the corresponding ions in the World Health Organization's (WHO) Guidelines for Drinking-water Quality, proving that the system can efficiently remove salt and major ions from seawater, and has excellent desalination performance and the potential to produce safe drinking water.

[0040] Figure 4 The BiOCl and Na4Mn9O provided in Example 1 are shown. 18 X-ray diffraction (XRD) patterns of the electrode materials. The results show that all diffraction peaks of both materials are consistent with the standard cards, and no obvious impurity peaks were observed, indicating that the prepared materials have high crystallinity and phase purity, and the target electrode materials were successfully obtained.

[0041] Figure 5 The microstructure characterization results of the electrode material provided in Example 1 are presented. Figure 5 a) in 5 and b) in 5 are respectively Na4Mn9O 18 Scanning electron microscope (SEM) and transmission electron microscope (TEM) images, Figure 5c) in 5 and d) in 5 are SEM and TEM images of BiOCl, respectively. SEM observation shows that Na4Mn9O 18 Exhibiting a short rod-like morphology, BiOCl shows a typical plate-like structure. TEM analysis further reveals that both materials have clear layered stacking or arrangement characteristics. This layered structure facilitates the rapid insertion and extraction of ions during charging and discharging, providing a favorable structural basis for salinity gradient energy collection and storage.

[0042] Figure 6 The results of rate performance and cycle stability tests of the system in simulated seawater electrolytes are presented. Figure 6 a) in the figure refers to the electrode at 0.5 mol·L⁻¹ -1 Different current densities (0.05 to 0.5 A·g) in NaCl solution -1 The constant current charge-discharge curves are shown. All curves exhibit an approximately symmetrical isosceles triangle shape, indicating that the electrode reaction is highly reversible. Notably, even at higher current densities, the charge-discharge plateau remains distinct and prolonged, reflecting the material's high specific capacity. This is mainly attributed to its layered structure, which provides abundant active sites and rapid diffusion channels for ion storage. Figure 6 b) in the figure represents the system at 0.5 A·g -1 Long-term cyclic stability tests were conducted at current density. After 1000 consecutive charge-discharge cycles, the system still maintained a coulombic efficiency of 97.89% and a capacity retention of 85%. This result fully demonstrates the excellent structural stability and electrochemical durability of the electrode material in seawater electrolyte, providing a key guarantee for the long-term stable operation of the system.

[0043] Figure 7 The system's performance in salinity gradient energy harvesting and conversion under continuous operation conditions was demonstrated. For example... Figure 7 As shown in a) of section 7, the system demonstrated good operational stability during the week-long test, continuously outputting an open-circuit voltage with a peak value of 166.69 mV. As can be seen from b) of section 7, the average daily energy output density reached 93.13 Wh·m³. -2 In section 7c), comprehensive calculations show that the overall energy conversion efficiency of the system during the simultaneous solar thermal freshwater production and salinity gradient power generation process can reach up to 1.28%, effectively demonstrating its synergistic advantages and practical application potential in "hydropower cogeneration".

[0044] In summary, this invention discloses a non-selective membrane mixing system for solar-powered seawater desalination and all-weather salinity gradient energy harvesting. The system includes cation and anion capture electrodes, a seawater chamber, and a photothermal evaporation assembly. The cation electrode is preferably sodium manganese oxide, and the anion electrode is preferably bismuth oxychloride. Under illumination, the photothermal evaporation assembly concentrates the seawater within the chamber, creating a localized high-salinity zone. Simultaneously, driven by the concentration gradient, sodium and chloride ions migrate to the anode and cathode, respectively, undergoing reversible insertion / extraction reactions, thereby generating a continuous current in the external circuit and simultaneously producing freshwater. In this study, the system maintained excellent capacity retention and high energy density after 1000 cycles, producing a large amount of freshwater. This research opens a new perspective for solving the energy crisis and freshwater scarcity problems.

[0045] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A hybrid system of non-selective membranes for solar seawater desalination and all-weather salinity power harvesting, characterized in that, The system comprises: a chamber containing seawater; a salinity gradient power generation module arranged at the bottom of the chamber, comprising a cation electrode and an anion electrode connected by an external circuit, the cation electrode and the anion electrode being immersed in the seawater in the chamber; a salinity gradient power generation module comprising an evaporator and a condenser, the evaporator floating in the seawater in the chamber, the condenser being arranged at the top of the chamber; wherein no ion-selective membrane is arranged between the cation electrode and the anion electrode.

2. The system of claim 1, wherein, The material of the cation electrode is a sodium ion intercalation material, and the material of the anion electrode is a chlorine ion capturing material.

3. The system of claim 2, wherein, The material of the cation electrode is sodium manganese oxide, and the material of the anion electrode is bismuth oxychloride.

4. The system of claim 1, wherein, The evaporator is a biomass carbonized material.

5. The system of claim 4, wherein, The evaporator is a biomass carbonized rattan.

6. The system of claim 1, wherein, The system is provided with a fresh water collection tank on the side, and the water vapor evaporated by the evaporator is condensed in the condenser to form liquid fresh water and then flows into the fresh water collection tank.

7. The use of the system of any one of claims 1-6 in solar seawater desalination and salinity gradient power conversion.

8. Use according to claim 7, characterized in that, The method of the use is: pouring seawater into the chamber, the cation electrode and the anion electrode being immersed in the seawater in the chamber, and the evaporator floating on the seawater; When in the daytime, the evaporator is heated by light and the temperature rises, the seawater starts to evaporate, the concentration of seawater in the chamber increases, a positive salinity gradient is generated, cations migrate to the cation intercalation electrode and intercalate, anions migrate to the anion capturing electrode and are captured, an electric current is generated in the external circuit, and water vapor is continuously generated and condensed into fresh water; When in the night, the light disappears, low-concentration seawater is poured into the chamber, and a reverse salinity gradient is generated; and under the driving of the reverse salinity gradient, cations migrate from the cation electrode to the seawater and are released, anions migrate from the anion electrode to the seawater and are released, and an electric current opposite to that in the first stage is generated in the external circuit.

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