MIL-100(Fe)-CdS-Sodium Alginate Composite Membrane for Long-Lasting Photocathode Protection in Seawater Environment, Its Preparation Method and Application

Abstract

This invention discloses a MIL-100(Fe)-CdS-sodium alginate composite film for long-term photocathode protection in seawater environments and its preparation method, belonging to the field of photocathode protection technology. The method includes the following steps: cleaning conductive glass; growing a MIL-100(Fe) thin film on the conductive glass using a hydrothermal method; growing a MIL-100(Fe)-CdS composite film on the surface of the MIL-100(Fe) thin film using a dip-coating method; and growing sodium alginate (SA) hydrogel on the surface of the MIL-100(Fe)-CdS using an in-situ drop-coating method. This invention utilizes the heterojunction of MIL-100(Fe) and CdS, and uses a loaded sodium alginate hydrogel as a protective layer, solving the problem of severe photoelectric conversion efficiency degradation in existing photoanodes during long-term seawater immersion. After coupling with a metal, the current generated under illumination causes cathodic polarization of the metal, and the hydrogel layer delays the loss of active materials. The synergistic effect of these two factors results in long-term stable photochemical cathodic protection performance in seawater environments.

Description

Technical Field

[0001] This invention relates to the field of photocathode technology, specifically to MIL-100(Fe)-CdS-sodium alginate for long-term photocathode protection in seawater environments, its preparation method, and its application. Background Technology

[0002] Photoelectrochemical cathodic protection is an innovative method that directly converts solar energy into electrical energy. Its basic principle involves attaching or coating a semiconductor material onto the surface of the metal to be protected. Under sunlight, electrons in the valence band of this material are excited by the light and transition to the conduction band, migrating to the metal surface and generating a polarization current, thereby inhibiting metal corrosion. Compared to traditional sacrificial anode and impressed current cathodic protection methods, this technology has significant advantages such as being environmentally friendly, requiring no additional energy consumption, and being low-cost, making it a highly promising marine engineering protection technology.

[0003] In photoelectrochemical cathodic protection systems, the photoelectric conversion efficiency and band structure of the photoanode determine its protective performance. In recent years, researchers have developed various semiconductor materials (such as WO3, TiO2, and ZnO) for this field. To overcome the drawbacks of single-component semiconductors, such as high photogenerated carrier recombination rates and narrow light absorption ranges, researchers have developed a series of modification strategies, such as morphology control, modified doping, and heterojunction design. Among these, constructing heterojunctions is considered an effective strategy to improve the photoelectric conversion efficiency of semiconductor materials and is widely used in the field of photoelectrochemical cathodic protection.

[0004] However, in practical marine engineering applications, existing photoanode materials generally face a severe challenge: when immersed in simulated seawater with complex compositions for extended periods, the photoanode exhibits poor structural stability due to the photocorrosion effect of the photocatalyst and the physical / chemical erosion by electrolyte ions. This leads to a rapid decline in photoelectric conversion efficiency, making it difficult to provide long-lasting and stable cathodic protection. Although there are reports on the construction of type II, Z-type, and S-type heterojunctions, these studies mostly focus on the construction and initial performance of the heterojunctions themselves, lacking effective solutions for improving the long-term stability of photoanodes in real marine corrosive environments. Therefore, how to construct a photoanode composite film that possesses both high-efficiency heterojunctions and maintains structural integrity and chemical stability in seawater environments has become a key bottleneck for the practical engineering application of photoelectrochemical cathodic protection technology.

[0005] Recent research has found that introducing hydrogels as functional modification layers can effectively address the aforementioned bottlenecks. First, hydrogels possess a unique three-dimensional porous network structure, enabling effective regulation of carrier transport through interface engineering, reducing charge transfer impedance at the heterojunction interface, and promoting efficient electron migration to the metal to be protected. Second, the high water content and porous nature of hydrogels provide rapid diffusion channels for electrolyte ions, ensuring the kinetics of the photochemical reaction. Finally, the hydrogel layer, as a flexible physical protective barrier, has a high elastic modulus and can absorb mechanical stress, preventing damage to the photoanode caused by seawater impact. Furthermore, the hydrogel can isolate seawater from direct contact with the photoanode, acting as a physical barrier.

[0006] Therefore, this invention prepares a MIL-100(Fe)-CdS-sodium alginate composite photoanode through a multi-step process. By conducting long-term simulated seawater immersion experiments on photoanodes loaded with sodium alginate hydrogel and those without sodium alginate hydrogel, the influence mechanism of hydrogel on the long-term stability of the composite photoanode was systematically investigated. Summary of the Invention

[0007] This invention provides a MIL-100(Fe)-CdS-sodium alginate composite film for long-term photocathode protection in seawater environments and its preparation method, in order to solve the problem of severe attenuation of photoelectric conversion efficiency induced by photocorrosion and physical / chemical erosion in complex marine environments in the prior art.

[0008] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows: A method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-lasting photocathode protection in seawater environments includes the following steps: (1) Clean the FTO conductive glass; (2) A MIL-100 (Fe) thin film is grown on the conductive glass obtained in step (1) by hydrothermal method; (3) A MIL-100(Fe)-CdS composite film was grown on the surface of the MIL-100(Fe) by dip-coating method; (4) The MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environment was obtained by in-situ growth drop coating method.

[0009] Specifically, step (1) involves placing the FTO conductive glass in an acetone solution and sonicating it for 30 minutes, then drying it for later use.

[0010] Specifically, step (2) involves dissolving ferric nitrate nonahydrate and 1,3,5-benzenetricarboxylic acid in deionized water, stirring at 800 rpm for 1-3 hours, transferring the mixture to the liner of a reaction vessel, placing the cleaned FTO conductive glass with the conductive side facing down in the above mixed solution, and reacting at 160-200℃ for 10-14 hours. After the reaction is complete, the MIL-100(Fe) film is removed under cooling conditions and washed three times with methanol and deionized water to obtain the MIL-100(Fe) film. Preferably, the stirring time is 2 hours, the reaction temperature is 180℃, and the reaction time is 12 hours.

[0011] The ferric nitrate nonahydrate solution and the 1,3,5-benzenetricarboxylic acid solution are dissolved in water to obtain a ferric nitrate nonahydrate solution with a molar concentration of 75-85 mmol / L and a 1,3,5-benzenetricarboxylic acid solution with a molar concentration of 60-70 mmol / L; preferably, the ferric nitrate nonahydrate solution has a molar concentration of 80.1 mmol / L and the 1,3,5-benzenetricarboxylic acid solution has a molar concentration of 66.6 mmol / L.

[0012] Specifically, step (3) involves placing the MIL-100(Fe) film in a solution of 0.08-0.12 mol / L cadmium nitrate and 0.08-0.12 mol / L sodium sulfide using a dip-coating method, repeating the operation 4-5 times, and then rapidly evaporating the film to obtain the MIL-100(Fe)-CdS film. Preferably, the molar concentration of cadmium nitrate and sodium sulfide is 0.1 mol / L.

[0013] Specifically, step (4) involves dissolving 1g of sodium alginate in 49 mL of deionized water at 60℃ and stirring at 2600 rpm for 3 hours in a water bath. The resulting sodium alginate solution is then applied to the surface of the MIL-100(Fe)-CdS composite membrane using a syringe with a drop of 0.2 ml. The resulting membrane is then immersed in a 0.01 g / ml calcium chloride solution for 30 minutes to complete ion crosslinking and obtain the MIL-100(Fe)-CdS-sodium alginate composite membrane.

[0014] A MIL-100(Fe)-CdS-sodium alginate composite membrane for long-lasting photocathode protection in seawater environments is prepared by the above-mentioned method.

[0015] Application of the MIL-100(Fe)-CdS-sodium alginate composite film prepared by the above method in photocathode protection.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention addresses the severe degradation of photoelectric conversion efficiency of photoanode thin films in complex marine environments due to photocorrosion and physical / chemical erosion. By loading a sodium alginate hydrogel layer, direct contact between the simulated seawater and the photoanode material is isolated, mitigating photocorrosion and physical / chemical erosion in complex marine environments. First, under long-term simulated seawater immersion, the sodium alginate hydrogel layer provides long-term protection for the prepared photoanode composite thin film. Second, the coupling potential between the prepared composite thin film and stainless steel undergoes cathodic polarization upon illumination, providing sufficient protection for the metal and exhibiting long-term stable photoelectrochemical cathodic protection performance in marine environments. Third, this invention reveals that sodium alginate hydrogel can serve as a protective layer for the long-term stable operation of photoanode thin films in complex marine environments. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein: Figure 1 XRD pattern of MIL-100(Fe)-CdS photoanode; Figure 2 The preparation process of MIL-100(Fe)-CdS-sodium alginate photoanode; Figure 3 EIS images of MIL-100(Fe), MIL-100(Fe)-CdS, and MIL-100(Fe)-CdS-sodium alginate photoanodes; Figure 4 The graph shows the photoinduced open-circuit short-term potential changes of MIL-100(Fe)-CdS photoanode and MIL-100(Fe)-CdS-sodium alginate photoanode after long-term immersion in simulated seawater under intermittent light. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0019] This embodiment provides a method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments, comprising the following steps: (1) Place the FTO conductive glass in an acetone solution and sonicate for 30 minutes, then dry it for later use; (2) Dissolve 1.94 g of ferric nitrate nonahydrate and 0.84 g of 1,3,5-benzenetricarboxylic acid in 60 ml of deionized water, stir at 800 rpm for 2 h, transfer to the liner of the reactor, and then place the cleaned FTO conductive glass with the conductive side down in the above mixed solution, and react at 180 °C for 12 h. After the reaction is completed, take out the MIL-100(Fe) film under cooling conditions, and wash it three times with methanol and deionized water respectively to obtain the MIL-100(Fe) film; (3) The MIL-100(Fe) film was placed in 0.1 mol / L cadmium nitrate solution and 0.1 mol / L sodium sulfide solution respectively by dip-coating method. The operation was repeated 4 times. After rapid evaporation, the MIL-100(Fe)-CdS film was obtained. (4) Dissolve sodium alginate in deionized water at 60℃, with a sodium alginate solution concentration of 2wt%, and stir at 2600 rpm for 3 hours in a water bath. Apply 0.2 ml of the obtained sodium alginate solution to the surface of the MIL-100(Fe)-CdS composite membrane using a syringe, and then immerse the resulting membrane in 0.01 g / ml calcium chloride solution for 30 min to complete ionic cross-linking and obtain the MIL-100(Fe)-CdS-sodium alginate composite membrane. Example 2

[0020] This embodiment provides a method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments, comprising the following steps: (1) Place the FTO conductive glass in an acetone solution and sonicate for 30 minutes, then dry it for later use; (2) Dissolve 1.82g of ferric nitrate nonahydrate and 0.76g of 1,3,5-benzenetricarboxylic acid in 50ml of deionized water, stir at 800 rpm for 1h, transfer to the liner of a reaction vessel, and then place the cleaned FTO conductive glass with the conductive side down in the above mixed solution, and react at 200℃ for 14h. After the reaction is completed, remove the MIL-100(Fe) film under cooling conditions, and wash it three times with methanol and deionized water respectively. The MIL-100(Fe) film is obtained. (3) The MIL-100(Fe) film was placed in 0.08 mol / L cadmium nitrate solution and 0.12 mol / L sodium sulfide solution respectively by dip-coating method. The operation was repeated 5 times and the film was quickly evaporated to dryness to obtain MIL-100(Fe)-CdS film. (4) Dissolve sodium alginate in deionized water at 60℃, with a sodium alginate solution concentration of 2wt%, and stir at 2600 rpm for 3 hours in a water bath. Apply 0.2 ml of the obtained sodium alginate solution to the surface of the MIL-100(Fe)-CdS composite membrane using a syringe, and then immerse the resulting membrane in 0.01 g / ml calcium chloride solution for 30 min to complete ionic cross-linking and obtain the MIL-100(Fe)-CdS-sodium alginate composite membrane. Example 3

[0021] This embodiment provides a method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments, comprising the following steps: (1) Place the FTO conductive glass in an acetone solution and sonicate for 30 minutes, then dry it for later use; (2) Dissolve 2.06 g of ferric nitrate nonahydrate and 0.88 g of 1,3,5-benzenetricarboxylic acid in 60 ml of deionized water, stir at 800 rpm for 3 h, transfer to the liner of the reactor, and then place the cleaned FTO conductive glass with the conductive side down in the above mixed solution, and react at 160 °C for 14 h. After the reaction is completed, take out the MIL-100(Fe) film under cooling conditions, and wash it three times with methanol and deionized water respectively to obtain the MIL-100(Fe) film; (3) The MIL-100(Fe) film was placed in 0.12 mol / L cadmium nitrate solution and 0.08 mol / L sodium sulfide solution respectively by dip-coating method. The operation was repeated 4 times and the film was quickly evaporated to dryness to obtain MIL-100(Fe)-CdS film. (4) Dissolve sodium alginate in deionized water at 60℃, with a sodium alginate solution concentration of 2wt%. Gradually increase the stirring speed in a water bath, finally stirring rapidly at 2600 rpm for 3 hours. Apply 0.2 ml of the obtained sodium alginate solution to the surface of the MIL-100(Fe)-CdS composite membrane using a syringe. Then immerse the resulting membrane in a 0.01 g / ml calcium chloride solution for 30 min to complete ionic cross-linking and obtain the MIL-100(Fe)-CdS-sodium alginate composite membrane.

[0022] Comparative Example 1 This embodiment provides a method for preparing a MIL-100(Fe) thin film, including the following steps: (1) Place the FTO conductive glass in an acetone solution and sonicate for 30 minutes, then dry it for later use; (2) Dissolve 1.94 g of ferric nitrate nonahydrate and 0.84 g of 1,3,5-benzenetricarboxylic acid in 60 ml of deionized water, stir at 800 rpm for 2 h, transfer to the liner of a reaction vessel, and then place the cleaned FTO conductive glass with the conductive side down in the above mixed solution, and react at 180 °C for 12 h. After the reaction is completed, remove the MIL-100(Fe) film under cooling conditions, and wash it three times with methanol and deionized water respectively to obtain the MIL-100(Fe) film.

[0023] Comparative Example 2 This embodiment provides a method for preparing a MIL-100(Fe)-CdS thin film, including the following steps: (1) Place the FTO conductive glass in an acetone solution and sonicate for 30 minutes, then dry it for later use; (2) Dissolve 1.94 g of ferric nitrate nonahydrate and 0.84 g of 1,3,5-benzenetricarboxylic acid in 60 ml of deionized water, stir at 800 rpm for 2 h, transfer to the liner of the reactor, and then place the cleaned FTO conductive glass with the conductive side down in the above mixed solution, and react at 180 °C for 12 h. After the reaction is completed, take out the MIL-100(Fe) film under cooling conditions, and wash it three times with methanol and deionized water respectively to obtain the MIL-100(Fe) film; (3) The MIL-100(Fe) film was placed in 0.1 mol / L cadmium nitrate solution and 0.1 mol / L sodium sulfide solution respectively by dip-coating method. The operation was repeated 4 times and the film was quickly evaporated to dryness to obtain MIL-100(Fe)-CdS film.

[0024] Experimental Example 1: XRD Testing of MIL-100(Fe)-CdS Composite Thin Film

[0025] Figure 1 The XRD patterns of MIL-100(Fe) and MIL-100(Fe)-CdS are shown in the figure.

[0026] Compared with the characteristic peaks of FTO, a new peak was observed at 9° in the MIL-100(Fe) material, which is a characteristic peak of MIL-100(Fe). A new peak at approximately 31.7° and 45.5° was observed in the MIL-100(Fe)-CdS material, which is attributed to the characteristic peaks of CdS. These phenomena indicate the successful preparation of MIL-100(Fe) and MIL-100(Fe)-CdS.

[0027] Experimental Example 2: Schematic diagram of the synthesis of MIL-100(Fe)-CdS-sodium alginate composite film

[0028] like Figure 2As shown, MIL-100(Fe) was grown on FTO via a hydrothermal method, exhibiting an ellipsoidal morphology. CdS was then grown on the MIL-100(Fe) surface using a dip-coating method. The CdS particles, with smaller sizes, were densely packed on the MIL-100(Fe) surface, increasing the contact area and facilitating charge migration. Sodium alginate hydrogel was grown on the MIL-100(Fe)-CdS surface via an in-situ drop-coating method. The sodium alginate hydrogel exhibits a three-dimensional network porous structure, which not only effectively protects the MIL-100(Fe)-CdS but also acts as an ion channel for liquid migration, promoting efficient carrier separation in the semiconductor.

[0029] Experimental Example 3: Electrochemical impedance data of MIL-100(Fe), MIL-100(Fe)-CdS, and MIL-100(Fe)-CdS-sodium alginate composite films were tested using an electrochemical workstation.

[0030] Electrochemical impedance spectroscopy (EIS) measurements of MIL-100(Fe), MIL-100(Fe)-CdS, and MIL-100(Fe)-CdS-sodium alginate films were performed using an electrochemical workstation (Gamry Reference 3000). The three-electrode method was employed, with a platinum sheet as the counter electrode, Ag / AgCl as the reference electrode, and the MIL-100(Fe), MIL-100(Fe)-CdS, and MIL-100(Fe)-CdS-sodium alginate composite films as the working electrodes. The measurements were conducted in a 3.5 wt% NaCl solution at a frequency range of 10 Hz. 4 Hz-10 -2 Hz, with a potential range of -0.5V to 0.5V.

[0031] like Figure 3 As shown, MIL-100(Fe) exhibits a large impedance arc radius (charge transfer resistance (R0)). ct The value is 3.01 × 10 5 The RΩ indicates that its charge transfer capability is relatively weak. In contrast, the RΩ of MIL-100(Fe)-CdS is much stronger. ct Reduced to 2.37 × 10 5 The above phenomena indicate that MIL-100(Fe) and CdS form a heterostructure, thereby significantly reducing the charge transfer resistance of the system. After introducing sodium alginate hydrogel, the R00 of the MIL-100(Fe)-CdS-sodium alginate system... ct No significant change occurred; it remained somewhere in between (R) ct It is 2.69 × 10 5 Ω, almost identical to the binary system). The above phenomenon indicates that the optimized hydrogel has almost no effect on the charge transfer resistance of the heterojunction.

[0032] Experiment Example 4: The protective effect of MIL-100(Fe)-CdS and MIL-100(Fe)-CdS-sodium alginate composite films on carbon steel under intermittent light after long-term immersion in simulated seawater was tested using an electrochemical workstation.

[0033] Electrochemical tests were conducted on MIL-100(Fe)-CdS and MIL-100(Fe)-CdS-sodium alginate composite films using an electrochemical workstation (model KOSTER CS350M). A three-electrode method was employed, with a platinum sheet as the counter electrode, Ag / AgCl as the reference electrode, and 304SS coupled to the MIL-100(Fe), MIL-100(Fe)-CdS, and MIL-100(Fe)-CdS-sodium alginate composite films as the working electrode. The photoinduced open-circuit potential (OCP) change was measured by intermittent light irradiation to indicate the photoelectrochemical cathodic protection performance of the materials. Both the corrosion cell and the photoanode cell used a 3.5 wt% NaCl solution with an intermittent light frequency of 50 s on and 50 s off. The light source system used was a PLS-SXE300E, simulating sunlight irradiation.

[0034] To evaluate the operational reliability of the composite films in a simulated marine environment, MIL-100(Fe)-CdS and MIL-100(Fe)-CdS-sodium alginate photoanodes were subjected to 24 h of testing in a 3.5% NaCl solution. Figure 4 b) Immersion experiment and intermittent light OCP test. In the initial stage of immersion ( Figure 4 a) Both MIL-100(Fe)-CdS and MIL-100(Fe)-CdS-sodium alginate exhibited keen photoresponse characteristics, with photoinduced negative shift potentials reaching approximately -0.49 V (vs. Ag / AgCl), demonstrating excellent initial photoelectric conversion efficiency. However, after immersion in 3.5% NaCl solution for 24 h, their operational stability showed a significant divergence. For the composite film without sodium alginate hydrogel, its photoelectric performance deteriorated drastically over time. Its OCP negative shift amplitude contracted significantly, and its protection potential rebounded sharply, indicating that the active component may have undergone photocorrosion or physical detachment from the substrate surface under long-term immersion in a strong electrolyte. In contrast, the photoanode incorporating sodium alginate hydrogel exhibited excellent long-term stability. Even after continuous immersion for 24 h, the system maintained a stable photoresponse, and its protection potential remained at a more negative level. This is attributed to the three-dimensional network structure of sodium alginate hydrogel protecting the composite film and inhibiting its photocorrosion and physical / chemical erosion.

[0035] As can be seen, this invention has obtained a MIL-100(Fe)-CdS-sodium alginate composite film that can be used for long-term photocathode protection through hydrothermal method, dip-coating method and in-situ growth drop coating method. Sodium alginate hydrogel, as a protective layer, effectively encapsulates the photoanode composite film material with its unique three-dimensional network structure. It not only blocks the penetration of ions in seawater, but also acts as a tough physical barrier to prevent material damage to the composite film caused by physical / chemical erosion.

[0036] In summary, the experimental results of Examples 1-4 all demonstrate that the MIL-100(Fe)-CdS-sodium alginate composite membrane of the present invention for long-term photocathode protection in seawater environments can provide photocathode protection current for stainless steel with low self-corrosion potential under long-term marine immersion.

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-lasting photocathode protection in seawater environments, characterized in that... Includes the following steps: (1) Clean the FTO conductive glass; (2) A MIL-100 (Fe) thin film is grown on the conductive glass obtained in step (1) by hydrothermal method; (3) A MIL-100(Fe)-CdS composite film was grown on the surface of the MIL-100(Fe) by dip-coating method; (4) The MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environment was obtained by in-situ growth drop coating method.

2. The method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments according to claim 1, characterized in that: Step (1) specifically involves placing the FTO conductive glass in an acetone solution and sonicating it for 30 minutes, then drying it for later use.

3. The method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments according to claim 1, characterized in that: Step (2) is as follows: dissolve ferric nitrate nonahydrate and 1,3,5-benzenetricarboxylic acid in deionized water, stir at 800 rpm for 1-3 hours and transfer to the liner of the reactor. Then place the cleaned FTO conductive glass with the conductive side down in the above mixed solution and react at 160-200℃ for 10-14 hours. After the reaction is completed, take out the MIL-100(Fe) film under cooling conditions and wash it three times with methanol and deionized water respectively to obtain the MIL-100(Fe) film.

4. The method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments according to claim 3, characterized in that: Ferric nitrate nonahydrate and 1,3,5-benzenetricarboxylic acid were dissolved in water to obtain a ferric nitrate nonahydrate solution with a molar concentration of 75-85 mmol / L and a 1,3,5-benzenetricarboxylic acid solution with a molar concentration of 60-70 mmol / L.

5. The method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments according to claim 1, characterized in that: Step (3) specifically involves placing the MIL-100(Fe) film in 0.08-0.12 mol / L cadmium nitrate solution and 0.08-0.12 mol / L sodium sulfide solution respectively using the dip-coating method, repeating the operation 4-5 times, and then rapidly evaporating to obtain the MIL-100(Fe)-CdS film.

6. The method for preparing a MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments according to claim 1, characterized in that: Step (4) is as follows: Dissolve 1g of sodium alginate in 49 mL of deionized water at 60℃ and stir at 2600 rpm for 3 hours in a water bath; use a syringe to drop the obtained sodium alginate solution onto the surface of the MIL-100(Fe)-CdS composite membrane, and then immerse the obtained membrane in 0.01g / ml calcium chloride solution for 30 minutes to complete ionic cross-linking and obtain the MIL-100(Fe)-CdS-sodium alginate composite membrane.

7. A MIL-100(Fe)-CdS-sodium alginate composite membrane for long-term photocathode protection in seawater environments is prepared by the method according to any one of claims 1-7.

8. The application of the MIL-100(Fe)-CdS-sodium alginate composite film prepared by any one of the preparation methods of claims 1-6 in photocathode protection.

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