A method for preventing cathode calcium and magnesium hydroxide deposition during seawater electrolysis

Abstract

The present invention belongs to the technical field of direct seawater electrolysis for hydrogen production, and specifically relates to a method for cathode calcium and magnesium hydroxide deposition during seawater electrolysis. Adding an organic metal chelating agent to seawater for reaction before using it for electrolysis can significantly improve the problem of cathode calcium and magnesium ion deposition. Furthermore, by controlling the type and concentration of the added organic chelating agent, the HER activity of the cathode can be significantly improved while suppressing deposition. For the recovery of calcium and magnesium minerals and metal chelating agents in seawater, the pH of the seawater is first increased by alkali to achieve the conversion of metal complexes into precipitates, and the magnesium and calcium minerals are recovered by filtration. The pH of the filtered solution is then lowered by acid to gradually precipitate the metal chelating agent, thereby achieving the recycling of the metal chelating agent.

Description

Technical Field

[0001] The invention belongs to the technical field of direct seawater electrolysis hydrogen production, and particularly relates to a method for preventing cathode calcium and magnesium hydroxide deposition during seawater electrolysis. Background Art

[0002] In the sustainable development energy strategy, hydrogen is considered to play a vital role. It has many advantages such as being renewable, efficient, and clean. Since hydrogen comes from abundant surface water resources, it does not have the same upper limit on reserves as fossil energy in terms of raw materials. However, existing water electrolysis technologies such as PEM water electrolysis, solid oxide water electrolysis, and alkaline water electrolysis all use purified fresh water as raw materials. Fresh water is a scarce resource on Earth. If seawater is used as the raw material for water electrolysis, this problem can be effectively solved. However, due to the complex composition of seawater, some new problems will arise during electrolysis. Among them, the cathode will produce a large amount of OH- in situ during the hydrogen evolution process, resulting in a high local pH. Calcium and magnesium ions will deposit and block the active sites, causing cathode inactivation.

[0003] There are currently two main solutions to the above problems. One is to prevent cations from migrating to the cathode surface by coating the cathode from the perspective of cations. For example, Ma et al. used porous carbon as a coating film to prepare a carbon-coated CoMoP catalyst for hydrogen evolution from seawater. The deposition phenomenon was significantly improved, and the durability was also increased to a certain extent. However, the coating would cover some active sites, resulting in a decrease in activity. Another solution is to control the pH of the solution through a buffer solution. Michael et al. added sodium hydrogen phosphate buffer solution to stabilize the pH of the electrolyte. However, studies have shown that a 0.01M buffer solution can only maintain -1 mA·cm -2 The pH value remains constant under high voltage, that is, the buffering capacity will be exceeded under high voltage. Summary of the Invention

[0004] To address the defects of the prior art, the present invention provides a new method for preventing cathode calcium and magnesium ion deposition during seawater electrolysis. An organic metal chelating agent is added to seawater for reaction before being used for electrolysis, which can significantly improve the problem of cathode calcium and magnesium ion deposition. The method has many advantages, such as simple operation, significant anti-deposition effect, and recyclability. In addition, by controlling the type and concentration of the added organic chelating agent, the HER activity of the cathode can be significantly improved while suppressing deposition. For the recovery of calcium and magnesium minerals and metal chelating agents in seawater, the pH of the seawater is first increased by an alkaline solution to achieve the conversion of metal complexes into precipitates, and the magnesium and calcium minerals are recovered by filtration. The pH of the filtered solution is then lowered by an acid solution to gradually precipitate the metal chelating agent, thereby achieving the recycling of the metal chelating agent.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] The present invention provides a method for preventing cathode calcium and magnesium ion deposition during seawater electrolysis. The method includes selecting a metal ion chelating agent, using a certain amount of the chelating agent, and recovering calcium and magnesium minerals and the metal ion chelating agent. The metal ion chelating agent is added to seawater to cause a complex reaction with calcium and magnesium ions before being used for water electrolysis to prevent deposition. This method can significantly improve the problem of cathode calcium and magnesium ion deposition.

[0007] The metal chelating agent is one or more combinations of organic metal chelating agents, and the organic metal chelating agent can be a small molecule organic chelating agent or a high molecular weight polymer chelating agent.

[0008] When the metal ion chelating agent is a small molecule organic chelating agent, the chelating agent and calcium and magnesium minerals in the electrolyzed seawater are recovered simultaneously.

[0009] Based on the above technical solution, the method includes the following steps:

[0010] (1) Add a certain amount of metal ion chelating agent into seawater and dissolve it by ultrasonication;

[0011] (2) magnetically stirring the solution for a certain period of time to allow the chelating agent to fully react with the metal calcium and magnesium ions, and then performing electrolysis;

[0012] (3) adding alkali to the electrolyzed seawater to adjust the pH until the metal precipitates from the chelating agent, thereby converting the metal complex into a precipitate, and filtering to recover the metal precipitate;

[0013] (4) Adding acid to the filtered solution to adjust the pH until the chelating agent precipitates, and filtering to recover the chelating agent.

[0014] Based on the above technical solution, preferably, in step (1), the molar ratio of the metal ion chelating agent to seawater is 1:1-5:1; and the ultrasonic power is 50-300W.

[0015] Based on the above technical solution, preferably, in step (2), the magnetic stirring power is 50-100 W, the speed range is 300-800 rpm, and the stirring time is 0.5-3 h.

[0016] Based on the above technical solution, preferably, the type of alkali in step (3) is solid sodium hydroxide or potassium hydroxide, and the pH is adjusted to 12-14.

[0017] Based on the above technical solution, preferably, the type of acid added in step (4) is hydrochloric acid, the concentration is 3-9 M, and the pH is adjusted to 1-3.

[0018] Based on the above technical solution, preferably, the chelating agent has a moderate complexation constant with calcium and magnesium ions. In an alkaline environment with a pH of 8 to 12, the metal ions preferentially react with the chelating agent for complexation, while at higher pH, the metal ions preferentially react with hydroxide ions for precipitation. The complexation constant of the chelating agent with calcium and magnesium ions is between 7.9 and 11.9.

[0019] Based on the above technical solution, preferably, the solubility of the metal chelating agent varies with pH, ​​and the solubility of the metal chelating agent in a solution with a pH of 8-12 can reach at least 0.08M; the solubility of the metal chelating agent in a solution with a pH of 1-3 is not higher than 0.001M.

[0020] Based on the above technical solution, preferably, the chelating agent remains stable during the hydrogen evolution reaction, that is, no redox reaction occurs within the range of -1.0-0 V (vs. RHE).

[0021] Based on the above technical solution, preferably, the chelating agent is stable in the pH range of 8 to 12, and remains in a stable state after chelating with the metal.

[0022] Based on the above technical solution, preferably, by controlling the type and concentration of the metal chelating agent, the hydrogen evolution reaction (HER) can be promoted to a certain extent while preventing calcium and magnesium deposition, which can significantly improve the HER activity of the cathode.

[0023] Based on the above technical methods, preferably, the small molecule organic chelating agent is sodium ethylenediaminetetramethylenephosphonate (EDTMPS), ethylenediaminetetraacetic acid salt (disodium and tetrasodium EDTA), sodium gluconate, etc.; the high molecular polymer chelating agent is polyacrylamide, polyhydroxy acrylic acid, alginic acid, etc. Since the organic metal salt has good solubility in the aqueous phase, and the carboxylate radical will combine with hydrogen ions to form organic acid and precipitate from the aqueous phase at low pH, it is beneficial to recover; secondly, the organic chelating agent itself is stable in nature and has strong complexing ability. If a small molecule organic metal chelating agent is selected, it can simultaneously promote the hydrogen evolution reaction when added to seawater at a concentration of 0.08-0.4M.

[0024] Based on the above technical method, preferably, the method is specifically as follows:

[0025] A 0.08-0.4M small molecule organic metal chelator is ultrasonically dissolved in seawater, magnetically stirred for 0.5-3 hours, and electrolyzed until a white precipitate begins to appear. KOH or NaOH solid is then added to the electrolyzed seawater to adjust the solution pH to 12-14. The solution is magnetically stirred for 0.5-3 hours, filtered, and the resulting solid is washed with deionized water for 0.5-2 hours and vacuum dried to obtain a metal hydroxide precipitate. 3-9M HCl is then added to the solution to adjust the pH to 1-3. The solution is magnetically stirred for 0.5-3 hours, filtered, and the chelate precipitates. The resulting solid is washed with deionized water for 0.5-2 hours and vacuum dried to achieve metal hydroxide precipitate.

[0026] Based on the above technical solution, preferably, the magnetic stirring speed is 300-800 rpm; and the vacuum drying conditions are 50-80° C. and 12-36 h.

[0027] A seawater electrolysis process includes the above-mentioned method of preventing cathode calcium and magnesium ion deposition by adding a metal chelating agent.

[0028] Beneficial effects:

[0029] (1) The technology of the present invention successfully solves the problem of cathode calcium and magnesium ion deposition in seawater electrolysis. By simply adding a metal chelating agent, the cathode durability of the seawater electrolysis cell is greatly improved. In addition, by regulating the pH change, the enrichment of calcium and magnesium minerals and the recycling of the metal chelating agent are achieved, which greatly reduces the cost problem caused by the chelating agent and has significant economic value.

[0030] (2) The organic small molecule metal chelating agent added in this application will have a certain promoting effect on the hydrogen evolution reaction when solving the problem of calcium and magnesium ion deposition. After the chelating agent is complexed with calcium and magnesium ions, during the electrolysis process, the cathode double layer changes from the original hydrated metal ions to a complex with a larger molecular volume, resulting in a thickening of the effective thickness of the double layer, a weakening of the interfacial electric field, and a lowering of the barrier for the recombination of interfacial water molecules, which is conducive to rearrangement and the establishment of ion transfer channels, greatly improving the cathode mass transfer, and having a certain promoting effect on the hydrogen evolution reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The chronopotentiographs obtained by electrolysis of Example 1 and Comparative Example 1 in seawater are shown in the insets. The electrode morphologies after electrolysis in seawater for the corresponding time are shown. a. Example 1, b. Comparative Example 1 DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0033] Example 1

[0034] Use ethylenediaminetetraacetic acid tetrasodium salt (EDTA tetrasodium salt) as a chelating agent to prevent cathode calcium and magnesium ion deposition, and implement it according to the following steps:

[0035] Step 1: Weigh 3.5 g of sea salt and 3.8 g of tetrasodium EDTA and ultrasonically dissolve them in 100 mL of deionized water. After magnetic stirring at 400 rpm for 1 hour, use this as the electrolyte. Use Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode to perform linear voltammetry and chronopotentiometry.

[0036] Step 2: After 120 hours of testing, the electrolyte was poured out and 5.6 g of KOH solid was added to adjust the pH to approximately 14. The mixture was stirred magnetically at 400 rpm for 2 hours. The resulting white precipitate was filtered, the solid was collected, washed with deionized water for 0.5 hours, and then vacuum dried at 60°C for 12 hours.

[0037] Step 3: Add 3 M HCl to the solution obtained after filtration in step 2 to adjust the solution to a pH of approximately 3. The mixture was stirred magnetically at 400 rpm for 2 h. The resulting white precipitate was filtered, the solid was collected, washed with deionized water for 0.5 h, and then vacuum dried at 60°C for 12 h.

[0038] Figure 1 It can be seen that at a current density of -30 mA·cm -2 When the electrolysis was continued for 120 h, the overpotential did not increase significantly. At the same time, it can be seen from the inset that there was no obvious deposition of calcium and magnesium ions on the cathode.

[0039] Example 2

[0040] The metal ion chelating agent recovered in Example 1 is reused to prevent the deposition of calcium and magnesium ions at the cathode, and the following steps are implemented:

[0041] Step 1: Weigh 1.46 g of the solid obtained in Step 3 of Example 1 and add it to 50 mL of 0.4 M NaOH solution. 1.75 g of sea salt was added to this solution and ultrasonically dissolved. The solution was magnetically stirred at 400 rpm for 2 h to prepare the electrolyte. Chronopotentiometry was performed using Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode.

[0042] Step 2: After 50 hours of testing, record and observe the changes on the cathode surface.

[0043] Example 3

[0044] Use ethylenediaminetetramethylenephosphonate (EDTMPS) as a chelating agent to prevent cathode calcium and magnesium ion deposition, and follow the steps below:

[0045] Step 1: Weigh 3.5 g of sea salt and 12.24 g of sodium ethylenediaminetetramethylenephosphonate and ultrasonically dissolve them in 100 mL of deionized water. After magnetic stirring at 500 rpm for 2 h, use the solution as the electrolyte. Linear voltammetry and chronopotentiometry are performed using Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode.

[0046] Step 2: After 50 hours of testing, the electrolyte was poured out and 0.56 g of KOH solid was added to adjust the pH to approximately 13. The mixture was stirred magnetically at 500 rpm for 3 hours. The resulting white precipitate was filtered, the solid was collected, washed with deionized water for 0.5 hours, and then vacuum dried at 60°C for 12 hours.

[0047] Step 3: Add 6 M HCl to the solution obtained after filtration in step 2 and adjust the pH of the solution to about 2. The mixture was stirred magnetically at 500 rpm for 1 hour. The resulting white precipitate was filtered and the solid was collected. The solid was washed with deionized water for 0.5 hours and then dried under vacuum at 60°C for 12 hours.

[0048] Example 4

[0049] Sodium gluconate is used as a chelating agent to prevent cathode calcium and magnesium ion deposition. The following steps are implemented:

[0050] Step 1: Weigh 3.5 g of sea salt and 6.54 g of sodium gluconate and dissolve them in 100 mL of deionized water. Stir the mixture magnetically at 600 rpm for 0.5 h and use it as the electrolyte. Perform linear voltammetry and chronopotentiometry using Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode.

[0051] Step 2: After 50 hours of testing, the electrolyte was poured out and 4 g of NaOH solid was added to adjust the pH to about 14. The mixture was stirred magnetically at 600 rpm for 1.5 hours. The resulting white precipitate was filtered, the solid was collected, washed with deionized water for 1 hour, and then vacuum dried at 60°C for 24 hours.

[0052] Step 3: Add 9 M HCl to the solution obtained after filtration in step 2 and adjust the pH of the solution to about 1. The reaction was carried out under magnetic stirring at 600 rpm for 1.5 h. The resulting white precipitate was filtered and the solid was collected. The solid was washed with deionized water for 0.5 h and then vacuum dried at 60°C for 12 h.

[0053] Example 5

[0054] Add polyacrylamide to seawater for electrolysis, and follow these steps:

[0055] Step 1: Weigh 3.5 g of sea salt and 20 g of polyacrylamide (Mw: 5 million) and dissolve them in 100 mL of deionized water. Stir at 400 rpm for 0.5 h to use as the electrolyte. Linear voltammetry and chronopotentiometry were performed using Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode.

[0056] Step 2: After 50 hours of testing, record the changes in the cathode surface during the test.

[0057] Comparative Example 1

[0058] Without adding any reagents, directly electrolyze seawater and follow the steps below:

[0059] Step 1: Weigh 3.5 g of sea salt and dissolve it in 100 mL of deionized water. Stir magnetically at 400 rpm for 0.5 h to use as the electrolyte. Perform linear voltammetry and chronopotentiometry using Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode.

[0060] Step 2: After 120 hours of testing, record the changes in the cathode surface during the test.

[0061] Comparative Example 2

[0062] Add 0.01M EDTA tetrasodium salt to seawater for electrolysis, and perform the following steps:

[0063] Step 1: Weigh 3.5 g of sea salt and 0.38 g of tetrasodium EDTA and dissolve them in 100 mL of deionized water. Stir at 400 rpm for 0.5 h to use as the electrolyte. Perform linear voltammetry and chronopotentiometry using Pt nanoparticles grown on nickel foam (Pt@Ni Foam) as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode.

[0064] Step 2: After 120 hours of testing, record the changes in the cathode surface during the test.

[0065] Step 3: Add 9 M HCl to the solution obtained after filtration in step 2 and adjust the pH of the solution to about 1. The mixture was stirred magnetically at 400 rpm for 2 h. The resulting white precipitate was filtered, the solid was collected, washed with deionized water for 0.5 h, and then vacuum dried at 60°C for 12 h.

[0066] The potential drop degree (ΔE=E) after 120 h of testing in Examples 1-3 and Comparative Examples 1-4 was compared based on the linear voltammetric scanning curve and the chronopotentiometry curve. after -Ebefore ), -10mA·cm -2 The overpotential and Tafel slope under the condition of IR are shown in Table 1.

[0067] Table 1. Comparison of various properties between Examples 1-3 and Comparative Examples 1-4

[0068]

[0069] As can be seen from Table 1, after adding the metal chelating agent EDTA tetrasodium salt, the electrolysis temperature of Example 1 was -30 mA·cm -2 The overpotential under the condition of -10mA·cm-1 was not significantly increased. On the contrary, the overpotential of comparative example 1 increased by 217mV, which was mainly due to the deposition of calcium and magnesium ions on the cathode covering the active sites. Examples 2-5 all added metal chelating agents, and the overpotential did not increase significantly after 50h constant current test. The overpotential of comparative example 2 increased by 189mV, which may be because the concentration of chelating agent was too low to completely complex the calcium and magnesium ions in seawater. Therefore, calcium and magnesium were still deposited during the electrolysis process, resulting in a decrease in cathode activity. Secondly, Examples 1-4 were -2 The overpotential under the condition of EDTA was significantly reduced from 231mV in Comparative Example 1 to below 200mV, and only 160mV after adding tetrasodium EDTA. Examples 1-4 also showed lower Tafel slopes. As can be seen from Example 5, although the macromolecular chelating agent can effectively prevent deposition, it has an inhibitory effect on the hydrogen evolution reaction, -10mA·cm -2 The overpotential under the conditions of catalytic oxidation is even higher than that of pure seawater. This may be because the macromolecular chelating agent occupies the entire double layer or is adsorbed on the cathode surface, making the double layer in a water-deficient state. At the same time, the cathode is severely adsorbed, resulting in the cathode being poisoned and the catalytic activity being greatly reduced. The Tafel slope even reaches 264 mV·dec. -1 , thus inhibiting the hydrogen evolution reaction.

[0070] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preventing cathode calcium and magnesium hydroxide deposition during seawater electrolysis, characterized in that: By adding metal ion chelating agents to seawater to complex calcium and magnesium ions and then using them for water electrolysis to prevent their deposition; The metal ion chelating agent is a small molecule organic chelating agent; When the metal ion chelating agent is a small molecule organic chelating agent, the chelating agent and calcium and magnesium minerals in the electrolyzed seawater are recovered simultaneously.

2. The method according to claim 1, characterized in that The method comprises the following steps: (1) dissolving the metal ion chelating agent in seawater; (2) magnetically stirring the solution for a certain period of time to allow the metal ion chelating agent to fully react with calcium and magnesium ions, and then performing electrolysis; (3) adding alkali to the electrolyzed seawater to adjust the pH until the metal is precipitated from the metal ion chelating agent, and filtering the recovered metal precipitate; (4) Adding acid to the filtered solution to adjust the pH until the metal ion chelating agent is precipitated, and filtering to recover the metal ion chelating agent.

3. The method according to claim 2, characterized in that In step (1), the molar ratio of the chelating agent to the calcium and magnesium ions in the seawater is 1:1-5:1; the dissolution is ultrasonic dissolution, and the ultrasonic power is 50-300W; In step (2), the magnetic stirring power is 50-100 W and the stirring time is 0.5-3 h; In step (3), the base is sodium hydroxide or potassium hydroxide solid, and the pH is adjusted to 12-14; The acid in step (4) is hydrochloric acid with a concentration of 3-9 M and a pH adjusted to 1-3.

4. The method according to claim 1 or 2, characterized in that The complex constant between the metal ion chelating agent and calcium and magnesium ions is 7.9-11.

9.

5. The method according to claim 1 or 2, characterized in that The solubility of the metal ion chelating agent in a solution with a pH of 8-12 can reach at least 0.08M; the solubility of the metal ion chelating agent in a solution with a pH of 1-3 is not higher than 0.001M.

6. The method according to claim 1, characterized in that The metal ion chelating agent remains stable during the hydrogen evolution reaction, that is, no redox reaction occurs within the range of -1.0-0 V (vs. RHE).

7. The method according to claim 1, characterized in that The metal ion chelating agent is stable in the pH range of 8 to 12 and remains in a stable state after chelating with the metal.

8. The method according to claim 1, characterized in that The small molecule organic chelating agent is at least one of sodium ethylenediaminetetramethylenephosphonate, ethylenediaminetetraacetate, and sodium gluconate.

9. The method according to claim 2, characterized in that Only small molecule organometallic chelates can promote hydrogen evolution reaction when added to seawater at a concentration of 0.08-0.4M.

10. A seawater electrolysis process, comprising the method of preventing cathode calcium and magnesium ion deposition by adding a metal chelating agent as described in any one of claims 1 to 9.

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