A method for electrolyzing seawater

Abstract

The present invention discloses a seawater electrolysis method, which is proposed to solve the problem that the chlorine evolution reaction (ClER) seriously affects the electrolysis efficiency of the oxygen evolution reaction (OER) and causes corrosion to the oxygen evolution electrode in the existing direct seawater electrolysis anode half process, and to solve the problem that the gas produced by the anode oxygen evolution reaction (OER) and chlorine evolution reaction (ClER) of the electrolytic cell and the cathode hydrogen evolution reaction (HER) process is seriously mixed. This method uses a two-step intermittent operation of seawater electroreduction hydrogen evolution and reversible auxiliary electrode thermal reduction, and performs the cathode hydrogen evolution reaction (HER) electrode process under the premise of suppressing the anode oxygen evolution reaction (OER) and chlorine evolution reaction (ClER) half reactions in the traditional direct seawater electrolysis, so that the electrolysis step mainly undergoes the hydrogen evolution reaction (HER), and high-purity hydrogen is produced from seawater.

Description

Technical Field

[0001] The invention belongs to the field of electrolysis, and in particular relates to a method for electrolyzing seawater. Background Art

[0002] "Green hydrogen" technology, which combines renewable energy power generation with water electrolysis hydrogen production technology, is expected to become the key to solving the problems of "carbon emissions" and "renewable energy consumption". Among them, offshore wind power and ocean beach photovoltaic power generation technology are important components of "green hydrogen" technology.

[0003] Among water electrolysis hydrogen production technologies, proton exchange membrane electrolysis and alkaline water electrolysis are relatively mature and have achieved industrial application. However, both use pure water or alkaline-treated pure water as raw materials. Freshwater resources are severely scarce in the operating environments of offshore wind power and offshore photovoltaic power generation. This situation will directly restrict the expansion of offshore wind and solar power generation, and thus the development of "green hydrogen" technology. Furthermore, freshwater is a scarce resource, and pure water electrolysis will be limited by raw material costs and resource reserves in the future. Seawater, on the other hand, accounts for 96.5% of the Earth's total water reserves, making it a nearly unlimited resource. Furthermore, research on high-performance electrode materials and process technologies for direct seawater electrolysis to produce hydrogen has recently been increasing.

[0004] Ning et al. (Materials Today Physics, 2021, 19.) reported an alkaline seawater electrolysis. Because the chlorine evolution reaction (ClER) could not be completely suppressed, the electrode showed a voltage decay of 47mV after only 100 hours of operation, which could not meet the development needs of current water electrolysis technology. Kuang et al. (Proc Natl Acad Sci USA, 2019, 116 (14): 6624-6629.) reported an alkaline simulated seawater electrolysis. The oxygen evolution electrode achieved long-term stable operation for more than 1000 hours. However, due to the construction of its metal sulfide chlorine-resistant structure, this electrode needs to be activated for more than 4 hours to achieve high-performance electrolysis, and cannot achieve rapid start and stop and adapt to the volatility of renewable energy power. In summary, the main challenge of direct seawater electrolysis technology lies in the inevitable competition between ClER and anode OER processes and the corrosion problems caused by them. This is because, although the equilibrium potential of the ClER half-reaction in seawater in an alkaline environment is approximately 480 mV higher than that of OER, meaning that ClER is more difficult to occur than OER, the inevitable local pH drop and chloride ion adsorption at high potentials still lead to local ClER and corrosion at the anode. Therefore, despite the remarkable achievements of numerous electrode studies in seawater electrolysis, the competitive reaction process occurring at the anode is always difficult to avoid. Summary of the Invention

[0005] Based on the above background technology, the present invention proposes a seawater electrolysis method that uses a two-step intermittent operation of electrocatalysis and thermal reduction, utilizing a reversible electrode with redox cycling characteristics through changes in the valence state of the active metal as the anode to achieve cathode HER while suppressing both the anodic OER and ClER reaction processes. This electrolysis method effectively avoids the competition and resulting corrosion problems of ClER in the anodic process in seawater environments; it also directly produces high-purity hydrogen, reducing the cost of the gas separation process.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A seawater electrolysis method comprises two steps:

[0008] (1) Cathode seawater electroreduction hydrogen evolution and anodic oxidation;

[0009] (2) Anode thermal reduction: Repeat the above intermittent operation steps to produce hydrogen from seawater through multiple cycles.

[0010] Based on the above technical solution, preferably, step (1) is "cold seawater feeding - passage - low-pressure electrolysis of seawater for hydrogen production - circuit breaker - washing water for gas collection", using cold seawater as raw material, forming a passage between the cathode, anode and power supply, controlling the electrolysis current density, and maintaining a certain working time; causing the cathode to undergo an electrochemical reduction reaction to convert seawater into the target product hydrogen; and causing the anode to undergo self-oxidation reaction, thereby suppressing the OER and ClER generated by the seawater during the anode oxidation; then the power supply is disconnected, the washing device is washed and the hydrogen in this process is collected.

[0011] Based on the above technical solution, preferably, step (2) is "hot seawater feeding - thermal reduction auxiliary electrode - washing water flushing"; while keeping the power supply disconnected, hot seawater is replaced as the raw material and maintained for a certain working time; the anode is spontaneously reduced in the hot seawater environment; at the end of the step, the device is washed, and then cold seawater is replaced as the raw material for electrolysis again, and so on, hydrogen is produced from seawater multiple times.

[0012] Based on the above technical solution, preferably, the electrolysis current density should be controlled at 10-200 mA / cm 2 .

[0013] Based on the above technical solution, preferably, the cold seawater is natural seawater treated with an alkaline substance or simulated seawater treated with an alkaline substance, and the alkaline substance is KOH, KHCO3, NaHCO3, borate or phosphate buffer, wherein the concentration of the alkaline substance is 0.1-6M and the temperature is 0-60°C.

[0014] Based on the above technical solution, preferably, the hot seawater is natural seawater treated with alkaline substances or simulated seawater treated with alkaline substances, and the alkaline substances are KOH, KHCO3, NaHCO3, borate or phosphate buffer, wherein the KOH concentration is 0.1-6M and the temperature is 80-100°C.

[0015] Based on the above technical solution, preferably, the simulated seawater is a solution composed of NaCl or commercial sea salt as a solute, wherein the NaCl concentration should be 0.4-0.6M.

[0016] Based on the above technical solution, preferably, maintaining a certain working time means that the working time of each step in the two-step intermittent operation process is 100-1000 seconds, and the operating cycles of the two steps should be kept consistent, that is, the two-step reaction is maintained for the same time.

[0017] Based on the above technical solution, preferably, the cathode should be an electrode material with good HER activity, such as NiMoN or CoMoP supported on nickel foam, a platinum sheet electrode, a hydrogen evolution electrode supported on porous carbon paper with Pt / C or Ptblack catalyst, etc.

[0018] Based on the above technical solution, preferably, the anode is a reversible electrode having the characteristic of being able to realize redox cycle by changing the valence state of active metals in an alkaline seawater environment, specifically a metal hydroxide, which is reversible through cyclic conversion between metal hydroxide / metal oxyhydroxide structures, and the metal is one, two or three of Ni, Co, Fe, Mn, Cr, V, Cu, Zn, Mo, Mg, and Al.

[0019] The above method of the present invention can be implemented by the following seawater electrolysis device, which includes a shell, a cathode and an anode arranged in the shell, and the cathode and the anode are connected to a power source outside the shell to form a passage;

[0020] The anode is a reversible electrode having redox cycling characteristics through changes in the valence state of the active metal, and the cathode should be an electrode material with good HER activity.

[0021] Based on the above technical solution, preferably, the cathode and anode electrode materials are fixed on one end of a metal rod containing a polytetrafluoroethylene coating, and the metal rod containing a polytetrafluoroethylene coating is fixedly installed on the top of the device from top to bottom and directly connected to the power supply.

[0022] Based on the above technical solution, preferably, the shell is provided with a feed port, a discharge port, an air outlet and a discharge port, and the feed port and the discharge port are controlled by a four-way valve, and the four-way valves are respectively connected to the seawater electrolysis device, the cold seawater storage tank, the hot seawater storage tank, and the washing water storage tank;

[0023] The seawater electrolysis device is directly connected to the hydrogen storage tank through the gas outlet; the discharge port of the seawater electrolysis device is connected to the washing water storage tank through a bypass.

[0024] Based on the above technical solution, preferably, the feed inlet of the seawater electrolysis device is located at the lower side of the device, the discharge port is located at the upper side of the device, the gas outlet is located at the center of the top of the device, and the discharge port is located at the center of the bottom of the device.

[0025] Based on the above technical solution, preferably, the overall structure of the seawater electrolysis device is a vertical reactor.

[0026] Based on the above technical solution, preferably, the shell of the seawater electrolysis device is sealed with a flange.

[0027] Based on the above technical solution, preferably, the seawater electrolysis device should be equipped with temperature indicating control (TIC) and liquid level indicating control (LIC) detectors or instruments for feedforward control of the control valve of the device feed pipeline.

[0028] Based on the above technical solution, preferably, the seawater electrolysis device should be equipped with safety valves for emptying and draining at the top and bottom, and the device emptying and draining safety valves are located at the top and bottom edges of the device respectively.

[0029] Based on the above technical solution, preferably, the entire seawater electrolysis device should be constructed of carbon steel coated with an insulating layer; for example, the carbon steel may be Q235R.

[0030] Based on the above technical solution, preferably, the relationship between the above seawater electrolysis device and the supporting equipment in the system is:

[0031] The flow indicator control (FIC) equipment of the four-way valves of the feed and discharge ports can directly perform feedforward control on the control valves of the cold seawater storage tank, hot seawater storage tank, wash water storage tank and the device feed pipeline;

[0032] In addition to the inlet and outlet pipelines, the cold seawater storage tank, hot seawater storage tank, and washing water storage tank are equipped with bypass pipelines. A closed-loop flow ratio control system is used between the bypasses of the three storage tanks (cold seawater storage tank, hot seawater storage tank, and washing water storage tank) to distribute the alkaline seawater circulating in the process to ensure sufficient feed reserves in each process;

[0033] Based on the above technical solution, preferably, the seawater electrolysis device and part of the pipelines in its system are heat-conducted in the form of heat exchangers, so that the waste heat in the hot seawater feed can be fully utilized.

[0034] In actual operation, the process method of the present invention preferably adopts an even number of seawater electrolysis devices working together to avoid the fluctuation adaptation problems and renewable energy waste problems caused by the periodicity of the two-step intermittent operation.

[0035] Beneficial effects

[0036] 1. This method is proposed to address the problem that the chlorine evolution reaction (ClER) seriously affects the electrolysis efficiency of the oxygen evolution reaction (OER) and causes corrosion to the oxygen evolution electrode in the anode half-process of the existing direct seawater electrolysis, as well as the serious mixing of gas produced by the anode OER and ClER with the cathode hydrogen evolution reaction (HER) process of the electrolysis cell. Through a two-step intermittent operation of seawater electroreduction hydrogen evolution and reversible auxiliary electrode thermal reduction, while suppressing the anode OER and ClER half-reactions in traditional direct seawater electrolysis, the cathode HER electrode process is carried out, so that the electrolysis step mainly produces HER, and high-purity hydrogen is produced from seawater under the condition of avoiding severe corrosion of the anode ClER and HER.

[0037] 2. It can effectively avoid ClER and corrosion of the anode during direct seawater electrolysis, and improve the electrolysis efficiency and service life of the electrode material in the seawater environment.

[0038] 3. The raw material cost is low, and the alkaline-treated seawater is used as direct feed, avoiding the cost burden of pretreatment process and high-value additives.

[0039] 4. The process cost is low. The OER in the electrolysis process is suppressed, so that the HER process can proceed efficiently. The purity of the prepared hydrogen is high, which can greatly reduce the cost consumption in the gas separation process.

[0040] 5. Operating at low voltage reduces the high energy consumption of the OER four-electron process, and the electrolysis efficiency of this process method is higher than that of the direct seawater electrolysis process. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the structure of the seawater electrolysis device.

[0042] In the figure: 1. device housing, 2. cathode, 3. anode, 4 and 5. metal rods with polytetrafluoroethylene coating, 6. DC power supply, 7. feed inlet, 8. discharge outlet, 9. air outlet, 10. discharge outlet.

[0043] Figure 2 The potential change curve during constant current electrolysis for 500 seconds at 100 mA in an alkaline seawater environment: the blue curve is the potential change curve of the first electrolysis, and the red curve is the potential change curve of the second electrolysis after the auxiliary electrode was reduced at 95°C for 500 seconds after the first electrolysis.

[0044] Figure 3 This is the gas chromatography result of H2 gas produced after the first electrolysis at a constant current of 100 mA for 500 seconds in an alkaline seawater environment.

[0045] Figure 4 This is the H2 gas chromatography result after the first electrolysis, the auxiliary electrode was reduced at 95℃ for 500 seconds, and then the second electrolysis was carried out at a constant current of 100 mA in an alkaline seawater environment for 500 seconds.

[0046] Figure 5 This is the gas chromatography result of O2 produced after the first electrolysis at a constant current of 100 mA for 500 seconds in an alkaline seawater environment.

[0047] Figure 6 The graph shows the O2 gas chromatography results after the first electrolysis, the auxiliary electrode was reduced at 95℃ for 500 seconds, and then the second electrolysis was performed at a constant current of 100 mA in an alkaline seawater environment for 500 seconds.

[0048] Figure 7 This is the gas chromatography H2 standard curve.

[0049] Figure 8 This is the gas chromatography O2 standard curve.

[0050] Figure 9 For Figure 1-6 Residual chlorine test results of alkaline seawater electrolyte after electrolysis under conditions of . DETAILED DESCRIPTION

[0051] The present invention will be further described below in conjunction with the accompanying drawings.

[0052] like Figure 1As shown, a seawater electrolysis device has an overall structure of a vertical reactor: it includes a shell 1, a cathode 2 and an anode 3 arranged in the shell, and the cathode 2 and anode 3 are connected to a DC power supply 6 outside the shell 1 to form a passage; the device shell 1 is sealed with a flange; the cathode and anode electrode materials 2 and 3 are fixed to one end of the rod by metal rods 4 and 5 with a polytetrafluoroethylene coating, and the rods are fixed from top to bottom on the top of the device and directly connected to the DC power supply 6; the shell is provided with a feed port 7, a discharge port 8, an air outlet 9 and a discharge port 10, and the feed port 7 and the discharge port 8 are controlled by a four-way valve, and the four-way valves are respectively connected to the seawater electrolysis device, the cold seawater storage tank, the hot seawater storage tank, and the washing water storage tank; the seawater electrolysis device is directly connected to the hydrogen storage tank via the air outlet 9; the discharge port 10 of the seawater electrolysis device is connected to the washing water storage tank via a bypass. The cathode 2 should be an electrode material with good HER activity, such as NiMoN or CoMoP supported on nickel foam, a platinum sheet electrode, or a hydrogen evolution electrode supported on porous carbon paper with Pt / C or Ptblack catalyst. The anode 3 is a reversible electrode capable of undergoing redox cycling in an alkaline seawater environment through changes in the valence state of the active metal. Specifically, it is a metal hydroxide, which is reversible through cyclic conversion between metal hydroxide / metal oxyhydroxide structures. The metals are one, two, or three of Ni, Co, Fe, Mn, Cr, V, Cu, Zn, Mo, Mg, and Al.

[0053] The feed port 7 of the seawater electrolysis device is located at the lower side of the device, the discharge port 8 is located at the upper side of the device, the gas outlet 9 is located at the center of the top of the device, and the discharge port 10 is located at the center of the bottom of the device;

[0054] The seawater electrolysis device should be equipped with temperature indicating control (TIC) and liquid level indicating control (LIC) detectors and instruments to perform feedforward control on the control valves of the device’s feed pipelines;

[0055] The seawater electrolysis device should be equipped with safety valves for venting and draining at the top and bottom of the device. The safety valves for venting and draining the device should be located at the top and bottom edges of the device respectively;

[0056] The entire seawater electrolysis device should be constructed of Q235R carbon steel coated with an insulating layer;

[0057] The relationship between the above-mentioned seawater electrolysis device and its supporting equipment in the system is as follows:

[0058] The flow indicator control (FIC) equipment of the four-way valves of the feed and discharge ports can directly perform feedforward control on the control valves of the cold seawater storage tank, hot seawater storage tank, wash water storage tank and the device feed pipeline;

[0059] In addition to the inlet and outlet pipelines, the cold seawater storage tank, hot seawater storage tank, and wash water storage tank are equipped with bypass pipelines. A closed-loop flow ratio control system is used between the bypasses of the three tanks to distribute the alkaline seawater circulating in the process to ensure sufficient feed reserves in each process.

[0060] The seawater electrolysis device and some of the pipes in its system conduct heat in the form of heat exchangers, so that the waste heat in the hot seawater feed can be fully utilized;

[0061] The process method of the present invention adopts an even number of seawater electrolysis devices working together in actual operation, avoiding the fluctuation adaptation problems and renewable energy waste problems caused by the periodicity of the two-step intermittent operation.

[0062] Based on the above device, the specific operating steps of this application method are:

[0063] Cold seawater feed: The cold seawater is seawater treated with potassium hydroxide (KOH) or simulated seawater, with a KOH concentration of 0.1-6M and a temperature of 0-60°C. In this step, the cold seawater enters the seawater electrolysis device from the cold seawater storage tank through the feed port 7, and the feed amount is 50-80% of the device volume;

[0064] Passage: The cathode and anode electrodes 2 and 3 in the seawater electrolysis device are connected to the power supply 6 through the polytetrafluoroethylene-coated metal rods 4 and 5 to form a passage;

[0065] Electrolysis of seawater to produce hydrogen at low current: Control the device voltage to keep it between 10-200mA / cm 2 Electrolysis is performed at a current density of 100-1000 seconds, and intermittent operation is maintained, causing an electrochemical reduction reaction at the cathode 2 to convert seawater into the target product, hydrogen. The anode 3 acts as a reversible auxiliary electrode to undergo an oxidation reaction, converting the metal hydroxide into a metal oxyhydroxide. The Faradaic efficiency of this oxidation reaction is high, so the occurrence of anode electrocatalytic OER and ClER in this step is largely suppressed. A portion of the generated hydrogen that is not absorbed by the seawater feed is transported from the device's outlet 9 to the hydrogen storage tank.

[0066] Disconnect: disconnect the cathode and anode electrodes 2 and 3 in the seawater electrolysis device from the DC power supply 6;

[0067] Wash water gas collection: The wash water is natural seawater or simulated seawater that has been alkaline-treated with potassium hydroxide (KOH), with a KOH concentration of 0.1-6M and a temperature of 0-60°C. In this step, the wash water enters the seawater electrolysis device from the wash water storage tank through the feed port 7, with the feed amount being 200-500% of the device volume, ensuring that the remaining gas product is completely transported along with the cold seawater and wash water through the discharge port 8 to the cold seawater storage tank. Hydrogen is desorbed and collected in the cold seawater storage tank. Subsequently, the remaining wash water in the device is returned to the wash water storage tank through the device discharge port 10 via a bypass for standby use.

[0068] Hot seawater feed: The hot seawater is natural seawater or simulated seawater that has been alkaline-treated with potassium hydroxide (KOH), with a KOH concentration of 0.1-6M and a temperature of 80-100°C. In this step, the hot seawater enters the seawater electrolysis device from the hot seawater storage tank through the feed port 7, and the feed amount is 50-80% of the device volume;

[0069] Thermal reduction auxiliary electrode: The electrode material is maintained in a hot seawater environment for the same intermittent operation time (100-1000 seconds) as in the low-pressure electrolysis of seawater for hydrogen production, so that the reversible auxiliary electrode 3 is spontaneously reduced from the metal oxyhydroxide to the metal hydroxide in the hot seawater environment;

[0070] Washing water flushing: The washing water is natural seawater or simulated seawater that has been alkaline-treated with potassium hydroxide (KOH), with a KOH concentration of 0.1-6M and a temperature of 0-60°C. In this step, the washing water enters the seawater electrolysis device from the washing water storage tank through the feed port 7 at a feed rate of 200-500% of the device volume, and flows through the discharge port 8 to the hot seawater storage tank, ensuring that the oxygen generated during the thermal reduction of the auxiliary electrode can be completely released and removed. Subsequently, the remaining washing water in the device returns to the washing water storage tank through the bypass 10 for standby use.

[0071] In the present application, cold seawater and hot seawater can be directly and continuously introduced into the electrolysis device by adding seawater of the desired temperature, or by placing the entire electrolysis device in an environment at the desired temperature to ensure that the seawater reaches the desired temperature, without further restrictions. The following describes a specific embodiment of the electrolysis method by controlling the temperature of alkaline simulated seawater in a water bath and then controlling the alkaline cold seawater or hot seawater feed to the seawater electrolysis device to illustrate the technical effects of the present invention.

[0072] In this embodiment, the material of the hydrogen evolution electrode, i.e., the cathode, of the seawater electrolysis device is a 2*2 cm Pt sheet electrode, and the material of the reversible auxiliary electrode, i.e., the anode, is a 2*3 cm nickel foam electrode directly grown with nickel cobalt layered double hydroxides (NiCo LDHs) (prepared with reference to the literature: Bifunctional Heterostructure Assembly of NiFe LDH Nanosheetson NiCoP Nanowires for Highly Efficient and Stable Overall Water Splitting. Advanced Functional Materials, 2018, 28(14).); the electrolysis raw material is alkaline simulated seawater, in which the concentration of KOH is 1 M and the concentration of NaCl is 0.5 M;

[0073] (1) alkaline simulated seawater is passed into a seawater electrolysis device, and the cathode, anode and DC power supply in the electrolysis device are connected to form a circuit. The device is operated in a constant temperature water bath at 15°C to realize the electrolysis of alkaline simulated cold seawater at a constant current of 100 mA using alkaline simulated cold seawater as a raw material. At this time, the electrolysis potential of the electrolysis device is about 1.8 V and is maintained for 500 seconds, so that the alkaline simulated seawater undergoes an electrochemical reduction reaction at the cathode to release hydrogen, and the anode, as a reversible auxiliary electrode, undergoes an oxidation reaction itself to convert the metal hydroxide into a metal hydroxide oxide. This step is the electrolysis process;

[0074] (2) collecting hydrogen from the sealed seawater electrolysis device during the electrolysis process, and passing the gas generated by the device at any stage during the electrolysis process into a hydrogen and oxygen phase chromatography to test its composition; then, disconnecting the seawater electrolysis device from the DC power supply;

[0075] (3) Take out the reversible auxiliary electrode and place the device in a 95°C constant temperature water bath to raise the electrolyte temperature to 95°C, that is, use alkaline hot seawater as the raw material, then place the reversible auxiliary electrode in the seawater electrolysis device and keep it for 500 seconds, so that the reversible auxiliary electrode is spontaneously reduced from metal hydroxide oxide to metal hydroxide in the alkaline thermal simulated seawater environment. Oxygen is generated during the thermal reduction of the auxiliary electrode. This step is the spontaneous reduction process of the reversible auxiliary electrode;

[0076] (4) Remove the reversible auxiliary electrode and place the seawater electrolysis device in a constant temperature water bath at 15°C;

[0077] (5) Repeat steps (1) to (4) once to perform the second seawater electrolysis operation.

[0078] like Figure 2As shown in the figure, during the first and second seawater electrolysis hydrogen production steps (constant current electrolysis of alkaline cold simulated seawater at 100 mA for 500 seconds), the potential remained basically stable within 500 seconds; the electrolysis potential was approximately 1.8 V. The reason for the improved performance of the second seawater electrolysis process compared to the first electrolysis process is that the reduction of the reversible auxiliary electrode after the first electrolysis activated the anode, that is, the metal hydroxide / metal oxyhydroxide conversion process was faster, resulting in a lower reaction overpotential;

[0079] like Figure 3 As shown, in the first electrolysis of seawater (electrolysis of alkaline cold simulated seawater at a constant current of 100 mA for 500 seconds), the chromatographic peak of H2 in the produced gas is a complete single peak, proving that the product of seawater electrolysis is high-purity H2;

[0080] like Figure 4 As shown, in the second electrolysis of seawater (after the first electrolysis of alkaline cold simulated seawater, the device was placed in a 95°C constant temperature water bath for reduction of the reversible auxiliary electrode, and the electrolyte was heated to 95°C and maintained for 500 seconds, then taken out and cooled to room temperature. It was then placed in a 15°C constant temperature water bath and electrolyzed alkaline seawater at a constant current of 100 mA for 500 seconds for the second electrolysis of alkaline cold simulated seawater), the chromatographic peak of H2 in the produced gas was a complete single peak, proving that the product of seawater electrolysis was high-purity H2;

[0081] like Figure 5 As shown, in the first electrolysis of seawater (electrolysis of alkaline cold simulated seawater at a constant current of 100 mA for 500 seconds), the concentration of O2 in the produced gas was about 7764.1 ppm, that is, the content of O2 in the produced gas was about 0.77641%, proving that the OER process was effectively suppressed;

[0082] like Figure 6 As shown, in the second electrolysis of seawater (after the first electrolysis, the device was placed in a constant temperature water bath at 95°C for reduction of the reversible auxiliary electrode, and the electrolyte was heated to 95°C and maintained for 500 seconds, then taken out and cooled to room temperature. It was then placed in a constant temperature water bath at 15°C and electrolyzed at a constant current of 100 mA for 500 seconds in alkaline simulated seawater), the concentration of O2 in the produced gas was about 19042.7 ppm, that is, the content of O2 in the produced gas was still less than 2%. Compared with the oxygen evolution Faraday efficiency of direct seawater electrolysis, the OER process was still effectively suppressed;

[0083] Figure 3 、 Figure 4 The gas chromatography results in the Figure 7 The H2 standard curve of gas chromatography is obtained. Figure 5 、 Figure 6 The gas chromatography results in the Figure 8 Derived from the O2 standard curve of gas chromatography;

[0084] like Figure 9 As shown, the residual chlorine test was performed on the alkaline simulated seawater electrolyte after the above two seawater electrolysis processes. The residual chlorine test paper showed that no chlorine gas or hypochlorite was generated in the electrolyte, and this was also proved by the fact that the starch indicator did not change color after adding excess NaI.

Claims

1. A method for electrolyzing seawater, characterized in that: The method comprises two steps: (1) cathode seawater electroreduction hydrogen evolution and anode oxidation; (2) anode thermal reduction; The above two steps are operated intermittently; In step (1), cold seawater is used as a raw material, a cathode, an anode and a power source form a path, the electrolysis current density is controlled, and a certain working time is maintained, so that the seawater undergoes an electrochemical reduction reaction at the cathode to precipitate hydrogen; the anode undergoes an oxidation reaction, the power source is disconnected, the washing device is cleaned and hydrogen is collected; In step (2), the power supply is disconnected, hot seawater is replaced as the raw material, and the operation is maintained for a certain period of time; the anode itself is reduced; the device is then washed; steps (1) and (2) are repeated to produce hydrogen from seawater multiple times; The anode is a reversible electrode having redox cycle characteristics through the change of active metal valence state; the reversible electrode is a metal hydroxide, and the metal of the metal hydroxide is a Ni and Co bimetallic; The electrolysis current density in step (1) is 10-200 mA / cm 2 .

2. The seawater electrolysis method according to claim 1, characterized in that: The cold seawater is natural seawater treated with alkaline substances or simulated seawater treated with alkaline substances, the alkaline substances are KOH, KHCO3, NaHCO3, borate or phosphate buffer, the alkaline substance concentration in the cold seawater is 0.1-6M, and the temperature of the cold seawater is 0-60°C.

3. The seawater electrolysis method according to claim 1, characterized in that: The hot seawater is natural seawater treated with alkaline substances or simulated seawater treated with alkaline substances. The alkaline substances are KOH, KHCO3, NaHCO3, borate or phosphate buffer. The concentration of the alkaline substances in the hot seawater is 0.1-6M, and the temperature of the hot seawater is 80-100°C.

4. The seawater electrolysis method according to claim 2 or 3, characterized in that: The simulated seawater is a solution formed when NaCl or commercial sea salt is used as a solute, and the NaCl concentration in the simulated seawater is 0.4-0.6M.

5. The seawater electrolysis method according to claim 1, characterized in that: The certain working time maintained in step (1) and step (2) is 100-1000 seconds, and the operation cycle of step (1) and step (2) is consistent.

6. The seawater electrolysis method according to claim 1, characterized in that: The cathode is an electrode material having HER activity.

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