An electrochemical system and method for the spontaneous discharge driven synthesis of calcium hydroxide

Abstract

This invention discloses an electrochemical system and method for spontaneously discharging calcium hydroxide. The electrochemical system includes independent negative and positive electrode chambers. The negative electrode chamber includes a gas diffusion electrode through which hydrogen gas is introduced and a negative electrode electrolyte containing CaCO3. The positive electrode chamber includes a NiOOH electrode and a positive electrode electrolyte. After the positive and negative electrodes are electrically connected, the intrinsic redox potential of the NiOOH electrode at the positive electrode and the hydrogen oxidation reaction at the negative electrode create a potential difference, resulting in spontaneous discharge. Hydrogen oxidation at the negative electrode generates hydrogen ions, dissolving CaCO3. NiOOH at the positive electrode is reduced to generate hydroxide ions. The spontaneously discharged negative and positive electrode electrolytes react to synthesize calcium hydroxide. This invention, by coupling hydrogen oxidation and NiOOH reduction, completely avoids external energy input and utilizes inherent electrochemical activity for spontaneous discharge preparation of calcium hydroxide. It aims to solve the high energy consumption and high carbon emission problems existing in the traditional cement industry for calcium hydroxide production, overcoming the high energy consumption bottleneck in existing electrolytic preparation processes.

Description

Technical Field

[0001] This invention relates to the field of calcium hydroxide synthesis technology, and more specifically, to an electrochemical system and method for the spontaneous discharge-driven synthesis of calcium hydroxide. Background Technology

[0002] The cement industry is the cornerstone of global infrastructure construction, but it is also a major consumer of energy and a major emitter of carbon. Its technological bottlenecks are mainly concentrated in the production of calcium hydroxide: 1. Disadvantages of the traditional calcination method: Currently, it heavily relies on calcining limestone at extreme high temperatures of ≥1450 ℃. This process not only consumes 3.0~3.5 GJ of energy per ton of clinker (accounting for more than 70% of the total energy consumption of the entire industry), but also directly releases a huge amount of CO2 into the atmosphere. 2. Limitations of existing electrochemical methods: Although Chiang et al. pioneered the idea of ​​using electrochemical pH gradient to dissolve calcium carbonate to prepare Ca(OH)2 at room temperature, achieving decarbonization (see Leah D. Ellis, Andres F. Badel, Miki L. Chiang, Richard J.-Y. Park, and Yet-Ming Chiang. Toward electrochemical synthesis ofcement-an electrolyzer-based process for decarbonating CaCO3 while producing useful gas streams. Proceedings of the National Academy of Sciences, 117(23):12584-12591, 2020), electrochemical technology is considered a highly promising alternative because it can operate under environmental conditions and avoids the combustion of fossil fuels. However, most existing electrolysis routes face serious energy consumption barriers. These devices require external high-voltage power input (typically between 1.8 V and 4.7 V) to drive the reaction, resulting in extremely poor economic efficiency.

[0003] Therefore, reducing the energy consumption of electrochemical synthesis of calcium hydroxide remains a pressing technical problem. In addition, the recycled value of existing decommissioned electrode materials (such as the NiOOH electrode in metal hydride batteries) is extremely low, necessitating the search for high-value reuse scenarios that can leverage their inherent electrochemical activity. Summary of the Invention

[0004] Due to the aforementioned deficiencies in existing technologies, this invention provides an electrochemical system and method for the spontaneous discharge-driven synthesis of calcium hydroxide. By cleverly coupling hydrogen oxidation on the negative electrode side and NiOOH reduction on the positive electrode side, it completely avoids external energy input and utilizes the inherent electrochemical activity of the chemical reaction to prepare calcium hydroxide through spontaneous discharge. This aims to solve the high energy consumption and high carbon emission problems existing in the traditional cement industry when producing the core precursor calcium hydroxide, and overcome the high energy consumption bottleneck in existing electrolytic preparation processes. Furthermore, this invention also provides a new approach for the high-value resource recovery and utilization of decommissioned NiOOH electrodes and discarded hydrogen.

[0005] To achieve the above objectives, in a first aspect, the present invention provides an electrochemical system for the spontaneous discharge-driven synthesis of calcium hydroxide, comprising a separate negative electrode chamber and a positive electrode chamber; the negative electrode chamber includes a gas diffusion electrode through which hydrogen-containing gas is introduced and a negative electrode electrolyte containing CaCO3; the positive electrode chamber includes a NiOOH electrode and a positive electrode electrolyte;

[0006] After the positive and negative electrodes are electrically connected, the intrinsic redox potential of NiOOH at the positive electrode and the oxidation reaction of hydrogen at the negative electrode create a potential difference, resulting in spontaneous discharge. Hydrogen oxidation at the negative electrode generates hydrogen ions, which dissolve CaCO3. NiOOH at the positive electrode is reduced to generate hydroxide ions. The electrolyte at the negative electrode after spontaneous discharge reacts with the electrolyte at the positive electrode to synthesize calcium hydroxide.

[0007] Unlike existing technologies that rely on external high-voltage power supplies, this invention ingeniously couples hydrogen oxidation on the negative electrode side and NiOOH reduction on the positive electrode side. This system completely avoids external energy input, instead utilizing the inherent electrochemical activity of the chemical reaction for spontaneous discharge, using the H₂ generated on the negative electrode side... + Dissolve CaCO3 and OH- produced on the positive electrode side - The reaction produces Ca(OH)₂. Addressing the issues of high overpotential, high energy consumption, and low proton generation selectivity associated with the proton production and oxygen evolution side reaction during water electrolysis, this method utilizes hydrogen oxidation at a gas diffusion electrode to directionally generate protons. This process avoids side reactions, has extremely low overpotential, and achieves higher proton generation efficiency, enabling faster and more efficient dissolution of CaCO₃. NiOOH possesses a layered hydroxide crystal structure, facilitating rapid surface transport of charge carriers. Its unique reduction potential perfectly matches the thermodynamic requirements of CaCO₃ dissolution and Ca(OH)₂ precipitation in this system, making it crucial for achieving spontaneous discharge. Compared to conventional cathodes (graphite, Pt / C, metal oxides, etc.) in existing electrochemical calcium hydroxide preparation processes, it exhibits unique advantages: Firstly, the intrinsic redox potential of NiOOH (0.49 V vs. SHE) and the hydrogen oxidation reaction (HOR) at the negative electrode create a self-generating potential difference, achieving zero external voltage-driven OH generation. -Firstly, the NiOOH reduction reaction generates OH-, whereas a conventional cathode requires an external voltage of 1.8 V or higher to complete the same reaction; secondly, the NiOOH reduction reaction directionally generates OH-. - No side reactions such as hydrogen evolution or hydrogen peroxide production, OH - Extremely high selectivity; conventional cathodes have many side reactions and cannot form efficient OH groups. - The reaction that produces it; thirdly, NiOOH is extremely stable in the alkaline environment of the system, and the reduction product Ni(OH)2 can be regenerated and reused.

[0008] Furthermore, the negative or positive electrolyte is an inert sodium or potassium salt solution; it is understood that "inert" here means that no redox reaction occurs within the electrochemical window of the system, such as one or two of NaCl, NaNO3, KCl, and KNO3; a salt bridge is provided between the negative and positive electrodes; after spontaneous discharge, the negative and positive electrolytes react to synthesize calcium hydroxide precipitate.

[0009] Furthermore, a cation exchange membrane (CEM) and an anion exchange membrane (AEM) are sequentially arranged between the negative and positive electrodes, dividing the electrochemical system into three chambers: the negative electrode chamber, the intermediate chamber, and the positive electrode chamber. Driven by kinetics, the Ca in the negative electrode electrolyte... 2+ OH- in the positive electrode electrolyte passes through the cation exchange membrane. - Passing through the anion exchange membrane, the two molecules converge in the intermediate chamber to form calcium hydroxide precipitate. Thus, the three-chamber reactor allows for continuous and uninterrupted production of calcium hydroxide.

[0010] Furthermore, the hydrogen-containing gas is industrial by-product hydrogen. Thus, no pretreatment is required, and the industrial by-product hydrogen can be directly reused, further reducing system costs and increasing economic value.

[0011] Furthermore, the hydrogen-containing gas is dry gas from petrochemical cracking or coke oven gas. The hydrogen content in dry gas from petrochemical cracking is generally below 30%, while the hydrogen content in coke oven gas is generally above 50%, both suitable as negative electrode active materials in this system. It is understandable that, considering the poisoning characteristics of different catalysts used in the gas diffusion electrode, hydrogen-containing gases such as dry gas from petrochemical cracking or coke oven gas can be purified to a certain extent.

[0012] Furthermore, the hydrogen-containing gas is produced by electrolyzing water using surplus renewable energy. This further enhances the economic value of the system.

[0013] Furthermore, the NiOOH electrode is the positive electrode obtained after charging a retired nickel-metal hydride battery. Thus, the recycled NiOOH electrode is directly used as the OH electrode. -This is an in-situ source of energy. This not only promotes the recycling of materials for metal hydride battery systems (in line with the principles of a circular economy), but also transforms waste into a key raw material for synthesizing green cement precursors.

[0014] Furthermore, the NiOOH electrode is a NiOOH electrode from a decommissioned electrochemical wastewater treatment system.

[0015] Furthermore, the gas diffusion electrode is obtained by mixing and grinding Pt / C powder, carbon black and PTFE solution in a certain proportion into a slurry, uniformly coating it on hydrophobic carbon cloth, drying it and pressing it.

[0016] Secondly, the present invention provides a method for synthesizing calcium hydroxide driven by spontaneous discharge, characterized in that it includes: electrically connecting the positive and negative electrodes of an electrochemical system as described above, wherein the electrochemical system generates spontaneous discharge; and the Ca in the negative electrode electrolyte after spontaneous discharge... 2+ and OH in the positive electrode electrolyte - The reaction synthesizes calcium hydroxide.

[0017] Compared with the prior art, the above invention has the following advantages or beneficial effects:

[0018] (1) Achieving spontaneous synthesis with "zero" external power input, significantly reducing energy consumption: Existing electrolytic synthesis of Ca(OH)2 typically requires a high external voltage of 1.8 V to 4.7 V to drive the reaction. This invention overturns this approach by coupling hydrogen oxidation at the negative electrode with NiOOH reduction at the positive electrode, utilizing its inherent electrochemical activity to spontaneously discharge. This process can synthesize Ca(OH)2 without any external power input, and can even output electrical energy, greatly reducing production energy consumption.

[0019] (2) Make flexible use of green hydrogen or even industrial by-product hydrogen as an energy source to further improve the economic value of the system.

[0020] (3) High-value closed-loop recycling of waste electrodes: After the decommissioning of existing metal hydride (MH) battery systems, their NiOOH electrodes often face the dilemma of low-value recycling; or the NiOOH auxiliary electrodes of electrochemical wastewater treatment systems that have been fully charged are easily wasted if left idle. This invention cleverly utilizes the reduction of NiOOH to generate OH - The intrinsic capabilities enable the high-value reuse of these waste electrodes in green chemical synthesis, which is highly consistent with the concept of circular economy.

[0021] (4) Physical isolation design effectively prevents electrode scaling: By using independent negative and positive electrode chambers and utilizing ion exchange membranes (or salt bridges) for selective ion separation, this invention can separate ions (Ca... 2+and OH - The precipitation reaction is specifically confined to other chambers, which fundamentally prevents harmful deposition (scaling) of precipitates on the electrode surface, thereby protecting the catalytic activity of the electrode and simplifying the product collection process. Attached Figure Description

[0022] The invention, its features and advantages will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0023] Figure 1 This is a schematic diagram illustrating the formation principle of the Ca(OH)2 precipitate product synthesized in Example 1 of the present invention;

[0024] Figure 2 This is a voltage-time curve of the electrochemical system during spontaneous discharge in Example 1 of the present invention;

[0025] Figure 3 Images of the negative electrode electrolyte, intermediate electrode electrolyte, and positive electrode electrolyte after spontaneous discharge in Embodiment 1 of the present invention;

[0026] Figure 4 This is a photograph of the precipitate collected after the mixing reaction in Example 1 of the present invention;

[0027] Figure 5 This is the X-ray diffraction (XRD) pattern of the precipitated product in Example 1 of the present invention;

[0028] Figure 6 This is a schematic diagram of the salt bridge electrochemical system design in Embodiment 2 of the present invention;

[0029] Figure 7 This is a voltage-time curve of the electrochemical system during spontaneous discharge in Example 3 of the present invention;

[0030] Figure 8 This is a voltage-time curve of the electrochemical system during spontaneous discharge in Example 4 of the present invention.

[0031] Reference numerals: 1. Electrolyte in the negative electrode chamber; 2. Electrolyte in the positive electrode chamber; 3. Electrolyte in the intermediate chamber. Detailed Implementation

[0032] The structure of the present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the invention. Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It will also be understood that, unless expressly defined herein, terms such as those defined in a general dictionary shall be interpreted as having the meaning consistent with their meaning in the relevant field context, and not as having an ideal or overly formal meaning.

[0033] This invention provides an electrochemical system and method for the spontaneous discharge-driven synthesis of calcium hydroxide. The electrochemical system includes a separate negative electrode chamber and a positive electrode chamber. The negative electrode chamber includes a gas diffusion electrode through which hydrogen-containing gas is introduced and a negative electrode electrolyte containing CaCO3. The positive electrode chamber includes a NiOOH electrode and a positive electrode electrolyte. After the positive and negative electrodes are electrically connected, the intrinsic redox potential of the NiOOH electrode at the positive electrode and the oxidation reaction of hydrogen at the negative electrode create a potential difference, resulting in spontaneous discharge. Hydrogen gas at the negative electrode is oxidized to generate hydrogen ions, dissolving CaCO3. NiOOH at the positive electrode is reduced to generate hydroxide ions. The spontaneously discharged negative electrode electrolyte and the positive electrode electrolyte react to synthesize calcium hydroxide.

[0034] Independent negative and positive electrode chambers can be two completely independent chambers connected to the electrolyte by a salt bridge, or they can be three chambers separated by a cation exchange membrane and an anion exchange membrane: a negative electrode chamber, an intermediate chamber, and a positive electrode chamber.

[0035] For independent two-chamber systems connected by a salt bridge, the negative or positive electrolyte is an inert sodium or potassium salt solution. It can be understood that "inert" here means that no redox reaction occurs within the electrochemical window of the system, such as one or two of NaCl, NaNO3, KCl, and KNO3.

[0036] In some embodiments, the hydrogen-containing gas is industrial by-product hydrogen, such as common petrochemical cracking by-product dry gas or coke oven gas, or hydrogen produced by electrolyzing water from waste renewable energy.

[0037] In some embodiments, the NiOOH electrode is the positive electrode obtained by charging a decommissioned nickel-metal hydride battery to 1.5V, or the NiOOH electrode of a decommissioned electrochemical wastewater treatment system.

[0038] As described above, this invention utilizes an electrochemical reactor coupling H2 oxidation and NiOOH reduction to spontaneously produce Ca(OH)2 without external voltage application, while also possessing the ability to output electrical energy (discharge). Furthermore, the use of ion-exchange membranes or salt bridges for ion-selective separation ensures that the precipitation reaction of reactant ions is confined to other chambers, fundamentally preventing harmful deposition (scaling) of precipitates on the electrode surface. This protects the catalytic activity of the electrodes and simplifies the product collection process. Simultaneously, the use of discarded hydrogen or spent NiOOH electrodes further enhances the economic efficiency of the electrochemical system.

[0039] The raw materials and solvents involved in the following effect examples are all commercially available, and the experimental equipment is also commercially available or custom-made. The usage methods are existing technologies that can be found through searches.

[0040] Example 1

[0041] This embodiment provides an electrochemical system for the synthesis of calcium hydroxide driven by spontaneous discharge and its application in the synthesis of calcium hydroxide.

[0042] See Figure 1 The experimental setup includes a negative proton chamber, an intermediate chamber, and a positive hydroxide ion chamber, each with a capacity of approximately 20 mL, and each chamber is filled with a 1M NaNO3 aqueous solution. The effective contact area between adjacent chambers is 4 × 4 cm². 2 Membrane selection: Commercially available LANCYTOM® CT-4 cation exchange membrane and LANCYTOM® AHT anion exchange membrane were selected respectively.

[0043] Positive electrode (NiOOH) preparation and activation: NiOOH electrodes were extracted from fully discharged commercial nickel-metal hydride batteries, attached with nickel foam current collectors, and pressed at 15 MPa for 30 seconds. Subsequently, they were charged and discharged four times in 1 M KOH electrolyte at an appropriate current (e.g., 20 mA), and finally charged to 1.5 V for activation and standby.

[0044] Preparation of gas diffusion electrode (GDE): 20 mg Pt / C powder, 100 mg carbon black and 1 mL 50 wt% PTFE solution were ground into a slurry and uniformly coated on the carbon layer of commercial hydrophobic carbon cloth (active area of ​​about 16 cm2). After baking at 60 °C for 2 hours, it was wrapped with aluminum foil and pressed at 15 MPa for 10 seconds.

[0045] The hydrogen-containing gas is an argon-hydrogen mixture containing 8 mol% H2, with a flow rate of 20 sccm.

[0046] After the positive and negative electrodes are electrically connected at room temperature, the intrinsic redox potential of NiOOH at the positive electrode and the potential difference formed by the oxidation reaction of hydrogen at the negative electrode create a potential difference, resulting in spontaneous discharge at a current of 10 mA; the discharge curve is shown below. Figure 2 As shown. During the discharge process, at the negative electrode: hydrogen gas undergoes an oxidation reaction at the GDE to generate protons: H2 → 2H. + + 2e - The generated H + The CaCO3 in the chamber is dissolved in situ with high efficiency, releasing free calcium ions: CaCO3 + 2H+ + → Ca 2+ + CO2↑+ H2O; Positive electrode: Waste NiOOH undergoes a reduction reaction at the cathode, generating hydroxide ions: NiOOH + H2O + e- - → Ni(OH)2 + OH - .

[0047] See Figure 2 The test recorded a curve showing a steady decrease in battery voltage over time, confirming spontaneous discharge behavior. After approximately 4 hours of testing at a 20 mA current, the solution in the intermediate chamber changed from clear to turbid. Under the influence of the electric field, Ca... 2+ Through CEM, OH - Passing through the AEM, the two converge in the intermediate chamber. When the concentration product exceeds its solubility product (K... sp ≈ 5.5 × 10 -6 During this process, a precipitation reaction occurs in the intermediate chamber, physically preventing the deposition and scaling of the product on the electrode surface: Ca 2+ + 2OH - → Ca(OH)₂↓. See also Figure 3 After spontaneous discharge, the electrolytes in the negative and positive electrode chambers (electrolyte 1 and electrolyte 2) were clear, while the mixed electrolyte 3 obtained by mixing in the intermediate chamber contained calcium hydroxide precipitate, fully confirming the effectiveness of this method. See also... Figure 4 and Figure 5 The collected precipitate showed clear Ca(OH)2 diffraction peaks by XRD characterization (due to incomplete washing of the product, there were also a small amount of diffraction peaks of NaNO3 electrolyte), further confirming the effectiveness of this scheme.

[0048] Example 2

[0049] This embodiment provides an electrochemical system for the synthesis of calcium hydroxide driven by spontaneous discharge and its application in the synthesis of calcium hydroxide.

[0050] See Figure 6 The electrochemical system comprises separate negative and positive electrode chambers. The negative electrode chamber includes a gas diffusion electrode through which hydrogen-containing gas is introduced and a 1M aqueous solution of NaNO3 containing CaCO3. The positive electrode chamber includes a NiOOH electrode and a 1M aqueous solution of NaNO3. The positive and negative electrode aqueous solutions are connected by a salt bridge.

[0051] The preparation process of the gas diffusion electrode (GDE), the preparation and activation process of the positive electrode (NiOOH), and the test conditions are similar to those in Example 1.

[0052] The system can also synthesize calcium hydroxide by mixing the positive and negative electrolytes after self-discharge. Furthermore, the system allows for more flexible reaction control by controlling the flow of fluid through valves.

[0053] Example 3

[0054] This embodiment provides an electrochemical system for the spontaneous discharge-driven synthesis of calcium hydroxide and its application in the synthesis of calcium hydroxide. The composition of the electrochemical system is similar to that of Example 1, except that the hydrogen-containing gas is an unpurified dry petrochemical cracking byproduct, with the composition of 15% H2 + 23% CO + 42% CH4 + 10% C2H6 + 4% CO2 + 3% C2H4 + 3% C3H8, and a flow rate of 40 sccm. See also Figure 7 The test recorded a curve showing a steady decrease in battery voltage over time, confirming that the electrochemical system can also achieve self-discharge and calcium hydroxide synthesis.

[0055] Example 4

[0056] This embodiment provides an electrochemical system for the spontaneous discharge-driven synthesis of calcium hydroxide and its application in the synthesis of calcium hydroxide. The composition of the electrochemical system is similar to that of Example 1, except that the hydrogen volume fraction in the hydrogen-containing gas is 8%, the flow rate is 20 sccm, and the test is conducted at a current of 20 mA for approximately 18 hours. See also Figure 8 The test recorded a curve showing a steady decrease in battery voltage over time, confirming that the electrochemical system supports long-term discharge and product synthesis.

[0057] The experimental results above demonstrate that this invention breaks away from the traditional thinking of "requiring external power to drive electrolysis," and innovatively proposes a three-chamber electrochemical system based on spontaneous discharge. This system directly utilizes the hydrogen oxidation reaction at the gas diffusion electrode and the reduction reaction at the recovered NiOOH electrode, through a spontaneous process of converting chemical energy into electrical energy, to simultaneously drive the dissolution of calcium carbonate and the preparation of Ca(OH)2. This solves the problems of high energy consumption and high carbon emissions in the traditional cement industry when producing the core precursor calcium hydroxide, and overcomes the high energy consumption bottleneck in existing electrolytic preparation processes.

[0058] Those skilled in the art should understand that variations can be implemented by combining existing technology with the above embodiments, which will not be elaborated here. Such variations do not affect the essence of the present invention, and will not be elaborated here either.

[0059] The preferred embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and the devices and structures not described in detail should be understood as being implemented in a conventional manner in the art. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the scope of the present invention. This does not affect the essential content 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 without departing from the content of the present invention's technical solutions still fall within the protection scope of the present invention.

Claims

1. An electrochemical system for the spontaneous discharge-driven synthesis of calcium hydroxide, characterized in that, It includes a separate negative electrode chamber and a positive electrode chamber; the negative electrode chamber includes a gas diffusion electrode through which hydrogen-containing gas is introduced and a negative electrode electrolyte containing CaCO3; the positive electrode chamber includes a NiOOH electrode and a positive electrode electrolyte; After the positive and negative electrodes are electrically connected, the intrinsic redox potential of NiOOH at the positive electrode and the oxidation reaction of hydrogen at the negative electrode create a potential difference, resulting in spontaneous discharge. Hydrogen oxidation at the negative electrode generates hydrogen ions, which dissolve CaCO3. NiOOH at the positive electrode is reduced to generate hydroxide ions. The electrolyte at the negative electrode after spontaneous discharge reacts with the electrolyte at the positive electrode to synthesize calcium hydroxide.

2. The electrochemical system for spontaneously discharging and synthesizing calcium hydroxide according to claim 1, characterized in that, The negative or positive electrolyte is an inert sodium or potassium salt solution; a salt bridge is provided between the negative and positive electrodes; the negative and positive electrolytes react together after spontaneous discharge to synthesize calcium hydroxide precipitate.

3. The electrochemical system for spontaneously discharging and synthesizing calcium hydroxide according to claim 1, characterized in that, A cation exchange membrane and an anion exchange membrane are sequentially arranged between the negative and positive electrodes, dividing the electrochemical system into three chambers: a negative electrode chamber, an intermediate chamber, and a positive electrode chamber. Driven by kinetics, the Ca in the negative electrode electrolyte... 2+ OH- in the positive electrode electrolyte passes through the cation exchange membrane. - Passing through the anion exchange membrane, the two converge in the intermediate chamber to form a calcium hydroxide precipitate.

4. The electrochemical system for spontaneously discharging-driven synthesis of calcium hydroxide according to any one of claims 1 to 3, characterized in that, The hydrogen-containing gas is industrial by-product hydrogen.

5. The electrochemical system for spontaneously discharging and synthesizing calcium hydroxide according to claim 4, characterized in that, The hydrogen-containing gas is dry gas from petrochemical cracking or coke oven gas.

6. The electrochemical system for spontaneously discharging-driven synthesis of calcium hydroxide according to any one of claims 1 to 3, characterized in that, The hydrogen-containing gas is produced by electrolyzing water using surplus renewable energy.

7. The electrochemical system for spontaneously discharging-driven synthesis of calcium hydroxide according to any one of claims 1 to 3, characterized in that, The NiOOH electrode is the positive electrode obtained by charging a retired nickel-metal hydride battery.

8. An electrochemical system for the spontaneous discharge-driven synthesis of calcium hydroxide according to any one of claims 1 to 3, characterized in that, The NiOOH electrode is a NiOOH electrode from a decommissioned electrochemical wastewater treatment system.

9. An electrochemical system for the spontaneous discharge-driven synthesis of calcium hydroxide according to any one of claims 1 to 3, characterized in that, The gas diffusion electrode is obtained by mixing and grinding Pt / C powder, carbon black and PTFE solution in a certain proportion into a slurry, uniformly coating it on hydrophobic carbon cloth, drying it and pressing it.

10. A method for synthesizing calcium hydroxide driven by spontaneous discharge, characterized in that, include: Electrically connecting the positive and negative electrodes of the electrochemical system as described in any one of claims 1 to 9, the electrochemical system undergoes spontaneous discharge; after the spontaneous discharge, the Ca in the negative electrode electrolyte... 2+ and OH in the positive electrode electrolyte - The reaction synthesizes calcium hydroxide.

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