An electrode assembly and capacitive deionization method for an asymmetric polarized capacitor deionization device

Abstract

This invention relates to the field of water treatment technology, specifically to an electrode assembly and a capacitive deionization method for an asymmetric polarized capacitive deionization device. The electrode assembly includes: a cathode unit whose active material is MOF-derived carbon obtained by co-heating MOF-5 with KCl and then washing with water; and an anode unit whose active material is KMnO4-modified MOF-derived carbon. This invention also provides a deionization method using this device. Through asymmetric polarization design, this invention allows the two electrodes to operate at their respective optimal voltages (1.4V and 1.2V), which is beneficial for fully utilizing the performance advantages of the two electrode materials. Furthermore, the device has a simple structure and low modification costs.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to an electrode assembly of an asymmetric polarized capacitor deionization device and a capacitor deionization method. Background Technology

[0002] Capacitive deionization (CDI) technology achieves desalination by applying a voltage to a pair of parallel electrodes, causing cations and anions in the water to migrate towards the positive and negative electrodes respectively and be adsorbed onto the porous electrode surfaces. A typical CDI device usually consists of a current collector, electrode plates, an ion exchange membrane, and a septum. The water to be treated flows through the channel between the positive and negative electrodes, and ion separation is completed under the action of an electric field. The desalination performance of a CDI device largely depends on the characteristics of the electrode materials. An ideal electrode material should have a high specific surface area, a reasonable pore size distribution, good conductivity, and electrochemical stability. In recent years, MOF-derived carbon materials have become an important research direction for CDI electrode materials due to their high specific surface area, controllable pore structure, and good electrochemical performance. However, on the one hand, existing CDI devices mostly adopt a symmetrical structure, that is, the positive and negative electrodes use the same electrode materials and are subjected to the same operating voltage. In such symmetrical CDI devices, if a single voltage is applied uniformly, it is difficult to ensure that two electrode materials with different optimal operating voltages are simultaneously in their optimal operating conditions, limiting the overall desalination performance of the system. On the other hand, although there are reports on asymmetric CDI devices and asymmetric electrode configurations, how to achieve differentiated power supply and balance the optimal operating conditions of the two electrodes in the same CDI device for two electrode materials with different optimal operating voltages still requires further research. Chinese patent literature (application number 202210311020.5) discloses a MOFs-derived porous carbon electrocatalyst, its preparation method, and its application. It employs multi-component molten salt systems such as KCl / KBr, KCl / LiCl, and KCl / NaCl, aiming to protect the catalyst precursor with molten salt for carbonization to prevent structural collapse and facilitate electrocatalytic applications. However, the porous materials prepared in this literature are mainly geared towards electrocatalytic applications and are not suitable for CDI electrode materials, especially showing poor performance in asymmetric CDI devices. Summary of the Invention

[0003] The purpose of this invention is to provide an electrode assembly and a capacitor deionization method for an asymmetric polarized capacitor deionization device, thereby solving the problem that existing symmetric CDI devices cannot simultaneously adapt to two electrode materials with different optimal operating voltages. To achieve the above objective, this invention provides the following technical solution:

[0004] An electrode assembly for an asymmetric polarized capacitor deionization device includes:

[0005] (1) A cathode unit comprising a cathode current collector and a cathode electrode sheet attached thereto, wherein the active material of the cathode electrode sheet is MOF-5 derived carbon; the preparation method of MOF-5 derived carbon includes: grinding and mixing MOF-5 and KCl at a mass ratio of 1:1 until uniform, and then heating at 5℃·min - ¹The product was calcined at 800℃ for 2 hours (the product obtained by calcination at 800℃ among 500~900℃ has the best performance). After cooling to room temperature, KCl was removed by washing with water. During the high-temperature carbonization process, the KCl molten salt occupies space and forms micropores, which are removed by washing with water, thus enriching the pore structure of MOF-5 derived carbon and increasing the specific surface area.

[0006] (2) An anode unit, comprising an anode current collector and an anode electrode sheet attached thereto, wherein the active material of the anode electrode sheet is KMnO4 modified MOF-derived carbon; the preparation method of KMnO4 modified MOF-derived carbon includes the following steps: first, MOF-5 is heated at 5℃·min - ¹The temperature was increased to 800℃ and calcined for 2 hours. After cooling to room temperature, CMOF-8 was obtained. CMOF-8 was then immersed in a 0.01 mol / L potassium permanganate aqueous solution and stirred at room temperature for full adsorption. The resulting mixture was then washed until the pH of the filtrate was constant. The solid product was then separated by filtration through a 0.45 μm microporous membrane and dried to obtain KMnO4 modified MOF-derived carbon.

[0007] When in use, the cathode unit is subjected to a working voltage of 1.4V and the anode unit is subjected to a working voltage of 1.2V.

[0008] Preferably, the cathode unit further includes a cation exchange membrane covering the surface of the cathode electrode sheet; the anode unit further includes an anion exchange membrane covering the surface of the anode electrode sheet.

[0009] Preferably, both the cathode electrode and the anode electrode are made by coating graphite paper with a mixture of active material, acetylene black and polytetrafluoroethylene in a mass ratio of 8:1:1.

[0010] The present invention also provides a method for desalination using the above-described apparatus, comprising:

[0011] (1) The water to be treated is introduced into the fluid channel of the device;

[0012] (2) Apply a first working voltage to the cathode unit and apply a second working voltage different from the first working voltage to the anode unit;

[0013] (3) Under the action of an electric field, cations in the water are adsorbed onto the cathode electrode plate, and anions are adsorbed onto the anode electrode plate.

[0014] Preferably, the first operating voltage is 1.4V and the second operating voltage is 1.2V.

[0015] Preferably, the inlet flow rate is 5 mL·min⁻¹.

[0016] Compared with the prior art, the present invention achieves the following beneficial effects:

[0017] (1) The present invention uses asymmetric polarization design to make KCl modified electrode and KMnO4 modified electrode serve as cathode and anode respectively, and operate at their respective preferred working voltages (1.4V and 1.2V), avoiding the problem of one electrode deviating from the optimal working condition when the same voltage is supplied, which is conducive to giving full play to the performance advantages of the two electrode materials.

[0018] (2) The KCl-modified and KMnO4-modified MOF-derived carbon electrode materials used in this invention have been optimized through comparative experiments: the preferred mass ratio of KCl is 1:1, and the preferred concentration of KMnO4 is 0.01 mol / L; at 50 mg·L -1 Under NaCl solution conditions, the two types of electrodes have ion removal rates of 98.9% and 99.1%, respectively, and the material performance is reliable. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the asymmetric polarization CDI device of the present invention.

[0020] Figure 2 Conductivity curves of KCl-modified MOF-derived carbon electrodes at different voltages to demonstrate their desalination performance.

[0021] Figure 3 Conductivity curves of the desalination effect of KMnO4-modified MOF-derived carbon electrode under different voltages. Detailed Implementation

[0022] To further understand the purpose, content, and advantages of this invention, specific embodiments of the invention are described in detail below. However, these embodiments are not limited to the examples described below and should be freely combined according to actual circumstances. The endpoints and values ​​of the ranges disclosed herein are not limited to the precise ranges and values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0023] The present invention will be further described in detail below with reference to the embodiments:

[0024] In some embodiments, the present invention provides an electrode assembly for an asymmetric polarized capacitor deionization device, comprising:

[0025] (1) A cathode unit comprising a cathode current collector and a cathode electrode sheet attached thereto, wherein the active material of the cathode electrode sheet is MOF-5 derived carbon; the preparation method of MOF-5 derived carbon includes: grinding and mixing MOF-5 and KCl at a mass ratio of 1:1 until uniform, and then heating at 5℃·min - ¹The temperature was rapidly increased to 800℃ for 2 hours for carbonization. After cooling to room temperature, KCl was removed by washing with water to obtain MOF-5-derived carbon active material, abbreviated as KCl-1. During the high-temperature carbonization process, KCl molten salt occupies space and forms micropores, which are removed by washing with water, thus enriching the pore structure of MOF-5-derived carbon and increasing its specific surface area.

[0026] (2) An anode unit, comprising an anode current collector and an anode electrode sheet attached thereto, wherein the active material of the anode electrode sheet is KMnO4 modified MOF-derived carbon; the preparation method of KMnO4 modified MOF-derived carbon includes the following steps: first, MOF-5 is heated at 5℃·min - ¹The temperature was increased to 800℃ and calcined for 2 hours. After cooling to room temperature, CMOF-8 was obtained. CMOF-8 was then immersed in a 0.01 mol / L potassium permanganate aqueous solution and stirred at room temperature for full adsorption. The resulting mixture was then washed until the pH of the filtrate was constant. The solid product was then separated by filtration through a 0.45 μm microporous membrane. The solid product was dried to obtain a black powder, which is KMnO4 modified MOF-derived carbon, denoted as KMnO4-0.01.

[0027] In the above embodiments, after modifying MOF-5 derived carbon in two different ways, it was found that: after KCl was involved in high-temperature co-heating treatment, the specific surface area and micropore volume of the material could be significantly improved; after CMOF-8 was impregnated with KMnO4, more oxygen-containing functional groups could be introduced on the material surface and manganese-containing active components could be formed, thereby improving hydrophilicity and capacitance performance.

[0028] The product with the best performance is obtained by calcination at 800℃ among the carbonization temperatures of 500~900℃ (e.g., 500℃, 600℃, 700℃, 750℃, 800℃, 850℃, 900℃) in step (1).

[0029] The following description uses more specific embodiments:

[0030] Example 1

[0031] An electrode assembly for an asymmetric polarized capacitor deionization device includes:

[0032] A cathode unit includes a cathode current collector and a cathode electrode sheet attached thereto, wherein the active material of the cathode electrode sheet is MOF-derived carbon;

[0033] An anode unit includes an anode current collector and an anode electrode sheet attached thereto, wherein the active material of the anode electrode sheet is KMnO4 modified MOF-derived carbon.

[0034] Asymmetric polarized capacitor deionization device, configuration diagram as shown in the figure. Figure 1 As shown, it also includes:

[0035] A spacer layer (washer 1, washer 2) is disposed between the cathode unit and the anode unit to form a fluid channel; a power supply unit is used to apply different operating voltages to the cathode unit and the anode unit respectively. The cathode current collector is a titanium plate 2, the cathode electrode sheet is carbon-containing graphite paper 2, the anode current collector is a titanium plate 1, and the anode electrode sheet is carbon-containing graphite paper 1.

[0036] The preparation of the active material for the cathode electrode sheet includes the following steps:

[0037] (1) Preparation of MOF-5: MOF-5 was prepared by solvothermal method. 5.95 g of Zn(NO3)2·6H2O and 1.15 g of H2BDC were weighed and added to 80 mL of DMF. After stirring for 30 min, the mixture was transferred to a 100 mL polytetrafluoroethylene liner and placed in a reaction vessel. The reaction was carried out at 120 °C for 14 h. After the reaction was completed, the mixture was cooled to room temperature, transferred to a centrifuge tube, centrifuged at 4000 rpm for 15 min, washed three times with DMF and three times with ethanol, filtered, and dried at 80 °C for 12 h to obtain MOF-5.

[0038] (2) Preparation of cathode active material: MOF-5 and KCl were ground and mixed evenly at mass ratios of 0.5:1, 1:1, and 2:1, respectively. The mixtures were placed in quartz boats and heated to 800℃ at a rate of 5℃·min⁻¹ in a tube furnace under argon protection for high-temperature carbonization for 2 hours, and then naturally cooled to room temperature. The carbonization product was repeatedly washed and filtered with deionized water until the conductivity of the filtrate was constant (residual KCl was removed after repeated washing and filtration with deionized water); the washed product was dried at 70℃ for 6 hours to obtain a black powder, which is MOF-derived carbon, denoted as KCl-1.

[0039] The preparation of KMnO4-modified MOF-derived carbon as the active material of the anode electrode sheet includes the following steps:

[0040] (1) Preparation of CMOF-8: First, MOF-5 was heated at 5℃·min - ¹The temperature was increased to 800℃ and calcined for 2 hours, then cooled to room temperature to obtain CMOF-8.

[0041] (2) Preparation of KMnO4-modified MOF-derived carbon: Take 5g of CMOF-8 prepared in step (1) and place it in 50mL of 0.005 mol / L, 0.01mol / L and 0.03 mol / L potassium permanganate aqueous solutions, respectively. Stir at 25℃ for 12h. Then wash the resulting mixture with deionized water 5 times until the pH of the filtrate is constant. After filtering through a 0.45μm microporous membrane, dry the solid product at 70℃ for 6h to obtain a black powder, which is KMnO4.

[0042] Modified MOF-derived carbons were designated as KMnO4-0.005, KMnO4-0.01, and KMnO4-0.03, respectively. Comparative experiments showed that the sample obtained under the 0.01 mol / L condition exhibited the best performance; therefore, KMnO4-0.01 was selected as the preferred anode active material. The role of KMnO4 modification is to introduce oxygen-containing functional groups and form manganese-containing active components on the material surface, thereby improving hydrophilicity and capacitance performance.

[0043] The cathode electrode (KCl-1 electrode) and the anode electrode (KMnO4-0.01 electrode) are made by coating graphite paper with a mixture of the above-mentioned active materials, acetylene black and polytetrafluoroethylene in a mass ratio of 8:1:1.

[0044] Example 2

[0045] The assembly and testing conditions for the asymmetric polarization CDI device are as follows:

[0046] Assemble an asymmetric polarized CDI device, the structure of which is as follows: Figure 1 As shown, the device consists of a cathode unit, an anode unit, and gaskets 1 and 2 positioned between them. The cathode unit includes a titanium plate current collector, a KCl-1 electrode sheet prepared in Example 1, and a cation exchange membrane covering the surface of the electrode sheet; the anode unit includes a titanium plate current collector, a KMnO4-0.01 electrode sheet prepared in Example 1, and an anion exchange membrane covering the surface of the electrode sheet; gaskets 1 and 2 are provided with fluid channels for the inflow and outflow of the water to be treated. The positive terminal of the power supply unit is electrically connected to titanium plate 1, and the negative terminal is electrically connected to titanium plate 2. The power supply unit can be independently set with two outputs to apply different working voltages to the cathode unit and the anode unit respectively. During the experiment, NaCl solution was used as the water to be treated. The inflow rate was controlled by a peristaltic pump, and the conductivity change of the solution was monitored in real time by a conductivity meter.

[0047] Example 3:

[0048] Under the apparatus and test conditions described in Example 2, the operating voltage and inlet water flow rate were optimized.

[0049] (1) The preferred working voltage is 50 mg·L -¹ NaCl solution and 5 mL·min - ¹The effect of different operating voltages on desalination efficiency was investigated under the condition of influent flow rate. The results showed that the KCl-1 electrode reached adsorption equilibrium at 1.4 V in 205 min, and the conductivity decreased to 1.6 μS·cm. - ¹; Under the same reaction time, the conductivity at 1.0 V is 74.7 μS·cm. - ¹, therefore, the preferred operating voltage for KCl-1 is 1.4 V. The conductivity of the KMnO4-0.01 electrode at 1.2 V decreased to 1.4 μS·cm⁻¹ and tended to equilibrium after 190 min, while the conductivity at 1.0 V and 1.4 V for the same time was 105.7 μS·cm⁻¹ and 19.2 μS·cm⁻¹, respectively. Therefore, the preferred operating voltage for KMnO4-0.01 is 1.2 V.

[0050] (2) Optimal Inlet Flow Rate: Under the aforementioned optimal operating voltage conditions, four inlet flow rates of 1, 5, 10, and 20 mL·min⁻¹ were further compared. The results showed that when the inlet flow rate was 5 mL·min⁻¹… - ¹At this time, the conductivity of the KCl-1 electrode decreased to 1.6 μS·cm after 205 min. - ¹; When the flow rate is increased to 20 mL·min - At time ¹, the conductivity increased to 51.8 μS·cm at the same time. - ¹; When the flow rate is reduced to 1 mL·min - At ¹, the conductivity was 79.5 μS·cm at 205 min. - ¹. This indicates that excessively high flow rates shorten the contact time between ions and the electrode, while excessively low flow rates are detrimental to mass transfer. Therefore, 5 mL·min⁻¹ is the preferred influent flow rate. In summary, the KCl-1 electrode and the KMnO4-0.01 electrode exhibit optimal desalination performance at 1.4 V and 1.2 V, respectively, which is a key basis for the asymmetric polarization design adopted in this application.

[0051] Example 4: Operation of an asymmetric polarized CDI device

[0052] Under the apparatus conditions described in Example 2, 50 mL of a solution with a concentration of 50 mg·L⁻¹ was taken. -A NaCl solution¹ was used as the water to be treated and introduced into the fluid channel of the device at a flow rate of 5 mL·min⁻¹ using a peristaltic pump. A working voltage of 1.4 V was applied to the cathode unit, and a working voltage of 1.2 V was applied to the anode unit. Under the influence of the electric field, Na⁺ in the solution passed through the cation exchange membrane (CMI-7000S, 5×9 cm, Hangzhou Huamo Technology Co., Ltd.) and was adsorbed onto carbon-containing graphite paper 2, while Cl⁻ passed through the anion exchange membrane (AMI-7001S, 5×9 cm, Hangzhou Huamo Technology Co., Ltd.) and was adsorbed onto carbon-containing graphite paper 1. The conductivity of the solution was monitored in real time using a conductivity meter. When the conductivity dropped to a stable value, the desalination process was completed. According to the applicant's experimental results, under these operating conditions, the KCl-1 electrode and the KMnO4-0.01 electrode exhibited ion removal rates of 98.9% and 99.1%, respectively.

[0053] Example 5: Operation under high-concentration influent conditions

[0054] The difference between this embodiment and Example 4 is that the water to be treated is a 750 mg·L⁻¹ NaCl solution. Under this concentration condition, the salt adsorption capacity of the KCl⁻¹ electrode and the KMnO₄⁻⁰.⁻¹ electrode can reach 15.8 mg·g⁻¹, respectively. - ¹ and 16.2 mg·g - ¹. During operation, the voltage can be adjusted appropriately near the optimal operating voltage based on the actual influent concentration to achieve better desalination results.

[0055] Specific capacitance was determined using cyclic voltammetry. A three-electrode system was employed, with a carbon-containing electrode sheet as the working electrode, a foil electrode as the counter electrode, and a saturated calomel electrode as the reference electrode. The voltage was scanned within a potential window of -1 to 2.5 V, and the specific capacitance was calculated based on the area enclosed by the cyclic voltammetry curves. The specific capacitance of different samples at a scan rate of 1 mV·s⁻¹ was compared and analyzed.

[0056] Comparative experiments show that:

[0057] In the preparation of MOF-derived carbon, an active material in carbon-containing graphite paper 2 as described in Example 1, the product obtained at 800°C exhibited the best performance when the calcination temperature varied within the range of 500–900°C. Furthermore, the applicant conducted a comparative experiment: first, MOF-5 was calcined at 5°C / min... - ¹The temperature was increased to 800℃ and calcined for 2 h, then cooled to room temperature to obtain CMOF-8, which has a specific surface area of ​​approximately 300.67 m²·g. -¹; KCl and CMOF-8 were then mixed evenly at a mass ratio of 1:1, and calcined at 800℃ for 2 h at a rate of 5℃·min⁻¹. After cooling to room temperature, residual KCl was removed by washing with water. The results showed that the performance of the active material prepared by the above route was significantly lower than that of the KCl-1 sample in Example 1.

[0058] In the preparation of MOF-derived carbon, the active material of carbon-containing graphite paper 2 in Example 1, the mass ratio of MOF-5 to KCl was modified to 0.5 and 2, respectively, resulting in samples KCl-0.5 and KCl-2. Comparative experiments showed that in the preparation of the cathode active material described in Example 1, when the mass ratio of MOF-5 to KCl was 0.5:1, 1:1, and 2:1, the resulting samples were denoted as KCl-0.5, KCl-1, and KCl-2, respectively. (At 1 mV·s) - At the specified scan rate, the specific capacitances of the three samples were 187.60, 231.13, and 152.72 F·g⁻¹, respectively, with KCl-1 exhibiting the highest capacitance. Furthermore, BET testing results showed that KCl-1 had a specific surface area of ​​933.33 m²·g⁻¹, an average pore size of 7.03 nm, a micropore volume of 0.13 cm³·g⁻¹, and a mesopore volume of 1.52 cm³·g⁻¹, significantly superior to the unmodified CMOF-8's 300.67 m²·g⁻¹, 8.25 nm, 0.03 cm³·g⁻¹, and 0.59 cm³·g⁻¹, respectively. Therefore, a MOF-5 to KCl mass ratio of 1:1 was determined to be the optimal ratio.

[0059] In the preparation of the anolyte active material described in Example 1, when the KMnO4 concentrations were 0.005 mol / L, 0.01 mol / L, and 0.03 mol / L, the resulting samples were denoted as KMnO4-0.005, KMnO4-0.01, and KMnO4-0.03, respectively. At a scan rate of 1 mV·s⁻¹, the specific capacitances of the three groups of samples were 210.53, 648.01, and 150.57 F·g⁻¹, respectively, with KMnO4-0.01 exhibiting the highest capacitance. Furthermore, BET test results showed that KMnO4-0.01 had a specific surface area of ​​770.93 m²·g⁻¹, an average pore size of 5.49 nm, a micropore volume of 0.05 cm³·g⁻¹, and a mesopore volume of 1.01 cm³·g⁻¹. Therefore, 0.01 mol / L was determined to be the preferred modification concentration.

[0060] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention. It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

Claims

1. An electrode assembly for an asymmetric polarized capacitor deionization device, characterized in that, include The cathode unit comprises a cathode current collector and a cathode electrode plate attached thereto. The active material of the cathode electrode plate is MOF-derived carbon. The preparation method of MOF-5-derived carbon includes: grinding and mixing MOF-5 and KCl at a mass ratio of 1:1 until uniform, and then heating at 5℃·min. - ¹The temperature was increased to 800℃ and carbonized for 2 hours. After cooling to room temperature, KCl was removed by washing with water to obtain MOF-5-derived carbon active material. The anode unit comprises an anode current collector and an anode electrode plate attached thereto. The active material of the anode electrode plate is KMnO4-modified MOF-derived carbon. The preparation method of KMnO4-modified MOF-derived carbon includes the following steps: first, MOF-5 is heated at 5℃·min - ¹The temperature was increased to 800℃ and calcined for 2 hours. After cooling to room temperature, CMOF-8 was obtained. CMOF-8 was then immersed in a 0.01 mol / L KMnO4 aqueous solution and stirred at room temperature for full adsorption. The resulting mixture was then washed until the pH of the filtrate was constant. The solid product was then separated by filtration through a 0.45 μm microporous membrane. The solid product was dried to obtain a black powder, which is KMnO4 modified MOF derived carbon. When in use, the cathode unit is subjected to a working voltage of 1.4V and the anode unit is subjected to a working voltage of 1.2V.

2. The electrode assembly of the asymmetric polarized capacitor deionization device according to claim 1, characterized in that, The cathode unit further includes a cation exchange membrane covering the surface of the cathode electrode sheet; the anode unit further includes an anion exchange membrane covering the surface of the anode electrode sheet.

3. The electrode assembly of the asymmetric polarized capacitor deionization method apparatus according to claim 1, characterized in that, The cathode electrode is made by coating graphite paper with a mixture of MOF-5 derived carbon, acetylene black and polytetrafluoroethylene in a mass ratio of 8:1:1; the anode electrode is made by coating graphite paper with a mixture of KMnO4 modified MOF derived carbon, acetylene black and polytetrafluoroethylene in a mass ratio of 8:1:

1.

4. A capacitor deionization method, characterized in that, Based on a capacitive deionization device, which employs the electrode assembly of the capacitive deionization device according to any one of claims 1 to 3, the process includes the following steps: A working voltage of 1.4V is applied to the cathode unit, and a working voltage of 1.2V is applied to the anode unit. The influent flow rate of the water to be treated is controlled at 5 mL / min. -1 .

5. The capacitor deionization method according to claim 4, characterized in that, The solute in the water to be treated is sodium chloride, and the concentration of sodium chloride is 50 mg·L⁻¹. -1 .

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