MEMS current-mode gas sensor

Abstract

The application belongs to the technical field of gas sensors, and particularly relates to a MEMS current type gas sensor. In use, the Nafion composite film is placed in the first hole, so that one side surface of the Nafion composite film is in contact with the surface of the metal electrode, the other side surface is in contact with the electrode contact, and the metal electrode is led out from the second hole; water is injected into the water injection hole of the array layer, the water flows through the electrode layer and is injected into the water tank assembly, and a preset current is applied to the metal electrode and the electrode contact located on the two sides of the Nafion composite film, so that the Nafion composite film reacts with water and the gas to be detected, and the detection of the gas to be detected is realized. The device designed in the application has a small volume, and the integration and portability of the MEMS gas sensor are improved.

Description

Technical Field

[0001] This invention belongs to the field of gas sensor technology, and particularly relates to a MEMS current-type gas sensor. Background Technology

[0002] Various gases exist in many fields, including daily life, healthcare, and industrial production. Some of these gases, such as H2S, H2, NH3, CO, and SO2, can harm human health when they reach certain concentrations. Inhaling hydrogen sulfide (H2S) can affect the eyes, respiratory system, and nervous system, and in severe cases, cause dizziness or even death. H2S mixes readily with air to form an explosive gas. Hydrogen (H2) is colorless, odorless, flammable, and explosive. It is highly ignitable when its volume fraction in air reaches 4.5%–75.6%, and its small and light molecules make it prone to leakage. Waste gases emitted into the atmosphere from industrial production include sulfur dioxide (SO2), ammonia (NH3), carbon monoxide (CO), and nitrogen dioxide (NO2), which are major components of pollutants. These pollutants not only damage the ecological environment, affect the climate, and produce smog and photochemical smog, but also harm human health, affecting vision and respiration and causing various diseases. Therefore, real-time safety monitoring of these gases is of great significance for effectively protecting people's lives and health and maintaining a beautiful living environment.

[0003] In recent years, with the rapid development of microelectronics technology, the development of microelectromechanical systems (MEMS) gas sensors has provided new ideas for real-time gas detection in complex environments. MEMS gas sensors utilize silicon wafer micromachining technology, offering advantages such as small size and low power consumption. However, single-component MEMS current-type gas sensors suffer from cross-sensitivity, making accurate measurement of multi-component gases impossible. To address this issue, most current MEMS gas sensors employ metal-oxide-semiconductor (MOS) gas sensors, requiring isothermal testing at high temperatures to acquire data. Prolonged operation can lead to baseline drift, resulting in a decrease in the gas-sensing performance of the MEMS gas sensor. Summary of the Invention

[0004] In view of this, the present invention aims to provide a MEMS current-type gas sensor to solve the problems of existing sensor arrays composed of multiple unit sensors, which are large in size, difficult to carry, and suffer from baseline drift due to high-temperature isothermal testing and prolonged operation, thus degrading the gas sensing performance of MEMS gas sensors. The present invention achieves qualitative and quantitative detection of multi-component gases and can be used at room temperature, improving the stability of MEMS gas sensors. The device designed in this invention is small in size, improving the integration and portability of MEMS gas sensors. This invention combines a current-type gas sensor with MEMS technology, requiring no heating or bias voltage, and generates direct current by converting the chemical energy of the detected gas into electrical energy, maintaining stable gas sensing performance even under long-term operation.

[0005] To achieve the above objectives, the technical solution created by this invention is implemented as follows:

[0006] A MEMS current-type gas sensor includes, from top to bottom, a protective shell, an array layer, an electrode layer, a water tank assembly, and a fixed shell. The protective shell and the fixed shell are fixedly connected, and a receiving cavity is provided between them. Electrode contacts are provided on the surface of the protective shell facing the array layer. The receiving cavity is used to house the array layer, the electrode layer, and the water tank assembly. Water injection holes are provided at both ends of the array layer, both ends of the electrode layer, and both ends of the water tank assembly. The water injection holes at the same end of the array layer, the electrode layer, and the water tank assembly are interconnected. The electrode layer includes a silicon plate and multiple metal electrodes located on the surface of the silicon plate facing the array layer. The multiple metal electrodes are arranged in an array. The electrode layer has multiple through holes, each corresponding to a metal electrode. Each through hole penetrates the first end of the corresponding metal electrode and penetrates the silicon plate located below the first end. The array layer has a first hole and a second hole. The first hole corresponds to the through hole of the metal electrode, and the second hole corresponds to the second end of the metal electrode.

[0007] In use, the Nafion composite membrane is placed in the first hole, with one side of the Nafion composite membrane in contact with the surface of the metal electrode and the other side in contact with the electrode contact. The metal electrode is led out from the second hole. Water is injected into the water injection hole of the array layer, and the water flows through the electrode layer into the water tank assembly. A preset current is applied to the metal electrodes and electrode contacts located on both sides of the same Nafion composite membrane, so that the Nafion composite membrane reacts with the water and the gas to be detected, thereby realizing the detection of the gas to be detected.

[0008] Furthermore, the water tank assembly includes a water channel structure layer and a silicon-sealed bottom layer that are adapted to each other. The water channel structure layer is engraved with a hollow water channel pattern, and one side surface of the water channel structure layer and the silicon-sealed bottom layer are bonded together to form a water storage cavity.

[0009] Furthermore, the first end and the second end of the metal electrode are distributed at both ends of the metal electrode along the length direction of the metal electrode.

[0010] Furthermore, the protective housing is provided with gas exchange holes, and the number of gas exchange holes is greater than the number of electrode contacts. The protective housing and the fixed housing are fixedly connected by bolts.

[0011] The MEMS current-type gas sensor also includes a protective cover, which is connected to the protective housing by an interference fit through a mating hole.

[0012] Furthermore, the Nafion composite membrane includes a Nafion membrane and metallic functional materials loaded on both sides of the Nafion membrane.

[0013] Furthermore, when the gas to be detected is hydrogen, the metallic functional material is composed of carbon fibers and palladium supported on the carbon fibers.

[0014] When the gas to be detected is sulfur dioxide, the metallic functional material is composed of carbon fiber and palladium supported on carbon fiber, or the metallic functional material is composed of copper and palladium supported on copper.

[0015] When the gas to be detected is nitrogen dioxide, the metallic functional material is composed of carbon fiber and molybdenum disulfide supported on the carbon fiber.

[0016] When the gas to be tested is ammonia, the metallic functional material is composed of carbon fiber and platinum supported on the carbon fiber, or rhodium and platinum supported on the rhodium.

[0017] Furthermore, the electrode contacts on the protective housing are configured to correspond to the Nafion composite film;

[0018] The water level in the water tank assembly is lower than the surface of the Nafion composite membrane that is in contact with the metal electrode.

[0019] Furthermore, the metal material used for the metal electrode is gold or copper.

[0020] Furthermore, the thickness of the sink assembly is greater than 3cm.

[0021] Furthermore, the array layer and the electrode layer are spaced at least 3 cm apart, and the electrode layer and the water tank assembly are spaced at least 3 cm apart.

[0022] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0023] This invention creates a MEMS current-type gas sensor that focuses on the qualitative and quantitative detection of multi-component gases, achieving system miniaturization and integration. Based on microelectronics process technology, an array-type MEMS gas sensor chip is designed to significantly reduce the size and power consumption of the gas-sensitive detection system. Furthermore, through the flexible design of the gas-sensitive materials on the array-type MEMS gas sensor chip, it exhibits differentiated response characteristics for different target gases, providing a core component for intelligent gas detection equipment in various future application scenarios. Simultaneously, the current-type gas sensor has the advantages of high sensitivity, good specificity, fast response speed, and zero power consumption. The gas sensor proposed in this invention can handle real-time gas monitoring in most environmental spaces. Attached Figure Description

[0024] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0025] Figure 1 This is a schematic diagram of the structure of the protective cover and the protective shell after interference fit according to an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram of the structure after removing the protective outer shell as described in an embodiment of the present invention;

[0027] Figure 3 A schematic diagram of the structure of the protective shell facing the array layer side surface as described in an embodiment of the present invention;

[0028] Figure 4 A schematic diagram of the array layer structure described in the embodiment of the present invention;

[0029] Figure 5 A schematic diagram of the structure of the electrode layer described in the embodiment of the present invention;

[0030] Figure 6 This is a schematic diagram of the waterway structure layer described in an embodiment of the present invention.

[0031] Explanation of reference numerals in the attached drawings: 1. Protective cover; 2. Protective outer shell; 3. Fixed outer shell; 4. Array layer; 4-1. First hole; 4-2. Second hole; 5. Water injection hole; 6. Through hole; 7. Electrode contact; 8. Gas exchange hole; 9. Electrode layer; 10. Metal electrode; 11. Waterway structure layer; 12. Waterway pattern. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0033] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0034] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0035] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0036] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0037] like Figures 1-6As shown, this invention provides a MEMS current-type gas sensor, comprising, from top to bottom, a protective housing 2, an array layer 4, an electrode layer 9, a water tank assembly, and a fixed housing 3. The protective housing 2 and the fixed housing 3 are fixedly connected, and a receiving cavity is provided between them. Electrode contacts 7 are provided on the surface of the protective housing 2 facing the array layer 4. The receiving cavity is used to house the array layer 4, the electrode layer 9, and the water tank assembly. Water injection holes are provided at both ends of the array layer 4, both ends of the electrode layer 9, and both ends of the water tank assembly. The water injection holes 5 at the same end of the array layer 4, the electrode layer 9, and the water tank assembly are interconnected. The electrode layer 9 includes a silicon plate and a plurality of metal electrodes 10 located on the surface of the silicon plate facing the array layer 4. The plurality of metal electrodes 10 are arranged in an array. The electrode layer 9 has a plurality of through holes 6, each through hole 6 corresponding to a metal electrode 10. Each through hole 6 penetrates the first end of the corresponding metal electrode 10 and penetrates the silicon plate located below the first end. The array layer 4 is provided with a first hole 4-1 and a second hole 4-2 (both the first hole 4-1 and the second hole 4-2 are through holes). The first hole 4-1 is provided corresponding to the through hole 6 of the metal electrode 10, and the second hole 4-2 is provided corresponding to the second end of the metal electrode 10.

[0038] In use, the Nafion composite membrane is placed in the first hole 4-1, so that one side of the Nafion composite membrane is in contact with the surface of the metal electrode 10 and the other side is in contact with the electrode contact 7, and the metal electrode 10 is led out from the second hole 4-2; water is injected into the water injection hole 5 of the array layer 4, and the water flows through the electrode layer 9 into the water tank assembly, and a preset current is applied to the metal electrode 10 and the electrode contact 7 located on both sides of the same Nafion composite membrane, so that the Nafion composite membrane reacts with the water and the gas to be detected, thereby realizing the detection of the gas to be detected.

[0039] It should be noted that this invention discloses a MEMS current-type gas sensor capable of simultaneously monitoring multiple gases. The gas sensor, based on a MEMS structure and a current-type gas sensor, includes a sensor housing cavity and an internal gas detection chip. The internal gas detection chip portion houses multiple Nafion membranes (a type of ion-exchange membrane based on perfluorosulfonic acid resin) made of different materials. Several different sensor electrode materials are employed to achieve specific real-time detection and quantitative analysis of multiple gases. The entire gas sensor device can be used at room temperature, solving the cross-sensitivity problem of single sensors and enabling the sensor chip to exhibit differentiated response characteristics for different target gases. This provides a core component for intelligent gas detection equipment in various future application scenarios.

[0040] Furthermore, this invention assembles multiple Nafion composite films into a sensor array. By changing the type of gas-sensitive material, the differences between the individual sensor units are enhanced, thereby improving the selective recognition capability for different gases. Therefore, multiple high-performance, high-stability sensor units with different high gas selectivities are integrated into the array. The difference in electrode materials mainly lies in the different metal materials supported on the carbon fibers, achieving differentiated responses to different gases. For example, Pd / C-Nafion material has high selectivity for H2, MoS2 / C-Nafion has high selectivity for NO2, Pd-Cu / C-Nafion has high selectivity for SO2, and Pt-Rh / C-Nafion has high selectivity for NH3, etc.

[0041] In some embodiments, the sink assembly includes a water channel structure layer 11 and a silicon-sealed underlayer that are adapted to each other. The water channel structure layer 11 is etched with a hollow water channel pattern 12. One side surface of the water channel structure layer 11 and the silicon-sealed underlayer are bonded together to form a water storage cavity.

[0042] Furthermore, the water channel structure layer 11 is made of polymer material by laser cutting. The water channel structure layer 11 and one side surface of the silicon-sealed bottom layer are bonded by laser cutting and glue connection. The water channel pattern 12 only needs to ensure that there is water directly below the through hole 6.

[0043] In some embodiments, the first end and the second end of the metal electrode 10 are distributed at both ends of the metal electrode 10 along the length direction of the metal electrode 10.

[0044] It should be noted that one side of the Nafion composite film is in contact with the surface of the metal electrode 10, and the other side is in contact with the electrode contact 7. The metal electrode 10 is led out from the second hole 4-2. The metal electrode 10 is the negative electrode, and the electrode where the electrode contact 7 is located is the positive electrode. The metal electrode 10 can be arranged in a 3×3 array, corresponding to the nine sets of the first hole 4-1 and the second hole 4-2 on the array layer 4.

[0045] In some embodiments, the protective housing 2 is provided with gas exchange holes 8, and the number of gas exchange holes 8 is greater than the number of electrode contacts 7. The protective housing 2 and the fixed housing 3 are fixedly connected by bolts.

[0046] The MEMS current-type gas sensor also includes a protective cover 1, which is connected to the protective housing 2 by means of an interference fit hole.

[0047] Furthermore, connecting holes are provided on both sides of the protective shell 2 and the fixed shell 3, and the connecting holes on the same side of the protective shell 2 and the fixed shell 3 are connected. The connecting holes are used to realize bolt fixing connection. The protective cover 1, the protective shell 2 and the fixed shell 3 are provided on one side. The positioning holes are used to realize the precise positioning of the three. The protective shell 2, the fixed shell 3 and the protective cover 1 are all made by photopolymerization printing.

[0048] In some embodiments, the Nafion composite membrane includes a Nafion membrane and a metallic functional material loaded on both sides of the Nafion membrane.

[0049] In some embodiments, when the gas to be detected is hydrogen, the metallic functional material is composed of carbon fiber and palladium supported on the carbon fiber.

[0050] When the gas to be detected is sulfur dioxide, the metallic functional material is composed of carbon fiber and palladium supported on carbon fiber, or the metallic functional material is composed of copper and palladium supported on copper.

[0051] When the gas to be detected is nitrogen dioxide, the metallic functional material is composed of carbon fiber and molybdenum disulfide supported on the carbon fiber.

[0052] When the gas to be tested is ammonia, the metallic functional material is composed of carbon fiber and platinum supported on the carbon fiber, or rhodium and platinum supported on the rhodium.

[0053] It should be noted that by using different gas-sensitive materials to fabricate the Nafion composite membrane, the sensor chip exhibits differentiated response characteristics for different target gases, enabling qualitative and quantitative detection of multiple gases.

[0054] In some embodiments, the electrode contacts 7 on the protective housing 2 are correspondingly arranged with the Nafion composite film;

[0055] The water level in the water tank assembly is lower than the surface of the Nafion composite membrane that is in contact with the metal electrode 10.

[0056] In some embodiments, the metal electrode 10 is made of gold or copper.

[0057] In some embodiments, the thickness of the sink assembly is greater than 3 cm.

[0058] In some embodiments, the array layer 4 and the electrode layer 9 are spaced at least 3 cm apart, and the electrode layer 9 and the water tank assembly are spaced at least 3 cm apart.

[0059] MEMS current-type gas sensors operate using traditional static testing methods. Wires leading from the metal electrode 10 and the corresponding electrode contact 7 are connected to the working and counter electrodes of an electrochemical workstation (CHI611C, Shanghai Instrument & Meter Co., Ltd., China). The short-circuit current of the sensor in the target gas is measured in Ampere-it curve mode without applying a bias voltage. All sensor gas sensitivity performance tests are conducted at laboratory room temperature (23-25℃) and 20%-25% relative humidity. Different concentrations of H2S, H2, SO2, NH3, CO, and NO2 are obtained by evacuating a certain volume of the corresponding dry standard gas into a 1 L glass bottle and then diluting it with laboratory air. The Nafion membrane allows protons to pass through in a hydrated state; when the sensor is exposed to the target gas, a corresponding reaction occurs near its metal electrode and the corresponding electrode contact.

[0060] The detection current of the sensor in air is defined as Ia, and the current that reaches a stable state in the target gas is defined as Ib. Due to the influence of factors such as catalyst efficiency and electrode thickness, Ib typically does not reach a stable value instantaneously, but rather gradually increases or decreases within a certain range before stabilizing. The difference between Ib and Ia is defined as the response value (ΔI), and the response / recovery time is defined as the time required to reach 90% of ΔI. The sensitivity to the sample gas is defined as the slope of the curve fitted by the gas concentration and ΔI.

[0061] Additionally, the top layer of the sensor chip used for gas detection can be a 3×3 array layer 4 (array layer 4 can be made of silicon wafer). The two holes at both ends of array layer 4 are water injection holes 5. In general silicon-based semiconductor processing, the silicon wafer is placed on a spin coater to suspend photoresist, then removed and placed on a hot plate at 120°C for 3 minutes to cure the photoresist. After completion, photolithographic patterning transfer is performed to transfer the designed mask pattern onto the silicon wafer. After photolithography, the silicon wafer is placed in a developer solution for development. After development, it is cleaned with deionized water. After cleaning, it is placed on a hot plate to dry the moisture at a temperature of 120°C for 3 minutes. The dried silicon wafer is then placed in an ICP etching device for etching to finally obtain the required array silicon holes. The second layer consists of a gold electrode layer thermally evaporated onto a silicon wafer. One end of the gold layer has a small hole to facilitate the reaction between the Nafion membrane and the underlying water molecules. The array layer 4 and electrode layer 9 are connected by laser-cut double-sided 3M adhesive. The cutting power is 7.2W, the cutting speed is 0.8, and the cutting frequency is 1 time. The third layer is the water channel structure layer 11, made by laser-cutting a polymer material. Both sides of the material are uniformly coated with 3M adhesive. The cutting power is 12W, the cutting speed is 0.8, and the cutting frequency is 1 time. To prevent sintering material from forming around the water tank edges during cutting, a layer of moistened filter paper is placed on top of the water tank layer during the cutting process. The fourth layer uses a silicon wafer as the silicon sealing base layer.

[0062] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0063] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A MEMS amperometric gas sensor, characterized by: The MEMS current-type gas sensor comprises a protective shell, an array layer, an electrode layer, a water tank assembly and a fixing shell which are sequentially arranged from top to bottom, wherein the protective shell and the fixing shell are fixedly connected, and a containing cavity is arranged between the protective shell and the fixing shell, and an electrode contact is arranged on a side surface of the protective shell facing the array layer; the containing cavity is used for placing the array layer, the electrode layer and the water tank assembly, water injection holes are arranged at both ends of the array layer, both ends of the electrode layer and both ends of the water tank assembly, and the water injection holes at the same end of the array layer, the electrode layer and the water tank assembly are communicated with each other; the electrode layer comprises a silicon plate and a plurality of metal electrodes arranged on a side surface of the silicon plate facing the array layer, the plurality of metal electrodes are arranged in an array, the electrode layer has a plurality of through holes, each through hole penetrates a first end of a corresponding metal electrode and penetrates the silicon plate below the first end, and the array layer is provided with a first hole and a second hole, the first hole is arranged corresponding to the through hole of the metal electrode, and the second hole is arranged corresponding to a second end of the metal electrode. In use, the Nafion composite film is placed in the first hole, one side surface of the Nafion composite film is in contact with the surface of the metal electrode, the other side surface is in contact with the electrode contact, and the metal electrode is led out from the second hole; water is injected into the water injection hole of the array layer, flows through the electrode layer and is injected into the water tank assembly, and a preset current is applied to the metal electrodes on both sides of the Nafion composite film and the electrode contact, so that the Nafion composite film reacts with water and the gas to be detected, and the detection of the gas to be detected is realized.

2. The MEMS amperometric gas sensor according to claim 1, characterized in that: The water tank assembly comprises a water channel structure layer and a silicon bottom sealing layer which are matched with each other, the water channel structure layer is engraved with a hollow water channel pattern, and the side surfaces of the water channel structure layer and the silicon bottom sealing layer are attached to form a water storage cavity.

3. The MEMS amperometric gas sensor of claim 1, wherein: The first end of the metal electrode and the second end of the metal electrode are distributed at both ends of the metal electrode along the length direction of the metal electrode.

4. The MEMS amperometric gas sensor of claim 1, wherein: The protective shell is provided with a gas exchange hole, and the number of the gas exchange holes is greater than the number of the electrode contacts, and the protective shell and the fixing shell are fixedly connected by bolts; The MEMS current-type gas sensor further comprises a protective cover which is connected with the protective shell through a matching hole by interference.

5. The MEMS amperometric gas sensor of claim 1, wherein: The Nafion composite film comprises a Nafion film and a metal functional material loaded on both side surfaces of the Nafion film.

6. The MEMS amperometric gas sensor of claim 1, wherein: When the gas to be detected is hydrogen, the metal functional material is composed of carbon fibers and palladium supported on the carbon fibers; When the gas to be detected is sulfur dioxide, the metal functional material is composed of carbon fibers and palladium supported on the carbon fibers, or the metal functional material is composed of copper and palladium supported on the copper; When the gas to be detected is nitrogen dioxide, the metal functional material is composed of carbon fibers and molybdenum disulfide supported on the carbon fibers; When the gas to be detected is ammonia, the metal functional material is composed of carbon fibers and platinum supported on the carbon fibers, or composed of rhodium and platinum supported on the rhodium.

7. The MEMS amperometric gas sensor of claim 1, wherein: The electrode contact on the protective shell is correspondingly arranged with the Nafion composite film; The water storage level in the water tank assembly is lower than the side surface of the Nafion composite film in contact with the metal electrode.

8. The MEMS amperometric gas sensor of claim 1, wherein: The metal material of the metal electrode is gold or copper.

9. The MEMS amperometric gas sensor of claim 2, wherein: The thickness of the sink assembly is greater than 3 cm.

10. The MEMS amperometric gas sensor of claim 1, wherein: The array layer and the electrode layer are spaced apart by no less than 3 cm, and the electrode layer and the sink assembly are spaced apart by no less than 3 cm.

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