Sensor unit for an air filter system and method of manufacturing a sensor unit

Abstract

This disclosure relates to a sensor unit for an air filtration system used to separate at least one harmful gas from an airflow. The sensor unit includes a first sensor. The first sensor includes two electrodes and a material layer connecting the electrodes. When the material layer comes into contact with the at least one harmful gas, the resistance of the material layer changes. The sensor unit further includes a processing unit configured to determine the time derivative of the resistance change of the material layer of at least the first sensor, and to generate an output signal based on a function of the determined time derivative. Furthermore, this disclosure also relates to a method of manufacturing the sensor unit.

Description

Technical Field

[0001] This disclosure relates to a sensor unit that can be used in conjunction with an air filter. The air filter is used to separate harmful gases, such as NH3 or NO2, from an airflow. This disclosure also relates to a method of manufacturing the sensor unit. Background Technology

[0002] The lifespan of a fuel cell largely depends on air cleanliness, as air is essential for the chemical reactions that occur within the fuel cell to generate electricity. Therefore, a cathode air filter is used to separate harmful gases, such as NH3, NO2, and SO2, from the airflow used in the fuel cell, as these gases can shorten its lifespan. For this purpose, a cathode air filter with a filter element containing activated carbon for adsorbing contaminants is used.

[0003] As the adsorbent load increases, it becomes impossible to completely adsorb the target gas, leading to gas permeation through the filter. Harmful gases then enter the fuel cell. This gas permeation should be prevented to avoid damage to the fuel cell. A sensor unit is used to detect gas permeation. If the sensor unit detects gas permeation, the filter element loaded with contaminants can be replaced. This enables predictive maintenance of the air filter. Summary of the Invention

[0004] Therefore, the purpose of this disclosure is to provide a sensor unit that can be used in an air filter system, which can be manufactured cost-effectively and can reliably detect harmful gases such as NH3 and NO2, thereby providing an indication for the necessary replacement of filter elements.

[0005] This enables predictive maintenance of air filter systems. The sensor unit can provide a warning when the saturation level of the filter element exceeds a saturation threshold. For example, the saturation threshold could be approximately 80%. The saturation threshold can be adjusted based on the application and operating mode of the air filter system.

[0006] The application of sensor units is not limited to fuel cell systems, but can also be used in other systems facing similar challenges.

[0007] The objective upon which this disclosure is based is achieved by a combination of the features described in claim 1. Embodiments of this disclosure can be obtained from the dependent claims.

[0008] According to this disclosure, the sensor unit has a processing unit. The processing unit is configured to determine the time derivative of the resistance change of the material layer of the first sensor. The processing unit then generates an output signal based on a function of the time derivative of the resistance change of the first sensor. The output signal can be used to send a message to the operator of the air filter or the air filter / fuel cell assembly, for example, that an excessive amount of harmful gas has been detected and the filter element needs to be replaced. The output signal can take various forms. It can be acoustic, visual, or in the form of data packets, and then transmitted to a device for further processing via a suitable interface.

[0009] The material layer of the first sensor can be polyaniline (pani) or metal oxide (MOx). The sensor unit can have at least one second sensor, wherein the material layer of the first sensor can be different from that of the second sensor. Different sensors react differently to different harmful gases, thus making it easier to distinguish between them.

[0010] In one embodiment, the time derivative or rate of change of resistance is used, for example, in the form of the time derivative of the relative resistance change of the first sensor and / or the second sensor. In determining the relative resistance change, the measured resistance R is associated, for example, with the initial resistance R0 of the material layer. The initial resistance R0 may be the resistance possessed by the material layer, made of polyaniline or metal oxide, before initial exposure to the harmful gas. If, due to exposure to the harmful gas, the resistance R increases by, for example, 20% compared to the initial resistance R0, the relative resistance change R / R0 is taken as 1.2.

[0011] For the hazardous gas NH3, it has been found that during finite-time exposure (e.g., exposure lasting one minute or several minutes), the R / R0 value rises directly at the start of exposure and then falls again after exposure ends, but does not return to its initial value (R / R0=1) even after a longer period following exposure. Due to sensor signal drift, if finite-time exposures are repeated multiple times at corresponding intervals, the drift accumulates, making it impossible to establish a simple correlation between the amount of hazardous gas during exposure and the sensor response based on the change in relative resistance. Conversely, if the time derivative of the relative resistance change is used, the sensor signal drift does not affect the measurement results. By using the time derivative of the relative resistance change Δ(R / R0) / Δt, the effect of sensor signal drift observed in the NH3 case can be eliminated, and measurement accuracy can be improved.

[0012] If the concentration of NH3 increases continuously from one exposure to the next, a good approximation of a linear correlation between the amount / concentration of hazardous gas and the time derivative is obtained in one embodiment. An increase in R / R0 can be identified as the exposure duration increases. This can cause problems if a correlation between R / R0 and the hazardous gas concentration is to be established. Conversely, the time derivative Δ(R / R0) / Δt is independent of the exposure duration.

[0013] Another advantage of using the time derivative of resistance change is that it eliminates the need for mathematical compensation for the effects of relative humidity and temperature. In fact, the time derivative of relative resistance change is unaffected by relative humidity and temperature because these two environmental parameters change relatively slowly in reality. Using the time derivative of resistance change can improve measurement accuracy and simplify the operation of the sensor unit.

[0014] For the harmful gas NO2, it has been shown that the resistivity of the polyaniline material layer also increases with temporary exposure to the gas, but even after a long waiting period following the end of exposure, the resistance remains virtually constant and does not fall back to R0. When exposed to the same concentration of the harmful gas again, R / R0 increases again by the same order of magnitude, thus identifying a cascading increase in R / R0. This makes it difficult to find a suitable correlation between R / R0 and the concentration of the harmful gas. However, it is easier to infer the concentration of the harmful gas by using the time derivative Δ(R / R0) / Δt.

[0015] In one embodiment, the processing unit is configured to generate an output signal based on whether the resistance change of the material layer of the first sensor exceeds a threshold S1. For example, the processing unit of the sensor unit generates an output signal only when the relative resistance change R / R0 is greater than S1 (and other conditions may also be met).

[0016] The processing unit can be configured to generate an output signal based on whether the time derivative of the resistance change of the first sensor exceeds a threshold S2. In one embodiment, the output signal depends only on this condition. However, the generation of the output signal can also depend on other conditions, such as the resistance change being greater than S1.

[0017] The processing unit can be configured to generate an output signal based on whether the time derivative of the resistance change of the material layer of the first sensor (and optionally, the second sensor) drops below a negative threshold S3. This criterion can be used to distinguish between NH3 and NO. When exposure to NH3 ends, the resistance of the sensor's material layer decreases again, so the time derivative of the relative resistance change takes a value less than zero; while during exposure to NO2, the resistance remains at an increased level, and even after the exposure ends and NO2 is no longer present on the material layer, the resistance does not decrease. Therefore, the time derivative here does not actually show a negative value or only shows a negative value close to zero, but its absolute value is not greater than the absolute value of the negative threshold S3.

[0018] In one embodiment, the output signal includes the dynamic resistance ratio R / R ref Or the output signal is based on the dynamic resistance ratio R / R ref Where R is the measured resistance of the material layer, R ref It is the dynamic baseline resistance. The dynamic baseline resistance R... ref This depends on the time derivative of the resistance change of the material layer, for example, the time derivative of the relative resistance change, and may vary over time. This is achieved by using the dynamic resistance ratio R / R... ref Used as an output signal or as the output signal, it can avoid or significantly reduce sensor signal drift, which may also be caused by changes in relative humidity or temperature.

[0019] In one embodiment, the dynamic baseline resistance R ref The definition is as follows: R is considered complete as long as the time derivative of the relative resistance change Δ(R / R0) / Δt (where R0 is the initial resistance) is less than the starting threshold Z1. ref This corresponds to the measured, time-varying resistance R or R(t) of the sensor material layer. If the time derivative of the relative resistance change Δ(R / R0) / Δt remains below the trigger threshold Z1, then the measured resistance R is compared with the dynamic baseline resistance R. ref The ratio is equal to 1 because the baseline resistance R ref This corresponds to the measured resistance R.

[0020] From the moment when the time derivative of the relative resistance change Δ(R / R0) / Δt exceeds the start-up threshold Z1, the dynamic baseline resistance R ref Based on the resistance R (Z1) measured when the threshold Z1 is exceeded. Baseline resistance R ref This is a temporary constant, therefore the dynamic resistance ratio R / R is different as the measured resistance R changes over time. ref The deviation is now 1. As long as this value does not fall below the termination threshold Z2, the dynamic baseline resistance remains at the value of R(Z1), or more precisely, at R(t). Z1 The value of ).

[0021] The initiation threshold Z1 is positive, while the termination threshold Z2 is negative or less than zero. When exposure to a harmful gas begins, the positive initiation threshold Z1 is exceeded, causing the resistance in the material layer to increase relatively rapidly. When exposure to the harmful gas ends, the resistance of the material layer drops rapidly again, causing the value to fall below the termination threshold Z2.

[0022] In one embodiment, the start threshold Z1 and the stop threshold Z2 are equal in size. In an alternative embodiment, the ratio of the start threshold Z1 to the stop threshold Z2 is in the range of 0.5 to 2.

[0023] Once this value drops below the termination threshold Z2, the dynamic baseline resistance R ref This corresponds again to the measured resistance R(t). R(t) and R ref The ratio is now 1 again and remains at this value until the time derivative of the relative resistance change, Δ(R / R0) / Δt, (again) exceeds the start-up threshold Z1. In this case, R ref The resistance value measured when the threshold Z1 is exceeded again will be set. If Δ(R / R0) / Δt subsequently falls below the termination threshold Z2 again, the measured resistance R(t) will be used again as the dynamic baseline resistance R. ref The result is the ratio R(t) / R ref It then equals 1 again.

[0024] In one embodiment, the output signal includes the concentration of the harmful gas (e.g., expressed in ppb or ppm), wherein the concentration is based on the dynamic resistance R / R. ref The calibration was performed.

[0025] Another object of this disclosure is to provide a method for manufacturing the above-described sensor unit, achieved by a combination of the features described in claim 5. Embodiments can be obtained from the dependent claims of claim 5.

[0026] The material layer of the first sensor can be subjected to acid surface treatment. This surface treatment constitutes a difference between the two material layers, provided that both sensors and their material layers are made of polyaniline in the same manner. In one embodiment, only the material layer of the first sensor is surface-treated, while the material layer of the second sensor is not. However, it is also conceivable that the material layer of the second sensor also undergoes a surface treatment different from that of the material layer of the first sensor. Again, the two material layers can be different and can provide different sensor signals when exposed to the same harmful gas.

[0027] For example, sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), phosphoric acid (H3PO4), or sulfonic acid can be used as acids. Surface treatment may include immersing the material layer of the first sensor in an aqueous solution of acid. This may be an acid solution of 1 mol / L to 10 mol / L (1 mol / L to 10 mol / L, or, for example, 3 mol / L to 7 mol / L).

[0028] For surface treatment, the material layer can be immersed in acid or an aqueous solution of acid for 5 to 90 seconds, for example, 10 to 60 or 20 to 40 seconds. Alternatively, immersion can be performed without applying a potential.

[0029] To construct the material layer, aniline can be polymerized by applying a dynamic potential. Aniline can be part of an aqueous acid solution. For example, polyaniline can be arranged as individual, stacked films on a plate-like carrier, on which electrodes are arranged. In one embodiment, the electrodes are interdigitated electrodes.

[0030] The material layer can be functionalized at least by adding a metal, metal alloy, or metal oxide (Pd, Ag, Pd / Sn, Zn) to the aqueous solution. For example, the material layer of the first sensor may have at least one functionalized film, while the material layer of the second sensor may not.

[0031] For chemical metal deposition, the material layer can be immersed in a dispersion containing solvent and metal nanoparticles. Attached Figure Description

[0032] This disclosure will be explained in more detail with reference to the embodiments shown in the accompanying drawings. Wherein: Figure 1 A schematic diagram of an air filter and a fuel cell is shown; Figure 2 An air filter with a first sensor and a second sensor is shown; Figure 3 The first sensor with two electrodes and a layer of polyaniline material is shown; Figure 4 The changes observed after exposure to the harmful gases NH3 and NO2 are shown. Figure 5 Showing by Figure 4 The process of relative resistance change over time caused by exposure to harmful gases; Figure 6 Showing by Figure 4 The change in the time derivative of the relative resistance caused by exposure to harmful gases; Figure 7 This demonstrates another process of change when exposed to the harmful gas NH3; Figure 8Showing by Figure 7 The process of relative resistance change caused by exposure to harmful gases; Figure 9 Showing by Figure 7 The change in the time derivative of the relative resistance caused by exposure to harmful gases; Figure 10 Displayed based on Figure 7 The process of change in dynamic resistivity when exposed to harmful gases; Figure 11 This demonstrates another process of change resulting from exposure to the harmful gas NO2; and Figure 12 Displayed based on Figure 11 The process of change in dynamic resistivity when exposed to harmful gases. Detailed Implementation

[0033] Figure 1 An air filter system 1 and a downstream fuel cell 2 are shown. The air filter system 1 includes an air filter with a filter element 10 and a sensor unit 20. An airflow 3 enters the air filter system 1 and passes through the air filter 10. Harmful gases such as nitrogen dioxide (NO2), ammonia (NH3), and / or sulfur dioxide (SO2) are separated from the airflow 3 by the air filter 10, which contains adsorbent material. The purified airflow 3' now passes through the sensor unit 20, which is used to check the effectiveness of the air filter 10. The sensor unit 20 is designed to detect harmful gases (e.g., NO2, NH3, and / or SO2) in the ppm and ppb ranges. The purified cathode airflow 3' enters the fuel cell 2, where oxygen in the airflow 3' reacts with an energy carrier such as hydrogen 4 to produce water 5, thereby generating electricity 6.

[0034] Figure 2 Sensor unit 20 is schematically shown. Sensor unit 20 includes a first sensor 21, a second sensor 22, and a processing unit 23. In processing unit 23, signals 24 and 25 from sensors 21 and 22 are processed into output signal 26. Output signal 26 can be transmitted, for example, to a subsequent processing device (not shown) via a suitable interface. Output signal 26 can then be used to obtain information, for example, that the concentration of a harmful gas is determined to be too high, and therefore the filter element 10 must be replaced.

[0035] Figure 3 The structure of the first sensor 21 is schematically shown. The structure of the second sensor 22 corresponds to that of the first sensor 21, therefore only the structure of the first sensor 21 will be discussed. Sensor 21 has a first electrode 27 and a second electrode 28, which are arranged on a plate-like carrier 29 made of glass. For example, the electrodes are made of gold.

[0036] Sensor 21 also has a material layer 30 made of polyaniline, which electrically connects the two electrodes 27 and 28. Material layer 30 comprises a single layer or film. Polyaniline is a conductive polymer. When material layer 30 comes into contact with NH3 or NO2 that may be present in the airflow 3', the conductivity of the material layer changes, thus changing the resistance between the first electrode 27 and the second electrode 28. The change in resistance can be measured via wire 31.

[0037] The difference between the material layer 30 of the first sensor 21 and the material layer 30 of the second sensor 22 is that the first sensor 21 and its material layer 30 are immersed in a sulfuric acid aqueous solution for a certain period of time. Compared with the untreated material layer of the second sensor 22, this immersion in the aqueous solution alters the properties (e.g., reactivity) of the material layer 30 of the first sensor 21.

[0038] For testing purposes, sensor unit 20 was exposed to the harmful gas NH3 and then, after a period of time, to the harmful gas NO2. Figure 4 The chart shows the selected concentration (in ppb), exposure duration (10 minutes), and the time interval (60 minutes) between the first exposure to NH3 and the second exposure to NO2. Airflow 3' was directed to the two sensors 21 and 22 at an NH3 concentration of 500 ppb and then to an NO2 concentration of 250 ppb after a 60-minute interruption.

[0039] Figure 5 The solid lines in the graph correspond to the response of the first sensor 21. The dashed lines correspond to the sensor response of the second sensor 22. The x-axis represents time (in minutes), and the y-axis represents the relative resistance change R / R0, where R0 corresponds to the initial resistance of the material layers of both sensors 21 and 22 before exposure to the harmful gas. It can be seen that the first sensor 21, after H2SO4 post-treatment, exhibits a stronger response upon exposure to NH3 than the second sensor 22, which was not post-treated with H2SO4. After exposure to NH3, the sensor signals of both sensors 21 and 22 at least substantially recovered to the same level.

[0040] When exposed to NO2, sensor 21, after being post-treated with H2SO4, showed almost no response, while the untreated sensor 22 exhibited a significant response (the relative resistance change R / R0 increased to approximately 1.25). However, after increasing to a value of 1.25, the relative resistance change remained at this level and did not return to any position close to the original value of 1.0. Figure 5The graph exemplarily shows the threshold S1 for the relative resistance change R / R0. Here, the threshold S1 is approximately 1.1. While both sensors reach this threshold when NH3 is applied, only the sensor signal of the untreated sensor 22 exceeds this threshold S1 when NO2 is applied.

[0041] Based on a comparison with the threshold S1 and the sensor signals from the two sensors 21 and 22, it can be determined in actual operation whether the sensor unit is exposed to NH3 or NO2. If both the first sensor 21 and the second sensor 22 show a sensor response higher than the threshold S1, it can be concluded that the harmful gas is NH3. The processing unit can then generate a corresponding output signal.

[0042] Conversely, if only the second sensor 22 shows a response above the threshold S1, it can be concluded that the harmful gas is NO2. The processing unit will then generate a correspondingly different output signal.

[0043] Figure 6 This shows the change of the time derivative of the relative resistance, Δ(R / R0) / Δt, over time. Figure 5 Similarly, this also shows the result of... Figure 4 The corresponding sensor responses caused by exposure are shown in the diagram.

[0044] Figure 6 It is clearly shown that bipolar pulses can be assigned to both sensors upon exposure to NH3. Both the first sensor (post-treated with H2SO4) and the second sensor 22 exhibit pulse peaks at the onset of NH3 exposure. At the end of the exposure, a negative pulse peak can be identified, with an amplitude approximately equivalent to the absolute value of the initial positive pulse peak.

[0045] Conversely, exposure to NO2 only resulted in unipolar pulses. No negative pulse peaks were detected at the end of NO2 exposure.

[0046] exist Figure 6 The diagram exemplarily shows a second threshold S2 for the positive pulse peak and a third negative threshold S3 for the negative pulse peak. The threshold values ​​can be the same or different. If both sensors 21 and 22 now detect bipolar pulses, where the corresponding positive pulse peak is above the second threshold S2 and the corresponding negative pulse peak is below the third threshold S3, this indicates that the harmful gas is NH3. By checking the corresponding conditions, the processing unit can generate an output signal derived therefrom.

[0047] If the sensor signals of the two sensors 21 and 22 do not have negative pulse peaks (the condition related to the threshold S3 is not met), and simultaneously meet the condition related to the relative resistance change R / R0, that is, the sensor signal of the second sensor 22 is greater than S1 and the sensor signal of the first sensor 21 is less than S1, then the processing unit determines the output signal, which indicates the presence of NO2.

[0048] Figure 7 The process of polyaniline sensor exposure to NH3 is shown. Starting from a concentration of 250 ppb, the gas concentration was gradually increased to 500, 1000, and 2000 ppb. There was a 5-minute interval between each 5-minute exposure phase of the hazardous gas.

[0049] Due to exposure to harmful gases, the resistance of the polyaniline sensor changes, such as... Figure 7 As shown.

[0050] Figure 8 The process of relative resistance change R / R0 over time is shown.

[0051] Figure 9 Showing Figure 8 The time derivative of the relative resistance change (Δ(R / R0) / Δt). Due to the first stage of exposure to harmful gases, the time curve of Δ(R / R0) / Δt exceeds the initiation threshold Z1 at approximately 5 to 6 minutes (see...). Figure 9 (The dotted line above). When the first exposure phase ends after approximately 5 minutes, the time derivative of the relative resistance change Δ(R / R0) / Δt drops below zero and then to the termination threshold Z2 (see...). Figure 9 Below the dashed line. The initiation threshold Z1 is between 0 and 0.2 s⁻¹, while the termination threshold is less than zero. In the second stage of hazardous gas exposure (approximately 15 to 20 minutes), the initiation threshold Z1 is first exceeded, and then the threshold falls below the termination threshold Z2 again. Exceeding the initiation threshold Z1 and falling below the termination threshold Z2 repeats in the third and fourth stages of hazardous gas exposure.

[0052] Figure 10 This shows the dynamic resistance ratio R / R ref The process of change. The numerator of the dynamic resistivity ratio is the resistance measured by the sensor. The denominator corresponds to the dynamic baseline resistance R. ref The dynamic baseline resistance R is determined by Δ(R / R0) / Δt and thresholds Z1 and Z2. ref It can be set and reset based on the time derivative Δ(R / R0) / Δt and thresholds Z1 and Z2.

[0053] As long as the activation threshold Z1 is not exceeded starting from time t=0, the dynamic baseline resistance R refThis corresponds to the measured resistance R(t). Therefore, until the time curve Δ(R / R0) / Δt first exceeds the Z1 value, the dynamic resistance ratio R / R ref The dynamic resistance ratio equals 1. After exceeding the start-up threshold Z1, the dynamic resistance ratio increases to approximately 1.15. During this phase, from approximately t=5 minutes to approximately t=10 minutes, the dynamic baseline resistance R... ref It is constant and corresponds to the resistance determined by the sensor when Z1 intersects the time curve Δ(R / R0) / Δt. At approximately t=10 minutes, when this value drops below the termination threshold Z2, the dynamic baseline resistance is again equal to the measured resistance R(t), thus making the ratio R / R0 constant. ref Returning to value 1. This is due to repeatedly exceeding Z1 and subsequently falling below Z2 (see...). Figure 9 Dynamic resistance ratio R / R ref (See Figure 10 The dynamic resistivity ratio R / R is reset to 1 after each exposure to hazardous gases. ref This can be compared with the value of exposure to harmful gases (see) Figure 7 This is related to the dynamic resistance ratio R / R, therefore, based on this relationship... ref By observing the time-varying process, the sensor unit can display the specific value of the harmful gas concentration.

[0054] Figure 11 Another time-varying process of exposure to the harmful gas NO2 was shown. The sensor with a metal oxide layer was exposed to six different concentration ranges (from 50 to 500 ppb). Figure 12 The time-varying process of the relative resistance change R / R0 (dashed line) and the dynamic resistance ratio R / R are shown. ref The time-varying process, where the value R / R ref It is similar to Figures 7 to 10 The example shown is calculated using R / Rr. ef The output signal can be used to compensate for sensor drift. Studies have also shown that using R / R... ref It can effectively compensate for the effects that may be caused by changes in relative humidity and temperature.

[0055] Determining the time derivative of the relative resistance change allows for a better and more accurate identification of the detected harmful gases. Since temperature and humidity conditions typically change slowly, mathematical compensation of the sensor signal is unnecessary when using the time derivative Δ(R / R0) / Δt. This results in a simple sensor unit with high measurement accuracy. Because only the material layer of one sensor is surface-treated, while the material layer structures of both sensors are otherwise identical, the processing unit of the sensor unit can distinguish between NH3 and NO2.

[0056] According to various embodiments, the sensor unit may include two or more sensors, such as three, four, or larger sensor arrays, each sensor having a material layer with a different composition and / or surface treatment. By providing multiple sensors with different sensitivities and selectivity specifically designed for NH3, NO2, and other hazardous gases, the processing unit can evaluate a richer range of resistance changes and time derivative patterns. This enables improved gas differentiation, more robust detection under diverse environmental conditions, and enhanced redundancy. If one sensor drifts or fails, the remaining sensors can still provide reliable information about the hazardous gas.

[0057] List of reference numerals 1. Air filter system 2 fuel cells 3 airflow (3' clean airflow) 4 hydrogen 5 water 6 energy 10 air filters 20 sensor units 21 First Sensor 22 Second Sensor 23 processing units 24 sensor signals 25 sensor signals 26 Output Signals 27 First Electrode 28 Second electrode 29 carriers 30 material layers Line 31

Claims

1. A sensor unit (20) for an air filtration system (1) for separating at least one harmful gas from an airflow (3), the sensor unit (20) comprising: A first sensor (21) includes two electrodes (27, 28) and a material layer (30) connecting the two electrodes (27, 28), wherein the resistance of the material layer (30) changes when the material layer (30) comes into contact with the at least one harmful gas; and The processing unit (23) is configured to determine the time derivative of the resistance change of the material layer (30) of the first sensor (21), and is further configured to generate an output signal (26) based on a function of the determined time derivative.

2. The sensor unit (20) according to claim 1, wherein, The material layer (30) is formed of polyaniline or metal oxide.

3. The sensor unit (20) according to claim 1 or 2, further comprising: The second sensor (22) includes two additional electrodes and an additional material layer connecting the two additional electrodes, wherein the material layer (30) of the first sensor (21) is different from the additional material layer of the second sensor (22). The processing unit (23) is further configured to determine the time derivative of the resistance change of another material layer of the second sensor (22), and is further configured to generate the output signal based on a function of the determined time derivative of the second sensor (22).

4. The sensor unit (20) according to claim 3, wherein, The processing unit (23) is further configured to identify the at least one harmful gas by comparing the resistance of the material layer (30) of the first sensor (21) and the resistance of the other material layer of the second sensor (22).

5. The sensor unit (20) according to any one of claims 1 to 4, wherein, The sensor unit (20) is configured to generate the output signal (26) based on whether the resistance change of the material layer (30) of the first sensor (21) exceeds a threshold S1.

6. The sensor unit (20) according to any one of claims 1 to 5, wherein, The sensor unit (20) is configured to generate the output signal (26) based on whether the time derivative of the resistance change of the material layer (30) of the first sensor (21) exceeds a threshold S2.

7. The sensor unit (20) according to any one of claims 1 to 6, wherein, The sensor unit (20) is configured to generate the output signal (26) based on whether the time derivative of the resistance change of the material layer (30) of the first sensor (21) drops below a negative threshold S3.

8. The sensor unit (20) according to any one of claims 1 to 7, wherein, The output signal (26) is based on the dynamic resistance ratio R / R ref Where R is the measured resistance of the material layer (30), R ref It is the dynamic baseline resistance of the material layer (30), which depends on the time derivative of the resistance change of the material layer (30).

9. The sensor unit (20) according to claim 8, wherein, The dynamic baseline resistance R ref The definition is as follows: R ref = The measured resistance R(t) is determined as long as the time derivative of the relative resistance change Δ(R / R0) / Δt with R0 as the initial resistance is less than the start-up threshold Z1. R ref = The resistance R(Z1) measured at the moment when the time derivative of the relative resistance change Δ(R / R0) / Δt exceeds the start-up threshold Z1, provided that the time derivative of the relative resistance change Δ(R / R0) / Δt does not fall below the termination threshold Z2; and R ref = The measured resistance R(t), once Δ(R / R0) / Δt drops below the termination threshold Z2.

10. The sensor unit (20) according to claim 8 or 9, wherein, The output signal (26) indicates the concentration of the at least one harmful gas, wherein the concentration is based on the dynamic resistance ratio R / R ref It is determined by calibration.

11. A method for manufacturing a sensor unit (20) according to any one of claims 1 to 10, wherein, The material layer (30) of the first sensor (21) is subjected to acid surface treatment.

12. The method according to claim 11, wherein, The acid is at least one of sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), phosphoric acid (H3PO4), and sulfonic acid.

13. The method according to claim 11 or 12, wherein, The surface treatment includes immersing the material layer (30) of the first sensor (21) in an aqueous solution of the acid at a concentration of 1 mol / L to 10 mol / L.

14. The method according to any one of claims 11 to 13, wherein, The material layer (30) is constructed by polymerizing aniline in an aqueous solution of acid by applying a dynamic potential.

15. The method according to claim 14, wherein, The material layer (30) is functionalized by incorporating at least one of a metal, a metal alloy, and a metal oxide into the aqueous solution.

16. The method according to claim 14 or 15, wherein, For chemical metal deposition, the material layer (30) is immersed in a dispersion containing solvent and metal nanoparticles.

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