Sensor based on In2O3 / NiFe2O4 combined electrode material and preparation method thereof

Abstract

The invention relates to the technical field of gas sensors, in particular to a sensor based on an In2O3 / NiFe2O4 combined electrode material and a preparation method of the sensor. According to the specific technical scheme, the preparation method comprises the following steps: (1) putting InCl3. 4H2O into an ethanol solution, carrying out magnetic stirring until the InCl3. 4H2O is fully dissolved, then adding urea, continuously stirring, then transferring into a high-pressure kettle, and keeping at 200-250 DEG C for 18-25 hours; cooling to room temperature, centrifuging, washing and drying the hydrothermal product, calcining at high temperature, cooling and grinding to obtain In2O3 powder; (2) adding A (NO3) 2.6 H2O and Fe (NO3) 3.9 H2O into deionized water, uniformly stirring, then adding citric acid hydrate and ethylene glycol, heating in a water bath at 70-90 DEG C, continuously magnetically stirring until a gel state is formed, calcining the gel at high temperature, cooling and grinding to obtain AFe2O4 powder; wherein A is one of Ni, Zn and Co. The YSZ is used as a solid electrolyte, and the sensor prepared by combining the two sensing electrodes can counteract response of NH3 and maintain high response to CO.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas sensor, in particular to a sensor based on In2O3 / NiFe2O4 combined electrode material and a preparation method thereof. BACKGROUND

[0002] Carbon monoxide is colorless, odorless and toxic. It can easily reduce the oxygen-carrying capacity of human hemoglobin, causing symptoms such as nausea, headache, and difficulty breathing. Even if 100 ppm CO is inhaled, it can cause people to suffocate or die. In order to realize the detection of CO, various types of sensors have been developed, such as resistance type, infrared type, optical type. However, these types of sensors have poor selectivity and stability, and are not suitable for application in industrial plants and automobile exhausts, and other harsh environments with many types of interfering gases. In contrast, YSZ-based potential sensors have the ability to detect gases at high temperatures, and have the advantages of high sensitivity, strong anti-oxygen interference ability, and good long-term stability.

[0003] Most YSZ-based potential sensors use a certain material as a sensitive electrode (SE) and Pt as a reference electrode (RE). The composition and microstructure of the SE material have a great influence on the adsorption of gases, catalytic processes, and electrochemical reactions, thereby affecting the sensitivity and response of the sensor. Therefore, the prerequisite for preparing a potential sensor with excellent sensing performance is to use appropriate SE materials. A variety of SE materials have been developed for detecting CO. Noble metals such as Au and Pt have high sensitivity to CO, but have poor selectivity and stability, so they are not suitable for practical application. There are also some metal oxides or composite metal oxides, such as In2O3, NiO, SnO2, and ZnO. However, single-SE sensors using pure single metal oxides as SEs have poor sensing performance, especially selectivity. Combining an electrode sensitive to both target and interfering gases with another electrode sensitive only to interfering gases in the same sensor can eliminate the response of interfering gases and enhance selectivity. For combined-SE sensors, there have been few reports in recent years on eliminating the interfering response of NH3 and improving the selectivity of CO.

[0004] In2O3 is a typical n-type semiconductor with a direct band gap of 3.55-3.75 eV. After modification, it can have high electrical conductivity, strong thermal stability, and good photoelectron performance, and is widely used in batteries, photocatalysis, and gas sensors. Pure In2O3 has excellent gas sensing performance, but its biggest disadvantage is lack of selectivity. If a single-SE sensor with an In2O3 electrode is used to detect CO in automobile exhaust, it is easy to be disturbed by other gases, especially NH3 and NOx. The selective catalytic reduction (SCR) system in a car usually sprays an aqueous urea solution to remove toxic gases NOx As much as possible to restore to N2 and H2O, but inevitably there will be a reduction agent injection excess ammonia leakage. Therefore, the main need to consider the interference of NH3. SUMMARY

[0005] In view of the deficiencies of the prior art, the application provides a sensor based on In2O3 / NiFe2O4 combined electrode material and a preparation method thereof.

[0006] The application found that the NiFe2O4 electrode almost does not respond to CO, and the response value of the electrode to NH3 is similar to that of the In2O3 electrode by comparing the sensing performance of several spinel ferrite electrodes. The combination-SEs sensor using the two electrodes shows excellent selectivity to CO. In addition, the sintering temperature of the electrode often affects the sensing performance of the sensor. Among all the sensing parameters, the response value is the basis for evaluating other parameters. The response of the combination-SEs sensor to CO is mainly affected by the adsorption characteristics and catalytic performance of the In2O3 electrode. The CO sensing performance of the combination-SEs sensor with In2O3 electrodes with different sintering temperatures is evaluated, and the sensing mechanism is analyzed.

[0007] To achieve the above object, the application is implemented by the following technical scheme: The application discloses a preparation method of an In2O3 / NiFe2O4 combined electrode material, comprising the following steps: (1) Preparation of In2O3: InCl3·4H2O is placed in an ethanol solution and fully dissolved under magnetic stirring, then urea is added and stirring is continued, then the mixture is transferred to an autoclave and kept at 200-250 DEG C for 18-25 h; after cooling to room temperature, the hydrothermal product is centrifuged, washed, dried and high-temperature calcined, and then ground after cooling to obtain In2O3 powder; (2) Preparation of spinel ferrite AFe2O4: A (NO3) 2·6H2O and Fe (NO3) 3·9H2O are added to deionized water and stirred uniformly, then hydrated citric acid and ethylene glycol are added, heated in a 70-90 DEG C water bath and continuously stirred under magnetic stirring until a gel state is presented, the gel is high-temperature calcined, cooled and ground to obtain AFe2O4 powder; wherein A is one of Ni, Zn and Co.

[0008] Preferably, in step (1), the molar ratio of InCl3·4H2O to urea is 1:12-18.

[0009] Preferably, in step (2), the molar ratio of A (NO3) 2·6H2O to Fe (NO3) 3·9H2O is 1:1-3, and the molar ratio of hydrated citric acid to ethylene glycol is 1:1.5-2.5.

[0010] Preferably, in step (1), the centrifugal speed is 5000-6000 r / min, the centrifugal time is 10-20 min; the drying temperature is 60-100℃, the drying time is 8-12 h; the high-temperature calcination temperature is 550-650℃, the heating rate is 1-3℃ / min, and the time is 2-4 h.

[0011] Preferably, in step (2), the high-temperature calcination temperature is 550-650℃, the heating rate is 1-3℃ / min, and the time is 1-3 h.

[0012] Correspondingly, a preparation method of a sensor of an In2O3 / NiFe2O4 combined electrode material prepared by the above preparation method, adopts YSZ as a solid electrolyte, screens AFe2O4 slurry on one side of YSZ, and then sintering at 950-1100℃ for 1-3 h, and taking out after cooling to room temperature; then screen In2O3 slurry on the other side of YSZ, and calcine at 750-1050℃ for 1-3 h, to prepare a combined-SEs sensor.

[0013] Correspondingly, a combined-SEs sensor prepared according to the above preparation method.

[0014] Correspondingly, an application of the combined-SEs sensor in detecting CO.

[0015] The present application has the following advantages: 1. In2O3 and NiFe2O4 are respectively synthesized by a hydrothermal method and a sol-gel method. In2O3 electrodes and NiFe2O4 electrodes are combined in the same sensor, which well makes up for the deficiency of a single-SE sensor with In2O3 electrodes in CO selectivity. At a working temperature of 500℃, the combined-SEs sensor with In2O3-SE sintered at 900℃ has the maximum response value of 58 mV and the maximum sensitivity of 38.9 mV / decade to 80 ppm CO. In addition, the response of the combined-SEs sensor to CO has good long-term stability.

[0016] 2. The method for preparing In2O3 and AFe2O4 sensitive electrode materials has the advantages of low cost, high efficiency, good repeatability, and high success rate.

[0017] 3. The solid electrolyte used in the combined-SEs sensor prepared by the present application is YSZ (5 mol% Y2O3 doped ZrO2), which has the advantages of high-temperature resistance, strong mechanical stability, and good ionic conductivity.

[0018] 4. Since In2O3-SE is sensitive to both CO and ammonia (NH3), while AFe2O4-SE' is only sensitive to NH3. Therefore, the two electrodes are screen-printed on both sides of the same YSZ substrate to make a combined-SEs sensor, which can offset the response of NH3 and maintain high response to CO. By adjusting the sintering temperature of In2O3-SE, the sensing performance of the combined-SEs sensor to CO is further improved. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 (a) single-SE sensor; (b) combined-SEs sensor based on YSZ; Figure 2 (a) single-SE sensor with different electrodes; (b) response of combined-SEs sensor with In2O3 electrode and NiFe2O4 electrode to 80 ppm different sample gases at 500℃; Figure 3 (a) XRD patterns of In2O3 electrode sintered at 800, 900, 1000℃; (b) XRD patterns of NiFe2O4 electrode sintered at 1000℃; Figure 4 SEM images of the surface morphology of In2O3 electrode sintered at (a) 800℃; (b) 900℃; (c) 1000℃; Figure 5 SEM images of the cross-section of In2O3 electrode sintered at (a) 800℃; (b) 900℃; (c) 1000℃; Figure 6 (a) surface; (b) cross-section SEM images of NiFe2O4 electrode sintered at 1000℃; Figure 7 Response transient curves of combined-SEs sensor with In2O3 electrode with different sintering temperatures at 500℃, (a) 800℃; (b) 900℃; (c) 1000℃; Figure 8 (a) CO sensitivity of combined-SEs sensor with sintering temperature of In2O3 electrode at 500℃; (b) effect on response / recovery time for 80 ppm CO; Figure 9 (a) response transient curves; (b) CO sensitivity; (c) response / recovery time for 80 ppm CO of combined-SEs sensor with 900℃ sintered In2O3 electrode at different working temperatures; Figure 10 Schematic diagram of the working principle of combined-SEs sensor; Figure 11are the polarization curves of (a) single-SE sensor with In2O3 electrode and NiFe2O4 electrode respectively; (b) combined-SEs sensor; Figure 12 are the response values of combined-SEs sensor with In2O3 electrode sintered at 900℃ under different oxygen contents to 5ppm and 80ppm CO at 500℃; Figure 13 are the results of long-term stability test of combined-SEs sensor. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be apparently and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0021] If not specifically indicated, the technical means used in the examples is the conventional means well known to those skilled in the art.

[0022] The present application discloses a preparation method of In2O3 / NiFe2O4 combined electrode material, comprising the following steps: (1) Preparation of In2O3: put InCl3·4H2O into ethanol solution and fully dissolve under magnetic stirring, then add urea and continue stirring, then move to autoclave and keep at 200-250℃ for 18-25h; after cooling to room temperature, centrifuge, wash and dry the hydrothermal product, then high-temperature calcine, cool and grind to obtain In2O3 powder; the molar ratio of InCl3·4H2O and urea is 1:12-18. The centrifugal speed is 5000-6000r / min, the centrifugal time is 10-20min; the drying temperature is 60-100℃, the drying time is 8-12h; the high-temperature calcination temperature is 550-650℃, the heating rate is 1-3℃ / min and the time is 2-4h.

[0023] (2) Preparation of spinel ferrite AFe2O4: A (NO3) 2·6H2O and Fe (NO3) 3·9H2O were added to deionized water and stirred uniformly, then citric acid hydrate and ethylene glycol were added, heated in a water bath at 70-90℃ and continuously stirred with a magnetic stirrer until a gel state was presented, the gel was calcined at high temperature, cooled and ground to obtain AFe2O4 powder; wherein A is one of Ni, Zn and Co. The molar ratio of A (NO3) 2·6H2O and Fe (NO3) 3·9H2O is 1:1-3, and the molar ratio of citric acid hydrate and ethylene glycol is 1:1.5-2.5. The temperature of high temperature calcination is 550-650℃, the heating rate is 1-3℃ / min, and the time is 1-3h.

[0024] A preparation method of a sensor of an In2O3 / NiFe2O4 combined electrode material prepared based on the above preparation method is disclosed, and the specific process is as follows: YSZ is used as a solid electrolyte, AFe2O4 slurry is screen printed on one side of YSZ, and then sintered at 950-1100℃ for 1-3h, and taken out after cooling to room temperature; then In2O3 slurry is screen printed on the other side of YSZ, and calcined at 750-1050℃ for 1-3h to prepare a combined-SEs sensor. Pt wires are adhered to the electrode surfaces of the two sides of the combined-SEs sensor using silver paste, and dried at 100-200℃ for 1-3h. The prepared combined-SEs sensor is used for detecting CO.

[0025] The preparation process of the AFe2O4 slurry and the In2O3 slurry is as follows: AFe2O4 powder or In2O3 powder is stirred uniformly with an organic binder (94wt% terpineol, 5wt% ethyl cellulose and 1wt% span 80) to obtain AFe2O4 slurry or In2O3 slurry. The mass ratio of AFe2O4 powder or In2O3 powder to the organic binder is 7:3.

[0026] The application will be further described below in combination with specific examples.

[0027] Example 1 Preparation of electrode material The chemical reagents used for synthesizing the electrode material are all purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd.

[0028] A preparation method of an In2O3 / NiFe2O4 combined electrode material includes the following steps: (1) In2O3 powder was prepared by hydrothermal method. 0.01 mol of InCl3•4H2O was put into a beaker containing appropriate amount of water and anhydrous ethanol (e.g. 50 mL of deionized water and 20 mL of anhydrous ethanol (concentration greater than 99.5%)) and magnetically stirred for 20 min until fully dissolved. 0.15 mol of urea was added and stirring was continued for 30 min, then the resulting mixed solution was transferred to an autoclave and kept at 220 °C for 20 h. After cooling to room temperature, the hydrothermal product was put into a centrifuge with a speed setting of 5500 r / min for 15 min. After washing the precipitate with deionized water and anhydrous ethanol, it was put into a drying oven and dried at 80 °C for 10 h. Then it was put into a muffle furnace and calcined at 600 °C for 3 h with a heating rate of 2 °C / min. After cooling, it was ground to obtain In2O3 powder.

[0029] (2) Spinel ferrite AFe2O4 (A = Ni, Zn, Co) was prepared by sol-gel method. Taking the preparation of NiFe2O4 powder as an example, 0.01 mol of Ni(NO3)2•6H2O and 0.02 mol of Fe(NO3)3•9H2O were added to deionized water and stirred until uniform. Then 0.03 mol of hydrated citric acid (C6H7O8•H2O) and 0.06 mol of ethylene glycol (C2H6O2) were added to the solution, which was heated in a water bath at 80 °C and continuously magnetically stirred until a gel state was formed. The gel was put into a muffle furnace and heated to 600 °C at a rate of 2 °C / min and calcined for 2 h. After cooling, it was ground to obtain NiFe2O4 powder. The preparation of other spinel ferrite powders (such as ZnFe2O4, CoFe2O4) was the same as the above process.

[0030] Example 2 Fabrication of sensors YSZ (5 mol% Y2O3-doped ZrO2) was used as solid electrolyte, and YSZ was sandwiched between a sensitive electrode (SE) and a reference electrode (RE). YSZ with a size of 9 mm x 9 mm x 0.35 mm was fabricated by tape casting process and sintered at 1480 °C for 2 h. Pt paste was screen-printed on one side of YSZ and calcined at 1200 °C for 2 h as RE. The electrode paste was obtained by stirring the electrode material powder with 30 wt% organic binder (94 wt% of terpineol, 5 wt% of ethyl cellulose and 1 wt% of Span 80) uniformly. After screen-printing In2O3 paste on the other side of YSZ, it was sintered at 900 °C for 2 h. The schematic diagram of the single-SE sensor with In2O3-SE prepared is shown in Figure 1 (a).

[0031] For single-SE sensors with spinel ferrite electrodes, the sintering temperature was set to 1000 °C after replacing the corresponding electrode paste, and the other preparation procedures were the same. Finally, NiFe2O4-SE sensors, ZnFe2O4-SE sensors and CoFe2O4-SE sensors were obtained.

[0032] Experimental example: The schematic diagram of the combination-SEs sensor is shown in FIG. 1 (b), which mainly consists of two sensitive electrodes (SE and SE') and YSZ. First, the NiFe2O4 paste was screen-printed on one side of the YSZ and sintered at 1000 °C for 2 h, and then removed after cooling to room temperature. Then, the In2O3 paste was screen-printed on the other side of the YSZ, and calcined at 800 °C, 900 °C and 1000 °C for 2 h, respectively, to prepare the combination-SEs sensor with In2O3 electrodes of different sintering temperatures, wherein the sintering temperature of the NiFe2O4-SE' was kept at 1000 °C. Figure 1

[0033] The Pt wires with a diameter of 0.2 mm and a length of 1 cm were adhered to the two electrode surfaces of the single-SE sensor and the combination-SEs sensor, respectively, using silver paste, and dried at 150 °C for 2 h.

[0034] The sensor test platform mainly consists of a gas cylinder (WUHAN NEWRADAR SPECIAL GAS CO., LTD.), a gas flow meter (CS200, Beijing Sevenstar Electronics Co., Ltd), an electrochemical workstation (PARSTAT3000A-DX, Princeton, USA), a tube furnace and a computer. Before testing the single-SE sensor, the sensor was first placed in a quartz tube, and the sensitive electrode and the reference electrode were connected to the positive and negative ports of the electrochemical workstation, respectively. For the test of the combination-SEs sensor, the In2O3-SE and the NiFe2O4-SE' were connected to the positive and negative ports of the electrochemical workstation, respectively. The quartz tube was fixed in the tube furnace, and the two ends of the quartz tube were connected to the gas outlet tube and the gas flow meter, respectively. The sensor was heated to different working temperatures (450 °C, 500 °C, 550 °C) by the tube furnace and kept for 5 min. Then, the base gas (10 vol.% O2+N2 balance) and the sample gas (5-80 ppm CO+10 vol.% O2+N2 balance) were alternately introduced into the quartz tube, and the total flow rate of the gas was controlled by the flow meter to be 0.5 L / min. Both electrodes of the single-SE sensor and the combination-SEs sensor were exposed to the same gas environment. When measuring the polarization curve, the voltage range was set to -0.35-0.35 V, and the scanning speed was set to 0.005 V / s.

[0035] Test results:​ 1. Response signals of single-SE sensors and combined-SE sensors At 500℃, the response values ​​of single-SE sensors equipped with In2O3 electrodes and several spinel ferrites to various sample gases with a concentration of 80 ppm were tested. The corresponding test results are as follows: Figure 2 As shown in (a), the In2O3-SE exhibits a response of -66 mV to CO, but its response to NH3 is also high at -48 mV. Combining the In2O3 electrode with an electrode sensitive only to NH3 can counteract the interfering response to NH3. Among several spinel ferrite electrodes as candidates, NiFe2O4-SE and In2O3-SE show the greatest difference in their responses to CO, while exhibiting similar response values ​​to other gases besides CO. This suggests that combining the In2O3 and NiFe2O4 electrodes in the same sensor (combined-SEs sensor) can theoretically significantly reduce the response to interfering gases while maintaining a high response to CO.

[0036] The response values ​​of the combined SEs sensor to various sample gases are as follows: Figure 2 As shown in (b), it exhibits a high response value of -58 mV to CO, similar to the response value (-66 mV) of a single-SE sensor with an In2O3 electrode. Furthermore, its response values ​​to NH3, CH4, and CO2 are only 9%, 5%, and 3% of the CO response value, respectively, demonstrating excellent selectivity for CO. In addition, since NiFe2O4-SE' produces almost no response signal to CO, the CO sensing performance of the combined-SEs sensor is mainly affected by various factors such as the adsorption characteristics, electrocatalytic activity, and gas-phase consumption of In2O3-SE. The sensing performance of the combined-SEs sensor can be further improved by adjusting the sintering temperature of In2O3-SE.

[0037] 2. Phase composition and microstructure of electrode materials XRD analysis was performed on each electrode material (D8A A25, Bruker, Germany) under Cu-Kα (0.15406 Å) radiation at 40 kV and 40 mA. The scan rate was 8° / min, ranging from 10° to 90°. The electrode morphology was observed at 15 kV using a henom Pharos G2 Desktop FEG-SEM (Thermo Fisher, USA).

[0038] The XRD patterns of In2O3 electrodes sintered at 800℃, 900℃, and 1000℃ are as follows: Figure 3(a) shows that there is no obvious difference in the XRD patterns of In2O3 electrodes sintered at different temperatures. All the diffraction peaks can be well matched with the standard card (PDF #00-006-0416). The XRD pattern of the NiFe2O4 electrode sintered at 1000 °C is shown in Figure 3 (b) shows that the diffraction peaks can be well matched with the standard card (PDF #00-054-0964). The sharp diffraction characteristics indicate that all the samples have good crystallinity. There are no other impurity peaks, meaning that the In2O3 and NiFe2O4 prepared by the hydrothermal method and the sol-gel method, respectively, are of high purity.

[0039] Figure 4 The surface morphologies of In2O3 electrodes sintered at different temperatures are shown. It can be seen that the In2O3 electrodes sintered at various temperatures are all porous, and the particle sizes are different, which may be caused by the different growth rates of the crystal nucleus during the hydrothermal reaction. With the increase of the sintering temperature, the cubic characteristics of the particles become more obvious, and the surface becomes smoother. For the In2O3 electrode sintered at 800 °C, there are many small particles attached to the surface of the large particles or filling in the gaps. When sintered at 900 °C, the particle morphology is relatively regular and the distribution is relatively uniform, and the porosity is high. When the sintering temperature reaches 1000 °C, the size of the particles becomes larger, and a certain degree of sintering phenomenon occurs.

[0040] The cross-sectional micrographs of In2O3 sintered at different temperatures are shown in Figure 5 At various sintering temperatures, the In2O3 electrodes are closely combined with YSZ, and the cross sections are relatively flat. The higher the sintering temperature, the lower the thickness of the electrode. At 900 °C, the thickness is about 11.1 μm.

[0041] Figure 6 (a) and (b) show the surface and cross-sectional morphologies of the NiFe2O4 electrode sintered at 1000 °C, respectively. It can be seen that the electrode surface presents a porous network structure and the pores are uniformly distributed. The cross section of the electrode appears many protrusions, which is because the difference in the thermal expansion coefficient between NiFe2O4 and YSZ is relatively large, and the electrode layer deforms when the stress is concentrated.

[0042] 3. Sensing performance of the combination-SEs sensor with In2O3-SEs with different sintering temperatures When the sensor is exposed to different gas environments, a potential difference will be generated between the two electrodes. The response transient curve is the curve of the sensor response signal changing with the concentration of the target gas. The transient response of the combination-SEs sensor with In2O3 electrodes sintered at different temperatures (800 °C, 900 °C, 1000 °C) in different concentrations of CO atmosphere is shown in Figure 7The working temperature chosen here is 500 °C, because the sensing performance of the combined-SEs sensor is the best at this temperature. When CO is introduced, the response curve moves to the negative direction and then reaches equilibrium. When the introduction of CO is stopped and the base gas environment is returned, the response curve returns to the initial value. The response value (△V) of the sensor is defined as the difference between the stable response values generated in the sample gas and the base gas, respectively (V sample gas -V base gas ). The combined-SEs sensor with the sintered In2O3 electrode at 900 °C has the maximum response value of -58 mV for 80 ppm CO, and it also has a response of -9 mV for 5 ppm CO.

[0043] The relationship between the response value and the logarithm of different CO concentrations (5 ppm-80 ppm) at 500 °C is shown in Figure 8 (a). It can be seen that the fitting line is linear, which is consistent with the mixed potential mechanism of the potential-type sensor. In addition, the slope of the fitting line represents the sensitivity of the sensor. The sensitivities of the combined-SEs sensors with the In2O3 electrodes sintered at 800 °C, 900 °C, and 1000 °C are -32.2 mV / decade, -38.9 mV / decade, and -18.2 mV / decade, respectively. The response / recovery time is also an important indicator for evaluating the sensing performance of the sensor, where the response time (t 90 ) is the time required for the output signal to reach 90% of the steady-state response value from the initial value, and the recovery time (t 10 ) is the time required for the output signal to return to 10% of the steady-state response value. As shown in Figure 8 (b), the response / recovery time of the combined-SEs sensor is shortened as the sintering temperature of the In2O3 electrode increases. The response time of the combined-SEs sensor with the In2O3 electrode sintered at 900 °C and 1000 °C is 16 s and 11 s, respectively, and the recovery time is 32 s and 23 s, respectively, with little difference. In contrast, the response time of the combined-SEs sensor with the In2O3 electrode sintered at 800 °C is slightly longer, at 26 s, but the recovery time is very long, reaching 68 s.

[0044] For potential-type sensors, the target gas undergoes an electrochemical reaction at the three-phase interface (TPB), generating a response signal. With constant electrolyte activity and reference electrode, three main factors influence sensing performance: electrocatalytic activity, the number of TPB sites, and the gas concentration at the TPB. Sintering temperature alters the microstructure of the In₂O₃ electrode, thus changing the sensing performance of the combined-SEs sensor. For an In₂O₃ electrode sintered at 800℃, the dense particle distribution and low porosity result in slow gas diffusion within the electrode layer, leading to a long response / recovery time. Sintering at 900℃ increases porosity, resulting in more TPB sites and more gas diffusion channels, leading to larger CO molecules participating in the electrochemical reaction at the TPB, thus improving sensitivity and response value. However, when the sintering temperature is increased to 1000℃, sintering occurs, reducing the specific surface area and weakening the particle's adsorption capacity for gas, thereby decreasing the response value and sensitivity. The response / recovery time, however, shortens slightly, possibly due to faster gas diffusion caused by the lower electrode layer thickness.

[0045] 4. The effect of operating temperature on the sensing performance of combined-SEs sensors The CO sensing performance of a combined-SEs sensor with an In2O3 electrode sintered at 900℃ was tested within an operating temperature range of 450℃-550℃. For example... Figure 9 As shown in (a), the response value of the combined-SEs sensor to each CO concentration decreases with increasing operating temperature. The response value to 80 ppm CO at 450 °C is -63 mV, while the response value at 550 °C is almost half that at 450 °C. For a low concentration of CO of 5 ppm, the sensor response value is -11 mV at 450 °C and -4 mV at 550 °C.

[0046] Plotting the response values ​​on the corresponding logarithmic scale of CO concentration on the coordinate axis shows that the response values ​​increase linearly with the increase of the logarithm of CO concentration. Figure 9 As shown in (b), the slope of the fitted line represents the sensitivity. When the operating temperature increases from 450°C to 500°C, the sensitivity decreases slightly from -44.1 mV / decade to -38.9 mV / decade. Upon further increasing the temperature to 550°C, the sensitivity drops sharply to -23.3 mV / decade. For the combined-SEs sensor, the difference in sensitivity at 450°C and 500°C is small, and both are worth considering.

[0047] The response / recovery time of the combined SEs sensor to 80ppmCO at different operating temperatures is as follows: Figure 9(c) shown. It can be seen that as the operating temperature rises from 450 °C to 550 °C, both the response time and the recovery time are dramatically shortened. At 450 °C, it takes 41 s for the output signal to reach 90% of the steady-state response value from the initial value, and it takes even longer, 67 s, to return to 10% of the steady-state response value, which is almost twice as long as at 500 °C. At 550 °C, the response / recovery times are the shortest, 10 s and 17 s, respectively.

[0048] Without changing the sensor electrodes and electrolyte, changing the operating temperature affects the adsorption capacity of the electrode material, the desorption rate, the gas-phase consumption of the target gas, the electrochemical reaction rate, etc. At low temperature (450 °C), the adsorption capacity of the particles to the gas molecules is strong, and the CO concentration reaching the TPB is large. Even though the electrocatalytic activity and the electrolyte ion conductivity are low at lower temperatures, the adsorption of the particles to the gas molecules is the main influencing factor, so both the sensitivity and the response value of the sensor are high. When the temperature rises to 500 °C, the electrochemical reaction and the desorption rate become faster, and the conductivity of the electrolyte becomes stronger, thus shortening the response / recovery time, but the adsorption capacity of the particles is somewhat weakened, resulting in a decrease in the response value. When the operating temperature reaches 550 °C, the response value and the sensitivity decrease dramatically, because at high temperature the gas-phase consumption is severe, and the adsorption capacity will also be weakened, the amount of CO molecules participating in the electrochemical reaction at the TPB is greatly reduced, resulting in a decrease in the sensitivity and the response value. The response / recovery time is very short because the increase in temperature speeds up the electrochemical reaction and the desorption rate, and the diffusion of the gas is also faster.

[0049] Overall, at an operating temperature of 500 °C, the combined-SE sensor can maintain a high sensitivity and a short response / recovery time with sufficient response value.

[0050] 5. Sensing mechanism For the single-SE sensor, when the base gas is introduced into the test tube, the sensor is in a state of thermodynamic equilibrium, and the sensitive electrode and the reference electrode are at the same equilibrium potential, so the response value generated is 0. When the sample gas containing CO and O2 is introduced, the response signal of the mixed-potential type sensor is mainly affected by two reactions. One is the gas-phase reaction as shown in equation (1), in the process of CO diffusion to the TPB, a part of it is oxidized in the porous SE layer, reducing the amount of CO molecules reaching the TPB. The other is the electrochemical reaction, the anodic reaction of CO oxidation on the SE as shown in equation (2) and the cathodic reaction of O2 reduction as shown in equation (3), which occur simultaneously at the TPB. On the RE, only the reduction reaction of O2 (equation (3)) occurs.

[0051] 2CO(ads) + O2(ads) → 2CO2(gas) (1) 2CO(ads) + 2O2- (YSZ)→2CO2(gas)+4e - (2) O2(ads)+4e - →2O 2- (YSZ) (3) When the anodic reaction (equation (2)) and the cathodic reaction (equation (3)) reach dynamic equilibrium, the reaction current is equal, and a mixed potential (E ) is generated. According to the Butler-Volmer equation, when the concentration of O2 is constant, there is a linear relationship with the logarithm of the CO concentration, as shown in equation (4). A single-SE sensor with a different electrode can also exhibit a similar linear relationship, but the values of and are different.

[0052] (4) where and are constants, and

[0053] For a combined-SEs sensor, the gas-phase reaction (equation (1)) occurs in each SE layer during gas diffusion, and two electrochemical reactions (equations (2) and (3)) occur at the TPB for each SE, as shown in Figure 10 . After the introduction of CO and a constant concentration of O2, the mixed potential is generated when the anodic reaction (equation (2)) and the cathodic reaction (equation (3)) of the In2O3-SE and the NiFe2O4-SE reach dynamic equilibrium, and the mixed potential (E ) of the combined-SEs sensor will also exhibit a linear relationship with the logarithm of the CO concentration, as shown in equation (5). When NH3 and a constant concentration of O2 are introduced, it can be represented by equation (6).

[0054] (5) (6) where , , , are constants corresponding to SE, , , , are constants corresponding to SE'. The constants corresponding to SE and SE' satisfy , , in theory, the interference response to NH3 can be offset while maintaining a high response to CO.

[0055] In₂O₃ and NiFe₂O₄ exhibit similar response values ​​to NH₃, such as... Figure 2 (a) Therefore, the combination of these two electrodes - the SEs sensor exhibits excellent selectivity for CO, such as Figure 2 (b).

[0056] The sensing mechanism can be further elucidated by measuring polarization curves, where the cathodic polarization curve is measured in the base gas, and the corrected anodic polarization curve is obtained by subtracting the base gas measurement from the sample gas measurement. For a single-SE sensor, the intersection of the corrected anodic and cathodic polarization curves is an estimate of the mixing potential. At the mixing potential, the oxidation and reduction reaction rates are equal, and the actual measured response is usually close to the estimated mixing potential. For combined-SE sensors, each SE undergoes an electrochemical reaction at the TPB, and both generate a mixing potential (SE = ...). SE' is By calculating the difference in the mixed potential value corresponding to each SE, the response value of the combined-SE sensor can be determined. The estimate is made as shown in equation (7).

[0057] (7) At an operating temperature of 500℃, the corrected anode and cathode polarization curves of a single-SE sensor with In2O3 and NiFe2O4 electrodes respectively for 80ppmCO are shown below. Figure 11 As shown in (a), the potential estimates are -69mV and -10mV, respectively, compared with... Figure 2 The measured values ​​of -66 mV and -8 mV in (a) are similar, indicating that both fabricated single-SE sensors conform to a mixed potential mechanism. The estimated response of the combined-SE sensor using these two electrodes to CO is ( According to equation (7), the value is -59mV. The polarization curve test results of the combined-SEs sensor are as follows: Figure 11 As shown in (b), its mixed potential estimate is -62mV, which is very close to the response estimate of -59mV and the actual measured value of -58mV, which means that the fabricated combined-SEs sensor also follows the mixed potential mechanism.

[0058] 6. The effect of oxygen In the automotive exhaust environment, changes in oxygen content often affect the sensor response performance. Therefore, it is necessary to test the stability of the combined-SEs sensor's response to CO under different oxygen contents. Figure 12The response values ​​of the combined-SEs sensor with a 900℃ sintered In2O3 electrode are shown to 5 ppm and 80 ppm CO in an oxygen content range of 2-12 vol%. It can be seen that the response values ​​for both concentrations decrease to some extent with increasing oxygen content, possibly because at the high temperature of 500℃, the increased oxygen content leads to greater gaseous CO consumption. Furthermore, using the response value at an oxygen content of 10 vol.% as a baseline, the change in response value is more significant in the 2-4 vol% oxygen content range, while the change is very small in the 4-12 vol% range. Since the oxygen content in automobile exhaust is generally 5-10 vol%, the combined-SEs sensor has sufficient resistance to oxygen interference in the automobile exhaust environment.

[0059] 7. Long-term stability The long-term stability of the sensor in harsh environments is of concern. A combined-SEs sensor with an In2O3 electrode sintered at 900℃ was tested for 20 days in CO sample gases containing different concentrations at 500℃, with response values ​​recorded daily. The test results are as follows: Figure 13 As shown, in a low CO concentration environment of 5 ppm, the response value hardly changes. In a higher CO concentration environment (80 ppm), the response value fluctuates to some extent, but the change does not exceed 5% of the initial measurement value. The different daily response values ​​of the same YSZ-based potential-type sensor under unchanged test conditions may be due to minor changes in the ion conduction characteristics of the electrolyte and the electrode interface state. Overall, the combined-SEs sensor exhibits good long-term stability and can be used for CO monitoring in automotive exhaust environments.

[0060] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing an In2O3 / NiFe2O4 combined electrode material, characterized in that: Includes the following steps: (1) Preparation of In2O3: InCl3·4H2O was placed in an ethanol solution and stirred magnetically until fully dissolved. Then urea was added and stirring was continued. The mixture was then transferred to an autoclave and kept at 200-250℃ for 18-25h. After cooling to room temperature, the hydrothermal product was centrifuged, washed, dried and calcined at high temperature. After cooling, it was ground to obtain In2O3 powder. (2) Preparation of spinel ferrite AFe2O4: A(NO3)2·6H2O and Fe(NO3)3·9H2O were added to deionized water and stirred evenly. Then, hydrated citric acid and ethylene glycol were added. The mixture was heated in a water bath at 70-90℃ and continuously stirred magnetically until it reached a gel state. The gel was calcined at high temperature, cooled and ground to obtain AFe2O4 powder. A is one of Ni, Zn and Co.

2. The preparation method according to claim 1, characterized in that: In step (1), the molar ratio of InCl3·4H2O to urea is 1:12-18.

3. The preparation method according to claim 1, characterized in that: In step (2), the molar ratio of A(NO3)2·6H2O and Fe(NO3)3·9H2O is 1:1-3, and the molar ratio of hydrated citric acid and ethylene glycol is 1:1.5-2.

5.

4. The preparation method according to claim 1, characterized in that: In step (1), the centrifugation speed is 5000-6000 r / min and the centrifugation time is 10-20 min; the drying temperature is 60-100℃ and the drying time is 8-12 h; the high-temperature calcination temperature is 550-650℃, the heating rate is 1-3℃ / min and the time is 2-4 h.

5. The preparation method according to claim 1, characterized in that: In step (2), the high-temperature calcination temperature is 550-650℃, the heating rate is 1-3℃ / min, and the time is 1-3h.

6. A method for preparing a sensor based on the In2O3 / NiFe2O4 combined electrode material prepared by the preparation method according to any one of claims 1-5, characterized in that: Using YSZ as a solid electrolyte, AFe2O4 paste was screen-printed onto one side of YSZ and sintered at 950-1100℃ for 1-3 hours. After cooling to room temperature, it was removed. Then, In2O3 paste was screen-printed onto the other side of YSZ and calcined at 750-1050℃ for 1-3 hours to prepare a combined-SEs sensor.

7. A combined SEs sensor prepared by the preparation method according to claim 6.

8. The application of the combined SEs sensor of claim 7 in CO detection.

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