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A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection
Journal Pre-proof A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection
Bin Yang, Jinlong Xu, Chao Wang, Jianzhong Xiao PII:
S0254-0584(20)30061-4
DOI:
https://doi.org/10.1016/j.matchemphys.2020.122679
Reference:
MAC 122679
To appear in:
Materials Chemistry and Physics
Received Date:
13 November 2019
Accepted Date:
17 January 2020
Please cite this article as: Bin Yang, Jinlong Xu, Chao Wang, Jianzhong Xiao, A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection, Materials Chemistry and
Physics (2020), https://doi.org/10.1016/j.matchemphys.2020.122679
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Journal Pre-proof
A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection Bin Yang, Jinlong Xu, Chao Wang, Jianzhong Xiao State Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China Abstract: The SmMn2O5 was prepared by sol-gel method and characterized by X-ray diffraction, BET and scanning electron microscope. A potentiometric methane sensor was fabricated at the sintering temperature of 900 °C by using SmMn2O5 as sensing electrode and YSZ as solid electrolyte. The response signal for different concentrations of methane was tested at 350-500 °C. With the increase of operating temperature from 350 °C to 500 °C, the methane sensitivity decreases gradually from -47.97 mV/decade to -13.96 mV/decade, meanwhile the response and recovery rates accelerate. At the optimal operating temperature of 400 °C, the response and recovery times are significantly shortened to 27 s and 33 s for 400 ppm CH4, respectively, while the sensitivity is only reduced slightly to -44.50 mV/decade. The linear relationship between the response values and the logarithm of CH4 concentration confirms the mixed potential mechanism of the prepared sensor. The sensor attached with SmMn2O5-SE has a good CH4 selectivity with small cross sensitivity to C3H8 and CO2, and it also shows good reproducibility and long-term stability, which indicate that there is a potential for SmMn2O5 to the methane leakage warning.
Keywords: Potentiometric sensor; SmMn2O5; Methane sensitivity; Response time; Selectivity
1. Introduction The chemical lifetime of methane is about ten years [1], and it would be enriched for a long time once it enters the atmosphere. Human activities, such as natural gas leakage, coal mining, garbage dumps and sewers are the main sources of methane in the atmospheric environment [2]. Methane
Corresponding author. [email protected] (C. Wang); [email protected] (J.Z. Xiao)
Journal Pre-proof can cause a more serious greenhouse effect than carbon dioxide [3], and the gradually rising atmospheric temperature may be related to increased methane content. It is also a flammable and explosive gas, which could cause an explosion when its volume concentration reaches the range of 4.9-15.4% [4], and most coal mine or kitchen explosions are related to methane leakage. In order to reduce the greenhouse effect and explosion risk, it is necessary to apply a technical means to monitor low concentrations of CH4 to provide early warning of leakage. Because it is colorless and odorless, the traditional method cannot achieve this purpose. So, it is eagerly desired to develop a portable, responsive, efficient and reliable methane sensor to protect the atmosphere and ensure the safety of life and property. According to the working principles, various types of methane sensors including semiconductor type [5-14], catalytic combustion type [15], optical spectroscopy type [16-18] and electrochemical type [19] have been reported. Among them, the semiconductor oxide sensor is the mainstream of current research, but it has disadvantages of poor selectivity [20] and high temperature stability [2123]. The electrochemical YSZ-based potentiometric sensor has been widely studied in automobile exhaust gas monitoring due to its simple structure, wide operating temperature range and stable performance [23, 24]. Compared with the semiconductor type, the potentiometric sensor has more advantages, such as better adaptability for harsh working environment and more stable performance, but there are few reports on the potentiometric CH4 sensor, so it is necessary to conduct in-depth research for it. For potentiometric gas sensor, the gas sensing properties of sensing material directly determine the response characteristics, so the selection of the sensing material is particularly important. When the sensor is tested in the target gas, the gas would adsorb in the electrode and diffuse to the threephase boundary (TPB) for electrode electrochemical reaction. Therefore, the adsorption characteristic and catalytic property of the sensing material directly influence the intensity of the electrode reaction, thereby affect the response signal of the sensor. So, it is necessary to choose the material with good catalytic activity and adsorption performance to prepare sensing electrode. It has been reported that SmMn2O5 is a highly efficient catalytic oxidation material with porous structure and has been extensively investigated in automobile exhaust gas treatment [25-28]. We tried to test its gas sensing performance and found it has considerable methane response signal. In this paper, SmMn2O5 was prepared by sol-gel method and sintered at 900 °C as the sensing
Journal Pre-proof electrode for YSZ-based potentiometric CH4 sensor. The CH4 sensing performances of the sensor were investigated at the operating temperatures of 350-500 °C. The effect of operating temperature on CH4 sensitivity and response/recovery rate were studied, and the cross sensitivities as well as the long-term stability of SmMn2O5-SE were also studied.
2. Experiment 2.1 Powder preparation and characterization of SmMn2O5 The SmMn2O5 was prepared by sol-gel method using Sm(NO3)3·6H2O, Mn(NO3)2, citric acid (C6H7O8·H2O) and ethylene glycol (C2H6O2) as raw materials, all of which were purchased from Sinopharm Chemical Reagent Co., Ltd, China. The citric acid and ethylene glycol with the molar ratio of 2:3 were weighed according to the stoichiometric ratio of 3:2 for citric acid and total metal ions. Sm(NO3)3.6H2O and Mn(NO3)2 with the molar ratio of 1:2 were dissolved in deionized water prepared by a ultra-pure water machine (PCDX-J-10, Chengdu, China), magnetically stirred to a homogeneous solution and heated in 80 °C water bath equipment (HWCL-3, Zhengzhou, China) . The citric acid and ethylene glycol were added to the obtained solution and stirred until the gel appeared. The gel was cleaned and filled into a crucible after heat treatment at 300 °C for 3 hours, and then calcined in a furnace (KSL-1700X-S, Hefei, China) at 900 °C for 6 hours in air. The obtained product was ground into fine powder and stored in a dry sealed container. The SmMn2O5 powder was characterized by X-ray diffraction (XRD) technique, recorded on a PANalytical X'Pert PRO diffractometer (PANalytical, Netherlands). A step length of 0.017° and scan speed of 8°/ min were used to collect data. Cu Kα was used with λ=1.5406 Å in the 2θ ranging from 20° to 80°. The instrument operated at 40 kV and 40 mA at room temperature. 2.2 Sensor preparation The potentiometric sensor consists of a 5 mol % Y2O3-doped ZrO2 (5YSZ, TOSOH, Japan) solid electrolyte, sensing electrode (SE) and reference electrode (RE), and its schematic is shown in Fig. 1. The solid electrolyte was prepared on a tape casting equipment (MSK-AFA-Ⅱ , Shenyang， China) and sintered at 1480 °C for 2 hours in air, and its final dimension was 10 mm × 10 mm × 0.3 mm. The reference electrode and Pt-collector were prepared by screen printing Pt slurry (82% Pt content, Yunnan, China) on both surfaces of YSZ electrolyte, dried at 180 °C for 2 hours and then
Journal Pre-proof sintered at 1200 °C for 2 h. The SmMn2O5 powder was uniformly mixed with organic binder to obtain the paste for preparing SE. This binder is a mixture containing 5 wt % ethocel, 94 wt % terpineol and 1 wt % span 80, which were all purchased from Sinopharm Chemical Reagent Co., Ltd, China. The paste was screen printed on the Pt-collector, dried at 180 °C for 2 hours and then sintered at 900 °C for 3 hours to obtain the sensing electrode. Pt wires (Pt1, Yunnan, China) with the diameter of 0.2 mm and the length of 10 mm were connected to both electrodes, respectively. The micro-morphology of SE was characterized by a field emission scanning electron microscope (FSEM, Gemini SEM 300, Germany) at 10 kV. 2.3 Sensor performance evaluation The sensing performances of the fabricated sensor were tested and recorded by the electrochemical workstation (VersaSTAT3, Princeton, USA). The sensor was connected to the electrochemical workstation and placed at the internal center of a quartz tube, and the section of the quartz tube where the sensor located was heated by a homemade tube furnace ( XMA-5000ZK, Wuhan, China) set at 350-500 °C. The quartz tube was connected to a gas flow device, which was controlled by a set of gas flow meters (MPA-80, Beijing Seven Star Electronics Company, China). The base gas consists of 10 vol. % oxygen and equilibrium nitrogen. The sample gases consist of 25-400 ppm CH4 with base gas. The total gas flow was 0.5 L / min. All gases used were obtained from nitrogen, air, CH4, C3H8 and CO2 (nitrogen mixed with 1000 ppm CH4 or C3H8 or CO2) under the control of the flow meters, which were all purchased from Jining XieLi Special Gas Co., Ltd, China. Both electrodes were exposed in the same gas atmosphere for testing. The response signal of the sensor was obtained by measuring the open circuit voltage between the two electrodes at different concentrations of target gas. The polarization curves from -100 mV to 0 mV in the base gas and sample gases were tested using a linear sweep speed of 2 mV / s.
3.
Results and discussion
3.1 Crystal phase of SmMn2O5 powder The crystal phase of the SmMn2O5 powder was characterized by XRD, as shown in Fig. 2. It can be found that the sample is well crystallized and the crystal phase is orthorhombic phase SmMn2O5, which is well matched with the standard card (ICCD 01-088-0374). No other diffraction peaks
Journal Pre-proof appear, indicating that the prepared powder is a pure phase. 3.2 Morphology of SmMn2O5-SE Fig. 3a shows surface morphology of SmMn2O5-SE sintered at 900 °C. It is observed that the size of the particles is homogeneous in the range of 100-200 nm. The sensing electrode has a threedimensional network structure composed of SmMn2O5 particles and pores. This porous structure is very beneficial for the target gas adsorption and diffusion to the TPB to participate in the electrode reaction. The micro-morphology of the interface between the electrolyte and SE is shown in Fig. 3b. It can be seen that the thickness of SE is uniform, about 40 μm. A clear and tightly bonded interface is formed between the electrolyte and the electrode. 3.3 Specific surface area and pore size distribution of SmMn2O5-SE The specific surface area and pore size distribution were tested to characterize the SmMn2O5-SE. It can be seen from Fig. 4 that the largest pore size is 170 nm, and the pore diameter above 40 nm account for half, which means that the SmMn2O5-SE has more large pores. The specific surface area of the electrode is 2.167 m2/g. It indicates that the electrode has a loose and porous structure, which is conducive to the adsorption and diffusion of the target gas in the electrode. 3.4 Effect of operating temperature on methane response characteristics 3.4.1 Effect on methane sensitivity The transient response signal curves of the sensor at the operating temperatures of 350 - 500 °C for different concentrations of methane are shown in Fig. 5. As the operating temperature increases, the platform value in the base gas gradually rises. The response signal gets more sensitive to the changes of CH4 concentration, and it’s easier for the signal value to reach the stabilized platform. With the increase of operating temperature from 350 °C to 500 °C, the sensor response signal gradually decreases for each methane concentration, the response value to 400 ppm methane reduces from -60.1 mV to -17.9 mV. But even at 500 °C, this sensor still has a readable signal value to 25 ppm methane, which is much lower than the concentration range of methane explosion, indicating that it could be applied for methane leak warning. At different operating temperatures, the difference in response value for the same concentration of methane may be due to the difference in gas adsorption capacity of SmMn2O5-SE. As the operating temperature increases, it is more difficult for
Journal Pre-proof methane molecules to be adsorbed in the electrode. So the amount of methane molecules that could reach the TPB gradually decreases, which eventually leads to a gradually decreasing response value. Fig. 6 shows the logarithmic relationship between the response signal value ΔV (Vsample gas-Vbase gas)
and the methane concentration at different temperatures. It can be found that the response signal
values exhibit a good linear relationship with the methane concentration at each temperature. This linear relationship helps to directly feedback the gas concentration through the signal value, which indicates SmMn2O5-SE has the potential for low concentration CH4 detection. This good linear relationship is a typical feature of the mixed-potential sensor. It can be seen that at 350 °C, the sensitivity (the slope of the line) is the highest, which is -47.97 mV/decade. As the operating temperature increases from 400 °C to 450 °C and 500 °C, the methane sensitivity gradually decreases from -44.50 mV/decade to -27.08 mV/decade and -13.96 mV/decade, respectively. 3.4.2 Effect on methane response/recovery rate In addition to sensitivity, response/recovery rate is also an important evaluation indicator for sensor performances. Fig. 7 shows the response and recovery times of the methane sensor at different operating temperatures for various concentrations of methane. It can be seen that the response and recovery processes are significantly accelerated as the operating temperature increases. For 400 ppm methane, the response time was shortened from 41 s at 350 °C to 8 s at 500 °C, and the recovery time was reduced from 53 s at 350 °C to 1 s at 500 °C. Such a fast response rate may be related to the excellent catalytic activity of SmMn2O5 [29]. The difference in response times for the same methane concentration at different operating temperatures may be due to the differences in reaction kinetics. As the temperature increases, the kinetic requirement of the electrode reaction could be more easily satisfied. The difference in the rate of the recovery process may be caused by the difference in the methane desorption capacity in the sensing electrode. As the operating temperature increases, it is easier for methane molecules to desorb from the electrode, ultimately resulting in a faster recovery rate. The sensor's response and recovery times to 400 ppm CH4 at 400 °C are significantly shortened to 27 s and 33 s compared with that of 350 °C, while the sensitivity is only reduced slightly to 44.50 mV/decade. Therefore, it is believed that 400 °C is the optimal operating temperature for the
Journal Pre-proof sensor attached with SmMn2O5-SE, so the subsequent gas sensing characteristic is investigated at 400 °C. 3.5 Sensing mechanism When the sensor is exposed to the sample gas, methane would adsorb and diffuse in the sensing electrode. During the diffusion process, the gas phase catalytic reaction would occur, and part of the methane molecules could be consumed, the remaining methane reaches the TPB and participates in the electrode reaction to generate a response signal. The reaction equation for the gas phase catalytic reaction is shown in Eq. (1), and the anode and cathode reaction equations for the electrode reaction are shown in Eq. (2) and Eq. (3). CH4 + 2O2 ⇋ CO2 + 2H2O
(1)
CH4 + 4O2- ⇋ CO2 + 2H2O
(2)
1/2O2 + 2e- ⇋ O2-
(3)
When the anode reaction rate and the cathode reaction rate are equal, a mixed potential is formed. It can be seen from Fig. 6 that the ΔV has a good linear relationship with the logarithm of methane concentration, indicating that the sensor conforms to the mixed potential mechanism. The polarization curve is usually used to verify whether a potentiometric sensor conforms to the mixed potential mechanism [30-34]. In order to further verify that the fabricated sensor conforms to this mechanism, the cathodic polarization was tested in the base gas and the anodic polarization was tested in the sample gases containing various concentrations of methane, the modified polarization curves are shown in Fig. 8. The intersection of the modified cathodic polarization curve and the anodic polarization curve is generally considered to be the theoretical mixed potential value. The theoretical and measured values are summarized in Table 1. It can be clearly found that the two values are close at the same methane concentration, indicating that the sensor conforms to the mixed potential mechanism. 3.6 Cross sensitivity The main component of coal mine gas and civil natural gas is methane, the small components are ethane, propane and butane, and the trace components are carbon dioxide, nitrogen and water. Usually these interfering gases introduce signal errors to gas sensor, so it is necessary to study the cross-sensitivity of these gases. The response values of this sensor to 400 ppm CH4, C3H8 and CO2
Journal Pre-proof at 400 °C are shown in Fig. 9. It could be found that the response value to CH4 is much larger than the response signal values to C3H8 and CO2, indicating that the sensor has good CH4 selectivity. It is necessary to point out that the response signal value to C3H8 reaches 40% of that to CH4, so the reduction of the cross sensitivity to C3H8 would be future research direction. However, the concentration of C3H8 is much lower than that of CH4 in coal mine gas and civil natural gas, moreover their signal directions are the same, so the interference it brings would be very limited. Based on the above analysis, it is believed that the sensor attached with SmMn2O5-SE can achieve the purpose of monitoring and leakage alarming for coal mine gas or civil natural gas. 3.7 Reproducibility and long-term stability Reproducibility and long-term stability are important indexes for evaluating whether a sensor could meet practical application. In this work, reproducibility was investigated by cyclical testing of the transient response to 400 ppm CH4, and the result is shown in Fig. 10a. It can be found that the signal curve of each test process is similar and the signal values fluctuate slightly, indicating that the sensor has good reproducibility. The long-term stability was assessed by multiple testing the sensor's response signal values to 400 ppm methane at 400 °C. The test period is up to 6 weeks and the test frequency is three times per week. The test results are shown in Fig. 10b, it can be seen that the signal fluctuations in the base gas and sample gas are very weak, demonstrating the good long-term stability of the sensor, which may be related to the excellent stability of the mullite phase structure of SmMn2O5 at high temperature [35].
4. Conclusion A new sensing material SmMn2O5 was prepared using sol-gel method for methane detection. Potentiometric type methane sensor based on YSZ solid electrolyte and SmMn2O5-SE was fabricated and evaluated at 350-500 °C. After sintering at 900 °C, the SmMn2O5 electrode exhibits a porous three-dimensional network structure with the specific surface area of 2.167 m2/g. As the operating temperature increases, the sensor response value gradually decreases for each methane concentration, and the sensitivity of the sensor gradually decreases from -47.97 mV/decade at 350 °C to -13.96 mV/decade at 500 °C. At the same time, the response and recovery time also significantly shortened as the operating temperature increased, from 41 s and 53 s at 350 °C to 8
Journal Pre-proof sand 1 s at 500 °C, respectively. The response characteristics of the sensor are in accordance with the mixed potential mechanism proved by polarization curve tests. The sensor showed good reproducibility, long-term stability and methane selectivity against C3H8 and CO2. Based on the good methane response characteristics of SmMn2O5-SE, it is believed that the SmMn2O5 has potential to leakage warning for coal mine and civil natural gas.
Acknowledgments This work is supported by the China Postdoctoral Science Foundation (Grant number: 2019M662603). The authors are deeply grateful to the Analytical and Testing Center of Huazhong University of Science and Technology for their hard works in XRD and SEM testing.
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Figure caption Fig.1. Schematic diagram of the prepared sensor. Fig.2. The XRD diffraction pattern of the SmMn2O5 powder. Fig.3. (a) Surface morphology of the SmMn2O5-SE; (b) micro-morphology of the interface between YSZ and SmMn2O5-SE. Fig. 4. The specific surface area and pore size distribution of the SmMn2O5-SE using the BET method. Fig.5. Transients response curve for the sensor attached with SmMn2O5-SE toward 25-400 ppm CH4 at 350-500 °C. Fig.6. The relationship between response values and logarithm of CH4 concentration for the sensor attached with SmMn2O5-SE at 350-500 °C. Fig.7. (a) The response times and (b) the recovery times of the sensor for different concentrations of methane at 350-500 °C. Fig.8. Modified polarization curves of the sensor attached with SmMn2O5-SE recorded in base gas and sample gases at 400 °C.
Journal Pre-proof Fig.9. Cross Sensitivities of the sensor attached with SmMn2O5-SE to CH4, C3H8 and CO2 of 400 ppm at 400 °C. Fig.10. (a) Reproducibility test and (b) long-term stability test of the sensor attached with SmMn2O5-SE.
Journal Pre-proof No conflict of interest exists in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors have approved the manuscript that is enclosed.
Journal Pre-proof Research highlights: 1. SmMn2O5 was used as sensing electrode of potentiometric methane sensor for the first time. 2. The lower detection limit of this sensor is even as low as 25 ppm. 3. The SmMn2O5 material has a good application prospect in the early warning of methane leakage.
Journal Pre-proof
Table 1. The comparison of the theoretical and the measured response values at 400 °C. CH4 concentration / ppm
ΔV / mV
200 ppm
Theoretical -38.0
Measured -39.6
400 ppm
-54.6
-56.0
Bin Yang, Jinlong Xu, Chao Wang, Jianzhong Xiao PII:
S0254-0584(20)30061-4
DOI:
https://doi.org/10.1016/j.matchemphys.2020.122679
Reference:
MAC 122679
To appear in:
Materials Chemistry and Physics
Received Date:
13 November 2019
Accepted Date:
17 January 2020
Please cite this article as: Bin Yang, Jinlong Xu, Chao Wang, Jianzhong Xiao, A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection, Materials Chemistry and
Physics (2020), https://doi.org/10.1016/j.matchemphys.2020.122679
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A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection Bin Yang, Jinlong Xu, Chao Wang, Jianzhong Xiao State Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China Abstract: The SmMn2O5 was prepared by sol-gel method and characterized by X-ray diffraction, BET and scanning electron microscope. A potentiometric methane sensor was fabricated at the sintering temperature of 900 °C by using SmMn2O5 as sensing electrode and YSZ as solid electrolyte. The response signal for different concentrations of methane was tested at 350-500 °C. With the increase of operating temperature from 350 °C to 500 °C, the methane sensitivity decreases gradually from -47.97 mV/decade to -13.96 mV/decade, meanwhile the response and recovery rates accelerate. At the optimal operating temperature of 400 °C, the response and recovery times are significantly shortened to 27 s and 33 s for 400 ppm CH4, respectively, while the sensitivity is only reduced slightly to -44.50 mV/decade. The linear relationship between the response values and the logarithm of CH4 concentration confirms the mixed potential mechanism of the prepared sensor. The sensor attached with SmMn2O5-SE has a good CH4 selectivity with small cross sensitivity to C3H8 and CO2, and it also shows good reproducibility and long-term stability, which indicate that there is a potential for SmMn2O5 to the methane leakage warning.
Keywords: Potentiometric sensor; SmMn2O5; Methane sensitivity; Response time; Selectivity
1. Introduction The chemical lifetime of methane is about ten years [1], and it would be enriched for a long time once it enters the atmosphere. Human activities, such as natural gas leakage, coal mining, garbage dumps and sewers are the main sources of methane in the atmospheric environment [2]. Methane
Corresponding author. [email protected] (C. Wang); [email protected] (J.Z. Xiao)
Journal Pre-proof can cause a more serious greenhouse effect than carbon dioxide [3], and the gradually rising atmospheric temperature may be related to increased methane content. It is also a flammable and explosive gas, which could cause an explosion when its volume concentration reaches the range of 4.9-15.4% [4], and most coal mine or kitchen explosions are related to methane leakage. In order to reduce the greenhouse effect and explosion risk, it is necessary to apply a technical means to monitor low concentrations of CH4 to provide early warning of leakage. Because it is colorless and odorless, the traditional method cannot achieve this purpose. So, it is eagerly desired to develop a portable, responsive, efficient and reliable methane sensor to protect the atmosphere and ensure the safety of life and property. According to the working principles, various types of methane sensors including semiconductor type [5-14], catalytic combustion type [15], optical spectroscopy type [16-18] and electrochemical type [19] have been reported. Among them, the semiconductor oxide sensor is the mainstream of current research, but it has disadvantages of poor selectivity [20] and high temperature stability [2123]. The electrochemical YSZ-based potentiometric sensor has been widely studied in automobile exhaust gas monitoring due to its simple structure, wide operating temperature range and stable performance [23, 24]. Compared with the semiconductor type, the potentiometric sensor has more advantages, such as better adaptability for harsh working environment and more stable performance, but there are few reports on the potentiometric CH4 sensor, so it is necessary to conduct in-depth research for it. For potentiometric gas sensor, the gas sensing properties of sensing material directly determine the response characteristics, so the selection of the sensing material is particularly important. When the sensor is tested in the target gas, the gas would adsorb in the electrode and diffuse to the threephase boundary (TPB) for electrode electrochemical reaction. Therefore, the adsorption characteristic and catalytic property of the sensing material directly influence the intensity of the electrode reaction, thereby affect the response signal of the sensor. So, it is necessary to choose the material with good catalytic activity and adsorption performance to prepare sensing electrode. It has been reported that SmMn2O5 is a highly efficient catalytic oxidation material with porous structure and has been extensively investigated in automobile exhaust gas treatment [25-28]. We tried to test its gas sensing performance and found it has considerable methane response signal. In this paper, SmMn2O5 was prepared by sol-gel method and sintered at 900 °C as the sensing
Journal Pre-proof electrode for YSZ-based potentiometric CH4 sensor. The CH4 sensing performances of the sensor were investigated at the operating temperatures of 350-500 °C. The effect of operating temperature on CH4 sensitivity and response/recovery rate were studied, and the cross sensitivities as well as the long-term stability of SmMn2O5-SE were also studied.
2. Experiment 2.1 Powder preparation and characterization of SmMn2O5 The SmMn2O5 was prepared by sol-gel method using Sm(NO3)3·6H2O, Mn(NO3)2, citric acid (C6H7O8·H2O) and ethylene glycol (C2H6O2) as raw materials, all of which were purchased from Sinopharm Chemical Reagent Co., Ltd, China. The citric acid and ethylene glycol with the molar ratio of 2:3 were weighed according to the stoichiometric ratio of 3:2 for citric acid and total metal ions. Sm(NO3)3.6H2O and Mn(NO3)2 with the molar ratio of 1:2 were dissolved in deionized water prepared by a ultra-pure water machine (PCDX-J-10, Chengdu, China), magnetically stirred to a homogeneous solution and heated in 80 °C water bath equipment (HWCL-3, Zhengzhou, China) . The citric acid and ethylene glycol were added to the obtained solution and stirred until the gel appeared. The gel was cleaned and filled into a crucible after heat treatment at 300 °C for 3 hours, and then calcined in a furnace (KSL-1700X-S, Hefei, China) at 900 °C for 6 hours in air. The obtained product was ground into fine powder and stored in a dry sealed container. The SmMn2O5 powder was characterized by X-ray diffraction (XRD) technique, recorded on a PANalytical X'Pert PRO diffractometer (PANalytical, Netherlands). A step length of 0.017° and scan speed of 8°/ min were used to collect data. Cu Kα was used with λ=1.5406 Å in the 2θ ranging from 20° to 80°. The instrument operated at 40 kV and 40 mA at room temperature. 2.2 Sensor preparation The potentiometric sensor consists of a 5 mol % Y2O3-doped ZrO2 (5YSZ, TOSOH, Japan) solid electrolyte, sensing electrode (SE) and reference electrode (RE), and its schematic is shown in Fig. 1. The solid electrolyte was prepared on a tape casting equipment (MSK-AFA-Ⅱ , Shenyang， China) and sintered at 1480 °C for 2 hours in air, and its final dimension was 10 mm × 10 mm × 0.3 mm. The reference electrode and Pt-collector were prepared by screen printing Pt slurry (82% Pt content, Yunnan, China) on both surfaces of YSZ electrolyte, dried at 180 °C for 2 hours and then
Journal Pre-proof sintered at 1200 °C for 2 h. The SmMn2O5 powder was uniformly mixed with organic binder to obtain the paste for preparing SE. This binder is a mixture containing 5 wt % ethocel, 94 wt % terpineol and 1 wt % span 80, which were all purchased from Sinopharm Chemical Reagent Co., Ltd, China. The paste was screen printed on the Pt-collector, dried at 180 °C for 2 hours and then sintered at 900 °C for 3 hours to obtain the sensing electrode. Pt wires (Pt1, Yunnan, China) with the diameter of 0.2 mm and the length of 10 mm were connected to both electrodes, respectively. The micro-morphology of SE was characterized by a field emission scanning electron microscope (FSEM, Gemini SEM 300, Germany) at 10 kV. 2.3 Sensor performance evaluation The sensing performances of the fabricated sensor were tested and recorded by the electrochemical workstation (VersaSTAT3, Princeton, USA). The sensor was connected to the electrochemical workstation and placed at the internal center of a quartz tube, and the section of the quartz tube where the sensor located was heated by a homemade tube furnace ( XMA-5000ZK, Wuhan, China) set at 350-500 °C. The quartz tube was connected to a gas flow device, which was controlled by a set of gas flow meters (MPA-80, Beijing Seven Star Electronics Company, China). The base gas consists of 10 vol. % oxygen and equilibrium nitrogen. The sample gases consist of 25-400 ppm CH4 with base gas. The total gas flow was 0.5 L / min. All gases used were obtained from nitrogen, air, CH4, C3H8 and CO2 (nitrogen mixed with 1000 ppm CH4 or C3H8 or CO2) under the control of the flow meters, which were all purchased from Jining XieLi Special Gas Co., Ltd, China. Both electrodes were exposed in the same gas atmosphere for testing. The response signal of the sensor was obtained by measuring the open circuit voltage between the two electrodes at different concentrations of target gas. The polarization curves from -100 mV to 0 mV in the base gas and sample gases were tested using a linear sweep speed of 2 mV / s.
3.
Results and discussion
3.1 Crystal phase of SmMn2O5 powder The crystal phase of the SmMn2O5 powder was characterized by XRD, as shown in Fig. 2. It can be found that the sample is well crystallized and the crystal phase is orthorhombic phase SmMn2O5, which is well matched with the standard card (ICCD 01-088-0374). No other diffraction peaks
Journal Pre-proof appear, indicating that the prepared powder is a pure phase. 3.2 Morphology of SmMn2O5-SE Fig. 3a shows surface morphology of SmMn2O5-SE sintered at 900 °C. It is observed that the size of the particles is homogeneous in the range of 100-200 nm. The sensing electrode has a threedimensional network structure composed of SmMn2O5 particles and pores. This porous structure is very beneficial for the target gas adsorption and diffusion to the TPB to participate in the electrode reaction. The micro-morphology of the interface between the electrolyte and SE is shown in Fig. 3b. It can be seen that the thickness of SE is uniform, about 40 μm. A clear and tightly bonded interface is formed between the electrolyte and the electrode. 3.3 Specific surface area and pore size distribution of SmMn2O5-SE The specific surface area and pore size distribution were tested to characterize the SmMn2O5-SE. It can be seen from Fig. 4 that the largest pore size is 170 nm, and the pore diameter above 40 nm account for half, which means that the SmMn2O5-SE has more large pores. The specific surface area of the electrode is 2.167 m2/g. It indicates that the electrode has a loose and porous structure, which is conducive to the adsorption and diffusion of the target gas in the electrode. 3.4 Effect of operating temperature on methane response characteristics 3.4.1 Effect on methane sensitivity The transient response signal curves of the sensor at the operating temperatures of 350 - 500 °C for different concentrations of methane are shown in Fig. 5. As the operating temperature increases, the platform value in the base gas gradually rises. The response signal gets more sensitive to the changes of CH4 concentration, and it’s easier for the signal value to reach the stabilized platform. With the increase of operating temperature from 350 °C to 500 °C, the sensor response signal gradually decreases for each methane concentration, the response value to 400 ppm methane reduces from -60.1 mV to -17.9 mV. But even at 500 °C, this sensor still has a readable signal value to 25 ppm methane, which is much lower than the concentration range of methane explosion, indicating that it could be applied for methane leak warning. At different operating temperatures, the difference in response value for the same concentration of methane may be due to the difference in gas adsorption capacity of SmMn2O5-SE. As the operating temperature increases, it is more difficult for
Journal Pre-proof methane molecules to be adsorbed in the electrode. So the amount of methane molecules that could reach the TPB gradually decreases, which eventually leads to a gradually decreasing response value. Fig. 6 shows the logarithmic relationship between the response signal value ΔV (Vsample gas-Vbase gas)
and the methane concentration at different temperatures. It can be found that the response signal
values exhibit a good linear relationship with the methane concentration at each temperature. This linear relationship helps to directly feedback the gas concentration through the signal value, which indicates SmMn2O5-SE has the potential for low concentration CH4 detection. This good linear relationship is a typical feature of the mixed-potential sensor. It can be seen that at 350 °C, the sensitivity (the slope of the line) is the highest, which is -47.97 mV/decade. As the operating temperature increases from 400 °C to 450 °C and 500 °C, the methane sensitivity gradually decreases from -44.50 mV/decade to -27.08 mV/decade and -13.96 mV/decade, respectively. 3.4.2 Effect on methane response/recovery rate In addition to sensitivity, response/recovery rate is also an important evaluation indicator for sensor performances. Fig. 7 shows the response and recovery times of the methane sensor at different operating temperatures for various concentrations of methane. It can be seen that the response and recovery processes are significantly accelerated as the operating temperature increases. For 400 ppm methane, the response time was shortened from 41 s at 350 °C to 8 s at 500 °C, and the recovery time was reduced from 53 s at 350 °C to 1 s at 500 °C. Such a fast response rate may be related to the excellent catalytic activity of SmMn2O5 [29]. The difference in response times for the same methane concentration at different operating temperatures may be due to the differences in reaction kinetics. As the temperature increases, the kinetic requirement of the electrode reaction could be more easily satisfied. The difference in the rate of the recovery process may be caused by the difference in the methane desorption capacity in the sensing electrode. As the operating temperature increases, it is easier for methane molecules to desorb from the electrode, ultimately resulting in a faster recovery rate. The sensor's response and recovery times to 400 ppm CH4 at 400 °C are significantly shortened to 27 s and 33 s compared with that of 350 °C, while the sensitivity is only reduced slightly to 44.50 mV/decade. Therefore, it is believed that 400 °C is the optimal operating temperature for the
Journal Pre-proof sensor attached with SmMn2O5-SE, so the subsequent gas sensing characteristic is investigated at 400 °C. 3.5 Sensing mechanism When the sensor is exposed to the sample gas, methane would adsorb and diffuse in the sensing electrode. During the diffusion process, the gas phase catalytic reaction would occur, and part of the methane molecules could be consumed, the remaining methane reaches the TPB and participates in the electrode reaction to generate a response signal. The reaction equation for the gas phase catalytic reaction is shown in Eq. (1), and the anode and cathode reaction equations for the electrode reaction are shown in Eq. (2) and Eq. (3). CH4 + 2O2 ⇋ CO2 + 2H2O
(1)
CH4 + 4O2- ⇋ CO2 + 2H2O
(2)
1/2O2 + 2e- ⇋ O2-
(3)
When the anode reaction rate and the cathode reaction rate are equal, a mixed potential is formed. It can be seen from Fig. 6 that the ΔV has a good linear relationship with the logarithm of methane concentration, indicating that the sensor conforms to the mixed potential mechanism. The polarization curve is usually used to verify whether a potentiometric sensor conforms to the mixed potential mechanism [30-34]. In order to further verify that the fabricated sensor conforms to this mechanism, the cathodic polarization was tested in the base gas and the anodic polarization was tested in the sample gases containing various concentrations of methane, the modified polarization curves are shown in Fig. 8. The intersection of the modified cathodic polarization curve and the anodic polarization curve is generally considered to be the theoretical mixed potential value. The theoretical and measured values are summarized in Table 1. It can be clearly found that the two values are close at the same methane concentration, indicating that the sensor conforms to the mixed potential mechanism. 3.6 Cross sensitivity The main component of coal mine gas and civil natural gas is methane, the small components are ethane, propane and butane, and the trace components are carbon dioxide, nitrogen and water. Usually these interfering gases introduce signal errors to gas sensor, so it is necessary to study the cross-sensitivity of these gases. The response values of this sensor to 400 ppm CH4, C3H8 and CO2
Journal Pre-proof at 400 °C are shown in Fig. 9. It could be found that the response value to CH4 is much larger than the response signal values to C3H8 and CO2, indicating that the sensor has good CH4 selectivity. It is necessary to point out that the response signal value to C3H8 reaches 40% of that to CH4, so the reduction of the cross sensitivity to C3H8 would be future research direction. However, the concentration of C3H8 is much lower than that of CH4 in coal mine gas and civil natural gas, moreover their signal directions are the same, so the interference it brings would be very limited. Based on the above analysis, it is believed that the sensor attached with SmMn2O5-SE can achieve the purpose of monitoring and leakage alarming for coal mine gas or civil natural gas. 3.7 Reproducibility and long-term stability Reproducibility and long-term stability are important indexes for evaluating whether a sensor could meet practical application. In this work, reproducibility was investigated by cyclical testing of the transient response to 400 ppm CH4, and the result is shown in Fig. 10a. It can be found that the signal curve of each test process is similar and the signal values fluctuate slightly, indicating that the sensor has good reproducibility. The long-term stability was assessed by multiple testing the sensor's response signal values to 400 ppm methane at 400 °C. The test period is up to 6 weeks and the test frequency is three times per week. The test results are shown in Fig. 10b, it can be seen that the signal fluctuations in the base gas and sample gas are very weak, demonstrating the good long-term stability of the sensor, which may be related to the excellent stability of the mullite phase structure of SmMn2O5 at high temperature [35].
4. Conclusion A new sensing material SmMn2O5 was prepared using sol-gel method for methane detection. Potentiometric type methane sensor based on YSZ solid electrolyte and SmMn2O5-SE was fabricated and evaluated at 350-500 °C. After sintering at 900 °C, the SmMn2O5 electrode exhibits a porous three-dimensional network structure with the specific surface area of 2.167 m2/g. As the operating temperature increases, the sensor response value gradually decreases for each methane concentration, and the sensitivity of the sensor gradually decreases from -47.97 mV/decade at 350 °C to -13.96 mV/decade at 500 °C. At the same time, the response and recovery time also significantly shortened as the operating temperature increased, from 41 s and 53 s at 350 °C to 8
Journal Pre-proof sand 1 s at 500 °C, respectively. The response characteristics of the sensor are in accordance with the mixed potential mechanism proved by polarization curve tests. The sensor showed good reproducibility, long-term stability and methane selectivity against C3H8 and CO2. Based on the good methane response characteristics of SmMn2O5-SE, it is believed that the SmMn2O5 has potential to leakage warning for coal mine and civil natural gas.
Acknowledgments This work is supported by the China Postdoctoral Science Foundation (Grant number: 2019M662603). The authors are deeply grateful to the Analytical and Testing Center of Huazhong University of Science and Technology for their hard works in XRD and SEM testing.
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Figure caption Fig.1. Schematic diagram of the prepared sensor. Fig.2. The XRD diffraction pattern of the SmMn2O5 powder. Fig.3. (a) Surface morphology of the SmMn2O5-SE; (b) micro-morphology of the interface between YSZ and SmMn2O5-SE. Fig. 4. The specific surface area and pore size distribution of the SmMn2O5-SE using the BET method. Fig.5. Transients response curve for the sensor attached with SmMn2O5-SE toward 25-400 ppm CH4 at 350-500 °C. Fig.6. The relationship between response values and logarithm of CH4 concentration for the sensor attached with SmMn2O5-SE at 350-500 °C. Fig.7. (a) The response times and (b) the recovery times of the sensor for different concentrations of methane at 350-500 °C. Fig.8. Modified polarization curves of the sensor attached with SmMn2O5-SE recorded in base gas and sample gases at 400 °C.
Journal Pre-proof Fig.9. Cross Sensitivities of the sensor attached with SmMn2O5-SE to CH4, C3H8 and CO2 of 400 ppm at 400 °C. Fig.10. (a) Reproducibility test and (b) long-term stability test of the sensor attached with SmMn2O5-SE.
Journal Pre-proof No conflict of interest exists in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors have approved the manuscript that is enclosed.
Journal Pre-proof Research highlights: 1. SmMn2O5 was used as sensing electrode of potentiometric methane sensor for the first time. 2. The lower detection limit of this sensor is even as low as 25 ppm. 3. The SmMn2O5 material has a good application prospect in the early warning of methane leakage.
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Table 1. The comparison of the theoretical and the measured response values at 400 °C. CH4 concentration / ppm
ΔV / mV
200 ppm
Theoretical -38.0
Measured -39.6
400 ppm
-54.6
-56.0