A potentiometric sensor based on SmMn2O5 sensing electrode for methane detection
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 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], [6], [7], [8], [9], [10], [11], [12], [13], [14]], catalytic combustion type [15], optical spectroscopy type [[16], [17], [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 [[21], [22], [23]]. 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 three-phase 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], [26], [27], [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 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.
Section snippets
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 an ultra-pure
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 appear, indicating that the prepared powder is a pure phase.
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.
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
Declaration of competing interest
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 co-authors 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.
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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