The principle and application of solid-state mixed electric potential gas sensor

1. Introduction As we all know, the YSZ potential sensor with Pt electrode at high temperature can generate a Nernst potential difference between the two electrodes. When the reducing gas and oxygen coexist, the measured potential will deviate from the equilibrium potential. Fleming first used the concept of mixed potential to explain the abnormal potential generated by the oxygen sensor in the presence of CO. He believes that the deviation from the Nernstian behavior is due to the first. The preface is well known that the YSZ potential sensor with Pt electrode can be There is a Nernst potential difference between the electrodes, and when reducing gas and oxygen coexist, the measured potential will deviate from the equilibrium potential. Fleming first used the concept of mixed potential to explain the abnormal potential generated by the oxygen sensor in the presence of CO. He believed that the deviation from Nernstian behavior was due to the cathode reduction reaction of oxygen and the anodic oxidation reaction of reducing gas simultaneously occurring on the electrode. The electric potential generated when the two reactions reach equilibrium is called the mixed electric potential [1]. Placing the working electrode of the oxygen sensor in a reducing atmosphere produces this non-Nernstian behavior, which opens up a study on the measurement of different gases based on the principle of mixed potential. As early as more than 20 years ago, someone began to study mixed potential devices for measuring the concentration of reducing gases such as CO, H2, and CH compounds. Shimizu et al. believe that the observed abnormal potential comes from the difference in the catalytic activity of different electrodes. They developed a YSZ oxygen sensor with Pt and Pd electrodes for measuring combustible gases in 1978, but it is higher than 500. The response at ℃ is not great, but the response at low temperature is unstable and the selectivity is poor. The reason is that the Pt electrode is a good oxidation catalyst, so that the reducing gas can be completely oxidized before reaching the three-phase interface at high temperature; while at low temperature, the oxidation of the gas is mainly limited by the low ion conductivity of YSZ . In order to solve these problems, other metals and metal alloys have also been used as alternative materials to improve the selectivity and sensitivity of sensors. V. Schule et al. found that the Pt/Au alloy electrode has a better response to CO and H2 at temperatures higher than 550°C [2]. In recent years, the development and research of various electrode materials and electrolytes for hybrid potential sensors have been very active [3, 6, 7]. Many device prototypes have been prepared, but commercial products that can be applied in practice have not yet appeared, because most devices Not enough long-term stability. The way to further improve these sensors is to replace the precious metal electrodes with metal oxides with better thermal and chemical stability. This not only enables the sensor to operate at a higher temperature, but also expands the types of target gases to be measured. Hybrid potential sensors are affected by the electrode material, electrode morphology, and the type of solid electrolyte. Many hybrid potential sensors based on Pt, Au electrodes and YSZ electrolyte work at temperatures higher than 400°C, but Au electrodes will quickly recrystallize and grow at high temperatures and lose their catalytic activity, which makes the sensor not stable for a long time at high temperatures. sex. The long-term thermal and chemical stability of the electrode and the reproducible sensor structure of the large-scale preparation are the main obstacles to the mixed potential gas sensor. The replacement of the metal electrode with the oxide electrode material with high temperature resistance and good activity is to improve the selectivity and long-term stability of the sensor. Provides prospects. 2. The working principle of the sensor A typical solid-state mixed potential sensor structure is shown in the figure. The sensor is composed of electrode 1/solid electrolyte/electrode 2. Electrode materials are generally metals such as Pt and Au and perovskite-type oxides such as WO3, LaFeO3 and LaSrMnO3; while solid electrolytes are mainly YSZ or CeO2. The two electrodes are placed on the same side of the measured mixed gas composed of oxygen and reducing gas, which eliminates the need for reference gas. The difference in the oxidation-reduction reaction rate of each electrode will produce different mixed electric potentials, and the response of the mixed electric potential device is the mixed electric potential difference between the two electrodes. When multiple electrochemical reactions occur on the electrode, the equilibrium potential is the mixed potential, which comes from the competition of each reaction on the electrode. The solid-state mixed-potential electrochemical sensor using electrode materials with different catalytic effects produces different equilibrium potentials on different electrodes. For low-concentration analysis gases, the possible control processes include mass transfer process and charge transfer process, while the control process of oxygen with higher partial pressure is mainly charge transfer process. These processes determine the various response modes of the mixed potential sensor.