Question

Explain the relationship between the potential of the working, counter, and reference electrode in an amperometric gas sensor. Explain how this relationship changes as the sensor is exposed to the target gas.

Answer 1

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chematic diagrams of Electrochemical-type gas sensor and Chemical reactions

Gas detection plays an important, even essential role in many areas, ranging from food safety to environmental monitoring, with one of the best known examples being fire alarms based on CO detection. Quantitative measurement of gases is based on a variety of physical or chemical principles. Examples of commercialized sensors include techniques using spectrometry, luminescence and electrochemistry as a basis of sensing. Amongst the various techniques, the electrochemical approach often shows significant advantages over the others. First and second, an electrochemical gas sensor provides high sensitivity at low cost. Third, their compact sizes allow for high portability. Fourth, only a small amount of energy is required to run the detector. On the other hand, the selectivity of electrochemical sensors is rarely perfect.

Electrochemical gas sensors are categorized as being either potentiometric or amperometric in nature. In the former, a potential, E, is established on a suitable electrode and related to the concentrations of the species giving rise to the potential via the concentrations of the Nernst equation. Accordingly potentiometric sensors respond to the logarithm of the concentrations.

Figaro Electrochemical-type gas sensor are amperometric fuel cells with two electrodes. The basic components of two electrode gas sensors are a working (sensing) electrode, a counter electrode, and an ion conductor in between them. When toxic gas such as carbon monoxide (CO) comes in contact with the working electrode, oxidation of CO gas will occur on the working electrode through chemical reaction with water molecules in the air (see Equation 1).

CO + H2O ? CO2? 2H+ + 2e- …?1?

Connecting the working electrode and the counter electrode through a short circuit will allow protons (H+) generated on the working electrode to flow toward the counter electrode through the ion conductor. In addition, generated electrons move to the counter electrode through the external wiring. A reaction with oxygen in the air will occur on the counter electrode (see Equation 2).

?1/2?O2 + 2H+ + 2e- ? H2O …?2?

The overall reaction is shown in Equation 3. Figaro Electrochemical-type gas sensor operate like a battery with gas being the active material for this overall battery reaction.

CO + ?1/2?O2 ? CO2 …?3?

By measuring the current between the working electrode and the counter electrode, this electrochemical cell can be utilized as a gas sensor.

Theoretical equation for CO detection

In order to measure the sensor’s output current, it must be connected to an external circuit. By controlling gas flowing toward the working electrode with diffusion film, output current flowing across the external circuit will be proportional to gas concentration (see Equation 4 and the chart at the right). The linear relationship of gas concentration to sensor output makes this technology ideal for gas sensing applications.

I = F × (A/?) × D × C × n …?4?

where: I: Sensor output F: Faraday constant A: Surface area of diffusion film ??Thickness of diffusion film D: Gas diffusion coefficient C: Gas concentration n: Number of reaction electrons

Features

The oxidation potential of CO gas (as expressed in Equation 1) is lower than the oxidation potential of the electrode?2H+ + 2e- ? H2), i.e. oxidation of CO has less noble potential than deoxidization. Since this reaction occurs easily, no external energy is needed to stimulate the sensor’s chemical reaction, unlike with three-electrode type sensors. As a result, this two-electrode type sensor offers superior characteristics for interference resistance, repeatability, and power consumption.