AN4348 datasheet(3/25 Pages) STMICROELECTRONICS

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AN4348

How does an electrochemical gas sensor work?

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How does an electrochemical gas sensor work?

An electrochemical gas sensor (see Figure 2) contains a gas membrane and two or three

electrodes in contact with an electrolyte. The sensor is impermeable to the electrolyte. Gas

enters the sensor through the gas membrane which limits the rate of gas diffusion

(therefore impacting the sensitivity of the sensor). When gas reaches the working electrode

(WE), a chemical reaction occurs: either oxidation (loss of electrons) occurs from the gases

CO, H2S, sulfur dioxide (SO2), and nitrogen monoxide (NO), or reduction (gain of electrons)

occurs from the gases O2, NO2, and chlorine (Cl2). The reaction depends on the sensor.

If oxidation occurs at the WE, the complimentary reaction (reduction) occurs at its counter

electrode (CE).

For example, in the case of a CO sensor (see Figure 2), oxidation occurs at its WE:

CO + H2O → CO2 + 2H

+ + 2e-

At the CE, the reaction is:

½O2 + 2H+ + 2e- → H2O (consumption of oxygen)

Figure 2: Electrochemical gas sensor (CO)

WE = working electrode (often called the "sensing electrode"), CE = counter electrode,

RE = reference electrode (see explanation below)

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