How do electrochemical gas sensors work?

Electrochemical Gas Sensors are used to detect and monitor low levels of toxic gases and oxygen levels in both Domestic and Industrial situations where it is essential to ensure that the air is safe to breathe. The most common type of Electrochemical sensor is the 3-Electrode fuel Cell.

Typical Electrochemical Sensor Layout

The air being measured diffuses into the cell through the diffusion barrier (capillary)and filters. when it comes into contact with the sensing electrode, the toxic gas present in the sample undergoes an electrochemical reaction. In the case of Carbon Monoxide, for example, the reaction is:

CO + H₂O => CO₂ + 2H⁺ + 2e^-

The Carbon Dioxide generated diffuses away into the air, whilst the positively charged hydrogen ions (H+) migrate into the electrolyte. The electrons generated charge the electrode but are removed as a small electric current by the external measuring circuit.

This oxidation reaction is balanced by a corresponding reduction reaction at the Counter Electrode:

O₂ + 4H⁺ + 4e^- => 2H₂O

So at one electrode, water is consumed whilst electrons are generated, and at the other, water is recreated and electrons are consumed. Neither reaction can occur if no carbon monoxide is present. By connecting the two electrodes, the small electric current generated between them is measured as directly proportional to the concentration of CO in  the air.

The Reference Electrode controls the whole process. It remains totally immersed in electrolyte, sees no gas and is not allowed to pass any current. The Reference Electrode always remains at the same electrochemical potential (known as it's "Rest-Air Potential", dependent on the material the electrode is made from, and the electrolyte used). The sensing electrode is electrically tied to the reference electrode ensuring it's potential will not change even when it is exposed to its determinand gas and generating current. Usually the potential of the sensing electrode is maintained at exactly the same value as the reference electrode, but for some gases and some applications, performance benefits are gained by maintaining the potential of the sensing electrode at a fixed level above or below the potential of the reference. This is known as "biased" operation.