How does an electrochemical gas sensor work?

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Figure 3: Electrochemical gas sensor (O2)

1.

WE = working electrode, CE = counter electrode, RE = reference electrode (see

explanation below)

Some electrical charges travel through the electrolyte. To keep the charge constant in the

electrolyte, some electrons flow through the circuitry (which could be as simple as a

resistor) connected to the sensor between the WE and the CE. In the case of the CO

sensor, H+ moves in the electrolyte from the WE to the CE whilst in the external circuitry,

e- moves from the WE to the CE. Effectively, the sensor generates a current flowing from

the CE to the WE. This is why the sensor is also called an amperometric sensor.

The current generated by these chemical reactions is proportional to the gas concentration.

So, in the presence of a targeted gas, a current flows through the CE and changes the

potential at the electrolyte/electrode interface. This can slightly impact the sensor response.

To maintain a stable potential difference between the WE and the electrolyte of the sensor,

some sensors are fitted with a third electrode in which no current flows (i.e. there is no

chemical reaction occurring at the surface). This electrode is called a reference electrode

(RE).