Detection Principle of CO Electrochemical Sensor for Gas Detectors
The electrochemical carbon monoxide gas sensor adopts a hermetically sealed structure, which consists of electrodes, filters, gas-permeable membranes, electrolyte, electrode lead pins and a housing.
Matched with alarm controllers, the CO sensor serves as the core sensing component of the alarm device, operating on the fundamental principle of potentiostatic electrolysis. When carbon monoxide diffuses into the gas sensor, an electric current is generated at the output terminal and fed to the sampling circuit inside the alarm unit, converting chemical energy into electrical energy. As the gas concentration fluctuates, the output current of the sensor changes proportionally. The signal is converted and amplified by the intermediate circuit of the alarm to drive various actuators, realizing audible, visual and electrical detection and alarm functions. Together with matching control equipment, it forms an environmental monitoring and alarming system.
When carbon monoxide passes through the air holes on the housing and diffuses via the gas-permeable membrane to the surface of the working electrode, CO undergoes oxidation under the catalytic effect of the working electrode. The chemical reaction equation is as follows:
\(\ce{CO + H2O -> CO2 + 2H+ + 2e-}\)
Hydrogen ions and electrons generated from the oxidation reaction at the working electrode migrate through the electrolyte to the counter electrode, which is spaced apart from the working electrode, and participate in the reduction reaction with oxygen dissolved in water. The reaction equation is:
\(\ce{\frac12 O2 + 2H+ + 2e- -> H2O}\)
A reversible redox reaction therefore takes place inside the sensor, with the overall reaction written as:
\(\ce{2CO + O2 -> 2CO2}\)
This continuous reversible redox reaction between the working electrode and the counter electrode creates a potential difference across the two electrodes.
Nevertheless, reactions occurring on both electrodes cause electrode polarization, making it difficult to stabilize the inter-electrode potential and limiting the measurable concentration range of carbon monoxide.
A reference electrode is introduced to maintain a constant potential between the electrodes. In a three-electrode electrochemical gas sensor, the output reflects the potential variation between the reference electrode and the working electrode. Since the reference electrode does not participate in any oxidation or reduction reactions, it stabilizes the inter-electrode potential (potentiostatic state), whereby potential deviation directly correlates with the CO concentration. The output current of the sensor is proportional to the gas concentration. The CO concentration can be measured by detecting the output current via an external circuit connected to the electrode leads, with an extensive linear measurement range. By connecting external signal acquisition, conversion and output circuits to the sensor, continuous detection and monitoring of carbon monoxide gas can be achieved.
