Using Electrochemical Gas Sensors for Detection of Carbon Monoxide / Hydrogen Sulfide

SGX electrochemical gas sensors determine gases of interest by generating a chemical reaction between oxygen present in the sensor and the gas. A small current is generated due to this chemical reaction and its amount varies in proportion to the gas concentration. The electrochemical sensor is actually a type of fuel cell in terms of operation.

Chemical Reactions in Electrochemical Sensors

Reactions take place on two electrodes and collectively execute the detecting process. These electrodes are composed of small discs of porous PTFE coated with a thin catalytic metal layer. At the working electrode, the reaction between the target gas and water takes place to generate either sulfuric acid or carbon dioxide, two electrons, and hydrogen ions. The resulting hydrogen ions and electrons are reacted with oxygen to form water on the counter electrode.
For instance,
Working Electrode: CO + H₂O → CO₂+ 2H+ +2e^-
Counter Electrode: ½O₂ + 2H+ + 2e^- → H₂O
Overall: CO + ½O₂ → CO₂
Electrodes have a finite catalytic activity that is changeable with temperature and time. Hence, the rate of diffusion of target gas into the sensor needs to be limited using a barrier for efficient reaction. This barrier is in the form of a capillary or tiny hole in the sensor housing (Figure 1).

Figure 1. The sensor housing

It is necessary to keep the electrodes apart and in a wetted condition with an acid electrolyte for effective transmission of an ionic current between them. Separators or discs of inert absorbent material are used for this purpose. Current collectors or thin metal strips connected to the pins link the electrodes to the external circuitry.

Role of the Reference Electrode

In comparison with the counter electrode, the working electrode has to operate at zero volts for optimum efficiency. The reaction of the electrode with the gas leads to voltage generation at the electrode surface and consequently affects the performance of the electrode due to the decreasing sensor response with respect to increasing gas concentration. This voltage change can be avoided and the working electrode can be maintained to run at zero volts when it is connected to a reference electrode.

Significance of Oxygen

Oxygen is essential for the counter electrode reaction and is present in the electrolyte in dissolved condition. Oxygen used in the reaction can be replenished by oxygen in the sample gas or by diffusing oxygen from gas through the housing at the back of the sensor.
If oxygen replenishment does not take place, the counter electrode reaction will change to a reaction generating hydrogen gas through reduction of protons. The sensor generally has adequate amount of oxygen to run for several hours in anaerobic conditions.

Cross-Sensitivity and Temperature Effects

Like other electrochemical sensors, SGX toxic electrochemical sensors may respond to gases other than the target gases due to oxidation or reduction of the gases on the working electrode.
The selection of an appropriate electrode or the use of an absorption filter directly at the backside of diffusion barrier facing the working electrode can reduce these unwanted responses. Each sensor datasheet describes the levels of cross-sensitivity in detail.
The sensor’s zero signal and the net sensitivity to the target gas are affected when the ambient temperature changes. The electrodes employed in SGX sensors have been designed to mitigate these temperature effects as much as possible. However, some effects will still be there.

Concentration Range and Outputs

Hydrogen sulfide and carbon monoxide can be measured in the ppm range with the SGX toxic sensors, thus enabling concentration measurements of these gases in the occupational exposure level. The sensor output is in the range of 55-850nA per ppm of gas.

About SGX Sensortech (IS) Ltd

SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost-effectiveness.
SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.
As an independent OEM supplier of gas sensors, we pride ourselves on providing customers with unrivalled product reliability and personal product support via specialist engineers.
SGX gas sensors are built to the highest standards with all pellistor and infrared gas sensors achieving ATEX and IECEx certification, SGX gas sensors are also UL and CSA approved.
Our product portfolio has continued to expand in technology and detectable gases used in a wide range of applications including:-

• Mining
• Oil and gas
• Confined space entry
• Indoor air quality
• Industrial area protection
• Leak detection

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