Operation Principle of Short-circuit Current type Proton Conductor CO Sensor

In the presence of CO mixed with air under the open-circuit condition, anodic oxidation of CO (2) and cathodic reduction of oxygen (3) proceed on the working electrode simultaneously at the same rate, forming a local cell.

CO + H₂O → CO₂ + 2H⁺ + 2e^-　...(2)

2H⁺ + 1/2O₂ + 2e^- → H₂O　 ...(3)

As a result, the potential of the working electrode changes from E1 to the mixed potential (EM), at which anodic current i(2) and cathodic current i(3) are equal in absolute value in a steady state (Figure 1). The volume of CO reaching the working electrode is limited by a diffusion control system, e.g., a gas diffusion control stainless film as used in the TGS5xxx series sensors, giving a diffusion limiting current (Figure 1), which is linearly proportional to CO concentration. Since only the reaction (1) takes place on the counter electrode, the potential of the counter electrode remains at E1. Thus, the sensing signal of this potentiometric (mixed-potential type) sensor can be given by EM - E1, which is proportional to the logarithm of CO concentration in air.

When the working electrode is electrically connected to the counter electrode with an external wiring to make a short-circuit condition in the presence of CO in air, the potentials of the two electrodes shift to the same value (Esc) in between E1 and EM, as shown in Figure 1. Since the potential of the working electrode changes in the direction the reaction (3) is decelerated, fewer protons are consumed on the working electrode. As a result, excess protons will transfer from the working electrode toward the counter electrode through the proton-conducting membrane. The number of the excess protons is proportional to CO concentration in air. And they are consumed by the reaction (3) on the counter electrode. This process is accompanied by a flow of the equivalent electrons from the working electrode to the counter electrode through an external wiring as a short-circuit current (sensor output current), which is also linearly proportional to CO concentration (Figures 2).

When the sensor returns to clean air, only the reaction (1) takes place on the working electrode, and current does not flow through the wiring because a potential difference between the two electrodes vanishes. Thus, the sensor shows reversible amperometric responses to CO.

Linear relationship between CO concentrations and sensor output currents makes this short-circuit current type CO sensor more accurate when compared to potentiometric CO sensors, the response of which is proportional to the logarithm of CO concentration in air. By calibrating the sensor output current with a known CO concentration, the sensor can be used for quantitative measurements of CO concentrations.

Features

Unlike conventional dry batteries, there is no consumption of active solid or liquid materials or electrodes of the sensor. This ensures this sensor’s excellent long-term stability and maintenance-free operation for a long period. This sensor works without a heater and produces self-generating output current, making it ideal for battery-operated CO detection equipment.