Amperometric electrochemical sensors are commonly used for the detection of carbon monoxide (CO) and hydrogen (H2) in air. The electrochemistry of heterogeneous CO and H2 oxidations is similar enough that the sensors show significant cross-sensitivities between the two gases. Thus, in applications where H2 is being monitored in the presence of CO, amperometric hydrogen sensors will produce false positive responses due to the presence of CO. This error is further aggravated by the fact that the sensor’s response to CO is typically at least twice that for hydrogen on a volumetric basis. Furthermore, chronic CO exposure will poison or fatigue the H2 sensor electrodes and reduce the sensor sensitivity toward hydrogen.
Amperometric H2 sensors and CO sensors are based on a sensing electrode (typically platinum) in intimate contact with a diffusion barrier separating the electrochemical cell that detects the analyte from the monitored environment. Additional filters of various types, placed between the environment and the diffusion barrier, are commonly employed in amperometric sensors for other compounds to prevent interfering substances from reaching the electrochemical cell where the sensing electrode detects the target analyte.
An additional electrode assembly is installed between the diffusion barrier and the sensing electrode to remove by adsorption the carbon monoxide from the diffusion-driven vapor flux between the monitored environment and the sensing electrode. This guard electrode prevents exposure of the sensing electrode to CO, eliminating any cross-sensitivity or poisoning effects. The electrical potential of the guard electrode may be allowed to float, or it may be held at a particular potential to enhance CO adsorption. At intervals when the adsorption capacity of the guard electrode has been exceeded, the potential of the guard electrode is changed so that adsorbed CO is oxidized to CO2 and the sorptive surface is regenerated. The sorptive capacity of the guard electrode need not be high, as the CO flux is a very small proportion of the CO in the monitored environment and the ultimate sensor life is not limited by sorptive capacity. The guard electrode may be constructed from a high-surface-area platinum material or another material that shows advantages for CO adsorption and surface regeneration.
This approach would lead to a cost-effective hydrogen sensor that does not exhibit cross-sensitivity toward the principal interferent for amperometric hydrogen sensors, and would also have a more stable quantitative response because of the elimination of potential CO poisoning of the sensing electrode. The sensor would have a longer life than one that used a non-regenerable filter.
This work was done by Charles H. Todd, Terrell Lee Morrison, Robert H. Howe, Kenneth P. Flynn, Thomas J. Stapleton, Wai Tak Lee, Daniel A. Gonzales, and Kenneth Carney of Hamilton Sundstrand Space Systems International, Inc. for Johnson Space Center. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-24968-1