5.2 Sensors and Actuators

Perfluorinated ionomers are being used in a wide variety of sensor applications. A commonly used carbon monoxide (CO) detector for instance uses a Nafion electrolyte [32,33]. Jiang et al. have reported on the use of Nafion in a sensor for both CO and methanol [34]. Stability of the response of these CO sensors has been a problem and has led to the recall of the "Nighthawk" and "Lifesaver" detectors in 1999 [35]. It is believed that it was due to the absorption of organic contaminants from the air or from packaging material into the Nafion electrolyte, where they may undergo acid catalyzed condensation reactions. One approach to overcome this problem is to cover the sensor with a thin layer of Teflon AF, which is quite permeable to CO [36]. In another approach, the Nafion electrolyte is converted to the sodium ion form. Whenever a sensor, using a fluorinated sulfonic ionomer as the electrolyte, is exposed to possibly contaminated gases, the conversion of the ionomer to the sodium (or lithium) form should be considered as an approach to long-term stability. In still another approach, the stability is improved by maintaining the Nafion electrolyte wet [37]. This same publication also describes a manufacturing technology suitable for inexpensive mass production. The AC impedance of a planar CO detector using Nafion as the electrolyte was studied by Mortimer et al. They found that the impedance of commercially available extruded film is lower than that of cast Nafion [38].

The...