A robust sensor based on the electrochemical oxidation of methanol has been developed for use in measuring the concentration of methanol dissolved in water. The sensor is expected to be particularly useful in continuous monitoring and control of the concentration of an aqueous solution of methanol metered into a liquid-feed, direct-oxidation methanol fuel cell.

Figure 1. This Methanol Sensor is based on electrochemical oxidation of methanol coupled with electrochemical reduction of water.

Other methanol sensors respond to changes in concentration too slowly to be useful for monitoring and control. Moreover, methanol sensors based on spectroscopy are relatively insensitive at high concentrations. The present sensor responds rapidly enough for monitoring and control. It also offers wide dynamic range; it can measure concentrations from 0.01 M to 5 M. Furthermore, it functions over the entire temperature range of liquid water at normal atmospheric pressure; that is, from 0 to 100 °C.

The sensor (see Figure 1) resembles a small liquid-feed, direct-oxidation fuel cell in some respects, but is operated in a different manner. It includes a polymer-electrolyte membrane coated on one side with a catalytic electrode composed of Pt/Ru-alloy powder and coated on the other side with a catalytic electrode composed of Pt black. Some details of the preparation of the Pt/Ru powder, the polymer-electrolyte membrane, and coating the membrane with the Pt/Ru powder were described in "Making Catalysts and Electrodes for Liquid-Feed Fuel Cells" (NPO-19893) NASA Tech Briefs, Vol. 20, No. 10 (October 1996), page 60. The coated membrane is pressed between sheets of the porous carbon paper. The resulting sandwich is mounted between graphite plates that act as both current collectors and structural supports. Circular openings in the graphite plates expose the sandwich to the aqueous solution of methanol.

Figure 2. The Electric Current in the Sensor increases with the applied potential and with the concentration of methanol in water. The current also depends on temperature. For a specified temperature and applied potential, the current thus serves an indication of the concentration of methanol.

When the Pt/Ru and Pt electrodes are connected to the positive and negative sides, respectively, of a dc power supply, electric current flows through the sensor, causing the following reactions:

CH3OH + H2O → CO2+ 6H+ + 6e¯ at the Pt/Ru electrode and 6H+ + 6e¯ → 3H2 at the Pt electrode.

As the applied potential increases, the rate of this reaction (and thus the electric current) becomes subject to limitation on the rate of transport of methanol to the surface of the Pt/Ru electrode. As the concentration of methanol increases, more methanol becomes available for transport, making it possible to sustain a larger transport-limited current. Thus, for a given applied potential, the current increases with the concentration of methanol (see Figure 2). This behavior is exploited in designing the methanol sensor; in the basic mode of operation, one applies a specified potential in the range in which current is strongly transport-limited (e.g. 0.63 V) and measures the current as an indication of the concentration of methanol.

The response of the sensor also depends on temperature. At each operating temperature of interest, the sensor is calibrated by applying the potential and measuring the current in the presence of aqueous solutions with known concentrations of methanol.

This work was done by Sekharipuram Narayanan, William Chun, and Thomas Valdez of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com  under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office JPL Mail Stop 122-116 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-2240

Refer to NPO-20125, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Sensor for Monotoring Concentration of Methanol

(reference NPO20125) is currently available for download from the TSP library.

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