Integrated electrochemical sensors are being developed for use in measuring concentrations of sulfur dioxide in gas mixtures. These sensors are based on the electrochemical oxidation of SO2 and measurement of the resulting electric current. Conceived for sensing SO2 in volcanic gases, sensors of this type could also be used to monitor SO2 in ambient air and in gases emitted during such industrial operations as metallurgical processing, refining of petroleum, and burning of coal.

Concentrations of SO2 can be measured by mass spectrometers, but these instruments are too power hungry, heavy, and bulky to be portable, and their response times are too long for taking repetitive measurements in unsafe locations. The developmental sensors are inherently simple and fast-responding and could be made small, lightweight, and robust for incorporation into portable instruments that could be operated in industrial and remote natural settings.

The Current in This Electrochemical Sensor is expected to be proportional to the concentration of SO2 in the gas to which the positive electrode is exposed.

A sensor of this type includes an integrated catalytic-electrode/ion-conducting-membrane/catalytic-electrode sandwich structure (see figure). The membrane is made of a proton-conducting polymer (for example, Nafion 117 or equivalent). The catalytic electrodes are made of porous carbon (graphite-based) paper substrates coated with catalysts in the form of platinum or platinum alloy powders. The electrodes on opposite sides of the membrane can be identical or different with respect to catalysts and treatments of the porous substrates. Such electrode/membrane/electrode sandwiches and techniques for fabricating them have been described in a number of previous articles in NASA Tech Briefs.

In operation, a dc potential is applied between the electrodes while the positive electrode is exposed to a gas mixture that contains SO2. At the positive electrode, SO2 in the presence of H2O is oxidized to sulfate ions, with production of hydrogen ions, in the following chemical reaction:

The hydrogen ions are conducted through the membrane to the negative electrode. Upon arrival at the negative electrode, the hydrogen ions react as follows:

2H + + 2e- --> H2

The current of hydrogen ions through the membrane gives rise to a measurable electric current in the wires connecting the electrodes to the source of applied potential. For a given fixed applied potential, the current is expected to be proportional to the concentration of SO2 in the gas in contact with the positive electrode.

The use of noble-metal catalysts and graphite-based substrates makes the electrodes resistant to chemical degradation. The Nafion 117 or equivalent membrane material is a sulfonic acid-based polymer that also resists chemical degradation.

An experimental sensor was made from a conditioned Nafion membrane and electrodes comprising porous carbon paper catalyzed with platinum black. The electrodes were bonded to the membrane. The resulting sandwich was mounted between two glass flanges with electrical leads attached to the electrodes. The active sensor area was a square about 1 in. (about 2.5 cm) on each side. In tests, potential in the range of 1 to 2 V was applied and currents were measured with and without exposure to SO2. Current responses on exposure to SO2 were significant, and the response time was about 1 s. Further studies to quantify responses and sensitivity to operating variables were under way at the time of reporting the information for this article.

This work was done by Sekharipuram Narayanan and Santosh Srivastava of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the category.

This Brief includes a Technical Support Package (TSP).
Integrated Electrochemical Sulfur Dioxide Sensors

(reference NPO20290) is currently available for download from the TSP library.

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