Fuel Cell Research Uses Alternative Approach

Sep 20, 2006 11:04 AM
By Mark Valentine, Technical Editor, Power Electronics Technology

During normal operation, the PEM is hydrated, and the hydrogen-side catalyst causes H2 to separate into 2 electrons and 2 protons. As the electrons from hydrogen flow through the circuit between the fuel cell terminals, the remaining protons of the hydrogen must travel through the water in the membrane, where they will combine with negative oxygen ions on the other side—the rate of proton transport, measured as a conductivity, determines the power output of the cell. This is the primary property screened with Meredith's system.

The protons from hydrogen supplied to the PEM fuel cell travel through the PEM as the electrons from the hydrogen travel through the electric circuit.

During a high-throughput conductivity test, the entire library is immersed in 18-MΩ/cm de-ionized, distilled water. The proton conductivity of each membrane composition, for example, at a set of chosen locations, is measured directly by using ac impedance spectroscopy. This is accomplished with a four-point probe that is scanned across the library and is lowered to contact the membrane at specified locations to make a measurement. Each of these measurements takes about 3 seconds, according to Dr. Meredith.

The 4-point conductivity sensor is used for high-throughput conductivity screening. The distance between pins is 635 μm, and the pin diameter is 100 μm (courtesy of Dr. Carson Meredith).

The thickness of the membrane at the measurement sites must be known to calculate conductivity for each composition in the library. Water absorption will affect the thickness of the membrane, which is measured with a spring-loaded micrometer (based on a resistive position sensor) that is also scanned across each library location. The instrument is sensitive to 0.5 microns.

In this picture the thickness measurement probe is ‘loaded’ on the sensor mount above the sample grid. The previous picture shows a close-up of the actual 4-point conductivity sensor that is placed on this instrument once the thickness has been measured (courtesy of Dr. Carson Meredith).

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