In-situ water measurements in polymer electrolyte fuel cells by fiber optic fluorescence spectroscopy

Date of Completion

January 2006

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Water management and control is a critical problem in polymer electrolyte membrane fuel cells. Techniques were previously unavailable to measure membrane water content inside fuel cells. We have measured in-situ membrane water content using fiber optic fluorescence spectroscopy. By preparing a membrane electrode assembly with an embedded optical fiber, in-situ membrane water content was obtained from the measured fluorescence response. ^ The Rbodamine-6G fluorophore was selected for its sensitivity to changes in Nafion® water content at fuel cell operating temperatures. The response was first calibrated over a range of membrane water content at 80°C. The fluorescence measurements indicated the membrane water content during the dynamic response of the fuel cell during startup, operating condition changes, and many small perturbations. These initial results showed a decrease in membrane water content as fuel cell current was increased. The decrease in water content was attributed to water uptake limitations. ^ Multiple optical fibers were embedded in the membrane to study membrane water variations under steady and transient cell operation. At steady state operation, the water content increased with an increase in distance from the oxygen inlet. Under transient operation, the membrane water content showed a fast inverse response to step changes in current, followed by a much slower approach to steady state behavior. In these carefully controlled experiments, the steady state membrane water content increased as current increased, in contrast to the initial results, which simply illustrated the inverse response characteristic at short times. ^ Rhodamine-6G in Nafion® was also used as a fiber optic fluorescence humidity sensor inside a fuel cell. Four sensors were incorporated in a commercial fuel cell stack to probe cathode side humidity variations. Significant variations in humidity at the four locations were observed due to uneven cell performance. The sensor response was reasonably quick as it followed the imposed step change in current. ^ These measurement techniques provide methods for obtaining fundamental thermodynamic and perhaps transport data in fuel cells under realistic conditions. For example, the equilibrium relationship between membrane phase water and vapor phase water is now available at several temperatures.^

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