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Diagnosis of the effects of thermal cycling on PEMFCS via electrochemical impedance spectroscopy Connell, Robert Michael

Abstract

A new method of applying electrochemical impedance spectroscopy (EIS) as a diagnostic tool was used to investigate the effects of thermal cycling on proton exchange membrane fuel cells. The focus of this thesis was the generation of hardware required to accomplish this goal. Unlike previous work, this study used a specialized load bank to draw the periodic current necessary for impedance measurements. This study also required the design and construction of new hardware to allow accelerated cycling of a single cell between ambient and operational temperatures. The equipment was tested (through experimentation) both to verify its functionality and to serve as a roadmap for a more thorough investigation into the effects of thermal cycling. It was hypothesized that cumulative damage due to thermal cycling would correspond to changes in cell impedance that were observable in characteristic frequency ranges. The equipment was used to perform galvanostatic impedance measurements in situ. Measurements were made at four different current densities (0.2, 0.4, 0.6, and 0.8A/cm²) over the frequency range 1 to lxlO⁶ Hz. Cell temperatures were cycled between 20-80°C or 30-90°C at a rate of 1°C per second for up to 2000 cycles. The recorded data represents the first reported measurements covering these ranges. The measured spectra were repeatable, produced results consistent with previous work and were as accurate as impedance measurements made using the traditional setup. Observed changes in impedance spectra as a function of thermal cycling were relatively minor but significant changes in impedance magnitude between 10 and 1000 Hz and changes in phase angle at frequencies greater than 10 kHz were measured. The effects of thermal cycling on MEA performance were minor with no irrecoverable damage recorded at moderate stack pressures of 120 PSI and degradation rates below 0.057mV/cycle at increased stack pressure of 350 PSI. These results suggest that stack sealing pressure is a factor in thermal degradation and is an avenue for continued investigation. Experimental data suggests that the main failure mode of thermal cycling is the formation of a contact resistance between the bipolar plates and current collectors due to degradation of the graphite. The contact resistance grew as large as 2xl0⁻⁴ Ωcm², leading to a potential drop of 0.25V, but available data was insufficient to calculate a growth rate.

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