SIGNIFICANCE OF HEAT DISSIPATION IN THE CATHODE CATALYST LAYER ON THE LIFETIME AND RELIABILITY OF POLYMER ELECTROLYTE FUEL CELLS

To study the dissipation of heat generated due to the formation of pinholes that cause local hotspots in the catalyst layer of the Polymer Electrolyte Fuel Cell, a two-phase non-isothermal model has been developed by coupling Darcy’s law with heat transport. The domain under consideration is a section of the membrane electrode assembly with a half-channel and a half-rib. Five potential locations where a pinhole might form were analyzed: at the midplane of the channel, midway between the channel midplane and the channel wall, at the channel or rib wall, midway between the rib midplane and the channel wall, at the midplane of the rib. In the first part of this work, a preliminary thermal model was developed. The model was then refined to account for the two-phase effects. A sensitivity study was done to evaluate the effect of the following properties on the maximum temperature in the domain: Catalyst layer thermal conductivity, the Microporous layer thermal conductivity, the anisotropy factor of the Catalyst layer thermal conductivity, the Porous transport layer porosity, the liquid water distribution and the thickness of the membrane and porous layers. Accounting for the two-phase effects, a slight cooling effect was observed across all hotspot locations. The thermal properties of the catalyst layer were shown to have a limited impact on the maximum temperature in the catalyst layer of new fuel cells without pinhole. However, as hotspots start to appear, thermal properties play a more significant role in mitigating the thermal runaway.

Polymer Light-Emitting Electrochemical Cells with Embedded Bipolar Electrodes: Visualizing Bipolar Electrochemistry in Solid State

The work presented in this thesis examines the properties of BPEs of various configurations and under different operating conditions in a large planar LEC system. Detailed analysis of time-lapsed fluorescence images allows us to calculate the doping propagation speed from the BPEs. By introducing a linear array of BPEs or dispersed ITO particles, multiple light-emitting junctions or a bulk homojunction have been demonstrated. In conclusion, it has been observed that both applied bias voltages and sizes of BPEs affected the electrochemical doping from the BPE. If the applied bias voltage was initially not sufficiently high enough, a delay in appearance of doping from the BPE would take place. Experiments of parallel BPEs with different sizes (large, medium, small) demonstrate that the potential difference across the BPEs has played a vital role in doping initiation. Also, the p-doping propagation distance from medium-sized BPE has displayed an exponential growth over the time-span of 70 seconds. Experiments with a linear array of BPEs with the same size demonstrate that the doping propagation speed of each floating BPE was the same regardless of its position between the driving electrodes. Probing experiments under high driving voltages further demonstrated the potential of having a much more efficient light emission from an LEC with multiple BPEs.