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Numerical Simulation of Interactive Transport in a Proton Exchange Membrane Fuel Cell

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Proton exchange membrane fuel cell (PEMFC) is clean electrochemical energy generation device that generates electricity by electrochemical reactions between hydrogen and oxygen and the only byproducts of this reaction are heat and water. PEMFCs are considered to be the most promising technology to replace internal combustion engines for automotive application however for the commercialization of PEMFCs many technical challenges need to be overcome including decreasing Pt loading, increasing high current density performance, better water and thermal management and cell durability. Understanding the transport inside the PEMFC components are crucial for solving the technical challenges. In PEMFCs, most of the cell components are of nano to micro meter length scale which makes experimental characterization and diagnostics extremely difficult. Numerical models built on fundamental physics are critical for further development of this technology.

In my doctoral research I took a comprehensive simulation approach by first building a single-phase model, followed by expanding the physics towards two-phase simulation, and completed by a full down-the-channel model.At first a single phase steady state 1-D modeling framework is developed which accounts accurate membrane water balance considering electro-osmotic drag and back diffusion, non-isothermal heat transfer inside thin PEMFC components, oxide coverage dependent ORR kinetics, proton transport loss in electrode with respect to I/C ratio, convective and diffusive gas transport, shorting and cross over current to accurately capture open circuit voltage, effects of land/channel geometry on transport and non-fickian resistance in electrode and microporous layer.

Next to expand the modeling capability in high humidity or high current density operations two-phase water transport in the GDL and in electrode is added to the 1-D modeling framework. To model liquid water transport in the gas diffusion media, a novel approach has been proposed that only requires the GDL porosity and tortuosity to formulate. To capture the effects of liquid water condensation in the electrode an empirical approach is adopted which correlates local water activity with catalyst utilization factor. The developed 1-D model establishes a robust modeling framework and it is validated with polarization and limiting current experimental results.

Finally, the 1-D two-phase model is expanded to a down-the-channel performance model to study the effects of stoichiometric flow rate, flow orientation (co-flow and counter-flow), channel pressure drop and coolant temperature on cell performance. The developed down-the-channel model can simulate species and current density distribution in along-the-channel direction and the results from developed 1+1-D model has been validated with experimental measurements from literature. In addition, parametric numerical studies on asymmetric GDL thickness, membrane thickness and asymmetric channel RH have been conducted to study internal water circulation mechanism. Down-the-channel simulation results show that counter-flow operation at low stoichiometric conditions enables efficient internal water circulation inside the cell, which improves membrane humidification and cell performance under extremely dry conditions. The findings from the newly developed model provide critical insights on the interaction of coupled heat and mass transfer, charge transport and down-the-channel distribution at realistic fuel cell operating conditions.

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