Characterization of Water Transport and Liquid Accumulation in Proton Exchange Membrane Fuel Cells
Creators: Wei, Fei
Publisher: name: University of Alberta Library
Year: 2024

<p>Hydrogen proton exchange membrane fuel cell (PEMFC) vehicles present a viable solution for decarbonizing the heavy-duty and long-distance transportation sector. They convert the chemical energy of hydrogen directly into electricity, with the only by-products being heat and water and, therefore, they are completely environmental-friendly. However, challenges related to hydrogen infrastructure and PEMFC cost and durability hinder their wider market-adoption. Enhancing the power density of PEMFCs can reduce their size and subsequent costs; however, high current density operation is limited by the cell’s ability to efficiently evacuate the water produced from the oxygen reduction reaction (ORR) in the cathode catalyst layer. Efficient removal of fast ORR product water at high currents, from the anode and cathode compartments of the cell, is imperative to prevent the reactant starvation arising from electrode flooding. Water accumulation has also been identified as a factor affecting catalyst degradation and as a result, the durability of PEMFCs. Therefore, a comprehensive understanding of water transport and liquid water accumulation during PEMFCs operation becomes paramount in improving their performance and durability and ultimately reducing their cost.</p>

<p>A water balance setup, enabling real-time quantification of water transport and accumulation inside operating PEMFCs, is developed and validated in this work (see Chapter 2). The setup integrates sensors for measuring relative humidity, temperature, and absolute pressure of the gas-vapor mixture, and dry reactant flow rate. Moreover, a fast and reliable approach is proposed for distinguishing between the water accumulated in the membrane and liquid water within the porous media. This method holds promise for real-time control strategies aimed at mitigating membrane dehydration and electrode flooding concerns.       </p>

<p>The established water balance setup was used to investigate several aspects of interest in the literature: (a) water transport in polymer electrolyte membranes (see Chapter 3); (b) the influence of operating conditions and the addition of a micro-porous layer to cathode gas diffusion layer on cell performance and water transport and accumulation (see Chapter 4); and (c) the influence of the cathode catalyst layer Nafion and platinum loadings on cell performance, water crossover between the electrodes and water accumulation in the membrane-electrode-assemble (see Chapter 5). </p>

<p>Analysis of water transport in proton-exchange (Nafion N211, N212, N115 and N117) and anion-exchange (Aemion AH1-HNN8-50-X, Fumapem FAA-3-30/50, and Versogen PiperION-A40) membranes confirmed that interfacial transport is the key limiting. Using the experimental setup in combination with a numerical model, water desorption rate and its activation energy were estimated. The water desorption rate of AH1-HNN8-50-X aligns closely with Nafion N115 and N117, and the activation energy for this process is similar. In contrast, FAA-3-30/50 and PiperION-A40 exhibit two to three times faster desorption and a lower activation energy.</p>

<p>The study of operating conditions and micro-porous layer (MPL) addition showed that changing operating conditions has a dramatic effect on the water transport across the membrane, while the ratio of water transported to produced water remained relatively constant with current density. Under very dry conditions, water moved from anode to cathode while increasing humidity and decreasing temperature enhanced the cathode-to-anode water crossover. Adding an MPL to the cathode gas diffusion layer increased the cathode-to-anode water crossover at all operating conditions, but the increase was not as pronounced as with changing operating conditions, resulting in only a significant performance increase at 60 C and 70% RH likely due to increased water vaporization and improved in-plane oxygen pathways. </p>

<p>Lastly, this work demonstrates that increasing the cathode Nafion loading reduces cathode-to-anode water crossover at all conditions, i.e., helps the cathode retain water. Cells with 20 wt.% Nafion loading were able to operate at the highest current density retaining less water inside the electrode. Varying the platinum loading had a negligible influence on water transport between the electrodes at all conditions. This contradicts the common hypothesis that thin CL will flood more quickly.</p>

<p>In summary, this thesis contributed a novel experimental setup and analysis strategy to be able to estimate the real-time water crossover and accumulation in an operating PEMFC. Using the setup, this thesis offers a comprehensive exploration of how modifications to PEMFC components impact the dynamics of water crossover and liquid water accumulation within the operating PEMFCs. The developed water balance setup stands as a reliable tool for real-time measurement of water fluxes and liquid water accumulations, providing insights into water management within operating PEMFCs. These insights can contribute to enhance PEMFCs performance and thus to reduce their cost.</p>