Impact of Polymer Electrolyte Membrane Properties on Transport, Performance and Durability

Abstract

The structure and properties of the polymer electrolyte membrane has a major impact on the MEA performance and durability. Polymers with different chemical structures can propose different mechanisms for water and proton transport which lead to differences in performance and durability. For membranes with similar chemical structures, factors such as thickness and equivalent weight (EW) are crucial in transport and therefore could cause differences in the performance. The focus of this work was a closer look at the impact of these properties, and their changes due to degradation, on the performance and durability of the MEA and its components. The impact of membrane thickness and equivalent weight was studied by comparing the water content of different PFSA membranes as a function of relative humidity (RH). The data was then used to further discuss the conductivity and hydrogen permeability of the membranes in different regimes of temperature and RH. The impact of membrane chemical degradation on transport property was studied by comparing the membrane conductivity, thickness, hydrogen permeability and performance before and after membrane degradation. To study the impact of membrane properties on cathode catalyst performance and degradation, the cathode degradation test was applied on MEAs with and without membrane degradation, and the results were compared. The water content (Lambda) of all membranes showed a type II isotherm increase by increasing the relative humidity. Lambda was found to be independent of membrane thickness and equivalent weight, while, as expected, the absolute water content (g water/g membrane) showed a decrease at higher equivalent weights. Increasing the relative humidity and operational temperature showed an increase in membrane conductivity. Higher thickness or higher equivalent weight both resulted in higher membrane resistance; supporting a more general trend which is the linear increase of membrane conductivity with increase in the absolute water content. Cathode degradation testing showed an impact on the ECSA loss and performance. The effect was more intense for thinner or lower EW membranes suggesting that membrane water content was the contributing factor. After the membrane degradation test, losses in membrane thickness, membrane conductivity, performance and ECSA were observed. In addition, the hydrogen crossover increased due to the membrane thinning. The membranes with pre-existing degradation showed less ECSA loss but increased performance losses. This trend was explained by the combination of higher catalyst layer ionomer resistance and platinum depletion which lead to higher catalyst layer ionic losses related to a reaction distribution shift further into the catalyst layer. Acknowledgements: Funding was provided by Department of Energy EERE Hydrogen and Fuel Cell Technology Program (Project DE-EE0006375).