Mechanical Aspects of Membrane Durability in PEM Fuel Cells

Proton-Exchange Membrane (PEM) Fuel Cells have the potential to contribute to the future of emission-free, efficient power systems for a wide range of applications. However, operational lifetime of the fuel cells is limited due to failure of cell components, with the membrane being one of the key ones. In PEFCs, chemical decomposition of the membrane due to the attacks of radicals formed during operation along with formation of pinholes, cracks, and delamination influenced by mechanical stressors leads to increased crossover of reactant gases and reduced power loss. Since the chemical and mechanical stressors are largely influenced by the temperature and humidity, it is difficult to determine what mechanisms dominate the membrane lifetime in operation. To understand underlying mechanisms of failure under chemical, mechanical, and combined chemical-mechanical degradation modes, and assess membrane lifetime in shorter time periods, accelerated stress testing (AST) methods are commonly employed. Swelling (or hydration) of the membrane is a key property in fuel cells since it controls both conductivity and stability of the membrane, which could be related to the cell performance and durability, respectively. Swelling-induced operational stresses in the membrane trigger failure mechanisms in the form of cracks or delamination, the impact of which could be further accelerated in the presence of chemical degradation.

In this talk, the mechanical aspects of the durability of the polymer fuel cell membranes will be discussed at multiple length-scales. First, membrane's mechanical response during cell operation will be discussed along with the effects of swelling and mechanical properties. We will elaborate on how membrane deformation could be related to the mechanical failure modes ranging from fatigue behavior to the growth of pinholes and cracks, depending on the operating conditions and stress-states. Strategies that could potentially improve membrane durability by tuning its properties will also be discussed. The results will be demonstrated using experimental data as well as modeling methodologies.

We will then show how the swelling and properties of membranes change with degradation by focusing on perfluorosulfonic-acid (PFSA) ionomers, the most widely studied membrane in PEM fuel cells. Changes in structure-property relationship of PFSA membranes due to degradation are highlighted based on the investigations on the morphology, mechanical properties and water uptake of degraded membranes. By studying the correlations between measured properties and structural features one can better understand the mechanical aspects of degradation. We will also elaborate how chemical and mechanical stressors could influence each other and control the overall membrane failure. Lastly, mechanistic approaches will be introduced to identify key membrane properties for mechanical durability, to simulate membrane deformation and failure, and to explore models for lifetime prediction.

Acknowledgements

The authors would like to thank the Los Alamos Fuel Cell Team for their help preparing the degraded membrane samples and for helpful discussions. This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02-05CH11231.