Progression in the Morphology of Fuel Cell Membranes upon Conjoint Chemical and Mechanical Degradation

Abstract

Ionomer membranes used to separate the electrodes in polymer electrolyte fuel cells are known to degrade both chemically and mechanically during regular fuel cell operation and may ultimately result in lifetime-limiting failure. The objective of the present work is to understand the effects of combined chemical and mechanical stresses on the mesoscale morphology of the membrane and its role in the overall degradation process. The mesoscale effects of sulfonic acid group loss and fluoride release on the phase segregated morphology of the membrane are analyzed using contrast-enhanced transmission electron microscopy and energy dispersive X-ray spectroscopy. The end-of-life ionic domain size of the ionomer is shown to be substantially enlarged compared to the pristine membrane state. Elemental mapping overlayed with the binary ionic and non-ionic morphology reveals mesoscopic void regions in the degraded material that are depleted of ionomer fluorine and carbon and considered susceptible to micro-crack initiation. A larger, severely degraded void region is also identified which contains evidence of hygrothermal stress induced localized ionomer crazing as a potential nucleation site for macroscopic fracture development. The synergetic effects of chemical and mechanical degradation on the progressive changes in the observed mesoscale morphology are discussed. Cost effectiveness, durability, and efficient performance are the essential requirements to be met for the commercialization of fuel cells. The properties of all key components comprising the fuel cell stack such as bipolar plates, gas diffusion layers, catalyst layers, and ion conducting membrane play a vital role in building the efficient and durable fuel cell system.2 One of the susceptible components responsible for the failure of polymer electrolyte fuel cells (PEFCs) is the ionomer membrane which conducts protons and separates the electrodes. The essential requirements of a standard fuel cell membrane are high proton conduction, good mechanical, chemical, and thermal stabilities, and low gas permeability. The most widely used electrolyte in PEFCs is the perfluorosulfonic acid (PFSA) ionomer membrane. The structure of the PFSA ionomer consists of hydrophobic tetrafluoroethylene main chains with periodic side chains terminated with hydrophilic sulfonic acid groups. When hydrated, the PFSA membrane exhibits a phase segregated network of ionic, water-filled hydrophilic domains which are necessary for enhanced proton conduction and non-ionic backbone dominated hydrophobic domains which are critical for the structural integrity of the material. The morphology of PFSA membranes can be studied using transmission electron microscopy (TEM), which is a direct imaging technique. The dehydrated morphology of a Pb 2+ ion-exchanged membrane viewed under the electron column has revealed phase-separated, dark hydrophilic ion-rich domains and bright polytetrafluoroethylene (PTFE)-like backbone domains.3 Recently, 2D and 3D images of PFSA ionomer were also reported using cry...

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

“…19 The strategies employed in these ASTs typically involved a combination of periods of OCV in hot/dry conditions and humidity cycling either sequentially [11][12][13][14]19 or simultaneously, 2,15,17,20 or simultaneous humidity and voltage cycling driven by current cycling. 18,21 These studies show that the combined stressors can significantly accelerate the membrane failure; 2,18 degrade the membrane's mechanical properties, water uptake behavior, or proton conductivity; [11][12][13][14] or increase the size of existing defects. 19 The synergistic pathways for membrane degradation by the combined mechanical and chemical stressors have been summarized in Kusoglu and Weber.…”

Nanoindentation Investigation of Temperature Effects on the Mechanical Properties of Nafion® 117

“…Today, a major concern in PFSA membranes is to continue to improve the sustainability and to extend their service life, which can be significantly reduced by the combined effects of changing chemical and mechanical environments [ 2 ]. During operation, chemical degradation and mechanical failure due to hydration and temperature cycles may be the most prominent factors restricting the durability of PEMs [ 3 , 4 , 5 , 6 ]. Therefore, it is very important to understand their mechanical stability changes with respect to operating conditions (humidity, temperature, etc.).…”

In-Situ Modeling of Chemical Membrane Degradation and Mitigation in Ceria-Supported Fuel Cells

“…[5][6][7][8][9][10][11][12] The resulting physical damage in the form of cracks and holes eventually causes hydrogen leaks across the membrane which is considered one of the main lifetime limiting failure modes in fuel cells. 4,[13][14][15][16] The primary chemical degradation in PEFCs is caused by reactions of reactive radicals with the PFSA ionomer membrane.17-21 For instance, hydrogen peroxide (H 2 O 2 ) can be generated via the twoelectron oxygen reduction reaction (ORR) resulting in hydroxyl radical (·OH) formation due to decomposition of hydrogen peroxide in the presence of metal contaminants. 22,23 The hydroxyl radical formed is highly reactive and can attack the ionomer membrane in terms of side chain cleavage and unzipping 18,20,21,24,25 which is responsible for the deteriorated physicochemical properties observed in degraded membranes.…”