Mechanical properties of catalyst coated membranes for fuel cells

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This work demonstrates an original approach to three-dimensional, high-resolution, non-destructive visualization of fuel cells during room temperature operation using a commercial micro-X-ray computed tomography (XCT) system. The novel application of 3D printing technology was used to create a customized housing fixture that allows for non-invasive imaging of an operational fuel cell. The use of 3D print materials also allows for rapid prototyping and intricate design of any experimental fixture with minimal cost. Demonstration of imaging capabilities is shown through in situ visualization of the internal MEA components and liquid water after fuel cell operation. Water imaging in fuel cells is particularly challenging using XCT due to small differences in X-ray absorption characteristics between constituent materials. Image processing operations are discussed which allows for the improved segmentation of solid and water to calculate porosity and saturation throughout the gas diffusion layer, as reported herein. Additionally, a membrane thickness increase of approximately 25% due to swelling under 100% relative humidity is shown, which has previously been difficult to determine. Overall, the results shown here illustrate the novel use of 3D printed materials and image processing steps to be further used in understanding of fuel cell devices through non-invasive imaging procedures by use in a commercial micro-XCT system. Polymer electrolyte fuel cells (PEFC) have significant potential for automotive applications as an alternate clean energy technology with zero emissions at the point of use. The many advantages include quick start-up time, low operating temperature, low weight, high efficiency and relatively simple design as compared to other clean energy solutions.1 Despite these advantages, however, a primary reason for the delay of this technology from meeting commercial automotive application standards is cost. Significant cost reductions are possible by increasing the power density, which is currently limited by liquid water related mass transport effects at high current densities.2 Higher power density cells also have an advantage for automotive applications due to a better form factor for vehicles. In other fuel cell technologies such as back-up power in extreme climates or mobile fuel cell applications, fuel cells operating at low temperature or low gas flow rates are susceptible to flooding. Flooding, or channel clogging, may lead to significantly reduced performance from inaccessibility of reactant gas flow as well as non-uniform reactant distribution in fuel cell stacks. 3,4 Proper understanding of water management in operating fuel cells is imperative to the design and implementation of higher power density fuel cells and is thus a highly researched field of PEFC performance.Current fuel cell diagnostic methods typically involve investigation by empirical testing and ex situ analysis. A common technique for fuel cell investigatory research is to use scanning electron microscopy (SEM), which provides hi...

Section: Resultsmentioning

confidence: 99%

3D Printed Flow Field and Fixture for Visualization of Water Distribution in Fuel Cells by X-ray Computed Tomography

White

Orfino

Hannach

et al. 2016

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“…14,16 Furthermore, the CCM tensile and dynamic mechanical properties were found to be more sensitive to temperature than RH. 14,16 The confinement imposed to the membrane by the catalyst and diffusion layers was shown to prevent in-plane expansion/contraction, hence resulting in lower tensile peak stress when compared to the pure membrane, while the through-plane stress and strain were increased during hydration. 17 In order to evaluate the durability and diagnose potential mitigation strategies at reasonable cost and time, accelerated stress testing (AST) is generally applied via aggravating the chemical and mechanical stressors including current density, cell voltage, temperature, and RH fluctuations.…”

mentioning

confidence: 99%

“…Mechanical properties of CCM composites were comprehensively investigated by our group and compared to those of the corresponding PFSA membranes. [14][15][16][17] By conducting tensile stress-strain and dehydration tests under a wide range of temperature and RH conditions, Goulet et al…”

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confidence: 99%

Microstructural and Mechanical Characterization of Catalyst Coated Membranes Subjected to In Situ Hygrothermal Fatigue

Alavijeh

Khorasany

Nunn

et al. 2015

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Catalyst coated membranes (CCMs) in polymer electrolyte fuel cells are subjected to mechanical stresses in the form of fatigue and creep that deteriorate the durability and lifetime of the cells. The present article aims to determine the effect of in-situ hygrothermal fatigue on the microstructure and mechanical properties of the CCM. The fatigue process is systematically explored by the application of two custom-developed accelerated mechanical stress test (AMST) experiments with periodic extraction of partially degraded CCMs. Cross sectional and top surface scanning electron microscope (SEM) images of the end-of-test CCMs reveal the formation of mechanically induced cracks and delamination due to cyclic tensile and compressive fatigue stress. Tensile and expansion tests are conducted at different stages of degradation to evaluate the evolution in the mechanical and hygrothermal properties of the CCM. The tensile test results indicate gradual reductions in final strain, ultimate tensile strength, and fracture toughness with increasing number of fatigue cycles. The decay in tensile properties is attributed to the microstructural damage and micro-cracks formed during the AMST. Moreover, it is shown that the hygrothermal expansion of the CCM is more sensitive to conditioning than mechanical degradation. Polymer electrolyte fuel cells (PEFCs) are a prime candidate to replace gasoline and diesel internal combustion engines for transportation applications due to their environmental benefits combined with rapid start-up, high efficiency, and high power density at relatively low operating temperature.1 The commercialization of PEFCs is dependent on the development of membrane electrode assemblies (MEA) capable of meeting the automotive industry durability targets.2 However, the current PEFC technology is facing insufficient longevity, mainly because of the deterioration of the proton exchange membrane (PEM) component.1 Hence, an essential step to accomplish the commercialization requirements for PEFCs is to enhance the membrane durability and lifetime. Among various types of membranes utilized in PEFCs, perfluorosulfonic acid (PFSA) ionomer membranes (e.g., Nafion from DuPont) are the most widely used materials due to the superior chemical stability attributed to the chemically inert C-F bonds of the polytetrafluoroethylene (PTFE) base structure. 3Chemical and mechanical degradation mechanisms are recognized as the principal root causes for lifetime limiting failures of PFSA ionomer membranes in fuel cells. Understanding of the degradation mechanisms, their interactions, and the corresponding failure modes could provide valuable insight toward decelerating the rate of the membrane degradation and thereby extend the lifetime.2 Chemical degradation is caused by the attack of radical species in the form of hydroxyl (•OH) and hydroperoxyl (•OOH) radicals generated through decomposition of hydrogen peroxide (H 2 O 2 ) by metal contaminants.2,4,5 Hydroxyl radicals also form as a by-product of the electrochemical reaction bet...

“…1,2 Chemical degradation initiates the overall degradation processes 3,4 and further damages the membrane when combined with mechanical stress, strain, and fatigue induced by hygrothermal fluctuations in the MEA. [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.…”

mentioning

confidence: 99%

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

Wong

Kjeang

2017

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Ceria-supported membrane electrode assemblies (MEAs) have recently been proposed to address chemical membrane degradation in polymer electrolyte fuel cells. Although ceria is known to effectively protect the membrane at open circuit voltage (OCV) conditions, its effectiveness has not been demonstrated for cell voltages below OCV and associated conditions relevant for field operation. In the present work, a comprehensive, transient in situ chemical degradation model for ceria stabilized MEAs is developed and applied to investigate the mitigation effectiveness of ceria additive. At high cell voltages, abundant Ce 3+ ions are available in the membrane to quench hydroxyl radicals which is the primary mitigation mechanism observed at OCV conditions. However, the mitigation is suppressed at low cell voltages, where electromigration drives Ce 3+ ions into the cathode catalyst layer (CL). Without an adequate amount of Ce 3+ in the membrane, the hydroxyl radical scavenging is significantly reduced, leading to a ten-fold reduction in mitigation effectiveness at cell voltages below 0.7 V. The simulated results also suggest that significant ceria precipitation may occur in the cathode CL due to the increased local Ce 3+ concentration at low to medium cell voltages. Ceria-supported MEAs may therefore experience higher rates of chemical membrane degradation at low cell voltages than at OCV. Hydrogen powered polymer electrolyte fuel cells (PEFCs) generally use perfluorosulfonic acid (PFSA) ionomer membranes to separate the two electrodes in the membrane electrode assembly (MEA). Their high proton conductivity at low temperatures, relatively low reactant permeation, and superior electrical insulation lead to high fuel cell performance. However, the ionomer membrane can be degraded in the fuel cell environment which reduces its stability and limits its lifetime.1,2 Chemical degradation initiates the overall degradation processes 3,4 and further damages the membrane when combined with mechanical stress, strain, and fatigue induced by hygrothermal fluctuations in the MEA. [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. Radical scavenging is therefore proposed to mitigate the chemical damage by quenching the radicals before they attack t...