There are three critical membrane degradation mechanisms that can lead to failure of polymer elec... more There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical degradation, mechanical degradation, and membrane shorting. This talk will focus on the individual failure modes; with attention dedicated to describing the fundamental model-based mechanistic understandings, appropriate accelerated stress tests that enable rapid screening and relevant in-situ diagnostics to track membrane health during fuel cell operation, as well as discussions of effective mitigation strategies to prevent or minimize the risk of failure caused by the specific modes of membrane degradation. The effect of simultaneous chemical and mechanical degradation will also be discussed. Both chemical and mechanical degradation of perfluorosulfonic acid (PFSA) membranes have been extensively studied, and effective mitigation strategies have been developed that can significantly extend PFSA proton exchange membrane (PEM) lifetimes. Accelerated st...
ASME 2011 9th International Conference on Fuel Cell Science, Engineering and Technology, 2011
The strength of the proton exchange membrane (PEM) and its ability to withstand cyclic, environme... more The strength of the proton exchange membrane (PEM) and its ability to withstand cyclic, environmentally induced stresses plays an important role in membrane integrity and consequently, fuel cell durability. In this study, pressure loaded blister tests are used to characterize the biaxial strength of a model commercially available membrane, Gore-Select® series 57, at the 90°C dry condition under ramped, constant and cyclic pressure loading. Gas leakage is used as the failure criterion, making it analogous to the stresses and failures seen during fuel cell operation. Three-dimensional digital image correlation is used to measure the deformation of the bulging membrane. A quasi-static, spherical pressure vessel approximation of the central portion of the blister then allows the biaxial stress state at this location of the PEM to be tracked through time. A damage accumulation model is applied to the data taken during constant pressure loading to determine the damage parameters. The mode...
When a proton exchange membrane (PEM) based fuel cell is placed in service, hygrothermal stresses... more When a proton exchange membrane (PEM) based fuel cell is placed in service, hygrothermal stresses develop within the membrane and vary widely with internal operating environment. These hygrothermal stresses associated with hygral contraction and expansion at the operating conditions are believed to be critical in membrane mechanical integrity and durability. Understanding and accurately modeling the viscoelastic constitutive properties of a
Temperature and humidity fluctuations in operating fuel cells impose significant biaxial stresses... more Temperature and humidity fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEMs) of a fuel cell stack. The strength of the PEM, and its ability to withstand cyclic environment-induced stresses, plays an important role in membrane integrity and consequently, fuel cell durability. In this study, a pressure loaded blister test is used to
The use of pressurized blister specimens to characterize the biaxial strength and durability of p... more The use of pressurized blister specimens to characterize the biaxial strength and durability of proton exchange membranes (PEMs) is proposed, simulating the biaxial stress states that are induced within constrained membranes of operating PEM fuel cells. PEM fuel cell stacks consist of layered structures containing the catalyzed PEMs that are surrounded by gas diffusion media and clamped between bipolar plates. The surfaces of the bipolar plates are typically grooved with flow channels to facilitate distribution of the reactant gases and water by-product. The channels are often on the order of a few millimeters across, leaving the sandwiched layers tightly constrained by the remaining lands of the bipolar plates, preventing in-plane strains. The hydrophilic PEMs expand and contract significantly as the internal humidity, and to a lesser extent, temperature varies during fuel cell operation. These dimensional changes induce a significant biaxial stress state within the confined membra...
Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) f... more Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) fuel cells is one of the major technological barriers to the commercialization of fuel cell vehicles. The cracks in the electrode layers of the MEA, referred to as mud-cracks, are potential contributors to the failure in the PEM. To investigate how these mud-cracks affect the mechanical durability of the MEA, pressure-loaded blister tests are performed at 90°C to determine the biaxial fatigue strength of Gore-Primea® series 57 MEA. In these volume-controlled tests, leaking rate is determined as a function of fatigue cycles. The failure is defined to occur when the leaking rate exceeds a specified threshold. Postmortem characterization using bubble point testing and field emission scanning electron microscopy (FESEM) was conducted to provide visual documentation of leaking failure sites. The analysis of the experimental leaking data indicates that the MEA has much shorter lifetimes at the ...
Many premature failures in proton exchange membrane (PEM) fuel cells are attributed to crossover ... more Many premature failures in proton exchange membrane (PEM) fuel cells are attributed to crossover of the reactant gas from microcracks in the membranes. The formation of these microcracks is believed to result from chemical and/or mechanical degradation of the constrained membrane during fuel cell operation. By characterizing the through-membrane leakage, we report failures resulting from crack formation in several PEMs mounted in 50cm2 fuel cell fixtures and mechanically stressed as the environment was cycled between wet and dry conditions in the absence of chemical potential. The humidity cycling tests also show that the failure from crossover leaks is delayed if membranes are subjected to smaller humidity swings. To understand the mechanical response of PEMs constrained by bipolar plates and subjected to changing humidity levels, we use Nafion® NR-111 as a model membrane and conduct numerical stress analyses to simulate the humidity cycling test. We also report the measurement of ...
ABSTRACT Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigu... more ABSTRACT Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigue stresses in response to changing operation conditions. Determination of these stresses is complicated as both constitutive properties of PEM and the hygrothermal strain in PEM are highly dependent on time, temperature and water absorption. In this study, by tracking the spherical bending curvature of a bimaterial strip composed of a 25-mu m thick PEM (Nafion) and a 250-mu m thick PEEK (polyether ether ketone) substrate using a Digital Image Correlation (DIC) system, the biaxial tensile and compressive stresses in the plane of the PEM during humidity cycles are recorded. Due to the relatively high bending stiffness ratio between PEEK and PEM, the hygral stresses in the PEM are determined from the bimaterial curvature without requiring knowledge of the constitutive properties of the PEM. As the strain in the PEM released due to bending of the bimaterial strip is limited, the stresses measured using the bimaterial strip are very close to that in a fully constrained membrane, mimicking those in a fuel cell. The hygral strain in a freestanding membrane placed next to the bimaterial structure is recorded at the same time using the DIC system. The full stress and strain histories during incremental relative humidity (RH) test between 10%RH and liquid wet as well as during larger RH cycles are presented. The hygral expansion of Nafion during the increasing and decreasing RH step changes exhibits an asymmetry that resembles the behavior of glassy polymers during the classic Kovacs temperature jump test. The stress history also reveals the RH-dependent viscoelasticity of Nafion.
There are three critical membrane degradation mechanisms that can lead to failure of polymer elec... more There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical degradation, mechanical degradation, and membrane shorting. This talk will focus on the individual failure modes; with attention dedicated to describing the fundamental model-based mechanistic understandings, appropriate accelerated stress tests that enable rapid screening and relevant in-situ diagnostics to track membrane health during fuel cell operation, as well as discussions of effective mitigation strategies to prevent or minimize the risk of failure caused by the specific modes of membrane degradation. The effect of simultaneous chemical and mechanical degradation will also be discussed. Both chemical and mechanical degradation of perfluorosulfonic acid (PFSA) membranes have been extensively studied, and effective mitigation strategies have been developed that can significantly extend PFSA proton exchange membrane (PEM) lifetimes. Accelerated st...
ASME 2011 9th International Conference on Fuel Cell Science, Engineering and Technology, 2011
The strength of the proton exchange membrane (PEM) and its ability to withstand cyclic, environme... more The strength of the proton exchange membrane (PEM) and its ability to withstand cyclic, environmentally induced stresses plays an important role in membrane integrity and consequently, fuel cell durability. In this study, pressure loaded blister tests are used to characterize the biaxial strength of a model commercially available membrane, Gore-Select® series 57, at the 90°C dry condition under ramped, constant and cyclic pressure loading. Gas leakage is used as the failure criterion, making it analogous to the stresses and failures seen during fuel cell operation. Three-dimensional digital image correlation is used to measure the deformation of the bulging membrane. A quasi-static, spherical pressure vessel approximation of the central portion of the blister then allows the biaxial stress state at this location of the PEM to be tracked through time. A damage accumulation model is applied to the data taken during constant pressure loading to determine the damage parameters. The mode...
When a proton exchange membrane (PEM) based fuel cell is placed in service, hygrothermal stresses... more When a proton exchange membrane (PEM) based fuel cell is placed in service, hygrothermal stresses develop within the membrane and vary widely with internal operating environment. These hygrothermal stresses associated with hygral contraction and expansion at the operating conditions are believed to be critical in membrane mechanical integrity and durability. Understanding and accurately modeling the viscoelastic constitutive properties of a
Temperature and humidity fluctuations in operating fuel cells impose significant biaxial stresses... more Temperature and humidity fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEMs) of a fuel cell stack. The strength of the PEM, and its ability to withstand cyclic environment-induced stresses, plays an important role in membrane integrity and consequently, fuel cell durability. In this study, a pressure loaded blister test is used to
The use of pressurized blister specimens to characterize the biaxial strength and durability of p... more The use of pressurized blister specimens to characterize the biaxial strength and durability of proton exchange membranes (PEMs) is proposed, simulating the biaxial stress states that are induced within constrained membranes of operating PEM fuel cells. PEM fuel cell stacks consist of layered structures containing the catalyzed PEMs that are surrounded by gas diffusion media and clamped between bipolar plates. The surfaces of the bipolar plates are typically grooved with flow channels to facilitate distribution of the reactant gases and water by-product. The channels are often on the order of a few millimeters across, leaving the sandwiched layers tightly constrained by the remaining lands of the bipolar plates, preventing in-plane strains. The hydrophilic PEMs expand and contract significantly as the internal humidity, and to a lesser extent, temperature varies during fuel cell operation. These dimensional changes induce a significant biaxial stress state within the confined membra...
Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) f... more Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) fuel cells is one of the major technological barriers to the commercialization of fuel cell vehicles. The cracks in the electrode layers of the MEA, referred to as mud-cracks, are potential contributors to the failure in the PEM. To investigate how these mud-cracks affect the mechanical durability of the MEA, pressure-loaded blister tests are performed at 90°C to determine the biaxial fatigue strength of Gore-Primea® series 57 MEA. In these volume-controlled tests, leaking rate is determined as a function of fatigue cycles. The failure is defined to occur when the leaking rate exceeds a specified threshold. Postmortem characterization using bubble point testing and field emission scanning electron microscopy (FESEM) was conducted to provide visual documentation of leaking failure sites. The analysis of the experimental leaking data indicates that the MEA has much shorter lifetimes at the ...
Many premature failures in proton exchange membrane (PEM) fuel cells are attributed to crossover ... more Many premature failures in proton exchange membrane (PEM) fuel cells are attributed to crossover of the reactant gas from microcracks in the membranes. The formation of these microcracks is believed to result from chemical and/or mechanical degradation of the constrained membrane during fuel cell operation. By characterizing the through-membrane leakage, we report failures resulting from crack formation in several PEMs mounted in 50cm2 fuel cell fixtures and mechanically stressed as the environment was cycled between wet and dry conditions in the absence of chemical potential. The humidity cycling tests also show that the failure from crossover leaks is delayed if membranes are subjected to smaller humidity swings. To understand the mechanical response of PEMs constrained by bipolar plates and subjected to changing humidity levels, we use Nafion® NR-111 as a model membrane and conduct numerical stress analyses to simulate the humidity cycling test. We also report the measurement of ...
ABSTRACT Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigu... more ABSTRACT Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigue stresses in response to changing operation conditions. Determination of these stresses is complicated as both constitutive properties of PEM and the hygrothermal strain in PEM are highly dependent on time, temperature and water absorption. In this study, by tracking the spherical bending curvature of a bimaterial strip composed of a 25-mu m thick PEM (Nafion) and a 250-mu m thick PEEK (polyether ether ketone) substrate using a Digital Image Correlation (DIC) system, the biaxial tensile and compressive stresses in the plane of the PEM during humidity cycles are recorded. Due to the relatively high bending stiffness ratio between PEEK and PEM, the hygral stresses in the PEM are determined from the bimaterial curvature without requiring knowledge of the constitutive properties of the PEM. As the strain in the PEM released due to bending of the bimaterial strip is limited, the stresses measured using the bimaterial strip are very close to that in a fully constrained membrane, mimicking those in a fuel cell. The hygral strain in a freestanding membrane placed next to the bimaterial structure is recorded at the same time using the DIC system. The full stress and strain histories during incremental relative humidity (RH) test between 10%RH and liquid wet as well as during larger RH cycles are presented. The hygral expansion of Nafion during the increasing and decreasing RH step changes exhibits an asymmetry that resembles the behavior of glassy polymers during the classic Kovacs temperature jump test. The stress history also reveals the RH-dependent viscoelasticity of Nafion.
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Papers by Craig Gittleman