Relative Humidity Cycling Test Research Articles

Predicting fatigue lifetimes of a reinforced membrane in polymer electrolyte membrane fuel cell using plastic energy

Limited mechanical durability remains a critical issue for the automotive application of polymer electrolyte membrane fuel cells (PEMFCs). Membrane fatigue cracking is a typical degradation mechanism that can lead to a failure of the PEMs. These cracks initiate and grow in the membrane due to hydration induced cyclic mechanical stresses, and the resistance to this type of damage is typically evaluated using relative humidity (RH) cycling as an accelerated stress test (AST) on the membrane. In this study, we simulate three RH cycling tests for a modern reinforced membrane. All the membrane material parameters used in the simulations were determined independently from the available experiments except for one: the material resistance to crack initiation, which is computed in terms of plastic energy in a representative unit fuel cell finite element model. This parameter was determined from one AST experimental data point and used to predict the cycle-to-failure of the membrane for the other two experimental conditions. Despite a number of assumptions used in the numerical analysis, the results show satisfactory agreement with experimental data. Although one test was used to calibrate the model, the proposed numerical scheme could replace numerous other long mechanical ASTs at the same temperature and humidity thereby significantly reducing the resources and time needed to evaluate the mechanical integrity of any particular modern membrane. Moreover, this model-based analysis could be used to correlate fuel cell operating conditions with membrane mechanical lifetime, thus aiding the development of system strategies in fuel cell vehicles for durability enhancement.

An Accelerated Mechanical Stress Test for Evaluating Fatigue Durability of Reinforced Fuel Cell Membranes

For evaluating the durability of membranes in polymer electrolyte fuel cells (PEFCs) within relatively short time, accelerated stress tests (AST) are used for applying chemical and mechanical stressors by means of current density, cell voltage, temperature, and relative humidity (RH) cycling. The U.S. Department of Energy (DOE) standardized RH cycling test is considered as a routine screening test for evaluating the mechanical durability under cyclic mechanical stresses [1]. Due to limitation in diffusion kinetics of water in the membrane, each RH cycle often takes several minutes that consequently restricts the magnitude of the induced mechanical stress. Therefore, conventional mechanical ASTs become time-consuming and expensive, especially as more durable membranes are being adopted.A typical mechanical degradation mitigation strategy is to insert an insulating and chemically-inert reinforcement layer, e.g., a thin microporous expanded polytetrafluoroethylene (ePTFE) mesh in the membrane to improve dimensional stability in response to RH-induced membrane expansion and contraction. The reinforcement layer provides additional mechanical strength and enables the use of thinner and more conductive ionomer that would otherwise have insufficient strength. When tested using mechanical ASTs, reinforced membranes possessed longer lifetime [2,3].Experimental work on application of ASTs to fuel cell membranes was previously performed [4] where an accelerated membrane durability test (AMDT) was developed to characterize membrane stability when subjected to chemical and mechanical degradation relevant to field operation. In another work [5], mechanical ASTs were performed via in situ RH cycling with longer dry cycles (for maximizing the fatigue stress amplitude) and at a higher temperature than the DOE protocol. Using these conventional protocols, it is expected that tests with reinforced membranes will continue beyond 20,000 cycles (as defined by DOE [1]) without reaching failure. An ex situ blister test method was used for bi-axial fatigue-creep testing by cycling air pressure [6,7]. In this case, the inherent hygroscopic membrane properties, i.e., water uptake and dimensional changes, which are the actual source of in situ fatigue and mechanical degradation in the constrained fuel cell environment, were not considered.In this work, a custom-developed mechanical AST procedure, based in part on the DOE-standardized AST for mechanical membrane degradation [1], was developed and used for evaluating the mechanical fatigue durability of reinforced membranes in a shorter time. Here, mechanical stressors were applied to the membrane using a sustained pressure differential (ΔP) combined with RH cycling at 80oC in a bi-axial, fuel cell inspired configuration. For validation of the developed method, denoted as ΔP-AMST, ePTFE-reinforced membranes were cycled using 4 min dry (0% RH) and 2 min wet (100% RH) cycles along with a ΔP ranging from 7 to 14 kPa. When the ΔP was applied, a transparent polycarbonate spacer, placed between the cathode and anode, allowed inflation of the membrane by means of a through-thickness hole, as shown in Figure 1. In order to prevent chemical degradation, nitrogen was used as the carrier gas in the ΔP-AMST instead of air, which is used in the DOE method [1]. Depending on the applied ΔP, reinforced membranes failed within ~10 to 10,000 RH cycles, the failure criterion being the loss of ΔP, indicative of major gas leakage through the degraded membrane. Therefore, compared to conventional mechanical ASTs, this novel mechanical AST could be used as a rapid and economical in situ alternative for evaluating the mechanical durability of advanced fuel cell membranes. Acknowledgements This research was supported by Mitacs, Automotive Fuel Cell Cooperation (AFCC), Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, and Simon Fraser University. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.

Effects of cycle duration and test hardware in relative humidity cycling of a polymer electrolyte membrane

In this study, relative humidity (RH) cycling accelerated stress testing (AST) is pursued on a supplier's catalyst coated membrane (CCM). The experimental design includes three RH cycles and two test cell/membrane electrode assembly (MEA) formats. All three cycles tested are shorter than the Department of Energy (DOE) protocol and model simulations indicate that deep and fast hydration swings could still be retained. Edge failure is observed in one test cell, which is in part responsible for the shorter lifetime in RH cycling as compared to using another test cell and suggests a test hardware harmonization if the lifetime is referenced to a target or compared between different materials. Nevertheless, the trends of H2 crossover versus the number of cycles and overall test time remain consistent for two hardware formats. This finding suggests that RH cycling could be effectively used to investigate a control factor, such as cycle duration, based on a single hardware and material, towards the development of system strategies on a fuel cell vehicle. Furthermore, since the H2 crossover versus RH cycling test time plots exhibit clear separation for three cycles, the frequency and duration of cycles both play a role in determining the membrane lifetime.

In-plane biaxial cyclic mechanical behavior of proton exchange membranes

The durability of a proton exchange membrane is affected by both mechanical degradation and chemical degradation. While fatigue and relative humidity cycling tests have been conducted to address mechanical degradation, the cyclic behavior that bridges the gap between the stress-strain response and fatigue behavior is not well established. The objective of this study is to understand the strain evolution during biaxial cyclic loading that resemble the actual stress state of the membrane. In particular, the effect of loading paths on strain evolution is examined to account for the stress state on strain accumulation. It is found that the constraint effect of stress in one direction on strain evolution in another direction strongly depends on the stress state of the membrane, and the equibiaxial stress state imposes the most significant constraint on strain evolution. Furthermore, the constraint effect induced by biaxial loading is more significant at higher relative humidity values. Moreover, high-stress amplitude cycle acts to retard strain accumulation in the subsequent low-stress amplitude cycle. The findings reported here will provide new evidence for an understanding of the fatigue behavior of a proton exchange membrane as well as durability modeling of proton exchange membrane fuel cells.

Shear strength degradation in claystones due to environmental effects

This note explores the influence of environmental effects, as those induced by cyclic changes in relative humidity, on the degradation of the shear strength parameters in Lilla claystone, a low-porosity clayey rock from northern Spain. The results of a comprehensive experimental programme, combining long-term relative humidity cycling tests with saturated direct shear tests, are described. A continuous monitoring of the evolution of volumetric strain during the previous relative humidity cycling is used to evaluate the swelling behaviour of the rock. Both undisturbed and degraded specimens are subjected to shearing at saturated conditions to determine the peak and post-rupture shear strength envelopes. The effects on rock brittleness and dilation angle are also analysed. Shear strength shows a strong dependence on the history of relative humidity cycling. In particular, the evolution of the peak shear strength parameters (c′ and φ′) seem to be related to the accumulated irreversible strains developed during each cycle. A damage law, recently proposed by the authors, is used to represent the progressive degradation of the shear strength parameters as a function of the accumulated irreversible strains.

Hygrothermal Aging of an ACA Attached PET Flex-on-Board Assembly

In this paper, accelerated temperature and humidity test methods were used to study the long-term reliability of a flex-on-board assembly attached with an anisotropically conductive adhesive (ACA). Test methods included constant 65°C/90% relative humidity (RH) (65/90) and 85°C/85%RH (85/85) tests, 10°C-65°C/90%RH cycling test with moisture condensation (65/90 cycling), and 85°C/85%RH humidity cycling test (85/85 cycling). The resistance of the ACA joints was measured in real time during testing. On basis of the resistance measurement results, failures occurred in the 65/90 cycling test due to ACA delamination and cracking. In addition, failures were also observed in the constant 85/85 test and 85/85 cycling test. However, these failures were mostly related to the properties of the polyethylene terephthalate flex and its embrittlement or latent defects. In addition, a 90° peel strength test was used to study the changes in the mechanical strength of the assembly. The peel strength results showed that with all the test methods, the largest drop in the mechanical strength occurred at the start of the test. After this, the peel strength in 65/90 and 65/90 cycling tests remained almost unchanged, whereas a more significant drop was observed with the 85/85 cycling test. The 85/85 test caused the flex to become brittle, which prevented peel testing. Based on the failure analysis of the peel tested samples, similar failure modes were observed with all the test methods used. With untested samples, failure mostly occurred at the interface between the adhesive and the flex, although cohesive as well as adhesion failures at the ACA-printed circuit board interface were also observed. After hygrothermal aging, the failure location changed toward the ACA-flex interface as testing progressed and, in most cases, finally resulted in the detachment of conductor traces from the flex.

Perfluorinated Sulfonic Acid Membrane and Membrane Electrode Assembly Degradation Correlating Accelerated Stress Testing and Lifetime Testing

An important step in achieving fundamental understanding of fuel cell failure mechanisms and development of technology to mitigate these failures is accomplished by analysis of directed lifetime and failure test results. Several lifetime, accelerated stress, and drive cycle test protocols have been developed and carried out. The two major ASTs that have been developed to evaluate membrane degradation are 1) Open circuit voltage tests, which are designed to accelerate chemical degradation, and 2) Relative humidity cycling tests, which are designed to accelerate mechanical degradation. The results from these tests have been compared to field tests. The ultimate goal is to use the laboratory tests to predict data in the field. An overall predictive decay model is being developed through a combination of specific modeling and tests.

Experimental Investigation of Air Relative Humidity (RH) Cycling Tests on MEA/Cell Aging in PEMFC Part I: Study of High RH Cycling Test With air RH at 62%/100%

AbstractThe effect of high air relative humidity (RH) cycling (RHC 62%/100%) on the degradation mechanisms of a single (5 × 5 cm2) proton exchange membrane fuel cells was investigated. The cell performance was compared to a cell operated at constant humidification (RHC = 62%). Runs were conducted over approximately 1,500 h at 0.3 A cm–2. The overall loss in cell performance for the high RH cycling test was 12 μV h–1 whereas it was at 3 μV h–1 under constant humidification. Impedance spectroscopy reveals that the ohmic and charge transfer resistances were little modified in both runs. H2 crossover measurement indicated that both high RH cycling and constant RH test did not promote serious effect on gas permeability. The electroactive surface loss for anode and cathode during high air RH cycling was more significant than at constant RH operation. The water uptake determined by 1H nuclear magnetic resonance within the membrane electrode assembly (MEA) after high RH cycling was reduced by 12% in comparison with a fresh MEA. Transmission electron microscopy showed bubbles and pinholes formation in the membrane, catalyst particles agglomeration (also observed by X‐ray diffraction), catalyst particles migration in the membrane and thickness reduction of the catalytic layers. Scanning electron microscopy was conducted to observe the changes in morphology of gas diffusion layers after the runs.

Experimental Investigation of Air Relative Humidity (RH) Cycling Tests on MEA/Cell Aging in PEMFC Part II: Study of Low RH Cycling Test with Air RH at 62%/0%

AbstractThe effect of low relative humidity (RH) cycling (RHC 62%/0%) on the degradation mechanisms of a single proton exchange membrane fuel cell (5 × 5 cm2) was investigated and compared to a cell operated at constant humidification (RHC = 62%). The overall cell performance loss was near 33 μV h–1, which is greater than the voltage decay under constant RH condition near 3 μV h–1. The electroactive surface was reduced but to an acceptable level. Impedance spectroscopy revealed that the ohmic and charge transfer resistances were reduced by the likely improved hydration of the ionomeric layer at the catalyst due to hydrogen crossover. This was so important that H2 starvation was finally responsible for the collapse of the cell after 650 h. Transmission electron microscopy showed occurrence of various phenomena, e.g., bubbles and pinholes formation in the membrane due to local overheat from hydrogen combustion at the cathode, and thickness reduction of catalytic layers. The water up take obtained by 1H NMR within the membrane electrode assembly (MEA) after low RH cycling reduced by 24% compared to a fresh MEA. Observations are also compared to those obtained at high RH cycling (RHC 62%/100%) presented in Part I of this study [1].

Fatigue and creep to leak tests of proton exchange membranes using pressure-loaded blisters

Abstract In this study, three commercially available proton exchange membranes (PEMs) are biaxially tested using pressure-loaded blisters to characterize their resistance to gas leakage under either static (creep) or cyclic fatigue loading. The pressurizing medium, air, is directly used for leak detection. These tests are believed to be more relevant to fuel cell applications than quasi-static uniaxial tensile-to-rupture tests because of the use of biaxial cyclic and sustained loading and the use of gas leakage as the failure criterion. They also have advantages over relative humidity cycling test, in which a bare PEM or catalyst coated membrane is clamped with gas diffusion media and flow field plates and subjected to cyclic changes in relative humidity, because of the flexibility in allowing controlled mechanical loading and accelerated testing. Nafion® NRE-211 membranes are tested at three different temperatures and the time–temperature superposition principle is used to construct stress-lifetime master curve. Tested at 90 °C, 2%RH extruded Ion Power® N111-IP membranes have a longer lifetime than Gore™-Select® 57 and Nafion® NRE-211 membranes.

Durability of Perfluorosulfonic Acid and Hydrocarbon Membranes: Effect of Humidity and Temperature

The effect of humidity on the chemical stability of two types of membranes [i.e., perfluorosulfonic acid type (PFSA, Nafion 112) and biphenyl sulfone hydrocarbon type, (BPSH-35)] was studied by subjecting the membrane electrode assemblies (MEAs) to open-circuit voltage (OCV) decay and potential cycling tests at elevated temperatures and low inlet-gas relative humidities. The BPSH-35 membranes showed poor chemical stability in ex situ Fenton tests compared to that of Nafion membranes. However, under fuel cell conditions, BPSH-35 MEAs outperformed Nafion 112 MEAs in both the OCV decay and potential cycling tests. For both membranes, (i) at a given temperature, membrane degradation was more pronounced at lower humidities and (ii) at a given relative humidity operation, increasing the cell temperature accelerated membrane degradation. Mechanical stability of these two types of membranes was also studied using relative humidity (RH) cycling. Due to decreased swelling and contraction during wet-up and dry-out cycles, Nafion 112 lasted longer than BPSH-35 membranes in the RH cycling test.