Humidity Cycling Tests Research Articles

External Reinforcement of Hydrocarbon Membrane By Combining Toughened Catalyst Layers and Interlocking Interfaces for High Mechanical Robustness of PEMFC

For decades, in polymer electrolyte membrane fuel cell (PEMFC) field, hydrocarbon (HC) membranes have been constantly studied to replace perfluorinated sulfonic acid membranes owing to their low cost and high fuel efficiency. However, adopting HC membranes to practical PEMFCs has been not successful due to their poor mechanical stability which causes a mechanical failure with generating pin-hole under repeated volume expansion/shrinkage during cell operation. Conventionally, the problem has been addressed by inserting a porous mechanical supporter in HC membrane which is denoted as ‘internal reinforcement’. However, the introduction of the inert support decreases proton conduction and increases membrane cost. Here we present an external reinforcement of HC membrane as a new strategy to enhance mechanical durability of HC membrane. It features the incorporation of mechanically tough porous fibrous network into catalyst layers and the strong connection of HC membrane and the toughened catalyst layers with introducing an interlocking interface. The mechanically toughened catalyst layers and interfaces can effectively mitigate the volume change of HC membrane, lowing the membrane failure. Under an accelerated humidity cycling test, the externally reinforced HC membrane exhibits an enhanced durability compared to un-reinforced counterpart. Furthermore, contrary to the internal reinforcement, the external reinforcement strategy does not cause any loss of proton conductivity of HC membrane. Therefore, the external reinforcement coupled with toughed catalyst layers and interfaces can provide an effective way to enhance durability with preserving the proton conductivity of pristine HC membrane.

In-situ membrane hydration measurement of proton exchange membrane fuel cells

Achieving proper membrane hydration control is one of the most critical aspects of PEM fuel cell development. This article describes the development and application of a novel 50 cm2 fuel cell device to study the in-situ membrane hydration by measuring the through-thickness membrane swelling via an array of linear variable differential transducers. Using this setup either as an air/air (dummy) cell or as a hydrogen/air (operating) cell, we performed a series of hydration and dehydration experiments by cycling the RH of the inlet gas streams at 80 °C. From the linear relationship between the under-the-land swelling and the over-the-channel water content, the mechanical constraint within the fuel cell assembly can suppress the membrane water uptake by 11%–18%. The results from the air/air humidity cycling test show that the membrane can equilibrate within 120 s for all RH conditions and that membrane can reach full hydration at a RH higher than 140% in spite of the use of a liquid water impermeable Carbel MP30Z microporous layer. This result confirms that the U.S. DOE's humidity cycling mechanical durability protocol induces sufficient humidity swings to maximize hygrothermal mechanical stresses. This study shows that the novel experimental technique can provide a robust and accurate means to study the in-situ hydration of thin membranes subject to a wide range of fuel cell conditions.

In Situ Observation of Deformation Behavior of Membrane Electrode Assembly Under Humidity Cycles

The mechanical reliability of the membrane electrode assembly (MEA) in polymer electrolyte fuel cells (PEFCs) is a major concern with respect to fuel cell vehicles. When PEFCs generate power, water is generated. The proton exchange membrane (PEM) swells in wet conditions and shrinks in dry conditions. These cyclic conditions induce mechanical stress in the MEA, and cracks are formed. Failure of the MEA can result in leaking of fuel gases and reduced output power. Therefore, it is necessary to determine the mechanical reliability of the MEA under various mechanical and environmental conditions. The purpose of the present paper is to observe the deformation behavior of the MEA under humidity cycles. We have developed a device in which the constrained condition of the GDL is modeled by carbon bars of 100 to 500 μm in diameter. The carbon bars are placed side by side and are pressed against the MEA. The device was placed in a temperature and humidity controlled chamber, and humidity cycles were applied to the specimen. During the tests, cross sections of the specimen were observed by microscope, and the strain was calculated based on the curvature of the specimen. The temperature in the test chamber was varied from 25 to 80 °C, and the relative humidity was varied from 50 to 100%RH, and the wet condition was also investigated. The results revealed that the MEA deformed significantly by swelling and residual deformation was observed under the dry condition, even for one humidity cycle. The crack formation criteria for one humidity cycle corresponded approximately with those of the static tensile tests. The results of the humidity cycle tests followed Coffin–Manson law, and the number of cycles until crack formation corresponded approximately with the results of the mechanical fatigue tests. These results will be valuable in the critical design of durable PEFCs.

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.

Effects of catalyst layer on the structural change of a membrane electrode assembly under humidity cycle tests

Structural changes in a polymer electrolyte membrane (PEM) and a membrane electrode assembly (MEA) of different catalyst thickness under humidity cycle tests were investigated. Nafion211 membrane was used in this study. The catalyst layer thickness of MEAs was set to 2–10 μm using catalyst paste of the same composition. Dimensional changes in the PEM and MEA due to swelling were measured, and dynamic mechanical analysis (DMA) of the membrane and MEA were carried out. Moreover, humidity cycle tests for the MEA were carried out according to the conditions specified in the FCCJ protocol, and structural changes in the MEA after the tests were examined. It was shown that the membranous structural change under humidity cycle tests tends to become small when the thickness of the catalyst layer is large.

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.

Reliability Analysis of RFID Tags in Changing Humid Environment

Radio frequency identification (RFID) tags with anisotropic conductive adhesive (ACA) joints are used in various applications where the environmental conditions may impair their reliability. Therefore, it is essential to investigate the effects of different environmental stresses on reliability and to analyze failure mechanisms. The purpose of this paper was to study the effects of changing humidity conditions on the performance of a passive ultrahigh frequency RFID tag with ACA joints. The tags were tested in a humidity cycling test where the humidity varied from 85%RH to 10%RH and temperature from 85 °C to 25 °C. Tags with four different sets of bonding parameters were tested. Significant differences in reliability between tags with different bonding parameters were observed. Different failure times, failure modes, and failure mechanisms were observed. Full-wave electromagnetic antenna simulations were used to support the analysis on the failure mechanisms manifesting themselves through changes in the common tag performance parameters. Changes in impedance matching between the chip and the antenna were found to be the most likely failure mechanism. The impedance matching may have changed due to cracks formed in the antenna during testing or due to chemical and mechanical changes in the ACA joints. It was observed that several failure factors might act simultaneously.

Pre-Stretched Low Equivalent Weight PFSA Membranes with Improved Fuel Cell Performance

Uniaxial stretching of recast perfluorsulfonic acid (PFSA) films was used to promote desirable morphological changes for 3M Company's 825 equivalent weight (EW) and 733 EW PFSA polymers. Stretching to a draw ratio (DR) of four was followed by a high temperature annealing step in order for the morphological changes to be permanent. For 825 EW PFSA, stretching increased the polymer crystallinity by 22.5%, with a reduction in methanol permeability and a small increase in proton conductivity. In direct methanol fuel cell tests at 60°C with 1.0 M methanol, the power density at 0.4 V with a DR = 4 stretched 825 EW membrane (72 mW/cm2) was considerably greater than that obtained with a solution-cast membrane (28 mW/cm2) or with a commercial Nafion 117 membrane (55 mW/cm2). For 733 EW PFSA, stretching promoted the formation of ordered ionic domains leading to an increase in proton conductivity. There was also a substantial increase in the polymer's α-transition temperature and an improvement in membrane mechanical properties after stretching. The lifetime of a stretched 733 EW PFSA membrane (DR = 4) was extended by a factor of 3.5 as compared to that of a solution-cast film in an open circuit voltage humidity cycling test at 80°C.

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.

Tin whisker analysis of an automotive engine control unit

Since the 1970s, automobile manufacturers have used electronics for safety–critical automobile control systems such as acceleration, braking, and steering. These electronic systems introduced functionalities in automobiles that were not feasible in a purely mechanical framework, such as anti-lock braking systems and electronic stability control. However, failures of these electronic systems can lead to fatalities, financial losses, legal ramifications, and reputational liabilities for manufacturers. Therefore, automotive electronics must be designed to have minimum failures during use.In this paper, materials used in an automotive engine control unit (ECU) from a 2008 Toyota Tundra truck were analyzed. It was found that pure tin with a nickel underlayer was used as the connector finish in the unit, and analysis revealed tin whiskers on the connector surface. The use of pure tin finishes in electronics can produce conductive tin whiskers capable of creating unintended electrical failures such as short circuits. To assess tin whisker growth and the use of nickel as an underlayer material, the engine control unit was subjected to a standard temperature–humidity cycling test. The connectors showed tin whisker growth after testing, raising additional reliability and safety concerns. Recommendations for the automotive industry and National Highway Traffic Safety Administration are offered.

Static Bending of Flip-Chip Structures Under Humid Conditions

The miniaturization of electronics poses challenges to reliability demands on devices. In applications where both high reliability and small size are demanded, reliability testing plays a critical role. Medical applications are one such example, and in this case, not only must the durability of electronics be considered but the safety of the device to the human body is also of vital importance. This paper considers the use of thin FR-4 substrates and thin silicon chips in medical applications. The joining is made using anisotropically conductive adhesives. The assemblies are protected using a parylene C conformal coating, which has been proved to be a biocompatible material and, thus, safe for medical applications. The assemblies are bent to a fixed radius during humidity testing. Nonbent and noncoated test lots are also studied for comparison. Two different humidity testing methods are used: the 85/85 constant humidity test and humidity cycling test. Furthermore, the test structures are simulated using finite element analysis. The results show the protective nature of parylene C coating and the differences between bent and nonbent structures. Parylene C coating seems to support the structure by lowering the shear and von Mises stresses formed, resulting in better reliability. Furthermore, the results show a clear difference in the two humidity testing methods, giving value to the standard 85/85 testing.

Reliability Analysis of UHF RFID Tags Under a Combination of Environmental Stresses

Accelerated environmental tests can be used to study the effects of environmental stresses on reliability. The environmental tests are commonly run in parallel. One test is performed for one set of test samples, and new sets of test samples are used for other possible tests. However, the subsequent use of different tests for the same set of test samples may describe the operational environment of the samples more accurately and give more reliable results in a short testing time. In addition, the different stresses may accelerate the effects of other stresses, and such behavior can only be perceived if combinations of environmental tests are used. Passive ultrahigh-frequency radio-frequency identification (RFID) tags were tested with different combinations of environmental tests. Some of the tags were subjected first to a bending test, to a constant humidity test, or to a temperature cycling test and then subjected to a humidity cycling test. As a reference, some of the tags were tested only in the humidity cycling test. Changes in the performance parameters of the RFID tags were examined during testing. Clear changes in the reliability of RFID tags with different test combinations were observed. The exposure to constant humidity test before the humidity cycling test significantly impaired the reliability of tags in the humidity cycling test. On the other hand, bending testing before the humidity cycling test had no noticeable effect. The temperature cycling test only slightly impaired the reliability of the tags in the humidity cycling test. In the failure analysis, no clear failure mechanisms were found. It is possible that several failure mechanisms affected the tags simultaneously during the humidity testing. This study demonstrated the importance of studying combinations of different environmental tests one after another on the same test samples. Although exposure to certain environmental stresses may not immediately cause a device to fail, it may impair the reliability when the device is later exposed to another environmental stress. Combinatory stress may be critical for the actual product, and therefore, their discovery during environmental testing is crucial.

Utilizing Modeling in a Reliability Study of ACA Joints in Humidity Tests

Radio frequency identification tags (RFID) with anisotropic conductive adhesive (ACA) joints are used in various applications where the environmental conditions may impair their reliability. Thus the effects of different environmental stresses on reliability need to be investigated. The purpose of this work was to study whether a relatively simple shear stress model can be utilized in reliability prediction of anisotropically conductive paste (ACP) joints in an accelerated humidity test on the basis of the information obtained from another humidity test. If modeling gives accurate results when studying reliability, the need for actual testing would decrease and thereby time and cost savings could be achieved. In this study, finite element models were made to calculate shear stresses in ACP joints induced by two different humidity tests. Additionally, experimental tests were performed and the results were compared with those of modeling. The test samples were RFID tags whose microchips were attached with ACP. A constant humidity test was used to study the effects of high humidity level and a humidity cycling test was used to examine the effects of constantly varying humidity. In the modeling it was observed that the selection of the stress-free temperature has a significant effect on the results. With three different stress-free temperatures, three different sets of results were obtained. Although the tags saturated in the extreme conditions of the humidity cycling test, according to modeling, the change in relative humidity level in the humidity cycling test did not increase the harshness of the test. However, the temperature change in the humidity cycling test increased the harshness.

Effect of time-dependent material properties on the mechanical behavior of PFSA membranes subjected to humidity cycling

A viscoelastic-plastic constitutive model is developed to characterize the time-dependent mechanical response of perfluorosulphonic acid (PFSA) membranes. This model is then used in finite element simulations of a representative fuel cell unit, (consisting of electrodes, gas diffusion layer and bipolar plates) subjected to standardized relative humidity (RH) cycling test conditions. The effects of hold times at constant RH, the feed rate of humidified air and sorption rate of water into the membrane on the stress response are investigated. While the longer hold times at high and low humidity lead to considerable redistribution of the stresses, the lower feed and sorption rates were found to reduce the overall stress levels in the membrane. The redistribution and reduction in stress magnitudes along with inelastic deformation during hydration eventually lead to development of residual tensile stresses after dehydration. Simulations indicate that these tensile stresses can be on the order of 9–10 MPa which may lead to mechanical degradation of the membrane. The simulation results show that time-dependent properties can have a significant effect on the in-plane stress response of the membrane.

Metal Phosphates as Intermediate Temperature Proton Conducting Electrolytes

A series of metal phosphates were synthesized and screened as potential proton conductor electrolytes for fuel cells and electrolysers operational at intermediate temperatures. Among the selected, niobium and bismuth phosphates exhibited a proton conductivity of 10-2 and 10-7 S cm-1, respectively, under the anhydrous atmosphere at 250 °C, showing close correlation with the presence of hydroxyl groups in the phosphate phases. At the water partial pressure of above 0.6 atm, both phosphates possessed a proton conductivity to a level of above 3 x 10-2 S cm-1. Reasonable stability of the proton conductivity was observed under either a constant low water partial pressure or under a humidity cycling test within a period of more than 80 hours.

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.

Effects of Cycling Humidity on the Performance of RFID Tags With ACA Joints

Radio frequency identification (RFID) tags with anisotropically conductive joints (ACAs) are used in different applications where the environmental conditions may impair their reliability. Thus the effects of different environmental stresses on reliability need to be investigated. The effects of high temperature and humidity may change the performance of the tags. More- over, the effects of constantly varying temperature and humidity conditions may be even more harmful. In this study, the effects of changing humidity conditions on the performance of a passive ultra high frequency (UHF) RFID tag with ACA joints were studied. The tags were tested in a humidity cycling test where humidity varied from 85%RH to 10% RH, and temperature from 85°C to 25°C. Tags with four different sets of bonding parameters were tested. Significant differences in the reliability between the tags with different bonding parameters were observed. The results were also compared with results from a corresponding constant humidity test where the humidity was 85%RH, and the temperature 85°C. The tags had different failure times, modes, and mechanisms in these two tests. Furthermore, the effects of bonding parameters on the reliability were different in these tests. According to this study, it is important to investigate the effects of changing humidity, when the reliability in different environments is investigated, but the constant humidity test cannot be replaced with the faster humidity cycling test.

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].

Flat Absorber Phosphorous Black Nickel Coatings for Space Applications

A new process of flat absorber black nickel alloy coating was developed on stainless steel by electrodeposition from a bath containing nickel, zinc and ammonium sulphates; thiocyanate and sodium hypophosphite for space applications. Coating process was optimized by investigating the effects of plating parameters, viz concentration of bath constituents, current density, temperature, pH and plating time on the optical properties of the black deposits. Energy dispersive X-ray spectroscopy showed the inclusion of about 6% phosphorous in the coating. The scanning electron microscopy studies revealed the amorphous nature of the coating. The corrosion resistance of the coatings was evaluated by the electrochemical impedance spectroscopy (EIS) and linear polarization (LP) techniques. The results revealed that, phosphorous addition confers better corrosion resistance in comparison to conventional black nickel coatings. The black nickel coating obtained from hypophosphite bath provides high solar absorptance (αs) and infrared emittance (ɛIR) of the order of 0.93. Environmental stability to space applications was established by the humidity and thermal cycling tests.

Study on the Sandstone Weathering Sensitivity Caused by the Changes of Temperature and Humidity

The nearby reservoir water impounding causes the water level rise of Minjiang River where Leshan Giant Buddha locate at. It may affects the microclimate of the Buddha area, and the possible microclimate changes mainly include the temperature and rainfall, for the reason of protecting the long-term stability of the cultural relics, it is necessary to assess the rock weathering sensitivity of the Leshan Giant Buddha sandstone because of these changes. Via the indoor temperature cycling test, humidity cycling test and humiture cycling test, the stone samples’ elastic wave velocity and mass are determined in different condition of temperature and humidity. And the influence to the Buddha stone weathering caused by the changes of temperature and humidity is analyzed.