Degradation Of Membrane Electrode Assembly Research Articles

Understanding the degradation of MEA in PEMFC: Definition of structural markers and comparison between laboratory and on‐site ageing

AbstractDifferent accelerated tests in 12 fuel cells stack were performed in laboratory, namely on/off, back‐up, and base‐load regimes. In parallel, membrane electrode assemblies (MEA) were integrated in two “on‐site” systems for GSM relay application. One of them was dedicated to base‐load power applications while the second fuel cells coupled with photovoltaic panels operated in semibase load mode. To investigate the influence of the power profiles on MEA degradation, over 80 CCB MEAs (5 layers) were studied at different scales using ex situ characterizations such as tensile tests, TGA‐MS, DMTA, and SEM. A series of complementary microstructural ageing markers were thereby identified. The isolated influence of dry‐wet cycling on MEA properties was also established after passive hydro‐thermal (HT) ageing performed continuously for 10 months in the laboratory. The changes of each marker as a function of HT ageing time permitted to define a temporal benchmark. Based on these indicators, the main changes occurred in the MEA properties appear after a 5 months dry‐humid cycling (up to about 1800 cycles). The trends observed were useful to compare and estimate the degree of degradation of each ageing tests. Thus, the accelerated tests performed in laboratory for at least 500 h in stack did not reveal systematic MEA modifications. On the contrary, the 1500 h “on‐site” system operation results in some MEA degradations which origins are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Diagnosis of MEA degradation under accelerated relative humidity cycling

The objective of this work is to identify the failure mode diagnosis protocols for polymer electrolyte membrane (PEM) fuel cells with the use of accelerated testing conditions. The single cells used in this work were constructed using commercial Ion Power ® membrane electrode assemblies (MEAs) and the performance degradation was studied under accelerated dynamic reactant relative humidity (RH) conditions. The influence of RH on cell performance was investigated, and a strong dependence of degradation with respect to relative humidity was found. RH cycling was seen to not only cause the gradual decrease in performance at the beginning of cell operation, but it was also related to the rapid decline in performance observed after 330 h operation. This change in degradation rate is seen a change in the material degradation failure mechanism at this point in the operational history of the cell. The increase in cell resistance, membrane crossover, fluoride release rate and decrease in electrochemical surface area (ESA) were also observed with time, and these results were correlated to change in degradation rate. Infra-Red (IR) imaging of an aged MEA was utilized to show varying temperature profiles and outline the possibility of cracks, tears or pinholes in the membrane.

Diagnosis of PTFE-Nafion MEA degradation modes by an accelerated degradation technique

SUMMARY Cost and durability are central issues in the commercialization of proton-exchange membrane fuel cells. This study used complex accelerated-degradation protocols to diagnose the degradation modes of low-cost polytetrafluoroethylene–Nafion membrane electrode assemblies (MEAs). Acceleration protocols included open-circuit voltage, relative humidity cycling, and load cycling. Failure modes included measurements of cyclic voltammetry, linear sweep voltammetry, polarization curve, and AC impedance. The four modes (stages) of degradation were determined to be (i) catalyst aging, (ii) creep deformation, (iii) pinhole formation, and (iv) membrane failure. During catalyst aging, the maximum power density decreased by 0.117 mW cm−2 cycle−1. After the 280th cycle, creep deformation occurred, and the maximum power density decreased by 0.227 mW cm−2 cycle−1. Pinholes led to membrane failure and a final dramatic loss of performance (−0.453 mW cm−2 cycle−1). Therefore, membrane failure is the major factor in the failure of MEAs. Copyright © 2010 John Wiley & Sons, Ltd.

The Impact of SO2 on the Degradation of MEA Components in PEMFCs

The impact of low level concentrations of sulfur dioxide (SO2) and SO2 recovery methods on the degradation of membrane electrode assembly (MEA) components of a polymer electrolyte membrane fuel cell (PEMFC) was studied. MEAs were exposed to 2ppm SO2 for various exposure times at constant current density of 0.6 A/cm2 and a cell temperature of 80oC. Severe performance degradation was observed due to SO2 exposure and cell performance only partially recovered. Ion Chromatography (IC) analysis of the fuel cell effluent water revealed increased fluorine emission due to the presence of SO2. Scanning Electron Microscopy (SEM) of MEA cross sections confirmed a drastic decrease in membrane thickness after exposure to SO2. Transmission Electron Microscopy (TEM) of catalyst sections showed no distinct impact on platinum agglomeration due to SO2 exposure but did confirm an increase in platinum agglomeration due to the application of a cyclic voltammetry recovery method.

Investigation of degradation behavior of membrane electrode assembly with polytetrafluoroethylene/Nafion composite membrane

The main purpose of this study is to investigate the accelerated degradation of the PTFE/Nafion membrane electrode assembly through open circuit voltage and relative humidity cycling. The state of a PTFE/Nafion membrane electrode assembly is evaluated in a fuel cell by monitoring the polarization curve, AC impedance, cyclic voltammetry, and linear sweep voltammetry data over time. The experimental results are then fitted to equations of an equivalent circuit. The results of the first 160 experimental cycles show that catalyst degradation is the main cause for the decay of the membrane electrode assembly. During the 160–520th cycles, the membrane electrode assembly experiences creep deformation, which is due to relative humidity cycling. During the 640–840th cycles, the degradation causes a gradual transition from minor to major membrane cracking; after which the combustion reaction dramatically accelerates membrane electrode assembly degradation.

Lifetime prediction through accelerated degradation testing of membrane electrode assemblies in direct methanol fuel cells

To expand the applications of direct methanol fuel cells (DMFCs), the testing time required to evaluate their durability must be shortened. In this article, we present a step-by-step, accelerated degradation test (ADT) procedure for simple application by fuel cell engineers to the membrane electrode assembly (MEA) in DMFCs. Using MEA degradation data obtained under high stress conditions, we provide a method to estimate the lifetime distribution for normal use conditions and derive optimal testing plans for further degradation tests. A bi-exponential model with random coefficients is introduced to represent the nonlinear deterioration path of the MEAs. Based on the lifetimes estimated from the bi-exponential model at higher temperatures, the lifetime distribution at normal use temperature is extrapolated using the Weibull–Arrhenius model as the lifetime-temperature relationship.

Membrane electrode assembly degradation by dry/wet gas on a PEM fuel cell

The vehicle that use a Polymer Electrolyte Membrane Fuel Cell (PEMFC) as a power source frequently experiences start up and shut down. Membrane Electrode Assembly (MEA) degradation by wet/dry gas repetition was studied for vehicle start up and shut down. The time of the wet/dry equilibrium state on the PEMFC was measured with High Frequency Resistance (HFR). The gas injection time was 20 min and 5 min for dry gas and wet gas, respectively. An experiment was carried out using electrochemical methods and a cross-section of the MEA was visualized with a Field Emission Scanning Electron Microscope (FE-SEM). After 1200 wet/dry cycles, the performance of the cell decreased by 45.7% to its current density of 800 mA/cm 2. Ohmic and charge transfer resistances of the cell increased in the Electrochemical Impedance Spectroscopy (EIS). The crossover current of hydrogen also increased in the linear sweep voltammetry (LSV). The reduction of the electrochemical active surface area (ECSA) was confirmed through cyclic voltammetry (CV). The interface among the membrane, catalyst layer, and gas diffusion layer was separated and significantly deteriorated compared with fresh MEA.

Degradation on a PTFE/Nafion membrane electrode assembly with accelerating degradation technique

Cost and durability are the main issues of Proton exchange membrane fuel cells (PEMFCs) commercializing. This study uses the accelerate degradation technique to analyze the durability of low cost PTFE/Nafion membrane electrode assembly (MEA). Before the MEA degradation experiment, the MEA must be activated at 65 °C until the performance is stable. Then increase the operation temperature to 80 °C. The experimental process for MEA degradation contains three steps in one cycle. The first step is to open circuit voltage (OCV) for 30 s under R.H. 100%. Then, set 0.6V for 150 s under R.H. 100%. The final step is to set 0.6V for 120 s under R.H. 0%. These three steps take around 5 min to complete. This MEA degradation experiment process includes the OCV, potential cycles, and R.H. cycles. This study uses the polarization curve, AC impedance, cyclic voltammetry (CV), linear sweep voltammetry (LSV), equations and equivalent circuit to analyze state of the MEA. At less than 160 experiment cycles, the result show that catalyst degradation is the main reason for the decay of MEA. After 280 cycles, the MEA begins to exhibit creep deformation due to the R.H. cycle. Electrochemical surface area and high frequency resistance can be used to estimate the degree of MEA degradation approximately.

Physical degradation of MEA in PEM fuel cell by on/off operation under nitrogen atmosphere

The durability of PEMFCs is one of the most important issues for application in automotive vehicles with a repeated start-up and shut-down system. The understanding of degradation phenomena such as causes, mechanisms and influence of working condition is essential to improving the performance and lifetime of PEMFC. We conducted on/off cyclic operation in a single cell configuration with ultra purity nitrogen gas to investigate the physical degradation of membrane electrode assembly (MEA). After on/off cycle operation for 100,000 cycles under different humid condition, the characteristics of the MEAs were examined by in situ and ex situ analyses techniques. The physical degradation of MEA by on/off cycling led to a change in the membrane-electrode interfacial structure, which is mainly attributed to the loss of cell performance.

Characterization of the Degradation in Membrane Electrode Assemblies Through Passive Electrical Measurements

A passive technique to characterize the degradation of membrane electrode assemblies is presented; it is based on simple measurement of electrical characteristics as a function of time. This method relies on the experimental evidence that the assembly behaves like a supercapacitor with a tremendous capacity in series with a small resistance. In a first approximation, a resistor-capacitor (RC) circuit can thus be utilized to model the charging and discharging behavior. The experimental data demonstrated that a more sophisticated equivalent circuit was necessary to understand the experimental results. The most favorable set of parameters was determined, thanks to a Monte Carlo type numerical analysis. Overall, a larger sensitivity to damage detection than the well-accepted electrochemical impedance spectroscopy is demonstrated that suggests a promising future to in situ detection of failure and understanding of degradation mechanisms.

Investigations of performance degradation and mitigation strategies in direct methanol fuel cells

This paper addresses a gradual performance loss that is encountered in a direct methanol fuel cell (DMFC) when it is subjected to a continuous operation at constant-load conditions for a period of 600 h. To gain insights into the physico-chemical origins of the degradation process, various analytical techniques are employed and characterization of the membrane electrode assembly (MEA) is carried out before and after the lifetime test. The results reveal that the performance degradation of MEA mainly stems from the degradations at the cathode, and this is further confirmed by individual impedance analyses of cathode and anode as well as by the observation on finding increased quantity of methanol exiting from the cathode with increased operational time. Supplement experiments with the cathodes containing either pre-oxidized Pt catalyst or a fractional amount of Ru catalyst offer new clues to understand the deactivation mechanism. The hydrophobicity losses of gas diffusion layers are prominent in the outlet regions compared to the inlet regions of the DMFC assembly. Further, a couple of restoration techniques are employed to evaluate performance recovery. A periodical on–off switching of applied load and an air-break technique are found to be effective to restore the performance loss that occurs during fuel cell operations.

Investigation of Stress and Water Distribution in Membrane Electrode Assembly (MEA) During Fuel Cell Operation

Investigation into the durability of fuel cells is a challenging issue due to the large number of parameters affecting membrane electrode assembly (MEA) integrity. The mechanical stresses induced in the membrane due to swelling associated with water absorption play an important role in the long-term failure of the MEA. Thus, the stresses and the water distribution in the MEA are coupled. To better understand the relationship between mechanical stresses and water absorption, we introduce a numerical model to simultaneously determine the swelling-induced stresses and the water distribution in the MEA under various cell operating conditions. Preliminary results suggest that open-circuit voltage (OCV) operations lead to the most severe environment for the mechanical response, whereas under current load, the stresses and the permanent deformation are on average lower and localized at the cathode side of the MEA. The results suggest that the mechanical behavior of the MEA must be taken into account in the investigations of MEA degradation to establish a complete understanding on the failure mechanisms and durability.

Cathode Platinum Degradation of Membrane Electrode Assembly Studied by Using Solid-State Electrochemical Cell

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Physical degradation of membrane electrode assemblies undergoing freeze/thaw cycling: Diffusion media effects

In this work, the effects of properties of diffusion media (DM) (stiffness, thickness and micro-porous layer (MPL)) on the physical damage of membrane electrode assembly (MEA) subjected to freeze/thaw cycling were studied. Pressure uniformity of the diffusion media onto the catalyst layer (CL) was determined to be a key parameter to mitigate freeze-induced physical damage. Stiffer diffusion media, enabling more uniform compression under the channels and lands, can mitigate surface cracks, but flexible cloth diffusion media experienced severe catalyst layer surface damage. The thickness of the diffusion media and existence of a micro-porous layer were not observed to be major factors to mitigate freeze-damage when the catalyst layer is in contact with liquid. Interfacial delamination between diffusion media and catalyst layers, but not between the catalyst layer and membrane, was observed. This permanent deformation of the stiff diffusion media in the channel locations as well as fractures of carbon fibers increased electrical resistance, and may increase water flooding, resulting in reduced longevity and operational losses. Although use of a freeze-tolerable MEA design (negligible virgin cracked catalyst layers with thinner reinforced membrane) [S. Kim, M.M. Mench, J. Power Sources, in press] with stiff diffusion media can reduce the freeze-damage in the worst case scenario test condition of direct liquid contact, extensive irreversible damage (diffusion media/catalyst layer interfacial delamination) was not completely prevented. In addition to proper material selection, liquid water contact with the catalyst layer should be removed prior to shutdown to a frozen state to permit long-term cycling damage and facilitate frozen start.

Development of shut-down process for a proton exchange membrane fuel cell

Several different shut-down procedures were carried out to reduce the degradation of membrane electrode assembly (MEA) in a proton exchange membrane fuel cell (PEMFC). The effects of close/open state of outlets of a single cell and application of a dummy load during the shut-down on the degradation of the MEA were investigated. Also, we elucidated the relationship between the thickness of the electrolyte membrane and the degradation of the MEA for different shut-down procedures. When a thin electrolyte membrane was used, the closer of outlets mitigated the degradation during on/off operation. For the thicker electrolyte membrane, the dummy load which eliminates residual hydrogen and oxygen in the electrodes should be applied to lower the degradation.

Effects of Pressurizing Operation on the Degradation of Membrane Electrode Assemblies in PEFCs

The correlation between the degradation of membrane electrode assembly (MEA) and the operation pressure during the load cycling test was studied. The precipitated Pt, obtained as Pt band, and the thickness of the membrane were observed in the section of the tested MEAs. The precipitated ratio of Pt was determined by a method that involved image processing. An increase in the relative volume of precipitated Pt and a reduction in the thickness of the membrane depending on the operation pressure were observed.

Effects of Pressurizing Operation on the Degradation of Membrane Electrode Assemblies in PEFCs

not Available.

Physical degradation of membrane electrode assemblies undergoing freeze/thaw cycling: Micro-structure effects

The objective of this work is to investigate physical damage of polymer electrolyte fuel cell (PEFC) materials subjected to freeze/thaw cycling. Effects of membrane electrode assembly micro-structures (catalyst layer cracking, membrane thickness, and membrane reinforcement) and diffusion media with micro-porous layers were analyzed by comparing scanning electron microscopy images of freeze/thaw cycled samples (−40 °C/70 °C) with those of virgin material and thermal cycled samples without freezing (5 °C/70 °C). Ex situ testing performed in this study has revealed a strong direction for the material choices in the PEFC and confirmed the previous computational model in the literature [S. He, M.M. Mench, J. Electrochem. Soc., 153 (2006) A1724–A1731; S. He, S.H. Kim, M.M. Mench, J. Electrochem. Soc., in press]. Specifically, the membrane electrode assemblies were found to be a source of water that can damage the catalyst layers under freeze/thaw conditions. Damage was found to occur almost exclusively under the channel, and not under the land (the graphite that touches the diffusion media). Conceptually, the best material to mitigate freeze-damage is a crack free virgin catalyst layer on a reinforced membrane that is as thin as possible, protected by a stiff diffusion media.

Investigation of hydrogen consumption in proton exchange membrane fuel cell with depth-reduced channels

The failure of membrane electrode assembly (MEA) relates to the fuel consumption for proton exchange membrane fuel cell(PEMFC). In this study, a complete 3-D computational model, including nine layers of the integrated fuel cell system, was developed to investigate the influences of depth-reduced channels on hydrogen consumption. Simulation results showed that the different combinations of depth caused dissimilar pressure profiles, pressure drop, and distribution of hydrogen concentration in the depth-reduced channels, and thus resulted in the variations of the current density and the hydrogen consumption. The maximum hydrogen mass flux in MEA was located at the area of channel entrance for all channels. This implied that the serious degradation of MEA always occurred around the channel entrance. In addition, a depth-reduced channel with a shorter and deeper front section was able to reduce the maximum hydrogen mass flux by about 3–4% with losing current density about 2%. These results suggest that a depth-reduced channel with a shorter and deeper front section can delay the serious degradation of MEA to prolong the lifetime. Copyright © 2007 John Wiley & Sons, Ltd.

Copper alloy bipolar plates for polymer electrolyte membrane fuel cell

Bipolar plates are one of the critical componets of the fuel cell. Excellent corrosion resistance and good conductivity are some of the desirable properties for better operation of fuel cells. Corrosion studies of copper–beryllium alloy (C-17200) were conducted in corrosion simulated fuel cell environment by chronoamperometry. The alloy selected was observed to show good corrosion resistance. The selected alloy was tested in an actual fuel cell. Standard parallel flow field design with 16 cm 2 surface area was machined on end plates. The single cell showed good performance without degradation of MEA and bipolar plates for 100 h at a constant load of 1 A. Characterization of membrane electrode assembly (MEA) and plates revealed no poisoning or degradation. Calculated cell resistance from the polarization curve was 0.0836 Ω which is comparable with SS316L under similar conditions.