A review of accelerated stress tests of MEA durability in PEM fuel cells

This paper is a review of recent work done on accelerated stress tests in the study of PEM fuel cell durability, with a primary focus on the main components of the membrane electrode assembly (MEA). The accelerated stressors for each component under different conditions are outlined, in an attempt to gain a detailed understanding of cell degradation with respect to microstructural change and performance attenuation in the perfluorosulfonic acid membrane, catalyst, and gas diffusion layers. Various techniques for evaluating the components' performance are presented, along with representative mitigation strategies. In addition, different degradation mechanisms proposed in recent publications are briefly reviewed.     

A review of polymer electrolyte membrane fuel cell durability test protocols

Durability is one of the major barriers to polymer electrolyte membrane fuel cells (PEMFCs) being accepted as a commercially viable product. It is therefore important to understand their degradation phenomena and analyze degradation mechanisms from the component level to the cell and stack level so that novel component materials can be developed and novel designs for cells/stacks can be achieved to mitigate insufficient fuel cell durability. It is generally impractical and costly to operate a fuel cell under its normal conditions for several thousand hours, so accelerated test methods are preferred to facilitate rapid learning about key durability issues. Based on the US Department of Energy (DOE) and US Fuel Cell Council (USFCC) accelerated test protocols, as well as degradation tests performed by researchers and published in the literature, we review degradation test protocols at both component and cell/stack levels (driving cycles), aiming to gather the available information on accelerated test methods and degradation test protocols for PEMFCs, and thereby provide practitioners with a useful toolbox to study durability issues. These protocols help prevent the prolonged test periods and high costs associated with real lifetime tests, assess the performance and durability of PEMFC components, and ensure that the generated data can be compared.     

Understanding and approaches for the durability issues of Pt-based catalysts for PEM fuel cell

The state-of-art understanding of durability issues (the degradation reasons and mechanisms, the influence of working conditions, etc.) of Pt-based catalysts for proton exchange membrane fuel cell (PEMFC) and the approaches for improving and studying catalyst durability are reviewed. Both carbon support and catalytic metals degrade under PEMFC conditions, respectively, through the oxidation of carbon and the agglomerate and the detachment from support materials of catalytic metals, especially under unnormal working conditions; furthermore, the degradation of carbon support and catalytic metals interact with and exacerbate one another. The working temperature, humidity, cell voltage (the electrode potential and the mode applied on the electrode), etc. can influence the catalyst durability. Carbons with high graphitization degree as support materials and alloying Pt with some other metals are proved to be effective ways to improve the catalyst durability. Time-effective and reliable methods for studying catalyst durability are indispensable for developing PEMFC catalysts.     

A review of PEM hydrogen fuel cell contamination: Impacts,mechanisms, and mitigation

This paper reviewed over 150 articles on the subject of the effect of contamination on PEM fuel cell. The contaminants included were fuel impurities (CO, CO₂, H₂S, and NH₃); air pollutants (NO_x, SO_x, CO, and CO₂); and cationic ions Fe³⁺ and Cu²⁺ resulting from the corrosion of fuel cell stack system components. It was found that even trace amounts of impurities present in either fuel or air streams or fuel cell system components could severely poison the anode, membrane, and cathode, particularly at low-temperature operation, which resulted in dramatic performance drop. Significant progress has been made in identifying fuel cell contamination sources and understanding the effect of contaminants on performance through experimental, theoretical/modeling, and methodological approaches. Contamination affects three major elements of fuel cell performance: electrode kinetics, conductivity, and mass transfer.     

A review of platinum-based catalyst layer degradation in proton exchange membrane fuel cells

Catalyst layer degradation has become an important issue in the development of proton exchange membrane (PEM) fuel cells. This paper reviews the most recent research on degradation and durability issues in the catalyst layers including: (1) platinum catalysts, (2) carbon supports, and (3) Nafion ionomer and interfacial degradation. The review aims to provide a clear understanding of the link between microstructural/macrostructural changes of the catalyst layer and performance degradation of the PEM fuel cell fueled with hydrogen under normal operating or accelerated stress conditions. In each section, different degradation mechanisms and their corresponding representative mitigation strategies are presented. Also, general experimental methods are classified and various investigation techniques for evaluating catalyst degradation are discussed.     

Fundamental mechanisms limiting solid oxide fuel cell durability

The fundamental issues associated with solid oxide fuel cell (SOFC) durability have been reviewed with an emphasis on general features in SOFCs and respective anode and cathode related phenomena. As general features, physicochemical properties and cell performance degradation/failure are correlated and bridged by the electrode reaction mechanisms. Particular emphasis is placed on the elemental behaviour of gaseous impurities and the possible role of liquids formed from gaseous substances. The lifetime of a state-of-the-art Ni cermet anodes is limited by a variety of microstructural changes, which mainly result from material transport-, deactivation- and thermomechanical mechanisms. Anode degradation can mainly be influenced by processing, conceptual and operating parameters. Designing a redox stable anode is currently one of the biggest challenges for small scale SOFC systems. Degradation mechanisms of different cathode materials are reviewed with a focus on the intrinsic degradation of doped lanthanum manganites (e.g. LSM) and doped lanthanum ferro-cobaltites (LSCF). Manganese-based perovskites can be regarded to be sufficiently stable, while for the better performing LSCF cathodes some intrinsic degradation was detected. New materials that are supposed to combine a better stability and high performance are also shortly mentioned.     

Detaching behaviors of catalyst layers applied in PEM fuel cells by off-line accelerated test

The detaching behavior of catalyst layers in membrane electrode assembly (MEA) for PEM fuel cells could affect the lifetime of both catalyst layers and membranes. However, this issue is always neglected. Therefore, the study of detaching behavior of catalyst layer is very conducive to investigate the failure mechanism of fuel cells. The detaching of catalyst layers was simulated by dipping membrane electrode assemblies (MEAs) into H₂O₂ solution with or without Fe²⁺. We observed the presence of detaching of catalyst layers and found the varied detaching behaviors with different accelerated testing solutions: a layered-type detaching behavior is shown for the catalyst layer treated with 30% H₂O₂ solution, whereas a crack-like detaching behavior in the case of 30% H₂O₂ solution with Fe²⁺ species (or Fenton's test). At the same time, the layered-type detaching of catalyst layers has a higher detaching rate than the crack-like detaching. These detaching behaviors should have an inherent link to degradation of recast-ionomer (Nafion) films in catalyst layers. In addition, the effect of detaching behaviors of catalyst layers on the lifetime of fuel cells has been studied by hydrogen crossover measurement, and shows that, for the crack-like detaching, the membrane has a shorter lifetime than that for the layered detaching.     

Comparison of the performance and durability of PEM fuel cells with different Pt-activated microporous layers

We report on a comparative study of the performance level of H₂/O₂ PEM fuel cells in which the catalytic layers containing Pt nanoparticles were deposited on the microporous layer side of gas diffusion electrodes, using three different deposition techniques: (i) by magnetron sputtering, (ii) by impregnation followed by chemical reduction (using either ethylene glycol or hydrogen or sodium borohydride as reducing agent), and (iii) by spraying a catalytic ink (containing either Pt/C or bulk Pt particles). The microstructure and chemical composition of the different catalytic layers has been determined by SEM, TEM, XRD and XPS analysis. Their electrochemical surface areas have been determined by cyclic voltammetry. The i-V curves have been measured and compared. A durability stress test based on cycles of potential was used to assess the degradation rate of the different catalytic layers and to rank performance.     

Comparison between two PEM fuel cell durability tests performed at constant current and under solicitations linked to transport mission profile

The first phase of a research program on PEMFC durability was devoted to the test of a 100 W stack operated in stationary regime during 1000 h. The second phase was dedicated to the test of another 100 W stack under dynamical current constraint. In this case, the load current cycle applied was computed from a standardised transportation mission profile and then adapted to the power of the investigated fuel cell. For both ageing experiments, the stack characterisations were based on polarisation curves, recorded for various stoichiometry rates, as well as on EIS measurements performed at regular time-spaced intervals throughout the ageing. Some analysis tools derived from the response surface methodology are employed to analyse and compare the results of the two durability experiments. Some numerical models are proposed for the degradation of the stack performances. They are finally used to optimise the fuel cell operating conditions versus ageing time.     

Degradation analysis of dead-ended anode PEM fuel cell at the low and high thermal and pressure conditions

In this study, it is demonstrated that operation of dead-ended anode fuel cell at high temperature and pressure reduce the durability of membrane electrode assembly. In such a way that after 9000 degradation cycles, the maximum power density under H₂/O₂ gas feed mode for the aged MEA at high temperature and pressure is dropped by 38.8%. While the maximum power density drop is 27.1% for the aged MEA at low temperature and pressure. Comparison of the electrochemical impedance spectroscopy responses of MEAs shows that during aging process, the charge transfer resistance increase rate is more at higher temperature and pressure. This suggests the more severe destruction of catalyst layer at higher temperature and pressure and is in agreement with the obtained values of electrochemical surface area from the cyclic voltammetry test. In addition, the transmission electron microscopy and scanning electron microscopy images show the further degradation of cathode catalyst layer and more sever Pt agglomeration at higher temperature and pressure.     

A review of the main parameters influencing long-term performance and durability of PEM fuel cells

This paper presents an overview of issues affecting the life and the long-term performance of proton exchange membrane fuel cells based on a survey of existing literature. We hope that this brief overview provides the engineers and researchers in the field with a perspective of the important issues that should be addressed to extend the life of next-generation fuel cells. Causes and fundamental mechanisms of cell degradation and their influence on long-term performance of fuel cells are discussed. Current research shows that main causes of short life and performance degradation are poor water management, fuel and oxidant starvation, corrosion and chemical reactions of cell components. Poor water management can cause dehydration or flooding, operation under dehydrated condition could damage the membrane whereas flooding facilitates corrosion of the electrodes, the catalyst layers, the gas diffusion media and the membrane. Corrosion products and impurities from outside can poison the cell. Thermal management is particularly important when the fuel cell is operated at sub-zero and elevated temperatures and is key at cold start-ups as well as when subjected to freezing conditions.     

The influence of the electrochemical stressing (potential step and potential-static holding) on the degradation of polymer electrolyte membrane fuel cell electrocatalysts

The understanding of the degradation mechanisms of electrocatalysts is very important for developing durable electrocatalysts for polymer electrolyte membrane (PEM) fuel cells. The degradation of Pt/C electrocatalysts under potential-static holding conditions (at 1.2 V and 1.4 V vs. RHE) and potential step conditions with the upper potential of 1.4 V for 150 s and lower potential limits (0.85 V and 0.60 V) for 30 s in each period [denoted as Pstep(1.4V_150s–0.85V_30s) and Pstep(1.4V_150s–0.60V_30s), respectively] were investigated. The electrocatalysts and support were characterized with electrochemical voltammetry, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Pt/C degrades much faster under Pstep conditions than that under potential-static holding conditions. Pt/C degrades under the Pstep(1.4V_150s–0.85V_30s) condition mainly through the coalescence process of Pt nanoparticles due to the corrosion of carbon support, which is similar to that under the conditions of 1.2 V- and 1.4 V-potential-static holding; however, Pt/C degrades mainly through the dissolution/loss and dissolution/redeposition process if stressed under Pstep(1.4V_150s–0.60V_30s). The difference in the degradation mechanisms is attributed to the chemical states of Pt nanoparticles: Pt dissolution can be alleviated by the protective oxide layer under the Pstep(1.4V_150s–0.85V_30s) condition and the potential-static holding conditions. These findings are very important for understanding PEM fuel cell electrode degradation and are also useful for developing fast test protocol for screening durable catalyst support materials.     

Effect of sub-freezing temperatures on a PEM fuel cell performance, startup and fuel cell components

The cold-start behavior and the effect of sub-zero temperatures on fuel cell performance were studied using a 25-cm² proton exchange membrane fuel cell (PEMFC). The fuel cell system was housed in an environmental chamber that allowed the system to be subjected to temperatures ranging from sub-freezing to those encountered during normal operation. Fuel cell cold-start was investigated under a wide range of operating conditions. The cold-start measurements showed that the cell was capable of starting operation at −5 °C without irreversible performance loss when the cell was initially dry. The fuel cell was also able to operate at low environmental temperatures, down to −15 °C. However, irreversible performance losses were found if the cell cathode temperature fell below −5 °C during operation. Freezing of the water generated by fuel cell operation damaged fuel cell internal components. Several low temperature failure cases were investigated in PEM fuel cells that underwent sub-zero start and operation from −20 °C. Cell components were removed from the fuel cells and analyzed with scanning electron microscopy (SEM). Significant damage to the membrane electrode assembly (MEA) and backing layer was observed in these components after operation below −5 °C. Catalyst layer delamination from both the membrane and the gas diffusion layer (GDL) was observed, as were cracks in the membrane, leading to hydrogen crossover. The membrane surface became rough and cracked and pinhole formation was observed in the membrane after operation at sub-zero temperatures. Some minor damage was observed to the backing layer coating Teflon and binder structure due to ice formation during operation.     

Stresses and their impacts on proton exchange membrane fuel cells: A review

The Proton Exchange Membrane (PEM) fuel cell is a promising substitute for combustion engines owing to its low level of output pollutants and high efficiency. Great efforts have been done toward the commercialization of PEM fuel cell. A number of review papers that investigate the electrochemical aspects of the fuel cell have already been published. However, the literature on the mechanical aspects is relatively limited. The durability of a PEM fuel cell is one of the significant factors that influence the industrialization of this technology. The PEM fuel cell is subjected to several mechanical stresses due to the different assembly procedures, operational and environmental aggressive conditions. Avoiding the high stress points is necessary for long term PEM fuel cell durability. The behavior of these generated stresses and how they affect each other is not well understood, including the compressive clamping stress, hygrothermal stress, freeze-thaw stress, and the stress due to vibration conditions. This paper reviews the developed stresses within the PEM fuel cell under different conditions. In addition, the various failure and damage mechanisms in the MEA, GDL, gas flow channel and bipolar plate due to these stresses are reviewed. The aforementioned stresses are discussed separately in the literature. The review shows that the combination of these stresses could be a key reason for the performance degradation and structural damage. This review suggests an effective tool to explore the correlation between the addressed stresses and to find out how they contribute to mechanical damage of PEM fuel cell systems and recommendations that can be implemented for improving the cell durability.     

Corrosion of metal bipolar plates for PEM fuel cells: A review

PEM fuel cells are of prime interest in transportation applications due to their relatively high efficiency and low pollutant emissions. Bipolar plates are the key components of these devices as they account for significant fractions of their weight and cost. Metallic materials have advantages over graphite-based ones because of their higher mechanical strength and better electrical conductivity. However, corrosion resistance is a major concern that remains to be solved as metals may develop oxide layers that increase electrical resistivity, thus lowering the fuel cell efficiency. This paper aims to present the main results found in recent literature about the corrosion performance of metallic bipolar plates.     

Degradation mitigation effects of pressure swing in proton exchange membrane fuel cells with dead-ended anode

High cost remains to be one of the primary obstacles for the commercialization of proton exchange membrane fuel cells (PEMFCs). To simplify the fuel cell system and reduce cost, dead-ended anode (DEA) is widely used. However, water and nitrogen can accumulate in the dead-ended anode, resulting in cell performance decrease and severe cell degradation. Anode pressure swing supply is a new technology which has been shown to be effective in reducing local water and nitrogen accumulation in the anode channel. In this work, the effects of pressure swing supply on fuel cell degradation have been experimentally studied. Two sets of experiments on the same fuel cell are conducted, one under conventional constant pressure operation and the other under pressure swing operation. Polarization curves show that pressure swing supply can significantly mitigate cell degradation during DEA operations. Electrochemical characterizations are performed to study the mechanisms of mitigations in cell degradation. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results show that pressure swing supply can significantly reduce electrolyte membrane degradation, but has no significant mitigation effect on the cathode catalyst degradation during DEA operation. Further examinations of the membrane-electrode-assembly (MEA) by scanning electron microscope (SEM) confirm the significant difference in membrane degradations since there is a very large difference in average thickness of the membranes after the degradation tests.     

Characterization and experimental results in PEM fuel cell electrical behaviour

A control oriented electrochemical static model of a proton exchange membrane fuel cell (PEMFC) stack is developed in this paper. Even though its validation is performed on a specific 7-cell PEMFC stack fed by humidified air and pure hydrogen, the methodology and fit parameters can be applied to different fuel cell systems with minor changes. The fuel cell model was developed combining theoretical considerations and semi-empirical analysis based on the experimental data. The proposed model can be successfully included into a larger dynamic subsystem to complete the power generation system.     

Innovative catalyst supports to address fuel cell stack durability

To successfully penetrate the automotive market, cost-efficient and durable fuel cell technologies are necessary to compete with existing mature technologies.     

Numerical investigation of delamination onset and propagation in catalyst layers of PEM fuel cells under hygrothermal cycles

Durability is one of the main obstacles that inhibits the commercialization of polymer electrolyte membrane (PEM) fuel cells for transport applications, in which the microstructure of the catalyst layers (CLs) deteriorates under dynamic loading operation. In this study, CLs’ naturally random porous structure is simplified to be a random three-phase microstructure consisting of ionomers, catalyst agglomerates and pores, and the onset and growth of delamination process between the ionomer and catalyst agglomerate is investigated numerically by considering the catalyst agglomerate as elastic while the ionomer is elasto-viscoplastic, influenced by the cell assembly force arising from the cell clamping and variations in temperature and relative humidity. It is found that increasing clamping stress delays the delamination onset but has marginal effect on delamination propagation. The amplitude of hygrothermal cycles is the dominating factor in delamination and more frequent startup/shutdown of PEM fuel cells alleviates the delamination. Correlation between the rate of plastic strain accumulation in the ionomer and the interface delamination has been observed.