Degradation Of Membrane Electrode Assembly Research Articles

Startup and Shutdown Study Applied in Polymer Electrolyte Membrane Fuel Cell System Operation

Automotive polymer electrolyte membrane (PEM) fuel cells (FC) have been increasingly under development, due to their ability to provide extended vehicle driving ranges at zero emission. However, PEM fuel cells still face several challenges, such as durability and cost. The lifetime of the PEM fuel cell stacks is determined by the durability of materials under fuel cell operation, in which catalyst, membrane and ionomer are exposed to strongly acidic and oxidizing conditions at elevated temperatures. In addition, the stack lifetime is affected by various operational modes and hydration conditions experienced through drive cycles, idling conditions and startup (SU) / shutdown (SD) cycles.1 System operation strategy can mitigate the stress to the materials and improve the lifetime of fuel cell stacks.This study investigated several SUSD cycles with a PtCo based membrane electrode assembly (MEA) in single cell testing. The schematics of SUSD cycles are shown in Figure 1. In general, a SUSD cycle includes four parts: FC operation, shutdown, soak, and startup. Figure 1(a) is an air-SUSD cycle, which represents a long term shutdown of the vehicle, when air penetrates into the anode. During startup, when hydrogen (H2) is supplied to the anode, there will be an air-hydrogen front at the anode, which causes substantial carbon corrosion and platinum (Pt) dissolution to occur on the cathode. Thus, in accelerated air-SUSD cycles, during the shutdown, air is introduced to the anode and the cell voltage drops to zero within a second. Then, the cell is allowed to soak under air/air for anode/cathode for 60 seconds. After the soak process, during SU, air supply to the anode is stopped and hydrogen is supplied to the anode. In order to improve the SUSD durability of PEM fuel cells in FC vehicles, hydrogen protection (H2-SUSD) has been proposed.2 Figure 2(b) presents a H2-SUSD cycle. In the soak portion of the cycle, the O2 in the cathode is consumed by the H2 permeated from the anode, when the cell voltage drops to zero slowly. At higher temperature, the voltage drop is faster. The H2-SUSD process reduces the development of loop currents within a cell that are associated with high half-cell potential reactions. Because high half-cell potentials are mostly avoided, the H2-SUSD suppresses severe carbon corrosion and Pt dissolution that are observed in air-SUSD cycles. However, we still observe carbon corrosion happening in H2-SUSD process, which is confirmed with catalyst layer failure analysis results. In order to prevent this carbon corrosion, in H2-SUSD, after shutdown, a small load is drawn to consume O2 at the cathode quickly and improve the H2 protection effect as shown in Figure 1(c). These SUSD processes are studied at different temperatures as well. For diagnostics, the voltage degradation at 0.02 A/cm2 and 1.2 A/cm2 and ECSA losses are recorded either at 100 cycle intervals for air-SUSD and 500 cycle intervals for H2-SUSD. Our results show that H2-SUSD with a load preserves the cell performance greatly. The MEA degradation results in these SUSD cycles provide a reference for the FC system team to develop automotive SUSD operation strategy, for example, to determine the minimum H2-protection time for the FC system operation. Y. C. Park, K. Kakinuma, M. Uchida, D. A. Tryk, T. Kamino, H. Uchida, and M. Watanabe, Electrochimica Acta, 91, 195 (2013).Y. Yamashita, S. Itami, J. Takano, K. Kakinuma, H. Uchida, M. Watanabe, A. Iiyama, and M. Uchida, J. Electrochem. Soc., 164 (4), F181 (2017). Figure 1

On the influence of methanol addition on the performances of PEM fuel cells operated at subzero temperatures

The storage and the operation of proton exchange membrane fuel cells (PEM FCs) at subzero temperatures result in the ageing and degradation of membrane-electrode assemblies (MEAs). In this study, we investigated the effect of a freeze-thaw temperature cycles (from ambient down to −80 °C) on various properties of PFSA membranes and on the performance level of MEAs. The beneficial effects resulting from the addition of methanol vapor to the hydrogen have been put into evidence. Small angle x-ray scattering (SAXS) data have been measured on membranes swollen with water and water-methanol mixtures, to identify possible microstructure differences. It was found that the addition of methanol tends to increase the ionic resistivity of the membranes but has a protective effect on the catalytic layers during the freeze-thaw cycles. The degradation rate of the catalytic layer was reduced by almost a factor of two as a result of optimal methanol addition.

Failure Analysis and Defects Characterization of Polymer Electrolyte Membrane Fuel Cells after Relative Humidity Cycling

Water content in the proton exchange membrane fuel cell (PEMFC) changes due to the spatiotemporal interplay of temperature, humidity, and electrochemical reactions. The humidity fluctuations result in membrane swelling and shrinking, and consequently, in-plane tensile stress development that causes mechanical failures [1,2]. In this study, commercial catalyst coated membranes (CCMs) were tested, under a relative humidity (RH) cycling accelerated stress test (AST), between oversaturated and fully-dry states. After the failure of the membrane, a conventional bubble jig test was performed on the membrane electrode assemblies (MEAs) to identify the defective regions. For further understanding of the morphology around the defects and the associated failure mechanism, the identified failure regions were examined using scanning electron microscopy (SEM) cross-section imaging and X-ray computed tomography (XCT) to be referenced with the beginning of life (BOL) sample [3]. The failed regions showed a wide range of defects, from small pinholes to fully-ruptured CCMs, in various forms and patches. There was good conformity between the size of the bubble obtained from the bubble jig test and the morphology of the defects in the images. Further, possible microstructural changes, i.e. porosity and ionomer distribution of the RH cycled catalyst layers were evaluated for the first time by scanning-transmission electron microscopy- energy dispersive spectroscopy (STEM-EDS) analysis along with an in-house developed quantitative method [3,4]. The STEM-EDS mapping revealed a changed ionomer distribution as well as a decrease in the catalyst layer porosity with respect to the BOL sample. The novel analysis approach revealed additional mechanisms of MEA degradation in the RH cycled MEAs, beside the membrane failure modes. To further explain the failure mechanisms, the nuclear magnetic resonance (NMR) was used to investigate the perfluorosulfonic acid (PFSA) chain and structure changes due to the RH cycling and differentiate any mechanical defect from chemical alteration of the chains in the membrane [5]. The NMR spectrum showed irregular broadening of the 19F peaks, which can be correlated to the mechanical changes in the fluorine chains of the membrane. Complementary characterization techniques such as Fourier transform infra-red spectroscopy (FTIR) and Raman spectroscopy analyses results will be shown for the catalyst layer to further explain the failure modes. Consequently, a comprehensive characterization methodology was developed for better understanding of the failure mode from the RH cycling AST.Keywords: RH cycling, Failure Analysis, Polymer Electrolyte Membrane, Fuel Cell Durability.

Performance and durability studies of perfluorosulfonic acid ionomers as binders in PEMFC catalyst layers using Electrochemical Impedance Spectroscopy

Perfluorosulfonic acids (PFSA) are the most widely used ionomers in Polymer electrolyte membrane fuel cells as both membrane and catalyst layer support. This study presents an electrochemical based analysis of using long-side-chain Nafion® and short-side-chain Aquivion® PFSA ionomers as binders in the catalyst-layers of PEMFC. Membrane electrode assemblies were designed with consistent components, varying only the catalyst ink's ionomer type from 28 wt% Aquivion® to 28 wt% Nafion®. The durability and performance profile of the resulting catalyst-coated-membranes made from using these PFSA ionomers as binders in the catalyst layers was investigated. Semi-empirical modeling shows that the Nafion® ionomer based binders give a catalyst-coated-membrane with a 46.6% higher ohmic resistance compared to using Aquivion® ionomers as binders. The power density analysis also showed that catalyst-coated-membranes made with Aquivion® ionomer based binder gives an output power that is 22.33% higher than catalyst-coated-membranes made with Nafion® ionomers as binders in the catalyst layers. The Aquivion® ionomer binder also shows higher catalyst utilization compared to the Nafion® ionomer binders. The Electrochemical Impedance Spectroscopy– Equivalent Circuit Model analysis showed that the membrane electrode assembly degradation is more pronounced in the Nafion® ionomer based binder compared to its Aquivion® counterpart. It is thus shown that Aquivion® ionomers are better catalyst binders compared to the Nafion® ionomers as they result in catalyst-coated-membranes that are better performing and more durable.

Effect of electrode Pt-loading and cathode flow-field plate type on the degradation of PEMFC

Abstract The electrode Pt-loading has an effect on the number of active sites and the thickness of catalyst layer, which has huge influence on the mass transfer and water management during dynamic process in PEMFCs. In this study, membrane electrode assemblies with different Pt-loadings were prepared, and PEMFCs were assembled using those membrane electrode assemblies with traditional solid plate and water transport plate as cathode flow-field plates, respectively. The performance and electrochemical surface area of cells were characterized to evaluate the membrane electrode assemblies degradation after rapid current-variation cycles. Scanning electron microscope and transmission electron microscope were used to investigate the decay of catalyst layers and Pt/C catalyst. With the increase of Pt-loading, the performance degradation of membrane electrode assemblies will be mitigated. But higher Pt-loading means thicker catalyst layer, which leads to a longer pathway of mass transfer, and it may result in carbon material corrosion in membrane electrode assemblies. The decay of Pt/C catalyst in cathode is mainly caused by the corrosion of carbon support, and the degradation of anode Pt/C catalyst is a consequence of migration and aggregation of Pt particles. And using water transport plate is beneficial to alleviating the age of cathode Pt/C catalyst.

Impact of Intermittent Operation on the Lifetime and Performance of a PEM Water Electrolyzer

In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen.1 However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions. In this work, we present a test protocol involving operation at high (3 Acm-2 geo) and low (0.1 Acm-2 geo) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrOx 2, the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM-WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrOx is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes.3 The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2).4 The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrOx.5 Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrOx. Interestingly, the polarization curve recorded directly after IrOx reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of cell performance over an extended number of OCV cycles due to Ir dissolution/re-precipitation occurs. In summary, this study will provide a better understanding of the MEA degradation mechanism occurring over an extended number of OCV cycles, which could result in a PEM-WE system when operated with intermittent renewable energy sources. This implies that hybridization strategies are required to maximize PEM-WE durability. In addition, PEM-WE load cycles to OCV may serve as an accelerated aging test. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. International Journal of Hydrogen Energy 2013, 38, 4901-4934.Weiß, A.; Bernt, M.; Siebel, A.; Rheinländer, P. J.; Gasteiger, H. A., ECS Meet. 232 2017, Abstr. # I01F-1648.Woods, R., Hydrogen Adsorption on Platinum, Iridium and Rhodium Electrodes at Reduced Temperatures and the Determination of Real Surface Area. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1974, 49, 217-226.Pickup, P. G.; Birss, V. I., A Model for Anodic Hydrous Oxide Growth at Iridium. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 220, 83-100.Reier, T.; Teschner, D.; Lunkenbein, T.; Bergmann, A.; Selve, S.; Kraehnert, R.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. Journal of The Electrochemical Society 2014, 161, F876-F882. Figure 1 Cyclic Voltammograms (CVs) recorded at 50mV/s during the accelerated degradation before cycling (black) and after 10 cycles (blue) at 80 °C, ambient pressure and 5 mL min-1 H2O (anode)/ 50 mL min-1 H2 (cathode) for an MEA with ~1.6 mgIr cm-²MEA anode and ~0.3 mgPt cm-²MEA cathode loading using a Nafion® 212 (~50 µm) membrane Figure 1

The Influence of the Platinum Loading and the Ionomer to Carbon Ratio on the Durability of the PEMFC

Cost and durability are the major barriers to the success of the Proton Exchange Membrane Fuel Cells (PEMFCs), and reducing the platinum loading of the catalyst layer could reduce the cost of the device. Thus, in this work, long-term durability tests in constant-current mode were performed using single-cells of 25 cm2 with Membrane Electrode Assemblies (MEAs) prepared with 20% Pt/C (BASF) catalyst and electrodes with platinum loadings of 0.4 and 0.1 mgPt.cm-2. Different ionomer to carbon ratios were also studied with the electrodes containing 0.1 mgPt.cm-2 of catalyst loading. To characterize the electrochemical and structural properties of the MEAs, cyclic voltammograms, electrochemical impedance spectroscopy and field-emission scanning electron microscopy were utilized before and after the long-term durability tests. Concerning the platinum loading, the electrodes prepared with 0.1 mg Pt cm-2 have lost proportionally more electrochemical surface area than the ones prepared with 0.4 mg Pt cm-2 but at the same time, the electrodes with the lowest platinum loading presented lower irreversible performance loss rate when using higher ionomer to carbon ratio. The analyses made by several electrochemical techniques have indicated that the raise of the ohmic and mass transport resistances are the factors that most contribute to the irreversible performance loss, meanwhile the charge transport resistance due to the electrodes flooding is the main responsible for the reversible performance loss. The proportion of ionomer in the catalytic layer was studied and it was possible to infer that the highest ionomer proportion studied has contributed to mitigate the MEA degradation, although it facilitates the occurrence of flooding in the electrodes. Finally, it was observed that the quality of the contact formed between the electrodes and the membrane has remarkable influence on the PEMFCs durability.

Evaluation of HT-PEM Fuel Cells via Load Cycling at High Current Densities

Polymer electrolyte membrane (PEM) fuel cells have a wide range of applications: stationary, transportation and portable communication [1]. Irrespective of type or operating temperature, for all fuel cells persist the same requirements like long term stability and reproducibility. To assemble an entire fuel cell stack, the utilized membrane electrode assemblies (MEA) should not only be the same type, they are also required to exhibit similar performances and power outputs. Vision of the German project QUALIFIX, funded by the Ministry of Economic Affairs and Energy (SC: 03Et6046A), is the development of consistent conditions for improved batch production to analyze and monitor quality of fuel cells, systems and components. With help of defined quality standards, the lifetime of fuel cell systems should be improved and cost-intensive losses can be minimized at an early production stage. Within QUALIFIX, the reproducibility of MEAs has been screened by evaluating several small batch series. One operation strategy, which has been used for this investigation, is load cycling between high current densities of 0.6 and 1.0 A/cm² and under equal test conditions. High temperature (HT) PEM MEAs with PBI-membranes of four different providers have been tested and their results will be presented in this work. All MEAs passed identical break-in procedures and activation phase under 0.3 A/cm². These start time intervals lasted 48 hours for each MEA respectively followed by initial characterization including polarization curves, impedance spectroscopy, cyclic and linear sweep voltammetry. Afterwards the load cycling operation started with 16 min of 1.0 A/cm² followed by 4 min of 0.6 A/cm². At the end of tests (EoT), after 500 h of operation, a second complete characterization has been performed. The characterizations have been replenished by ante- and post-mortem analysis via micro-computed tomography (µ-CT) and by product water and MEA analysis to provide phosphoric acid concentration changes. In Figure 1 the voltage changes for four MEAs from different suppliers are displayed as function of time. Indeed, all four MEAs are from the same type but their performances differ strongly; only MEA B and D show a good and similar performance but unequal degradation rates. All degradation rates are listed in Table 1. Table 1: Degradation rates at 0.3 A/cm² of load cycling tests after 500 h of operation. MEA Degradation Rate [µV/h] A -37 B +36 C -94 D -6 In addition, another MEA of the batches of MEA B and D have been adducted for inner-batch evaluations and comparisons. For both MEA-types (two different providers), the degradation rates vary within the batches of identical assemblies and under same operation conditions of load cycling at high current densities. Hence the production of MEAs with reproducible performances seem to be still of major concern. The in-situ electrochemical and ex-situ post- and ante-mortem evaluation (e.g. MEA acid content and µ-CT investigations) of all tested MEAs will be presented and compared. Figure 1: Voltage changes as function of time under load cycling conditions, (0.6 A/cm²: 4 min, 1.0 A/cm²: 16 min, 160 °C) under H2 and air supply (λH2=1.5; λair=2.0), MEAs of 4 different suppliers.

A Study on Degradation of Membrane Electrode Assembly By Operating Conditions of Fuel Cell Vehicle

In generally, degradation of membrane & electrode assembly(MEA) according to driving conditions of fuel cell electric vehicle focused on analyzing the state of catalyst, membrane, gas diffusion layer before and after durability evaluation . At present, the exact mechanism for degradation of membrane electrode assembly(MEA) according driving conditions of fuel cell vehicle does not hold and the mechanism were approximately the level that analogize the whole out of a part. In case of the degradation of MEA by operating modes, it was shown on three sources; (1) the loss of apparent catalytic activity according to the time(ηa), (2) conductivity loss by decreasing ion conductivity per the time, (3) the loss of rate of mass transfer according to the time. In other words, the equation about degradation of MEA can be seen that the basic form can be created by analyzing the polarization according to time.In this study, the 10 driving modes were devised by cell voltage behavior according to driving conditions of fuel cell vehicle. The operating modes were basically designed by high current, middle current and low current. Also, these modes were considered about the most commonly used driving power and time variation in accordance with the current change. The result of analyzing the MEA degradation by current/voltage variation was as below. The activation overpotential shows a tendency to be rapidly decreased in early state and decrease or increase gradually without reference to operating conditions. The ohmic overpotential tends to decrease in proportion to the time. OCV(Open Circuit Voltage) shows the result of decreasing rapidly in high current mode than low current mode. In this paper, we conducted an analysis of the equation for degradation of MEA according to the time, current/voltage variation. In conclusions, the system of fuel cell vehicle must be operated in high current range in order to prevent degradation of the MEA in similar load change condition. It is considered to be occurred in oxidation of electrode catalyst under high voltage condition.

Voltage Cycling Induced Losses in Electrochemically Active Surface Area and in H2/Air-Performance of PEM Fuel Cells

The influence of voltage cycling profiles on the degradation of MEAs (membrane electrode assemblies) for PEM (proton exchange membrane) fuel cells was investigated using MEAs with carbon supported catalysts. The loss of electrochemically active surface area (ECSA) of the cathode electrodes significantly increased with the upper potential limit. The ECSA loss was correlated with the performance decrease for hydrogen/air polarization curves, revealing purely kinetic losses at low and additional mass-transport related voltage losses at high platinum surface area normalized current densities (≡specific current densities). The absolute voltage losses only depended on the specific current density, independent of the extent of ECSA loss and the geometric current density.

Catalyst-Layer Ionomer Imaging of Fuel Cells

Investigation of membrane/electrode assembly (MEA) microstructure has become an essential step to optimize the MEA components and manufacturing processes or to study the MEA degradation. For these investigations, transmission electron microscopy (TEM) is a tool of choice as it provides direct imaging of the different components. TEM is then widely used for analyzing the catalyst nanoparticles and their carbon support. However, the ionomer inside the electrode is more difficult to be imaged. The difficulties come from the fact that the ionomer forms an ultrathin layer surrounding the carbon particles and in addition, these two components, having similar density, present no difference in contrast. In this work, we show how the recent progresses in TEM techniques as spherical aberration (Cs) corrected HRTEM, electron tomography and X-EDS elemental mapping provide new possibilities for imaging this ionomer network and consequently to study its degradation.

Effects of temperature and backpressure on the performance degradation of MEA in PEMFC

The performance characteristics of proton exchange membrane fuel cells (PEMFCs) can be affected by a degradation of the membrane-electrode-assembly (MEA). In this paper, the performance degradation of MEA in a single cell PEMFC was examined at 80 °C and a backpressure of 10 psi after 100 h of operational time using open circuit voltage (OCV) conditions. A control experiment was conducted by testing the MEA at ambient temperature without backpressure. Both electrochemical impedance spectroscopy (EIS) and an OCV analysis showed that both the conductivity and the performance of the cell operating at 80 °C were less than the control values after 100 h of operation. In contrast, an application of 10 psi of backpressure improved the cell conductivity and can maintain OCV in compare with ambient and 80 °C conditions. Characterizations of the pre- and post-operation of the MEA (cathode, anode and membrane) by scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed that the MEA performance degradation at 80 °C after 100 h resulted mainly from carbon corrosion combined with Pt agglomeration and dissolution, producing a PtS compound with reduced activity at both the cathode and the anode and also drying and cracking of the membrane.

Characterization of Ionomer Thin Film Distributions in PEM Fuel Cell Catalyst Layers Using Advanced Analytical Microscopy

Polymer electrolyte membrane (PEM) fuel cell performance degradation, particularly associated with the cathode catalyst layer, can be directly attributed to the stability and durability of individual material constituents comprising the membrane electrode assemblies (MEAs), including the electrocatalyst, catalyst support, and ionomer films. The structural and chemical changes of these MEA constituents are quantified via advanced electron microscopy methods to elucidate the materials-specific degradation mechanisms contributing to performance loss, with the ultimate goal being microstructural and compositional improvements that will enhance MEA durability. Research efforts at Oak Ridge National Laboratory are focused on the high-resolution microstructural and microchemical characterization of as-fabricated fuel cell materials (e.g., the individual materials constituents and the same materials incorporated in fresh MEAs), MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components, and field-aged MEAs. These studies are used to establish critical processing-microstructure-performance correlations and to elucidate the materials factors contributing to MEA degradation, performance loss, and failure. Recently, high-resolution analytical microscopy methods have been used to directly image/map the distribution and chemistry of the ionomer layers within catalyst electrodes, both on a mm-level for assessing “bulk” ionomer dispersions and at the nm-level to understand the continuity and uniformity of thin ionomer films. This presentation will focus on understanding ionomer distributions as a function of initial ionomer chemistry (e.g., solvents used), electrocatalyst content and dispersion, and type of carbon support used. Additionally, the stability of the ionomer films in cathode catalyst layers after ASTs designed for catalyst degradation and carbon corrosion will be evaluated. ________________________________________________________________________ Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility.

(Invited) Understanding Fuel Cell Materials Degradation Via Advanced Electron Microscopy Techniques

Polymer electrolyte membrane (PEM) fuel cell performance degradation, particularly associated with the cathode catalyst layer, can be directly attributed to the stability and durability of individual material constituents comprising the membrane electrode assemblies (MEAs), including the electrocatalyst, catalyst support, recast ionomer, polymer membrane, and gas diffusion-microporous layers. The structural and chemical changes of these MEA constituents are being quantified via advanced electron microscopy methods to elucidate the specific degradation mechanisms contributing to performance loss, with the ultimate goal being microstructural and compositional improvements that will enhance MEA durability. Research efforts at Oak Ridge National Laboratory have been focused on the high-resolution microstructural and microchemical characterization of as-fabricated fuel cell materials (e.g., the individual materials constituents and the same materials incorporated in fresh MEAs), these materials/MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components, and field-aged MEAs. These studies are used to establish critical processing-microstructure-performance correlations and to elucidate the individual materials changes contributing to measured MEA degradation, performance loss, and failure. Analytical microscopy methods have been used to mechanistically explain the degradation of various carbon support structures, to quantify Pt loss due to migration, catalyst coalescence, and to understand the structure, distribution, and composition of thin ionomer films within catalyst layers. Understanding and quantifying the structural and compositional changes of the materials comprising the MEA during electrochemical-aging will allow for the implementation of processing changes and critical materials development that are required for optimizing PEM fuel cell durability and performance. ________________________________________________________________________ Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is an Office of Science User Facility.

Effect of relative humidity cycles accompanied by intermittent start/stop switches on performance degradation of membrane electrode assembly components in proton exchange membrane fuel cells

The performance degradation of membrane electrode assembly (MEA) components in proton exchange membrane fuel cell (PEMFC) is studied by designing relative humidity (RH) cycles accompanied by intermittent start/stop switches. Cathode catalyst activity, permeability and resistance of proton exchange membrane (PEM) as well as cell performance are monitored during the test procedure. The interfaces of MEA, the catalyst particle distribution near the cathode inlet are characterized by SEM and TEM, respectively. The results demonstrate both the overall H2 permeability and crossover current of PEM are doubled compared with its initial properties. Signs of PEM degradation, including periodical thinning, cracks and pinholes formation, are observed after 300 RH cycles and 40 times of start/stop switches. The average Pt particle size increases by more than 75%, and the cathode electrochemical surface area decreases by 48% after the test procedure. Meanwhile, the cathode catalyst layer becomes looser due to the dissolution of some smaller Pt particles and catalyst agglomeration in the RH cycles and the high potential during the intermittent start/stop switches. The membrane resistance demonstrates downshift variation during the RH cycles. PEMFC performance, however, decays due to the chemical and electrochemical attack as well as the mechanical stresses.

Insight into the Degradation of HT-PEMFCs Containing Tungsten Oxide Catalyst Support Material for the Anode

Besides high fabrication costs, degradation effects are a major drawback of high temperature proton exchange membrane fuel cells which hinder their successful commercialization so far. Detailed investigation of degradation effects is a key issue for being able to understand and prevent the occurring processes. In this work electron microscopy based methods are utilized to study the degradation of membrane electrode assemblies (MEA) that underwent different operation modes and runtimes in single cells and fuel cell stacks. One focus is placed on WO3−x based catalyst support material on the anode which is a promising electrode material to substitute the widely used high surface area carbon. Via energy dispersive X-ray measurements the elemental distribution in the MEAs before and after operation is analyzed. Instead of individual nanoparticles the Pt catalyst forms several hundred nanometer large, crystalline, 3D-networks of rod-shaped nanoparticles on the WO3−x grains. The nominal 2000 h in-situ long-term fuel cell operation tests reveal that the performance of the MEAs with WO3−x based anodes lies 10 mV below an MEA with standard high surface area carbon. However, the degradation behavior of the MEAs containing WO3−x outperforms the conventional used MEA and therefore confirms the usability of WO3−x as catalyst support.

Investigation of MEA degradation in a passive direct methanol fuel cell under different modes of operation

Direct methanol fuel cell (DMFC) durability tests were conducted in three different operational modes: continuous operation with constant load (LT1), on–off operation with constant load (LT2) and on–off operation with variable load (LT3). Porous carbon nanofiber (CNF) anode layers were employed in three sets of single passive DMFCs; each membrane electrode assembly (MEA) was run continuously in durability testing for 3000h. The objective of this study is to investigate the degradation mechanisms in an MEA with a porous CNF anode layer under different modes of operation. The polarization curves of single passive DMFCs before and after durability tests were compared. The degradation of DMFC performance under the cyclic LT1 mode was much more severe than that of LT2 and LT3 operation. The loss of maximum power density after degradation tests was 49.5%, 28.4% and 43.7% for LT1, LT2 and LT3, respectively. TEM, SEM and EDS mapping were used to investigate the causes of degradation. The lower power loss for LT2 was mainly attributed to the reversible degradation caused by poor water discharge, which thus reduced the air supply. Catalyst agglomeration was especially observed in LT1 and LT3 and is related to carbon corrosion due to possible fuel starvation. The loss of active catalyst area was a major cause of performance degradation in LT1 and LT3. In addition to this, the dissolution and migration of Ru catalyst from the anode to cathode was identified and correlated with degraded cell performance. In the DMFC, the carbon nanofiber anode catalyst support exhibited higher performance stability with less catalyst agglomeration than the cathode catalyst support, carbon black. This study helps understand and elucidate the failure mechanism of MEAs, which could thus help to increase the lifetime of DMFCs.

SAXS Analysis of Catalyst Degradation in High Temperature PEM Fuel Cells Subjected to Accelerated Ageing Tests

AbstractThe loss in performance during fuel cell operation is one of the critical factors that hamper fuel cells commercialization. This paper presents a research activity related to high temperature polymer electrode membrane fuel cell (HT‐PEMFC) degradation. The aim of the study is to investigate catalyst degradation of membrane electrode assemblies (MEAs) subjected to load cycles. Two HT‐PEM MEAs have been subjected to accelerated ageing tests based on load cycling. The cycles profile has been chosen in order to enhance catalyst degradation. Both the tests show a fuel cell performance loss lower than 30 mV after 100,000 cycles at 600 mA cm−2. In order to analyze the catalyst evolution, synchrotron small angle X‐ray scattering (SAXS) has been employed. The catalyst degradation of the two conditioned samples has been compared with the data obtained from a new MEA that has been used as reference sample. The SAXS results showed a mean size increase of the platinum nanoparticles up to the 100%.

Electrochemical Characterization of Sb-Doped SnO2 as Electrocatalyst Support for Electrochemical Energy Conversion Systems

Electrochemical energy conversion systems, mainly Unitized Regenerative Fuel Cells (URFC) and Solid Polymer Electrolyzer (SPE), are considered to have a promising future due to the generation of hydrogen in a clean and sustainable way. However, the design of oxygen electrodes is the main limiting factor for an efficient performance. In this matter, most of the studies have been focused in the development of efficient electrocatalyst for the oxygen reduction and water electrolysis reactions, and the use of electrocatalysts supports different from conventional carbon-based materials, which is susceptible of corrosion at the URFC and SPE operation conditions. Corrosion resistance supports, with improved surface area and conductivity, would reduce the electrocatalyst loading,the membrane electrode assembly degradation rate, and the global construction cost of the systems. The present work deals with the synthesis and electrochemical characterization of a Sb – doped SnO2 (ATO) for their potential use as electrocatalyst support in the oxygen electrode in electrochemical energy conversion systems. The synthesis of the support was performed by the sol –gel technique with a surface area modification with dodecylamine as surfactant followed by a thermal treatment. Electrochemical stability was evaluated by cyclic voltammetry (CV) with 250, 400 and 500 cycles in a wide range (0 – 2.0 V vs ENH), while the chronoamperometry (CA) was evaluated at high anodic potential (1.8 V vs ENH) in a continuous period of time of 5 h and 8 h. The results revealed that the Sb doped SnO2 support, present good electrical properties and electrochemical stability at a higher potential of 1.8 V vs. NHE in the presence of O2, as compared to other carbon-based and commercial sub-stoichiometric titanium oxide (Ebonex ®) supports.

PEM Fuel Cell MEA Structure Degradation

The durability of MEAs (Membrane Electrode Assemblies) of PEM (Polymer Electrolyte Membrane) fuel cells is a major barrier to the commercialization of these systems for transportation applications [1]. Power transients, shut-down/start-up, temperature cycling, RH (relative humidity) cycling, are all operating conditions imposed by the rigid requirements for transportation applications. These operational transients have the effect of inducing a number of chemical and structural degradation mechanisms on MEAs changing their performance. One of the major degradation involves the electrocatalyst, including corrosion of the catalyst support leading to structural changes in the catalyst layer. These structural changes can cause changes in the PEM fuel cell performance due to loss of catalyst utilization, electronic/protonic conductivity, and changes in the mass transport of the gases and product water. A primary cause for changes in the structure of the catalyst layer is due to the corrosion of carbonaceous electrode supports leading to electrode collapse, reduced active catalyst area, and increased surface hydrophilicity. The carbons typically used as the support for Platinum catalysts in proton exchange membrane fuel cells (PEMFCs) are vulnerable to electrochemical corrosion, especially during potential excursions generated by startup/shutdown cycling and local anode starvation [2].Cathode catalyst layer thinning is regularly observed and increases losses associated with mass transport. This catalyst layer thinning is coupled with loss of porosity in the catalyst layer. The graphitic content of catalyst supports increases during the operation of fuel cells at high potentials which is likely due to a decrease of amorphous carbon due to corrosion. Support corrosion also leads to detachment and agglomeration of catalyst particles, while weakening of the carbon structure allows collapse of electrode pores and severely limits gas transport [3]. The electrode structure degradation has been shown to occur in microscopically localized regions. An example of the localized degradation is demonstrated in Figure 1 after testing utilizing a simulated Drive Cycle Test prescribed by DOE/Fuel Cell Tech. Team [4]. The fresh catalyst layer (Fig. 1a) shows a mostly graphitic carbon support with well distributed Pt nanoparticles. After Drive Cycle operation, TEM post-analysis shows localized bands of carbon corrosion which are correlated increased Pt particle sizes and closer Pt-Pt interparticle spacings (Figure 1b). Typically, these oxidized regions of carbon surround non-oxidized regions. Regions of non-oxidized catalyst retain their graphitic structure and are correlated with smaller Pt particle sizes.Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume. This loss of cathode layer porosity is shown in Figure 2, where Figure 2a is a digital TEM image of the fresh cathode catalyst layer, 2b is after operation at 1.2V for 10 hours, and 2c is after 40 hours operation at 1.2V. The initial catalyst layer porosity is approximately 40%, decreasing to 33% over 10 hours, and is only about 7% after 40 hours.The air/hydrogen (or hydrogen/air) fronts during shut-down/start-up have been shown to cause high potentials leading to carbon support corrosion and catalyst layer degradation. The anode catalyst layer ability to conduct the oxygen reduction reaction (ORR) plays a major role in MEA degradation during H2/Air start-stops. By limiting the anode participation in ORR during these phenomena, the degradation can be reduced. Other mitigation techniques can also be employed.The catalyst layer fabrication process using different solvents can have a dramatic effect on the performance/durability of fuel cell electrodes, although the catalyst ECSA durability is largely unaffected. Catalyst loading has a significant impact on performance of MEAs made via decal process using a more traditional water/alcohol/glycerol mixture in the ink. The fact that the ECSA shows similar decreases during the potential cycling indicates that other changes in performance are occurring. Further measurements show that the mass transport resistance is decreasing during the potential cycling. Enhanced performance and impedance behavior of ultra-low loading cathode with potential cycling have similarity with performance improvement with HelOx (or air flow increase). Thus, electrode structural changes occur during potential cycling which favor oxygen diffusion, and lower mass transport resistance.