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6 figures, 3 tables., Perfluorinated sulfonic acid (PFSA) polymers such as Nafion® are widely used for both electrolyte membranes and ionomers in the catalytic layer of membrane-electrode assemblies (MEAs) because of their high protonic conductivity, σH, as well as chemical and thermal stability. The use of PFSA polymers with shorter side chains and lower equivalent weight (EW) than Nafion®, such as Aquivion® PFSA ionomers, is a valid approach to improve fuel cell performance and stability under drastic operative conditions such as those related to automotive applications. In this context, it is necessary to optimize the composition of the catalytic ink, according to the different ionomer characteristics. In this work, the influence of the ionomer amount in the catalytic layer was studied, considering the dispersing agent used to prepare the electrode (water or ethanol). Electrochemical studies were carried out in a single cell in the presence of H2-air, at intermediate temperatures (80–95 °C), low pressure, and reduced humidity ((50% RH). %). The best fuel cell performance was found for 26 wt.% Aquivion® at the electrodes using ethanol for the ink preparation, associated to a maximum catalyst utilization., This research was funded by the European Union’s Seventh Framework Programme (FP7/2007–2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant no 303452 (IMPACT).

The membrane-electrodes assembly (MEA) is the core of the Polymer Electrolyte Fuel Cell (PEFC). It consists of a membrane, catalytic (CL) and gas diffusion layers (GDL). In order to manufacture MEAs with suitable performance, a hot-pressing procedure is generally used. The relevant parameters are the temperature, pressure and time of hot-pressing. Such variables need to be adjusted as a function of the type of ionomer used in the catalytic layer and membrane. In this study, an evaluation of the temperature of hot-pressing was carried out and its influence on MEA electrochemical performance was assessed. In particular, preparation trials of MEAs were carried out with reinforced experimental membranes based on Aquivion® short-side-chain PFSA (by Solvay Specialty Polymers). The membranes were coupled to gas diffusion electrodes, and MEAs were manufactured using different temperatures for the hot-pressing procedure in order to evaluate their influence on the electrochemical performance of PEFCs, in the temperature range of 80–95 °C, with low relative humidity of the reactant gases. The electrochemical performance of the prepared MEAs was tested in a H2/Air 25 cm2 single cell in terms of polarization curves and accelerated stress test (AST).

6 Figuras, 4 Tablas, The large-scale application of polymer electrolyte membrane fuel cells (PEMFCs) requires a reduction of the costs and an improvement in performance and stability. Particularly, the automotive market needs the PEMFC works under harsh conditions (high temperature, low relative humidity, etc.), maintaining a good performance level and low degradation. These operating conditions permit to mitigate the constraints concerning the thermal and water management allowing both a simplification and a volume reduction of the system inside the car with a strong impact on costs and reliability. However, PEMFCs require the development of the components such as catalysts, ionomers and membranes with a proper stability. In this work, PtCo/KB and Pt/KB electrocatalysts were investigated with the aim of verifying the performance and stability, coupling them with an innovative reinforced Aquivion® membrane. An optimization of the catalytic ink composition was carried out, according to the different ionomers characteristics to be used in the catalytic layer in order to reduce the degradation level. The stability is assessed by considering the variations of the performance and electrochemical parameters, such as electrochemical surface area (ECSA) and mass activity (jm), at the beginning and end of accelerated stress test procedures., The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant no 303452 (IMPACT).

Nanocrystalline platinum with different morphologies is synthesized via electrochemical deposition technique. The nucleation mechanism and its structural effect over the electrodeposited Pt on carbon electrodes have been systematically studied. Powder X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy are employed to study nucleation, oxidation states, and Pt structure growth on a carbon electrode. This study reports significant development of Pt metal nanoparticles with different morphologies such as a sphere, flower, core-flower, and rod-like structure directly fabricated on carbon electrode while tuning the deposition parameters such as current density, time, temperature, pH during the deposition process. The proposed electrochemical route represents a superior fabrication procedure for large-scale electrode fabrication compared to a conventional method for preparing membrane electrode assemblies for fuel cell stacks.

Composite membranes based on sulphonated Polyetheretherketone (sPEEK) and 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) were developed for portable applications. A sulphonation degree of 65% and different loadings (0-5wt%) of TPyP porphyrin were used. Spectroscopic studies (UV-Vis and fluorescence emission), ionic exchange capacity, water uptake, structural and morphological analyses were carried out. A good interaction between the filler and the polymer was found, probably due to specific supramolecular interactions among the nitrogenous atoms present in the periphery of the porphyrin ring and sulphonic groups of polymer. The membrane with the lowest loading (~1wt%) of TPyP, shows slightly lower water uptake and unaltered ? values than the reference membrane, resulting in quite unaltered proton conductivity also in anhydrous conditions. Electrochemical tests were performed in a PEFC 25cm2 single cell at room temperature and anhydrous hydrogen/humidified air. A power density of 93mW/cm2 was found in anhydrous conditions for sample with the lowest loading of TPyP.

Perfluorinated sulphonic acid (PFSA) polymers such as Nafion® are widely used for both electrolyte membrane and ionomer in the catalytic layer because of their high proton conductivity (σH) as well as chemical and thermal stability. When PFSA polymers alternative to Nafion® with a different EW are used, it is necessary to optimize the composition of the catalytic ink, considering the different ionomer characteristics. Moreover, fuel cell operating conditions play an important role regarding the catalytic layer optimization. In this work, the influence of ionomer amount in the catalytic layer was evaluated. The electrochemical studies were carried out in a single cell in H2–O2 at intermediate temperatures (80 °C–130 °C) by using a fumapem® F-950 membrane as an electrolyte. A hydro-alcoholic dispersion of fumion® F was also used as an ionomer in the electrode catalytic structure with an amount ranging from 30 to 60%. Under operating conditions finalised to automotive applications (intermediate temperature, low relative humidity and low pressure of gases) an improvement of performance in terms of maximum power density was obtained by using optimized ionomer content.

Understanding the freezing mechanism of liquid water within polymer electrolyte fuel cells (PEFC) is crucial for its commercialization for mass market. Although various publications investigated the cold start capability of PEFCs, contradictory behaviors during cold starts at freezing temperatures have been published in literature. Interestingly, differences can be mainly identified between large and small size fuel cells: the latter have a significantly higher cold start capability than large size fuel cells. In order to understand the influence of different active areas, we present measurements of cold starts performed with cells sizes of 1 and 50 cm2 using the same materials, including a large count of experiments (>200) on the 1 cm2 cells to perform statistical analysis. The cold start capability is strongly reduced with an increasing size of the active area, and the operating times changes from stochastic values for small cells to reproducible values for large cells. Both effects can be explained by the higher probability of the aggregate state transition from super-cooled water to ice in large cells. Consequently this paper highlights the implications of downscaling PEFCs to draw conclusions for technical relevant cold-starts., Journal of the Electrochemical Society, 162 (10), ISSN:0013-4651, ISSN:1945-7111

Micro-porous layers (MPLs) play an important role in the water management of polymer electrolyte fuel cells (PEFCs), however, the detailed mechanism of how the produced water is drained from these layers is not well understood. This paper observed the cross-sectional distribution of liquid water inside the cathode MPL to elucidate details of the phase state of the water transported through the MPL. The freezing method and cryo-scanning electron microscope (cryo-SEM) are used for the observations; the freezing method enables immobilization of the liquid water in the cell as ice forms by the freezing, and the cryo-SEM can visualize the water distribution in the vicinity of the MPL at high resolution without the ice melting. It was shown that no liquid water accumulates inside the MPL in operation at 35 °C, while the pores of the MPL are filled with liquid water under very low cell temperature operation, at 5 °C. These results indicate that the produced water passes through the MPL not as a liquid but in the vapor state in usual PEFC operation. Additionally, liquid water at the interface between the MPL and a catalyst layer (CL) was identified, and the effect of the interfacial contact on the water distribution was examined. Keywords: PEFC, MPL, Direct observation, Water transport, Phase state

To overcome the fundamental disadvantages of conventional cathodes for polymer electrolyte fuel cells (PEFCs), such as dissolution and migration of platinum-based catalysts and consumption of the carbon catalyst-support, a substantially novel gas-diffusion cathode has been proposed. The electrode was made of a porous oxide catalyst, which was coated on the inner and outer surfaces of a macro-porous titanium sheet substrate, using neither a platinum catalyst nor carbon catalyst-support. The suitability of this cathode for PEFCs, tested using a highly porous iridium oxide strongly coated on the macro-porous titanium sheet substrate, was confirmed by successful power generation. The gas-diffusion electrode functioned not only as the cathode but also as an anode of a PEFC., Article, ELECTROCHIMICA ACTA. 105:224-229 (2013)

Proton transport properties of a partially protonated poly(aspartic acid)/sodium polyaspartate (P-Asp) were investigated. A remarkable enhancement of proton conductivity has been achieved in the thin film. Proton conductivity of 60-nm-thick thin film prepared on MgO(100) substrate was 3.4 × 10(-3) S cm(-1) at 298 K. The electrical conductivity of the oriented thin film was 1 order of magnitude higher than the bulk specimen, and the activation energies for the proton conductivity were 0.34 eV for the oriented thin film and 0.65 eV for the pelletized sample, respectively. This enhancement of the proton transport is attributable to the highly oriented structure on MgO(100) substrate. This result proposes great potential for a new strategy to produce a highly proton-conductive material using the concept of an oriented thin film structure without strong acid groups.

Adequate water management represents one of the main challenges in the design and operation of polymer electrolyte membrane fuel cells. In this work, the influence of inlet gas humidification on cell performance is investigated by in-situ current density measurements obtained using the segmented cell approach. Particular attention is paid to the combined effect of cell temperature and relative humidity of the anode and cathode feed streams. When operated at 80 degrees C and low humidity conditions, the cell is seen to undergo a severe voltage decline that is not observed at 60 degrees C. The analysis shows that the variation with temperature of the water uptake rate of the gaseous streams plays a key role in determining the observed differences in performance stability. In the case of 60 degrees C operation, the water uptake rate of the cathode stream at 50% inlet relative humidity is roughly 30% of its value at 80 degrees C at the same humidification level, resulting in a significantly lower drying capacity. A simple balance of water model, able to explain the observed cell behavior, is finally presented and discussed. This work has been partially supported by the European Union's Seventh Framework Programme (FP7/2007-20013) for Fuel Cell and Hydrogen Joint Technology Initiative under grants no 303446 and 325239, and by Project ENE2011-24574 of the Spanish Ministerio de Economía y Competitividad. Authors would also like to thank H. Sander at DLR and P. García-Ibarra at the Dept. de Física Matemática y de Fluidos (UNED) for helpful suggestions and discussions.

The durability of polymer exchange membrane (PEM) fuel cells, under a wide range of operational conditions, has been attracting intensive attention, as durability is one of the largest barriers for commercialization of this promising technology. In the present work, membrane electrode assembly (MEA) degradation of a four-cell stack with Nafion membranes of different thicknesses, including N117, N115, NR212, and NR211, was carried out for 1000 h under idle conditions. By means of several on-line electrochemical measurements, the performance of the individual cells was analyzed at different times during the degradation process. The results indicate that the cells with thinner membranes have a lower open circuit voltage (OCV) due to the higher fuel crossover. Before degradation, the thickness of the membranes correlates with performance of the cell. However, with the advancement of degradation, the performance of cells with thinner membranes degraded much faster than those with thicker membranes, especially after 800 h of operation. The fast performance degradation for thinner membranes is evident by a dramatic increase in hydrogen crossover indicating membrane thinning or pinhole formation.

In order to optimize the electrode composition and performance of Polymer Fuel Cells and to reduce the production cost of membrane electrode assemblies (MEAs), different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt). The electrochemical characterization was performed by recording current–voltage curves and electrochemical impedance spectra (EIS) in the galvanostatic mode of operation at 500 mA cm −2 . The evaluation of the measured impedance spectra with an adequate equivalent circuit shows that the cathode of the fuel cell is very sensitive to the electrode composition whereas the contribution of the anode is very small and invariant to the electrode composition. Furthermore, it could be shown for the first time using electrolyte powder in the electrodes that the charge transfer of the cathode decreasing monotonically with increasing electrolyte content in the cathode. These findings suggest that with increasing electrolyte content in the electrodes, in particular in the cathode, the utilization degree of the catalyst increasing linearly with increasing electrolyte content in the electrode.

A fluorescence probe method, which enables highly sensitive detection of OH radicals (·OH) by forming a stable fluorescent product, was applied to a model reaction system of polymer electrolyte fuel cells (PEFCs). 'OH was successfully detected under open-circuit condition with a homemade test cell having reservoirs of a coumarin probe solution at both anode and cathode electrodes. The experiments with H 2 O 2 and Fe 2+ in the probe solution suggested that the ·OH is produced via electrode reduction and oxidation of H 2 O 2 at the anode and cathode, respectively.

Low cost metals are quite attractive as bipolar plate of polymer electrolyte fuel cell (PEFC). However, low cost metals easily corrode due to the absence of good passivation films. In order to give high electrical conductivity and corrosion resistance to low cost metals such as carbon steel (S25C), carbon-coat was carried out by plasma-assisted chemical vapor deposition. Carbon-coated samples were characterized by XRD, Raman spectroscopy, and interfacial contact resistance. Based on the results of polarization measurement, the combination of carbon-coat and electroless nickel plating was found to be effective for decreasing of anodic dissolution of base metal.

The functional group p -benzensulfonic acid ( pb -SO 3 H) was chemically attached to the surfaces of two different carbon supports, XC72 and BP2000, to improve carbon support corrosion resistance. An accelerated durability test (ADT) utilizing a rotating disk electrode (RDE) system was used to study the corrosion of Pt catalysts using these two carbon supports, Pt/FXC72-SO 3 H and Pt/FBP2000-SO 3 H, under simulated fuel cell conditions. The results showed that the functional group pb -SO 3 H can effectively reduce the performance decay of these catalysts by enhancing the corrosion resistance of the carbon support and promoting the stability of the Pt nanoparticles. The ECASA and ORR current measurements of these catalysts indicate that the effects of the functional group ( pb -SO 3 H) on improving carbon corrosion resistance were more significant for the BP2000 than for the XC72. TME imaging revealed that the size of the Pt nanoparticles was significantly reduced and the particle distribution was improved in a polymer electrolyte fuel cell (PEFC).

The most common method used to characterise the electrochemical performance of fuel cells is the recording of current/voltage U(i) curves. Separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques like electrochemical impedance spectroscopy (EIS). The application of EIS is an approach to determine parameters which have proved to be indispensable for the characterisation and development of all types of fuel cell electrodes and electrolyte electrode assemblies. In addition to EIS semi-empirical approaches based on simplified mathematical models can be used to fit experimental U(i) curves. By varying the operating conditions of the fuel cell and by the simulation of the measured EIS with an appropriate equivalent circuit, it is possible to split the cell impedance into electrode impedances and electrolyte resistance. Integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell (PEFC) and the assignment of voltage loss to the different processes. In case of using a three electrode cell configuration with a reference electrode, one can directly determine the corresponding overvoltage. For the evaluation of the measured impedance spectra the porous electrode model of Gohr [3] was used. This porous electrode model includes different impedance contributions like impedance of the interface porous layer/pore, interface porous layer/electrolyte, interface porous layer/bulk, impedance of the porous layer and impedance of the pores filled by electrolyte.

The formation of leakages in the membrane in PEFC is a relatively frequent failure mode which leads to a significant loss of performance or loss of the fuel cell functionality. The leakages occur due to a malfunction during the manufacturing process of the membrane or the membrane-electrode-assembly (e.g. penetration of a carbon fiber from the GDL), the assembling the fuel cell (mechanical stress) or be dry-wet cycling of the membrane. Leakages in the membrane allow the reactant gases to cross the membrane and to react directly without generation of electricity. This reaction takes place on the catalyst surfaces like the electrochemical reactions. Consequently, at the leakage a local mixed potential will be formed. Caused by the electrical in-plane conductivity of the electrodes, gas diffusion layer, bipolar plates and membrane the local potential influences the residual electrode, and consequently, an electrochemical mass conversion depending on the local potentials starts. In current density measurements the local electrochemical mass conversion is measured. Alterations in the current density distributions are related to varying local operating conditions. For a cell at open circuit condition the total (integral) current is zero. However, in the cell at open circuit conditions different current directions will be occur in the cell in some areas. This can be clearly observed in the current density distribution measurements. Typically in the cell at open circuit conditions, the currents in the individual segments are all nearly zero, because the local conditions in the cell do not varies significantly. In contrast, if the cell has a leakage, at the leakage a mixed potential will be formed. Consequently, the local conditions in the cell at the leakage are significant changed compared with the undamaged areas. The lateral differences in the local potentials induce equalizing currents which can clearly detect in the current density measurement. In the current density measurement the equalizing current in the bipolar plate, which is the main equalizing current caused by the better in-plane conductivity compared with the in-plane conductivity of the membrane and the gas diffusion layer, is measured. Under load the current density distribution are changed with the cell voltages. If the cell has a leakage the changes are more significant as in an undamaged cell. The current density measurements on the cell with a leakage show clearly, that current density measurements are a powerful tool for error diagnostic in fuel cells.

The analysis of the processes in fuel cells is a main topic at the German Aerospace Center (DLR). The degradation of cell components - especially electrodes - and changes during start- up processes are of foremost interest. For this purpose physical and electrochemical methods are used individually and in combination. In this paper, studies using a combination of methods were performed on different fuel cell types. The examples in the field of low temperature fuel cells, namely alkaline fuel cells (AFC), polymer electrolyte fuel cells (PEFC), and direct methanol fuel cells (DMFC), are mainly focused on the alteration of the electrodes due to electrochemical operation; for solid oxide fuel cells (SOFC) an example for the investigation of the local operating conditions is given.

A sufficient life time of the membrane-electrode assemblies (MEA) for polymer electrolyte fuel cells (PEFC) is necessary for using PEFC in mobile or stationary power supplies. During the life time, the electrochemical performance should be stable and the operation behavior should not change. But in real operation, the electrochemical performance decreases and the operation behavior changes. These effects are induced by a degradation of fuel cell components, which makes necessary to study the structural and chemical changes of single components affected by electrochemical stressing for understanding the degradation mechanisms and effects. The catalyst in the electrodes is the essential component dominating the fuel cell reaction. In both electrodes, platinum is used as catalyst, therefore changes of the catalyst or catalyst distribution must be investigated. The MEAs were operated in single cells in full automatic test facilities. After fuel cell operation, electrodes and MEAs were characterized physically by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and chemisorption measurements. An alteration of the platinum distribution and a migration of the platinum was observed after fuel cell operation.

The influence of carbon monoxide poisoning on the platinum and platinum–ruthenium anode in a polymer electrolyte fuel cell was investigated using electrochemical impedance spectroscopy (EIS). EIS is a very useful method for the characterisation of fuel cells. Therefore, impedance measurements of the cell under constant load were performed at periodic time intervals. Due to the poisoning effect of the carbon monoxide, the system changes its state during the experiment. The reconstruction of quasi-causal spectra was made possible using enhanced numerical procedures, especially the time course interpolation and the Z-HIT refinement. The reconstructed impedance spectra show a strong time dependence and exhibit pseudo-inductive contributions at the low-frequency part of the spectra which increase during the experiment. The analysis of the spectra suggests that the pseudo-inductive behaviour can be attributed to a surface relaxation process of the anode. Furthermore, the influence of the carbon monoxide on the electrochemical behaviour of the contaminated fuel cell may be interpreted by means of a Faraday impedance in addition to a potential-dependent hindrance of the charge transfer.

Alkaline fuel cells (AFC) are an interesting alternative to polymer electrolyte fuel cells (PEFC). In AFC no expensive platinum metal is necessary; silver can be used for the oxygen reduction reaction (cathode catalyst). To enhance its catalytic activity, silver is used in high surface area forms such as porous gas diffusion electrodes (GDE). The gas diffusion electrodes (cathodes) of AFC were prepared by a reactive powder-polymer mixing and rolling (RMR) technology. They consist of a mixture of a silver catalysts and polytetrafluorethylene (PTFE) as organic binder rolled onto a metal web. Different silver catalysts, e.g. silver deposited onto PTFE, silver oxide or silver catalyst formed from Raney silver, can be used. In gas diffusion electrodes hydrophobic and hydrophilic pore systems are required. In AFC the hydrophilic system allows the penetration of the electrolyte into the electrode and the transport of the ions to or from the reaction zone; the hydrophobic pore system is required for the transport of the oxygen to the reaction zone. In addition, to give the mechanical stability, the PTFE in the electrodes forms a hydrophobic pore system, whereby the spin webs of PTFE fibers in and on the electrodes are formed during preparation. For technical use of AFC the long-term stability of AFC components is important, especially that of the gas diffusion electrodes. In order to analyse in detail the long term behaviour - the degradation (ageing) process of silver electrodes during oxygen reduction, electrochemical impedance spectroscopy (EIS) has been performed [1]. The silver GDE has been operated galvanostatically at -100 mA/cm2 in 30 wt % KOH at 70°C with pure oxygen (1 bar) over 1400 h and impedance spectra were recorded at the open circuit potential (OCP), at the beginning daily and after 2 weeks in 48 and 72 h intervals respectively. The evaluation of the measured impedance spectra was performed with the simulation software program. The spectra can be simulated by an equivalent circuit which consists of a series combination of uncompensated electrolyte resistance (Rel), charge transfer resistance (Rct,C), double layer capacity (Cdl) and infinite diffusion impedance (Warburg impedance, W). From the charge (time) dependency of the impedance parameters, the ageing effects can be related to the decrease of the double layer capacity, probably a decrease of the electrochemically active surface and the increase of the Warburg impedance, suggesting that diffusion of reacting gas is hindered. In addition, the fuel cell cathodes were physically characterized by x-ray photoelectron spectroscopy (XPS). By successive measurement of XP spectra and ion etching the surface depth profiles were recorded. Due to electrochemical operation the chemical composition is changed. The PTFE in the electrode is partially decomposed, which can be seen in the depth profile of the electrochemically stressed electrode by the enhanced decrease of the fluorine concentration. Similar, the electrochemical induced decomposition of PTFE is also observed for other low temperature fuel cell electrodes like AFC or PEFC anodes. The second difference in the depth profiles is that the recorded silver concentration is increased. This is induced by the decomposition of the PTFE and the resulting lower concentration of the elements related to PTFE. The decomposition of the PTFE changes the hydrophobic/hydrophilic behavior and in this way it affects directly the three phase zone between catalyst, electrolyte and gas phase relevant for the reaction. Typically, the decomposition of PTFE reduces the hydrophobic property and so the electrode will be more flooded by the liquid electrolyte. Consequently the gas transport in the electrode is more hindered. In addition, the thickness of the three phase zone will be reduced.

For the study of electrochemical and transport mechanisms in polymere electrolyte fuel cells (PEFC) electrodes and for a further development of PEFC electrodes it is important to characterize these electrodes. The characterization of the electrodes was performed by electrochemical analytical as well as physical methodes on both single electrodes and electrode-membrane assemblies (MEA). In addition to voltage-current characteristics the electrodes were electrochemically measured by cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry. To determine the pore systems nitrogen adsorption and mercury porosimetry were used. Chemical composition and microstructure of the electrodes were studied by surface science methodes like scanning electron microscopy or X-ray induced photoelectron spectroscopy.

Gas diffusion electrodes for electrochemical devices frequently consist of polytetrafluorethylene (PTFE) and are carbon black supported catalysts. The hydrophilic and hydrophobic character determines the moisturewetting wetting behavior, which is important for the media transports in the gas diffusion electrodes. The main components of gas diffusion electrodes are an organic binder, typical PTFE, and a catalyst, for applications with acid electrolyte a noble metal catalyst, which frequently is supported by carbon black. The chemical surface composition of electrodes prepared in different ways from a mixture of carbon black and PTFE and mechanically and thermally modified were determined by X-ray photoelectron spectroscopy (XPS). The XPS measurements show differences between electrodes wet prepared from a suspension and electrodes prepared with the dry preparation technique of the DLR (Deutsches Zentrum fur Luft- und Raumfahrt). During the preparation of the electrodes by the dry preparation technique the carbon black is enriched on the surface. Due to the mechanical stressing the enrichment of the carbon black is reduced. By tempering the PTFE concentration on the surface can be increased. The results of the XPS measurements were related to the wetting behavior; a concentration of approximately 20 weight% (wt%) PTFE is the limit between hydrophilic and hydrophobic surfaces.

Fuel cells allow an environmentally friendly and highly efficiently conversion of chemical energy to electricity and heat. Therefore, they have a high potential to become important components of an energy-efficient and sustainable economy. The main challenges in the development of fuel cells are cost reduction and long-term durability. Whereas the cost can be significantly reduced by innovative mass production, the knowledge to enhance the lifetime sufficiently is not available. Surface science analysis methods used for the characterization of the new and used electrodes can be use to determine the alterations in the fuel cell components and in this way to identify the degradation processes, but they do not allow to quantify the influence of the alterations in the electrodes on the electrochemical performance. For this purpose electrochemical methods are necessary; especially the electrochemical impedance spectroscopy (EIS) allows to separate the performance losses individually and to assign them to different components and processes of the cell via a model, whereas the choice of the right model can be problematic. Two important and distinct structural degradation processes were identified by surface analysis of the electrodes before and after fuel cell operation: first, the decomposition of poly tetra-fluoro-ethylene (PTFE) which is used as an organic binder and as a hydrophobic agent in the electrodes and second, a change of the structure of the catalysts. The observed decomposition of the PTFE is associated with a decrease of the hydrophobicity of the electrode. A loss of hydrophobicity influences drastically the required operation conditions and leads to a more critical water management of the fuel cell. In contrast, the alteration of the catalysts structure in the electrodes causes an irreversible decrease of the electrochemical performance. In polymer electrolyte fuel cells (PEFCs) a particle agglomeration of the platinum catalysts at the cathodes is detected. With EIS the effect of two different degradation processes in the membrane-electrode-assembly was quantified. During continuous operation the degradation of the PTFE induces an approximately two times higher performance loss compared with the performance loss related to the agglomeration of the platinum catalyst.