Combined electrochemical and surface analysis investigation of degradation processes in polymer electrolyte membrane fuel cells

Electrochimica Acta 52 (2007) 2328–2336

Combined electrochemical and surface analysis investigation of degradation processes in polymer electrolyte membrane fuel cells M. Schulze ∗ , N. Wagner, T. Kaz, K.A. Friedrich Deutsches Zentrum f¨ur Luft- und Raumfahrt e.V. Institut f¨ur Technische Thermodynamik, Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany Received 16 March 2006; received in revised form 9 May 2006; accepted 16 May 2006 Available online 30 October 2006

Abstract 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. © 2006 Elsevier Ltd. All rights reserved. Keywords: PEFC; Degradation; Hydrophobicity; Catalysts; EIS; XPS

1. Introduction Due to their high electrical efficiency and their high total (electricity and heat) efficiency fuel cells will be used in future as environmentally friendly generator for electrical power and heat. Low temperature fuel cells with polymer electrolytes can be used for many applications in the range of performance of a few mW and a couple of 100 kW [1]. All types of fuel cells exhibit a decrease of the electrochemical performance with operation time, however, the degradation

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rates for different fuel cell types are quite different. The degradation also depends strongly on the operating conditions. In PEFC the active part is the membrane-electrode assemblies. The electrodes have the function to catalyze the reaction and to allow a facile transport of the reactants and educts. The reaction in each electrode takes place in the three-phase zone formed by the catalyst, the electrolyte and reactants. For the transport of the reactants to the reaction zone in the electrodes and the transport of the educts out of the electrodes a porous structure is needed. A hydrophilic pore system is required for the transport of water and a hydrophobic pore system for the transport of gases. Therefore, a highly porous structure with specific surface properties is needed. The reaction zone in a PEFC is determined by the electrode-membrane interface and is independent of the

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reaction conditions, but the water balance is critical for the electrochemical performance: the electrolyte membrane needs a high water content for a good ion conductivity, but a flooding of the pore system inhibiting gas transport should be avoided. As a consequence, the balance of hydrophobic and hydrophilic properties of the materials determines the operation conditions and the electrochemical performance. Because of its high chemical stability poly tetra-fluoro-ethylene (PTFE) is typically used to provide a hydrophobic character to the electrodes and the hydrophobicity of the electrodes will be adjusted by the concentration of the PTFE [2]. The structure of the electrodes depends on the preparation process [2] and [3]. The PTFE amount which determines the hydrophobic character of the electrodes can be determined by XPS [2]. An improvement of the long-term durability of low temperature fuel cells is necessary as lifetimes between a few thousand and a few ten thousand hours, depending on the application, are required. During the lifetime not only the electrochemical performance but also the operation conditions should not be changed significantly. For this purpose, it is necessary that hydrophobic properties stay constant during the fuel cell operation. In principle, PTFE is not only strongly hydrophobic, but it has a very high stability in aggressive chemical environmental conditions. However, the decomposition of PTFE during the fuel cell operation was observed in different low-temperature fuel cell types [4]. This paper is focused on the simultaneously determination of degradation processes of both electrodes—anode and cathode. In addition more degradation processes take place in fuel cells, e.g. the degradation of the membranes or the corrosion of metallic bipolar plates. The investigation of these processes, however, is not the objective of this paper. 2. Experimental 2.1. Sample-preparation For the experiments commercial electrodes and electrodes prepared with the DLR dry spraying technique were used. The DLR dry spraying technique [3,5–9] is a consequent further development of the manufacturing technique for alkaline fuel cell electrodes [4,10–16], with a new deposition type of the powder mixture for the reaction layer. In both types of electrodes, commercial electrodes and the DLR electrodes, a carbon supported platinum catalyst is used. In contrast to the preparation technique of commercial electrodes – formed from a liquid suspension which contains catalyst and additives – the DLR manufacturing technique is a dry process avoiding solutions and complicated process steps. Different MEA were prepared by the DLR dry spraying technique. In the investigated MEA Nafion® 1135 membranes are used, coated with a mixture of 80 wt.% carbon supported platinum catalyst (20 wt.% Pt on Vulcan XC-72 from E-Tek) and 20 wt.% PTFE (Hostaflon® TF 2053), corresponding to a total catalyst loading of 0.12 mg Pt/cm2 electrode, 0.25 mg Pt/cm2 electrode and 1.0 mg Pt/cm2 electrode. This corresponds to very low catalyst loading used in order to accelerate degradation pro-

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cesses and to facilitate the determination of degradation mechanism. As gas diffusion layers single sided E-Tek backings are used on both electrodes (anode and cathode). In addition, some experiments were performed with commercial electrodes (E-Tek electrodes with 20 wt.% Pt in the carbon supported platinum catalyst). To prepare all membrane electrode assemblies (MEA) for the fuel cell operation the electrodes were hot pressed at 160 ◦ C, and 1.6 MPa for 3 min with a Nafion® membrane from Du Pont, with exception of the MEA for analyzing the electrodes by X-ray photoelectron spectroscopy. 2.2. Electrochemical characterization The MEAs were electrochemically tested in single cell configuration. The active area of the MEAs was 4.8 cm × 4.8 cm = 23 cm2 . The tests were performed in automated fuel cell test facilities [3,17]. The test facilities are designed to allow the control of operation conditions in a wide range of values. The membrane electrode assemblies are electrochemically characterized in the test facilities by recording current–voltage (V–i) curves, using chronopotentiometry, measurement of the local current densities and electrochemical impedance spectroscopy (EIS). Some of tests were performed in a segmented cell. This test cell is segmented into 16 areas on the anode side, whereas the cathode is not segmented. With this set-up the current density in the cell can be measured on-line in fuel cell operation. The fuel cell tests were performed at various operation conditions. In all tests the operation temperature was 80 ◦ C. As fuel gas pure hydrogen was used, the operation pressure at the anode as well as at the cathode was 2.0 bar. The operation mode at the anode was “dead end” (100% utilization), with various purging intervals. The current density for operation was 500 mA/cm2 with exception of the long-term experiment shown in Fig. 1 (300 mA/cm2 ) and in the experiment with the flooded cell, descript below. The oxygen/air flow at the gas outlet of the fuel cell was kept constant. The stoichiometry of the oxygen was 3.

Fig. 1. Long-term operation with interrupts and drying of the MEA (Nafion 1135, Pt 0.2 mg/cm2 each electrode prepared by the DLR dry spraying technique) at T = 70 ◦ C, pH2 = 2 bar dead end, pAir = 2 bar, λAir = 2 and a loading of i = 300 mA/cm2 .

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The MEA investigated by XPS was operated with pure oxygen. After operation the electrodes were separated from the membrane and analyzed by XPS, whereby the electrochemical experiment was finished with an operable MEA (active electrodes). For the scanning electron microscopy (SEM) investigations the MEAs prepared by the DLR dry spraying technique were operated in single cells under normal fuel cell operation conditions with pure oxygen at the cathode as well as at special conditions. A MEA with two equal electrodes which have a Ptloading of 0.25 mg/cm2 and 1.0 mg/cm2 platinum on a single sided backing was hot pressed with a Nafion 1135 membrane. The fuel cell tests were performed in the segmented cell. With this set-up the current density in the cell can be measured online in fuel-cell operation. The MEA was electrochemically operated at 80 ◦ C with pure hydrogen and oxygen. The gases were not humidified. The anode side was purged by opening the outlet valve with pulses of 0.5 s every 900 s and similarly the outlet valve on the cathode side was opened every 120 s for 0.5 s. For the start-up the MEA was humidified directly with liquid water injection within the test cell and afterwards stressed electrochemically at 500 mV cell voltage. In a second experiment the operation mode was changed after 80 h. The anode was operated in dead-end mode without purging of the anode and the cycle time between the purging of the cathode gas volume was increased to 240 s. With these operation conditions the anode side was flooded by reaction water and the cell current decreased. The current density measurement has clearly shown that the cell passivation starts from the lower part and extents to the top with increasing level of liquid water on the anode side. So, the decreasing current of the cell can be related to a decreasing active area of the anode. After 30 h under these operation conditions the anode outlet valve was opened and water was blown out from the anode side. For the long-term tests and the EIS investigations the MEA with the loading of 0.25 mg Pt/cm2 electrode was operated galvanostatically at a constant current density of 500 mA/cm2 . The cell was operated with air as oxidant. For these tests the gas flows were set constant and the air and hydrogen pressure in the cell were regulated; the anode as well as the cathode was not purged. At particular time intervals electrochemical impedance spectra were recorded in the frequency range from 100 mHz to 100 kHz. The EIS measurements were performed with a IM6 electrochemical workstation in combination with a power potentiostat (PP240), both from Zahner electric, Germany applying a small sine-wave (AC) signal distortion of 10 mV amplitude. 2.3. Physical characterization After preparation and after electrochemical operation in the fuel cell the MEAs were investigated by scanning electron microscopy [18] combined with energy dispersive X-ray spectroscopy (EDX) [18], X-ray photoelectron spectroscopy (XPS) [18]. For the SEM and EDX measurements a Zeiss Gemini microscope (LEO) was used in combination with a NORAN VOYAGER 3000 EDX-detector. The SEM used allows

high magnification imaging at low and high beam voltages (1–30 keV). The XPS measurements were performed in a XSAM 800 (Kratos)—the XPS equipment is described in more details in [19]. XPS yields information about the chemical composition of the surface and the method shows a very high surface sensitivity. In contrast, EDX has a low surface sensitivity and yields information about the bulk composition of the range close to the surface. SEM yields information about the surface structure and gives information about the bulk character. By alternation recording XP spectra and ion etching the surface (Ar+ -ions, 2.5 keV) depth profiles of the chemical composition were measured. 3. Results and discussion The Fig. 1 shows an operation of a PEFC over a period of 1600 h with occasional interruptions of the operation and concurrent drying of the cell. The measurement shows that an interruption of the operation and drying of the cell leads to an increase in performance for some time, but that the following decrease of the electrochemical performance is accelerated depending on the total operation time. The measurement implies that a drying of the cell after extensive operation is not as beneficial as in the beginning phase of the operation of the cell. The most likely explanation for such behaviour is a change in the water balance of the fuel cell. An alteration of the hydrophobic character was also observed for PEFC electrodes operated with hydrogen as well as with methanol as fuel [20]. In earlier work it was found that the decomposition of the PTFE measured by XPS is more significant on the anodes than on the cathodes [3]. It is assumed that by the PTFE decomposition, the hydrophobicity of the electrodes decreases and the water balance in the PEFC becomes more critical. A partial loss of the electrochemical performance can be assign to this degradation mechanism. Some of the degradation mechanisms which are discussed in detail in this paper have already been reported before [3,21]. The new results of this paper are the combination of ex situ measurements with impedance spectroscopy and the quantitative determination of losses in the cell and their correlation with the anode or/and cathode and degradation processes. 3.1. XPS-measurements The surface compositions of the commercial electrodes and of the electrodes prepared by the DLR dry spraying technique are different. The depth profile measurements of commercial electrodes shows that a polymer film covers the surfaces; whereas the depth profile of the electrode prepared by the DLR dry spraying technique shows a very homogeneous distribution of the electrode components [3]. Consequently the platinum concentrations determined during depth profiling of both electrode types are different: for the commercial electrodes an increasing platinum concentration during depth profiling is observed, in contrast for the DLR electrodes a constant platinum concentration is observed [3].

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The XPS depth profiles of the commercial electrodes before and after electrochemical stressing differ significantly. For a non-hot pressed membrane-electrode-assembly the platinum concentration on the surface of the anode is strongly changed; the platinum concentration is significantly decreased [21]. For the understanding this experiment it is important that the MEA was not hot pressed and consequently the contact resistance between anode and membrane was higher as in a normal MEA. A higher contact resistance is also associated with a reduced interface area between electrode and membrane. As a consequence at same total current a much higher local current density are expected leading to a significant migration of ions at the non-hot pressed MEA compared with a hot-pressed MEA. In a PEFC with a hot-pressed MEAs the main losses are on the cathode, therefore, normally on the cathodes higher electrical field gradients are than in the anodes due to the large overpotentials on the cathode. The investigation of the alterations of the platinum catalysts has shown that the potential gradient has a strong influence of the migration of the platinum [21]. But normally hot pressed MEAs were investigated by scanning electron microscopy; XPS investigations could not perform because the hot-pressed MEAs cannot be separated in a defined way. XPS investigations of the polymers in gas diffusion electrodes have demonstrated that the PTFE and Nafion can be partially decomposed by the electrochemical stressing [4,19,22–26]. The C1s spectra of three electrodes are presented in Fig. 2. The measurements were performed on commercial electrodes which have a polymer covered surface. Two of the electrodes were electrochemically stressed in a fuel cell operation. The MEA was not hot pressed, so the contact of membranes and electrode was weak and the electrodes could be separated from the MEA after operation in the fuel cell. Due to the absence of a reproducible separation process for electrodes of well prepared, hot pressed MEA, XPS measurements on used electrodes cannot be performed with certain reproducible results. Two different binding states of the carbon can clearly be distinguished. The first at the higher binding energy is related to the PTFE and the other is related to the carbon in the carbon black. The spectrum at the bottom is the spectrum of an unused electrode. The carbon is mainly bonded in the PTFE. The XP spectrum of the electrode, which was used as oxygen electrode, is nearly unchanged as compared to that of a new electrode. The carbon in the graphite state has increased a little bit in this electrode. On the electrode, which was used as the anode, this state has increased strongly. This indicates a change of the PTFE, an effect that is stronger on the anode. Similarly decomposition of the PTFE during electrochemical operation was observed for rolled electrodes for alkaline fuel cells in previous XPS studies [19] and for the PEFC electrodes, which were used in H2 SO4 [22]. Alike the decomposition of the PTFE in gas diffusion electrodes the polymer electrolyte membrane can be also partially decomposed by the electrochemical loading [24,25]. A change of the PTFE in the electrode or the backing induced by the electrochemical operation can explain the change of the influence of the humidification. This hypothesis should be checked with further measurements. Caused by the strong correlation between the PTFE concentration determined by XPS measurement and the hydropho-

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Fig. 2. C1s X-ray photoelectron spectra of new electrode (bottom), and electrodes used in a PEFC cell as cathode (middle) and as anode (top).

bic/hydrophilic behaviour a loss of hydrophobicity induced by the electrochemical stressing of the electrodes can be derived from this investigation. Especially on the anode the hydrophobic character decreases. 3.2. Characterization by scanning electron microscopy Considering the problem to separate reproducibly the electrodes from membrane-electrode assemblies prepared with the DLR spraying technique, the electrodes in DLR MEAs were investigated by scanning electron microscopy. These investigations were performed on cross-sections of stressed and unstressed MEAs. In contrast to the XPS measurements the surface sensitivity of the EDX analysis is significantly lower, but the SEM/EDX gives additional information about the structure. Therefore, the information about the platinum distribution determined by XPS and SEM/EDX measurements can differ. Cross section of a used MEA operated under normal conditions, meaning with purging of anode and cathode, and an unused electrode were investigated by scanning electron microscopy. The different layers in the MEA, gas diffusion layer at the top and at the bottom, the reaction layers of the anode and of the cathode imaged bright and the polymer electrolyte membrane in the middle can be clearly distinguished. Fig. 3 shows reaction zones imaged by SEM with a higher magnification of such

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Fig. 3. Scanning electron microscopy images of the reaction layers of a new electrode (on top), an used anode (middle) and an used cathode (bottom) with 20,000 fold magnification (left side) and 100,000 fold magnification (right side); the cell was operated under normal conditions with pure hydrogen and air.

cross sections. On the left side the reaction zone was imaged with a magnification of 20,000 and on the right side of the same region was imaged with 100,000. At the used beam energy of 20 keV in the SEM images the polymer structures become transparency and the image is more determined by the distribution of the heavy elements like platinum. At the top the reaction layer of a fresh MEA is displayed, in the middle the anodic reaction layer of the operated MEA and at the bottom the corresponding cathodic reaction layer. The SEM images of the new and the operated anodic reaction layers are very similar and show a highly dispersed platinum distribution, which is imaged as a bright cloud. In contrast to the anodic reaction layer, in the cathodic reaction layer an accumulation of the highly dispersed platinum can be observed yielding sharp bright points, which indicates growth of some of the platinum particles in the cathodic reaction layer. The more significant

formation of bigger platinum clusters in the cathode than in anode was also observed in the X-ray diffraction study [27]. This indicates that the platinum distribution is changed significantly by the fuel cell operation. In a further experiment the operation conditions were set that one part of the gas volume was flooded by water and controlled by the current density measurements in the segmented cell (second experiment). The cycle time between purging the cathode was increases to 240 s, the purging of the anode was stopped; the anode side was flooded by reaction water by diffusion of reaction water from the cathode. During the operation time the water level increases until the cell is completely flooded. The current distribution measured with the segmented cell shows that the cell is flooded starting from bottom. The current density maximum shifts from the middle of the active area at the beginning of the experiment to the region at the highest point in the cell. After this experiment the MEA did

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not regain the former electrochemical performance. Under these conditions the accumulation of platinum was observed only in the cathodic reaction layer opposite to the flooded anodic gas volume, in the other parts of the cathodic reaction layer this significant alteration of the platinum distribution could not be observed. An agglomeration of Pt was also observed from other groups, e.g. in an experiment with fuel starvation at constant current whereas the polarity was reserved a significant agglomeration was observed on the anode [28]. It is well known that platinum can be dissolved in acids at high potentials [29]. The dissolution of platinum supported on carbon was observed in cycle experiments potentials between 0.87 and 1.2 V (NHE) [30,31]. On the backside of the gas diffusion layers (GDLs) of the cathodes deposited particles were observed after this experiments. Therefore, additionally to the SEM investigations of the cross sections, the backsides of the GDL directed to the bipolar or end plates were investigated. The particles on the backside of the cathode GDL were identified by EDX as platinum oxide. Fig. 4 shows such a platinum oxide particle with a size of approx-

Fig. 4. Scanning electron microscopy images of a platinum oxide particle formed during fuel cell operation on the backside of the cathode gas diffusion layer in a flooded area, the cell was operated under dead end conditions with pure hydrogen and oxygen.

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imately 50 ␮m. The platinum oxide particles are formed during the fuel cell experiments. The distribution of the big platinum oxide particles is strongly inhomogeneous; particularly on the area of the cathode backing which were opposite to the water flooded anode area where the big particles were formed. In the area opposite to the “dry” anode part, no big platinum oxide particles were found. On the anode backing no platinum or platinum oxide could be observed. An alteration of the platinum distribution during cyclic voltammetry measurements was also found with nuclear magnetic resonance (NMR) [31]. 3.3. Characterization by electrochemical impedance spectroscopy (EIS) To investigate in detail the degradation mechanism of the electrodes in a PEFC single cell, a MEA was operated in a longtime test, galvanostatically, at a current density of 500 mA/cm2 . From time to time altogether 13 EIS measurements were performed without interrupting the fuel cell operation the corresponding measuring time is indicated by arrows in Fig. 5. As shown in Fig. 5, after an operation time of 1000 h an additional nearly linear time dependent voltage loss of about 140 mV could be observed, corresponding to a voltage loss (degradation) rate of 140 ␮V/h equivalent to a power density loss rate of 70 ␮W/h/cm2 . The recorded EIS measurements, represented in Fig. 6 as Bode diagram also shows a strong time dependency. A linear increase of the impedance with operation time, especially in the higher frequency range (1–20 kHz) of the impedance spectra could be observed. After a complete shut down of the cell after an operation time of 1000 h and restarting the cell after 24 h at OCV, the cell showed nearly the same performance as at the beginning of the long-time test and the impedance spectra recorded directly after the restart shows similar values as the impedance spectra recorded at the beginning of the lifetime test. After shut down for 24 h and restarting cell operation the lifetime test was continued for other 720 h at the same operating conditions as before, similar to the experiment shown in Fig. 1. During this second test period a more pronounced voltage loss

Fig. 5. Time dependency of cell voltage during long-term operation at 80 ◦ C and 500 mA/cm2 , arrows indicating the time when EIS is measured.

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Fig. 8. Time dependence of Rct(A) during the long-time test.

Fig. 6. Bode diagram of EIS measured at different times during 1000 h at 80 ◦ C and 500 mA/cm2 .

(degradation) was observed corresponding to a degradation rate of 270 ␮V/h. An equivalent circuit can be applied for the simulation of the measured impedance spectra of the PEFC. The measured impedance spectra can be described by a model (equivalent circuit) of elementary impedance elements. The numeral values of the parameters are calculated by a fitting procedure of the model to the measured data. The measured data are represented in the Bode diagram (Fig. 6) as symbols and the modelled curves as lines. By varying the experimental conditions such as current load, temperature, gas composition and humidification measured cell impedance can be split up into anode impedance, cathode impedance and electrolyte resistance without using reference electrodes. The variation of the experimental conditions is also a useful method to confirm the accuracy of the equivalent circuit. The equivalent circuit was derived by changing different operating conditions of the PEFC like gas composition, cell voltage and electrodes, for details see [32–34]. Besides a series resistance (membrane resistance RM ), the equivalent circuit (Fig. 7) contains three time constants of parallel R/C. In the equivalent circuit Rct(A) and Rct(C) are related to charge transfer resistance on the anode and on the cathode, the capacitances Cdl(A) and Cdl(C) are related to the double layer

Fig. 7. Equivalent circuit used for the evaluation of measured EIS.

capacity of both electrodes. The diffusion processes will be simulated also by an RC-element (RN and CN ) and the membrane resistance by RM . In the simulation the capacitance (C) was replaced by CPE (CPE = constant phase element) due to the porous structure of the electrodes. The cathode can be described using two time constants, one for the charge transfer through the double layer (Rct(C) /CPEdl(C) , the exponent of the CPE is around 0.85, for an exponent of 1 the CPE is equal with the capacitance) and one for the finite diffusion of water with a Nernst-impedance like behaviour (R(N) /CPE(N) , the exponent of the CPE is around 0.95). The time constant of the anode (Rct(A) /CPEdl(A) , the exponent of the CPE is around 0.80) is given by the charge transfer through the anode double layer. The resistance of the membrane RM = 5.65 ± 0.10 m and the resistance related to the diffusion RN = 2.50 ± 0.5 m were both nearly constant during the whole experiment. The most time sensitive impedance elements are the charge transfer resistance of the anode Rct(A) and the charge transfer resistance of the cathode Rct(C) . The time dependence of Rct(A) is shown in Fig. 8. The contribution of Rct(A) to the overall cell impedance increased at the beginning of the experiment from 2.5 m to nearly 9 m after 1000 h of operation. After a new start up of the cell, the resistance reached nearly at the same value (3.0 m) as at the beginning of the experiment. The time dependence of Rct(C) is shown in Fig. 9. The contribution of Rct(C) to the overall cell impedance increased at the beginning of the experiment from 10.5 m to nearly 16 m after 1000 h of

Fig. 9. Time dependence of Rct(C) during the long-time test.

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Fig. 10. Breakdown of voltage (performance) losses resulting from long-term operation into reversible and irreversible degradation after evaluation of EIS.

operation. After the new start up of the cell, the charge transfer resistance of the cathode decreased at a value of 14 m, thus the reversible part, defined as the difference between the value after 1000 h of operation (16 m) and the value after start up (14 m) is 2 m. The irreversible part is defined as the difference between the value after start up and the value at the beginning of the longterm operation (10.5 m). Taking into account that the surface area of the cell is 23 cm2 and the current density is 0.5 A/cm2 we obtain the total current of 11.5 A. Using Ohm’s low and the exact resistance values from Figs. 8 and 9 we are able to calculate and to separate the voltage losses of the cell during long-term operation into reversible and irreversible voltage losses and furthermore into contributions of the anode and cathode. The result of the separation into different voltage losses is shown in Fig. 10. From this we can calculate that total voltage loss of the cell during 1000 h of operation is 137.4 mV. The greatest part of the voltage loss is a reversible loss (88.8 mV) and 48.6 mV is an irreversible voltage loss. The voltage loss related to the anode can be revoked and the irreversible part of voltage loss is small at the anode (5.4 mV) compared to the irreversible voltage loss at the cathode (43.2 mV). 4. Conclusions The electrochemical performance of low temperature fuel cells degrades during operation. This degradation can be separated into reversible (recoverable after shut downs) and irreversible parts. The combination of electrochemical investigations of the electrodes and electrode-membrane-assemblies during the fuel cell operation with interface characterization by physical methods facilitates the identification of the important degradation processes. The different processes are associated with reversible or irreversible performance degradation. In order to quantify the effect of the different degradation processes the MEAs were investigated by electrochemical impedance spectroscopy. The EIS measurements allow to separate the losses of the electrochemical performance and to distinguish between the degradation processes in the anode and in the cathode. The EIS measurement after interruption of the fuel

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cell operation allows also to distinguish between the reversible and the irreversible degradation processes. The surface science investigations allow to identify the degradation processes. Two different degradation processes were identified: the agglomeration of the platinum catalyst mainly in the cathode and the disintegration of the PTFE and the correlated decrease of the hydrophobic degree. The loss of the hydrophobicity is more significant on the anode than on the cathode. The combination of the electrochemical and surface analytic methods allows to come to a better understanding of the degradation process and an interpretation of the change of the electrochemical performance during long-term operation without a predetermined model. The irreversible degradation is probably attributable to a catalyst structure change which is detected by surface analytical methods. The platinum catalyst agglomerates during fuel cell operation. As consequence the active surface of the catalyst in the cathode decreases and the electrochemical performance decreases concurrently. In contrast to the decrease of the hydrophobicity the loss of active surface area cannot be compensated by modification of the operation conditions. This degradation is therefore an irreversible process. Under extreme operation conditions when the electrodes are flooded with water the platinum can moves across the gas diffusion layer. This indicates that the mobility of the platinum is related to a liquid water phase. The reversible degradation is related to the decomposition of the PTFE and respectively the decrease of the hydrophobicity and the correlated alteration of the water balance. This degradation process will mainly affect the performance of the anodes. So the electrochemical performance increases after an interrupt of operation and drying the fuel cell. The changed hydrophobicity or the related alteration of the PTFE yields a decrease of the electrochemical performance which can be compensated by modification of the water balance, e.g. the water balance can be modified by adding of purging intervals and adapting the periods between purging and the length of the purging intervals. The mechanism of the decomposition of the PTFE due to fuel cell operation is not known and should be investigated in future studies. The quantification of the effects of both degradation processes has shown that the reversible degradation processes are more significant than the irreversible processes. Therefore, it is very important for long-term experiments and for the long-term stability to take the experimental conditions into account which are used to determine degradation values; especially if the experimental set-up eliminates the effect of the reversible degradation process. The both important components of the electrode – platinum catalyst and PTFE – are not long-term stable. Therefore, alternative materials are necessary for new materials to give the anodes hydrophobic character and new catalyst materials for the cathodes. Acknowledgements The authors thank the ministry of research and education of the Germany for the financial support of the projects “Langlebige

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