Ultralow Degradation Rates in HT-PEM Fuel Cells. Oldenburg, Carl-von-Ossietzky Str. 15, Oldenburg, Germany. Czech Republic

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1 Ultralow Degradation Rates in HT-PEM Fuel Cells M. Rastedt a, F.J. Pinar a, P. Wagner a, H.R. García b, T. Steenberg b, H.A. Hjuler b, M. Paidar c and K. Bouzek c a NEXT ENERGY EWE Research Centre for Energy Technology at the University of Oldenburg, Carl-von-Ossietzky Str. 15, Oldenburg, Germany b Danish Power Systems ApS, Egeskovvej 6C, 3490 Kvistgård, Denmark c University of Chemistry and Technology-Prague, Technická 5, Praha 6-Dejvice, Czech Republic One of the major advantages of polybenzimidazole (PBI) based high temperature polymer electrolyte (HT-PEM) fuel cells compared to the low temperature representatives of this type of fuel cell technology is the higher tolerance against impurities like CO. Nevertheless lifetime and durability are still an issue. In this work, an improvement in degradation rates and therefore an extension of lifetime of HT-PEM fuel cells will be presented. Extended long term tests have been performed at different facilities under similar test conditions. After 3,000 h of operation, an average degradation rate of -1.7 µv/h has been achieved. One of the tests is still under operation; this MEA already reached a lifetime more than 9,000 hours with an actual degradation rate of -3 µv/h. Introduction The vision of the European project CISTEM (Construction of Improved HT-PEM MEAs and Stacks for Long Term Stable Modular CHP Units) is to develop an improved fuel cell based combined heat and power (CHP) technology, which is suitable for fitting into large scale peak shaving systems in relation to wind mills, natural gas and SMART grid applications. The technology should be integrated with localized power/heat production in order to utilize the heat from the fuel cell via district heating and should deliver an electrical output of up to 100 kw el. Additionally, the CHP system is fuel flexible, which has been already successfully tested on single cell level (1-3). The unit is designed modularly, with fuel cell modules consisting of two 4 kw el stacks and one reformer. This strategy of numbering up will achieve an optimal adaption of the CHP system size to a very wide area of applications like different building sizes. Within CISTEM an ideal new fuel cell technology for the special requirements of a CHP system in relation to efficiency, costs and lifetime will be developed. Durability and degradation rates of HT- PEM fuel cells are still an issue (4) and are the most important tasks of this project and are in the focus of this work. The desired lifetime target of HT-PEM fuel cells adds up to 40,000 hours for stationary applications (5-7). The best degradation rate for this kind of HT-PEMFC technology ever published has been found to be -4.9 µv/h for a 2,500 hours steady-state long term operation test was published by Yu et al. in 2008 (8). Moreover, Schmidt et al. have published similar degradation (-5.1 µv/h) rates (9). In another publication, Schmidt et al. pointed out that the requirements for membranes in HT-PEM

2 fuel cells to achieve an extended durability are a very challenging endeavor due to the required mechanical and chemical stability and needed capability for sufficient proton conductivity without humidification (10). These fairly high degradation rates, compared with the LT-PEM fuel cells, and therefore the succinct lifetime are the major reasons that the world-wide commercialization of HT-PEM fuel cell is rather slowly advancing (4, 7, 11). The purpose of the CISTEM project is to show the proof of concept of high temperature PEM-MEA technology for large combined heat and power (CHP) systems. The project was chosen by the Fuel Cell and Hydrogen Joint Undertaking (FCH-JU) to also advance lifetime through ultralow degradation rates of less than -4 µv/h. The tests have been performed with Dapozol -G55 MEAs at the facilities of three different partners (DPS, University of Chemistry and Technology Prague (UCTP), NEXT ENERGY (NEXT)). The distribution of new MEAs and testing at three different lab facilities under agreed identical operating conditions support and verify achieved degradation results perfectly. Electrochemical investigation with polarization curves have been performed at Begin of Life (BoL), every 1,000 hours of operation and at the End of Test (EoT). The characterization has been completed with ante- and post-mortem microcomputed tomography (µ-ct). To verify the influence of different test conditions on the degradation rates, these experiments have been compared with constant load (0.3 A/cm²) long term tests under different fuel and oxidant compositions and performed with complete in-situ electrochemical characterization procedures (polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry and linear sweep voltammetry). These investigations enable the determination of the best possible operating conditions for HT- PEM fuel cells to achieve ultralow degradation rates. Membrane Electrode Assembly Experimental All ultralow degradation experiments at each facility have been carried out with new Dapozol -G55 MEAs with polybenzimidazole (PBI) membrane and Pt/C based electrodes developed and provided by Danish Power Systems (Denmark). The membrane polymer electrolyte of the G55-MEAs consists of phosphoric acid doped meta-pbi; the acid content is approx. 8 to 9 phosphoric acid molecules per repeat unit (monomer) of PBI (12). These membranes underwent thermal treatments; in this point these MEAs differ from the State-of-the-Art (SoA)-MEAs from Danish Power Systems used in the experiments. Non-woven carbon cloth has been inserted as Gas Diffusion Layer (GDL). The nominal active surface area adds up to 25 cm 2. Due to proprietary issues, the exact active surface area and further details about the MEA assembly cannot be specified here.

3 Cell Compression Unit and Fuel Cell Test Station The fuel cell test equipment differs from each facility. A detailed description is there given for each lab: At DPS facilities the MEAs are tested in customized fuel cell test units built in-house. The used flow fields were 5-fold serpentine flow fields (SFF) along with gold plated brass current collectors. The single cell is compressed by tightening the fuel cell unit to 2 N m thus; the usage of spring washers on the aluminum end plates ensures constant pressure operation. Appropriate Teflon gaskets are used on both anode and cathode side. At UCTP, an in-house manufactured test station was used. The used cell with endplates with triple serpentine flow field from high density pure graphite was provided by WonATech (Korea). The compression was realized by torque wrench (5 N m) and appropriate thickness of flat PTFE gasket. During the experiment the stoichiometry was held constant with 1.3 for pure hydrogen and 2 for air. At NEXT ENERGY, commercially available cell compression units (CCU) balticfuelcells (Germany) have been used within the fuel cell test stations and electrochemical analysis. An accurately adjustable compression force is executed by the CCUs via a hydraulically controlled piston onto the active surface area of the MEA. The CCU systems have been operated in constant pressure mode with a nominal contact pressure of 0.75 MPa (13). The used flow fields were serpentine flow field (SFF) with a land area of 13.4 cm² for each side. Therefore the absolute contact pressure amounts to 1.4 MPa. A more detailed discussion can be found here (14). A displacement sensor simultaneously monitors changes in MEA thickness with a sensitivity of ± 1 µm and was used only for the fuel cell test NEXT-2. NEXT ENERGY operated two test benches commercially available (Evaluator C50- LT and C1000-LT from Fuel Con AG, Germany (13)) in combination with the CCU s from balticfuelcells. During the experiments, constant stoichiometry was chosen for all fuel cell tests: 1.5 for pure hydrogen, 1.3 for dry or wet reformate on the anode side. For the cathode gas supply, a stoichiometry factor of 2 was set for air and a factor of 2.85 for oxygen enriched air. Long term tests were usually conducted for 1,000 h (of operation or until reaching end of life (EoL = 90% BoL, begin of life). The required duration for extended long term testing was min 2,000 hours.

4 Fuel Cell Test Procedure Table I summarizes the evaluated tests and their operation conditions: Table I: List of abbreviation of the evaluated test conditions. Abbreviation DPS UCTP NEXT-UL NEXT-1 NEXT-2 NEXT-FS Test conditions Extended long term test (H 2 /air) at 0.3 A/cm², IV-curve every 1,000 h, performed at the facilities of DPS, runtime > 2,000 h Extended long term test (H 2 /air) at 0.3 A/cm², IV-curve every 1,000 h, performed at the facilities of UCTP, runtime > 2,000 h Extended long term test (H 2 /air) at 0.3 A/cm², IV-curve every 1,000 h, performed at the facilities of NEXT ENERGY, runtime > 2,000 h Long term test (H 2 /oxygen enriched air) at 0.3 A/cm², full MEA weekly characterization Long term test (wet reformate (54% H % CO % H 2 O)/oxygen enriched air) at 0.3 A/cm², full MEA weekly characterization Long term test at 0.3 A/cm², fuel switching with 1 h H 2 and 5 h synthetic reformate (78% H 2 and 22% CO 2 ) /oxygen enriched air, full MEA characterization weekly The fuel cell test procedure is divided into two sections: The extended long term testing under the required conditions pronounced by the FCH JU and the long term test at constant load for different gas compositions. Extended Long Term Testing (Ultralow Degradation Target). The test conditions for these extended long term test have been defined as: Single cell test with 25 cm²-meas Constant current density: 0.3 A/cm² Hydrogen and air supply (λ=1.5/2) Temperature: 160 C Min. test duration: 2,000 hours Min. MEAs with ultralow degradation rates: 3. The target is to realize ultralow degradation rates achieving less than -4 µv/h. At DPS a long term test was carried out at 0.3 A/cm 2 after undergoing fuel cell activation at 0.2 A/cm 2 approximately 120 hours, which is considered Beginning of Life (BoL). Three polarization curves were obtained in galvanostatic mode up to approximately 0.6 A/cm 2 at 120, 1,000 and 2,000 hours. The electrochemical characterization was performed by using a DC load H&H ZSxx series 500W. The operational temperature is fixed to 160 C. The stoichiometries of pure hydrogen and air were set to 1.5/2 respectively, by digital mass flow controllers (SLA5850, Brooks Instruments).

5 UCTP realized the test under conditions defined by the FCH JU. After break-in period the constant current density 0.3 A/cm 2 was set. Each 1,000 h an IV curve was recorded. All other electrochemical characterization was realized by use Autolab PGSTAT302N (Netherlands) with 20 A booster. Post mortem analysis is still not possible at this point of time because the MEA is still under operation. At NEXT ENERGY several extended long term tests under the required conditions have been performed. For the whole series of measurements a constant current density of 0.3 A/cm² and temperature of 160 C were chosen. The extended long term test (NEXT UL) was operated under hydrogen and air, while the gas composition for the other three tests differs. Long Term Testing. At NEXT ENERGY facilities another three long term tests have been carried out with different gas compositions than the previous extended long term tests, but with SoA MEAs (no thermal treatment). All tests were performed at constant load conditions (0.3 A/cm²) and 160 C. The oxidant composition for these investigations was identical: oxygen enriched air (30% O 2 ). The test NEXT-1 was performed with pure hydrogen and NEXT-2 with wet reformate (54% H % CO % H 2 O) as fuel. NEXT-FS was a fuel switching test and consist of switching the fuel between pure hydrogen and dry reformate operation (78% H % CO 2 ) over time. A fuel switching cycle consists of 5 hours operation with synthetic reformate followed by 1 hour operation with pure hydrogen. The MEAs were mounted in cell fixtures and placed into the CCUs. The fuel cell was heated up under gas supply of nitrogen (0.3 L/min) at both sides until 120 C has been reached. After this point, the fuel and oxidant supply was started and a current density of 0.3 A/cm² was applied. Afterwards the cell was heated up to 160 C. The following break-in period lasted 100 h. After this activation phase, an initial electrochemical characterization with polarization curves has been performed for the extended long term test. For the additional long term tests the characterization has been expanded and the following methods (13, 15, 16) have been executed: Polarization curves; Electrochemical impedance spectroscopy (EIS); Cyclic voltammetry (CV); Linear sweep voltammetry (LSV). EIS-, CV- and LSV-measurements have been performed using an external potentiostat type Modulab 2100A from Solartron Analytical, United Kingdom (17). For EIS measurements, a potentiostatic mode was kept and an AC voltage was applied with an amplitude perturbation of 10 mv r.m.s. The applied frequency range varied between 100 mhz and 100 khz. During the CV measurements, nitrogen was passed through the cathode (working electrode, 0.1 L/min) and hydrogen through the anode (counter and pseudo reference electrode, 0.1 L/min); several CV scans from 0.05 to 1.0 V with a scan rate of 100 µv/s have been performed. For the LSV investigations, the flows on both electrodes have been increased to 0.3 L/min and the LSV was performed with a potential between the initial rest and 0.5 V and a sweep rate of 2 mv/s.

6 This test procedure was repeated weekly for the long term tests. For the extended long term tests (DPS, UCTP and NEXT UL) only polarization curves were iterated every 1,000 hours. After finishing the break-in procedure and initial characterization, the fuel cell operation has been switched back to constant current density of 0.3 A/cm² and was only interrupted to perform the characterization mentioned before. At the end of test, a final characterization has been performed. Micro Computed Tomography The µ-ct investigations have been performed ex-situ ante- and post-mortem and have been carried out with a micro-computed X-ray tomography system (Skyscan 1172 Desktop-Micro-CT, Bruker, Belgium). The µ-ct allows the imaging of complete sample volumes by taking a large number of radiographs at different angles. Subsequent image processing creates three dimensional representations of the recorded data. For sample preparation, MEAs have been cut with a standard punching tool with a diameter of 6 mm. The setting of parameters used in the µ-ct is specified in Table II. The thickness values of all single MEA layers have been calculated from min. 10 values of five cross sectional 2D images (sagittal and coronal) with help of the software Dataviewer. The 3D-images and models are generated via CT Vox. The average pore diameters have been determined by the 3D- and individual 3D-analyses of a centered GDL layer of both electrodes by CT Analyser. The binary thresholds have been set to 20 and 80 for all µ-ct analysis procedures. TABLE II. µ-ct operational parameters. Parameter Value Acceleration voltage (kv) Sample size Ø (mm) 6 Rotation step ( ) 0.2 Random movement 10 Averaging 4 Optical resolution (µm) Duration 2 h10 min Temperature ( C) Results and Discussion In the first part of this section we will focus on the results of the fuel cell testing, electrochemical characterization of the long term test operated under different gas compositions and the polarization curves of the extended long term test. We will emphasize the outcome of the micro-computed tomography investigations and the pore size and porosity calculation in the second part of this section. Fuel Cell Testing The fuel cell voltage behavior as function of time for all tested MEAs, long term and extended long term tests, are shown in Figure 1. The break-in phase is not shown here for

7 most of these fuel cell tests, the only exception is the experiment executed at DPS. The weekly MEA characterization is displayed by black points for the experiments performed at NEXT ENERGY, for Figure 1c) the black points represent the executions of the polarization curves and in Figure 2d) f) the complete electrochemical MEA characterizations are displayed through these dots. Extended Long Term Testing (Ultralow Degradation Target). Figure 1a) represent the extended long term test performed at the facility of DPS. The duration of this test sums up to 2,900 h (without break-in phase). The first 30 hours belong to the break-in phase where current density was selected to be 0.2 A/cm 2 and initial polarization curves. It is clearly visible, that this test went through many disturbances. At a runtime of approx. 750 h of operation, a power shut-down occurred, which seemed not to affect the MEA immediately, the fuel cell voltage exhibited only a slight increase. At operation time 1370 h, the MEA suffered fuel starvation which caused a drop of performance. After this event, the MEA followed a relaxation phase whereby the MEA was held at a lower current density of 0.2 A/cm² for 160 hours and finally set again to the operational conditions of 0.3 A/cm 2. The signal at 2,500 h was a response to polarization curve investigations on second experimental set up. The test was finished after 3,000 hours of accumulative operation and resulted into a degradation rate of -4.7 µv/h. In Figure 1c), the extended long term test at UCTP is shown. Up to now, this test is still running and has passed 9,000 hours of operation. As it is visible from the curve, the cell operated approximately 100 h at OCV shortly after Begin of Life (BoL). It caused drying of the cell and it took over 200 h to recover the cell performance. The highest performance was found just before this failure but it is evident that the cell performance increases continually up to that time. Several other load shut-downs (back to OCV mode) occurred during the test but only for short periods of time. The gap in data between 3,670 h and 4,440 h was caused by a failure in data recording, but the cell continued to operate under constant current density 0.3 A/cm 2. Due the electricity switch off at hour 2,640 of operation, the cell was flushed by nitrogen and cooled down. The restart did not show any change in cell performance but as shown on Figure 1d) OCV decreased significantly, but the performance drop at the current density of 0.3 A/cm² was much less pronounced. The degradation rate for this long term test is -3 µv/h after 9,000 h of operation. The last extended long term test was operated at NEXT ENERGY and is displayed in Figure 1e). The break-in phase is not shown; the graph starts directly at BoL. While comparing the initial fuel cell voltage of this test with the starting value of the UCTP- (0.62 V) and the DPS-test (0.59 V), it can be seen, that the MEA NEXT UL exhibits the lowest value (0.56 V). Nevertheless, fuel cell performance is increasing over time and reached its maximum after 1,000 hours of operation, right after the second polarization curve. From this time on till the end of operation (3,000 h), the fuel cell voltage slightly decreased over time. During this test no irregularities occurred, i. e. fuel starvation or electrical black-out periods. The overall degradation rate is still positive (+2 µv/h) which means the fuel cell performance was still higher than BoL after 3,000 h of operation.

8 Figure 1. Fuel cell voltage as function of time under extended long term test conditions under H 2 and air, 160 C, ambient pressure, load = 0.3 A/cm², Dapozol -G55-MEAs, SFF, a) DPS, c) UCTP, e) NEXT UL and selected points of polarization curves for these fuel cells, b) DPS, d) UCTP and f) NEXT UL. The Figures 1b), d) and f) show the selected point of the polarization curves for the extended long term tests. In Figure 1b), it can be seen that the fuel cell performance was higher than BoL one. As discussed in Figure 1a), the first fuel cell shut-down of DPS test did not affect the fuel cell performance but the next shut-down and hydrogen starvation periods may be the cause of the lower fuel cell performance after 2,000 h of operation. Figure 1d) shows UCTP performance for each characterization. In this case, the MEA displayed an activation phase for roughly 3,000 h of operation, subsequently degradation phase started. It can be also observed that at 3,000 h the OCV was stepwise reduced as a consequence of an electrical blackout so hydrogen crossover through the membrane electrolyte might increase, although no LSV was recorded for this kind of testing to

9 investigate this membrane property. Nevertheless, it seems that hydrogen crossover does not have a high impact on fuel cell performance for this kind of fuel cell technology (13). The test performed at the facilities of NEXT ENERGY (Figure 1f)) has shown an increasing performance up to 1,000 h (day 42) of operation as well as Figure 1c) has shown. Therefore, this MEA has shown a shorter activation phase than the one tested in UCTP facilities. All mentioned degradation rates of the extended long term tests are listed in Table III, the average degradation rate for all extended long term test is -1.7 µv/h at 3,000 h of operation. This value and the degradation rate of -3.0 µv/h for hour test (UCTP) represent the lowest ever published degradation rates for phosphoric acid based PBI HT- PEM fuel cell technology. With these values the ultralow degradation rate target has been reached. TABLE III. Degradation rates of long term tests under constant load of 0.3 A/cm² with different reactant gases and of extended long term tests with BoA MEAs. MEA tested Degradation rate at different operation times 2,000 h 3,000 h 9,000 h DPS -5.3 µv/h -4.7 µv/h - UCTP +3.8 µv/h -2.4 µv/h -3.0 µv/h NEXT UL µv/h +2.0 µv/h - Long Term Testing. Figure 2a) shows long term test with pure hydrogen and 30% oxygen enriched air. This test went through a weekly complete electrochemical characterization. The fuel cell voltage has been improved during the first week of the test. It can be also observed that after the first MEA characterization (165 h), voltage has shown a step-down behavior followed with constant values over time until the second MEA characterization (335 h). Therefore, MEA characterization techniques may accelerate degradation of fuel cell materials as stated in previous investigations (4). From this point, voltage has developed a smooth linear reduction over time and the overall voltage loss rate has been μv/h (1,000 h) which is much higher than the degradation rates observed in the Extended Long-Term test (see Table III) as MEA characterization was conducted smoother than for the present testing. The second long term test with verified gas supply is displayed in Figure 2c). This test has a runtime of 1,000 hours with wet reformate (54% H % CO % H 2 O) and 30% oxygen enriched air. The starting voltage value of 0.58 V directly after the break-in phase is comparable with the one from NEXT UL. During the first approx. 350 hours of operation a slight increase of the fuel cell voltage can be observed. After the third MEA characterization a voltage stepwise decrease has been measured, this behavior continues after every following electrochemical characterization in a reduced manner.

10 Figure 2. Fuel cell voltage as function of time under long term test conditions under H 2 and air, 160 C, ambient pressure, load = 0.3 A/cm², Dapozol -G55-MEAs, SFF, a) NEXT-1 under H 2 and 30% O 2 enriched air, c) NEXT-2 under wet reformate (54% H % CO % H 2 O) and 30% O 2 enriched air and e) NEXT-FS fuel switching test with 5 h synthetic reformate (78% H 2 and 22% CO 2 ) and 1 h H 2 alternating/30% O 2 enriched air and selected points of polarization curves for these fuel cells, b) NEXT-1, d) NEXT-2 and f) NEXT FS, j in A/cm²: 0 (black), 0.3 (red), 0.5 (blue), 0.6 or 0.7 (grey). The last long term test is the one operated under fuel switching conditions, presented in Figure 2e). A difference in voltage between the operation with hydrogen and the operation with synthetic reformate can be recognized, this hydrogen-gain constitutes 10 mv for the first cycle and 13 mv for the last cycle. In the first week of fuel cell test duration, the voltage increases until the second MEA characterization, after 200 hours the voltage starts to decrease, after each characterization a slight stepwise decrease can also

11 be observed. This ends up in a degradation rate of µv/h for hydrogen and µv/h for synthetic reformate and 1,650 h of operation. In Figure 2b), d) and f), the selected point of polarization curves for the three longterm tests carried out at different operation conditions are presented. In general, the activation phase of the three tests is shorter than the one shown by the extended long term tests of the previous sections. Thus, larger number of MEA characterizations and different operation conditions than pure hydrogen and air may result to enhance degradation of materials, so degradation starts at an early stage. After the activation phase, the tests performed under H 2 /30%O 2 (Figure 2 b)) and fuel switching (Figure 2f)) underwent a performance loss, while the test performed with humidified fuel (Figure 2d)) kept almost constant. Therefore, fuel humidification may improve proton conductivity and lifetime. Nevertheless, this behavior should be further investigated. The degradation rates of the evaluated long term tests are presented in Table IV: TABLE IV. Degradation rates of long term tests under constant load of 0.3 A/cm² with different reactant gases and of extended long term tests. MEA tested Degradation rate Operation time [h] NEXT-1 NEXT-2 NEXT FS Micro Computed Tomography µv/h -7.6 µv/h µv/h (H 2 ) µh/h syn. ref. 1,050 1,050 1,650 1,650 With aid of the post-mortem ex-situ μ-ct investigations, it is possible to visualize mechanical defects and degradation processes on a microscopic scale. In addition to postmortem analyses, an ante-mortem µ-ct-investigation of a pristine MEA from the same type has been performed. Figure 3 displays the two-dimensional images from the reconstructed data set of the extended long term test of DPS and NEXT ENERGY and of NEXT-1. Next to the transaxial images of the membrane and the cathode catalyst layer, a cross-section of the complete MEA through all layers (sagittal or coronal) is presented. These 2D-images have been also used to determine the post-mortem layer thicknesses with the help of the software program Dataviewer. The average layer thickness values are listed in Table IV. The reconstructed 2D-images of the reference MEA are shown in the undermost row of Figure 3, mirror symmetry with the membrane as center can clearly be observed. The pristine MEA shows similar thickness values for both electrodes and does not exhibit defects or irregularity. The post-mortem images for the MEA tested under fuel cell conditions reveal cracks within the both catalyst layers. These cracks might support the phosphoric acid flooding in the BPP channels, as stated by Eberhardt et al. (18). In this paper the cathode catalyst layer will be show exemplarily, the images for both cathode layers (anode and cathode) are similar.

12 While comparing the cross sections of all tests (DPS, NEXT UL and -1) and of the pristine MEA, it can be determined that the long term test with weekly full characterization show the highest amount of defects and additionally delamination. The catalyst layers of all post-mortem MEA exhibit, next to the increase of cracks, a modification and irregularity in the thickness, some areas show thinning, especially for the MEA tested at DPS, and other parts of catalyst layer reveal a slight increase in thickness (see red circles in Figure 3). But nevertheless, thickness values of both electrode layers (anode and cathode) are quite similar for both extended long term test and the reference MEA, but for the MEA tested at DPS membrane thinning can be observed while examining the cross section (Figure 3) and the membrane thickness values (Table V). Reason might be the interruptions during operation. But it must be pointed out that the coronal and sagittal 2D-images do not reveal any defects like pinholes. In the 3D µ-ct images presented in Figure 4, it can be observed that the NEXT UL-MEA shows, except from the cracks in catalyst layer, nearly no modification, even the mirror symmetry was retained. This is in conjunction with the thickness values presented in Table III. Figure 3. Reconstructed 2D-images (Dataviewer) of the complete MEA, the cathode catalyst layer and the membrane gained by post-mortem µ-ct measurements for DPS, NEXT UL and NEXT-1 and of pristine MEA. For the long term test under pure hydrogen and 30% oxygen enriched air (NEXT-1) the delamination of MEA-layers are clearly seen at the 2D- and 3D- reconstructed images (Figure 3 and 4), but next to the typical drying cracks in the catalyst layers no further mechanical defects can be observed. The membrane exhibits a stable, defect-free and homogenous behavior while the CL- and GDL-layers for both electrodes increased and the micro porous layer thickness reduced. The NEXT-1 MEA manifests the most changes

13 (Table V) and defects in CL (Figure 3). These observations are in agreement with the results from electrochemical investigations. Table V. Average layer thicknesses, determined by Dataviewer. Long Term Test under constant load (0.3 A/cm²) Reference ante-mortem DPS NEXT UL NEXT-1 GDL Cathode 177.4±13 µm 181.1±14 µm 190.7±7 µm 204.8±16 µm MPL Cath. 54.5±9 µm 52.2±7 µm 53.5±6 µm 47.6±6 µm CL Cathode 24.0±6 µm 23.0±8 µm 28.9±6 µm 42.8±9 µm Membrane 56.7±8 µm 41.5±5 µm 54.4±11 µm 61.0±7 µm CL Anode 24.4±9 µm 22.7±5 µm 31.9±6 µm 30.7±10 µm MPL Anode 59.3±6 µm 59.8±7 µm 49.7±8 µm 46.3±10 µm GDL Anode 172.1±13 µm 187.3±23 µm 169.5±12 µm 210.9±17 µm Total 560.4±27 µm 570.0±20 µm 577.8±22 µm 644.1±21 µm Table VI. Pore spaces and porosities, determined by CT Analyzer. MEA Anode/ Cathode Open pore space / µm Total pore space / µm Open porosity [%] * Closed porosity [%] * Total porosity [%] * DPS Anode DPS Cathode NEXT UL Anode NEXT UL Cathode NEXT-1 Anode NEXT-1 Cathode Reference MEA Anode Reference MEA Cathode * rounded values In Table VI and VII the post-mortem porosity and average pore sizes of the MEAs DPS, NEXT UL and NEXT-1 and the ante-mortem values from the same type of MEA are displayed. The pristine MEA, DPS and NEXT-1 show similar values, total porosity of 27 to 37 %, in which the main part is allotted to closed porosity. The extended long term test with NEXT UL behaves totally different; this MEA exhibits a total porosity of 66 %, which is nearly completely affected through open pore areas. The same can be monitored for the pore sizes, for all tests the pore sizes are in the same range, except for the NEXT MS test; here the pore sizes are twice as high compared with the pristine MEA. For DPS operated under the same conditions like NEXT UL, a slight increase (1-3 µm) can be observed as well, while the post-mortem pore size values of the NEXT-1 matches perfectly with the values of the reference MEA.

14 Figure 4. Reconstructed 3D-images (CT Vox ) of the complete MEA, the cathode catalyst layer and the membrane gained by post-mortem µ-ct measurements for DPS, NEXT MS and NEXT-1 and of pristine MEA. TABLE VII. Average Pore-diameter, determined by CT Analyzer. Reference ante-mortem DPS NEXT MS NEXT-1 GDL Cathode 8.1±1 µm 9.1±1 µm 16.4±1 µm 7.8±1 µm GDL Anode 8.2±1 µm 11.5±1 µm 16.8±1 µm 8.1±1 µm Conclusion The single cell fuel cells presented in this work reached ultralow degradation rates of averaged -1.7 µv/h. The test at UCTP has reached -3 µv/h after more 9,000 h of operation. Both values represent the lowest ever published degradation rates for phosphoric acid based PBI HT-PEM fuel cell technology. The fuel cell activation phase of these thermally cured new Dapozol -G55 MEAs may last up to 3,000 h which may also support achieving longer lifetimes. MEA characterization techniques (incl. CV and LSV) and uncontrolled shut-down periods enhance degradation of fuel cell materials. The MEA which underwent the complete electrochemical characterization revealed a slight increase of catalyst and gas diffusion layer-thicknesses, shown via µ-ct investigations. Acknowledgments The authors would like to thank the European Commission as this work was supported by the Seventh Framework Programme through the project CISTEM (Grant Agreement

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