Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging

Energy xxx (2016) 1e10

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Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging Tobias Arlt a, c, *, Merle Klages b, Matthias Messerschmidt b, Joachim Scholta b, Ingo Manke a a b c

Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109, Berlin, Germany Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg, Helmholtzstraße 8, 89081, Ulm, Germany €t Berlin, Hardenbergstr. 36, 10623, Berlin, Germany Technische Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 June 2016 Received in revised form 30 September 2016 Accepted 19 October 2016 Available online xxx

The influence of artificial ageing of gas diffusion layers (GDLs) on the cell performance was investigated using high resolution synchrotron radiography. State-of-the-art GDLs of the type SIGRACET® SGL 25BC were aged for 0 h, 16 h and 24 h in a hydrogen peroxide solution before they were assembled in the fuel cells. In-operando radiographic measurements were combined with voltage and contact angle measurements. Correlations between applied ageing conditions, GDL water saturation and cell performance were revealed. Hereby, all cell operating conditions were tested several times to estimate the reproducibility of in-operando radiographic fuel cell measurements. Water films at the GDL-membrane and at the GDL-flow field interfaces were found and attributed to MPL cracks and large pores in the GDL structure. The combination of these cracks and pores are assumed to play a crucial role for blocked gas paths, leading to an undersupply with reactants and an increased humidification of the membrane. It is shown that water agglomerations directly impact the membrane resistance. We assume that the hydrophobicity of the fibers inside the GDL is more important for the cell performance than water agglomerations at the membrane-GDL interface. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Polymer electrolyte membrane fuel cell In-operando synchrotron radiography Artificial ageing Gas diffusion layer Water management

1. Introduction Polymer-electrolyte-membrane (PEM) fuel cells are actually the most promising type of fuel cells for automotive applications, since they offer high current densities [1e4] and e compared to other types of fuel cells e good dynamic response. In spite of the research carried out in recent years, water management in PEM fuel cells is still a problem which needs to be investigated [5e15]. One of the most critical problem is the discharge of product water out of the cell. The gas diffusion layer (GDL) plays a decisive role as it represents the interface between water production areas (catalyst layers) and water removal through the flow fields. Transport of water within the GDL is very complex phenomena and was analyzed very detailed [16] by means of optical observations (by using acrylic

* Corresponding author. Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109, Berlin, Germany. E-mail address: [email protected] (T. Arlt).

glass inlays in the endplates of a cell) [17e22] or radiographic measurements using different types of radiation. Latter method is well-established nowadays since it enables for analyzing fuel cells with minor intrusions of the cell design and cell materials, only. CFD simulation of the gas-water-management of a fuel cell is another powerful tool that was applied by a couple of research groups [23e26]. Radiographic measurements often served to obtain boundary conditions for simulations or have been used to verify simulations [27]. Another possibility to modify/optimize the GDL structure is to insert pores or cracks artificially into the GDL and MPL [28,29]. We investigated the water management of these perforations in former publications [30]. It was found that pores can serve as drainage volumes and mainly show eruptive behavior of water volumes, resulting in better gas supply of the membrane. With this method Mench et al. could improve the cell performance of 6% for current densities ranging from 0.2 to 1.4 Acm
2 [31]. In addition, the proton conductivity of the membrane is essential for the cell's performance and is directly influenced by its water management [32]. Park et al. have shown that self-humidifying

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membranes may overcome some of these issues [33]. The fibers of the GDL are often impregnated with polytetrafluorethylene (PTFE) in order to increase its hydrophobic property [34]. The hydrophobicity of the outer surface GDL can be measured by contact angle measurements. The outer contact angle is influenced by both surface and structural effects and is usually between 140 and 165 . Especially the inner surfaces of the GDL, i.e. fibers inside the GDL and the corresponding inner contact angle, have a great impact on the water management of the GDL material. The inner contact angle can be obtained e.g. under use of inverse gas chromatography methods (iGC) [35]. As well known, several tasks have to be fulfilled by the GDL [36,37]: thermal [38] and electrical conduction, gas supply and humidification of the membrane [39,40] and specific water outtake into the channels [41]. Especially gas supply and water removal capabilities are essential for high performance of fuel cells since the media concentration and membrane humidity are very sensitive parameters for the fuel cell performance [42e47]. The hydrophobicity properties of the GDLs are changing over operation time due to various effects, exemplarily PTFE loss [48,49]. Therefore it is important to compare pristine and aged GDLs with respect to water management and cell performance. Investigating the impact of ageing in-operando is very time consuming [50,51]. As one possibility, artificial ageing can be performed to reduce the time required [43,52]. In addition, artificial ageing benefits to focus on one specific ageing effect only. Radiographic and tomographic imaging are well-established measurement techniques. They are suitable for in-operando investigation of whole fuel cells [41,53e71] and for analyses of single components such as GDLs [72e76]. In this paper, we present in-operando synchrotron radiographic measurements [75e77], performed with fuel cells containing GDLs that were artificially aged. For the presented measurements, a SIGRACET® SGL 25BC GDL was used. The GDLs were aged for different periods of time: 0 h (pristine), 16 h and 24 h [78]. The operation schedule (see chapter 3.2) was run up to three times. This enables to estimate the reproducibility of in-operando radiographic measurements of PEM fuel cells, which has so far been analyzed sparsely. Radiographic measurements were combined with voltage measurements that were simultaneously performed. Contact angle measurements were performed at the very same GDL before cell assembling and are compared with the results from the water distribution measurements. 2. Artificial ageing of gas diffusion layers GDL ageing can be performed by in-operando long-term ageing and by artificial ageing. Several measurements and simulations were already performed in order to understand ageing phenomena in more detail [50,79,80]. Most ageing methods are very time consuming and mostly affected by more than one ageing effect (for example membrane poisoning, GDL or seal degradation). As an advantage, the delivered ageing-dependent parameters are directly related to real cell operation. However, they do not allow considering the impact of a single ageing effect on the cell performance individually. To avoid a superposition of many effects, a specific ageing effect has to be separated from the others. In the current investigation only ageing effects of GDLs affecting the cell performance shall be considered. All other parameters have to be kept constant. Therefore we had chosen an artificial ex-situ ageing method for GDL ageing. Significant differences between pristine and long-term aged GDLs are both loss of PTFE content and changing PTFE distribution. For instance, the hydrophobic property of GDLs - and therewith the water management of a fuel cell changes with varying PFTE content [81e85]. In operating fuel cells, PTFE loss occurs mostly due to high cell potentials, i.e. above 0.6 V

[86,87] which occurs more likely at the cathode, for long periods of time. One opportunity to reproduce the loss of PTFE in a shorter period of time (i.e. artificial ageing) is a treatment with hydrogen peroxide (H2O2) [78,88e90]. As proposed by Anderton et al. hydrogen peroxide can be produced at the cathode [91] and cross over to the anode where further reactions e such as formation of free radicals (Haber-Weiss-reaction [92]) e can proceed following the suggestions by LaConti et al. [93]. Another product of reactions with hydrogen peroxide is the oxidation of carbon to carbon dioxide that can corrode the GDL material and the catalyst support, leading to changing properties of the GDL material and loss of catalyst. Also the electrode potential is influenced by ageing as showed by Aoki et al. [89]. The comprehensive assessment of the preferred reduction mechanisms is not possible from the literature, since both supporters of the cathode degradation [94], as well as the anode degradation [95] are present. Hence the study of the impact of hydrogen peroxide on the GDLs at both sides is of great interest. 3. Experimental 3.1. Artificial ageing GDL ageing has been performed prior to cell assembly under use of the following procedure: Two GDLs per batch (one for anode side, another one for cathode side) were put into hot hydrogen peroxide solution (90  C, 30 wt % H2O2 solution) for a predetermined period of time (16 h and 24 h) [96,97]. During ageing, the H2O2 concentration of the solution was checked every eight hours using potassium permanganate. Only minor deviations of H2O2 content of less than 10% of the initial concentration were measured (resulting in 27e33% H2O2 solution during total ageing time). After ageing, the GDLs have been washed for 1 h in pure water and finally dried for 16 h at 80  C. In the next step, the GDLs were assembled into cell, whereby fresh cell components (expect the GDLs) have been used for all cells. 3.2. Fuel cell operation Three test cells with an active area of 3.89 cm2 were assembled with different artificially aged GDL materials both at anode side and at cathode side. The GDLs inside the fuel cell described as “0 h cell” were not intentionally aged (pristine), the GDL inside the “16 h cell” has been aged for 16 h, and the GDL inside the “24 h cell” for 24 h in a hydrogen peroxide solution. All other operating parameters during the ageing process of the three cells have been kept the same. All three fuel cells were operated at three different operating conditions: The conditioning at 50% r.h. at OCV (referred to as C1) was done to get an equal starting point for each measurement and a quasi-dry radiograph without swelling of the membrane. The swelling of the membrane induces a movement of single components, e.g. GDLs and catalyst layers, and thus makes the water thickness determination more difficult. The operating conditions were run twice to check the reproducibility of cell potentials and water volumes. The first condition step to 1.5 A cm
2 at 50% r.h. (referred to as C2) was done to detect the difference of water thickness due to electrochemical reaction through drawing current. The second step at 1.5 A cm
2 from 50 to 100% r.h. (referred to as C3) was conducted to investigate the water thickness difference due to gas humidification. All cells were equipped with a GORE™ PRIMEA®5761 MEA, including 0.45 mg cm
2 Platinum at the cathode side and 0.40 mg cm
2 Platinum/Ruthenium at the anode side as catalyst. The thickness of the membrane between electrodes in pristine

Please cite this article in press as: Arlt T, et al., Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.10.061

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(dry) condition was 18 mm. Due to water uptake during cell operation a higher membrane thickness can be measured in Figs. 2 and 3. The micro porous layer (MPL) of the GDLs contained 23 wt % PTFE. The channel width and height as well as the rib width were 500 mm. A four channel parallel flow field geometry was used at anode and cathode side. Humidification plays a crucial role for the water management of the membrane, as the performance of fuel cells is improved with an increase of membrane water saturation. Several works have already been performed with humidified and not humidified gases to optimize the performance with a reasonable dimension of the whole fuel cell system [98,99]. Within this study, we investigated the change of water saturation in the GDLs due to their ageing levels. Especially the changing hydrophobicity of both substrate and MPL influences the water management in the GDLs and, consequently, the fuel cell operation [60,100,101]. 3.3. Synchrotron radiography The radiographic measurements were performed at the electron storage ring BESSY II at Helmholtz-Zentrum Berlin, Germany, at the tomography station [102]. A pixel size of 2.1 mm was chosen to obtain both, a large field of view and an appropriate spatial resolution. The optical setup consisted of a pco4000 camera (4008  2672 pixels) in combination with a microscopic optic. The X-ray energy was tuned to 17 keV using a Si-W multilayer monochromator. A 20 mm-thick CWO scintillator was used. For appropriate normalization results and continuous data acquisition, one bright field sequence was taken before each operation condition and another sequence after each operation condition. The data normalization and water extraction was done with self-created IDL code while the reconstruction was using the software Octopus [103]. A special cell design meets high X-ray transmittance and ideal cell properties as cell temperature and homogenous gas supply over the whole active region; the active region is marked by a blue rectangle in Fig. 1. Two notches were shaped into the endplates and partly into the flow fields to provide high transmittances. For these measurements, the lower notch was used, see synchrotron beam in Fig. 1a. Water thickness is obtained by applying the Lambert-Beer law on the radiographs [104]. All radiographs taken at one operation condition of the cell Ir have been normalized to a radiograph of a dry cell I0

Ir ¼ I0 $e
mH2O dH2O where by mH2O is the attenuation coefficient of water and dH2O the unknown water thickness. This was done for each operation condition of the cell and every repetition of the measuring program C1C3. Averaged water thicknesses as given in Table 2 corresponds to the average of the water thickness of all repetitions at a distinct operation condition. Occasionally negative values can be yield since all images obtain pixel noise and radiation artifacts (i.e. scattering effects). These values are less than the measurement errors. 4. Results After different periods of ageing in liquid hydrogen peroxide solution the contact angles of the GDLs were measured ex-situ (see Table 1) to investigate the change in hydrophobicity due to ageing. This was done before cell assembly. Typical deviations of contact angle measurements are around ±3 . A Krüss DSA1-Goniometer was used to measure the outer contact angle of the GDLs. After contact angle measurement, the cells were assembled to

Fig. 1. a) Fuel cell design and imaging setup. The active region in the center of the cell is marked by blue rectangle. 1-scintillator, 2- magnifying optic, 3-mirror, 4-microscopic optic, 5-CCD camera. b) Labeled raw radiograph, taken during cell operation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Contact angles of SIGRACET® SGL 25BC of pristine material and after 16 h and 24 h of ageing. Ageing duration/h

MPL side/

Fiber side/

0 16 24

165.2 ± 4.4 160.3 ± 2.0 160.2 ± 2.4

160.4 ± 1.7 158.2 ± 2.1 155.5 ± 3.7

perform in-situ radiographic measurements. All cell operating conditions were performed several times (see Table 2). More precisely, the sequence C1-C3 was performed with one cell up to three times. Then the cell was changed and the sequence was performed with cell 2 three times consecutive. There was no time gap between the repetitions of the sequence. The repetitions were done to estimate the reproducibility of radiographic measurements. Between the repetitions, influencing factors as material changes, cell assembly or changing measurement equipment can be neglected. The cells were operated for 30 min at condition C1 (open circuit conditions using 50% humidified gas supply) before water producing conditions C2 (50% relative gas humidification, 1.5 A cm
2) and C3 (100% relative gas humidification, 1.5 A cm
2) were performed. This ensures comparable cell condition and water saturation in the cells before operating conditions without load. The average water thicknesses (“Avg. water thickness” in Table 2) were calculated implying all measured repetitions of each operating condition whereby the standard deviation indicates the error. Negative water thicknesses can be explained by not totally dry cells that were taken as references for the normalization process of the radiographs. Especially cells with highly parallelized flow field characteristics are difficult to get completely dried. Most differences (expect that at “0 h cell, C2, cathode” and “24 h

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Table 2 Overview of water thicknesses (which have been averaged over the last 10 min of each operating condition) in anode and cathode GDL for “0 h cell” and “24 h cell”. “Avg. water thickness” includes all repetitions of each cell and operating condition. The standard deviation was used as error indication.

“0 h cell” Repetition 1 Repetition 2 Avg. water thickness “24 h cell” Repetition 1 Repetition 2 Repetition 3 Avg. water thickness

C2, anode/mm

C2, cathode/mm

C3, anode/mm

C3, cathode/mm

0.05 0.11 0.08 ± 0.03

0.26
0.03 0.12 ± 0.15

0.16 0.14 0.15 ± 0.01

0.60 0.90 0.75 ± 0.15

1.30 0.32 1.25 0.96 ± 0.45

0.89 0.54 0.49 0.64 ± 0.18

1.57 1.25 1.22 1.35 ± 0.16

0.68 0.80 0.65 0.71 ± 0.06

cell, C2, anode”) of water thickness between different operating conditions are larger than the standard deviations, which were used for error calculation. Thus water thicknesses, averaged over some minutes of cell operation, are expressive for the conditions. It is remarkable that measurements of dynamic systems also implicate peculiarities of limited duration, for example water droplet forming in the channels. This also effects water agglomerations in the GDL and the cell potential. Radiographic measurements using synchrotron X-ray radiation were applied for visualizing water volumes in-operando. Test cells were equipped with GDLs of different periods of ageing. The temporal resolved water distribution of the test cells is shown in Figs. 2 and 3 from top to bottom while the duration of the operating conditions are given in minutes from the left to the right. The water distribution for gases with 50% r.h. is shown in Fig. 2, while the water distribution for gases with 100% r.h. is shown in Fig. 3. The GDL water is represented by gray value in the range from
0.4 mm (black) to 1.5 mm (white). Negative water thickness values were caused by areas of GDL fiber movement which cannot be corrected (i.e. measurement artifacts) and by areas, affected by beam refraction artifacts. Significant differences in the water amount can be observed for all test cells. Cell “0 h cell”, containing the pristine GDL, shows just few water agglomerations in anode and cathode GDLs and in the membrane. Even after 15 min of operation (at steady state

condition) any water can hardly be observed. Minor deviations can be found between the upper and the lower part of the images of cell “0 h cell”. The higher amount of water at the bottom part of the images can be explained by gravitational force. The radiographs of cell “16 h cell” show first film-like structures at the GDL-flow field interface after 2 min of operation. Such films were analyzed in previous works and have been located under the rib structure of the flow field [105]. After 5 min of operation, film increases and finally forms water drops in the anodic channels (after 15 min). Low humidified gases allow for easier evaporation of water out of the GDL. Especially cell “24 h cell” shows water films at the GDLmembrane interface as well at the GDL-channel interface. Already after one minute of operation at 50% r.h. water films were developed and increased during further 4 min. First water drops are formed after 5 min of operation at anode and cathode side whereas a higher water amount can be found in the cathode GDL than in the anode GDL. This test cell at the presented operating condition exhibits the most water droplet forming and droplet outtake of all test cells at any analyzed cell conditions. Comparable results were found for operating conditions at totally humidified gases (Fig. 3). Cell “0 h cell” exhibits nearly no water inside the GDL nor in the channel structure. Small water films were only found at the GDL-membrane interface. (Note: Horizontal beam artifacts are visible over the entire radiograph and could not

Fig. 2. Water distribution in fuel cells. From left to right: operating time at condition C2, from up to bottom: ageing duration of applied GDLs. Gray values represent water thickness in the range of
0.4e1.5 mm (CH e channel, MEA e membrane electrode assembly).

Please cite this article in press as: Arlt T, et al., Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.10.061

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Fig. 3. Water distribution in the fuel cells (second measurement series for validation). From left to right: operating time at condition C3, from up to down: duration of prior GDL ageing. Gray values represent water thickness in the range of
0.4e1.5 mm.

be corrected in this case). Developing water drops as well as water film at GDL-membrane and GDL-flow field interfaces are clearly visible for cell “16 h cell”. Water drops were found again in the channels, especially at steady state conditions after 15 min of operation with 100% humidified gases. The water outtake capacity of this GDL seems to be compromised by the 16 h ageing procedure. Cell “24 h cell” shows comparable water distributions as cell “16 h cell”. Cell “16 h cell” even exhibits more water drops on the channels. Drops are hardly recognizable at the cathode side of cell “24 h cell”. Distinct water films at the GDL-membrane and at the GDL-flow field interfaces were found. No water drops can be observed in the channels; only thin water films, located at the rear channel wall of the anode flow field, can be observed. The temporal development of GDL water and cell potentials during repetition 1 are given in Figs. 4e6. Since more than one couple of parameters within these measurements can be compared, Figs. 5 and 6 show a more comprehensive comparison of the data that is already shown in Fig. 4. Red lines in Fig. 4 show the water evolution during membrane conditioning operating conditions; no load was applied during these operating conditions but the cells were supplied with 50% humidified gases in order to humidify the membrane. Therewith imaging artifact, resulting from in-operando membrane swelling, can largely be avoided [106]. An unstable cell potential was measured for the “24 h cell” at C2. The curve characteristic is accompanied by large fluctuations in the water thickness, caused by temporal inhomogeneous droplet formation and evolution. This droplet evolution can be seen in the lower section of Fig. 2. Large droplets can block one or more of the four channels and therewith can lead to an undersupply of the active region, resulting in cell potential break-down as shown in Fig. 4c. For a comparison of the test cells, a detailed analysis at steady state conditions, which were reached after approximately 15 min of operation at each operating condition, is necessary. Thus, the last 10 min of each operating condition were averaged computationally with respect to the cell potential and the GDL water thicknesses. Measured water thicknesses, cell potentials and high frequency resistances are given in Table 3. Channel water and membrane

Fig. 4. Temporal development of water volumes and cell potentials (black lines) for a) “0 h cell”, b) “16 h cell” and c) “24 h cell”. Red lines: C1, green lines: C2, and blue lines: C3. Dotted lines show the anode water, straight lines show the cathode water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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are similar, the measured values are comparable. The resistance can be directly correlated to the change in membrane resistance due to varying water inventory of the cells. A graphical comparison of Table 3 is given in Fig. 7. Water thicknesses are presented in bar charts, while cell potentials are visualized by point charts. Fig. 7c shows the electrical resistance of the membrane as a function of the anode and cathode water content. Larger area of the circles are related to higher electrical resistances of the membrane. A clear distinction of the membrane resistance, as well as of the water thicknesses at anode and cathode, between the pristine and the aged GDLs is elucidated.

5. Discussion

Fig. 5. a) Anode water saturation and b) cathode water saturation of all cells. (Operating condition “C1” was run for only few minutes.)

Fig. 6. Temporal evolution of water thickness at a) C2 and b) C3 shown for anode and cathode side of all cells. A e anode, C e cathode.

water are not considered. Averaged values, averaged over all repetitions, are given in Table 3. The resistances are measured using a AC-Ohmmeter MR2212W from Schuetz Messtechnik GmbH with a frequency of 1 kHz. As the single components of the three fuel cells

It was shown that water agglomerations in the GDL have a great impact on the cell performance, especially on the electric resistance of the membrane. Proton conductivity can be enhanced by increasing water in the membrane, but increasing water at the GDL-membrane interface can also lead to mass-transport limitations due to local flooding or film-formations. High frequency resistances were measured at 1 kHz using an AC-Ohmmeter. Therewith, the membrane resistance directly correlated to the HF-resistance could be analyzed independently from resistances of other cell components [107,108]. Cell “0 h cell” with the pristine GDL showed the least water volumes in the GDL as well as the least membrane resistance. The analysis shows that the membrane was well humidified by water stored in the GDL pores, especially at 100% humidified gas conditions resulting in a HF-resistance of around 0.034 U cm2. Zawodzinski et al. also pointed out that high water amount in the membrane lead to higher conductivity and lower membrane resistance, respectively [109]. This is in good accordance with the pristine membrane of our measurements. Less GDL water was measured for lower gas humidification, resulting in less membrane humidification and lower cell potential, consequentially. The membrane resistance increased for cell “16 h cell” for both cell conditions C2 and C3 above 0.16 U cm2. This increase is accompanied with a significant drop of cell potential. An increase of water volume was measured in the GDLs simultaneously, whereby more water was found in the GDL for condition C2. Due to the high gas humidification, the product water cannot evaporate at the anode during condition C3 and therewith it is mainly liquid. This high amount of water (especially at the cathode) can lead to mass transport limitations and finally to the lowest cell potential for this cell. This is in line with the water amounts measured for condition C3 of the “16 h cell”: the cell contains less water but a higher cell potential. Comparing the cell with aged membranes to the cell with pristine GDL, the resistance does not decrease much when increasing the gas humidification to 100% r.h. for the cell with the aged membranes. The GDLs properties had still changed after 24 h ageing in that kind, which slightly increased the cell potential at 50% r.h. and 100% r.h. as well. Due to the same HFR measured for C2 and C3 the membrane is already saturated with water at condition C2 (50% r.h.). This applies to both cells that are equipped with aged membrane. If the membrane is totally saturated (as for C2 for “16 h cell” and “24 h cell”), a further increase of GDL water does not affect the membranes humidification anymore. However, these changes are not reflected by prior contact angle measurements, as given in Table 1. Note that the contact angle measurements only feature the properties of the outer GDL surface (i.e. interfaces to bipolar plate or membrane), and do not represent the surface properties inside the GDL, i.e. the surface properties of large pores which can act as transport paths for product water. Thus we assume the properties of these pores to be responsible for €tter et al. had shown that membrane humidification, too. Marko

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Table 3 Average (including all repetitions) GDL water thickness for 50% r.h. (C2) and 100% r.h. (C3), cell potential (CP) at 1.5 Acm
2 and high frequency resistance (H-FR), simultaneously measured during radiography. “A” e anode, “C” e cathode. Cell

“0 h cell” “16 h cell” “24 h cell”

C2

C3

dH2O/mm

U/mV

MR/Ucm2

dH2O/mm

U/mV

H-FR/Ucm2

C: 0.27/A: 0.06 C: 1.09/A: 0.38 C: 0.89/A: 0.13

327 287 351

0.064 0.164 0.116

C: 0.61/A: 0.17 C: 1.28/A: 0.61 C: 0.69/A: 1.58

348 302 348

0.034 0.161 0.115

Fig. 7. a) Water thicknesses and b) cell potentials for different load conditions. Both were averaged over last 10 min of each operating condition. c) Correlates the water thickness of the anode and cathode GDL with the electrical resistance of the membrane (larger area of a circle is related to a higher resistance).

especially large pores have a very special impact on water agglomerations in the GDL near the flow field channel as well as at the GDL-membrane interface [41]. A decrease of membrane resistance and cell potential could be observed between 50% r.h. and for 100% r.h. gas supplies. Song et al. reported about same effect [110]: In this work, the decrease was lead back to a gradient of moles of water per mole of sulfonic acid sites inside the membrane. They conclude that the interface between the membrane and the anode catalyst layer becomes a liquid-equilibrated phase, resulting in a rapid improvement of anode reaction kinetics. We assume that such liquid-equilibrated phase can be formed due to the surface properties of inner parts of the GDL, which lead to water retention in large pores.

Burheim et al. analyzed a GDL of the same type [111]. They applied a comparable ageing procedure which also lead to PTFEloss and focused on the contact angle, the thickness, the throughplane thermal conductivity and the PTFE content of the GDL. A decrease of the PTFE content was found that involved an increase of the thermal conductivity and a decrease of the contact angle. They assume that the PTFE mainly remained at fiber to fiber contact regions. A higher PTFE-loss after longer ageing times can also explain the increase of the voltage as shown in Fig. 7b since the electrical conductivity can be increased with less PTFE on the fiber surfaces. Such correlation have also been found by other groups. Sadeghifar et al. made some interesting experiments with a couple of SGL GDL-series at different pressures with varying PTFE loadings and MPL [112]. Here again, the presence of PTFE decreased the thermal conductivity. Water films observed in Figs. 2 and 3 next to the membrane and next to the flow fields can be explained by analyzing the structure of the SIGRACET® SGL 25BC GDL. The three-dimensional structure of this GDL is shown in Fig. 8a). The picture was obtained through an ex-situ synchrotron X-ray radiation tomography applying a pixel size of 0.9 mm. To emulate the compression condition during ex-situ tomography as it occurs in real fuel cells, the applied compression device [113,114] was equipped with two bipolar-like plates whereby one plate featured a channel profile. The GDL was compressed to 80% of its original thickness. Fig. 8a) shows the fiber side of the GDL with large pores between the fibers and an uneven MPL surface. These features can be found in the cross section views in Fig. 8bed) again. Yellow arrows in illustration b) mark large pores at the GDL-membrane interface and at the GDL-rib interface which can be filled with product water. Starting at those pores, there are many cracks in the MPL, which can be seen as conducting channels for water between the GDL structure and the GDL-membrane €tter et al. had already interface (blue arrows in Fig. 8). Marko shown that those cracks play a crucial role for the water management in GDLs [41,64]. Large pores in between GDL fibers were found to act as water storage capacities by Ref. [115], observable as water films in through-plane view as we applied in this publication. Therewith, we assume the observed water films between the flow fields and the membrane in Figs. 2 and 3 originates from the large pore structure of the GDL due to the special structure of the SIGRACET® SGL 25BC gas diffusion layer. All measurements which were taken to prove the reproducibility of radiographic measurements (shown in Table 2), are in good agreement with each other. The measurement errors are mostly lower than differences of water thickness for different operating conditions. For small water agglomerations in the GDLs (especially for anode GDL at 50% r.h.), a larger deviation was observed. This can be explained by water drops in the channels, which can increase water agglomeration in the GDL.

6. Conclusions In-operando

radiographic

measurements

have

been

Please cite this article in press as: Arlt T, et al., Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.10.061

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Fig. 8. a) X-ray synchrotron tomogram of a pristine SIGRACET® SGL 25BC looking at the fiber side. b)-d) Cross sections of GDL. Top: interface to membrane, bottom: flow field. Yellow arrows: large pores and uneven MPL surface allows for water agglomerations at GDL-membrane and GDL-flow field interfaces. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

performed to analyze the impact of GDLs with different ageing states (i.e. PTFE loss) on the fuel cell performance. The SIGRACET® SGL 25BC is of special interest because of the PTFE around the fibers and inside the MPL. Three cells, equipped with differently aged GDLs have been analyzed. X-ray synchrotron imaging measurements were used to quantify the water amounts during operation and were combined with electrochemical measurements (voltage and high frequency resistance). Additionally, apriori contact angle measurements were performed. Ageing of GDLs has a significant impact on the water amount and distribution in the GDLs. However, we found that the correlation between ageing and water distribution is very complex and does not results just in a simple increase of water amounts. We have used an artificial ageing procedure that can be performed ex-operando within short periods of time without causing other ageing effects in the cell. This allows for a separation of GDL ageing effects from others. Summarizing, the presented radiographic measurements mostly showed good reproducibility when repeated several times. Thus, most changes of measured GDL water volumes due to changing operating condition can be seen as significant. We assume the origin of the deviation of the water volumes between the repetitions to be based on statistical fluctuations caused by the small time slot that is observed during radiography. However, transport paths or pore filling effects observed should not be affected by this limitation. A decrease of outer contact angles was measured prior to radiographic measurements for increasing ageing periods. The influence of ageing (and therewith with decreased contact angles) was also found to be dependent on the operating condition of the fuel cell, especially on the anode gas humidification. 50% r.h. and 100% r.h. were chosen for anode and cathode gas humidification, while a current of 1.5 A cm
2 was drawn. The water amount in an operating cell containing the investigated type of GDL had shown a very sensitive impact on changing contact angles. We assume that surface properties of fibers inside the GDL, which were not measurable with goniometry-based contact angle measurement methods, have a larger effect on water agglomerations and membrane humidification than contact angles measured at the outer surface of the GDLs. A three-dimensional data set of the GDL was taken in order to explain the origin of water films at the GDLmembrane and the GDL-flow field interfaces. The pore structure of the GDL is assumed to be responsible for these water films. In especial the combination of cracks in the MPL and large pores in the GDL might have a strong effect on the water management in the GDL and membrane.

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Please cite this article in press as: Arlt T, et al., Influence of artificially aged gas diffusion layers on the water management of polymer electrolyte membrane fuel cells analyzed with in-operando synchrotron imaging, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.10.061