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Effect of humidity and thermal cycling on the catalyst layer structural changes in polymer electrolyte membrane fuel cells
Energy Conversion and Management 189 (2019) 24–32
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effect of humidity and thermal cycling on the catalyst layer structural changes in polymer electrolyte membrane fuel cells
T ⁎
Yafei Changa,1, Jing Liua,1, Ruitao Lia, Jian Zhaob, Yanzhou Qina, Junfeng Zhanga, Yan Yina, , ⁎ Xianguo Lia,b, a
State Key Laboratory of Engines, Tianjin University, 135 Yaguan Road, Tianjin 300350, China 20/20 Laboratory for Fuel Cell and Green Energy RD&D, Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer electrolyte membrane fuel cell Catalyst layer Degradation Relative humidity cycling Thermal cycling
Catalyst layer structural changes in polymer electrolyte membrane fuel cells have significant impact on the cell performance and durability. In this study, ex-situ experiments are designed to investigate the effect of humidity and/or thermal cycles on the structural changes of catalyst layers. The relative humidity and temperature are controlled by an environmental chamber and the catalyst layer structure is characterized by scanning electron microscopy and optical microscopy. The experimental results indicate that crack growth and development, catalyst agglomerate detachment, and surface bulges are the main structural changes of the catalyst layers. Applying relative humidity and thermal cycling simultaneously causes the most significant crack growth, while applying thermal cycling alone causes no appreciable changes. This indicates that the absolute humidity is the key parameter for the crack growth. Through cyclic voltammetry analysis, it is shown that the electrochemical active surface area decreases from 64.1 m2 g−1 to 49.1 m2 g−1 after 500 combined relative humidity and thermal cycles. Analyses of electrochemical impedance spectroscopy show that the charge transfer resistance and ohmic resistance increase significantly after 500 combined relative humidity and thermal cycles, causing the cell performance degradation.
1. Introduction Performance and durability are the two main technical challenges preventing polymer electrolyte membrane (PEM) fuel cells from widespread commercial applications [1–4]. Catalyst layer (CL) is the key component of PEM fuel cells thus the degradation of CL may influence the performance and durability significantly. The degradation mechanism of the CL mainly includes Pt particle agglomeration and growth, Pt loss and migration, active sites contamination and carbon corrosion which has been fully addressed in the previous studies [5–7]. Recent studies indicate that the microstructure of the catalyst layers (CLs) has a significant impact on the mass transport properties, hence the performance and durability of the PEM fuel cells [8–10]. Of particular interest is the structural change of the CLs under the long-term operation. The CL is conventionally composed of ionomer, carbonsupported platinum catalyst (Pt/C) and void region [11], in which the electrochemical reaction and transport of multi-phase fluids, electron, proton and energy (heat) occur spontaneously. Therefore, small
changes in the CL microstructure affect the cell performance and durability dramatically. One of the most frequently observed structural changes in CLs is the growth and propagation of the cracks. The cracks in the CLs can be induced by many factors during the CL fabrication, such as the evaporation of the solvent, poor handling, and uneven mechanical stresses [12,13]. Kusoglu et al. [13] found that residual in-plane stress often exists in the membrane, which may lead to the formation of cracks in the membrane. As the CLs are only a few microns thick and closely attached to the membrane, a tiny crack in the membrane can lead to the occurrence of implicated cracks in the CLs. Kai et al. [14] observed cracks formed on the CL surface when they performed static tensile tests and low-cycle fatigue tests on the membrane electrode assemblies (MEAs). Their results indicated that the cracks are caused by the deformation mismatch between the membrane and CL. To investigate the mechanism of crack formation and growth in the electrodes, Jang et al. [15] proposed an electromechanical diagnostic method to correlate the electrical resistance with the crack area density, by which the growth of
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Corresponding authors. E-mail addresses: [email protected] (Y. Yin), [email protected] (X. Li). 1 First author with equal contribution. https://doi.org/10.1016/j.enconman.2019.03.066 Received 12 January 2019; Received in revised form 11 March 2019; Accepted 22 March 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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caused by the CL structural changes also needs to be clarified. Therefore, the objective of the present study is to investigate the CL structural changes under RH and/or thermal cycling. Three sets of exsitu experiments are conducted to study the structural changes in the CLs under the RH cycling alone, the thermal cycling alone, and the combined RH and thermal cycling. The crack evolution process is recorded by a scanning electron microscopy (SEM) and the surface profile is captured by optical microscopy (OM). Through the ex-situ experiments, the dominant factor that influence the crack propagation can be obtained and some other CL structural changes can be observed. Then the in-situ polarization curves are measured for the samples before and after tests to investigate the influence of the CL structural changes on the fuel cell performance. The changes in the cell resistance and electrochemical surface area (ECSA) are studied by the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements, respectively.
the cracks in the electrodes can be quantitatively measured. However, during the practical cell operation, variations in the electrical resistance can be influenced by many factors such as the membrane hydration levels and operating temperature changes, and cannot be unambiguously related to the formation and growth of the cracks in the CLs or electrodes. The crack formation and propagation in the CLs are closely related to the ionomer swelling and shrinking. The ionomer in the CLs acts as a binding material and is used to transport protons during the electrochemical reaction. The ionomer is required to be well-hydrated, similar to the membrane, to maintain high conductivity. Internal or external humidification methods are normally applied to increase the hydration level of the ionomer [16]. However, the volume of the ionomer is very sensitive to the hydration level, i.e., the ionomer swells when it is wet and shrinks when it is dry. A frequent swelling and shrinking of the ionomer may cause the detachment of the catalyst agglomerate [10] and further lead to the crack formation and propagation in the electrodes. Uchiyama et al. [17] found that the cracks may arise due to the buckling deformation of the membrane when humidity changes in cycles. Their results indicated that the gas diffusion layer (GDL) with a high rigidity and a narrow flow channel are preferable to better distribute the clamping force applied to the MEA. There are contradictory views on the role of CL cracks in the practical fuel cell operation. Pestrak et al. [18] reported that the cracks in the electrodes may cause stress concentration which may damage the membrane, causing gas leakage issues. The cracks break the continuity of the CL and thus increase the resistance for the electron and proton conduction [12]. In addition, the cracks may also inhibit the multiphase transport [19] and affect the three-phase interface regions [20] which negatively affect the performance. However, Karst et al. [21] reported that the cracks may be beneficial for the fuel cell due to the enhanced water management. Nevertheless, they conducted the experiment for a microscale PEM fuel cell without the GDL which is an important component for the water management, thus their conclusion may not be suitable for large scale PEM fuel cells. Despite the cracks, structural defects such as pinholes and delamination between the CL and membrane are also frequently encountered during the fuel cell operation [12]. Tavassoli et al. [22] reported that the delamination may enhance the membrane thinning which is one of the major factors causing the membrane degradation. Banan et al. [23,24] found that the delamination length at the CL–membrane interface propagates significantly due to the environmental vibrations and hygrothermal cycles. In reality, the CL surface is not smooth but rough, which leads to imperfect contact with the adjacent components, such as GDL or membrane. This would cause the so-called “contact resistance” which is one of the important parts of ohmic losses [25]. The contact resistance may be further increased during the long-term dynamic operation because of the interfacial delamination between the CL and membrane [23] or between the CL and GDL [26]. In addition, the gaps between the different layers may act as water pooling sites which may hinder the gas transport. The accumulated water in the gaps may cause more severe damages, e.g., the frost heave under sub-zero operating conditions [26]. For vehicular applications, the dynamic operating conditions may accelerate the CL degradation process significantly [10,27,28]. Under dynamic operating conditions, the CLs suffer harsh driving cycles including variations in the relative humidity (RH), temperature, and potential, which may cause material and/or structure degradation of the CLs [27]. It is found that the RH cycling may not only cause the performance decrease at the beginning of the cell operation, but also be responsible for the drastic performance degradation after the long-term operation [28]. Through modeling studies, it is reported that the RH and thermal cycles may affect the CL microstructure [29] and the delamination inside the MEA [30]. However, few studies investigate the effect of long term dynamic operating conditions on the CL structural changes experimentally, and the fuel cell performance degradation
2. Experimental 2.1. Catalyst layer preparation The CL cannot stand alone because of its structure and thickness, thus two methods are usually adopted to prepare the CL: one method is to deposit the CL ink on the GDL (the so-called Catalyst Coated Substrate (CCS)); the other method is to deposit the CL ink on the membrane (Catalyst Coated Membrane (CCM)). Considering that the membrane is very sensitive to the thermal and RH cycles which may affect the experimental results, a commercially available PEM fuel cell electrode (GDL + CL) manufactured by the CCS method from Shanghai Hesen Inc. is employed in this study. The CL is made from Nafion ionomer (25 wt%) and Pt/C particles (the Pt weight ratio is 30% in Pt/C particles). The Pt loading is 0.4 mg cm−2 and the thickness of the CL is about 10 μm. Samples for the surface morphology characterization are prepared in a square shape with the dimensions of 5 mm × 5 mm, and the samples for the in-situ performance testing are prepared with the size of 2 cm × 2 cm. 2.2. Relative humidity and thermal cycling An environmental chamber (Espec, SH-222) is used to change the RH and/or temperature periodically. For the real fuel cell operation, the operating temperature and RH are assumed to be 85 °C and 90% respectively. The ambient temperature and RH are assumed to be 25 °C and 30% respectively. Thus in this study, the temperature ranges between 25 °C and 85 °C, while the RH ranges between 30% and 90% in the startup and shutdown cycles. To investigate the effect of RH and thermal cycling separately, three sets of experiments are conducted as shown in Table 1. For each experiment, the RH and temperature change between State 1 to State 2 back and forth. In Experiment 1, temperature is kept constant and only RH is changed; in Experiment 2, the environmental chamber is kept as dry as possible (also referred to as 0% RH hereafter) and only temperature is changed; and in Experiment 3, Table 1 Operating conditions for the three sets of the experiments under the relative humidity (RH) or/and thermal cycles. State 1
Experiment 1 Experiment 2 Experiment 3
State 2
Number of cycles
T, °C
RH, %
T, °C
RH, %
85 25 25
30 0 30
85 85 85
90 0 90
500 500 500
Notes: Experiment 1: applying RH cycles alone with constant temperature. Experiment 2: applying thermal cycles alone with dry ambient air. Experiment 3: applying combined RH and thermal cycles. 25
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Fig. 1. Schematic of ex-situ experimental procedure.
under the specified RH and thermal cycles. After every 100 cycles, the samples are taken out for observation of the surface structural changes at the same locations. 2.4. Performance evaluation The fuel cell electrode samples are assembled into a single cell with Nafion 212 membrane. The fuel cell is fixed by bolts and nuts with the same torque to ensure the even distribution of clamping force. Single cell performance tests are conducted at temperature of 75 °C and 100% RH using the fuel cell test station (Greenlight, G60). The back pressure is set to be 50 kPag. Hydrogen is supplied to the anode at a flow rate of 0.1 L min−1 while oxygen is applied to the cathode at a flow rate of 0.2 L min−1. Each sample is measured three times at different times and days and three samples are measured for each condition. The final polarization curves are obtained by taking the average values of the polarization curves in the same set and the standard deviation is less than 5.0%. The EIS and CV curves are measured by an electrochemical workstation (Zahner, Zennium-E). The frequency for the EIS measurement is scanned from 100 mHz to 100 kHz at a constant current density of 50 mA cm−2 and the amplitude is 50 mA. CV measurements are conducted at 30 °C and 100% RH. The voltage ranges from 0.05 V to 1.2 V, and the scan rate is 0.02 V s−1. 3. Results and discussion The ex-situ experimental results are presented in terms of RH cycling alone, thermal cycling alone, and the combined RH and thermal cycling. The in-situ experimental results are provided in terms of the cell performance, EIS, and CV before and after the tests. Fig. 2. Scanning electron microscopy (SEM) images of the cracks before and after 500 relative humidity (RH) cycles at the constant temperature of 85 °C.
3.1. Effect of relative humidity cycling In order to study the effect of RH cycling alone on the CL structural change, the chamber temperature is kept constant at 85 °C and the RH is varied between 30% and 90% in cycles. Fig. 2 shows the change of CL surface cracks before and after 500 RH cycles. It is observed the cracks originally existed in the CLs varies from a few microns to tens of microns. After 500 cycles, the preexisted cracks are slightly enlarged. The values of crack length are shown in Fig. 3. It is seen that the length of cracks increases by about 13%–30% due to the 500 RH cycles. The crack growth is likely due to the swelling and shrinking of the ionomers [10] during the RH cycling. At the RH of 90%, the ionomer is maintained at the high hydration level, which means the ionomer volume is at a large value. When the moisture is partially removed, e.g., at a RH of 30%, the ionomer is dehydrated to some extent, during which the
both RH and temperature are changed simultaneously. A total of 500 cycles (consuming about 350 h) are applied for each experiment. 2.3. Ex-situ observation The ex-situ experimental procedure is shown in Fig. 1. The samples are stuck on the sample stage. The morphology changes for the CL samples before and after cycling tests are observed using SEM (Hitachi, S-4800) and OM (Keyence, VHX-S550E). Four locations are selected as the observation area in convenience of finding the same location after RH and/or thermal cycles periodically. After obtaining the initial surface morphology, the samples are put into the environmental chamber 26
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Fig. 3. Changes of crack length before and after 500 relative humidity (RH) cycles at the constant temperature of 85 °C.
Fig. 4. Scanning electron microscopy (SEM) images of a crack before and after 500 thermal cycles at the constant relative humidity (RH) of 0%.
ionomer shrinks. A frequent swelling and shrinking of the ionomer will contribute to the growth of the pre-existed cracks. In the process of fuel cell operation, the CL cracks may act as stress concentration position which may lead to the membrane degradation. Pinhole or tears may occur in the membrane under the CL cracks due to the large stress and strain [18]. The cracks may also inhibit the multiphase transport [19] and decrease the three-phase interface regions [20] which is undesirable for the fuel cell performance. In addition, it is also reported that hydrogen and oxygen might flow easily into the cracks and thus radicals generate in the “crack space”. These radicals may accelerate the membrane degradation under the cracks [17]. To avoid the crack formation and growth, the ionomer content and the mechanical properties are important factors to be considered. According to the work of Li et al. [31], the mechanical properties of the ionomer (or membrane) can be improved by heat treatment due to the enhancement in the crystallinity of the fluorinated backbones. Thermal annealing could effectively stiffen the ionomer because of the sulfonic anhydrides formation between molecules [32]. The crack growth process may also be influenced by the CL thickness and Pt loading [15].
thermal cycling alone). This means the extent of swelling and shrinking of the ionomer is more significant than those under the other two experimental conditions. In addition, it is interesting to find that a new crack forms after 300 cycles besides the preexisted Crack 1. The formation of the new crack is likely due to the unevenly distributed CL compositions, especially ionomer. It is known that the volume of the ionomer is very sensitive to the operating condition, especially to the RH. Ionomer absorbs water when the RH and temperature are at high values, leading to the swelling of the ionomer. While at low RH and temperatures, ionomer loses water resulting in the shrinking in volume, which is known as dehydration. Cyclic hydration and dehydration may cause permanent plastic strain in the ionomer, resulting in the deformation of the agglomerate including ionomer, carbon support and Pt particles [29]. At locations where the materials are distributed highly unevenly, agglomerates can be easily broken apart from each other and thus some defects (such as cracks) arise in the CL. Comparing to the flat and continuous locations, the crack tip is more vulnerable because agglomerates distribute more unevenly around it. Internal stress and strain during the swelling/shrinking process may change the structure of the crack tip, leading to the crack propagation. The propagation process of a crack tip is shown in Fig. 7. It is noted that the crack tip changes slowly at the first 200 cycles. After 300 cycles, the crack tends to propagate along two paths. From 300 to 400 cycles, the crack tip propagates significantly from the left path while the right path is blocked by the agglomerate (as shown in the red circle). After 500 cycles, the crack becomes much wider and agglomerate may be detached from the bulk layer which will be discussed later. The decrease in the interconnections of the electrical pathways caused by the crack may lead to poor fuel cell performance and durability [15]. Despite the change of crack length, another interesting finding is the detachment of the agglomerate. The agglomerate detachment phenomenon can be clearly observed in Fig. 8. The left two figures are taken from the newly formed crack shown in Fig. 2 at different magnification scales. After 200 more cycles (500 cycles totally) are applied to the sample, the “bridge” at the middle position of the crack detaches from the crack, which is believed to be caused by the swelling and shrinking of the agglomerate as mentioned above. This further decreases the interconnections and cut the pathways for electrons and protons transfer, resulting in higher ohmic resistance. Moreover, the connectivity of the ionomer and Pt/C particles may be reduced because of the detachment, leading to the decrease in the three-phase regions. From Fig. 8, it is also observed that the width of the crack increases from 0.71 µm to 1.12 µm (increased by about 57.7%), thus the detached
3.1.1. Effect of thermal cycling In order to investigate the effect of thermal cycling alone on the structural changes in CLs, the environmental chamber is kept as dry as possible so that the effect of moisture is excluded; and the change in the temperature ranges from 25 to 85 °C, which is the typical temperature range of the cell operation. The change of the surface cracks is displayed in Fig. 4. It is shown that comparing to the initial crack, the crack after 500 thermal cycles do not change much even with a large magnification scale. Because the influence of humidity has been excluded, this result suggests that the influence of temperature on the crack morphology is negligible within the range from 25 °C to 85 °C, which is the typical operation temperature of PEM fuel cells. In addition, no obvious catalyst agglomerate growth is found after 500 thermal cycles. 3.2. Effect of the combined relative humidity and thermal cycling Fig. 5 shows the change of cracks after the combined RH and thermal cycles. It is seen that the cracks increase significantly and the length of the cracks before and after RH and thermal cycles is summarized in Fig. 6. After 500 cycles, the cracks are changed dramatically, and become about 2–6 times longer than their initial length. This is likely because the molar fraction of water in the ionomer at different temperature and RH conditions varies even more significantly than under the conditions in the other two experiments (that is, either RH or 27
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Fig. 5. Scanning electron microscopy (SEM) images of the cracks before and after the combined relative humidity (RH) and thermal cycles.
Fig. 6. Change of crack length before and after the combined relative humidity (RH) and thermal cycles.
agglomerate may drop into the crack and combine with other agglomerates. In the study of Jang et al. [15], the crack widening stage comes after the crack propagation stage. However, the crack propagation and crack widening occur at the same time in this study, which may be mainly due to the different loading conditions. The surface profile of the CL is captured by the OM. Fig. 9(a) shows the surface profile of the CL before and after 500 combined RH and thermal cycles. Different colors stand for different surface heights. Comparing the two images shown in Fig. 9(a), it is noticed that in the center area of the image, a new crack is formed after 500 RH and thermal cycles. At the left-top corner, the existing cracks become more
Fig. 7. Evolution of a crack tip under combined relative humidity (RH) and thermal cycles.
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operation process. Actually, delamination between the CL and membrane has been observed by many researchers [12,22,26]. Cracks in the CLs and delamination between the CL and membrane are considered as two of the main factors that may accelerate the membrane degradation [22]. In addition, CL delamination from the membrane leads to more severe gas crossover, and may accelerate the membrane thinning [22]. It should be noticed that in this study, the stress state of the CL is different from that in a real fuel cell. For the real fuel cell operation, the surface profile variation may be not so significant because of the “clamping pressure”. For the three sets of the designed experiments conducted, changing in RH and temperature simultaneously leads to the most drastic changes in CL, especially crack growth and propagation. It is believed that the dimensional change of the ionomer may take the main responsibility for the structural change. Lu et al. [33] measured the mechanical behavior of Nafion membrane under different RH and temperature conditions. The Young’s modulus of the membrane decreases with the increase in RH and temperature; hence the dimension of the membrane increases significantly under the same condition. Though the mechanical properties of the Nafion ionomer in the CL is not exactly the same with the membrane, it is still shown that the volume of the ionomer is highly dependent on the RH and temperature [34]. The ionomer swells at high RH and temperature and shrinks when the RH or temperature goes down. From the result of this study, it is indicated that the CL structural change is not sensitive to the temperature variation. Comparing to Experiment 1, cracks in Experiment 3 propagate more significantly. The key parameter may be the “absolute humidity” rather than the relative humidity or temperature. Increasing the temperature while keeping the RH constant leads to an increase in the absolute humidity, thus the ionomer/membrane swells. Fig. 10 shows the variation of absolute humidity for the three experiments. In the case of Experiment 1, the absolute humidity changes between 105.11 g m−3 and 315.34 g m−3; for Experiment 2, the absolute humidity keeps close to zero; and for Experiment 3, the absolute humidity changes between 6.93 g m−3 and 315.34 g m−3. Larger variation in the amplitude of absolute humidity
Fig. 8. Detachment of agglomerate and the growth of crack width due to the combined relative humidity (RH) and thermal cycles.
significant in length and width. However, the interesting finding is that the surface becomes “raised” at locations adjacent to the new crack formation, which is referred as “surface bulges”. The height of the CL surface along the center line (as marked in Fig. 9(a)) is shown in Fig. 9(b). It is shown that the height of the CL surface after 500 RH and thermal cycles is higher than the initial height by as large as 3.3 µm. This may be because that the edge of the newly formed cracks tends to go upward due to the internal stress and strain, and the surface around the cracks is raised with the cracks together. In practice, the CL contacts with the membrane for the transport of water and protons. However, the interfacial gaps between the membrane and CL may exist due to the surface roughness. The raise of the CL surface profile may contribute to the delamination between the CL and membrane in the fuel cell
Fig. 9. (a) Catalyst layer (CL) surface profile before and after 500 relative humidity (RH) and thermal cycles. (b) Variation of the surface height along the line in the middle of (a). 29
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Fig. 10. Variation in the absolute humidity for the three sets of experiments.
leads to more significant propagation and growth of cracks. It should be noted that in this ex-situ experiment, the CL can swell and shrink freely without constraint. In the real operating conditions, CL is physically constrained within a fuel cell, the volume variation may build up in-plane and through-plane stress which will definitely accelerate the degradation process. Fig. 12. Profile of cyclic voltammograms (CVs) for the fresh sample and aged samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments.
3.3. Fuel cell performance analysis As it is almost impossible to separate the electrode samples perfectly without any damages from the membrane after an in-situ test, several nominally identical samples are tested instead in this section. Fig. 11 shows the cell performance of a fresh sample and samples after RH and/or thermal cycles. It is found that after 500 thermal cycles (Experiment 2), the performance almost stays unchanged. However, after 500 RH cycles (Experiment 1) or 500 combined RH and thermal cycles (Experiment 3), the performance decreases in some degree. At a current density of 1000 mA cm−2, the voltage drops by about 5.92% after 500 RH cycles alone (Experiment 1) and 14.57% after 500 RH and thermal cycles combined (Experiment 3). As discussed earlier, the performance degradation is mainly caused by the structural changes of CL including the crack propagation, agglomerate detachment, and surface bulges. To obtain the change in the ECSA, CV is conducted for each sample at a temperature of 30 °C with fully humidified H2 and N2 supplied to the anode and cathode, respectively. The CV profiles (as shown in Fig. 12) display the characteristic peaks of the electrochemical redox which occurs on the catalyst surface. The hydrogen desorption area (0.0–0.4 V vs. NHE) can be used as a measure of active surface area of the catalyst that is available for the electrochemical reactions. In Fig. 12, the hydrogen desorption area decreases clearly for Experiment 1 and Experiment 3, which may be mainly caused by the agglomerate
Fig. 13. Electrochemical surface area (ECSA) for the fresh sample and aged samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments.
detachment and crack propagation, leading to the decrease in the threeboundary area. Fig. 13 displays the ECSA for the different experiments. After 500 RH cycles (Experiment 1), the ECSA drops from 64.1 m2 g−1 to 53.1 m2 g−1, and after 500 RH and thermal cycles (Experiment 3), the ECSA drops to 49.1 m2 g−1. This explains the performance variation shown in Fig. 11. Fig. 14 shows the impedance spectra for the different experiments in the form of Nyquist plots, under 75 °C and 100% RH with a backpressure of 50 kPag. In the case of Experiment 2, the impedance spectrum stays almost the same with the fresh one. For Experiment 1 and Experiment 3, the increases in high frequency resistance (ohmic resistance) might be caused by the crack propagation and agglomerate detachment. Also, the surface bulges of the CL related with larger interfacial gaps between the CL and membrane may also contribute to the increase in the ohmic resistance. The low frequency resistance (charge transfer resistance) increases to a large extent especially for Experiment 3. This can be contributed by the decrease in the ECSA to a large degree, thus a larger polarization loss occurs to maintain the same current density.
Fig. 11. Polarization curves for the fresh and aged samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments conducted in this study. 30
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[2]
[3]
[4]
[5]
[6]
[7]
Fig. 14. Impedance spectra for the fresh sample and samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments.
[8]
[9]
4. Conclusions In this study, the effects of relative humidity and/or thermal cycles on the structural changes of catalyst layers in polymer electrolyte membrane fuel cells have been investigated experimentally. Relative humidity and thermal cycling are applied either simultaneously or individually in three sets of ex-situ experiments, and 500 cycles are applied for each experiment. Crack propagation and growth, and catalyst agglomerate detachment are observed through scanning electron microscopy, and surface bulges of the catalyst layers due to the crack formation and propagation are investigated through optical microscopy. After 500 relative humidity cycles while keeping the temperature constant, the crack length increases by about 13%–30%. After 500 thermal cycles with dry ambient air, the crack shows no significant changes. However, with 500 combined relative humidity and thermal cycles, the crack length can be 2–6 times larger than the initial length, much more significant than the crack growth observed in the other two experiments when relative humidity or temperature cycling is applied separately. This indicates that the absolute humidity is the key parameter that influences the crack propagation and growth. The results of cyclic voltammetry show that the electrochemical active surface area decreases from 64.1 m2 g−1 to 49.1 m2 g−1 after 500 combined relative humidity and thermal cycles, consistent with the catalyst layer structural changes; both the charge transfer resistance and ohmic resistance increase significantly through electrochemical impedance spectroscopy analysis, suggesting simultaneous relative humidity and thermal cycling has a significant impact on the cell performance degradation, which is observed through the cell performance test.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
Declaration of interest None.
[21]
Acknowledgements [22]
This work was supported by the National Natural Science Foundation of China (21875161), the National Key R&D Program of China (2018YFB0105601), the Natural Science Foundation of Tianjin, China (17JCZDJC31000), the Ontario-China Research and Innovation Fund (OCRIF Round 3) and the Natural Sciences and Engineering Research Council of Canada via a Discovery Grant.
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Effect of humidity and thermal cycling on the catalyst layer structural changes in polymer electrolyte membrane fuel cells
T ⁎
Yafei Changa,1, Jing Liua,1, Ruitao Lia, Jian Zhaob, Yanzhou Qina, Junfeng Zhanga, Yan Yina, , ⁎ Xianguo Lia,b, a
State Key Laboratory of Engines, Tianjin University, 135 Yaguan Road, Tianjin 300350, China 20/20 Laboratory for Fuel Cell and Green Energy RD&D, Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer electrolyte membrane fuel cell Catalyst layer Degradation Relative humidity cycling Thermal cycling
Catalyst layer structural changes in polymer electrolyte membrane fuel cells have significant impact on the cell performance and durability. In this study, ex-situ experiments are designed to investigate the effect of humidity and/or thermal cycles on the structural changes of catalyst layers. The relative humidity and temperature are controlled by an environmental chamber and the catalyst layer structure is characterized by scanning electron microscopy and optical microscopy. The experimental results indicate that crack growth and development, catalyst agglomerate detachment, and surface bulges are the main structural changes of the catalyst layers. Applying relative humidity and thermal cycling simultaneously causes the most significant crack growth, while applying thermal cycling alone causes no appreciable changes. This indicates that the absolute humidity is the key parameter for the crack growth. Through cyclic voltammetry analysis, it is shown that the electrochemical active surface area decreases from 64.1 m2 g−1 to 49.1 m2 g−1 after 500 combined relative humidity and thermal cycles. Analyses of electrochemical impedance spectroscopy show that the charge transfer resistance and ohmic resistance increase significantly after 500 combined relative humidity and thermal cycles, causing the cell performance degradation.
1. Introduction Performance and durability are the two main technical challenges preventing polymer electrolyte membrane (PEM) fuel cells from widespread commercial applications [1–4]. Catalyst layer (CL) is the key component of PEM fuel cells thus the degradation of CL may influence the performance and durability significantly. The degradation mechanism of the CL mainly includes Pt particle agglomeration and growth, Pt loss and migration, active sites contamination and carbon corrosion which has been fully addressed in the previous studies [5–7]. Recent studies indicate that the microstructure of the catalyst layers (CLs) has a significant impact on the mass transport properties, hence the performance and durability of the PEM fuel cells [8–10]. Of particular interest is the structural change of the CLs under the long-term operation. The CL is conventionally composed of ionomer, carbonsupported platinum catalyst (Pt/C) and void region [11], in which the electrochemical reaction and transport of multi-phase fluids, electron, proton and energy (heat) occur spontaneously. Therefore, small
changes in the CL microstructure affect the cell performance and durability dramatically. One of the most frequently observed structural changes in CLs is the growth and propagation of the cracks. The cracks in the CLs can be induced by many factors during the CL fabrication, such as the evaporation of the solvent, poor handling, and uneven mechanical stresses [12,13]. Kusoglu et al. [13] found that residual in-plane stress often exists in the membrane, which may lead to the formation of cracks in the membrane. As the CLs are only a few microns thick and closely attached to the membrane, a tiny crack in the membrane can lead to the occurrence of implicated cracks in the CLs. Kai et al. [14] observed cracks formed on the CL surface when they performed static tensile tests and low-cycle fatigue tests on the membrane electrode assemblies (MEAs). Their results indicated that the cracks are caused by the deformation mismatch between the membrane and CL. To investigate the mechanism of crack formation and growth in the electrodes, Jang et al. [15] proposed an electromechanical diagnostic method to correlate the electrical resistance with the crack area density, by which the growth of
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Corresponding authors. E-mail addresses: [email protected] (Y. Yin), [email protected] (X. Li). 1 First author with equal contribution. https://doi.org/10.1016/j.enconman.2019.03.066 Received 12 January 2019; Received in revised form 11 March 2019; Accepted 22 March 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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caused by the CL structural changes also needs to be clarified. Therefore, the objective of the present study is to investigate the CL structural changes under RH and/or thermal cycling. Three sets of exsitu experiments are conducted to study the structural changes in the CLs under the RH cycling alone, the thermal cycling alone, and the combined RH and thermal cycling. The crack evolution process is recorded by a scanning electron microscopy (SEM) and the surface profile is captured by optical microscopy (OM). Through the ex-situ experiments, the dominant factor that influence the crack propagation can be obtained and some other CL structural changes can be observed. Then the in-situ polarization curves are measured for the samples before and after tests to investigate the influence of the CL structural changes on the fuel cell performance. The changes in the cell resistance and electrochemical surface area (ECSA) are studied by the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements, respectively.
the cracks in the electrodes can be quantitatively measured. However, during the practical cell operation, variations in the electrical resistance can be influenced by many factors such as the membrane hydration levels and operating temperature changes, and cannot be unambiguously related to the formation and growth of the cracks in the CLs or electrodes. The crack formation and propagation in the CLs are closely related to the ionomer swelling and shrinking. The ionomer in the CLs acts as a binding material and is used to transport protons during the electrochemical reaction. The ionomer is required to be well-hydrated, similar to the membrane, to maintain high conductivity. Internal or external humidification methods are normally applied to increase the hydration level of the ionomer [16]. However, the volume of the ionomer is very sensitive to the hydration level, i.e., the ionomer swells when it is wet and shrinks when it is dry. A frequent swelling and shrinking of the ionomer may cause the detachment of the catalyst agglomerate [10] and further lead to the crack formation and propagation in the electrodes. Uchiyama et al. [17] found that the cracks may arise due to the buckling deformation of the membrane when humidity changes in cycles. Their results indicated that the gas diffusion layer (GDL) with a high rigidity and a narrow flow channel are preferable to better distribute the clamping force applied to the MEA. There are contradictory views on the role of CL cracks in the practical fuel cell operation. Pestrak et al. [18] reported that the cracks in the electrodes may cause stress concentration which may damage the membrane, causing gas leakage issues. The cracks break the continuity of the CL and thus increase the resistance for the electron and proton conduction [12]. In addition, the cracks may also inhibit the multiphase transport [19] and affect the three-phase interface regions [20] which negatively affect the performance. However, Karst et al. [21] reported that the cracks may be beneficial for the fuel cell due to the enhanced water management. Nevertheless, they conducted the experiment for a microscale PEM fuel cell without the GDL which is an important component for the water management, thus their conclusion may not be suitable for large scale PEM fuel cells. Despite the cracks, structural defects such as pinholes and delamination between the CL and membrane are also frequently encountered during the fuel cell operation [12]. Tavassoli et al. [22] reported that the delamination may enhance the membrane thinning which is one of the major factors causing the membrane degradation. Banan et al. [23,24] found that the delamination length at the CL–membrane interface propagates significantly due to the environmental vibrations and hygrothermal cycles. In reality, the CL surface is not smooth but rough, which leads to imperfect contact with the adjacent components, such as GDL or membrane. This would cause the so-called “contact resistance” which is one of the important parts of ohmic losses [25]. The contact resistance may be further increased during the long-term dynamic operation because of the interfacial delamination between the CL and membrane [23] or between the CL and GDL [26]. In addition, the gaps between the different layers may act as water pooling sites which may hinder the gas transport. The accumulated water in the gaps may cause more severe damages, e.g., the frost heave under sub-zero operating conditions [26]. For vehicular applications, the dynamic operating conditions may accelerate the CL degradation process significantly [10,27,28]. Under dynamic operating conditions, the CLs suffer harsh driving cycles including variations in the relative humidity (RH), temperature, and potential, which may cause material and/or structure degradation of the CLs [27]. It is found that the RH cycling may not only cause the performance decrease at the beginning of the cell operation, but also be responsible for the drastic performance degradation after the long-term operation [28]. Through modeling studies, it is reported that the RH and thermal cycles may affect the CL microstructure [29] and the delamination inside the MEA [30]. However, few studies investigate the effect of long term dynamic operating conditions on the CL structural changes experimentally, and the fuel cell performance degradation
2. Experimental 2.1. Catalyst layer preparation The CL cannot stand alone because of its structure and thickness, thus two methods are usually adopted to prepare the CL: one method is to deposit the CL ink on the GDL (the so-called Catalyst Coated Substrate (CCS)); the other method is to deposit the CL ink on the membrane (Catalyst Coated Membrane (CCM)). Considering that the membrane is very sensitive to the thermal and RH cycles which may affect the experimental results, a commercially available PEM fuel cell electrode (GDL + CL) manufactured by the CCS method from Shanghai Hesen Inc. is employed in this study. The CL is made from Nafion ionomer (25 wt%) and Pt/C particles (the Pt weight ratio is 30% in Pt/C particles). The Pt loading is 0.4 mg cm−2 and the thickness of the CL is about 10 μm. Samples for the surface morphology characterization are prepared in a square shape with the dimensions of 5 mm × 5 mm, and the samples for the in-situ performance testing are prepared with the size of 2 cm × 2 cm. 2.2. Relative humidity and thermal cycling An environmental chamber (Espec, SH-222) is used to change the RH and/or temperature periodically. For the real fuel cell operation, the operating temperature and RH are assumed to be 85 °C and 90% respectively. The ambient temperature and RH are assumed to be 25 °C and 30% respectively. Thus in this study, the temperature ranges between 25 °C and 85 °C, while the RH ranges between 30% and 90% in the startup and shutdown cycles. To investigate the effect of RH and thermal cycling separately, three sets of experiments are conducted as shown in Table 1. For each experiment, the RH and temperature change between State 1 to State 2 back and forth. In Experiment 1, temperature is kept constant and only RH is changed; in Experiment 2, the environmental chamber is kept as dry as possible (also referred to as 0% RH hereafter) and only temperature is changed; and in Experiment 3, Table 1 Operating conditions for the three sets of the experiments under the relative humidity (RH) or/and thermal cycles. State 1
Experiment 1 Experiment 2 Experiment 3
State 2
Number of cycles
T, °C
RH, %
T, °C
RH, %
85 25 25
30 0 30
85 85 85
90 0 90
500 500 500
Notes: Experiment 1: applying RH cycles alone with constant temperature. Experiment 2: applying thermal cycles alone with dry ambient air. Experiment 3: applying combined RH and thermal cycles. 25
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Fig. 1. Schematic of ex-situ experimental procedure.
under the specified RH and thermal cycles. After every 100 cycles, the samples are taken out for observation of the surface structural changes at the same locations. 2.4. Performance evaluation The fuel cell electrode samples are assembled into a single cell with Nafion 212 membrane. The fuel cell is fixed by bolts and nuts with the same torque to ensure the even distribution of clamping force. Single cell performance tests are conducted at temperature of 75 °C and 100% RH using the fuel cell test station (Greenlight, G60). The back pressure is set to be 50 kPag. Hydrogen is supplied to the anode at a flow rate of 0.1 L min−1 while oxygen is applied to the cathode at a flow rate of 0.2 L min−1. Each sample is measured three times at different times and days and three samples are measured for each condition. The final polarization curves are obtained by taking the average values of the polarization curves in the same set and the standard deviation is less than 5.0%. The EIS and CV curves are measured by an electrochemical workstation (Zahner, Zennium-E). The frequency for the EIS measurement is scanned from 100 mHz to 100 kHz at a constant current density of 50 mA cm−2 and the amplitude is 50 mA. CV measurements are conducted at 30 °C and 100% RH. The voltage ranges from 0.05 V to 1.2 V, and the scan rate is 0.02 V s−1. 3. Results and discussion The ex-situ experimental results are presented in terms of RH cycling alone, thermal cycling alone, and the combined RH and thermal cycling. The in-situ experimental results are provided in terms of the cell performance, EIS, and CV before and after the tests. Fig. 2. Scanning electron microscopy (SEM) images of the cracks before and after 500 relative humidity (RH) cycles at the constant temperature of 85 °C.
3.1. Effect of relative humidity cycling In order to study the effect of RH cycling alone on the CL structural change, the chamber temperature is kept constant at 85 °C and the RH is varied between 30% and 90% in cycles. Fig. 2 shows the change of CL surface cracks before and after 500 RH cycles. It is observed the cracks originally existed in the CLs varies from a few microns to tens of microns. After 500 cycles, the preexisted cracks are slightly enlarged. The values of crack length are shown in Fig. 3. It is seen that the length of cracks increases by about 13%–30% due to the 500 RH cycles. The crack growth is likely due to the swelling and shrinking of the ionomers [10] during the RH cycling. At the RH of 90%, the ionomer is maintained at the high hydration level, which means the ionomer volume is at a large value. When the moisture is partially removed, e.g., at a RH of 30%, the ionomer is dehydrated to some extent, during which the
both RH and temperature are changed simultaneously. A total of 500 cycles (consuming about 350 h) are applied for each experiment. 2.3. Ex-situ observation The ex-situ experimental procedure is shown in Fig. 1. The samples are stuck on the sample stage. The morphology changes for the CL samples before and after cycling tests are observed using SEM (Hitachi, S-4800) and OM (Keyence, VHX-S550E). Four locations are selected as the observation area in convenience of finding the same location after RH and/or thermal cycles periodically. After obtaining the initial surface morphology, the samples are put into the environmental chamber 26
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Fig. 3. Changes of crack length before and after 500 relative humidity (RH) cycles at the constant temperature of 85 °C.
Fig. 4. Scanning electron microscopy (SEM) images of a crack before and after 500 thermal cycles at the constant relative humidity (RH) of 0%.
ionomer shrinks. A frequent swelling and shrinking of the ionomer will contribute to the growth of the pre-existed cracks. In the process of fuel cell operation, the CL cracks may act as stress concentration position which may lead to the membrane degradation. Pinhole or tears may occur in the membrane under the CL cracks due to the large stress and strain [18]. The cracks may also inhibit the multiphase transport [19] and decrease the three-phase interface regions [20] which is undesirable for the fuel cell performance. In addition, it is also reported that hydrogen and oxygen might flow easily into the cracks and thus radicals generate in the “crack space”. These radicals may accelerate the membrane degradation under the cracks [17]. To avoid the crack formation and growth, the ionomer content and the mechanical properties are important factors to be considered. According to the work of Li et al. [31], the mechanical properties of the ionomer (or membrane) can be improved by heat treatment due to the enhancement in the crystallinity of the fluorinated backbones. Thermal annealing could effectively stiffen the ionomer because of the sulfonic anhydrides formation between molecules [32]. The crack growth process may also be influenced by the CL thickness and Pt loading [15].
thermal cycling alone). This means the extent of swelling and shrinking of the ionomer is more significant than those under the other two experimental conditions. In addition, it is interesting to find that a new crack forms after 300 cycles besides the preexisted Crack 1. The formation of the new crack is likely due to the unevenly distributed CL compositions, especially ionomer. It is known that the volume of the ionomer is very sensitive to the operating condition, especially to the RH. Ionomer absorbs water when the RH and temperature are at high values, leading to the swelling of the ionomer. While at low RH and temperatures, ionomer loses water resulting in the shrinking in volume, which is known as dehydration. Cyclic hydration and dehydration may cause permanent plastic strain in the ionomer, resulting in the deformation of the agglomerate including ionomer, carbon support and Pt particles [29]. At locations where the materials are distributed highly unevenly, agglomerates can be easily broken apart from each other and thus some defects (such as cracks) arise in the CL. Comparing to the flat and continuous locations, the crack tip is more vulnerable because agglomerates distribute more unevenly around it. Internal stress and strain during the swelling/shrinking process may change the structure of the crack tip, leading to the crack propagation. The propagation process of a crack tip is shown in Fig. 7. It is noted that the crack tip changes slowly at the first 200 cycles. After 300 cycles, the crack tends to propagate along two paths. From 300 to 400 cycles, the crack tip propagates significantly from the left path while the right path is blocked by the agglomerate (as shown in the red circle). After 500 cycles, the crack becomes much wider and agglomerate may be detached from the bulk layer which will be discussed later. The decrease in the interconnections of the electrical pathways caused by the crack may lead to poor fuel cell performance and durability [15]. Despite the change of crack length, another interesting finding is the detachment of the agglomerate. The agglomerate detachment phenomenon can be clearly observed in Fig. 8. The left two figures are taken from the newly formed crack shown in Fig. 2 at different magnification scales. After 200 more cycles (500 cycles totally) are applied to the sample, the “bridge” at the middle position of the crack detaches from the crack, which is believed to be caused by the swelling and shrinking of the agglomerate as mentioned above. This further decreases the interconnections and cut the pathways for electrons and protons transfer, resulting in higher ohmic resistance. Moreover, the connectivity of the ionomer and Pt/C particles may be reduced because of the detachment, leading to the decrease in the three-phase regions. From Fig. 8, it is also observed that the width of the crack increases from 0.71 µm to 1.12 µm (increased by about 57.7%), thus the detached
3.1.1. Effect of thermal cycling In order to investigate the effect of thermal cycling alone on the structural changes in CLs, the environmental chamber is kept as dry as possible so that the effect of moisture is excluded; and the change in the temperature ranges from 25 to 85 °C, which is the typical temperature range of the cell operation. The change of the surface cracks is displayed in Fig. 4. It is shown that comparing to the initial crack, the crack after 500 thermal cycles do not change much even with a large magnification scale. Because the influence of humidity has been excluded, this result suggests that the influence of temperature on the crack morphology is negligible within the range from 25 °C to 85 °C, which is the typical operation temperature of PEM fuel cells. In addition, no obvious catalyst agglomerate growth is found after 500 thermal cycles. 3.2. Effect of the combined relative humidity and thermal cycling Fig. 5 shows the change of cracks after the combined RH and thermal cycles. It is seen that the cracks increase significantly and the length of the cracks before and after RH and thermal cycles is summarized in Fig. 6. After 500 cycles, the cracks are changed dramatically, and become about 2–6 times longer than their initial length. This is likely because the molar fraction of water in the ionomer at different temperature and RH conditions varies even more significantly than under the conditions in the other two experiments (that is, either RH or 27
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Fig. 5. Scanning electron microscopy (SEM) images of the cracks before and after the combined relative humidity (RH) and thermal cycles.
Fig. 6. Change of crack length before and after the combined relative humidity (RH) and thermal cycles.
agglomerate may drop into the crack and combine with other agglomerates. In the study of Jang et al. [15], the crack widening stage comes after the crack propagation stage. However, the crack propagation and crack widening occur at the same time in this study, which may be mainly due to the different loading conditions. The surface profile of the CL is captured by the OM. Fig. 9(a) shows the surface profile of the CL before and after 500 combined RH and thermal cycles. Different colors stand for different surface heights. Comparing the two images shown in Fig. 9(a), it is noticed that in the center area of the image, a new crack is formed after 500 RH and thermal cycles. At the left-top corner, the existing cracks become more
Fig. 7. Evolution of a crack tip under combined relative humidity (RH) and thermal cycles.
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operation process. Actually, delamination between the CL and membrane has been observed by many researchers [12,22,26]. Cracks in the CLs and delamination between the CL and membrane are considered as two of the main factors that may accelerate the membrane degradation [22]. In addition, CL delamination from the membrane leads to more severe gas crossover, and may accelerate the membrane thinning [22]. It should be noticed that in this study, the stress state of the CL is different from that in a real fuel cell. For the real fuel cell operation, the surface profile variation may be not so significant because of the “clamping pressure”. For the three sets of the designed experiments conducted, changing in RH and temperature simultaneously leads to the most drastic changes in CL, especially crack growth and propagation. It is believed that the dimensional change of the ionomer may take the main responsibility for the structural change. Lu et al. [33] measured the mechanical behavior of Nafion membrane under different RH and temperature conditions. The Young’s modulus of the membrane decreases with the increase in RH and temperature; hence the dimension of the membrane increases significantly under the same condition. Though the mechanical properties of the Nafion ionomer in the CL is not exactly the same with the membrane, it is still shown that the volume of the ionomer is highly dependent on the RH and temperature [34]. The ionomer swells at high RH and temperature and shrinks when the RH or temperature goes down. From the result of this study, it is indicated that the CL structural change is not sensitive to the temperature variation. Comparing to Experiment 1, cracks in Experiment 3 propagate more significantly. The key parameter may be the “absolute humidity” rather than the relative humidity or temperature. Increasing the temperature while keeping the RH constant leads to an increase in the absolute humidity, thus the ionomer/membrane swells. Fig. 10 shows the variation of absolute humidity for the three experiments. In the case of Experiment 1, the absolute humidity changes between 105.11 g m−3 and 315.34 g m−3; for Experiment 2, the absolute humidity keeps close to zero; and for Experiment 3, the absolute humidity changes between 6.93 g m−3 and 315.34 g m−3. Larger variation in the amplitude of absolute humidity
Fig. 8. Detachment of agglomerate and the growth of crack width due to the combined relative humidity (RH) and thermal cycles.
significant in length and width. However, the interesting finding is that the surface becomes “raised” at locations adjacent to the new crack formation, which is referred as “surface bulges”. The height of the CL surface along the center line (as marked in Fig. 9(a)) is shown in Fig. 9(b). It is shown that the height of the CL surface after 500 RH and thermal cycles is higher than the initial height by as large as 3.3 µm. This may be because that the edge of the newly formed cracks tends to go upward due to the internal stress and strain, and the surface around the cracks is raised with the cracks together. In practice, the CL contacts with the membrane for the transport of water and protons. However, the interfacial gaps between the membrane and CL may exist due to the surface roughness. The raise of the CL surface profile may contribute to the delamination between the CL and membrane in the fuel cell
Fig. 9. (a) Catalyst layer (CL) surface profile before and after 500 relative humidity (RH) and thermal cycles. (b) Variation of the surface height along the line in the middle of (a). 29
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Fig. 10. Variation in the absolute humidity for the three sets of experiments.
leads to more significant propagation and growth of cracks. It should be noted that in this ex-situ experiment, the CL can swell and shrink freely without constraint. In the real operating conditions, CL is physically constrained within a fuel cell, the volume variation may build up in-plane and through-plane stress which will definitely accelerate the degradation process. Fig. 12. Profile of cyclic voltammograms (CVs) for the fresh sample and aged samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments.
3.3. Fuel cell performance analysis As it is almost impossible to separate the electrode samples perfectly without any damages from the membrane after an in-situ test, several nominally identical samples are tested instead in this section. Fig. 11 shows the cell performance of a fresh sample and samples after RH and/or thermal cycles. It is found that after 500 thermal cycles (Experiment 2), the performance almost stays unchanged. However, after 500 RH cycles (Experiment 1) or 500 combined RH and thermal cycles (Experiment 3), the performance decreases in some degree. At a current density of 1000 mA cm−2, the voltage drops by about 5.92% after 500 RH cycles alone (Experiment 1) and 14.57% after 500 RH and thermal cycles combined (Experiment 3). As discussed earlier, the performance degradation is mainly caused by the structural changes of CL including the crack propagation, agglomerate detachment, and surface bulges. To obtain the change in the ECSA, CV is conducted for each sample at a temperature of 30 °C with fully humidified H2 and N2 supplied to the anode and cathode, respectively. The CV profiles (as shown in Fig. 12) display the characteristic peaks of the electrochemical redox which occurs on the catalyst surface. The hydrogen desorption area (0.0–0.4 V vs. NHE) can be used as a measure of active surface area of the catalyst that is available for the electrochemical reactions. In Fig. 12, the hydrogen desorption area decreases clearly for Experiment 1 and Experiment 3, which may be mainly caused by the agglomerate
Fig. 13. Electrochemical surface area (ECSA) for the fresh sample and aged samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments.
detachment and crack propagation, leading to the decrease in the threeboundary area. Fig. 13 displays the ECSA for the different experiments. After 500 RH cycles (Experiment 1), the ECSA drops from 64.1 m2 g−1 to 53.1 m2 g−1, and after 500 RH and thermal cycles (Experiment 3), the ECSA drops to 49.1 m2 g−1. This explains the performance variation shown in Fig. 11. Fig. 14 shows the impedance spectra for the different experiments in the form of Nyquist plots, under 75 °C and 100% RH with a backpressure of 50 kPag. In the case of Experiment 2, the impedance spectrum stays almost the same with the fresh one. For Experiment 1 and Experiment 3, the increases in high frequency resistance (ohmic resistance) might be caused by the crack propagation and agglomerate detachment. Also, the surface bulges of the CL related with larger interfacial gaps between the CL and membrane may also contribute to the increase in the ohmic resistance. The low frequency resistance (charge transfer resistance) increases to a large extent especially for Experiment 3. This can be contributed by the decrease in the ECSA to a large degree, thus a larger polarization loss occurs to maintain the same current density.
Fig. 11. Polarization curves for the fresh and aged samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments conducted in this study. 30
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Fig. 14. Impedance spectra for the fresh sample and samples after 500 relative humidity (RH) and/or thermal cycles in the three experiments.
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4. Conclusions In this study, the effects of relative humidity and/or thermal cycles on the structural changes of catalyst layers in polymer electrolyte membrane fuel cells have been investigated experimentally. Relative humidity and thermal cycling are applied either simultaneously or individually in three sets of ex-situ experiments, and 500 cycles are applied for each experiment. Crack propagation and growth, and catalyst agglomerate detachment are observed through scanning electron microscopy, and surface bulges of the catalyst layers due to the crack formation and propagation are investigated through optical microscopy. After 500 relative humidity cycles while keeping the temperature constant, the crack length increases by about 13%–30%. After 500 thermal cycles with dry ambient air, the crack shows no significant changes. However, with 500 combined relative humidity and thermal cycles, the crack length can be 2–6 times larger than the initial length, much more significant than the crack growth observed in the other two experiments when relative humidity or temperature cycling is applied separately. This indicates that the absolute humidity is the key parameter that influences the crack propagation and growth. The results of cyclic voltammetry show that the electrochemical active surface area decreases from 64.1 m2 g−1 to 49.1 m2 g−1 after 500 combined relative humidity and thermal cycles, consistent with the catalyst layer structural changes; both the charge transfer resistance and ohmic resistance increase significantly through electrochemical impedance spectroscopy analysis, suggesting simultaneous relative humidity and thermal cycling has a significant impact on the cell performance degradation, which is observed through the cell performance test.
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Declaration of interest None.
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Acknowledgements [22]
This work was supported by the National Natural Science Foundation of China (21875161), the National Key R&D Program of China (2018YFB0105601), the Natural Science Foundation of Tianjin, China (17JCZDJC31000), the Ontario-China Research and Innovation Fund (OCRIF Round 3) and the Natural Sciences and Engineering Research Council of Canada via a Discovery Grant.
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