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Identification of performance degradations in catalyst layer and gas diffusion layer in proton exchange membrane fuel cells
Journal of Power Sources 449 (2020) 227580
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Identification of performance degradations in catalyst layer and gas diffusion layer in proton exchange membrane fuel cells Xu Zhang a, Yupeng Yang b, Xuyang Zhang a, Hongtan Liu a, * a b
Clean Energy Research Institute, Department of Mechanical and Aerospace Engineering, University of Miami, Coral Gables, FL, 33146, USA Key Laboratory Electronic Equipment Structure Design, Ministry of Education, Xidian University, Xi’an, Shaanxi, 710071, China
H I G H L I G H T S
� Degradations in CL and GDL are successfully isolated. � Major performance degradation results from CL degradations. � Degradations in both CL and GDL contribute to increased mass transport losses. � Degradation in GDL leads to severe mass transport loss at high RH. � Oxygen transport resistance increases significantly in CL, not in GDL. A R T I C L E I N F O
A B S T R A C T
Keywords: Proton exchange membrane fuel cell (PEMFC) Degradation Mass transport loss Gas diffusion layer Catalyst layer Oxygen transport resistance
Degradations occur in both the catalyst layer (CL) and gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFC) due to different mechanisms. Identification of the locations and mechanisms of degradations is critical in mitigating them. In this study, performance degradation of a PEMFC is examined using an accel erated stress test (AST) protocol. Using a novel methodology of combining fresh and aged MEAs with either fresh or aged GDLs, the origins of losses in either CL or GDL are identified. Increase in mass transport resistance caused by increases in water saturation and in oxygen transport resistance are further isolated. The results show although kinetic degradation accounts for the primary performance decay, mass transport losses increase significantly due to the deterioration in water transport and increase in oxygen transport resistances. The in crease in mass transport resistance in CL is greater than in GDL. The aged CL tends to retain more liquid water, causing significant decrease in actual porosity. Water transport in aged GDL also deteriorates and leads to in crease in mass transport loss, especially under high relative humidity (RH). Oxygen transport resistance increases significantly in aged CL. However, there is no apparent increase in oxygen transport resistance in aged GDLs.
1. Introduction Performance degradation in a proton exchange membrane fuel cell (PEMFC) can result from both the catalyst layer (CL) and gas diffusion layer (GDL). Beside the kinetic degradation in the CL, the changes in mass transport in the carbon-based porous layers are also responsible for performance degradations of a PEMFC [1,2]. Supply of reactants to the reaction sites within the CL though gas diffusion layer (GDL) and micro porous layer (MPL), and effective management of water balance in the porous components ensures continuous reaction in the CL and low mass transport loss [3,4]. However, the carbon-based material is exposed to oxidative environment at the cathode of a PEMFC and damages in
different forms thus cannot be avoided. Material degradation can be exacerbated significantly when transient high electrode interfacial po tential is created due to either frequent start-up/shut-down cycling, highly cyclic load operation, reactant starvation or water flooding [5–7]. Beside the well-documented kinetics degradation caused by catalyst support corrosion in the CL, another important source of deterioration is the changes in mass transport inside the porous components [8,9]. The gas transport and water management inside the porous components, including the CL and GDL, can be severely affected when material degradation occurs [10,11]. Oxygen transport in the porous components of a PEMFC could be severely obstructed due to damages of the pore space caused by many
* Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (H. Liu). https://doi.org/10.1016/j.jpowsour.2019.227580 Received 18 October 2019; Received in revised form 1 December 2019; Accepted 6 December 2019 Available online 25 January 2020 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 449 (2020) 227580
degradation mechanisms [12,13]. The interconnected network of the carbon particles was found to be severely damaged or corroded, creating isolated pores, unfavourable pore size distribution, and embedded platinum particles [14,15]. For instance, the porosity of cathode CL is found to drop from 46 � 1% to 22 � 2% after a cycling durability test, corresponding to an almost 52% reduction measured by Ghosh et al. [15,16]. Meyer et al. [13] used EIS as a diagnosis tool to detect the reduction of catalyst activity and the changes in mass transport resis tance. The mass transport resistance was found to first decrease, then increase during carbon corrosion. Another detrimental consequence is the creation of a more hydrophilic surface due to formation of diverse carbon oxide groups and increase of the surface roughness (structural defects) [17]. Water balance inside the CL pore space can be affected and the propensity of flooding increases substantially [18,19]. Both the in crease in gas diffusion resistance and deterioration of water manage ment can lead to significant increase in mass transport losses [16,20,21]. Degradation in GDLs leads to further deterioration in water transport [22–24]. GDLs are a highly porous carbon-fibre based paper or cloth wet-proofed with polytetrafluoroethylene (PTFE) to enhance water management capability. Park et al. [22] reviewed the degradation issues of GDLs in PEMFC and emphasized that degradation of GDLs leads to severe deterioration of water transport at cathode. Yu et al. [11] studied the hydrophobicity loss of GDL caused by electrochemical corrosion, which is likely to be associated to the loss of carbon material and PTFE. Ha et al. [25] found that the carbon component of the GDL is easily corroded while PTFE component is not affected by electric potential. They showed the reduction of GDL thickness and severe structure damages in the centre of GDL. Damages of GDL pore structure and hy drophobicity loss jeopardize water removal ability and result in increase of mass transport losses. Due to compact configuration of a single fuel cell, in situ decoupling of performance degradation originating in the CL from that in the GDL are difficult. It is commonly presumed that the most severe degradation occurs in the CL due to its multiple function and harsher environment [26,27]. The GDLs are not considered as an additional source of per formance degradation in most in situ studies. However, the contribution of GDL degradation to the overall performance has been confirmed since it is also exposed to the similar corrosive conditions [28,29]. Besides, the transports of liquid water inside the two cathode porous components are in quite sophisticated balance and mutually influenced [30,31]. Thus, degradations in both components and their effects on overall mass transport loss need to be elucidated. Deterioration of water transport in the porous components further exacerbates the material degradation, since water flooding is an essential reason for the high electrode inter facial potential and water is a reactant for degradation reactions [32, 33]. Hence, degradations of individual components and their contribu tions to overall performance should be carefully clarified, providing foundations for pertinent specific mitigation strategies. Besides the in crease in mass transfer resistance due to increased water saturation in the porous layers, damages of microstructure can also cause increased oxygen transport loss in the pore space. As stated above, mass transport in the porous layer can be severely affected by pore microstructure damages and hydrophobicity loss in a PEMFC. The increase of mass transport resistance could originate either from the CL and/or the GDL and due to different mechanisms. In this work, experiments are conducted to investigate the performance loss in a single cell using an accelerated stress test (AST) protocol. By replacing the GDL in an aged cell with a fresh GDL and replacing an aged GDL in a fresh cell, the mass transport losses in CL and GDL are successfully decoupled and thus the origins of the increase in mass transport resis tance are isolated. To further isolate the increases in mass transfer resistance due to reduction of oxygen diffusion in the damaged pore spaces from that caused by increased liquid water saturation, oxygen transport resistances are also measured under relatively dry condition.
2. Experimental 2.1. Experimental system Two fuel cells with identical components are assembled using Nafion-212 based membrane electrode assemblies. Platinum loading on the cathode and anode sides are both 0.4 mg cm 2 and the active area of the electrode is 16 cm2. The MEA is sandwiched between two carbon papers (Toray 060, Toray Industries. Inc., Japan). The MEA, GDLs and gaskets are assembled by a single cell testing hardware with graphite bipolar plates with single serpentine flow fields. The depth of the flow channels is 1 mm and the width of channels and land are both 2 mm. The cathode and anode reactant flows arrangement is co-flow. The single cell is connected to a fuel cell testing station (FCTS-16, Fuel Cell Technolo gies, Inc., USA) for operating conditions controls and a potentiostat/ galvanostat (HCP-803, Bio-Logic, Inc., France) for electrochemical characterizations. 2.2. Cell conditioning and performance characterization Prior to the performance characterizations, both cells are fully acti vated by cycling the cell voltage between open circuit voltage and 0.2 V with 0.05 V increments every 5 min. Then polarization curves and electrochemical impedance spectroscopy (EIS) plots are obtained. The polarization curves are obtained by sweeping the voltage from OCV to 0.2 V at a rate of 1 mV s 1. EISs are measured at different current densities to identify different types of losses. The AC-amplitudes applied are 5% of the cell current and the frequency ranges from 10 kHz to 100 mHz. When the current density of the cell does not change, i.e. when the cell exhibits nearly same performance, the data are used as fresh cell performance. All the above experiments are conducted at 343K and ambient pressure. Hydrogen and air flow rates are kept constant at 168sccm and 560sccm (standard cubic centimeter per minute), respec tively. Hydrogen is 100% humidified and the relative humidity of air is varied in different sets of experiments to identify the origin and effects of mass transport loss on cell performance. After the initial characterizations, a high potential holding protocols, i.e.,1.4 V, is applied to the first cell as the AST condition to cause carbon corrosion in the fuel cell [34]. Fully saturated hydrogen and nitrogen with flow rates of 200 sccm are used to purge the anode and cathode during the AST, respectively. The AST stops after 15 h due to severe performance degradation. After the AST, a performance recovery pro cedure is performed prior to the performance characterization to recover the portion of reversible degradation accumulated [35]. Then, the characterizations are performed under the same conditions as those used for the fresh cell to identify the origin of degradation. To decouple the mass transport resistances resulted from the CL from that of the GDL, the aged GDL from the first cell, the cell has undergone AST degradation process, is removed carefully. The removal of the aged cathode GDL without any effect on the other component of cell is ach ieved by using a relatively low clamping pressure (6 N m) in assembling the cell. This clamping pressure was obtained by repeated trials of different clamping pressures to ensure reproducible cell performances when the GDL is removed and re-assembled or replaced, while still achieving good cell performances. Note that the clamping pressure de pends on the cell hardware designs and the various materials used, such as MEA, gasket, etc. In this study, GDLs without MPL are used to further avoid possible adhesion between the MPL and the CL during disas sembly. Then, a fresh GDL removed from the second cell, the cell has only gone through the activation process, is assembled into the aged cell. The performances for the cell before and after the GDL replacement are measured under different cathode RHs. To ensure repeatability, the as sembly with aged GDL and fresh GDL are repeated and same perfor mances are obtained. Similarly, the aged GDL from the first cell (the aged cell) is assembled into the second cell (the fresh cell), and perfor mances are recorded with both the fresh and aged GDLs. The procedure 2
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of the GDL replacement experiment is shown schematically in Fig. 1.
Δc is oxygen concentration difference from the channel to the reaction sites in CL (mol⋅m 3), and i is current density (A⋅m 2). When the limiting current density is reached under dry condition, the oxygen concentration on active sites can be assumed to be zero. The inlet oxygen concentration is obtained by
2.3. Oxygen transport resistance measurement To further elucidate the mechanisms of mass transport losses during degradation, oxygen transport resistance is measured and isolated using the limiting current technique. The measurement is conducted using a cell with an area of 0.8 cm2 (1 cm � 0.8 cm). A pair of flow fields with 0.8 mm channel and 0.75 mm land are used. Hydrogen flow rate of 800 sccm is maintained at the anode and nitrogen flowrate of 2500 sccm is maintained at the cathode. Pure oxygen is introduced and mixed into the cathode inlet tube to obtain diluted oxygen with dry mole fractions of 0.99%, 1.48%,1.96% and 2.44%, respectively. The measurement con ditions are: cell temperature 353 K, pressure 150 kPa, anode RH 60%, cathode RH 50%. The limiting current is obtained by sweeping the voltage from 0.6V to 0.05V at a rate of 1 mV s 1. Total oxygen transport resistance can be calculated by RO2 ¼
4FΔc i
cO2; inlet ¼ xO2; dry
P
Pw RT
(2)
where P is total pressure (Pa), Pw is vapor partial pressure (Pa), xO2; dry is dry oxygen mole fraction, R is gas constant, T is temperature (K). The mechanism of oxygen transport resistance can be found in the literatures [36,37]. To isolate the origins (the CL or GDL) of oxygen transport resistance, the measurements are performed following the same pro cedure shown in Fig. 1. 3. Results and discussion 3.1. Performance characterization and decoupling
(1)
First, the effect of repeated assembly procedure on cell performance is examined. Fig. 2 displays the polarization curves before disassembly and after reassembly for the fresh cell and aged cell, respectively. It can be seen that the disassembly and reassembly procedures do not affect the cell performance. The good consistence indicates that this methodology can be used to decouple the contributions of degradations from the CL and GDL. To locate the origin of mass transport loss, polarization curves are measured for four cell configurations under different air RHs, as shown in Fig. 3 (a) and Fig. 3 (b). The first cell is the fresh cell, i.e. it has a fresh MEA and a fresh GDL; the second cell has a fresh MEA and an aged cathode GDL; the third cell is an aged cell, i.e. it has both an aged MEA and an aged GDL; the fourth cell has an aged MEA and a fresh GDL. As expected, the drastic kinetic degradation is clearly shown from the large difference in cell current densities in the high voltage region between the cells with fresh MEAs and those with aged MEAs. Such results are consistent with the data in literatures and the main mecha nism is the corrosion of carbon support in the CL results in severe Pt particle detachment and losses [21,38]. Besides, almost overlapping polarization curves in the high voltage regions before and after GDL replacement indicate that the kinetic degradation within cathode CL is not affected by the replacement of cathode GDL. The polarization curves exhibit quite distinct degradation phenom ena under different air RH. Under a drier condition (air RH at 50%), the polarization curves barely show any difference in mass transport losses
where RO2 is oxygen transport resistance (s⋅m 1), F is Faraday constant,
Fig. 1. Experimental methodology to isolate the mass transport losses in cathode GDL from CL.
Fig. 2. Effect of repeated disassembly and assembly on cell performance. 3
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an aged GDL is used in a fresh cell, the mass transport loss increases. These results show that the aged GDLs hold more water, which also confirms that in an aged cell the GDL also experience degradation (microstructure damage, hydrophobicity loss, etc.). However, comparing to the fresh cell performance, replacing the aged GDL in the aged cell with a fresh GDL the cell performance does not increase significantly, indicating that the primary performance degra dation must come from the cathode CL, including both kinetic degra dation and increase in mass transport losses. This is reasonable since the electrochemical condition for the cathode CL is more severe in addition to the catalytic effect of Pt on the carbon corrosion reaction. 3.2. Impedance analysis To identify different types of losses, EIS is employed to characterize the four different cells with different configurations. First, EISs are measured at RH 50% and current density of 150 mA cm 2, and the re sults are presented in Fig. 4. At a relative dry condition of 50% RH and low current density, the results in Fig. 3 (a) show that the mass transport loss is minimal. The arcs solely represent the charge transfer resistance and the intercept at the real axis is the high frequency resistance (HFR). The EIS spectra of the aged cell exhibit significant increases in HFR and charge transfer resistance compared to the fresh cell. In addition to the apparent significant kinetic degradation, the ohmic loss also increases significantly. The increase in ohmic resistance can result from the degraded CL [8,10,40] as well as the increase in contact resistance be tween CL and GDL due to layer delamination. Replacement of the aged GDL in the aged cell with a fresh GDL results in an overlapping EIS spectrum with the original one, which is consistent with the results shown in the polarization curves (Fig. 3). These results further confirm that the experimental methodology of replacement of GDLs with disassembly/reassembly of the cells has no effect on the performance of the cell. For the fresh MEA and an aged GDL combination, the spectra present similar charge transfer resistance, but slightly lower HFR, confirming the results of the polarization curves that the aged GDL retains more water and benefits membrane hydration. The EIS spectra are quite similar when air RH increases from 50% to 75% and current density remains at 150 mA cm 2, as shown in Fig. 5 (a). The EIS spectra at a higher current density of 500 mA cm 2 show dra matic increases in both the mass transport resistance and charge transfer resistance for the aged cells (Fig. 5 (b)). Due to porosity and hydro phobicity losses, both the aged CL and aged GDL retain more water in the pore spaces and the thus mass transport resistance increases. It can be clearly seen that replacing the aged GDL with a fresh GDL the mass transport resistance decreases remarkably. This confirms that degrada tion does occurs in a GDL and the main effect is increased liquid water retention. Since at lower air RH the mass transport resistance between a fresh and an aged GDL is not significant (Fig. 4), the large difference in mass transport resistance between the fresh and aged GDLs under higher
Fig. 3. Comparisons of polarization curves of the four different cell configu rations, under (a) cathode RH 50% and (b) cathode RH 75%.
between the cell with a fresh GDL and an aged GDL (Fig. 3 a). The performance degradation is dominated by kinetic degradation and ohmic resistance increase. When a fresh GDL is used to replace the aged GDL in the aged cell, the cell performance hardly shows any improve ment. This indicates that the aged GDL has negligible contribution to the overall performance degradation under a drier condition. It is inter esting to notice that the cell performance actually increases slightly when an aged GDL is used to replace the GDL in a fresh cell (Fig. 3 a, upper two curves). Such an unexpected result is likely caused by reduced ohmic resistance. For an aged GDL, carbon corrosion in GDL causes hydrophobicity loss and possibly some structure damages [39], which facilitate the retention of produced water in the cell, leading to better membrane hydration under dry conditions and lower ohmic resistance. When a higher air RH, i.e., 75%, is used, apparent mass transport limitation exists in the large current density region for all the four cells due to higher liquid water saturation in the porous layers. It is very clear that the cells with the aged GDLs have higher mass transport losses. In the large current density region, for the aged cell, when the aged GDL is replaced by a fresh GDL, the mass transport loss is clearly reduced; while
Fig. 4. EIS measured at 150 mA cm 2, and with air RH at 50%. 4
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occurs due to both kinetic degradation and increase in mass transport loss in CL, as well as increase in mass transport loss in GDL. The dete rioration of water management capability and damages to the pore structures of both the CL and GDL are responsible for the even worse performance loss at high RH conditions. 3.3. Oxygen transport resistance Besides the increase in mass transport loss due to the higher water saturation in the CL and GDL, oxygen diffusion in the pore space can also be affected due to microstructure damages even under low humidity conditions. To further isolate the mass transport losses due to oxygen diffusion in the damaged pore space from those caused by liquid water saturation, oxygen transport resistance is measured under dry condition. Under relative dry condition, oxygen diffusion is only affected by pore structure and the liquid water content in pore space is negligible. Under such conditions, the total oxygen transport resistance can be measured using the limiting current density technique. By maintaining constant oxygen fraction in the channel, the oxygen transport resistance can be obtained from the oxygen concentration in the channel and the limiting current density using Eq. (1). The principle of measuring oxygen transport resistance using Eq. (1) was proposed by Baker et al. [37] and used by many researchers to study oxygen diffusion in multi-scale porous structures [36,41,42]. Fig. 6 shows the limiting current den sities for the different configurations shown in Fig. 1. And the results show that for the aged cells, the limiting current densities are signifi cantly lower compared to those for the fresh cells. Fig. 7 shows the re sults of limiting current density measured under four different oxygen concentrations, i.e. 0.99%, 1.48%,1.96% and 2.44%. The results show that the difference between the fresh and aged GDL in all the configu rations is a slight increase in limiting current density, indicating that the primary increase in oxygen transport limitation is caused by the aged CL. By linear fitting, the oxygen transport resistances can be obtained by Eq. (1) and the results are given in Table 1. The results show that the total oxygen transport resistance increases substantially for the aged MEA, and it comes primarily from the CL, while the GDL’s contribution is very minor. In the CL, oxygen transport involves those in larger secondary pores between the agglomerates, primary pores (<10 nm) within the individual porous agglomerates, and dissolution and diffusion in the ionomer [43–45]. With large portion of Pt particle imbedded in the micro pores in the agglomerate, oxygen transport resistance arises from molecular diffusion, Knudsen diffusion and oxygen permeation in the ionomer [16,42,46]. The loss of porosity and damages to microstructure in the CL definitely impede the accessibility of oxygen to the reaction sites. Due to the significantly larger pore size of the GDL, oxygen
Fig. 5. EIS measured with air RH at 75%, (a) current density at 150 mA cm 2, (b) replacing the aged GDL in the aged cell with a fresh GDL, and (c) replacing the GDL with an aged GDL in the fresh cell.
air RH must be caused by the increased liquid water saturation in an aged GDL, and such an increase can also cause an increase in liquid water saturation in the CL. The aged GDL is then used to replace the cathode GDL in a fresh cell and EISs are measured at 500 mA cm 2, and 750 mA cm 2, and the results are as shown in Fig. 5 (c), respectively. The results further show that the aged GDL introduces significantly higher mass transport resis tance at higher air RH and higher current densities. The results above confirm that severe performance degradation
Fig. 6. Limiting current density measurement. 5
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Journal of Power Sources 449 (2020) 227580
Fig. 8. Oxygen transport resistance versus oxygen concentration.
Fig. 7. Limiting current density versus oxygen concentration.
� Aged GDL also retain more liquid water due to mainly loss of hy drophobicity, causing significant increases in mass transport resis tance under high cathode RH. � Oxygen transport resistance in the CL increases substantially due to microstructure damages, but no apparent change in oxygen transport resistance is observed in the GDL.
Table 1 Fitting results of Fig. 7 and oxygen transport resistance. Fresh MEA þ Fresh GDL Fresh MEA þ Aged GDL Aged MEA þ Aged GDL Aged MEA þ Fresh GDL
Slope
Intercept
RO2 (s m 1)
0.4946 0.4732 0.1435 0.1590
0.0054 0.0072 0.0309 0.0269
78.03 81.56 269.0 242.7
Declaration of competing interest
diffusion is barely affected even though degradation has occurred as revealed from the results shown in Sections 3.1 and 3.2. Theoretically the intercepts in Fig. 7, corresponding to the limiting current density under zero oxygen concentration, should be zero. Oxy gen transport resistance thus can be calculated by the limiting current density under individual oxygen concentration independently, and they should be identical. Fig. 7 and Table 1 show that the intercepts are almost negligible for the fresh cell and even for the cell with fresh MEA and aged GDL. Oxygen transport resistances calculated at different ox ygen concentrations remain constant and are similar to those obtained from the slopes, as shown in Fig. 8. However, significant residuals exist for the cell with the aged MEA. The limiting current density residual is due to the increase of water saturation in the CL. In the aged CL, the loss of porosity and hydrophobicity immobilize the produced water in the pores in proximity to the reaction sites. With increase of water pro duction rates at higher oxygen concentrations, more water trapped in CL pore space would further hinder oxygen supply. Consequently, oxygen transport resistance increases continuously with oxygen concentration in the aged MEA as shown in Fig. 8.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The financial supports from Bengi & Oya Veziroglu Endowed Fund for Clean Energy and the Albert W. Dorgan Memorial Fund are gratefully acknowledged. References [1] A.Z. Weber, R.L. Borup, R.M. Darling, P.K. Das, T.J. Dursch, W. Gu, D. Harvey, A. Kusoglu, S. Litster, M.M. Mench, R. Mukundan, J.P. Owejan, J.G. Pharoah, M. Secanell, I.V. Zenyuk, J. Electrochem. Soc. 161 (2014) F1254–F1299. [2] R. Banerjee, S.G. Kandlikar, Int. J. Hydrogen Energy 40 (2015) 3990–4010. [3] K. Jiao, X. Li, Prog. Energy Combust. Sci. 37 (2011) 221–291. [4] H. Li, Y.H. Tang, Z.W. Wang, Z. Shi, S.H. Wu, D.T. Song, J.L. Zhang, K. Fatih, J. J. Zhang, H.J. Wang, Z.S. Liu, R. Abouatallah, A. Mazza, J. Power Sources 178 (2008) 103–117. [5] N. Yousfi-Steiner, P. Moçot� eguy, D. Candusso, D. Hissel, J. Power Sources 194 (2009) 130–145. [6] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. Kimijima, N. Iwashita, Chem. Rev. 107 (2007) 3904–3951. [7] F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Fuel Cells 8 (2008) 3–22. [8] A.P. Hitchcock, V. Berejnov, V. Lee, M. West, V. Colbow, M. Dutta, S. Wessel, J. Power Sources 266 (2014) 66–78. [9] D. Spernjak, J. Fairweather, T. Rockward, R. Mukundan, R.L. Borup, Polym. Electrolyte. Fuel Cells 11 (2011) 741–750, 41. [10] J.D. Fairweather, D. Spernjak, A.Z. Weber, D. Harvey, S. Wessel, D.S. Hussey, D. L. Jacobson, K. Artyushkova, R. Mukundan, R.L. Borup, J. Electrochem. Soc. 160 (2013) F980–F993. [11] S. Yu, X. Li, S. Liu, J. Hao, Z. Shao, B. Yi, RSC Adv. 4 (2014) 3852–3856. [12] A.G. Star, T.F. Fuller, J. Electrochem. Soc. 164 (2017) F901–F907. [13] Q. Meyer, Y. Zeng, C. Zhao, J. Power Sources 437 (2019) 226922. [14] H. Schulenburg, B. Schwanitz, N. Linse, G.G. Scherer, A. Wokaun, J. Krbanjevic, R. Grothausmann, I. Manke, J. Phys. Chem. C 115 (2011) 14236–14243. [15] A.G. Star, T.F. Fuller, ECS. Transact. 69 (2015) 431–441. [16] S. Ghosh, H. Ohashi, H. Tabata, Y. Hashimasa, T. Yamaguchi, J. Power Sources 362 (2017) 291–298.
4. Conclusions In this study, the performance degradation originated from CL and GDL in a single PEMFC is successfully isolated and characterized using a novel methodology. To separate the mass transport loss resulting from increased water saturation and oxygen diffusion in CL and GDL, cell performance, impedance analysis and oxygen transport resistance are measured. The following conclusions can be obtained from this study: � CL contributes to primary performance degradation due to both mass transport resistance increases and kinetic degradation. � Aged CLs tend to retain more liquid water due to loss of hydropho bicity and/or pore structure damages.
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7
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Identification of performance degradations in catalyst layer and gas diffusion layer in proton exchange membrane fuel cells Xu Zhang a, Yupeng Yang b, Xuyang Zhang a, Hongtan Liu a, * a b
Clean Energy Research Institute, Department of Mechanical and Aerospace Engineering, University of Miami, Coral Gables, FL, 33146, USA Key Laboratory Electronic Equipment Structure Design, Ministry of Education, Xidian University, Xi’an, Shaanxi, 710071, China
H I G H L I G H T S
� Degradations in CL and GDL are successfully isolated. � Major performance degradation results from CL degradations. � Degradations in both CL and GDL contribute to increased mass transport losses. � Degradation in GDL leads to severe mass transport loss at high RH. � Oxygen transport resistance increases significantly in CL, not in GDL. A R T I C L E I N F O
A B S T R A C T
Keywords: Proton exchange membrane fuel cell (PEMFC) Degradation Mass transport loss Gas diffusion layer Catalyst layer Oxygen transport resistance
Degradations occur in both the catalyst layer (CL) and gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFC) due to different mechanisms. Identification of the locations and mechanisms of degradations is critical in mitigating them. In this study, performance degradation of a PEMFC is examined using an accel erated stress test (AST) protocol. Using a novel methodology of combining fresh and aged MEAs with either fresh or aged GDLs, the origins of losses in either CL or GDL are identified. Increase in mass transport resistance caused by increases in water saturation and in oxygen transport resistance are further isolated. The results show although kinetic degradation accounts for the primary performance decay, mass transport losses increase significantly due to the deterioration in water transport and increase in oxygen transport resistances. The in crease in mass transport resistance in CL is greater than in GDL. The aged CL tends to retain more liquid water, causing significant decrease in actual porosity. Water transport in aged GDL also deteriorates and leads to in crease in mass transport loss, especially under high relative humidity (RH). Oxygen transport resistance increases significantly in aged CL. However, there is no apparent increase in oxygen transport resistance in aged GDLs.
1. Introduction Performance degradation in a proton exchange membrane fuel cell (PEMFC) can result from both the catalyst layer (CL) and gas diffusion layer (GDL). Beside the kinetic degradation in the CL, the changes in mass transport in the carbon-based porous layers are also responsible for performance degradations of a PEMFC [1,2]. Supply of reactants to the reaction sites within the CL though gas diffusion layer (GDL) and micro porous layer (MPL), and effective management of water balance in the porous components ensures continuous reaction in the CL and low mass transport loss [3,4]. However, the carbon-based material is exposed to oxidative environment at the cathode of a PEMFC and damages in
different forms thus cannot be avoided. Material degradation can be exacerbated significantly when transient high electrode interfacial po tential is created due to either frequent start-up/shut-down cycling, highly cyclic load operation, reactant starvation or water flooding [5–7]. Beside the well-documented kinetics degradation caused by catalyst support corrosion in the CL, another important source of deterioration is the changes in mass transport inside the porous components [8,9]. The gas transport and water management inside the porous components, including the CL and GDL, can be severely affected when material degradation occurs [10,11]. Oxygen transport in the porous components of a PEMFC could be severely obstructed due to damages of the pore space caused by many
* Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (H. Liu). https://doi.org/10.1016/j.jpowsour.2019.227580 Received 18 October 2019; Received in revised form 1 December 2019; Accepted 6 December 2019 Available online 25 January 2020 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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degradation mechanisms [12,13]. The interconnected network of the carbon particles was found to be severely damaged or corroded, creating isolated pores, unfavourable pore size distribution, and embedded platinum particles [14,15]. For instance, the porosity of cathode CL is found to drop from 46 � 1% to 22 � 2% after a cycling durability test, corresponding to an almost 52% reduction measured by Ghosh et al. [15,16]. Meyer et al. [13] used EIS as a diagnosis tool to detect the reduction of catalyst activity and the changes in mass transport resis tance. The mass transport resistance was found to first decrease, then increase during carbon corrosion. Another detrimental consequence is the creation of a more hydrophilic surface due to formation of diverse carbon oxide groups and increase of the surface roughness (structural defects) [17]. Water balance inside the CL pore space can be affected and the propensity of flooding increases substantially [18,19]. Both the in crease in gas diffusion resistance and deterioration of water manage ment can lead to significant increase in mass transport losses [16,20,21]. Degradation in GDLs leads to further deterioration in water transport [22–24]. GDLs are a highly porous carbon-fibre based paper or cloth wet-proofed with polytetrafluoroethylene (PTFE) to enhance water management capability. Park et al. [22] reviewed the degradation issues of GDLs in PEMFC and emphasized that degradation of GDLs leads to severe deterioration of water transport at cathode. Yu et al. [11] studied the hydrophobicity loss of GDL caused by electrochemical corrosion, which is likely to be associated to the loss of carbon material and PTFE. Ha et al. [25] found that the carbon component of the GDL is easily corroded while PTFE component is not affected by electric potential. They showed the reduction of GDL thickness and severe structure damages in the centre of GDL. Damages of GDL pore structure and hy drophobicity loss jeopardize water removal ability and result in increase of mass transport losses. Due to compact configuration of a single fuel cell, in situ decoupling of performance degradation originating in the CL from that in the GDL are difficult. It is commonly presumed that the most severe degradation occurs in the CL due to its multiple function and harsher environment [26,27]. The GDLs are not considered as an additional source of per formance degradation in most in situ studies. However, the contribution of GDL degradation to the overall performance has been confirmed since it is also exposed to the similar corrosive conditions [28,29]. Besides, the transports of liquid water inside the two cathode porous components are in quite sophisticated balance and mutually influenced [30,31]. Thus, degradations in both components and their effects on overall mass transport loss need to be elucidated. Deterioration of water transport in the porous components further exacerbates the material degradation, since water flooding is an essential reason for the high electrode inter facial potential and water is a reactant for degradation reactions [32, 33]. Hence, degradations of individual components and their contribu tions to overall performance should be carefully clarified, providing foundations for pertinent specific mitigation strategies. Besides the in crease in mass transfer resistance due to increased water saturation in the porous layers, damages of microstructure can also cause increased oxygen transport loss in the pore space. As stated above, mass transport in the porous layer can be severely affected by pore microstructure damages and hydrophobicity loss in a PEMFC. The increase of mass transport resistance could originate either from the CL and/or the GDL and due to different mechanisms. In this work, experiments are conducted to investigate the performance loss in a single cell using an accelerated stress test (AST) protocol. By replacing the GDL in an aged cell with a fresh GDL and replacing an aged GDL in a fresh cell, the mass transport losses in CL and GDL are successfully decoupled and thus the origins of the increase in mass transport resis tance are isolated. To further isolate the increases in mass transfer resistance due to reduction of oxygen diffusion in the damaged pore spaces from that caused by increased liquid water saturation, oxygen transport resistances are also measured under relatively dry condition.
2. Experimental 2.1. Experimental system Two fuel cells with identical components are assembled using Nafion-212 based membrane electrode assemblies. Platinum loading on the cathode and anode sides are both 0.4 mg cm 2 and the active area of the electrode is 16 cm2. The MEA is sandwiched between two carbon papers (Toray 060, Toray Industries. Inc., Japan). The MEA, GDLs and gaskets are assembled by a single cell testing hardware with graphite bipolar plates with single serpentine flow fields. The depth of the flow channels is 1 mm and the width of channels and land are both 2 mm. The cathode and anode reactant flows arrangement is co-flow. The single cell is connected to a fuel cell testing station (FCTS-16, Fuel Cell Technolo gies, Inc., USA) for operating conditions controls and a potentiostat/ galvanostat (HCP-803, Bio-Logic, Inc., France) for electrochemical characterizations. 2.2. Cell conditioning and performance characterization Prior to the performance characterizations, both cells are fully acti vated by cycling the cell voltage between open circuit voltage and 0.2 V with 0.05 V increments every 5 min. Then polarization curves and electrochemical impedance spectroscopy (EIS) plots are obtained. The polarization curves are obtained by sweeping the voltage from OCV to 0.2 V at a rate of 1 mV s 1. EISs are measured at different current densities to identify different types of losses. The AC-amplitudes applied are 5% of the cell current and the frequency ranges from 10 kHz to 100 mHz. When the current density of the cell does not change, i.e. when the cell exhibits nearly same performance, the data are used as fresh cell performance. All the above experiments are conducted at 343K and ambient pressure. Hydrogen and air flow rates are kept constant at 168sccm and 560sccm (standard cubic centimeter per minute), respec tively. Hydrogen is 100% humidified and the relative humidity of air is varied in different sets of experiments to identify the origin and effects of mass transport loss on cell performance. After the initial characterizations, a high potential holding protocols, i.e.,1.4 V, is applied to the first cell as the AST condition to cause carbon corrosion in the fuel cell [34]. Fully saturated hydrogen and nitrogen with flow rates of 200 sccm are used to purge the anode and cathode during the AST, respectively. The AST stops after 15 h due to severe performance degradation. After the AST, a performance recovery pro cedure is performed prior to the performance characterization to recover the portion of reversible degradation accumulated [35]. Then, the characterizations are performed under the same conditions as those used for the fresh cell to identify the origin of degradation. To decouple the mass transport resistances resulted from the CL from that of the GDL, the aged GDL from the first cell, the cell has undergone AST degradation process, is removed carefully. The removal of the aged cathode GDL without any effect on the other component of cell is ach ieved by using a relatively low clamping pressure (6 N m) in assembling the cell. This clamping pressure was obtained by repeated trials of different clamping pressures to ensure reproducible cell performances when the GDL is removed and re-assembled or replaced, while still achieving good cell performances. Note that the clamping pressure de pends on the cell hardware designs and the various materials used, such as MEA, gasket, etc. In this study, GDLs without MPL are used to further avoid possible adhesion between the MPL and the CL during disas sembly. Then, a fresh GDL removed from the second cell, the cell has only gone through the activation process, is assembled into the aged cell. The performances for the cell before and after the GDL replacement are measured under different cathode RHs. To ensure repeatability, the as sembly with aged GDL and fresh GDL are repeated and same perfor mances are obtained. Similarly, the aged GDL from the first cell (the aged cell) is assembled into the second cell (the fresh cell), and perfor mances are recorded with both the fresh and aged GDLs. The procedure 2
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of the GDL replacement experiment is shown schematically in Fig. 1.
Δc is oxygen concentration difference from the channel to the reaction sites in CL (mol⋅m 3), and i is current density (A⋅m 2). When the limiting current density is reached under dry condition, the oxygen concentration on active sites can be assumed to be zero. The inlet oxygen concentration is obtained by
2.3. Oxygen transport resistance measurement To further elucidate the mechanisms of mass transport losses during degradation, oxygen transport resistance is measured and isolated using the limiting current technique. The measurement is conducted using a cell with an area of 0.8 cm2 (1 cm � 0.8 cm). A pair of flow fields with 0.8 mm channel and 0.75 mm land are used. Hydrogen flow rate of 800 sccm is maintained at the anode and nitrogen flowrate of 2500 sccm is maintained at the cathode. Pure oxygen is introduced and mixed into the cathode inlet tube to obtain diluted oxygen with dry mole fractions of 0.99%, 1.48%,1.96% and 2.44%, respectively. The measurement con ditions are: cell temperature 353 K, pressure 150 kPa, anode RH 60%, cathode RH 50%. The limiting current is obtained by sweeping the voltage from 0.6V to 0.05V at a rate of 1 mV s 1. Total oxygen transport resistance can be calculated by RO2 ¼
4FΔc i
cO2; inlet ¼ xO2; dry
P
Pw RT
(2)
where P is total pressure (Pa), Pw is vapor partial pressure (Pa), xO2; dry is dry oxygen mole fraction, R is gas constant, T is temperature (K). The mechanism of oxygen transport resistance can be found in the literatures [36,37]. To isolate the origins (the CL or GDL) of oxygen transport resistance, the measurements are performed following the same pro cedure shown in Fig. 1. 3. Results and discussion 3.1. Performance characterization and decoupling
(1)
First, the effect of repeated assembly procedure on cell performance is examined. Fig. 2 displays the polarization curves before disassembly and after reassembly for the fresh cell and aged cell, respectively. It can be seen that the disassembly and reassembly procedures do not affect the cell performance. The good consistence indicates that this methodology can be used to decouple the contributions of degradations from the CL and GDL. To locate the origin of mass transport loss, polarization curves are measured for four cell configurations under different air RHs, as shown in Fig. 3 (a) and Fig. 3 (b). The first cell is the fresh cell, i.e. it has a fresh MEA and a fresh GDL; the second cell has a fresh MEA and an aged cathode GDL; the third cell is an aged cell, i.e. it has both an aged MEA and an aged GDL; the fourth cell has an aged MEA and a fresh GDL. As expected, the drastic kinetic degradation is clearly shown from the large difference in cell current densities in the high voltage region between the cells with fresh MEAs and those with aged MEAs. Such results are consistent with the data in literatures and the main mecha nism is the corrosion of carbon support in the CL results in severe Pt particle detachment and losses [21,38]. Besides, almost overlapping polarization curves in the high voltage regions before and after GDL replacement indicate that the kinetic degradation within cathode CL is not affected by the replacement of cathode GDL. The polarization curves exhibit quite distinct degradation phenom ena under different air RH. Under a drier condition (air RH at 50%), the polarization curves barely show any difference in mass transport losses
where RO2 is oxygen transport resistance (s⋅m 1), F is Faraday constant,
Fig. 1. Experimental methodology to isolate the mass transport losses in cathode GDL from CL.
Fig. 2. Effect of repeated disassembly and assembly on cell performance. 3
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an aged GDL is used in a fresh cell, the mass transport loss increases. These results show that the aged GDLs hold more water, which also confirms that in an aged cell the GDL also experience degradation (microstructure damage, hydrophobicity loss, etc.). However, comparing to the fresh cell performance, replacing the aged GDL in the aged cell with a fresh GDL the cell performance does not increase significantly, indicating that the primary performance degra dation must come from the cathode CL, including both kinetic degra dation and increase in mass transport losses. This is reasonable since the electrochemical condition for the cathode CL is more severe in addition to the catalytic effect of Pt on the carbon corrosion reaction. 3.2. Impedance analysis To identify different types of losses, EIS is employed to characterize the four different cells with different configurations. First, EISs are measured at RH 50% and current density of 150 mA cm 2, and the re sults are presented in Fig. 4. At a relative dry condition of 50% RH and low current density, the results in Fig. 3 (a) show that the mass transport loss is minimal. The arcs solely represent the charge transfer resistance and the intercept at the real axis is the high frequency resistance (HFR). The EIS spectra of the aged cell exhibit significant increases in HFR and charge transfer resistance compared to the fresh cell. In addition to the apparent significant kinetic degradation, the ohmic loss also increases significantly. The increase in ohmic resistance can result from the degraded CL [8,10,40] as well as the increase in contact resistance be tween CL and GDL due to layer delamination. Replacement of the aged GDL in the aged cell with a fresh GDL results in an overlapping EIS spectrum with the original one, which is consistent with the results shown in the polarization curves (Fig. 3). These results further confirm that the experimental methodology of replacement of GDLs with disassembly/reassembly of the cells has no effect on the performance of the cell. For the fresh MEA and an aged GDL combination, the spectra present similar charge transfer resistance, but slightly lower HFR, confirming the results of the polarization curves that the aged GDL retains more water and benefits membrane hydration. The EIS spectra are quite similar when air RH increases from 50% to 75% and current density remains at 150 mA cm 2, as shown in Fig. 5 (a). The EIS spectra at a higher current density of 500 mA cm 2 show dra matic increases in both the mass transport resistance and charge transfer resistance for the aged cells (Fig. 5 (b)). Due to porosity and hydro phobicity losses, both the aged CL and aged GDL retain more water in the pore spaces and the thus mass transport resistance increases. It can be clearly seen that replacing the aged GDL with a fresh GDL the mass transport resistance decreases remarkably. This confirms that degrada tion does occurs in a GDL and the main effect is increased liquid water retention. Since at lower air RH the mass transport resistance between a fresh and an aged GDL is not significant (Fig. 4), the large difference in mass transport resistance between the fresh and aged GDLs under higher
Fig. 3. Comparisons of polarization curves of the four different cell configu rations, under (a) cathode RH 50% and (b) cathode RH 75%.
between the cell with a fresh GDL and an aged GDL (Fig. 3 a). The performance degradation is dominated by kinetic degradation and ohmic resistance increase. When a fresh GDL is used to replace the aged GDL in the aged cell, the cell performance hardly shows any improve ment. This indicates that the aged GDL has negligible contribution to the overall performance degradation under a drier condition. It is inter esting to notice that the cell performance actually increases slightly when an aged GDL is used to replace the GDL in a fresh cell (Fig. 3 a, upper two curves). Such an unexpected result is likely caused by reduced ohmic resistance. For an aged GDL, carbon corrosion in GDL causes hydrophobicity loss and possibly some structure damages [39], which facilitate the retention of produced water in the cell, leading to better membrane hydration under dry conditions and lower ohmic resistance. When a higher air RH, i.e., 75%, is used, apparent mass transport limitation exists in the large current density region for all the four cells due to higher liquid water saturation in the porous layers. It is very clear that the cells with the aged GDLs have higher mass transport losses. In the large current density region, for the aged cell, when the aged GDL is replaced by a fresh GDL, the mass transport loss is clearly reduced; while
Fig. 4. EIS measured at 150 mA cm 2, and with air RH at 50%. 4
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occurs due to both kinetic degradation and increase in mass transport loss in CL, as well as increase in mass transport loss in GDL. The dete rioration of water management capability and damages to the pore structures of both the CL and GDL are responsible for the even worse performance loss at high RH conditions. 3.3. Oxygen transport resistance Besides the increase in mass transport loss due to the higher water saturation in the CL and GDL, oxygen diffusion in the pore space can also be affected due to microstructure damages even under low humidity conditions. To further isolate the mass transport losses due to oxygen diffusion in the damaged pore space from those caused by liquid water saturation, oxygen transport resistance is measured under dry condition. Under relative dry condition, oxygen diffusion is only affected by pore structure and the liquid water content in pore space is negligible. Under such conditions, the total oxygen transport resistance can be measured using the limiting current density technique. By maintaining constant oxygen fraction in the channel, the oxygen transport resistance can be obtained from the oxygen concentration in the channel and the limiting current density using Eq. (1). The principle of measuring oxygen transport resistance using Eq. (1) was proposed by Baker et al. [37] and used by many researchers to study oxygen diffusion in multi-scale porous structures [36,41,42]. Fig. 6 shows the limiting current den sities for the different configurations shown in Fig. 1. And the results show that for the aged cells, the limiting current densities are signifi cantly lower compared to those for the fresh cells. Fig. 7 shows the re sults of limiting current density measured under four different oxygen concentrations, i.e. 0.99%, 1.48%,1.96% and 2.44%. The results show that the difference between the fresh and aged GDL in all the configu rations is a slight increase in limiting current density, indicating that the primary increase in oxygen transport limitation is caused by the aged CL. By linear fitting, the oxygen transport resistances can be obtained by Eq. (1) and the results are given in Table 1. The results show that the total oxygen transport resistance increases substantially for the aged MEA, and it comes primarily from the CL, while the GDL’s contribution is very minor. In the CL, oxygen transport involves those in larger secondary pores between the agglomerates, primary pores (<10 nm) within the individual porous agglomerates, and dissolution and diffusion in the ionomer [43–45]. With large portion of Pt particle imbedded in the micro pores in the agglomerate, oxygen transport resistance arises from molecular diffusion, Knudsen diffusion and oxygen permeation in the ionomer [16,42,46]. The loss of porosity and damages to microstructure in the CL definitely impede the accessibility of oxygen to the reaction sites. Due to the significantly larger pore size of the GDL, oxygen
Fig. 5. EIS measured with air RH at 75%, (a) current density at 150 mA cm 2, (b) replacing the aged GDL in the aged cell with a fresh GDL, and (c) replacing the GDL with an aged GDL in the fresh cell.
air RH must be caused by the increased liquid water saturation in an aged GDL, and such an increase can also cause an increase in liquid water saturation in the CL. The aged GDL is then used to replace the cathode GDL in a fresh cell and EISs are measured at 500 mA cm 2, and 750 mA cm 2, and the results are as shown in Fig. 5 (c), respectively. The results further show that the aged GDL introduces significantly higher mass transport resis tance at higher air RH and higher current densities. The results above confirm that severe performance degradation
Fig. 6. Limiting current density measurement. 5
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Journal of Power Sources 449 (2020) 227580
Fig. 8. Oxygen transport resistance versus oxygen concentration.
Fig. 7. Limiting current density versus oxygen concentration.
� Aged GDL also retain more liquid water due to mainly loss of hy drophobicity, causing significant increases in mass transport resis tance under high cathode RH. � Oxygen transport resistance in the CL increases substantially due to microstructure damages, but no apparent change in oxygen transport resistance is observed in the GDL.
Table 1 Fitting results of Fig. 7 and oxygen transport resistance. Fresh MEA þ Fresh GDL Fresh MEA þ Aged GDL Aged MEA þ Aged GDL Aged MEA þ Fresh GDL
Slope
Intercept
RO2 (s m 1)
0.4946 0.4732 0.1435 0.1590
0.0054 0.0072 0.0309 0.0269
78.03 81.56 269.0 242.7
Declaration of competing interest
diffusion is barely affected even though degradation has occurred as revealed from the results shown in Sections 3.1 and 3.2. Theoretically the intercepts in Fig. 7, corresponding to the limiting current density under zero oxygen concentration, should be zero. Oxy gen transport resistance thus can be calculated by the limiting current density under individual oxygen concentration independently, and they should be identical. Fig. 7 and Table 1 show that the intercepts are almost negligible for the fresh cell and even for the cell with fresh MEA and aged GDL. Oxygen transport resistances calculated at different ox ygen concentrations remain constant and are similar to those obtained from the slopes, as shown in Fig. 8. However, significant residuals exist for the cell with the aged MEA. The limiting current density residual is due to the increase of water saturation in the CL. In the aged CL, the loss of porosity and hydrophobicity immobilize the produced water in the pores in proximity to the reaction sites. With increase of water pro duction rates at higher oxygen concentrations, more water trapped in CL pore space would further hinder oxygen supply. Consequently, oxygen transport resistance increases continuously with oxygen concentration in the aged MEA as shown in Fig. 8.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The financial supports from Bengi & Oya Veziroglu Endowed Fund for Clean Energy and the Albert W. Dorgan Memorial Fund are gratefully acknowledged. References [1] A.Z. Weber, R.L. Borup, R.M. Darling, P.K. Das, T.J. Dursch, W. Gu, D. Harvey, A. Kusoglu, S. Litster, M.M. Mench, R. Mukundan, J.P. Owejan, J.G. Pharoah, M. Secanell, I.V. Zenyuk, J. Electrochem. Soc. 161 (2014) F1254–F1299. [2] R. Banerjee, S.G. Kandlikar, Int. J. Hydrogen Energy 40 (2015) 3990–4010. [3] K. Jiao, X. Li, Prog. Energy Combust. Sci. 37 (2011) 221–291. [4] H. Li, Y.H. Tang, Z.W. Wang, Z. Shi, S.H. Wu, D.T. Song, J.L. Zhang, K. Fatih, J. J. Zhang, H.J. Wang, Z.S. Liu, R. Abouatallah, A. Mazza, J. Power Sources 178 (2008) 103–117. [5] N. Yousfi-Steiner, P. Moçot� eguy, D. Candusso, D. Hissel, J. Power Sources 194 (2009) 130–145. [6] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. Kimijima, N. Iwashita, Chem. Rev. 107 (2007) 3904–3951. [7] F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Fuel Cells 8 (2008) 3–22. [8] A.P. Hitchcock, V. Berejnov, V. Lee, M. West, V. Colbow, M. Dutta, S. Wessel, J. Power Sources 266 (2014) 66–78. [9] D. Spernjak, J. Fairweather, T. Rockward, R. Mukundan, R.L. Borup, Polym. Electrolyte. Fuel Cells 11 (2011) 741–750, 41. [10] J.D. Fairweather, D. Spernjak, A.Z. Weber, D. Harvey, S. Wessel, D.S. Hussey, D. L. Jacobson, K. Artyushkova, R. Mukundan, R.L. Borup, J. Electrochem. Soc. 160 (2013) F980–F993. [11] S. Yu, X. Li, S. Liu, J. Hao, Z. Shao, B. Yi, RSC Adv. 4 (2014) 3852–3856. [12] A.G. Star, T.F. Fuller, J. Electrochem. Soc. 164 (2017) F901–F907. [13] Q. Meyer, Y. Zeng, C. Zhao, J. Power Sources 437 (2019) 226922. [14] H. Schulenburg, B. Schwanitz, N. Linse, G.G. Scherer, A. Wokaun, J. Krbanjevic, R. Grothausmann, I. Manke, J. Phys. Chem. C 115 (2011) 14236–14243. [15] A.G. Star, T.F. Fuller, ECS. Transact. 69 (2015) 431–441. [16] S. Ghosh, H. Ohashi, H. Tabata, Y. Hashimasa, T. Yamaguchi, J. Power Sources 362 (2017) 291–298.
4. Conclusions In this study, the performance degradation originated from CL and GDL in a single PEMFC is successfully isolated and characterized using a novel methodology. To separate the mass transport loss resulting from increased water saturation and oxygen diffusion in CL and GDL, cell performance, impedance analysis and oxygen transport resistance are measured. The following conclusions can be obtained from this study: � CL contributes to primary performance degradation due to both mass transport resistance increases and kinetic degradation. � Aged CLs tend to retain more liquid water due to loss of hydropho bicity and/or pore structure damages.
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