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Nafion composite membrane utilizing accelerated stress technique
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique Cheng Wang a,*, Jianbo Zhang b, Shubo Wang a, Sijia Hao a, Jianqiu Li b, Zongqiang Mao a, Zhiming Mao d, Minggao Ouyang b, Zhixiang Liu c,** a
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, People's Republic of China State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, People's Republic of China c School of Material Science and Energy Engineering, Foshan University, Foshan, 528000, Guangdong, People's Republic of China d Beijing Sino Hydrogen Technology Co., Ltd., Beijing 100084, People's Republic of China b
article info
abstract
Article history:
Durability is one of the key issues in commercialization of proton exchange membrane fuel
Received 23 January 2016
cell (PEMFC). The main purpose of this study is to investigate the degradation mechanism
Received in revised form
of Membrane Electrode Assembly (MEA) based on PTFE/Nafion composite membrane in
29 April 2016
PEMFC through the MEA/Stack Durability Protocol developed by the Fuel Cell Technical
Accepted 29 April 2016
Team (FCTT) of the Freedom CAR and Fuel Partnership. The accelerated life test lasted for
Available online xxx
300 h, with voltage decay rate of the MEA about 0.48 mV/h at operating current density 100 mA/cm2 being achieved. After the acceleration experiment, degradation mechanisms
Keywords:
for the MEA based on PTFE/Nafion composite membrane were analyzed in detail by
Proton exchange membrane fuel cell
hydrogen-thermal experiment, SEM, TEM, EDS and Ion Chromatographic test. The exper-
Durability
imental results show that pinhole formed at outlet position of the composite membrane
Accelerated stress test
leads to hydrogen crossover current density exceeding 21 mA/cm2 at 210 h, with F and
MEA degradation mechanism
SO2 4 concentrations sharply increased around 200 h in discharge water of fuel cell from IC results, therefore, membrane failure is the major factor in the failure of MEA. Moreover, the micro-structural damage in the MEA and Pt particles growing up along flow channel even appearing at membrane could be observed from SEM, TEM and EDS results, which play important role in the deterioration of performance and stability for PEMFC. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to its benefits of high efficiency, high power density, zero emission, scalability and fast start-up, Proton Exchange Membrane Fuel Cell (PEMFC) is a promising power source for
future transportation applications [1,2]. When a PEMFC system works as the automotive power, it suffers from the frequent start-up and shut-down or nearly random power load cycling, and confronts with durability issue in the practical road conditions. It is well known that for PEMFC
* Corresponding author. Tel./fax: þ86 10 62797595. ** Corresponding author. Tel./fax: þ86 10 82854648. E-mail addresses: [email protected] (C. Wang), [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.ijhydene.2016.04.215 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
technology to become commercially viable, it's durability must be improved to the level 5000 h (equivalent to 150,000 miles of driving) with less than 10% loss of performance according to DOE's Fuel Cell Technical Team Roadmap [2]. Membrane Electrode Assemblies (MEAs) play an important role in the performance, durability and cost of PEMFC. Generally, thin PTFE/Nafion composite membrane with high proton conductivity and good mechanical property is widely used in MEA manufacturing. Recently, experimental studies have been undertaken to evaluate the durability of MEA based on PTFE/ Nafion composite membrane under different accelerated life test (AST) techniques. Literatures show that the degradation can be accelerated with many methods such as startestop cycles, open-circuit voltage operation, relative humidity cycling and load cycling [3,4]. Jao et al. demonstrated four modes of degradation for MEA by acceleration protocols including opencircuit voltage, relative humidity cycling and load cycling, the maximum power density decreased by 0.227 mW/cm2/cycle [5]. Inaba et al. showed an increase in crossover current density from 0.8 to 11 mA/cm2 by operating at open-circuit voltage for 60 days, current density at 0.6 V decreased from 1.1 to 0.4 A/cm2 after load cycling operation for 900 h [6]. Three mechanisms for MEA based on PTFE/Nafion composite membrane were proposed by Jung et al. [7] to be: 1) separation of the catalyst layer from the membrane due to creep deformation; 2) separation of the outer Nafion layer film from the core PTFE/Nafion membrane due to creep deformation; 3) degradation of the Nafion plane (or Nafion dissolution) from the PTFE surface. In our studies, a complex accelerated stress test protocol “the MEA/Stack Durability Protocol” developed by the Fuel Cell Technical Team (FCTT) of the Freedom CAR and Fuel Partnership was used to assess durability of a commercial MEA based on PTFE/Nafion composite membrane (produced by Sunrise Power Co Ltd). The preliminary AST results of the MEA using the DOE MEA/Stack Durability Protocol have been reported in our previous article [8]. The performance degradation of the MEA has been diagnosed by electrochemical methods such as polarization curve, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) during the durability test in our previous article [8]. This article aims to further investigation of degradation mechanisms and failure modes of the MEA with hydrogen-thermal experiment, SEM, TEM, EDS and Ion Chromatographic analysis.
membrane) based on a 20 mm composite membrane with 0.6 mg Pt/cm2 and two pieces of GDLs (gas diffusion layer, SGL 25BC). Vulcanizing silicone was used on either side of membrane to provide fuel cell sealing. The assembly torque was 5 N m by the eight lubricated bolts of the fuel cell hardware. Reactant gas humidity, gas flow rate, cell temperature and discharge load were all controlled from a HS-150 Fuel Cell Test Equipment (Hephas Energy Co., Ltd.). The IMEA/Stack Durability ProtocolJ developed by FCTT of the FreedomCAR and Fuel Partnership was adopted for life test of the MEA. The test protocol program code was input in the interface of test software of HS Fuel cell test equipment for automatically measuring. During the AST of the MEA, 99.999% pure hydrogen fuel and dry air were fed into anode and cathode respectively with a mode of co-flow. The fuel cell temperature was maintained constantly at 80 C using the electric heating plate. The reactants were stoichiometrically controlled with outlet pressures maintained at ambient pressure, and were humidified at fuel cell temperature. To prevent water vapor from condensating on the surface of inlet pipe connecting fuel cell, the reactant gases lines into the fuel cell were heated 10 above the cell operating temperature. After 8 h activation operation, AST of fuel cell started up.
Hydrogen-thermal test For clearly identifying the failure of the MEA is attributed to the formation of pinhole, the hydrogen-thermal experiment was carried out. The failure MEA was fixed on the graphite substrate with one side sealed for introducing hydrogen into chamber at 20 kPa, and another side of the MEA was exposed to the air, which was employed for the test temperature surface by infrared probe measurement. To better understanding local temperature variation, the temperature test grid from the hydrogen inlet to the outlet direction was equally
Experimental Hardware and protocol for AST of the MEA The life test was performed in 25 cm2 single cell test fixture (Hephas Energy Co., Ltd.) with polar plates, gold-plating copper current collectors and super hard aluminum end plates. The flow fields of both anode and cathode were triple channel serpentine designs machined into graphite plates (Shanghai Hong Feng Industrial Company Limited) and were assembled in co-flow orientation. The channel depth, channel width, and rib width of the serpentine flow field plate were all 1 mm. The cell was assembled with a MEA manufactured by Sun-rise Power Ltd. In this study, the MEA consists of a CCM (catalyst coating
Fig. 1 e The temperature test grids in active area of the failure MEA.
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
patterned to 5 5 partitions on the entire active area of the MEA, as shown in Fig. 1.
Post-test analysis SEM is a common tool for morphology analysis of both GDL and CCM, which delivers surface images with high resolution in a wide range of magnifications. The failure MEA samples were prepared in small strips, which were cut after frozen in the liquid nitrogen to get clear cross-sectional image and the interface morphology between electrode and membrane. The thickness changes of the MEA were carried out by SEM (Nissei Sangyo Co. Ltd., Hitachi model S-4200), and the thicknesses of electrodes and the distributions of Pt catalyst were observed by SEM (JSM-6301F) and EDX (OXFORD). To confirm the migration of Pt particles into the membrane, TEM measurements were carried out using a JEOL JEM 1010 instrument. The drain water from cathode side of cells under AST was respectively collected for F or SO2 4 emission rate analysis by Ion Chromatograph (HIC-6A, Japan Shimadzu).
Results and discussions Pinhole formation analysis in the failure MEA Temperature test grids from the hydrogen inlet to the outlet direction of the failure MEA are divided into a pattern with 5 5 partitions in Fig. 1. If a pinhole is formed though the composite membrane in the failure MEA, the hydrogen permeated through the pinhole will be catalytically oxidized by the oxygen in the air, which could produce extra heat and be measured by hydrogen-thermal test. The pinhole on the failure MEA can be detected by measuring the temperature on each partition with infrared thermometer. The results of hydrogen-thermal test are shown in Fig. 2. Fig. 2a gives the temperature distribution on the surface of the failure MEA while hydrogen gas is just introduced, and the temperature at the outlet site on the surface of the failure MEA is 10 C higher than that at inlet site. After hydrogen flows for
3
30 min, the temperature at the gas outlet site on the surface of the failure MEA is much higher than other partitions, and the maximum temperature reaches about 70 C, which confirms the existence of the pinhole damage at this gas outlet site in the failure MEA based on PTFE/Nafion composite membrane.
Water quality analysis In this section, we present the major anions analysis for water produced from cathode of PEMFC during AST. Table 1 provides the average concentrations of the F and SO24- from PTFE/ Nafion composite membrane degradation evolving over time. It can be seen from Table 1 that there are F and SO2 4 in the water produced from PEMFC, with much higher concentration of F ion than that of SO2 4 , which indicates that the main chain, side chain and end groups of the perfluorosulfonic acid polymer are all involved in the degradation reactions. It is known that the intermediates such as hydrogen peroxide, OH radicals produced during the fuel cell working process are potential species that damage perfluorosulfonic acid polymer in the composite membrane by attacking at the alpha hydrogen [9], release of F and SO2 4 undoubtedly is a proof for change of the perfluorosulfonic acid polymer in the composite membrane or ionomer in catalyst layer. Cathode F emission rate increased substantially upon the time, but the measured is sharply increased around concentration of F and SO2 4 200 h in the AST, indicating the composite membrane experiences the relatively high degradation rate in this stage, which could be associated with the formation of pinhole in the composite membrane and the enhanced hydrogen permeability related to the local area of the membrane electrode membrane material degradation.
Analysis of GDL change Fig. 3 presents the images of enlarged GDL surfaces, with Fig. 3a and c showing the fresh GDL surface, and Fig. 3b and d giving these degraded surfaces after the AST. The GDL consists of carbon paper and water management layer (WML), whose key function is to disperse the reactant gases evenly
Fig. 2 e Temperature distribution on the surface of the failure MEA (a):Initial state after hydrogen-thermal test; (b): 30 min after hydrogen-thermal test. Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Table 1 e Major anions analyzed for membrane degradation. Anion Fluoride (F) Sulfate (SO2 4 )
Cumulative accelerated test time (h)
Unit
0
50
100
150
200
230
250
270
0.53 0.06
0.97 0.07
0.93 0.17
1.47 0.05
6.07 2.25
1.60 1.20
1.93 0.05
5.46 12.83
ug/mL ug/mL
Fig. 3 e SEM micrographs of GDL surface before and after AST (a) and (c): fresh GDL; (b) and (d): GDL after AST.
onto the CCM. The water management can be facilitated by the WML expelling excess water from the fuel cell and guarantee necessary water within the cell, therefore the catalyst flooding can be avoided and proton transport property of membrane can be optimized. Typical results can be observed that the surface of WML become rough and large voids are produced, and the carbon fiber of the carbon paper is exposed to the surface of GDL. There was one more distinct feature that the coating materials such as carbon black and PTFE covered on the carbon fiber have disappeared. This is mainly due to the PTFE loss in the acidic conditions, where the reaction is approximately 10% faster compared with the neutral conditions [10]. Furthermore, the reaction gas on the pore surface of GDL also leads to the degradation and erosion on both carbon black and PTFE, which results in the damage of the micro structure of the GDL, hence the mass transfer characteristics of the GDL is seriously damaged. Therefore the structural change and the degradation of the GDL during the AST leads to the deterioration of stability performance of this fuel cell. A greater voltage drop at current density 1.2 A/cm2 with test time is observed in our previous work [8], which is caused by mass transport
limitations, and the loss of hydrophobicity would weaken the water discharge of the GDL and lead to the reactant gases transport limitation.
Analysis of CCM change The SEM micrographs of the CCM, which are taken before and after life time test, are shown in Fig. 4. Fig. 4(a) shows the flat and porous surface of the fresh CCM, and Fig. 4(b) shows the degradation CCM, where the surface combined with the shedding debris from GDL and exhibited cracks compacted state surface could be observed. The sectional SEM micrographs are shown in Fig. 5, it can be seen from Fig. 5(a) that the CCM has a sandwiched structure with five layers, where the middle layer is reinforced matrix of porous PTFE film, outer layers on both sides are perfluorosulfonic acid membrane, and the outermost layers are made up of catalysts and Nafion ionomer. Fig. 5(bed) compare morphology of local positions for the failure CCM along the distribution of the flow field. It is evidenced that the Pt/C catalysts are uniformly dispersed on the Fresh CCM. In contrast, the failure CCM shows aggregated catalysts in the
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Fig. 4 e SEM micrographs of CCM surface before and after AST (a): fresh CCM; (b):Failure CCM.
Fig. 5 e Cross-section SEM images of CCM before and after ASTs (a): fresh CCM; (b): local failure CCM at the middle of flowfield; (c): local failure CCM at the outlet, and (d): local failure CCM at inlet.
layer as highlighted. More obviously the appearance of cracks in most regions of the catalyst layer is observed on the failure CCM. By considering the malfunction of the fuel cell, it is assumed that these structural changes may be associated with the mechanical strain caused by pressure drop and temperature change during the operation. Fig. 5 indicates the thickness of the failure CCM is reduced by 4 mm than that of the fresh CCM, which is mainly due to the reduced thickness of the reinforced matrix in composite membrane. This may be explained by the reaction at the cathode, eCH2e þ 2H2 / eCH2e þ 2HF, namely the hydrogen reacts with the perfluorosulfonic acid polymer skeleton in the reinforced matrix. The thickness of the reinforced matrix is reduced to less than 3 mm at the outlet, and the partial peeling
phenomenon can be observed in Fig. 5 (as the circles highlighted). The thickness of the failure CCM at outlet gives less change than that at inlet, but the cathode shows some local cracks, which is consistent with the linear sweep test [8]. Meanwhile, it can be seen, the cathode thicknesses at both outlet zone and inlet zone are significantly reduced, which is mainly concerned with the oxidation corrosion of the C support of Pt/C, and the Pt is oxidized to form the PtO, which is further dissolved in the acidic composite membrane. For further making clear of the microstructure of the cathode, the cross-sectional SEM images of the failure CCM are presented in Fig. 6. Cathode catalyst layer mainly consists of nanoscale secondary particles, which are made of nano Pt/ C catalyst particles formed in perfluorosulfonic acid polymer
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Fig. 6 e Cross-section SEM micrographs of cathode in the failure CCM.
adhesive. Secondary particles in the cathode present more uniform distribution than that in the failure one, but it's clearly observed that in the failure CCM the formation of large pores, destruction of the electron and proton transfer channel in cathode, result in a big resistance for charge transfer, which is consistent with the AC impedance test [8].
EDS test at sectional CCM Fig. 7 gives the Pt concentration distribution over crosssection of the failure CCM. Weak Pt peaks could be observed
in the composite membrane both the local inlet position and local outlet position, which indicates that Pt diffusion from catalyst layer to the composite occurs during AST. Considering the potential effect on Pt dissolution, the process of Pt dissolution most likely occurs on the cathode side. The Pt precipitation in the composite membrane occurs via the reduction of Pt ions by hydrogen that has crossed over from the anode. Pt particles precipitation in the composite membrane may act as catalyst for OH free radical direct generation without the H2O2 intermediate and cause membrane degradation.
Fig. 7 e Platinum EDS line scan analysis over cross-section of the failure CCM (a): local position at inlet; (b): local position at outlet. Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Fig. 8 e TEM micrographs of fresh and tested catalyst (a): fresh catalyst; (b): catalyst in cathode at the inlet position after AST; (c): catalyst in cathode at the middle of flow field after AST; (d): catalyst in cathode at the outlet position after AST.
TEM analysis of catalyst particles The Pt morphology along the flow field before and after AST was characterized by TEM and showed in Fig. 8. The average Pt diameter of the fresh Pt/C catalyst is about 3e4 nm. After AST, the average Pt diameter in cathode at the inlet position is approximately 5e6 nm, the average Pt diameter in cathode at the middle of the flow field is approximately 6e7 nm, and the average Pt diameter in cathode at the outlet position is about 7e8 nm, therefore, the size of Pt particles is increased gradually along the flow field, the agglomeration phenomenon of Pt particles becomes more obvious. One interesting phenomenon is that along the inlet to the outlet of the flow field, the catalyst Pt degradation shows an increasing trend. This is explained that Pt degradation process of dissolution and migration is accelerated by the increasing water content along the flow field under AST conditions, especially the saturated water conditions could lead to the acceleration of small Pt particles dissolving in the ionomer phase and re-depositing on larger particles. Therefore, flooding at the outlet of the flow field is very likely to take place and it leads to the maximum degradation of the Pt catalyst.
current density 100 mA/cm2. During AST, the amount of released F ion is much more than that of SO2 4 in the water produced from PEMFC, but the measured concentration of F and SO2 was sharply increased around 200 h in the AST, 4 which could be associated with the formation of pinhole in the composite membrane and verified by the results of hydrogenthermal test. Post-test analysis of the failure MEA indicated that damage of the micro structure and hydrophobicity of WML in the GDL resulted to the deterioration of stability performance of PEMFC due to mass transport limitations. The thickness reducing of the failure CCM was mainly due to the reinforced matrix tinning in composite membrane, the formation of large pores in catalyst layer in CCM resulted in a big resistance for charge transfer due to destruction of the electron and proton transfer channel in cathode, and weak Pt peaks could be observed in the composite membrane both the local inlet position and local outlet position, which indicated that Pt diffusion from catalyst layer to the composite occurs during AST. One interesting phenomenon was observed that the Pt catalyst particles growing up along the inlet to the outlet of the flow field showed an increasing trend, which could be explained that Pt degradation process of dissolution and migration is accelerated by the increasing water content along the flow field.
Conclusions In this study, a comprehensive analysis of failure mechanism for MEA based on PTFE/Nafion composite membrane was analyzed by “the MEA/Stack Durability Protocol” developed by the Fuel Cell Technical Team (FCTT) of the Freedom CAR and Fuel Partnership. The accelerated life test lasted for 300 h, voltage decay rate of the MEA is about 0.48 mV/h at operating
Acknowledgments This work was financially supported by The National Natural Science fund project of China (No. U1462112, 21573122), the Program of International Science and Technology Cooperation (2013DFG41460; 2013DFG60080), the State Key Basic Science
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Research Project of China (No. 2012CB215401) and State key laboratory of automotive safety and energy (No: ZZ2014-031). [6]
references [7] [1] Cheng Wang, Shubo Wang, Jianbo Zhang, Jianqiu Li, Jianlong Wang, Minggao OuYang. The durability research on the proton exchange membrane fuel cell for automobile application. Prog Chem 2015;27(4):424e35. [2] Cheng Wang, Shubo Wang, Jianbo Zhang, Jianqiu Li, Minggao OuYang, Jianlong Wang. The key materials and components for proton exchange membrane fuel cell. Prog Chem 2015;27(2e3):310e20. [3] Zhang S, Yuan XZ, Hin JNC, Haijiang Wang, Jinfeng Wu, Friedrich KA, et al. Effects of open-circuit operation on membrane and catalyst layer degradation in proton exchange membrane fuel cells. J Power Sources 2010;195:1142e8. [4] De Bruijn FA, Dam VAT, Janssen GJM. Review: durability and degradation issues of PEM fuel cell components. Fuel Cells 2008;8:3e22. [5] Jao Ting-Chu, Jung Guo-Bin, Ke Shih-Tsung, Chi Pei-Hung, Chan Shih-Hung. Diagnosis of PTFE-Nafion MEA degradation
[8]
[9]
[10]
modes by an accelerated degradation technique. Int J Energy Res 2011;35:1274e83. Inaba M, Kinumoto T, Kiriake M, Umebayashi R, Tasaka A, Ogumi Z. Gas crossover and membrane degradation in polymer electrolyte fuel cells. Electrochim Acta 2006;51:5746e53. Jao Ting-Chu, Jung Guo-Bin, Ke Shih-Tsung, Tzeng Wei-Jen, Su Ay. Degradation mechanism study of PTFE/Nafion membrane in MEA utilizing an accelerated degradation technique. Int J Energy Res 2012;37:13623e30. Liu Mingyang, Wang Cheng, Xie Fucheng, Mao Zongqiang. A polymer electrolyte fuel cell life test using accelerating degradation technique. Int J Hydrogen Energy 2013;38(25):11011e6. Panchenko A, Dilger H, Kerres J, Hein M, Ullrich A, Kaz T. Insitu spin trap electron paramagnetic resonance study of fuel cell processes. Phys Chem Chem Phys 2004;6(11):2891e4. Kim Han-Sang, Lee Eunsook, Jyoung Jy-Young, Min Kyoungdoug, Kim Han-Sang, Lee Eunsook, et al. Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells. Int J Hydrogen Energy 2011;36(19):12427e35.
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique Cheng Wang a,*, Jianbo Zhang b, Shubo Wang a, Sijia Hao a, Jianqiu Li b, Zongqiang Mao a, Zhiming Mao d, Minggao Ouyang b, Zhixiang Liu c,** a
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, People's Republic of China State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, People's Republic of China c School of Material Science and Energy Engineering, Foshan University, Foshan, 528000, Guangdong, People's Republic of China d Beijing Sino Hydrogen Technology Co., Ltd., Beijing 100084, People's Republic of China b
article info
abstract
Article history:
Durability is one of the key issues in commercialization of proton exchange membrane fuel
Received 23 January 2016
cell (PEMFC). The main purpose of this study is to investigate the degradation mechanism
Received in revised form
of Membrane Electrode Assembly (MEA) based on PTFE/Nafion composite membrane in
29 April 2016
PEMFC through the MEA/Stack Durability Protocol developed by the Fuel Cell Technical
Accepted 29 April 2016
Team (FCTT) of the Freedom CAR and Fuel Partnership. The accelerated life test lasted for
Available online xxx
300 h, with voltage decay rate of the MEA about 0.48 mV/h at operating current density 100 mA/cm2 being achieved. After the acceleration experiment, degradation mechanisms
Keywords:
for the MEA based on PTFE/Nafion composite membrane were analyzed in detail by
Proton exchange membrane fuel cell
hydrogen-thermal experiment, SEM, TEM, EDS and Ion Chromatographic test. The exper-
Durability
imental results show that pinhole formed at outlet position of the composite membrane
Accelerated stress test
leads to hydrogen crossover current density exceeding 21 mA/cm2 at 210 h, with F and
MEA degradation mechanism
SO2 4 concentrations sharply increased around 200 h in discharge water of fuel cell from IC results, therefore, membrane failure is the major factor in the failure of MEA. Moreover, the micro-structural damage in the MEA and Pt particles growing up along flow channel even appearing at membrane could be observed from SEM, TEM and EDS results, which play important role in the deterioration of performance and stability for PEMFC. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to its benefits of high efficiency, high power density, zero emission, scalability and fast start-up, Proton Exchange Membrane Fuel Cell (PEMFC) is a promising power source for
future transportation applications [1,2]. When a PEMFC system works as the automotive power, it suffers from the frequent start-up and shut-down or nearly random power load cycling, and confronts with durability issue in the practical road conditions. It is well known that for PEMFC
* Corresponding author. Tel./fax: þ86 10 62797595. ** Corresponding author. Tel./fax: þ86 10 82854648. E-mail addresses: [email protected] (C. Wang), [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.ijhydene.2016.04.215 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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technology to become commercially viable, it's durability must be improved to the level 5000 h (equivalent to 150,000 miles of driving) with less than 10% loss of performance according to DOE's Fuel Cell Technical Team Roadmap [2]. Membrane Electrode Assemblies (MEAs) play an important role in the performance, durability and cost of PEMFC. Generally, thin PTFE/Nafion composite membrane with high proton conductivity and good mechanical property is widely used in MEA manufacturing. Recently, experimental studies have been undertaken to evaluate the durability of MEA based on PTFE/ Nafion composite membrane under different accelerated life test (AST) techniques. Literatures show that the degradation can be accelerated with many methods such as startestop cycles, open-circuit voltage operation, relative humidity cycling and load cycling [3,4]. Jao et al. demonstrated four modes of degradation for MEA by acceleration protocols including opencircuit voltage, relative humidity cycling and load cycling, the maximum power density decreased by 0.227 mW/cm2/cycle [5]. Inaba et al. showed an increase in crossover current density from 0.8 to 11 mA/cm2 by operating at open-circuit voltage for 60 days, current density at 0.6 V decreased from 1.1 to 0.4 A/cm2 after load cycling operation for 900 h [6]. Three mechanisms for MEA based on PTFE/Nafion composite membrane were proposed by Jung et al. [7] to be: 1) separation of the catalyst layer from the membrane due to creep deformation; 2) separation of the outer Nafion layer film from the core PTFE/Nafion membrane due to creep deformation; 3) degradation of the Nafion plane (or Nafion dissolution) from the PTFE surface. In our studies, a complex accelerated stress test protocol “the MEA/Stack Durability Protocol” developed by the Fuel Cell Technical Team (FCTT) of the Freedom CAR and Fuel Partnership was used to assess durability of a commercial MEA based on PTFE/Nafion composite membrane (produced by Sunrise Power Co Ltd). The preliminary AST results of the MEA using the DOE MEA/Stack Durability Protocol have been reported in our previous article [8]. The performance degradation of the MEA has been diagnosed by electrochemical methods such as polarization curve, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) during the durability test in our previous article [8]. This article aims to further investigation of degradation mechanisms and failure modes of the MEA with hydrogen-thermal experiment, SEM, TEM, EDS and Ion Chromatographic analysis.
membrane) based on a 20 mm composite membrane with 0.6 mg Pt/cm2 and two pieces of GDLs (gas diffusion layer, SGL 25BC). Vulcanizing silicone was used on either side of membrane to provide fuel cell sealing. The assembly torque was 5 N m by the eight lubricated bolts of the fuel cell hardware. Reactant gas humidity, gas flow rate, cell temperature and discharge load were all controlled from a HS-150 Fuel Cell Test Equipment (Hephas Energy Co., Ltd.). The IMEA/Stack Durability ProtocolJ developed by FCTT of the FreedomCAR and Fuel Partnership was adopted for life test of the MEA. The test protocol program code was input in the interface of test software of HS Fuel cell test equipment for automatically measuring. During the AST of the MEA, 99.999% pure hydrogen fuel and dry air were fed into anode and cathode respectively with a mode of co-flow. The fuel cell temperature was maintained constantly at 80 C using the electric heating plate. The reactants were stoichiometrically controlled with outlet pressures maintained at ambient pressure, and were humidified at fuel cell temperature. To prevent water vapor from condensating on the surface of inlet pipe connecting fuel cell, the reactant gases lines into the fuel cell were heated 10 above the cell operating temperature. After 8 h activation operation, AST of fuel cell started up.
Hydrogen-thermal test For clearly identifying the failure of the MEA is attributed to the formation of pinhole, the hydrogen-thermal experiment was carried out. The failure MEA was fixed on the graphite substrate with one side sealed for introducing hydrogen into chamber at 20 kPa, and another side of the MEA was exposed to the air, which was employed for the test temperature surface by infrared probe measurement. To better understanding local temperature variation, the temperature test grid from the hydrogen inlet to the outlet direction was equally
Experimental Hardware and protocol for AST of the MEA The life test was performed in 25 cm2 single cell test fixture (Hephas Energy Co., Ltd.) with polar plates, gold-plating copper current collectors and super hard aluminum end plates. The flow fields of both anode and cathode were triple channel serpentine designs machined into graphite plates (Shanghai Hong Feng Industrial Company Limited) and were assembled in co-flow orientation. The channel depth, channel width, and rib width of the serpentine flow field plate were all 1 mm. The cell was assembled with a MEA manufactured by Sun-rise Power Ltd. In this study, the MEA consists of a CCM (catalyst coating
Fig. 1 e The temperature test grids in active area of the failure MEA.
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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patterned to 5 5 partitions on the entire active area of the MEA, as shown in Fig. 1.
Post-test analysis SEM is a common tool for morphology analysis of both GDL and CCM, which delivers surface images with high resolution in a wide range of magnifications. The failure MEA samples were prepared in small strips, which were cut after frozen in the liquid nitrogen to get clear cross-sectional image and the interface morphology between electrode and membrane. The thickness changes of the MEA were carried out by SEM (Nissei Sangyo Co. Ltd., Hitachi model S-4200), and the thicknesses of electrodes and the distributions of Pt catalyst were observed by SEM (JSM-6301F) and EDX (OXFORD). To confirm the migration of Pt particles into the membrane, TEM measurements were carried out using a JEOL JEM 1010 instrument. The drain water from cathode side of cells under AST was respectively collected for F or SO2 4 emission rate analysis by Ion Chromatograph (HIC-6A, Japan Shimadzu).
Results and discussions Pinhole formation analysis in the failure MEA Temperature test grids from the hydrogen inlet to the outlet direction of the failure MEA are divided into a pattern with 5 5 partitions in Fig. 1. If a pinhole is formed though the composite membrane in the failure MEA, the hydrogen permeated through the pinhole will be catalytically oxidized by the oxygen in the air, which could produce extra heat and be measured by hydrogen-thermal test. The pinhole on the failure MEA can be detected by measuring the temperature on each partition with infrared thermometer. The results of hydrogen-thermal test are shown in Fig. 2. Fig. 2a gives the temperature distribution on the surface of the failure MEA while hydrogen gas is just introduced, and the temperature at the outlet site on the surface of the failure MEA is 10 C higher than that at inlet site. After hydrogen flows for
3
30 min, the temperature at the gas outlet site on the surface of the failure MEA is much higher than other partitions, and the maximum temperature reaches about 70 C, which confirms the existence of the pinhole damage at this gas outlet site in the failure MEA based on PTFE/Nafion composite membrane.
Water quality analysis In this section, we present the major anions analysis for water produced from cathode of PEMFC during AST. Table 1 provides the average concentrations of the F and SO24- from PTFE/ Nafion composite membrane degradation evolving over time. It can be seen from Table 1 that there are F and SO2 4 in the water produced from PEMFC, with much higher concentration of F ion than that of SO2 4 , which indicates that the main chain, side chain and end groups of the perfluorosulfonic acid polymer are all involved in the degradation reactions. It is known that the intermediates such as hydrogen peroxide, OH radicals produced during the fuel cell working process are potential species that damage perfluorosulfonic acid polymer in the composite membrane by attacking at the alpha hydrogen [9], release of F and SO2 4 undoubtedly is a proof for change of the perfluorosulfonic acid polymer in the composite membrane or ionomer in catalyst layer. Cathode F emission rate increased substantially upon the time, but the measured is sharply increased around concentration of F and SO2 4 200 h in the AST, indicating the composite membrane experiences the relatively high degradation rate in this stage, which could be associated with the formation of pinhole in the composite membrane and the enhanced hydrogen permeability related to the local area of the membrane electrode membrane material degradation.
Analysis of GDL change Fig. 3 presents the images of enlarged GDL surfaces, with Fig. 3a and c showing the fresh GDL surface, and Fig. 3b and d giving these degraded surfaces after the AST. The GDL consists of carbon paper and water management layer (WML), whose key function is to disperse the reactant gases evenly
Fig. 2 e Temperature distribution on the surface of the failure MEA (a):Initial state after hydrogen-thermal test; (b): 30 min after hydrogen-thermal test. Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Table 1 e Major anions analyzed for membrane degradation. Anion Fluoride (F) Sulfate (SO2 4 )
Cumulative accelerated test time (h)
Unit
0
50
100
150
200
230
250
270
0.53 0.06
0.97 0.07
0.93 0.17
1.47 0.05
6.07 2.25
1.60 1.20
1.93 0.05
5.46 12.83
ug/mL ug/mL
Fig. 3 e SEM micrographs of GDL surface before and after AST (a) and (c): fresh GDL; (b) and (d): GDL after AST.
onto the CCM. The water management can be facilitated by the WML expelling excess water from the fuel cell and guarantee necessary water within the cell, therefore the catalyst flooding can be avoided and proton transport property of membrane can be optimized. Typical results can be observed that the surface of WML become rough and large voids are produced, and the carbon fiber of the carbon paper is exposed to the surface of GDL. There was one more distinct feature that the coating materials such as carbon black and PTFE covered on the carbon fiber have disappeared. This is mainly due to the PTFE loss in the acidic conditions, where the reaction is approximately 10% faster compared with the neutral conditions [10]. Furthermore, the reaction gas on the pore surface of GDL also leads to the degradation and erosion on both carbon black and PTFE, which results in the damage of the micro structure of the GDL, hence the mass transfer characteristics of the GDL is seriously damaged. Therefore the structural change and the degradation of the GDL during the AST leads to the deterioration of stability performance of this fuel cell. A greater voltage drop at current density 1.2 A/cm2 with test time is observed in our previous work [8], which is caused by mass transport
limitations, and the loss of hydrophobicity would weaken the water discharge of the GDL and lead to the reactant gases transport limitation.
Analysis of CCM change The SEM micrographs of the CCM, which are taken before and after life time test, are shown in Fig. 4. Fig. 4(a) shows the flat and porous surface of the fresh CCM, and Fig. 4(b) shows the degradation CCM, where the surface combined with the shedding debris from GDL and exhibited cracks compacted state surface could be observed. The sectional SEM micrographs are shown in Fig. 5, it can be seen from Fig. 5(a) that the CCM has a sandwiched structure with five layers, where the middle layer is reinforced matrix of porous PTFE film, outer layers on both sides are perfluorosulfonic acid membrane, and the outermost layers are made up of catalysts and Nafion ionomer. Fig. 5(bed) compare morphology of local positions for the failure CCM along the distribution of the flow field. It is evidenced that the Pt/C catalysts are uniformly dispersed on the Fresh CCM. In contrast, the failure CCM shows aggregated catalysts in the
Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Fig. 4 e SEM micrographs of CCM surface before and after AST (a): fresh CCM; (b):Failure CCM.
Fig. 5 e Cross-section SEM images of CCM before and after ASTs (a): fresh CCM; (b): local failure CCM at the middle of flowfield; (c): local failure CCM at the outlet, and (d): local failure CCM at inlet.
layer as highlighted. More obviously the appearance of cracks in most regions of the catalyst layer is observed on the failure CCM. By considering the malfunction of the fuel cell, it is assumed that these structural changes may be associated with the mechanical strain caused by pressure drop and temperature change during the operation. Fig. 5 indicates the thickness of the failure CCM is reduced by 4 mm than that of the fresh CCM, which is mainly due to the reduced thickness of the reinforced matrix in composite membrane. This may be explained by the reaction at the cathode, eCH2e þ 2H2 / eCH2e þ 2HF, namely the hydrogen reacts with the perfluorosulfonic acid polymer skeleton in the reinforced matrix. The thickness of the reinforced matrix is reduced to less than 3 mm at the outlet, and the partial peeling
phenomenon can be observed in Fig. 5 (as the circles highlighted). The thickness of the failure CCM at outlet gives less change than that at inlet, but the cathode shows some local cracks, which is consistent with the linear sweep test [8]. Meanwhile, it can be seen, the cathode thicknesses at both outlet zone and inlet zone are significantly reduced, which is mainly concerned with the oxidation corrosion of the C support of Pt/C, and the Pt is oxidized to form the PtO, which is further dissolved in the acidic composite membrane. For further making clear of the microstructure of the cathode, the cross-sectional SEM images of the failure CCM are presented in Fig. 6. Cathode catalyst layer mainly consists of nanoscale secondary particles, which are made of nano Pt/ C catalyst particles formed in perfluorosulfonic acid polymer
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Fig. 6 e Cross-section SEM micrographs of cathode in the failure CCM.
adhesive. Secondary particles in the cathode present more uniform distribution than that in the failure one, but it's clearly observed that in the failure CCM the formation of large pores, destruction of the electron and proton transfer channel in cathode, result in a big resistance for charge transfer, which is consistent with the AC impedance test [8].
EDS test at sectional CCM Fig. 7 gives the Pt concentration distribution over crosssection of the failure CCM. Weak Pt peaks could be observed
in the composite membrane both the local inlet position and local outlet position, which indicates that Pt diffusion from catalyst layer to the composite occurs during AST. Considering the potential effect on Pt dissolution, the process of Pt dissolution most likely occurs on the cathode side. The Pt precipitation in the composite membrane occurs via the reduction of Pt ions by hydrogen that has crossed over from the anode. Pt particles precipitation in the composite membrane may act as catalyst for OH free radical direct generation without the H2O2 intermediate and cause membrane degradation.
Fig. 7 e Platinum EDS line scan analysis over cross-section of the failure CCM (a): local position at inlet; (b): local position at outlet. Please cite this article in press as: Wang C, et al., Degradation study of Membrane Electrode Assembly with PTFE/Nafion composite membrane utilizing accelerated stress technique, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.215
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Fig. 8 e TEM micrographs of fresh and tested catalyst (a): fresh catalyst; (b): catalyst in cathode at the inlet position after AST; (c): catalyst in cathode at the middle of flow field after AST; (d): catalyst in cathode at the outlet position after AST.
TEM analysis of catalyst particles The Pt morphology along the flow field before and after AST was characterized by TEM and showed in Fig. 8. The average Pt diameter of the fresh Pt/C catalyst is about 3e4 nm. After AST, the average Pt diameter in cathode at the inlet position is approximately 5e6 nm, the average Pt diameter in cathode at the middle of the flow field is approximately 6e7 nm, and the average Pt diameter in cathode at the outlet position is about 7e8 nm, therefore, the size of Pt particles is increased gradually along the flow field, the agglomeration phenomenon of Pt particles becomes more obvious. One interesting phenomenon is that along the inlet to the outlet of the flow field, the catalyst Pt degradation shows an increasing trend. This is explained that Pt degradation process of dissolution and migration is accelerated by the increasing water content along the flow field under AST conditions, especially the saturated water conditions could lead to the acceleration of small Pt particles dissolving in the ionomer phase and re-depositing on larger particles. Therefore, flooding at the outlet of the flow field is very likely to take place and it leads to the maximum degradation of the Pt catalyst.
current density 100 mA/cm2. During AST, the amount of released F ion is much more than that of SO2 4 in the water produced from PEMFC, but the measured concentration of F and SO2 was sharply increased around 200 h in the AST, 4 which could be associated with the formation of pinhole in the composite membrane and verified by the results of hydrogenthermal test. Post-test analysis of the failure MEA indicated that damage of the micro structure and hydrophobicity of WML in the GDL resulted to the deterioration of stability performance of PEMFC due to mass transport limitations. The thickness reducing of the failure CCM was mainly due to the reinforced matrix tinning in composite membrane, the formation of large pores in catalyst layer in CCM resulted in a big resistance for charge transfer due to destruction of the electron and proton transfer channel in cathode, and weak Pt peaks could be observed in the composite membrane both the local inlet position and local outlet position, which indicated that Pt diffusion from catalyst layer to the composite occurs during AST. One interesting phenomenon was observed that the Pt catalyst particles growing up along the inlet to the outlet of the flow field showed an increasing trend, which could be explained that Pt degradation process of dissolution and migration is accelerated by the increasing water content along the flow field.
Conclusions In this study, a comprehensive analysis of failure mechanism for MEA based on PTFE/Nafion composite membrane was analyzed by “the MEA/Stack Durability Protocol” developed by the Fuel Cell Technical Team (FCTT) of the Freedom CAR and Fuel Partnership. The accelerated life test lasted for 300 h, voltage decay rate of the MEA is about 0.48 mV/h at operating
Acknowledgments This work was financially supported by The National Natural Science fund project of China (No. U1462112, 21573122), the Program of International Science and Technology Cooperation (2013DFG41460; 2013DFG60080), the State Key Basic Science
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Research Project of China (No. 2012CB215401) and State key laboratory of automotive safety and energy (No: ZZ2014-031). [6]
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