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Effect of air purging and dry operation on durability of PEMFC under freeze/thaw cycles Kah-Young Song, Hee-Tak Kim* Samsung SDI Co., LTD, 428-5 Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-577, Republic of Korea
article info
abstract
Article history:
The effect of air purging and dry operation on durability of polymer electrolyte membrane
Received 25 March 2011
fuel cell (PEMFC) under repeated freeze/thaw cycles between 20 C and 60 C was
Received in revised form
investigated. The cathode air purging and the operation with dry air feed were highly
14 June 2011
effective to mitigate freeze damage. The removal of the air purging of the anode
Accepted 17 June 2011
compartment did not lead to the degradation of the anode catalyst layer. It is of practical
Available online 23 July 2011
importance, because the air purging of the anode could cause carbon corrosion of the cathode. The performance degradation by the freeze/thaw cycles was associated with the
Keywords:
increased charge transfer and mass transfer resistances. After the freeze/thaw cycles, any
Polymer electrolyte membrane fuel
discernable morphological changes were not observed in the scanning electron micro-
cell
scopic images of the anode, the membrane and the membrane/electrode interface,
Freeze/thaw cycle
however, mechanical damage of the poly(tetrafluoroethylene) phases in the cathode
Freeze damage
diffusion layer was detected.
Air purging
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Membrane electrode assembly
1.
Introduction
One of the remaining technical challenges for polymer electrolyte membrane fuel cell (PEMFC) commercialization for outdoor applications is to achieve storage at sub-zero temperature without damage. The durability of membrane electrode assembly (MEA) under freeze/thaw cycles has been attracting a lot of attention because it is most likely that the water produced in the cathode freezes at sub-zero temperature and the mechanical stress induced by ice formation leads to irreversible degradation of MEA. There exist conflicts in the literature regarding freeze damage of MEA. One important result shows that fuel cells dried to remove all liquid water during the shutdown experience neither observable physical damage nor electrochemical losses by freezing [1e5]. However, there are conflicting results in case of a cell with no significant purge. Some reported
reserved.
physical damage, performance loss, and electrochemical loss (electrochemical surface area, interfacial and charge transfer resistance increase) [2,6,7]. Physical damage includes membrane failure (holes and cracks), catalyst cracks and delamination, pore distribution change, and gas diffusion media fracture. Others observed no significant performance loss even without dry purge during the shutdown [8]. The conflict is expected to be due to the difference in remaining water content inside MEA and the difference in a critical water content above which freeze damage becomes significant. If the porous catalyst layer or diffusion layer can withstand the stress generated by ice formation, freezing damage could not be significant even without dry purging. One the other hand, the development of water removal process during shutdown is highly important to prevent any possible freeze damage in practical applications. A frequently used method to avoid freezing damage of PEMFC is air purging
* Corresponding author. Tel.: þ82 31 288 4570. E-mail address: [email protected] (H.-T. Kim). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.095
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2.
Experimental
2.1.
Freeze/thaw cycles and purging experiment
Single cells were assembled with a 26 cm2-sized commercial catalyst coated membrane (PRIMEA 5761, W.L. Gore & Associates) based on Pt/C (cathode) and PtRu/C (anode), a pair of gas diffusion media (35BC, SGL), a pair of Teflon gaskets and a pair of graphite blocks with triple serpentine flow fields for air and H2 feeds. The Pt loading for the cathode was 0.40 mg cm2. The Pt and Ru loading for the anode were 0.30 and 0.15 mg cm2, respectively. The thickness of the membrane was 18 mm. The channel depth, channel width and land width of the triple serpentine were 1.00, 0.79, 0.91 mm, respectively. The flow fields for anode- and cathode reactants were mirror images. PtRu/C and Pt/C were used as the anodeand cathode catalysts, respectively. Three different cells (one each with WO/NP, WO/AP, and DO/AP) with different operating and purging logics are conducted as depicted in Fig. 1. NP and AP represent no purging and air purging of the cathode, respectively. The AP process includes cooling of the cell down to 30 C with closing any inlet
100 80
Cell Operation WO/NP & WO/AP : IV (80%/80%(A/C)) DO/AP : IV (80%/80%(A/C)) + CC (80%/dry(A/C))
60 40
Cooling/purging (30min) WO/NP : No purging WO/AP & DO/AP : Cathode Purging with dry air
20
1cycle
o
Temperature (C)
of the cathode and anode compartments prior to low temperature storage. It is simple and effective method. However, the generation of the hydrogen and air boundary at the anode leads to the increase of cathode potential and consequent carbon corrosion at the cathode [9,10]. Therefore, purging of anode compartment with dry air is problematic. If the anode is not damaged through freeze/thaw cycles even in the absence of air purging of anode, it is possible to mitigate freeze damage by purging of only cathode without suffering from carbon corrosion at cathode. Since the recent publications on freezing damage of the MEA concentrated on the changes of cathode and membrane, it has remained unclear that freeze damage of the anode happens in the case of no anode purging. The present study deals with the durability of PEMFC MEA during freeze/thaw cycles with various purging logics and operating conditions. The main objective of this work is to (i) to quantify the freeze damage of the anode and the cathode individually, in the case of no purging, (ii) to check the effect of removing anode purging on the freeze damage, and (iii) to investigate the effect of the operation with dry air feed on freeze damage. The operation with dry air feed would reduce bulk liquid water in the cathode, and could provide better freeze/thaw durability. Three different operation and shutdown logics are applied; the first does not include any purging, the second has air purging of cathode and the third has additional operation with dry air feed prior to air purging of cathode. Freeze/thaw cycle from 20 C to 60 C was repeated 20 times. After each cycle, IV polarization was measured at 60 C to monitor power evolution with cycle number. In order to elucidate degradation mechanism, extensive electrochemical analysis based on cyclic voltammetry (CV), CO stripping voltammetry and electrochemical impedance spectroscopy (EIS) was made for the MEAs before and after 20 cycles. Postmortem analysis based on scanning electron microscopy (SEM) was also conducted after the freeze/thaw cycles.
0 -20 Storage at –20o C (2hrs)
-40
Time (hr) Fig. 1 e Temperatures profile of the freeze/thaw cycle.
and outlet of cathode and anode compartments, and the subsequent injection of dry air to the cathode compartment at flow rate of 500 sccm for 30 min while closing inlet and outlet of anode compartment. The wet operation (WO) corresponds to the measurement of IV polarization at 60 C with 80% humidified air and H2 feeds. The detailed description for the IV polarization measurement is given in the next section. The dry operation (DO) means a constant-current operation at 300 mA cm2 for 1 h, subsequent to WO, with dry air feed at stoichiometry of 2.0 and with 80% humidified hydrogen at stoichiometry of 1.2. The stoichiometry of 2.0 for cathode air feed was selected for stable operation with dry air feed; when the stoichiometry is higher than 2.0, cell drying happens leading to a voltage decay. The freeze/thaw cycles consist of the repetitive steps of cell operation at 60 C and low temperature storage at 20 C. The cell cooling from 20 C to 20 C, the storage at 20 C for 2 h and the cell heating from 20 C to 60 C were conducted in a temperature controlled oven. All the inlets and outlets were closed during the cooling, low temperature storage and heating process. The cooling and heating rates were 0.67 C/ min. WO step was applied for WO/NP and WO/AP; DO step only for DO/AP.
2.2.
IV Polarization measurements
IV Polarization curves were obtained by stepping the current density, allowing the cell to stabilize, and measuring the cell voltage. Two minutes were spent at each current density with the cell voltage collected every 2 s. For hydrogen, a constant mass flow rate of 43.5 sccm was used for current density below 200 mA cm2; above 200 mA cm2, a constant stoichiometry of 1.2 was used. An air flow at a constant mass flow rate of 215.6 sccm was used for current density below 200 mA cm2; above 200 mA cm2, a constant stoichiometry of 2.5 was used. The cell temperature was controlled to remain at 60 C.
2.3.
CV and CO stripping voltammetry measurement
CV of the single cell was carried out to determine electrochemical area (ECA) of the cathode using a potentiostat (SI
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2.4.
EIS measurement
EIS was conducted to determine ohmic resistance, charge transfer resistance and mass transfer resistance in the frequency range of 10 kHz to 100 mHz with 10 steps per decade using a potentiostat (SI 1287, Solartron Analytical) combined with a frequency response analyzer (SI 1255B, Solartron Analytical) and a power booster (SI 1290, Solartron Analytical). The amplitude of the sinusoidal voltage signal did not exceed 10 mV. The cathode was used as a working electrode and the anode as a counter electrode. The counter electrode also served as a reference electrode, because the over-potential of hydrogen oxidation at the counter electrode is negligible.
2.5. Relative humidity measurement with mass spectroscopy The relative humidity (RH) of the cathode effluent during dry air purging of the cathode compartment was determined from mass spectroscopy (HPR20, Hiden Analytical). The mass spectroscopy was connected to the outlet of the cathode and the mass spectrogram was in-situ measured during dry air purging. Air feeds with various RH were injected to the mass spectroscopy, and the relationship between the RH of air feed and the intensity ratio of water and nitrogen peaks in the mass spectrogram. The plot of the intensity ratio of water and nitrogen peaks as a function of RH shows an excellent linearity. From the relationship, the RH of the cathode effluent during dry air purging was in-situ determined.
2.6.
Contact angle measurement
The contact angles of diffusion layers before and after the freeze/thaw cycles were measured by the sessile-drop test using a contact angle system (DSA 100, Kru¨ss) at room temperature. For each measurement, 15 ml pure water droplet was made by placing the tip of the syringe close to the diffusion layer surface. Then the contact angle was measured at about 15 s after the droplet attached to the surface of the diffusion layer. For better accuracy, measurements of contact
angles were performed at five different random regions on each sample, and then the average value was determined.
2.7.
Postmortem analysis
The SEM images of the diffusion layers and the catalyst coated membranes after 20th freeze/thaw cycle were obtained by using Hitachi S-4700 SEM at Korea Basic Science Institute in Jeonju.
3.
Results and discussion
3.1. cycles
Performance degradation during the freeze/thaw
Ambient air of which RH is 4% at 60 C is used as a cathode purging gas in this work. It carries water vapor from MEA while passing through the cathode compartments. As MEA is dried, the amount of water transferred from MEA to purging gas is reduced. Therefore, RH of the cathode effluent of purging gas qualitatively informs the amount of water remained in MEA during cathode air purging. In order to investigate the effects of the air purging and dry operation, the amount of water in the cathode effluent during air purging was monitored for WO/AP and DO/AP with in-situ mass spectroscopy and expressed as RH at 60 C as shown in Fig. 2. The amount of water in cathode effluent is quite different for WO/AP and DO/AP. At the onset of cathode air purging, the RH of cathode effluent is 49% and 27% for WO/AP and DO/AP, respectively. After the cathode air purging for 30 min, the RH is reduced to 30% and 7% for WO/AP and DO/AP, respectively. The in-situ mass spectroscopy results clearly show that the amount of water remained in MEA before the freezing step is increased in this order: DO/AP < WO/AP < WO/NP. These also confirm that the dry operation before shutdown effectively shortens purging time required to reduce the residual water content to a certain low level. The evolution of IV polarizations before with the freeze/ thaw cycle is displayed in Fig. 3. WO/NP exhibits gradual
60
Relative humidity at 60o C of cathode effluent (%)
1287, Solarton Analytical). For measuring the CV of cathode, it was considered as the working electrode and fed with nitrogen, whereas the anode that was fed with pure hydrogen functioned as the counter and reference electrodes. Hydrogen and nitrogen were 80% humidified at the cell temperature of 60 C. The potential was scanned between 0.08 V and 0.80 V at a scan rate of 20 mV s1. The ECA of the anode was determined from CO stripping voltammetry using a potentiostat (SI 1287, Solarton Analytical). CO was adsorbed on the surface of the anode catalyst by flowing 100 ppm CO in hydrogen at a flow of 50 sccm through the anode compartment for 1 h, while holding the anode potential at constant value (0.1 V vs. DHE). By keeping the potential at the same value, the gas was switched to N2 for 15 min (flow rate: 95 sccm), to remove CO traces from the gas phase. After that, the potential was scanned from 0.1 to 0.8 V and then back to 0.1 V at scan rate of 5 mV s1. The potential scan was repeated two more times.
WO/AP DO/AP
50
40
30
20
10
RH of Purging Gas : 4% 0 0
10
20
30
40
50
60
Time (min) Fig. 2 e Evolution of the RH of the cathode effluent during cathode dry air purging for WO/AP and DO/AP.
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increases of voltage loss over whole range of current densities. Performance degradation is also observed for WO/AP, however, it is less significant than that for WO/NP. In the comparison of WO/NP and WO/AP, the most pronounced
a 1.0
RH 80% air operation/Cathode air purging operation/No purging
0th 1st 5th 10th 15th 20th
0.9
Voltage (V)
0.8 0.7
difference is found at high current density regime (1000e1700 mA cm2). WO/NP exhibits much larger voltage loss with the freeze/thaw cycles at the high current density regime compared to WO/AP. It indicates that the freeze/thaw cycles render WO/NP to be prone to mass transfer limitation. Meanwhile, DO/AP does not reveal any appreciable change with the freeze/thaw cycles. In conjunction with the mass spectroscopic analysis, it can be stated the more the residual water is removed before low temperature storage, the better freeze/thaw durability is. Also, it is confirmed that that the operation with dry air feed coupled with the cathode purging with dry air is highly effective in alleviating the freeze damage problem.
3.2. Electrochemical analysis on the performance degradation
0.6 0.5 0.4 0.3
0
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2
Current Density (mA/cm )
b 1.0
RH 80% air operation/Cathode air purging
0th 1st 5th 10th 15th 20th
0.9
Voltage (V)
0.8 0.7
One of the possible causes for the performance degradation by the freeze/thaw cycles could be a structural destruction of the catalyst layer, which should accompany a reduction of ECA. In the potential range of 0.1e0.3 V, currents from hydrogen adsorption to Pt surface in cathodic scan and hydrogen desorption from Pt surface in anodic scan are shown (see Fig. 4). Significant decrease of hydrogen adsorption and desorption peak area is found after the freeze/thaw cycles for all the cells. From the average of the adsorption charge and the desorption charge, cathode ECA is calculated. Anode ECA
a 0.2
0.6
Initial After 20 cycles
0.1 0.5
0.0 0.4 0.3
-0.1 0
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b
Current Density (mA/cm2)
0.2
Current (A)
c 1.0
Dry air operation/Cathode air purging
0th 1st 5th 10th 15th 20th
0.9
Voltage (V)
0.8 0.7
Initial After 20 cycles
0.1 0.0 -0.1 -0.2
c 0.2
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Initial After 20 cycles
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0
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Current Density (mA/cm ) Fig. 3 e Evolution of polarization curves with the freeze/ thaw cycles for (a) WO/NP, (b) WO/AP and (c) DO/AP.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (V) Fig. 4 e Cyclic voltammograms of the cathode of (a) WO/NP, (b) WO/AP and (c) DO/AP.
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Fig. 5 e CO stripping voltammograms of the anode of (a) WO/NP, (b) WO/AP and (c) DO/AP.
is measured by means of the CO stripping voltammetry (see Fig. 5). Since PtRu catalyst used in the anode does not reveal clear hydrogen adsorption/desorption peak, CO stripping voltammetry is used to determine anode ECA instead of CV. From the area of the CO oxidation peak shown in anodic scan, anode ECA is calculated. Contrary to cathode catalyst layer, anode catalyst layer show no appreciable change in CO stripping voltammetry with the freeze/thaw cycles for all the cells. The values for cathode and anode ECA are listed in Table 1. The differences in the initial ECA values come from MEA variation. The ECA losses for WO/NP and WO/AP are much larger than that for DO/AP. The cathode ECA is decreased from 34.5 to 23.2 m2 g1 for WO/NP, from 39.8 to 25.6 m2 g1 for WO/ AP and from 43.0 to 34.0 m2 g1 for DO/AP. The value for %
cathode ECA retention was 67.2, 64.3 and 79.1 for WO/NP, WO/ AP and DO/AP, respectively. The larger cathode ECA losses for WO/NP and WO/AP compared that for DO/AP strongly indicates that freezing of the residual water leads to mechanical degradation of the cathode catalyst layer. A 21% reduction of cathode ECA for DO/AP after the freeze/thaw cycles is unexpected result, because DO/AP does not show performance degradation. We found that the freeze damage is not the only reason for cathode ECA loss; Pt dissolution is another reason for the cathode ECA loss. In the next section, the evidence for Pt dissolution will be disposed. It is interesting to note that the anode ECA is nearly not changed even when anode purging is omitted. The value for % anode ECA retention was 101.3, 98.3 and 100.8 for WO/NP, WO/ AP and DO/AP, respectively. It means that the freeze damage of anode is quite small even when anode purging is not conducted. The result is practically important, because problematic anode air purging, which is known to cause cathode carbon corrosion, could be eliminated. The water transfer from anode to cathode via electro-osmotic drag or thermoosmotic drag would be responsible for this behavior. Fig.6 shows the Nyquist plots of the impedances for WO/ NP, WO/AP and DO/AP before and after the freeze/thaw cycles collected at three different current densities (100, 300, and 500 mA cm2). The impedance spectra are characterized by the following features: (i) the intercept in the high frequency domain on the Z0 axis, which corresponds to ohmic resistance (Rm), (ii) the linear region starting from the intercept on the Z0 axis, which represents the contribution from ionic conduction through the catalyst layer, (iii) the semicircle of higher frequencies originating from the charge transfer process (the cathodic oxygen reduction), and (iv) the semicircle of lower frequencies originating from mass transfer processes with the contributions from proton diffusion, gas diffusion, or both [11,12]. With decreasing frequency, these contributions are evident in the following order: ionic conduction through the membrane, ionic conduction through the catalyst layer, charge transfer reaction, and mass transfer process. Since the hydrogen oxidation reaction is much faster than that of the oxygen reduction reaction, the contribution from the anode is generally ignored. For all the cells, the ohmic resistance identified from the intercept in the high frequency domain on the Z0 axis mainly representing the membrane resistance is not changed after the cycles (Fig. 6 (a)e(c) and Table 2). The notable changes after the cycles are found in the semi-circles those are attributed to charge transport process and mass transport process. The
Table 1 e Values for cathode and anode ECA before and after the freeze/thaw cycles. ECA
Before the freeze/thaw cycles (m2g1 -Pt) After the freeze/thaw cycles (m2 g1) % ECA retention after the freeze/thaw cycles
WO/NP
WO/AP
DO/AP
Cathodea
Anodeb
Cathodea
Anodeb
Cathodeb
Anodeb
34.5 1.0 23.2 1.0 67.2
111.9 3.5 113.4 3.5 101.3
39.8 1.0 25.6 1.0 64.3
102.1 3.5 100.4 3.5 98.3
43.0 1.0 34.0 1.0 79.1
96.0 3.5 96.8 3.5 100.8
a The cathode ECA was determined with a conversion factor of 210 mC/cm2. b The anode ECA was determined with a conversion factor of 420 mC/cm2.
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Fig. 6 e Impedances of (a) WO/NP, (b) WO/AP, and (c) DO/AP at various current densities at 0th and 20th freeze/thaw cycle; (d) fitting of impedance with the equivalent circuit: Solid lines represent fits of the equivalent circuit.
Table 2 e Electrical parameters obtained by the fitting of the impedances at 0th and 20th for WO/NP, WO/AP and DO/AP.
WO/NP before the cycle WO/NP after the cycle WO/AP before the cycle WO/AP after the cycle DO/AP before the cycle DO/AP after the cycle
Current density mA/cm2
R1 (Rm) U cm2
R2 (Rct) U cm2
CPE2-T F/cm2
CPE2-P
R3 (Rtr) U cm2
CPE3-T F/cm2
CPE3-P
100 300 500 100 300 500 100 300 500 100 300 500 100 300 500 100 300 500
0.226 0.224 0.217 0.231 0.234 0.228 0.236 0.234 0.235 0.233 0.232 0.226 0.229 0.242 0.229 0.229 0.240 0.240
0.565 0.257 0.195 0.601 0.280 0.249 0.507 0.221 0.164 0.526 0.239 0.176 0.503 0.214 0.177 0.503 0.213 0.180
0.043 0.043 0.050 0.024 0.032 0.041 0.040 0.042 0.044 0.032 0.036 0.040 0.046 0.044 0.049 0.039 0.036 0.042
0.939 0.956 0.929 0.950 0.982 0.941 0.951 0.964 0.963 0.965 0.966 0.958 0.949 1.005 0.975 0.963 1.027 0.993
0.033 0.107 0.120 0.041 0.139 0.138 0.039 0.084 0.111 0.046 0.089 0.114 0.043 0.112 0.133 0.046 0.110 0.134
2.049 0.679 0.473 0.934 0.598 0.396 1.892 0.867 0.561 1.760 0.740 0.522 1.778 0.785 0.414 1.617 0.674 0.493
1.37 1.16 1.08 1.44 1.08 1.12 1.35 1.16 1.05 1.24 1.19 1.06 1.32 1.12 1.11 1.32 1.15 1.08
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semi-circles become larger after the cycles for WO/NP and WO/AP cells. DO/AP cell does not reveal appreciable changes in the semi-circles after the cycles. For quantitative analysis, the circuit model that includes resistance and constant phase element (CPE) for charge transport and mass transport process is used. By fitting the circuit model to the semi-circles, the parameters are determined. The range of impedances fitted with the circuit model is typically marked in Fig. 6 (d). Since ionic transport through porous catalyst layer is not reflected in the circuit model, the linear region is out of the fitting range.
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The results of fitting are given in Table 2. For WO/NP, both Rct and Rtr are increased irrespective of current densities with the freeze/thaw cycles. The difference in Rct is 0.036, 0.023 and 0.054 for 100, 300, and 500 mA cm2, respectively, and the difference in Rtr is 0.008, 0.032 and 0.018 for 100, 300 and 500 mA cm2, respectively. WO/AP also shows the increased Rct and Rtr even though the differences are much smaller than those for WO/NP. DO/AP does not show appreciable increase of these resistances. The large increase of Rtr for WO/NP is in good agreement with the increased mass transfer limitation informed from the IV polarization.
Fig. 7 e SEM images of the backing layers after the 20 freeze/thaw cycles: (a) the cathode of WO/NP, (b) the anode of WO/NP, (c) the cathode of WO/AP, (d) the anode of WO/AP, (e) the cathode of DO/AP and (f) the anode of DO/AP.
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WO/NP reveals larger increase in Rct than WO/AP in spite of the same degree of ECA loss for the two cells. It can be explained by the larger increase in Rtr for WO/NP. The charge transfer rate is affected by how fast the reactants are supplied to the catalyst surface. Since the concentration of the reactant at the catalyst surface is reduced when mass transfer is limited, Rct is increased. According to the impedance analysis, the performance degradation for WO/NP is associated with a loss of cathode ECA together with an increased mass transfer limitation. In order to find the reason for the increase of mass transfer limitation, postmortem analysis was conducted. SEM investigation of diffusion layer, catalyst layer, and the interface between catalyst layer and diffusion layer was made for WO/NP, WO/AP and DO/AP.
layers. The PTFE content is 5 wt% for the backing layer and 23 wt% for the micro-porous layer. Fig. 7 shows the SEM images of the cathode and anode backing layers for WO/NP, WO/AP and DO/AP after the freeze/thaw cycles. In the backing
3.3. Postmortem analysis of the MEAs after the freeze/ thaw cycles The diffusion layers used in this work consist of a backing layer and a micro-porous layer. As a water repelling agent, poly(tetrafluoroethylene) (PTFE) is incorporated in the two
Fig. 8 e SEM images of the micro-porous layers after the 20 freeze/thaw cycles: (a) the cathode of WO/NP, (b) the anode of WO/NP.
Fig. 9 e SEM images of the membrane/catalyst layer interface after the 20 freeze/thaw cycles: (a) the membrane/ cathode interface of WO/NP, (b) the membrane/cathode interface of WO/AP, and (c) the membrane/cathode interface of DO/AP.
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layers, PTFE fills some parts of pores forming a phase. It appears that a defect in PTFE phase is generated after the freeze/thaw cycles as indicated in Fig. 7. The defect is more frequently found in the SEM images of WO/NP and WO/AP than in that of DO/AP. In addition, the cathode diffusion layers exhibit larger number of defects than the anode diffusion layers. Therefore, it is highly probable that the defect is originated from the freezing of the residual water. Since the damage of the PTFE phase should accompany the change in hydrophobicity of the diffusion layer, water contact angle can be a measure of the freeze damage. The water contact angle of the pristine backing layer is 152 . After the freeze/thaw cycles, the water contact angle of the cathode backing layer is changed to 118 , 125 and 135 , for WO/NP, WO/AP and DO/AP, respectively. By contrast, the water contact angle of the anode backing layer is 139 , 143 and 137 , for WO/NP, WO/AP and DO/AP, respectively. The results for water contact angle confirm that the mechanical degradation of cathode diffusion layer happens during the freeze/thaw cycles and it becomes significant when the amount of residual water is large. The freeze damage of the diffusion layer was reported by Wang et al. [13]. They observed the pore side distribution of the MEA after cold
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start up at 15 C. Large pores (10e100 micron) were generated, indicating that pores becomes larger and the number of the pores increases due to volume change. The change of water contact angle of cathode diffusion layer strongly correlates with the performance degradation. Water flooding due to the loss of hydrophobicity would be responsible for the large voltage loss in the high current density regime and the large increase in Rtr for WO/NP. The anode diffusion layer loses its hydrophobicity to some extent, however, the degree of hydrophobicity is not different for WO/ NP, WO/AP and DO/AP. It means that the decrease in water contact angle for anode backing layer is not due to freeze damage but due to physico-chemical changes during cell assembly and IV measurements. No notable morphological change is detected in the SEM images of micro-porous layer for WO/NP, WO/AP and DO/ AP. The SEM images for WO/NP are typically shown in Fig. 8. The water contact angle of the pristine micro-porous layer is not measureable because the surface of the layer is so hydrophobic as not to hold water droplet. After the freeze/thaw cycles, the water contact angle of the cathode micro-porous layer is reduced to 141 , 139 and 138 for WO/NP, WO/AP and DO/AP, respectively. The water contact
Fig. 10 e SEM images of the catalyst layers after the 20 freeze/thaw cycles: (a) the cathode of WO/NP, (b) the anode of WO/NP, (c) the cathode of WO/AP, and (d) the cathode of DO/AP.
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angle of the anode micro-porous layer is 145 , 142 and 142 , for WO/NP, WO/AP and DO/AP, respectively. Contrary to the backing layer, micro-porous layer does not show significant difference among the different purging methods. Since the micro-porous layer has higher PTFE content and smaller pore size than the backing layer, the residual water in MEA would remain in the backing layer rather than in the micro-porous layer. The interfacial adhesion between the membrane and the catalyst layer is preserved after the freeze/thaw cycles for WO/NP, WO/AP, and DO/AP cells. The SEM images of the interface of the membrane and the cathode catalyst layer are typically shown in Fig. 9. These results coincide with previous work. According to Luo et al. [14], no visible physical damage and destruction was observed by SEM images of the cross section of CCM. Kim et al. [15] reported that test cells thermally 30 cycled between 40 and 70 C in water submerged conditions revealed interfacial delamination between diffusion layer and catalyst layer but not between the catalyst layer and membrane. As mentioned above, the Pt band coming from dissolution of Pt and subsequent reduction by hydrogen crossing over the membrane is detected in the membrane near the cathode side for all the samples. The Pt dissolution is not induced by freeze/thaw cycles because the normal operation cycles from 20 C to 60 C without the freeze/thaw step also shows the Pt band. It is evident that Pt dissolution is, at least, partly responsible for the ECA loss. The SEM images of the catalyst layers after the freeze/ thaw cycles are shown in Fig. 10. Both the cathode- and the anode catalyst layers do not reveal any visible mechanical destruction in sub-micron scale. Considering the large cathode ECA losses for WO/NP and WO/AP, the mechanical degradation happens in a smaller dimension.
4.
Conclusions
The investigation of the performance degradation during the freeze/thaw cycles with varying purging logic and operation condition has been carried out. It clarifies three points. First, the increased water content remained in MEA leads to the larger performance degradation by the freeze/thaw cycles, and the performance degradation is originated from the cathode ECA loss and increased mass transfer limitation. Second, the freeze damage of the anode catalyst layer is quite small even when the anode air purging is not conducted. Third, the purging of only cathode compartment coupled with operation with dry air feed is effective to mitigate freeze damage.
references
[1] McDonald RC, Mittelsteadt CK, Thompson EL. Effects of deep temperature cycling on nafion 112 membranes and membrane electrode assemblies. Fuel Cells 2004;4(3):208e13. [2] Guo Q, Qi Z. Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies. Journal of Power Sources 2006;160(2):1269e74. [3] Hou J, Yu H, Zhang S, Sun S, Wang H, Yi B, et al. Analysis of PEMFC freeze damage at 20 C after gas purging. Journal of Power Sources 2006;162(1):513e20. [4] Hou J, Yu H, Yi B, Xiao Y, Wang H, Sun S, et al. Comparative study of PEM fuel cell storage at 20 C after gas purging. Electrochemical and Solid State Letters 2007;10:B11. [5] Cho EA, Ko JJ, Ha HY, Hong SA, Lee KY, Lim TW, et al. Effects of water removal on the performance degradation of PEMFCs repetitively brought to <0 C. Journal of Electrochemical Society 2004;151(5):A661e5. [6] Cho EA, Ko JJ, Ha HY, Hong SA, Lee KY, Lim TW, et al. Characteristics of the PEMFC repetitively brought to temperatures below 0 C. Journal of Electrochemical Society 2003;150(12):A1667e70. [7] Yan Q, Toghiani H, Lee Y, Liang K, Causey H. Effect of subfreezing temperatures on a PEM fuel cell performance, startup and fuel cell components. Journal of Power Sources 2006;160(2):1242e50. [8] Wilson MS, Valerio JA, Gottesfeld S. Low platinum loading electrodes for polymer electrolyte fuel cells fabricated using thermoplastic ionomers. Electrochimica Acta 1995;40(3): 355e63. [9] Reiser CA, Bregoli L, Patterson TW, Yi JS, Yang JDL, Perry ML, et al. A reverse-current decay mechanism for fuel cells. Electrochemical Solid State Letter 2005;8:A273. [10] Tang H, Qi Z, Ramani M, Elter JF. PEM fuel cell cathode carbon corrosion due to the formation of air/fuel boundary at the anode. Journal of Power Sources 2006;158(2):1306e12. [11] Perez J, Gonzalez ER, Ticianelli EA. Impedance studies of the oxygen reduction on thin porous coating rotating platinum electrodes. Journal of Electrochemical Society 1998;145(7): 2307e13. [12] Springer TE, Zawodzinski TA, Wilson MS, Gottesfeld S. Characterization of polymer electrolyte fuel cells using AC impedance spectroscopy. Journal of Electrochemical Society 1996;143(2):587e99. [13] Wang H, How J, Yu H, Sun S. Effects of reverse voltage and subzero startup on the membrane electrode assembly of a PEMFC. Journal of Power Sources 2007;165(1):287e92. [14] Luo M, Huang C, Liu W, Luo Z, Pan M. Degradation behaviors of polymer electrolyte membrane fuel cell under freeze/thaw cycles. International Journal of Hydrogen Energy 2010;35(7): 2986e93. [15] Kim S, Mench MM. Physical degradation of membrane electrode assemblies undergoing freeze/thaw cycling: microstructure effects. Journal of Power Sources 2007;174(1): 206e20.
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Effect of air purging and dry operation on durability of PEMFC under freeze/thaw cycles Kah-Young Song, Hee-Tak Kim* Samsung SDI Co., LTD, 428-5 Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-577, Republic of Korea
article info
abstract
Article history:
The effect of air purging and dry operation on durability of polymer electrolyte membrane
Received 25 March 2011
fuel cell (PEMFC) under repeated freeze/thaw cycles between 20 C and 60 C was
Received in revised form
investigated. The cathode air purging and the operation with dry air feed were highly
14 June 2011
effective to mitigate freeze damage. The removal of the air purging of the anode
Accepted 17 June 2011
compartment did not lead to the degradation of the anode catalyst layer. It is of practical
Available online 23 July 2011
importance, because the air purging of the anode could cause carbon corrosion of the cathode. The performance degradation by the freeze/thaw cycles was associated with the
Keywords:
increased charge transfer and mass transfer resistances. After the freeze/thaw cycles, any
Polymer electrolyte membrane fuel
discernable morphological changes were not observed in the scanning electron micro-
cell
scopic images of the anode, the membrane and the membrane/electrode interface,
Freeze/thaw cycle
however, mechanical damage of the poly(tetrafluoroethylene) phases in the cathode
Freeze damage
diffusion layer was detected.
Air purging
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Membrane electrode assembly
1.
Introduction
One of the remaining technical challenges for polymer electrolyte membrane fuel cell (PEMFC) commercialization for outdoor applications is to achieve storage at sub-zero temperature without damage. The durability of membrane electrode assembly (MEA) under freeze/thaw cycles has been attracting a lot of attention because it is most likely that the water produced in the cathode freezes at sub-zero temperature and the mechanical stress induced by ice formation leads to irreversible degradation of MEA. There exist conflicts in the literature regarding freeze damage of MEA. One important result shows that fuel cells dried to remove all liquid water during the shutdown experience neither observable physical damage nor electrochemical losses by freezing [1e5]. However, there are conflicting results in case of a cell with no significant purge. Some reported
reserved.
physical damage, performance loss, and electrochemical loss (electrochemical surface area, interfacial and charge transfer resistance increase) [2,6,7]. Physical damage includes membrane failure (holes and cracks), catalyst cracks and delamination, pore distribution change, and gas diffusion media fracture. Others observed no significant performance loss even without dry purge during the shutdown [8]. The conflict is expected to be due to the difference in remaining water content inside MEA and the difference in a critical water content above which freeze damage becomes significant. If the porous catalyst layer or diffusion layer can withstand the stress generated by ice formation, freezing damage could not be significant even without dry purging. One the other hand, the development of water removal process during shutdown is highly important to prevent any possible freeze damage in practical applications. A frequently used method to avoid freezing damage of PEMFC is air purging
* Corresponding author. Tel.: þ82 31 288 4570. E-mail address: [email protected] (H.-T. Kim). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.095
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2.
Experimental
2.1.
Freeze/thaw cycles and purging experiment
Single cells were assembled with a 26 cm2-sized commercial catalyst coated membrane (PRIMEA 5761, W.L. Gore & Associates) based on Pt/C (cathode) and PtRu/C (anode), a pair of gas diffusion media (35BC, SGL), a pair of Teflon gaskets and a pair of graphite blocks with triple serpentine flow fields for air and H2 feeds. The Pt loading for the cathode was 0.40 mg cm2. The Pt and Ru loading for the anode were 0.30 and 0.15 mg cm2, respectively. The thickness of the membrane was 18 mm. The channel depth, channel width and land width of the triple serpentine were 1.00, 0.79, 0.91 mm, respectively. The flow fields for anode- and cathode reactants were mirror images. PtRu/C and Pt/C were used as the anodeand cathode catalysts, respectively. Three different cells (one each with WO/NP, WO/AP, and DO/AP) with different operating and purging logics are conducted as depicted in Fig. 1. NP and AP represent no purging and air purging of the cathode, respectively. The AP process includes cooling of the cell down to 30 C with closing any inlet
100 80
Cell Operation WO/NP & WO/AP : IV (80%/80%(A/C)) DO/AP : IV (80%/80%(A/C)) + CC (80%/dry(A/C))
60 40
Cooling/purging (30min) WO/NP : No purging WO/AP & DO/AP : Cathode Purging with dry air
20
1cycle
o
Temperature (C)
of the cathode and anode compartments prior to low temperature storage. It is simple and effective method. However, the generation of the hydrogen and air boundary at the anode leads to the increase of cathode potential and consequent carbon corrosion at the cathode [9,10]. Therefore, purging of anode compartment with dry air is problematic. If the anode is not damaged through freeze/thaw cycles even in the absence of air purging of anode, it is possible to mitigate freeze damage by purging of only cathode without suffering from carbon corrosion at cathode. Since the recent publications on freezing damage of the MEA concentrated on the changes of cathode and membrane, it has remained unclear that freeze damage of the anode happens in the case of no anode purging. The present study deals with the durability of PEMFC MEA during freeze/thaw cycles with various purging logics and operating conditions. The main objective of this work is to (i) to quantify the freeze damage of the anode and the cathode individually, in the case of no purging, (ii) to check the effect of removing anode purging on the freeze damage, and (iii) to investigate the effect of the operation with dry air feed on freeze damage. The operation with dry air feed would reduce bulk liquid water in the cathode, and could provide better freeze/thaw durability. Three different operation and shutdown logics are applied; the first does not include any purging, the second has air purging of cathode and the third has additional operation with dry air feed prior to air purging of cathode. Freeze/thaw cycle from 20 C to 60 C was repeated 20 times. After each cycle, IV polarization was measured at 60 C to monitor power evolution with cycle number. In order to elucidate degradation mechanism, extensive electrochemical analysis based on cyclic voltammetry (CV), CO stripping voltammetry and electrochemical impedance spectroscopy (EIS) was made for the MEAs before and after 20 cycles. Postmortem analysis based on scanning electron microscopy (SEM) was also conducted after the freeze/thaw cycles.
0 -20 Storage at –20o C (2hrs)
-40
Time (hr) Fig. 1 e Temperatures profile of the freeze/thaw cycle.
and outlet of cathode and anode compartments, and the subsequent injection of dry air to the cathode compartment at flow rate of 500 sccm for 30 min while closing inlet and outlet of anode compartment. The wet operation (WO) corresponds to the measurement of IV polarization at 60 C with 80% humidified air and H2 feeds. The detailed description for the IV polarization measurement is given in the next section. The dry operation (DO) means a constant-current operation at 300 mA cm2 for 1 h, subsequent to WO, with dry air feed at stoichiometry of 2.0 and with 80% humidified hydrogen at stoichiometry of 1.2. The stoichiometry of 2.0 for cathode air feed was selected for stable operation with dry air feed; when the stoichiometry is higher than 2.0, cell drying happens leading to a voltage decay. The freeze/thaw cycles consist of the repetitive steps of cell operation at 60 C and low temperature storage at 20 C. The cell cooling from 20 C to 20 C, the storage at 20 C for 2 h and the cell heating from 20 C to 60 C were conducted in a temperature controlled oven. All the inlets and outlets were closed during the cooling, low temperature storage and heating process. The cooling and heating rates were 0.67 C/ min. WO step was applied for WO/NP and WO/AP; DO step only for DO/AP.
2.2.
IV Polarization measurements
IV Polarization curves were obtained by stepping the current density, allowing the cell to stabilize, and measuring the cell voltage. Two minutes were spent at each current density with the cell voltage collected every 2 s. For hydrogen, a constant mass flow rate of 43.5 sccm was used for current density below 200 mA cm2; above 200 mA cm2, a constant stoichiometry of 1.2 was used. An air flow at a constant mass flow rate of 215.6 sccm was used for current density below 200 mA cm2; above 200 mA cm2, a constant stoichiometry of 2.5 was used. The cell temperature was controlled to remain at 60 C.
2.3.
CV and CO stripping voltammetry measurement
CV of the single cell was carried out to determine electrochemical area (ECA) of the cathode using a potentiostat (SI
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2.4.
EIS measurement
EIS was conducted to determine ohmic resistance, charge transfer resistance and mass transfer resistance in the frequency range of 10 kHz to 100 mHz with 10 steps per decade using a potentiostat (SI 1287, Solartron Analytical) combined with a frequency response analyzer (SI 1255B, Solartron Analytical) and a power booster (SI 1290, Solartron Analytical). The amplitude of the sinusoidal voltage signal did not exceed 10 mV. The cathode was used as a working electrode and the anode as a counter electrode. The counter electrode also served as a reference electrode, because the over-potential of hydrogen oxidation at the counter electrode is negligible.
2.5. Relative humidity measurement with mass spectroscopy The relative humidity (RH) of the cathode effluent during dry air purging of the cathode compartment was determined from mass spectroscopy (HPR20, Hiden Analytical). The mass spectroscopy was connected to the outlet of the cathode and the mass spectrogram was in-situ measured during dry air purging. Air feeds with various RH were injected to the mass spectroscopy, and the relationship between the RH of air feed and the intensity ratio of water and nitrogen peaks in the mass spectrogram. The plot of the intensity ratio of water and nitrogen peaks as a function of RH shows an excellent linearity. From the relationship, the RH of the cathode effluent during dry air purging was in-situ determined.
2.6.
Contact angle measurement
The contact angles of diffusion layers before and after the freeze/thaw cycles were measured by the sessile-drop test using a contact angle system (DSA 100, Kru¨ss) at room temperature. For each measurement, 15 ml pure water droplet was made by placing the tip of the syringe close to the diffusion layer surface. Then the contact angle was measured at about 15 s after the droplet attached to the surface of the diffusion layer. For better accuracy, measurements of contact
angles were performed at five different random regions on each sample, and then the average value was determined.
2.7.
Postmortem analysis
The SEM images of the diffusion layers and the catalyst coated membranes after 20th freeze/thaw cycle were obtained by using Hitachi S-4700 SEM at Korea Basic Science Institute in Jeonju.
3.
Results and discussion
3.1. cycles
Performance degradation during the freeze/thaw
Ambient air of which RH is 4% at 60 C is used as a cathode purging gas in this work. It carries water vapor from MEA while passing through the cathode compartments. As MEA is dried, the amount of water transferred from MEA to purging gas is reduced. Therefore, RH of the cathode effluent of purging gas qualitatively informs the amount of water remained in MEA during cathode air purging. In order to investigate the effects of the air purging and dry operation, the amount of water in the cathode effluent during air purging was monitored for WO/AP and DO/AP with in-situ mass spectroscopy and expressed as RH at 60 C as shown in Fig. 2. The amount of water in cathode effluent is quite different for WO/AP and DO/AP. At the onset of cathode air purging, the RH of cathode effluent is 49% and 27% for WO/AP and DO/AP, respectively. After the cathode air purging for 30 min, the RH is reduced to 30% and 7% for WO/AP and DO/AP, respectively. The in-situ mass spectroscopy results clearly show that the amount of water remained in MEA before the freezing step is increased in this order: DO/AP < WO/AP < WO/NP. These also confirm that the dry operation before shutdown effectively shortens purging time required to reduce the residual water content to a certain low level. The evolution of IV polarizations before with the freeze/ thaw cycle is displayed in Fig. 3. WO/NP exhibits gradual
60
Relative humidity at 60o C of cathode effluent (%)
1287, Solarton Analytical). For measuring the CV of cathode, it was considered as the working electrode and fed with nitrogen, whereas the anode that was fed with pure hydrogen functioned as the counter and reference electrodes. Hydrogen and nitrogen were 80% humidified at the cell temperature of 60 C. The potential was scanned between 0.08 V and 0.80 V at a scan rate of 20 mV s1. The ECA of the anode was determined from CO stripping voltammetry using a potentiostat (SI 1287, Solarton Analytical). CO was adsorbed on the surface of the anode catalyst by flowing 100 ppm CO in hydrogen at a flow of 50 sccm through the anode compartment for 1 h, while holding the anode potential at constant value (0.1 V vs. DHE). By keeping the potential at the same value, the gas was switched to N2 for 15 min (flow rate: 95 sccm), to remove CO traces from the gas phase. After that, the potential was scanned from 0.1 to 0.8 V and then back to 0.1 V at scan rate of 5 mV s1. The potential scan was repeated two more times.
WO/AP DO/AP
50
40
30
20
10
RH of Purging Gas : 4% 0 0
10
20
30
40
50
60
Time (min) Fig. 2 e Evolution of the RH of the cathode effluent during cathode dry air purging for WO/AP and DO/AP.
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increases of voltage loss over whole range of current densities. Performance degradation is also observed for WO/AP, however, it is less significant than that for WO/NP. In the comparison of WO/NP and WO/AP, the most pronounced
a 1.0
RH 80% air operation/Cathode air purging operation/No purging
0th 1st 5th 10th 15th 20th
0.9
Voltage (V)
0.8 0.7
difference is found at high current density regime (1000e1700 mA cm2). WO/NP exhibits much larger voltage loss with the freeze/thaw cycles at the high current density regime compared to WO/AP. It indicates that the freeze/thaw cycles render WO/NP to be prone to mass transfer limitation. Meanwhile, DO/AP does not reveal any appreciable change with the freeze/thaw cycles. In conjunction with the mass spectroscopic analysis, it can be stated the more the residual water is removed before low temperature storage, the better freeze/thaw durability is. Also, it is confirmed that that the operation with dry air feed coupled with the cathode purging with dry air is highly effective in alleviating the freeze damage problem.
3.2. Electrochemical analysis on the performance degradation
0.6 0.5 0.4 0.3
0
200
400
600
800
1000
1200
1400
1600
1800
2
Current Density (mA/cm )
b 1.0
RH 80% air operation/Cathode air purging
0th 1st 5th 10th 15th 20th
0.9
Voltage (V)
0.8 0.7
One of the possible causes for the performance degradation by the freeze/thaw cycles could be a structural destruction of the catalyst layer, which should accompany a reduction of ECA. In the potential range of 0.1e0.3 V, currents from hydrogen adsorption to Pt surface in cathodic scan and hydrogen desorption from Pt surface in anodic scan are shown (see Fig. 4). Significant decrease of hydrogen adsorption and desorption peak area is found after the freeze/thaw cycles for all the cells. From the average of the adsorption charge and the desorption charge, cathode ECA is calculated. Anode ECA
a 0.2
0.6
Initial After 20 cycles
0.1 0.5
0.0 0.4 0.3
-0.1 0
200
400
600
800
1000
1200
1400
1600
-0.2
1800
b
Current Density (mA/cm2)
0.2
Current (A)
c 1.0
Dry air operation/Cathode air purging
0th 1st 5th 10th 15th 20th
0.9
Voltage (V)
0.8 0.7
Initial After 20 cycles
0.1 0.0 -0.1 -0.2
c 0.2
0.6
Initial After 20 cycles
0.1 0.0
0.5
-0.1
0.4
-0.2 0.3
0
200
400
600
800
1000
1200
1400
1600
1800
2
Current Density (mA/cm ) Fig. 3 e Evolution of polarization curves with the freeze/ thaw cycles for (a) WO/NP, (b) WO/AP and (c) DO/AP.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (V) Fig. 4 e Cyclic voltammograms of the cathode of (a) WO/NP, (b) WO/AP and (c) DO/AP.
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Fig. 5 e CO stripping voltammograms of the anode of (a) WO/NP, (b) WO/AP and (c) DO/AP.
is measured by means of the CO stripping voltammetry (see Fig. 5). Since PtRu catalyst used in the anode does not reveal clear hydrogen adsorption/desorption peak, CO stripping voltammetry is used to determine anode ECA instead of CV. From the area of the CO oxidation peak shown in anodic scan, anode ECA is calculated. Contrary to cathode catalyst layer, anode catalyst layer show no appreciable change in CO stripping voltammetry with the freeze/thaw cycles for all the cells. The values for cathode and anode ECA are listed in Table 1. The differences in the initial ECA values come from MEA variation. The ECA losses for WO/NP and WO/AP are much larger than that for DO/AP. The cathode ECA is decreased from 34.5 to 23.2 m2 g1 for WO/NP, from 39.8 to 25.6 m2 g1 for WO/ AP and from 43.0 to 34.0 m2 g1 for DO/AP. The value for %
cathode ECA retention was 67.2, 64.3 and 79.1 for WO/NP, WO/ AP and DO/AP, respectively. The larger cathode ECA losses for WO/NP and WO/AP compared that for DO/AP strongly indicates that freezing of the residual water leads to mechanical degradation of the cathode catalyst layer. A 21% reduction of cathode ECA for DO/AP after the freeze/thaw cycles is unexpected result, because DO/AP does not show performance degradation. We found that the freeze damage is not the only reason for cathode ECA loss; Pt dissolution is another reason for the cathode ECA loss. In the next section, the evidence for Pt dissolution will be disposed. It is interesting to note that the anode ECA is nearly not changed even when anode purging is omitted. The value for % anode ECA retention was 101.3, 98.3 and 100.8 for WO/NP, WO/ AP and DO/AP, respectively. It means that the freeze damage of anode is quite small even when anode purging is not conducted. The result is practically important, because problematic anode air purging, which is known to cause cathode carbon corrosion, could be eliminated. The water transfer from anode to cathode via electro-osmotic drag or thermoosmotic drag would be responsible for this behavior. Fig.6 shows the Nyquist plots of the impedances for WO/ NP, WO/AP and DO/AP before and after the freeze/thaw cycles collected at three different current densities (100, 300, and 500 mA cm2). The impedance spectra are characterized by the following features: (i) the intercept in the high frequency domain on the Z0 axis, which corresponds to ohmic resistance (Rm), (ii) the linear region starting from the intercept on the Z0 axis, which represents the contribution from ionic conduction through the catalyst layer, (iii) the semicircle of higher frequencies originating from the charge transfer process (the cathodic oxygen reduction), and (iv) the semicircle of lower frequencies originating from mass transfer processes with the contributions from proton diffusion, gas diffusion, or both [11,12]. With decreasing frequency, these contributions are evident in the following order: ionic conduction through the membrane, ionic conduction through the catalyst layer, charge transfer reaction, and mass transfer process. Since the hydrogen oxidation reaction is much faster than that of the oxygen reduction reaction, the contribution from the anode is generally ignored. For all the cells, the ohmic resistance identified from the intercept in the high frequency domain on the Z0 axis mainly representing the membrane resistance is not changed after the cycles (Fig. 6 (a)e(c) and Table 2). The notable changes after the cycles are found in the semi-circles those are attributed to charge transport process and mass transport process. The
Table 1 e Values for cathode and anode ECA before and after the freeze/thaw cycles. ECA
Before the freeze/thaw cycles (m2g1 -Pt) After the freeze/thaw cycles (m2 g1) % ECA retention after the freeze/thaw cycles
WO/NP
WO/AP
DO/AP
Cathodea
Anodeb
Cathodea
Anodeb
Cathodeb
Anodeb
34.5 1.0 23.2 1.0 67.2
111.9 3.5 113.4 3.5 101.3
39.8 1.0 25.6 1.0 64.3
102.1 3.5 100.4 3.5 98.3
43.0 1.0 34.0 1.0 79.1
96.0 3.5 96.8 3.5 100.8
a The cathode ECA was determined with a conversion factor of 210 mC/cm2. b The anode ECA was determined with a conversion factor of 420 mC/cm2.
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Fig. 6 e Impedances of (a) WO/NP, (b) WO/AP, and (c) DO/AP at various current densities at 0th and 20th freeze/thaw cycle; (d) fitting of impedance with the equivalent circuit: Solid lines represent fits of the equivalent circuit.
Table 2 e Electrical parameters obtained by the fitting of the impedances at 0th and 20th for WO/NP, WO/AP and DO/AP.
WO/NP before the cycle WO/NP after the cycle WO/AP before the cycle WO/AP after the cycle DO/AP before the cycle DO/AP after the cycle
Current density mA/cm2
R1 (Rm) U cm2
R2 (Rct) U cm2
CPE2-T F/cm2
CPE2-P
R3 (Rtr) U cm2
CPE3-T F/cm2
CPE3-P
100 300 500 100 300 500 100 300 500 100 300 500 100 300 500 100 300 500
0.226 0.224 0.217 0.231 0.234 0.228 0.236 0.234 0.235 0.233 0.232 0.226 0.229 0.242 0.229 0.229 0.240 0.240
0.565 0.257 0.195 0.601 0.280 0.249 0.507 0.221 0.164 0.526 0.239 0.176 0.503 0.214 0.177 0.503 0.213 0.180
0.043 0.043 0.050 0.024 0.032 0.041 0.040 0.042 0.044 0.032 0.036 0.040 0.046 0.044 0.049 0.039 0.036 0.042
0.939 0.956 0.929 0.950 0.982 0.941 0.951 0.964 0.963 0.965 0.966 0.958 0.949 1.005 0.975 0.963 1.027 0.993
0.033 0.107 0.120 0.041 0.139 0.138 0.039 0.084 0.111 0.046 0.089 0.114 0.043 0.112 0.133 0.046 0.110 0.134
2.049 0.679 0.473 0.934 0.598 0.396 1.892 0.867 0.561 1.760 0.740 0.522 1.778 0.785 0.414 1.617 0.674 0.493
1.37 1.16 1.08 1.44 1.08 1.12 1.35 1.16 1.05 1.24 1.19 1.06 1.32 1.12 1.11 1.32 1.15 1.08
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semi-circles become larger after the cycles for WO/NP and WO/AP cells. DO/AP cell does not reveal appreciable changes in the semi-circles after the cycles. For quantitative analysis, the circuit model that includes resistance and constant phase element (CPE) for charge transport and mass transport process is used. By fitting the circuit model to the semi-circles, the parameters are determined. The range of impedances fitted with the circuit model is typically marked in Fig. 6 (d). Since ionic transport through porous catalyst layer is not reflected in the circuit model, the linear region is out of the fitting range.
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The results of fitting are given in Table 2. For WO/NP, both Rct and Rtr are increased irrespective of current densities with the freeze/thaw cycles. The difference in Rct is 0.036, 0.023 and 0.054 for 100, 300, and 500 mA cm2, respectively, and the difference in Rtr is 0.008, 0.032 and 0.018 for 100, 300 and 500 mA cm2, respectively. WO/AP also shows the increased Rct and Rtr even though the differences are much smaller than those for WO/NP. DO/AP does not show appreciable increase of these resistances. The large increase of Rtr for WO/NP is in good agreement with the increased mass transfer limitation informed from the IV polarization.
Fig. 7 e SEM images of the backing layers after the 20 freeze/thaw cycles: (a) the cathode of WO/NP, (b) the anode of WO/NP, (c) the cathode of WO/AP, (d) the anode of WO/AP, (e) the cathode of DO/AP and (f) the anode of DO/AP.
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WO/NP reveals larger increase in Rct than WO/AP in spite of the same degree of ECA loss for the two cells. It can be explained by the larger increase in Rtr for WO/NP. The charge transfer rate is affected by how fast the reactants are supplied to the catalyst surface. Since the concentration of the reactant at the catalyst surface is reduced when mass transfer is limited, Rct is increased. According to the impedance analysis, the performance degradation for WO/NP is associated with a loss of cathode ECA together with an increased mass transfer limitation. In order to find the reason for the increase of mass transfer limitation, postmortem analysis was conducted. SEM investigation of diffusion layer, catalyst layer, and the interface between catalyst layer and diffusion layer was made for WO/NP, WO/AP and DO/AP.
layers. The PTFE content is 5 wt% for the backing layer and 23 wt% for the micro-porous layer. Fig. 7 shows the SEM images of the cathode and anode backing layers for WO/NP, WO/AP and DO/AP after the freeze/thaw cycles. In the backing
3.3. Postmortem analysis of the MEAs after the freeze/ thaw cycles The diffusion layers used in this work consist of a backing layer and a micro-porous layer. As a water repelling agent, poly(tetrafluoroethylene) (PTFE) is incorporated in the two
Fig. 8 e SEM images of the micro-porous layers after the 20 freeze/thaw cycles: (a) the cathode of WO/NP, (b) the anode of WO/NP.
Fig. 9 e SEM images of the membrane/catalyst layer interface after the 20 freeze/thaw cycles: (a) the membrane/ cathode interface of WO/NP, (b) the membrane/cathode interface of WO/AP, and (c) the membrane/cathode interface of DO/AP.
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layers, PTFE fills some parts of pores forming a phase. It appears that a defect in PTFE phase is generated after the freeze/thaw cycles as indicated in Fig. 7. The defect is more frequently found in the SEM images of WO/NP and WO/AP than in that of DO/AP. In addition, the cathode diffusion layers exhibit larger number of defects than the anode diffusion layers. Therefore, it is highly probable that the defect is originated from the freezing of the residual water. Since the damage of the PTFE phase should accompany the change in hydrophobicity of the diffusion layer, water contact angle can be a measure of the freeze damage. The water contact angle of the pristine backing layer is 152 . After the freeze/thaw cycles, the water contact angle of the cathode backing layer is changed to 118 , 125 and 135 , for WO/NP, WO/AP and DO/AP, respectively. By contrast, the water contact angle of the anode backing layer is 139 , 143 and 137 , for WO/NP, WO/AP and DO/AP, respectively. The results for water contact angle confirm that the mechanical degradation of cathode diffusion layer happens during the freeze/thaw cycles and it becomes significant when the amount of residual water is large. The freeze damage of the diffusion layer was reported by Wang et al. [13]. They observed the pore side distribution of the MEA after cold
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start up at 15 C. Large pores (10e100 micron) were generated, indicating that pores becomes larger and the number of the pores increases due to volume change. The change of water contact angle of cathode diffusion layer strongly correlates with the performance degradation. Water flooding due to the loss of hydrophobicity would be responsible for the large voltage loss in the high current density regime and the large increase in Rtr for WO/NP. The anode diffusion layer loses its hydrophobicity to some extent, however, the degree of hydrophobicity is not different for WO/ NP, WO/AP and DO/AP. It means that the decrease in water contact angle for anode backing layer is not due to freeze damage but due to physico-chemical changes during cell assembly and IV measurements. No notable morphological change is detected in the SEM images of micro-porous layer for WO/NP, WO/AP and DO/ AP. The SEM images for WO/NP are typically shown in Fig. 8. The water contact angle of the pristine micro-porous layer is not measureable because the surface of the layer is so hydrophobic as not to hold water droplet. After the freeze/thaw cycles, the water contact angle of the cathode micro-porous layer is reduced to 141 , 139 and 138 for WO/NP, WO/AP and DO/AP, respectively. The water contact
Fig. 10 e SEM images of the catalyst layers after the 20 freeze/thaw cycles: (a) the cathode of WO/NP, (b) the anode of WO/NP, (c) the cathode of WO/AP, and (d) the cathode of DO/AP.
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angle of the anode micro-porous layer is 145 , 142 and 142 , for WO/NP, WO/AP and DO/AP, respectively. Contrary to the backing layer, micro-porous layer does not show significant difference among the different purging methods. Since the micro-porous layer has higher PTFE content and smaller pore size than the backing layer, the residual water in MEA would remain in the backing layer rather than in the micro-porous layer. The interfacial adhesion between the membrane and the catalyst layer is preserved after the freeze/thaw cycles for WO/NP, WO/AP, and DO/AP cells. The SEM images of the interface of the membrane and the cathode catalyst layer are typically shown in Fig. 9. These results coincide with previous work. According to Luo et al. [14], no visible physical damage and destruction was observed by SEM images of the cross section of CCM. Kim et al. [15] reported that test cells thermally 30 cycled between 40 and 70 C in water submerged conditions revealed interfacial delamination between diffusion layer and catalyst layer but not between the catalyst layer and membrane. As mentioned above, the Pt band coming from dissolution of Pt and subsequent reduction by hydrogen crossing over the membrane is detected in the membrane near the cathode side for all the samples. The Pt dissolution is not induced by freeze/thaw cycles because the normal operation cycles from 20 C to 60 C without the freeze/thaw step also shows the Pt band. It is evident that Pt dissolution is, at least, partly responsible for the ECA loss. The SEM images of the catalyst layers after the freeze/ thaw cycles are shown in Fig. 10. Both the cathode- and the anode catalyst layers do not reveal any visible mechanical destruction in sub-micron scale. Considering the large cathode ECA losses for WO/NP and WO/AP, the mechanical degradation happens in a smaller dimension.
4.
Conclusions
The investigation of the performance degradation during the freeze/thaw cycles with varying purging logic and operation condition has been carried out. It clarifies three points. First, the increased water content remained in MEA leads to the larger performance degradation by the freeze/thaw cycles, and the performance degradation is originated from the cathode ECA loss and increased mass transfer limitation. Second, the freeze damage of the anode catalyst layer is quite small even when the anode air purging is not conducted. Third, the purging of only cathode compartment coupled with operation with dry air feed is effective to mitigate freeze damage.
references
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