- Email: [email protected]
Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells Taehun Ha a, Junhyun Cho a, Jaeman Park a, Kyoungdoug Min a,*, Han-Sang Kim b, Eunsook Lee c, Jy-Young Jyoung c a
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, South Korea Department of Automotive Engineering, Seoul National University of Science and Technology, South Korea c Hyupjin I&C, South Korea b
article info
abstract
Article history:
The gas diffusion layer (GDL) is important for maintaining the performance of polymer
Received 20 March 2011
electrolyte membrane (PEM) fuel cells, as its main function is to provide the cells with
Received in revised form
a path for fuel and water. In this study, the mechanical degradation process of the GDL was
1 June 2011
investigated using a leaching test to observe the effect of water dissolution. The amount of
Accepted 17 June 2011
GDL degradation was measured using various methods, such as static contact angle
Available online 20 July 2011
measurements and scanning electron microscopy. After 2000 h of testing, the GDL showed structural damage and a loss of hydrophobicity. The carbon-paper-type GDL showed
Keywords:
weaker characteristics than the carbon-felt-type GDL after dissolution because of the
Polymer electrolyte membrane fuel
structural differences, and the fuel cell performance of the leached GDL showed a greater
cell
voltage drop than that of the fresh GDL. Contrary to what is generally believed, the
Gas diffusion layer
hydrophobicity loss of GDL was not caused by the decomposition of polytetrafluoro-
Durability
ethylene (PTFE).
Degradation
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells are well known as a source of clean alternative power. In particular, the polymer electrolyte membrane (PEM) fuel cell is recognized as the most suitable fuel cell type for use in vehicles in terms of start ability, operating temperature, and quick response to load change [1,2]. However, for the PEM fuel cell to be commercially viable, several obstacles must be overcome, including durability, hydrogen production, infrastructure, standards, and costs. Among these barriers, the poor durability is the most critical issue. This poor durability is caused by the degradation of fuel cell components, the most sensitive and weakest of which is the membrane electrode assembly. Recently, many researchers have investigated the
degradation of both the membrane and the catalyst. Collier et al. [3] showed that the membrane keeps the fuel and oxidant separated, which prevents the two gases (air and hydrogen) from mixing. The membrane must be able to withstand harsh conditions, including active catalysts, high temperatures or temperature fluctuations, strong oxidants, and reactive radicals. The catalyst layer experiences CO poisoning [4e6], carbon corrosion [7e9], and catalyst particle growth [10]. Another major factor contributing to this issue, the degradation of the gas diffusion layer (GDL), remains largely unstudied. The carbon fiber structure is also exposed to tough environmental conditions, such as high temperature, air and water flow, and electric potential. Under these conditions, the GDL loses its hydrophobicity; in addition, the GDL’s
* Corresponding author. Tel.: þ82 2 880 1661; fax: þ82 2 883 0179. E-mail address: [email protected] (K. Min). 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.2011.06.096
12428
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
weight decreases [11,12], which can cause an increase in the electric contact resistance and a decrease in water discharge and gas permeation. Therefore, the loss of GDL hydrophobicity can result in a breakdown of the uniformity of the power density and reduce the overall performance of the fuel cell because the GDL directly affect the mass transport mechanism and transient response of the PEM fuel cell [13,14]. Despite this fact, only a few papers have discussed GDL degradation. Lee and Merida [15] observed a change in the mechanical characteristics of the GDL under both steady state and freezing conditions, and Wu et al. [16] showed the effects of elevated temperature and flow rate on GDL durability. Borup et al. [17] observed a loss of GDL hydrophobicity and a change in the polytetrafluoroethylene (PTFE) concentration. Wood et al. [18] used scanning electron microscopy (SEM) to identify noticeable changes in the microstructure of the GDL’s PTFE particles after extended operation. However, there is evidence to support the fact that PTFE decomposition is not the only cause of hydrophobicity loss, and PTFE decomposition does not occur to a high degree. In this work, other possibilities responsible for the hydrophobicity loss and the processes of GDL degradation are suggested. Furthermore, GDL degradation is the result of very complex processes. Therefore, it is necessary to separate each sign of degradation, identify their causes, and investigate their individual effects. GDL degradation can be divided into two categories: mechanical degradation and chemical degradation. Mechanical degradation is physical damage resulting from dissolution in water and erosion by gas flow. There are two sources of water in PEM fuel cells: external and internal. Humidified air and hydrogen are supplied, and they can condense under saturated conditions. Water is also produced by electrochemical reactions and can cause the dissolution of the GDL. During fuel cell operation, air and hydrogen are always flowing in the fuel cell. These gases contact the GDL’s surface directly and pass through to the inside. Therefore, this gas flow can cause degradation and erosion of the GDL. Chemical degradation denotes carbon corrosion. Many components of PEM fuel cells are made of carbon, and under certain conditions (such as start-up, shut-down or local fuel starvation), the carbon reacts with the water and is washed away as a form of the carbon dioxide [19,20], which causes structural breakdown of the fuel cell components, including the GDL. Chen et al. [21] demonstrated GDL degradation as the result of carbon corrosion under simulated fuel cell conditions. Our research team is investigating all three of these conditions (water dissolution, gas erosion, carbon corrosion), but this paper focuses on the dissolution effect and the changes in the properties of the GDL, which were assessed using various techniques.
Table 1 e Property measurement intervals. Property
Measurement time
Static contact angle SEM TGA Performance test
0/168/336/720/1440/2160 (h) Before/after Before/after Before/after
be a more harsh condition that would accelerate the degradation of GDL). Deionized water and 10% v/v sulfuric acid were used as the test liquids. Sulfuric acid was selected to represent the acidic conditions of PEM fuel cells. Samples 6 cm 6 cm in size were used, and six samples were immersed in a 1-L bottle of liquid. After the test, changes in the properties were assessed using various methods, such as static contact angle measurements, scanning electron microscopy, thermogravimetric analysis, and fuel cell performance testing. Among these measurement methods, the static contact angle measurement was very important because it reflected the GDL’s hydrophobicity. The GDL is manufactured to be hydrophobic because hydrophobicity increases a capillary pressure of the GDL pores, and this capillary pressure is a driving force for water removal. Immediate water removal is crucial because a poor water removal prevents gas diffusion and results in performance degradation. This capillary pressure can be expressed as a function of the static contact angle [22]. 3 1 2 JðsÞ pc ¼ scosðqc Þ K
(1)
JðsÞ ¼ 1:417 s 2:120 s2 þ 1:263 s3
(2)
s denotes the surface tension, 3 is the porosity, and K represents the permeability. Because of the relationship between capillary pressure and the static contact angle, measurement of the static contact angle can be used to assess GDL degradation. To measure the static contact angle, a water droplet was made with a micro syringe pump (Harvard Apparatus Pump 11), and a SUGITOH zoom lens (TS-93001) and halogen lamp (150 W) were used to observe the behavior of the water droplet. The measurement of droplet height and contact angle was performed with NEX MEASURE software. As the droplet height increases, the static contact angle increases, and, theoretically, the contact angle is defined as the static contact angle when the droplet height equals zero. Therefore, to obtain the static contact angle of one sample, the contact angle with various heights of droplets must be measured, and
Table 2 e GDLs used in the experiments.
2.
Experimental
To investigate the dissolution effect, a leaching test was performed. GDL samples were immersed in test liquids and heated up to 80 C, which is slightly higher than the typical operating temperature of PEM fuel cells (the operating temperature in automobiles is 65 C, but we expected 80 C to
Property A B C D
Structure type
MPL existence
PTFE (wt. %)
Felt Paper Felt Paper
O O
5 5 0 0
PTFE denotes the property of GDM. (MPL is not included).
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Table 3 e Experimental conditions of the performance test. Property Active area Temperature Humidity Pressure S/R
Condition 5 5 cm2 65 C 100% Ambient 2.0 (cathode)/1.5 (anode)
extrapolation using these data points must be performed. However, this process requires too much time and effort, so the height of the water droplet was kept constant at 1.3 mm, and the contact angle was measured at that point. Fieldemission scanning electron microscopy (Carl Zeiss SUPRA 55VP) was used to observe the microscale changes in the GDL structure, and thermogravimetric analysis (TA Instruments Q5000 IR) was used for quantitative analysis. The total test period was 2000 h. The static contact angle measurement intervals are shown in Table 1, and Table 2 shows the GDLs used in this experiment. A dry process was applied before the evaluating the properties of the GDLs to avoid the effects of water. To observe durability characteristics with structural differences, both carbon-paper-type and carbon-felt-type GDLs were used. A non-PTFE-coated GDL was also studied to determine the effect of PTFE on the durability of the GDL; this comparison was made because decomposition of the PTFE is generally considered to be the cause of the loss of hydrophobicity [23]. Among the experimental GDLs, type A and type B contained a microporous layer (MPL). All of the test samples were commercially available GDLs.
12429
The performance test was conducted with precise temperature, humidity and pressure control, and the specific conditions are represented in Table 3.
3.
Results and discussion
3.1.
Change of static contact angle
The static contact angle of a water droplet for each GDL sample was measured before and after the leaching test, and physical dissolution and a loss of hydrophobicity occurred during the test. Fig. 1 shows the change of the static contact angle with respect to time, leaching solution, and GDL type. The macroporous substrate side and the MPL side are represented separately. The numbers on the right side show the total decrease in the contact angle in each case. After 2000 h of testing, the carbon-paper-type (type B) GDL showed more damage than the carbon-felt-type (type A) GDL at the substrate surface because of the different structural characteristics of these two types of GDLs. The carbon-felt-type GDL had a three-dimensional twisted structure, unlike the carbonpaper-type GDL; this difference was the result of the different manufacturing process used, such as cross lapping and needle punching. In addition, the carbon-paper-type GDLs consisted of relatively short carbon fibers and had many cut ends of fibers. Due to these structural differences, the carbon-felttype GDL was resistant to dissolution and showed less physical damage and loss of hydrophobicity during the leaching test. The change in hydrophobicity was greater on the MPL side than on the substrate side, and acidic conditions accelerated the hydrophobicity loss, which was approximately 10%
Fig. 1 e Static contact angles at (a) the substrate surface and (b) the MPL surface of a carbon-felt-type GDL, and at (c) the substrate surface and (d) the MPL surface of a carbon-paper-type GDL.
12430
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Fig. 2 e Static contact angles of a non-PTFE-coated GDL for (a) a carbon-felt-type GDL and (b) a carbon-paper-type GDL.
faster under acidic conditions than under neutral conditions. There was one more noticeable characteristic: the rate of GDL degradation decreased with time, and after approximately 1000 h, the hydrophobicity loss reached a steady state because the weak parts of the GDL structure had already been dissolved. After 1000 h, the remaining strong structures were not affected. This hydrophobicity loss at the early stage is inevitable, so the initial properties of the GDL are very important. The decomposition of the PTFE is generally considered to be responsible for the loss of hydrophobicity [17,18,23]. In contrast to this hypothesis, the SEM results show that PTFE did not decompose remarkably during the test; this is discussed in more detail in a later chapter. For comparison, a non-PTFE-coated GDL was also involved in this study. Fig. 2 shows the static contact angle change of a non-PTFE-coated GDL. Both the carbon-felt-type (type C) and carbon-papertype (type D) GDLs were tested for 1000 h. If PTFE decomposition is main reason for the hydrophobicity loss, as stated in previous works, the non-PTFE GDLs should not show any change in the static contact angle. However, our results demonstrate that there is another factor responsible for the GDL hydrophobicity loss. The static contact angles changed approximately 7e10 after 1000 h, which was a greater change than that for the PTFE-coated GDL. Not only did the non-PTFEcoated GDL lose hydrophobicity, but the extent of the loss of hydrophobicity was greater than that for the PTFE-coated GDL. A hypothesis regarding the cause of this loss of hydrophobicity is presented in this paper, but further research and additional analysis are needed.
Fig. 3 e Substrate surface SEM images of the non-PTFE-coated GDL with different GDL conditions and under different magnifications: (a) fresh GDL, magnification of 1,0003; (b) fresh GDL, magnification of 3,0003; (c) leached GDL, magnification of 3,0003; and (d) leached GDL, magnification of 5,0003.
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
3.2.
Analysis with scanning electron microscopy
To better understand the GDL degradation process, the GDL structure before and after the leaching test were observed by SEM. Fig. 3 shows SEM images of enlarged GDL surfaces. Fig. 3a and b show the fresh non-PTFE GDL surface (carbon-felt-type GDL, type C), and Fig. 3c and d show these surfaces after leaching. The GDL consisted of carbon fibers (region ‘A’ in Fig. 3d) and fillers (region ‘B’ in Fig. 3d), which were sharp and rough. The fillers were injected in the middle of the fabrication process to fill the empty spaces of the GDL structure. There was one more distinct feature: the smooth surface material that covered the carbon filler was carbonized resin (region ‘C’ in Fig. 3b), which is a sort of glue. The phenolic resin is impregnated to the carbon fiber structure with carbon fillers and then, phenolic resin is converted to amorphous carbon structure after heat treatment in the GDL making process. This carbonized resin combines carbon fibers and carbon fillers and consequently strengthen the GDL structure. And the carbonized resin has hydrophobic characteristics as shown in the Fig. 2 which is the contact angle results of the non-PTFE coated GDL. Most of the carbon filler was covered with carbonized resin, as shown in Fig. 3a and b. However, after 2000 h of leaching, it was hard to find traces of the carbonized resin in some areas; only the carbon fiber and carbon filler remained in these regions. It appears that the resin was washed away during the leaching test. Fig. 4 shows the SEM images of PTFE-coated GDLs (type A). PTFE also presented a smooth surface like that of the carbonized
12431
resin but with a small wave pattern on its surface. The relatively bright surface is the PTFE (region ‘D’ in Fig. 4), and the dark surface is the carbonized resin (region ‘C’ in Fig. 4b). The PTFE remained intact after the leaching test (region ‘D’ in Fig. 4c and d), but the other carbon surfaces changed and became contaminated. In this sample, carbonized resin was not entirely washed away, but the surfaces were worn, and large particles that were attached via carbonized resin were detached and removed. Through this SEM analysis of the GDL surface, the results of the static contact angle analysis (which showed that the decomposition of the PTFE was not the reason for the hydrophobicity loss) were confirmed qualitatively. Additionally, Fig. 5 shows an enlarged crack on the MPL before and after the test; generally, MPL has many cracks on its surface, and wear of branches and protrusions was mainly observed on a crack surface.
3.3.
Thermo gravimetric analysis
To confirm the loss of carbon parts by water dissolution with a more quantitative analysis, a thermogravimetric analyzer (TGA) was used, which detects the mass change of a sample while the temperature increased from room temperature to approximately 800 C. The PTFE started to be pyrolyzed at a temperature of approximately 150 C and was pyrolyzed entirely at 620 C in a nitrogen environment. Fig. 6 shows the weight percentage and heat flow of PTFE during the thermogravimetric analysis. The mass proportion decreased to zero when the temperature reached 620 C Fig. 7a (type A) and
Fig. 4 e Substrate surface SEM images of the PTFE-coated GDL with different GDL conditions and under different magnifications: (a) fresh GDL, magnification of 3,0003; (b) fresh GDL, magnification of 5,0003; (c) leached GDL, magnification of 2,0003; and (d) leached GDL, magnification of 3,0003.
12432
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Fig. 5 e SEM images of the MPL surface with different GDL conditions and under different magnifications: (a) fresh GDL, magnification of 5,0003; (b) fresh GDL, magnification of 10,0003; (c) leached GDL, magnification of 5,0003; and (d) leached GDL, magnification of 10,0003.
7b (type B) give the TGA results for two GDL samples, and the mass proportion did not decrease to zero at 620 C because GDL contains carbon structures in addition to PTFE. The carbon structures (carbon fiber, filler, carbonized resin, MPL structure) were pyrolyzed at a temperature of higher than 620 C; therefore, weight proportion decreased continuously after the complete pyrolysis of the PTFE. The numbers on the graphs indicate the decrease in the mass percentage between 150 C and 620 C; this difference represents the mass of the PTFE in the sample. The mass remaining at a temperature of 620 C includes all of other carbon structures (all structures of the GDL except the PTFE). The starting point of measurement was 150 C instead of 0 C because the initial mass decrease was due to water evaporation. The results for both the fresh sample and the leached sample (with both DI water and sulfuric acid) are shown together, and the numbers are the average values of three measurements. The leached sample showed a higher proportion of PTFE mass than the fresh sample did, which indicated that the carbon structure accounted for a smaller proportion. In the TGA results, relative content of the PTFE in the GDL means relative portion of the PTFE. Because there are only PTFE and carbon in the GDL, increase of relative portion of the PTFE means decrease of relative portion of the carbon. Because the PTFE cannot be produced during the leaching test, increase of relative content of the PTFE means loss of carbon component and that component is hydrophobic carbonized resin. Therefore, the
carbon structures, such as the carbonized resin, were more damaged than the PTFE, resulting in a reduction in mass as high as a 0.5e1%. In addition, the leached sample treated with sulfuric acid showed a greater loss of carbon than the sample treated with DI water.
3.4.
GDL degradation effect on fuel cell performance
The structural changes and the degradation of the GDL during the leaching test were observed, and the dissolution effect on
Fig. 6 e TGA results for PTFE.
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Fig. 7 e TGA results for (a) carbon-felt-type GDL and (b) carbon-paper-type GDL.
12433
the GDL was also investigated. However, the question we sought to answer was whether these losses and degradation affect fuel cell performance. The loss of hydrophobicity lowers the ability to discharge the water and therefore causes flooding within the GDL. In addition, the static contact angle provides a measure of the stability of a water droplet. As the static contact angle decreases, the contact area of the water droplet on the surface increases and the droplet becomes more stable [24]. The high stability of the water droplet makes it difficult to detach the droplet from the GDL surface; therefore, a loss of hydrophobicity can lead to channel clogging. As a result, a voltage instability or a voltage drop of the fuel cell can occur. In this study, unit cell performance tests were also performed with a fresh GDL and a leached GDL to investigate the effect of GDL degradation. The experimental temperature was 65 C, which is the operating temperature of fuel cells for automobiles, and the relative humidity (RH) of the inlet gases was controlled to be 100% and 50% by an external humidifier. A unit fuel cell 5 cm 5 cm in size was used in the test, and the stoichiometric ratios were fixed at 2.0 (cathode) and 1.5 (anode). Commercially available membranes and electrodes were used, and the general serpentine channel configuration was adopted in this experiment. All of the performance tests were repeated five times. Fig. 8a shows the voltageecurrent curve of the unit fuel cell with the carbon-felt-type GDL (type A) at 100% RH. Until the middle of the load range, the fresh GDL and the leached GDL had similar performance levels, but once a current
Fig. 8 e Current-voltage curves of unit fuel cells with different GDL types and under different humidity conditions: (a) carbon-felt-type GDL, RH 100%; (b) carbon-felt-type GDL, RH 50%; (c) carbon-paper-type GDL, RH 100%; and (d) carbonpaper-type GDL, RH 50%.
12434
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
density of approximately 1 was reached, the leached GDL began to demonstrate a greater voltage drop as the current density increased (0.07 V difference at 1.5 Acm2). This voltage drop in the high-load region was caused by mass transport limitations; therefore, it seems that the loss of hydrophobicity reduced the water discharge of the GDL and caused the air transport limitation. In contrast, Fig. 8b shows the result for the same GDL at 50% RH. The general performance was lower than that at 100% RH because the water content of the membrane is a crucial factor for ionic conduction. However, the voltage drop in the high-load region for the leached sample was smaller than that at 100% RH. The greatest difference in the voltage drop between the leached GDL and the fresh GDL was 0.03 V at 1.5 Acm2. In the relatively low-humidity condition, water saturation did not occur as seriously as it did in the high-humidity condition, so the mass transport limitation also did not increase remarkably. This tendency can be verified with the results for the type B GDL. Fig. 8c shows the voltageecurrent curve with a relative humidity of 100%, and Fig. 8d shows this curve for a relative humidity of 50%. The additional voltage drop at 1.5 Acm2 due to the leached GDL amounted to 0.07 V at 100% RH, whereas the additional voltage drop was 0.02 V at 50% RH.
4.
Conclusions
The GDL degradation caused by water dissolution was investigated with various analysis methods, such as static contact angle measurement, scanning electron microscopy, thermogravimetric analysis, and unit cell performance testing. To observe only the water dissolution effect, the leaching test was adopted. During the test, hydrophobicity decreased continuously and reached a steady state after approximately 1000 h. The contact angle was reduced by approximately 5% of the initial contact angle. The carbon-paper-type GDL showed weaker characteristics than the carbon-felt-type GDL after dissolution because of the structural differences; acidic conditions accelerated the degradation, but not remarkably. Contrary to what is generally believed, the hydrophobicity loss of GDL was not caused by the decomposition of PTFE: the non-PTFE-coated GDL also showed hydrophobicity loss. By SEM analysis, it was confirmed that the decomposition of PTFE did not occur during the leaching test. The TGA results supported this phenomenon and showed the loss of carbon instead of the PTFE. It seems that some kind of structural change (mainly occurring in the carbonized resin) and mechanical wear of the carbon surface caused the loss of hydrophobicity. Further investigations are required to assess the involvement of other factors, such as chemical composition or contamination. The structural damage and losses due to dissolution impacted the fuel cell performance as well. The general currentevoltage curves were observed before and after the leaching test. The performance of the leached GDL showed a greater voltage drop than that of the fresh GDL (up to 0.08 V) due to the loss of hydrophobicity of the GDL.
Acknowledgments This work was supported by the New & Renewable Energy R&D program (2008-N-FC12-J-01-2-100) under the Ministry of Knowledge Economy.
references
[1] Larminie J, Dicks A. Fuel cell Systems Explained. New York: John Wiliey & Sons; 2003. [2] Ning Q, Xuan D, Kim Y. Modeling and control Strategy Development for fuel cell Hybrid vehicles. Int J Automotive Technol 2010;11:229e38. [3] Collier A, Wang H, Yuan XZ, Zhang J, Wilkinson DP. Degradation of polymer electrolyte membranes. Int J Hydrogen Energy 2006;31:1838e54. [4] Baschuk JJ, Li XG. Modelling CO poisoning and O2 bleeding in a PEM fuel cell anode. Int J Energy Res 2001;25:695e713. [5] Jiang R, Kunz HR, Fenton JM. Electrochemical oxidation of H2 and H2/CO mixtures in higher temperature (Tcell>100 C) proton exchange membrane fuel cells: electrochemical impedance spectroscopy. J Electrochem Soc 2005;152: A1329e40. [6] Yan W, Chu H, Lu M, Weng F, Jung G, Lee C. Degradation of proton exchange membrane fuel cells due to CO and CO2 poisoning. J Power Sources 2009;188:141e7. [7] Knights SD, Colbow KM, St-Pierre J, Wilkinson DP. Aging mechanisms and lifetime of PEFC and DMFC. J Power Sources 2004;127:127e34. [8] Taniguchi A, Akita T, Yasuda K, Miyazaki Y. Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation. J Power Sources 2004;130:42e9. [9] Lim KH, Oh H, Jang S, Ko Y, Kim H, Kim H. Effect of operating conditions on carbon corrosion in polymer electrolyte membrane fuel cells. J Power Sources 2009;193:575e9. [10] Zhai Y, Zhang H, Xing D, Shao Z. The stability of Pt/C catalyst in H3PO4/PBI PEMFC during high temperature life test. J Power Sources 2007;164:126e33. [11] Borup R, Davey J, Wood D, Garzon F, Inbody M, Guidry D. PEM fuel cell durability; 2004. DOE Hydrogen Program: FY 2004 Progress Report. [12] Kangasniemi KH, Condit DA, Jarvi TD. Characterization of Vulcan electrochemically oxidized under simulated PEM fuel cell conditions. J Electrochem Soc 2004;151:E125e32. [13] Cho J, Kim H-S, Min K. Transient response of a unit protonexchange membrane fuel cell under various operating conditions. J Power Sources 2008;185:118e28. [14] Cho J, Ha T, Park J, Kim H-S, Min K, Lee E, et al. Analysis of transient response of a unit proton-exchange membrane fuel cell with a degraded gas diffusion layer. Int J Hydrogen Energy 2011;36:6090e8. [15] Lee C, Merida W. Gas diffusion layer durability under steadystate and freezing conditions. J Power Sources 2007;164:141e53. [16] Wu J, Martin JJ, Orfino FP, Wang H, Legzdins C, Yuan X, et al. In situ accelerated degradation of gas diffusion layer in proton exchange membrane fuel cell: Part I: effect of elevated temperature and flow rate. J Power Sources 2010;195:1888e94. [17] Borup R, Davey J, Garzon F, Wood D, Welch P, More K. PEM fuel cell durability with transportation transient operation. ECS Trans 2006;3:879e86. [18] Wood DL, Davey JR, Atanassov P, Borup RL. PEMFC component characterization and its relationship to masstransport overpotentials during long-term testing. ECS Trans 2006;3:753e63.
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
[19] Reiser CA, Bregoli L, Patterson TW, Yi JS, Yang JD, Perry ML, et al. A Reverse-current Decay mechanism for fuel cells. Electrochem Solid-State Lett 2005;8:A273e6. [20] Noto H, Kondo M, Otake Y, Kato M. Development of fuel cell Hybrid vehicle by Toyota -Durability. Proceedings of the SAE International; 2009. Apr 20-23. 2009-01-1002. [21] Chen G, Zhang H, Ma H, Zhong H. Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions. Int J Hydrogen Energy 2009;34:8185e92.
12435
[22] Dullien FAL. Porous Media: Fluid transport and pore structure. New York: Academic Press; 1992. [23] Schulze M, Wagner N, Kaz T, Friedrich KA. Combined electrochemical and surface analysis investigation of degradation processes in polymer electrolyte membrane fuel cells. Electrochim Acta 2007;52:2328e36. [24] Ha T, Kim B, Kim H, Min K. Investigation on the liquid water droplet instability in a simulated flow channel of PEM fuel cell. J Mech Sci Technol 2008;22:1030e6.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Experimental study of the effect of dissolution on the gas diffusion layer in polymer electrolyte membrane fuel cells Taehun Ha a, Junhyun Cho a, Jaeman Park a, Kyoungdoug Min a,*, Han-Sang Kim b, Eunsook Lee c, Jy-Young Jyoung c a
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, South Korea Department of Automotive Engineering, Seoul National University of Science and Technology, South Korea c Hyupjin I&C, South Korea b
article info
abstract
Article history:
The gas diffusion layer (GDL) is important for maintaining the performance of polymer
Received 20 March 2011
electrolyte membrane (PEM) fuel cells, as its main function is to provide the cells with
Received in revised form
a path for fuel and water. In this study, the mechanical degradation process of the GDL was
1 June 2011
investigated using a leaching test to observe the effect of water dissolution. The amount of
Accepted 17 June 2011
GDL degradation was measured using various methods, such as static contact angle
Available online 20 July 2011
measurements and scanning electron microscopy. After 2000 h of testing, the GDL showed structural damage and a loss of hydrophobicity. The carbon-paper-type GDL showed
Keywords:
weaker characteristics than the carbon-felt-type GDL after dissolution because of the
Polymer electrolyte membrane fuel
structural differences, and the fuel cell performance of the leached GDL showed a greater
cell
voltage drop than that of the fresh GDL. Contrary to what is generally believed, the
Gas diffusion layer
hydrophobicity loss of GDL was not caused by the decomposition of polytetrafluoro-
Durability
ethylene (PTFE).
Degradation
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells are well known as a source of clean alternative power. In particular, the polymer electrolyte membrane (PEM) fuel cell is recognized as the most suitable fuel cell type for use in vehicles in terms of start ability, operating temperature, and quick response to load change [1,2]. However, for the PEM fuel cell to be commercially viable, several obstacles must be overcome, including durability, hydrogen production, infrastructure, standards, and costs. Among these barriers, the poor durability is the most critical issue. This poor durability is caused by the degradation of fuel cell components, the most sensitive and weakest of which is the membrane electrode assembly. Recently, many researchers have investigated the
degradation of both the membrane and the catalyst. Collier et al. [3] showed that the membrane keeps the fuel and oxidant separated, which prevents the two gases (air and hydrogen) from mixing. The membrane must be able to withstand harsh conditions, including active catalysts, high temperatures or temperature fluctuations, strong oxidants, and reactive radicals. The catalyst layer experiences CO poisoning [4e6], carbon corrosion [7e9], and catalyst particle growth [10]. Another major factor contributing to this issue, the degradation of the gas diffusion layer (GDL), remains largely unstudied. The carbon fiber structure is also exposed to tough environmental conditions, such as high temperature, air and water flow, and electric potential. Under these conditions, the GDL loses its hydrophobicity; in addition, the GDL’s
* Corresponding author. Tel.: þ82 2 880 1661; fax: þ82 2 883 0179. E-mail address: [email protected] (K. Min). 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.2011.06.096
12428
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
weight decreases [11,12], which can cause an increase in the electric contact resistance and a decrease in water discharge and gas permeation. Therefore, the loss of GDL hydrophobicity can result in a breakdown of the uniformity of the power density and reduce the overall performance of the fuel cell because the GDL directly affect the mass transport mechanism and transient response of the PEM fuel cell [13,14]. Despite this fact, only a few papers have discussed GDL degradation. Lee and Merida [15] observed a change in the mechanical characteristics of the GDL under both steady state and freezing conditions, and Wu et al. [16] showed the effects of elevated temperature and flow rate on GDL durability. Borup et al. [17] observed a loss of GDL hydrophobicity and a change in the polytetrafluoroethylene (PTFE) concentration. Wood et al. [18] used scanning electron microscopy (SEM) to identify noticeable changes in the microstructure of the GDL’s PTFE particles after extended operation. However, there is evidence to support the fact that PTFE decomposition is not the only cause of hydrophobicity loss, and PTFE decomposition does not occur to a high degree. In this work, other possibilities responsible for the hydrophobicity loss and the processes of GDL degradation are suggested. Furthermore, GDL degradation is the result of very complex processes. Therefore, it is necessary to separate each sign of degradation, identify their causes, and investigate their individual effects. GDL degradation can be divided into two categories: mechanical degradation and chemical degradation. Mechanical degradation is physical damage resulting from dissolution in water and erosion by gas flow. There are two sources of water in PEM fuel cells: external and internal. Humidified air and hydrogen are supplied, and they can condense under saturated conditions. Water is also produced by electrochemical reactions and can cause the dissolution of the GDL. During fuel cell operation, air and hydrogen are always flowing in the fuel cell. These gases contact the GDL’s surface directly and pass through to the inside. Therefore, this gas flow can cause degradation and erosion of the GDL. Chemical degradation denotes carbon corrosion. Many components of PEM fuel cells are made of carbon, and under certain conditions (such as start-up, shut-down or local fuel starvation), the carbon reacts with the water and is washed away as a form of the carbon dioxide [19,20], which causes structural breakdown of the fuel cell components, including the GDL. Chen et al. [21] demonstrated GDL degradation as the result of carbon corrosion under simulated fuel cell conditions. Our research team is investigating all three of these conditions (water dissolution, gas erosion, carbon corrosion), but this paper focuses on the dissolution effect and the changes in the properties of the GDL, which were assessed using various techniques.
Table 1 e Property measurement intervals. Property
Measurement time
Static contact angle SEM TGA Performance test
0/168/336/720/1440/2160 (h) Before/after Before/after Before/after
be a more harsh condition that would accelerate the degradation of GDL). Deionized water and 10% v/v sulfuric acid were used as the test liquids. Sulfuric acid was selected to represent the acidic conditions of PEM fuel cells. Samples 6 cm 6 cm in size were used, and six samples were immersed in a 1-L bottle of liquid. After the test, changes in the properties were assessed using various methods, such as static contact angle measurements, scanning electron microscopy, thermogravimetric analysis, and fuel cell performance testing. Among these measurement methods, the static contact angle measurement was very important because it reflected the GDL’s hydrophobicity. The GDL is manufactured to be hydrophobic because hydrophobicity increases a capillary pressure of the GDL pores, and this capillary pressure is a driving force for water removal. Immediate water removal is crucial because a poor water removal prevents gas diffusion and results in performance degradation. This capillary pressure can be expressed as a function of the static contact angle [22]. 3 1 2 JðsÞ pc ¼ scosðqc Þ K
(1)
JðsÞ ¼ 1:417 s 2:120 s2 þ 1:263 s3
(2)
s denotes the surface tension, 3 is the porosity, and K represents the permeability. Because of the relationship between capillary pressure and the static contact angle, measurement of the static contact angle can be used to assess GDL degradation. To measure the static contact angle, a water droplet was made with a micro syringe pump (Harvard Apparatus Pump 11), and a SUGITOH zoom lens (TS-93001) and halogen lamp (150 W) were used to observe the behavior of the water droplet. The measurement of droplet height and contact angle was performed with NEX MEASURE software. As the droplet height increases, the static contact angle increases, and, theoretically, the contact angle is defined as the static contact angle when the droplet height equals zero. Therefore, to obtain the static contact angle of one sample, the contact angle with various heights of droplets must be measured, and
Table 2 e GDLs used in the experiments.
2.
Experimental
To investigate the dissolution effect, a leaching test was performed. GDL samples were immersed in test liquids and heated up to 80 C, which is slightly higher than the typical operating temperature of PEM fuel cells (the operating temperature in automobiles is 65 C, but we expected 80 C to
Property A B C D
Structure type
MPL existence
PTFE (wt. %)
Felt Paper Felt Paper
O O
5 5 0 0
PTFE denotes the property of GDM. (MPL is not included).
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Table 3 e Experimental conditions of the performance test. Property Active area Temperature Humidity Pressure S/R
Condition 5 5 cm2 65 C 100% Ambient 2.0 (cathode)/1.5 (anode)
extrapolation using these data points must be performed. However, this process requires too much time and effort, so the height of the water droplet was kept constant at 1.3 mm, and the contact angle was measured at that point. Fieldemission scanning electron microscopy (Carl Zeiss SUPRA 55VP) was used to observe the microscale changes in the GDL structure, and thermogravimetric analysis (TA Instruments Q5000 IR) was used for quantitative analysis. The total test period was 2000 h. The static contact angle measurement intervals are shown in Table 1, and Table 2 shows the GDLs used in this experiment. A dry process was applied before the evaluating the properties of the GDLs to avoid the effects of water. To observe durability characteristics with structural differences, both carbon-paper-type and carbon-felt-type GDLs were used. A non-PTFE-coated GDL was also studied to determine the effect of PTFE on the durability of the GDL; this comparison was made because decomposition of the PTFE is generally considered to be the cause of the loss of hydrophobicity [23]. Among the experimental GDLs, type A and type B contained a microporous layer (MPL). All of the test samples were commercially available GDLs.
12429
The performance test was conducted with precise temperature, humidity and pressure control, and the specific conditions are represented in Table 3.
3.
Results and discussion
3.1.
Change of static contact angle
The static contact angle of a water droplet for each GDL sample was measured before and after the leaching test, and physical dissolution and a loss of hydrophobicity occurred during the test. Fig. 1 shows the change of the static contact angle with respect to time, leaching solution, and GDL type. The macroporous substrate side and the MPL side are represented separately. The numbers on the right side show the total decrease in the contact angle in each case. After 2000 h of testing, the carbon-paper-type (type B) GDL showed more damage than the carbon-felt-type (type A) GDL at the substrate surface because of the different structural characteristics of these two types of GDLs. The carbon-felt-type GDL had a three-dimensional twisted structure, unlike the carbonpaper-type GDL; this difference was the result of the different manufacturing process used, such as cross lapping and needle punching. In addition, the carbon-paper-type GDLs consisted of relatively short carbon fibers and had many cut ends of fibers. Due to these structural differences, the carbon-felttype GDL was resistant to dissolution and showed less physical damage and loss of hydrophobicity during the leaching test. The change in hydrophobicity was greater on the MPL side than on the substrate side, and acidic conditions accelerated the hydrophobicity loss, which was approximately 10%
Fig. 1 e Static contact angles at (a) the substrate surface and (b) the MPL surface of a carbon-felt-type GDL, and at (c) the substrate surface and (d) the MPL surface of a carbon-paper-type GDL.
12430
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Fig. 2 e Static contact angles of a non-PTFE-coated GDL for (a) a carbon-felt-type GDL and (b) a carbon-paper-type GDL.
faster under acidic conditions than under neutral conditions. There was one more noticeable characteristic: the rate of GDL degradation decreased with time, and after approximately 1000 h, the hydrophobicity loss reached a steady state because the weak parts of the GDL structure had already been dissolved. After 1000 h, the remaining strong structures were not affected. This hydrophobicity loss at the early stage is inevitable, so the initial properties of the GDL are very important. The decomposition of the PTFE is generally considered to be responsible for the loss of hydrophobicity [17,18,23]. In contrast to this hypothesis, the SEM results show that PTFE did not decompose remarkably during the test; this is discussed in more detail in a later chapter. For comparison, a non-PTFE-coated GDL was also involved in this study. Fig. 2 shows the static contact angle change of a non-PTFE-coated GDL. Both the carbon-felt-type (type C) and carbon-papertype (type D) GDLs were tested for 1000 h. If PTFE decomposition is main reason for the hydrophobicity loss, as stated in previous works, the non-PTFE GDLs should not show any change in the static contact angle. However, our results demonstrate that there is another factor responsible for the GDL hydrophobicity loss. The static contact angles changed approximately 7e10 after 1000 h, which was a greater change than that for the PTFE-coated GDL. Not only did the non-PTFEcoated GDL lose hydrophobicity, but the extent of the loss of hydrophobicity was greater than that for the PTFE-coated GDL. A hypothesis regarding the cause of this loss of hydrophobicity is presented in this paper, but further research and additional analysis are needed.
Fig. 3 e Substrate surface SEM images of the non-PTFE-coated GDL with different GDL conditions and under different magnifications: (a) fresh GDL, magnification of 1,0003; (b) fresh GDL, magnification of 3,0003; (c) leached GDL, magnification of 3,0003; and (d) leached GDL, magnification of 5,0003.
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
3.2.
Analysis with scanning electron microscopy
To better understand the GDL degradation process, the GDL structure before and after the leaching test were observed by SEM. Fig. 3 shows SEM images of enlarged GDL surfaces. Fig. 3a and b show the fresh non-PTFE GDL surface (carbon-felt-type GDL, type C), and Fig. 3c and d show these surfaces after leaching. The GDL consisted of carbon fibers (region ‘A’ in Fig. 3d) and fillers (region ‘B’ in Fig. 3d), which were sharp and rough. The fillers were injected in the middle of the fabrication process to fill the empty spaces of the GDL structure. There was one more distinct feature: the smooth surface material that covered the carbon filler was carbonized resin (region ‘C’ in Fig. 3b), which is a sort of glue. The phenolic resin is impregnated to the carbon fiber structure with carbon fillers and then, phenolic resin is converted to amorphous carbon structure after heat treatment in the GDL making process. This carbonized resin combines carbon fibers and carbon fillers and consequently strengthen the GDL structure. And the carbonized resin has hydrophobic characteristics as shown in the Fig. 2 which is the contact angle results of the non-PTFE coated GDL. Most of the carbon filler was covered with carbonized resin, as shown in Fig. 3a and b. However, after 2000 h of leaching, it was hard to find traces of the carbonized resin in some areas; only the carbon fiber and carbon filler remained in these regions. It appears that the resin was washed away during the leaching test. Fig. 4 shows the SEM images of PTFE-coated GDLs (type A). PTFE also presented a smooth surface like that of the carbonized
12431
resin but with a small wave pattern on its surface. The relatively bright surface is the PTFE (region ‘D’ in Fig. 4), and the dark surface is the carbonized resin (region ‘C’ in Fig. 4b). The PTFE remained intact after the leaching test (region ‘D’ in Fig. 4c and d), but the other carbon surfaces changed and became contaminated. In this sample, carbonized resin was not entirely washed away, but the surfaces were worn, and large particles that were attached via carbonized resin were detached and removed. Through this SEM analysis of the GDL surface, the results of the static contact angle analysis (which showed that the decomposition of the PTFE was not the reason for the hydrophobicity loss) were confirmed qualitatively. Additionally, Fig. 5 shows an enlarged crack on the MPL before and after the test; generally, MPL has many cracks on its surface, and wear of branches and protrusions was mainly observed on a crack surface.
3.3.
Thermo gravimetric analysis
To confirm the loss of carbon parts by water dissolution with a more quantitative analysis, a thermogravimetric analyzer (TGA) was used, which detects the mass change of a sample while the temperature increased from room temperature to approximately 800 C. The PTFE started to be pyrolyzed at a temperature of approximately 150 C and was pyrolyzed entirely at 620 C in a nitrogen environment. Fig. 6 shows the weight percentage and heat flow of PTFE during the thermogravimetric analysis. The mass proportion decreased to zero when the temperature reached 620 C Fig. 7a (type A) and
Fig. 4 e Substrate surface SEM images of the PTFE-coated GDL with different GDL conditions and under different magnifications: (a) fresh GDL, magnification of 3,0003; (b) fresh GDL, magnification of 5,0003; (c) leached GDL, magnification of 2,0003; and (d) leached GDL, magnification of 3,0003.
12432
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Fig. 5 e SEM images of the MPL surface with different GDL conditions and under different magnifications: (a) fresh GDL, magnification of 5,0003; (b) fresh GDL, magnification of 10,0003; (c) leached GDL, magnification of 5,0003; and (d) leached GDL, magnification of 10,0003.
7b (type B) give the TGA results for two GDL samples, and the mass proportion did not decrease to zero at 620 C because GDL contains carbon structures in addition to PTFE. The carbon structures (carbon fiber, filler, carbonized resin, MPL structure) were pyrolyzed at a temperature of higher than 620 C; therefore, weight proportion decreased continuously after the complete pyrolysis of the PTFE. The numbers on the graphs indicate the decrease in the mass percentage between 150 C and 620 C; this difference represents the mass of the PTFE in the sample. The mass remaining at a temperature of 620 C includes all of other carbon structures (all structures of the GDL except the PTFE). The starting point of measurement was 150 C instead of 0 C because the initial mass decrease was due to water evaporation. The results for both the fresh sample and the leached sample (with both DI water and sulfuric acid) are shown together, and the numbers are the average values of three measurements. The leached sample showed a higher proportion of PTFE mass than the fresh sample did, which indicated that the carbon structure accounted for a smaller proportion. In the TGA results, relative content of the PTFE in the GDL means relative portion of the PTFE. Because there are only PTFE and carbon in the GDL, increase of relative portion of the PTFE means decrease of relative portion of the carbon. Because the PTFE cannot be produced during the leaching test, increase of relative content of the PTFE means loss of carbon component and that component is hydrophobic carbonized resin. Therefore, the
carbon structures, such as the carbonized resin, were more damaged than the PTFE, resulting in a reduction in mass as high as a 0.5e1%. In addition, the leached sample treated with sulfuric acid showed a greater loss of carbon than the sample treated with DI water.
3.4.
GDL degradation effect on fuel cell performance
The structural changes and the degradation of the GDL during the leaching test were observed, and the dissolution effect on
Fig. 6 e TGA results for PTFE.
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
Fig. 7 e TGA results for (a) carbon-felt-type GDL and (b) carbon-paper-type GDL.
12433
the GDL was also investigated. However, the question we sought to answer was whether these losses and degradation affect fuel cell performance. The loss of hydrophobicity lowers the ability to discharge the water and therefore causes flooding within the GDL. In addition, the static contact angle provides a measure of the stability of a water droplet. As the static contact angle decreases, the contact area of the water droplet on the surface increases and the droplet becomes more stable [24]. The high stability of the water droplet makes it difficult to detach the droplet from the GDL surface; therefore, a loss of hydrophobicity can lead to channel clogging. As a result, a voltage instability or a voltage drop of the fuel cell can occur. In this study, unit cell performance tests were also performed with a fresh GDL and a leached GDL to investigate the effect of GDL degradation. The experimental temperature was 65 C, which is the operating temperature of fuel cells for automobiles, and the relative humidity (RH) of the inlet gases was controlled to be 100% and 50% by an external humidifier. A unit fuel cell 5 cm 5 cm in size was used in the test, and the stoichiometric ratios were fixed at 2.0 (cathode) and 1.5 (anode). Commercially available membranes and electrodes were used, and the general serpentine channel configuration was adopted in this experiment. All of the performance tests were repeated five times. Fig. 8a shows the voltageecurrent curve of the unit fuel cell with the carbon-felt-type GDL (type A) at 100% RH. Until the middle of the load range, the fresh GDL and the leached GDL had similar performance levels, but once a current
Fig. 8 e Current-voltage curves of unit fuel cells with different GDL types and under different humidity conditions: (a) carbon-felt-type GDL, RH 100%; (b) carbon-felt-type GDL, RH 50%; (c) carbon-paper-type GDL, RH 100%; and (d) carbonpaper-type GDL, RH 50%.
12434
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
density of approximately 1 was reached, the leached GDL began to demonstrate a greater voltage drop as the current density increased (0.07 V difference at 1.5 Acm2). This voltage drop in the high-load region was caused by mass transport limitations; therefore, it seems that the loss of hydrophobicity reduced the water discharge of the GDL and caused the air transport limitation. In contrast, Fig. 8b shows the result for the same GDL at 50% RH. The general performance was lower than that at 100% RH because the water content of the membrane is a crucial factor for ionic conduction. However, the voltage drop in the high-load region for the leached sample was smaller than that at 100% RH. The greatest difference in the voltage drop between the leached GDL and the fresh GDL was 0.03 V at 1.5 Acm2. In the relatively low-humidity condition, water saturation did not occur as seriously as it did in the high-humidity condition, so the mass transport limitation also did not increase remarkably. This tendency can be verified with the results for the type B GDL. Fig. 8c shows the voltageecurrent curve with a relative humidity of 100%, and Fig. 8d shows this curve for a relative humidity of 50%. The additional voltage drop at 1.5 Acm2 due to the leached GDL amounted to 0.07 V at 100% RH, whereas the additional voltage drop was 0.02 V at 50% RH.
4.
Conclusions
The GDL degradation caused by water dissolution was investigated with various analysis methods, such as static contact angle measurement, scanning electron microscopy, thermogravimetric analysis, and unit cell performance testing. To observe only the water dissolution effect, the leaching test was adopted. During the test, hydrophobicity decreased continuously and reached a steady state after approximately 1000 h. The contact angle was reduced by approximately 5% of the initial contact angle. The carbon-paper-type GDL showed weaker characteristics than the carbon-felt-type GDL after dissolution because of the structural differences; acidic conditions accelerated the degradation, but not remarkably. Contrary to what is generally believed, the hydrophobicity loss of GDL was not caused by the decomposition of PTFE: the non-PTFE-coated GDL also showed hydrophobicity loss. By SEM analysis, it was confirmed that the decomposition of PTFE did not occur during the leaching test. The TGA results supported this phenomenon and showed the loss of carbon instead of the PTFE. It seems that some kind of structural change (mainly occurring in the carbonized resin) and mechanical wear of the carbon surface caused the loss of hydrophobicity. Further investigations are required to assess the involvement of other factors, such as chemical composition or contamination. The structural damage and losses due to dissolution impacted the fuel cell performance as well. The general currentevoltage curves were observed before and after the leaching test. The performance of the leached GDL showed a greater voltage drop than that of the fresh GDL (up to 0.08 V) due to the loss of hydrophobicity of the GDL.
Acknowledgments This work was supported by the New & Renewable Energy R&D program (2008-N-FC12-J-01-2-100) under the Ministry of Knowledge Economy.
references
[1] Larminie J, Dicks A. Fuel cell Systems Explained. New York: John Wiliey & Sons; 2003. [2] Ning Q, Xuan D, Kim Y. Modeling and control Strategy Development for fuel cell Hybrid vehicles. Int J Automotive Technol 2010;11:229e38. [3] Collier A, Wang H, Yuan XZ, Zhang J, Wilkinson DP. Degradation of polymer electrolyte membranes. Int J Hydrogen Energy 2006;31:1838e54. [4] Baschuk JJ, Li XG. Modelling CO poisoning and O2 bleeding in a PEM fuel cell anode. Int J Energy Res 2001;25:695e713. [5] Jiang R, Kunz HR, Fenton JM. Electrochemical oxidation of H2 and H2/CO mixtures in higher temperature (Tcell>100 C) proton exchange membrane fuel cells: electrochemical impedance spectroscopy. J Electrochem Soc 2005;152: A1329e40. [6] Yan W, Chu H, Lu M, Weng F, Jung G, Lee C. Degradation of proton exchange membrane fuel cells due to CO and CO2 poisoning. J Power Sources 2009;188:141e7. [7] Knights SD, Colbow KM, St-Pierre J, Wilkinson DP. Aging mechanisms and lifetime of PEFC and DMFC. J Power Sources 2004;127:127e34. [8] Taniguchi A, Akita T, Yasuda K, Miyazaki Y. Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation. J Power Sources 2004;130:42e9. [9] Lim KH, Oh H, Jang S, Ko Y, Kim H, Kim H. Effect of operating conditions on carbon corrosion in polymer electrolyte membrane fuel cells. J Power Sources 2009;193:575e9. [10] Zhai Y, Zhang H, Xing D, Shao Z. The stability of Pt/C catalyst in H3PO4/PBI PEMFC during high temperature life test. J Power Sources 2007;164:126e33. [11] Borup R, Davey J, Wood D, Garzon F, Inbody M, Guidry D. PEM fuel cell durability; 2004. DOE Hydrogen Program: FY 2004 Progress Report. [12] Kangasniemi KH, Condit DA, Jarvi TD. Characterization of Vulcan electrochemically oxidized under simulated PEM fuel cell conditions. J Electrochem Soc 2004;151:E125e32. [13] Cho J, Kim H-S, Min K. Transient response of a unit protonexchange membrane fuel cell under various operating conditions. J Power Sources 2008;185:118e28. [14] Cho J, Ha T, Park J, Kim H-S, Min K, Lee E, et al. Analysis of transient response of a unit proton-exchange membrane fuel cell with a degraded gas diffusion layer. Int J Hydrogen Energy 2011;36:6090e8. [15] Lee C, Merida W. Gas diffusion layer durability under steadystate and freezing conditions. J Power Sources 2007;164:141e53. [16] Wu J, Martin JJ, Orfino FP, Wang H, Legzdins C, Yuan X, et al. In situ accelerated degradation of gas diffusion layer in proton exchange membrane fuel cell: Part I: effect of elevated temperature and flow rate. J Power Sources 2010;195:1888e94. [17] Borup R, Davey J, Garzon F, Wood D, Welch P, More K. PEM fuel cell durability with transportation transient operation. ECS Trans 2006;3:879e86. [18] Wood DL, Davey JR, Atanassov P, Borup RL. PEMFC component characterization and its relationship to masstransport overpotentials during long-term testing. ECS Trans 2006;3:753e63.
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 3 6 ( 2 0 1 1 ) 1 2 4 2 7 e1 2 4 3 5
[19] Reiser CA, Bregoli L, Patterson TW, Yi JS, Yang JD, Perry ML, et al. A Reverse-current Decay mechanism for fuel cells. Electrochem Solid-State Lett 2005;8:A273e6. [20] Noto H, Kondo M, Otake Y, Kato M. Development of fuel cell Hybrid vehicle by Toyota -Durability. Proceedings of the SAE International; 2009. Apr 20-23. 2009-01-1002. [21] Chen G, Zhang H, Ma H, Zhong H. Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions. Int J Hydrogen Energy 2009;34:8185e92.
12435
[22] Dullien FAL. Porous Media: Fluid transport and pore structure. New York: Academic Press; 1992. [23] Schulze M, Wagner N, Kaz T, Friedrich KA. Combined electrochemical and surface analysis investigation of degradation processes in polymer electrolyte membrane fuel cells. Electrochim Acta 2007;52:2328e36. [24] Ha T, Kim B, Kim H, Min K. Investigation on the liquid water droplet instability in a simulated flow channel of PEM fuel cell. J Mech Sci Technol 2008;22:1030e6.