3.1 Chemical degradation of e-PTFE by Fenton test.
Fig. 1 shows the fluoride emission rate (FER) in e-PTFE during Fenton test depending on the temperature of the Fenton reaction. The FER increased as the reaction temperature increased. It is due to increase in reaction rate that occurs with the temperature rise. The FER of e-PTFE is relatively low because e-PTFE support has chemical stability. However, this result shows that even though e-PTFE is chemically stable, it can still be degraded by radicals.
FT-IR spectroscopy was used to analyze the molecular bond change in the e-PTFE support after the Fenton test (Fig. 2). Clear changes were observed at 1,148 and 1210 cm-1, corresponding to symmetric and asymmetric vibrations of CF2 groups in the PTFE [25]. With an increase in the Fenton reaction temperature, the absorbance of CF2 groups decreased, which correlates with the FER results. As the reaction temperature increases, the degradation rate of radicals/H2O2 increases, leading to a greater breakage of CF2 bond of the e-PTFE support. This shows that the chemical degradation caused by radicals can break molecular bond of the e-PTFE.
3.2 Change of physical structure of e-PTFE after Fenton test.
Fig. 3 shows the change in the surface of the e-PTFE support before and after the Fenton test. After the Fenton test, significant structural deformations occurred in e-PTFE support. Tangled fibrils adhered to each other at junctions, nodes and fibrils were thickened, the micropores between the fibrils enlarged, and the number of micropores decreased. As the temperature of the Fenton test increased, these deformations became more pronounced. As the nodes and fibrils become thicker, the number of adjacent nodes and fibrils simultaneously decreases. Therefore, the increase in thickness appears to be a result of the adjacent fibrils and nodes sticking together due to degradation. Consequently, this result shows that the chemical degradation of radicals transforms the dense microstructure of the e-PTFE support.
3.3 Change of mechanical properties of e-PTFE after Fenton test.
Fig. 4 shows Stress-Strain (S-S) curve of the support after the Fenton test at 80℃, and table 1 presents its tensile mechanical properties. The Young's modulus decreased from 40.8 MPa to 39.5 MPa before and after the Fenton test. Young’s modulus indicates the stiffness of an elastic material, which depends on the force of interatomic bonds [26]. During the Fenton test, the radicals attack the CF2 bonds of the e-PTFE support. The weakened interatomic bonds of the support led to a reduction in Young’s modulus. When the stiffness of a support decreases, it becomes more susceptible to deformation. This implies that the dimensional stability of e-PTFE becomes lower after the Fenton test. Dimensional stability is crucial in a reinforced membrane, as increased dimensional changes can make it easier for cracks to appear in the membrane. Additionally, the yield stress decreased from 1.7 MPa to 1.5 MPa, and the yield strain decreased from 3.2% to 3.0% after the Fenton test. The ultimate stress decreased from 31.6 MPa to 26.4 MPa. Consequently, the elasticity and the toughness of the e-PTFE support decreased after the Fenton test. We suggest that the overall changes in the physical properties of e-PTFE after chemical degradation are attributed to the altered structure of the support following degradation. The collapse of the densely intertwined micro-fibril structures due to degradation has resulted in a reduction in physical strength. These results show that e-PTFE, which is chemically stable, can undergo changes in its physical structure through chemical degradation by radicals.
Table 1. Tensile mechanical properties of e-PTFE support after Fenton test.
|
Young’s Modulus
(MPa)
|
Yield Stress
(MPa)
|
Yield Strain (%)
|
Ultimate Stress (MPa)
|
Ultimate Strain (%)
|
Pristine
|
40.8
|
1.7
|
3.2
|
31.6
|
64.4
|
After Fenton test
|
35.9
|
1.5
|
3.0
|
26.4
|
59.0
|
3.4 Electrochemical degradation of reinforced membrane by OCV holding.
A MEA was fabricated using an e-PTFE support to investigate the electrochemical degradation of the e-PTFE support during actual fuel cell operation. Subsequently, OCV holding, which is an AST protocol to evaluation the electrochemical durability of MEA, was conducted in a unit cell [Fig 5]. The OCV holding was carried out under conditions of low humidity, high temperature, and oxygen atmosphere at the cathode to accelerate membrane degradation [5, 27, 28]. Fig. 6 shows the changes in the I-V curve during the OCV holding. The performance of cell decreased with OCV holding time. The current density at 0.6 V decreased to 26% and OCV decreased to 5% at OCV holding 144 h, as compared to the initial state. These performance decreases indicate that the electrochemical degradation of MEA. Notably, the sharp drop in OCV after OCV holding 144 h implies that hydrogen permeability of membrane increases due to electrochemical degradation.
Fig. 7 shows the changes in LSV during OCV holding. Until OCV holding 96 h, HCCD hardly changed, and it even decreased. This is attributed to the occurrence of S-O-S cross-linking within the membrane during degradation [29]. In early stages of electrochemical degradation, sulfone groups in membrane form the S-O-S cross-linking. This bridge creates a dense membrane structure, which reduces hydrogen crossover. After OCV holding 96 h, there was a slight increase in HCCD, attributed to the breaking of S-O-S bridges as a result of degradation. After OCV holding 144 h, HCCD doubled compared to its initial value. This increase of HCCD is attributed to the reduction in membrane thickness of the MEA caused by electrochemical degradation.
To investigate the degradation of the support within the MEA after OCV holding, e-PTFE support was extracted using propanol. The MEA was immersed in 2-propanol solution(99.5 %, SAMCHUN) for 30 minutes to eliminate ionomer and catalyst. Subsequently, the e-PTFE support was extracted and air-dried for approximately 6 hours at room temperature and atmospheric pressure. During the e-PTFE support extraction process, e-PTFE shrinks as it is immersed in the solution and dries. This shrinkage occurs because propanol causes a shrinkage in the morphology of the e-PTFE [30]. As the result, the extracted e-PTFE support undergoes inevitable morphological changes, leading to variations in FT-IR, SEM, and S-S curve analyses compared to the pristine state (Fig. 1S, 2S, 3S). The changes of e-PTFE after extraction are described in detail in supplementary material. Therefore, to clearly understand the changes in the e-PTFE support after MEA degradation, the e-PTFE supports were extracted from both non-degraded MEA and degraded MEA, respectively, followed by an analysis of e-PTFE degradation.
Fig. 8 shows the changes of the FT-IR spectrum of the e-PTFE support in degraded MEA during OCV holding. No peaks associated with the ionomer were observed in the extracted e-PTFE support [25]. Therefore, the e-PTFE supports were successfully extracted from MEA. After OCV holding, the absorbance of CF2 groups in the e-PTFE support decreased similarly to that observed after Fenton reaction. However, the reduction of CF2 peak in e-PTFE was greater after OCV holding than after the Fenton test. This result shows that long-term degradation during PEMFC operation can sufficiently deteriorate even the e-PTFE support present inside the membrane. Furthermore, when the Pt catalyst is dissolved and deposited in membrane by electrode degradation during PEMFC operation, the chemical degradation inside membrane can be accelerated, because radicals can be generated at the Pt catalyst inside the membrane [31-33]. In such cases, the support in the membrane may undergo more severe chemical degradation. Therefore, the chemical durability of the membrane support is significantly important during long-term operation.
3.5 Change of mechanical structure after OCV holding.
Fig. 9 shows the structure change of e-PTFE after OCV holding. After OCV holding, the e-PTFE support in the membrane was deformed similarly observed to the results after Fenton test. Adjacent nodes and fibrils were stuck to each other, thickening material. The number of micropores decreased, and the pore size was larger. These results show that the electrochemical degradation during PEMFC operation deteriorates the e-PTFE support chemically and deforms the microstructure.
3.6 Change of mechanical properties and structure of e-PTFE after OCV holding.
Fig. 10 shows the change in the S-S curve of e-PTFE support after OCV holding, and the change in mechanical properties is shown in Table 2. After OCV holding, The Young’s modulus of the e-PTFE support decreased from 382 MPa to 301 MPa, yield stress decreased from 11.3 MPa to 6.9 MPa, and ultimate stress decreases from 100.2 MPa to 82.2 MPa. The yield strain decreases from 2.5% to 1.8%, ultimate strain decreases from 54.9% to 41.0%. This trend is similar to that observed after the Fenton test, however, the reduction rate is greater. This result correlates with the result of FT-IR. It is the result that the more chemical degradation leads the more decrease of mechanical properties. When the elasticity and toughness of the support decrease, it becomes easily deformable and cannot withstand mechanical stresses. Therefore, the chemically degraded support can easily lead failure caused by mechanical stress during PEMFC operation.
Consequently, our results show that the e-PTFE support is deteriorated chemically during PEMFC operation, and this chemical degradation lead reduction of mechanical strength. In the reinforced membrane, the mechanical properties of the support are important to enhance the membrane of mechanical strength. However, the chemical durability of the membrane support during PEMFC operation is also crucial because the chemical and mechanical degradation occur simultaneously during PEMFC operation. Therefore, we suggest that to achieve the long-term durability targets of PEMFC, the chemical durability as well as the mechanical durability of the membrane support should be studied.
Table 2. Tensile mechanical properties of e-PTFE support after OCV holding.
|
|
Young’s Modulus
(MPa)
|
Yield Stress
(MPa)
|
Yield Strain
(%)
|
Ultimate Stress (MPa)
|
Ultimate Strain
(%)
|
Pristine
|
382
|
11.3
|
2.5
|
100.2
|
54.8
|
After OCV holding
|
301
|
6.9
|
1.8
|
82.2
|
41.0
|