Membrane Durability and Degradation under Dry Fuel Cell Operation Conditions. Jing Li, Keping Wang and Yunsong Yang

Transcription

1 / The Electrochemical Society Membrane Durability and Degradation under Dry Fuel Cell Operation Conditions Jing Li, Keping Wang and Yunsong Yang Automotive Fuel Cell Cooperation, Burnaby, BC V5J 5J8, Canada Membranes are a critical component of the fuel cell stack and must be durable in a wide range of operating conditions. In the present study, PFSA membranes were evaluated using accelerated stress testing (AST) methods of OCV and cyclic OCV under dry conditions at 95 C. PFSA membrane degradation rates were determined by measuring fluorine release rate (FRR) and thickness change under different fuel cell operating conditions. The FRR was found to increase with a decrease in inlet relative humidity (RH) and an increase in stack temperature. The degradation active energy (derived from the Arrherius equation) was approximately JK -1 mol -1. A cyclic OCV test was designed to simulate membrane degradation caused by both mechanical and chemical stresses. Mechanically reinforced membranes showed much longer lifetime in the COCV test as compared to membranes without reinforcement. Under present operating conditions, membrane degradation is shown to initiate from cathode side. Introduction Proton exchange membrane (PEM) is a critical component of the fuel cell stack and must be durable in order to tolerate a wide range of operating conditions including low to high humidity and wide temperature range (-40 to 100 C for transportation applications). For automotive application, it must be able to perform over the full range of system operating conditions with less than 10% loss of performance by the end of life after 5000 hours operation. Cost, durability and performance under high temperature and dry conditions are major challenges in PEM development. Low operating temperature and high humidity requirements of current membranes add complexity to the fuel cell system and impact the system cost and durability. Increased operation temperature and decreased inlet gas relative humidity to near zero are required to lower system cost. However, even current perfluorosulfonic acid (PFSA) membranes that have excellent chemical stability, degrade very quickly with increased operation temperature and decreased inlet gas relative humidity (RH) (1-8). There are many studies regarding membrane degradation and failure mechanisms under different fuel cell operation conditions (1-17). The main causes for membrane failures are: 1) chemical/electrochemical degradation; 2) cation contamination; 3) mechanical stress from hydration/dehydration and pressure difference; 4) manufacture defects. Fuel cell operation conditions significantly impact on membrane durability. The present study aims to evaluate membrane durability under automotive application conditions. In the present study, PFSA membranes were evaluated using accelerated stress tests (AST) by holding the stack at the open circle voltage (OCV) under both Downloaded on to IP address. Redistribution subject to ECS 385 terms of use (see ecsdl.org/site/terms_use)

2 constant and cycling RHs. PFSA membrane degradation rate was determined under different fuel cell operating conditions and the degradation mechanisms were discussed. Experimental A 3-cell stack with 50 cm 2 of active area hardware was used in the AST. AFCC standard anode and cathode GDEs were used to make MEAs for all membrane evaluations. In order to minimize the effects of catalyst on membrane evaluation, a high catalyst loading was used in the GDEs (0.7 mg/cm 2 for cathode and 0.3 mg/cm 2 for anode). Gas flow rate was 3.5 standard liters per minute (slpm) for hydrogen at anode and 11 slpm for air at cathode in the OCV test. Inlet gas pressures at both anode (H 2 ) and cathode (air) were 3 bars (abs.). A cyclic OCV (COCV) test was designed to simulate membrane degradation caused by both of mechanical stressing and chemical degradation. The COCV includes mechanical cycles by changing RHs from dry to wet under OCV plus a loading period at 0.5 A/cm 2 and one chemical degradation period by holding at OCV under dry condition. Test temperature of COCV was 95 C. Chemical degradation rate of membranes was determined by measuring fluorine release rate (FRR) during OCV holding test. The FRR at both anode and cathode was monitored through on-line analysis of gas outlet water conductance and off-line fluorine content in the drain water using a fluorine ion selective electrode. Membrane thickness change before and after degradation was measured from SEM images of cross sections of the MEAs at the beginning and the end of life (BOL and EOL) or pulled out at the middle of test. The open circle voltage, hydrogen crossover and gas leakage of the stack were monitored during OCV and COCV tests and the end of life of the membranes at OCV and COCV tests was determined by either OCV <0.75 at OCV test and <0.8 at COCV test or the stack leak rate > 5 ccm/min. Chemical degradation of membranes Results and discussions In the car driving, the fuel cell stack often is in idling or low loading states. There is approximately more than 30% of time running at the state of cell voltage >0.85 V, which is close to OCV state. OCV lifetime of a membrane under certain stack operation conditions is an important index for stack durability. In the idling or low loading states, the stake is under high potential and membrane is dry due to less or no water produced in the operations, resulting in high membrane degradation rate. Fig. 1 shows FRR of the DuPont s Nafion NRE211 membrane at 80 C under 1) condition operating at 95% RH, 1A/cm2 loading, 2) OCV holding at 95% RH and 3) OCV holding at 30% RH. The FRR of the membrane was the lowest under an operation with 1A/cm 2 of loading, then increased under OCV holding at 95% RH. The highest FRR was found to be in an OCV holding at 30% RH. Obviously, the higher potential of the cell had and the drier the membrane was, the higher degradation of the membrane had. Downloaded on to IP address. Redistribution subject to ECS 386 terms of use (see ecsdl.org/site/terms_use)

3 Fluoride release rate (mol/s) 3.E-08 2.E-08 1.E-08 95% RH 1A/cm 2 loading 95% RH OCV 30% RH, OCV Cathode Anode 0.E Time (hrs) Figure 1. FRR (mol/s) of cathode and anode of DuPont s NRE211 membrane under 95% RH and 1A/cm 2 loading (left), 95% RH at OCV and 30% RH at OCV. Stack temperature: 80 C To identify the RH effect on membrane durability, the stack with NRE211 membrane was holding at OCV under different RHs at 95 C. The fluorine release rate (FRR) was found to increase linearly with a RH decrease from 80% to 30% at 95 C, as shown in Fig. 2. The time to fail of the membrane in the OCV holding test decreased from 223 hours at 80% RH to 66 hours at 30% RH, which was 4 times lower with the RH change. FRR (mol/s) 8.00E E E E OCV lifetime (h) 0.00E RH % Figure 2. FRR (mol/s) at cathode and OCV lifetimes of NRE211 vary with RHs at 95 C in the OCV test. Liu et al. reported that the FRR increased more than one order magnitude when the RH decreased from 100% to 25%. (3) Inaba et al. also found that the FRR increased with a decrease of RH in a OCV holding test (8). The next generation fuel cell system for automotive application is required to run under low inlet gas RH (without humidifier). It means that the membrane has to work under more critical environment. A significant 0 Downloaded on to IP address. Redistribution subject to ECS 387 terms of use (see ecsdl.org/site/terms_use)

4 improvement in membrane chemical stability is required to meet the fuel cell durability target. The FRR of the NRE211 membrane at OCV holding test under 30%RH increased exponentially with temperatures. The OCV lifetime of NRE211 decreased also with temperature rise and was about 5 time shorter as temperature rises from 70 C to 95 C under 30% RH, as shown in Fig.3. A degradation active energy of the membrane (derived from Arrherius plot) is approximately JK -1 mol FRR Log (mol/s) OCV lifetime (h) /T (K) Figure 3. FRR (mol/s) at cathode and OCV lifetimes of NRE211 test vary with temperature at 30% RH in the OCV test. Membrane degradation under cyclic OCV Membrane often is stressed by swelling and shrinking due to hydration and dehydration during the fuel cell operating, consequently, forming micro-cracks in the membranes under mechanical stress as well as chemical degradation. The cyclic OCV test was designed to simulate the membrane degradation caused by both of mechanical and chemical stresses. A mechanical reinforced PFSA membrane from DuPont was used in the COCV test to compare with un-reinforced NRE211. The cell potentials and FRR of both samples are shown in Fig. 4. The NRE211 membrane lasted 12 cycles (~81 hrs) in the COCV test that was more than 3 times shorter than the mechanical reinforced membrane (42 cycles). Obviously, the mechanical reinforced membrane exhibits much better durability due to less swelling in x and y dimensions. Cell voltage changes with test time are also shown in the Fig. 4. A solid line at top presents OCV and a dashed line presents a cell voltage at the 0.5 A/cm 2 loading. Two lines are almost parallel to each other. It implies that the performance drop in the COCV test is due to the OCV loss. Downloaded on to IP address. Redistribution subject to ECS 388 terms of use (see ecsdl.org/site/terms_use)

5 2.0E-07 FRR, reinforced membrabe FRR, NRE211 Voltage, reinforced membrane Voltage, NRE211 Cathode FRR (mol/sec) 1.5E E E Voltagr (V) 0.0E Time (h) Figure 4. FRR (mol/s) at cathode and COCV lifetimes of NRE211 and a reinforced PFSA membrane. Top solid lines are OCV and dashed lines present the cell voltages at 0.5 A/cm 2 loading. Membrane failure and analysis The studies on the membrane degradation in PEM fuel cell operation have been carried out and some fundamental understandings on the failure mechanisms have been reported (8-17). One of the mechanisms regarding the chemical degradation of PEM involves the formation of hydroperoxide, H 2 O 2 which decomposes to form hydroperoxyl ( OOH) and hydroxyl ( OH) radicals. H 2 O 2 can form on catalyst surface via chemical reaction 1 Or electrochemical reaction 2 O 2 + H2 H 2 O 2 (1) O 2 + 2H + +2e- H 2 O 2 E 0 = 0.67 V (2) H 2 O 2 diffuses into membrane from catalyst layer/membrane interface and chemically decomposes into hydroxyl radical in the present of Fenton metal ions: H 2 O 2 + M 2+ M 3+ + OH +OH - (3) Although PFSA membranes do not contain R-H group in the polymer chain, a trace amount of polymer end groups with residual H-containing terminal bonds, like -COOH, could be attacked by the peroxide radical and initiate decomposition (9, 17). Hence, once decomposition begins at one end group, a complete PFSA unit is decomposed to HF, CO2, and low-molecular weight compounds by the radical depolymerization reactions, called the unzipping mechanism following the reaction 4-6, resulting in damage of the membrane (9, 11, 17). Downloaded on to IP address. Redistribution subject to ECS 389 terms of use (see ecsdl.org/site/terms_use)

6 R-CF 2 COOH + OH R-CF 2 + CO 2 + H 2 O (4) R-CF 2 + OH R-CF 2 OH R-COF + HF (5) R-COF + H 2 O R-COOH + HF (6) In recent years, membrane manufactures have made a great progress to replace unstable end groups with chemical stable fluorinated groups to prevent from radical attack. For example, the FRR of chemical stabilized PFSA membrane DuPont NRE211 is only ~1/10 of the non-stabilized DuPont NRE112 in the Fenton test. Even if NRE211 has chemical stabilized end group, it last only 66 hours in the OCV test at 30% RH and 95 C. Results from a few groups (8-11), have also demonstrated that membrane degradation was much fast under dry conditions, as we saw in the OCV holding tests. The chain unzipping reaction mechanism mentioned above has no sensitive to environment RH. That means hydroxyl radical could attack other weak group in polymer under dry condition to cause membrane degradation, not only the end groups. Frank Coms (9) analyzed bond energies of C-C, C-O, C-S, C-F and C-H bonds in various chemical structures. It revealed that a weak F 3 C-SO 3 H could be attacked by hydroxyl radical under dry condition. He proposed a chain scission via sulfonyl radical initiation mechanism to explain membrane degradation under dry condition. λ (NH2O/NSO3-) NRE211 SSC PFSA EW 830 Equilibrium with liquid water Equilibrium with liquid water water vapor activity or P/P 0 Figure 5. Water uptakes of NRE211 and a short side chain (SSC) PFSA (EW=830) while equilibrating with water vapor under different water vapor pressures and with liquid water. Based on the thermochemical calculation, ionized R-SO - 3 group is more stable than R- SO 3 H in which hydrogen bonds to SO 3 group via a hydrogen bond. When RH is lower - than 30%, as shown in Fig 5, number of water molecules per SO 3 is less than the threshold value of three that is the minimum requirement for ionization (18). Acidic proton resides on -SO 3 group to form R-SO 3 :H rather than R-SO H+ (H 2 O)n ions. R- SO 3 :H is unstable and can be abstracted by OH to form -SO 3 which possesses an Downloaded on to IP address. Redistribution subject to ECS 390 terms of use (see ecsdl.org/site/terms_use)

7 extremely weak C-S bond and rapidly cleaves forming SO 3 and a fluororadical. Propagation of side chain fluororadical induces a main chain scission under dry condition, resulting in membrane degradation (9). Membrane could be dry if the inlet gas humidification is not enough or the system has no humidifier. Dry membrane could occur also if the stack is operating at low loading or in idling, or start up from low temperature. Locally, membrane could be drier at the inlet than at outlet. As discussed above, dry membrane could degrade fast. It has been observed that membrane thinning to fail often occurs at the gas inlet in the fuel cell driving cycle tests. Hydrogen peroxide formation and decomposition plays a very important role in membrane degradation. Where the hydrogen peroxide forms is related to amount and rate of hydrogen and oxygen crossover. Ohma et al. (5) found that the FRR at the anode side was higher than FRR at the cathode side when 100% oxygen gas was used in the cathode and 5% or 100% H 2 gas was used in the anode. However, the FRR is vise verse when air was used in the cathode and H 2 used in the anode. High oxygen partial pressure leads a high oxygen crossover rate, resulting in the reaction interface shifting to anode side. In the present OCV holding test, air was used in the cathode and H 2 was used in anode. It can be predicted that the reaction interface could be close to cathode because of high diffusion rate of hydrogen. In the experiments, we did found that the FRR at cathode was much higher than that at anode, as shown in Fig. 6. It means that there is higher membrane degradation rate at the cathode than at the anode. 3.0E E-08 Cathode FRR FRR (mol/sec) 2.0E E E-08 Anode FRR 5.0E E Time (h) Figure 6. FRR at cathode (top) and anode (Bottom) in OCV holding test at 95 C and 30% RH. Flow rate of Air/H 2 = 11/3.5 slpm. To prove a fast degradation of the membrane at cathode, a 5-cell stack was tested under OCV at 95 C and 30% RH. The MEAs were pulled out from the 5-cell stack at different time for SEM analysis. The pulled-out MEA was replaced with a new MEA to keep the stack always having 5-cells. The pulled-out MEAs then were sectioned for SEM analysis. In order to distinguish degradation region, the reinforcement layer was used as reference line to separate anode and cathode regions (Fig. 9a). The thicknesses Downloaded on to IP address. Redistribution subject to ECS 391 terms of use (see ecsdl.org/site/terms_use)

8 changes of the ionomer at the anode and cathode regions were measured individually from SEM images. Fig. 7 shows that overall membrane thickness decreased with time linearly in the OCV holding test. At the first 120 hours, the membrane thickness decrease was mainly due to the cathode region ionomer degradation. The thickness of ionomer at the cathode region decreased at almost the same rate as the rate of whole membrane thinning, while the ionomer thickness at the anode region kept constantly without significantly change. Ionomer at the cathode side almost corroded away at about 130 hours, as shown in Fig. 9b. From then, ionomer thinning at the anode region started at a detectable rate. Even though, the reaction interface was still closer to cathode. After 190 hours, the ionomer at both anode and cathode regions almost corroded away, only remaining the reinforcement layer in most area, as shown in Fig. 9c. It can be deduced from the facts above that the membrane degradation reaction interface was close to cathode electrode where the hydrogen peroxide concentration in the membrane was higher than at anode side. This is because that hydrogen diffusion through the membrane is much faster than oxygen and that H 2 and air were used in the present experiment. It should be noted that the OCV lifetime of the reinforced membrane was approximately 4 times longer than the membrane without reinforcement. It could be attributed to the chemical inert reinforcement layer that prevented membrane failure from crack formation and gas crossover cathode side thickness anode side thickness whole membrane thickness membrane thickness (um) OCV time (h) Figure 7. Membrane thickness and ionomer thickness at anode and cathode changed with time at the OCV holding test. Downloaded on to IP address. Redistribution subject to ECS 392 terms of use (see ecsdl.org/site/terms_use)

9 anode a b c Reinforced cathod Figure 8. SEM images of the MEA cross sections after the OCV holding test. a) Cross section of the MEA after 24 hours of the OCV test. There is ionomer at both anode and cathode regions; b) Cross section of the MEA after 130 hours of the OCV test. There is no ionomer at cathode region but ionomer at anode region still exists. c) Cross section of the MEA after 190 hours of the OCV test. There is no ionomer at both regions at most area, only remaining reinforcement layer. We also observed that membrane thinning occurred more close to sealing edge in the OCV and COCV tests. Membrane predominant thinning and microcrack fracture has usually been observed in local stress concentrated regions, such as the transition region, the edge of seals or the edge the flow channel where the polymer electrolyte may stretch by the pinching action of the flow field lands. Except mechanical stress, the failure near the boundary region may come from peroxide radical chemical degradation by reactant gas reached through imperfect sealing of gaskets (4, 10). An edge protection film introduced during MEA lamination between the membrane and the catalyzed substrate could mitigate the MEA failure at the sealing edges and increase the MEA durability (4). Conclusion When a fuel cell car is operating at the idling or low loading states, the stack is under high potential and the membrane is dry due to less or no water produced in the operating, especially operating under low capacity of humidification or without humidification. Membrane degradation rate was much higher under these operations. The OCV lifetime of the NRE211 membrane in the OCV holding test was 4 times lower with RH change from 80% to 30%. Even with reinforcement technology that has the OCV lifetime approximately 4 times longer than the membrane without reinforcement, improvements in durability are still required to meet the target of automotive application. In the present experiment conditions, membrane degradation was close to the cathode electrode. An chemical inert reinforcement layer can significantly increase the membrane durability, therefore, the MEA lifetime. Acknowledgments Authors would like to thank Jason Yang and Si Rim Kim in help of OCV tests and Rajeev Vohra for his comments and useful discussions. Downloaded on to IP address. Redistribution subject to ECS 393 terms of use (see ecsdl.org/site/terms_use)

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