Highly Durable MEA for PEMFC Under High Temperature and Low Humidity Conditions. Eiji Endoh a. Yokohama, JAPAN

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1 / , copyright The Electrochemical Society Highly Durable MEA for PEMFC Under High Temperature and Low Humidity Conditions Eiji Endoh a a Research Center, Asahi Glass Co., Ltd Hazawacho, Kanagawaku, Yokohama, JAPAN The world s first highly durable perfluorinated polymer based MEA for PEMFC, under high temperature and low humidity, has been developed. The newly developed MEA, which is composed of a new perfluorinated polymer composite (NPC), reduces the degradation rate to 1/100-1/1000 compared to the conventional MEA. The NPC MEA can be operated for more than 4000 hours at 120ºC and 50% relative humidity (RH). Introduction In the development of polymer electrolyte membrane fuel cells (PEMFC), the durability of the membrane is one of the most important issues. For automotive utilization of PEMFC, high-temperature operations between ºC and low humidity conditions are required. For the membranes of PEMFC, perfluorosulfonic acid (PFSA) polymers are extensively used due to their considerably higher chemical stability over hydrocarbon polymers. However, conventional PFSA polymers have glass transition temperatures around 80ºC, which is subject to critical breakdown of the membrane at high temperatures. Additionally, conventional PFSA polymers suffer from degradation under low humidity conditions even at 80ºC. In order to develop a durable membrane electrode assembly (MEA), it is of the utmost importance that the degradation mechanism is understood. The degradation study of the conventional MEA under low humidity OCV conditions was conducted, and carbon radicals were observed within the catalyst layers of the degenerated MEA (1). The radical species, which generated carbon radicals, was identified as the hydroxyl radical. Due to these results, it was confirmed that the hydroxyl radical is the main cause of the MEA degradation. In order to overcome these challenges, extensive research has been conducted. The world s first highly durable perfluorinated polymer based MEA, under high temperature and low humidity, has been developed (2)(3). The newly developed MEA, which is composed of a new perfluorinated polymer composite (NPC), reduces the degradation rate to 1/100-1/1000 compared to the conventional MEA. The NPC MEA can be operated for more than 4000 hours at 120ºC and 50% relative humidity (RH). This development has opened up a new frontier for the application of the highly stable perfluorinated polymer composite MEA for automotive utilization. Downloaded on to IP address. Redistribution subject to ECS terms 9 of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

2 Membrane Electrode Assembly (MEA) Experimental The electrocatalysts for oxygen reduction and hydrogen oxidation were Pt/C or Pt alloy/c. The cathode Pt loading values and anode Pt loading values were varied. The control MEAs were prepared by hot-pressing the electrodes onto the Flemion SH50 membranes (thickness: 50 µm). The NPC MEAs were prepared by hot-pressing the electrodes onto the NPC membranes (thickness: 40 µm or 25 µm). The geometrical area of the electrode was 25 cm 2. Evaluation of cell performances Current voltage curves and durability tests of the MEAs were measured by utilizing a single cell with a square-shaped active area (25 cm 2 ). The cell temperature was kept constant at 120ºC. Utilization of hydrogen and air was 50/50%. For the measurement of the current voltage curves, the cell pressures were varied in value from 127 to 280 kpa, and relative humidity of the inlet gas (both hydrogen and air) was varied from 25 to 100%. The OCV tests were conducted with the same cell. Confirmation of hydrogen peroxide formation Air which contained 1.3% hydrogen gas was bubbled into the aqueous dispersion of Ketjen black, and the formation of hydrogen peroxide was examined via titration. Identification of radical species 30 mg of commercially available Pt/C catalyst or Ketjen black was dispersed in 1 ml of ultra pure water, which contained 200 µl of 5,5-Dimethyl-1-Pyrroline-N-Oxide (DMPO). Next, air which contained 1.3% hydrogen gas was bubbled into the dispersion for 20 minutes. The filtrate of each dispersion was analyzed by ESR. The ESR analyses were conducted with a JEOL ESR Spectrometer JES-TE300 at room temperature. Results and Discussion 1. Clarification of the MEA degradation mechanism In order to develop a durable membrane electrode assembly (MEA), it is of the utmost importance that the degradation mechanism is understood. The degradation study of the conventional MEA under low humidity OCV test was conducted (1). Figure 1 indicates the ESR spectra of the degraded MEA at liquid nitrogen temperature. Large ESR signals at 327 mt were observed within the catalyst layers of the degenerated MEA. The signals observed at 327 mt were assigned to carbon radicals. A probable carbon radical formation mechanism was the abstraction of hydrogen, which was bonded directly to carbon atoms, by the hydroxyl or hydroperoxyl radicals. In order to identify the radical species, model tests were conducted. First, air which contained 1.3% hydrogen gas was bubbled into an aqueous dispersion of Ketjen black, and the formation of hydrogen peroxide was confirmed. Next, the radical species which 10

3 were generated during the decomposition of hydrogen peroxide were identified. Commercially available carbon supported Pt catalyst or Ketjen black was added into the hydrogen peroxide solution, which contained DMPO, and the generation of hydroxyl radical was confirmed by ESR measurement. Initial sample Figure 1 ESR spectra of the degraded MEA at liquid nitrogen temperature The successive experiment of the generation and decomposition of the hydrogen peroxide was conducted. Figure 2 indicates the ESR hyperfine spectra of the DMPO adducts, which reacted with the respective radicals. OOH HO CH3 H Figure 2 ESR hyperfine spectra of the DMPO adducts 11

4 Figure 3 indicates the ESR spectra of the Pt/C catalyst dispersed solution. Figure 4 indicates the ESR spectra of Ketjen black dispersed solution. Both spectra indicated the formation of radicals, which corresponded with the hydroxyl radical. Field/mT Figure 3 ESR spectra of Pt/C catalyst dispersed solution Figure 4 ESR spectra of Ketjen black dispersed solution From these experimental results, the degradation mechanism of the MEA under OCV test can be concluded. At the cathode, H 2 O 2 is chemically formed, and a hydroxyl radical is generated by the decomposition of the H 2 O 2. At the anode, H 2 O 2 is chemically or electrochemically generated, and a hydroxyl radical is similarly generated by the decomposition of the H 2 O 2. The hydroxyl radical will attack the ionomer, carbon support of the catalysts and membrane. Due to these results, it was elucidated that the MEA degradation proceeds via chemical attack by the hydroxyl radical, which is generated in the cathode and anode. The degradation mechanism of MEA under OCV condition is summarized as follows. 12

5 At the Cathode H 2 + O 2 H 2 O 2 (Chemically formed) H 2 O 2 HO (Chemically generated) At the Anode H 2 + O 2 H 2 O 2 (Chemically formed) 2H + + O 2 + 2e H 2 O 2 (Electrochemically formed) H 2 O 2 HO (Chemically generated) 2. Performance of NPC MEA In order to overcome these challenges, extensive research has been conducted, and a highly durable membrane under high temperature and low humidity conditions has been developed (2)(3). The newly developed membrane is composed of a new perfluorinated polymer composite (NPC). Figure 5 indicates the current voltage curves of the NPC MEA-I at 120ºC, and at various relative humidities. The cathode Pt loading was set to 0.6 mg /cm 2. Both the anode and cathode were subjected to similar relative humidity. The cell pressure was varied in order to maintain the oxygen concentration in the cathode gas to 16%. The NPC MEA-I showed good performance at 100% RH. The MEA was also able to operate at a lower relative humidity of 25% RH and 127 kpa. I-V Curve of NPC MEA-I at 120 Cell Voltage[V] Current Density[A/c m2 ] RH100% 280kPa RH80% 240kPa RH60% 200kPa RH50% 180kPa RH40% 156kPa RH30% 140kPa RH25% 127kPa Figure 5. Current voltage curve of the NPC MEA-I at 120ºC. Reactant gases were H 2 and air. Utilization of H 2 and air was 50/50%. 13

6 Figure 6 indicates the open circuit voltage durability test at 120ºC, 18% RH and ambient pressure. The control MEA failed within 10 hours of operation releasing a high amount of fluoride ion. The NPC MEA-I showed excellent stability over 1000 hours, and the fluoride ion release rate was approximately 2 x 10-8 g (F - ) cm -2 hr -1, which was less than 1% of the control MEA. This result demonstrates the exceptional chemical stability of the NPC MEA-I against degradation caused by the peroxide radicals at 120ºC and low humidity. Open Ciruit Voltage (V) NPC MEA-I OCV 100 Control MEA OCV 80 Control MEA F- ion release rate NPC MEA-I F- ion release rate Duration (hrs) F- ion release rate (µg/day cm 2 ) Figure 6. OCV durability test at 120ºC, 18% RH. Reactant gases were H 2 and air. Cell pressure was ambient. Figure 7 indicates the constant current durability test at 120ºC and 50% RH. The current density was 0.2 A/cm 2. The pressure of the cell was kept constant at 200 kpa abs, and the utilization of H 2 /air was 50/50%. The control MEA failed within 100 hours of operation releasing a high amount of fluoride ion. The NPC MEA-I showed excellent stability and was operated for more than 4000 hours. The fluoride ion release rate of the NPC MEA-I was approximately 2 x 10-8 g (F - ) cm -2 hr -1. The fluoride ion release rate of the conventional MEA operated at 70ºC and 100% RH varies from 1 x 10-8 to 10 x 10-8 g (F - ) cm -2 hr -1 (4). The fluoride ion release rate of The NPC MEA-I which was operated at 120ºC and 50% RH, was similar to the fluoride ion release rate of the conventional MEA operated at 70ºC and 100% RH. The degradation rate of the NPC MEA-I for 4000 hours of operation at 120ºC and 50% RH was approximately 75 µv/hr. The degradation rate of the NPC MEA-I operated at 70ºC and 100% RH was approximately 3 µv/hr. Therefore, the degradation rate of the NPC MEA-I at 120ºC and 50% RH was approximately 25 times higher than the degradation rate of the MEA at 70 º C and 100% RH. In order to investigate the degradation mechanism, this test was terminated and the MEA was analyzed. 14

7 Control MEA Cell voltage NPC MEA-I Cell voltage Cell Voltage ( V) Control MEA F- ion release rate NPC MEA-I F- ion release rate F- ion release rate (µg/day cm 2 ) Duration(hrs) 0 Figure 7 Durability of MEAs at 120ºC, 50% RH, 0.2 A/cm 2 and 200 kpa. Reactant:H 2 /air, Utilization:50/50%, Cathode Pt:0.6 mg/cm 2, Anode Pt:0.3 mg/cm 2. Figure 8 indicates the SEM image of the cross-section of the NPC MEA-I. The thickness of the membrane was 40 µm, and the cathode layer was 18 µm. The membrane thickness remained constant after 4000 hours of operation at 120ºC. However, thickness of the cathode catalyst layer decreased to approximately 8 µm. Initial Test MEA (Upper Portion) Cathode 18μm Cathode 8μm Membrane 40μm 4,000hrs operation Membrane 40μm Anode Anode Figure 8. SEM image of the cross-section of the NPC MEA-I. Figure 9 indicates the TEM image of the cathode layer cross-section of the test NPC MEA-I. Carbon support of the cathode catalyst decreased significantly, and large platinum particles were dispersed in the catalyst coating ionomer. Figure 10 indicates the schematic illustration of the cathode layer cross-section. 15

8 This occurred during 4000 hours of operation at 120ºC, and resulted in the decrease of the cathode catalyst layer thickness. This may be due to the oxidation of the carbon support by the hydrogen peroxide, which was formed during the oxygen reduction reaction. Substantial crystallite growth of Pt catalyst was observed in that region. XRD measurement indicated that the crystallite size of the Pt catalyst increased from 2 nm to 8 nm. The electrochemical cathode Pt surface area decreased by 97% after 4000 hours of operation at 120ºC. Therefore, the performance loss of the MEA was attributed to the degradation of the cathode catalyst. Cathode layer (Thickness:8µm) Membrane (Thickness:40µm) Figure 9. TEM image of the cathode layer cross-section. Initial Test MEA Gas diffusion layer Gas diffusion layer CO 2 Pt crystallite Membrane Membrane Carbon support Figure 10. Schematic illustration of the cathode layer cross-section 16

9 In order to overcome these challenges, improvements on the cathode catalysts are being conducted. Figure 11 indicates the durability of the NPC MEA-VI which is composed of an originally developed cathode catalyst at 120ºC, 50% RH, 200 kpa abs and 0.2 A/cm 2. The cathode Pt loading was 0.2 mg/cm 2 and the thickness of the NPC membrane was 25 µm. The MEA showed excellent stability for 3500 hours. Due to the malfunction of the test bench after 3500 hours of operation, a slight degradation of the performance was observed. The degradation rate was below 3 microvolts/hr over 3500 hours of operation. An additional benefit of the catalyst was the reduction of the Pt loading to 0.2 mg/cm E Cell Voltage ( V) E-07 6E-07 4E-07 2E-07 F- release rate (g/cm 2 hr) E Duration (hrs) Figure 11 Durability of NPC MEA-VI at 120ºC, 50%RH, 0.2 A/cm 2 and 200 kpa. Reactant:H 2 /air, Utilization:50/50%, Cathode Pt:0.2 mg/cm 2, Anode Pt:0.2 mg/cm 2. Thickness of the NPC membrane:25 µm. In order to make the NPC MEA feasible for automotive usage, focus will be on the improvement of the durability of the cathode catalysts. Additionally, focus will be on the increase of the NPC membrane conductivity to allow for operation at 120ºC and at a much lower relative humidity. Conclusions The world s first highly durable perfluorinated polymer based MEA for PEMFC has been developed. The MEA can be operated for more than 4000 hours at 120ºC and 50% relative humidity. This development has opened up a new frontier for the application of highly stable perfluorinated ion-exchange polymer composite (NPC) for automotive usage. Additionally, this development will solve the degradation problem of the ion-exchange polymer for stationary usage which will significantly extend the operational life of the PEMFC system. 17

10 Acknowledgements The author would like to acknowledge the contributions of Mr. Hisao Kawazoe, Mr. Satoru Honmura, Mr. Shinji Terazono, Dr. Hardiyanto Widjaja, Mr. Yasuyuki Takimoto and Ms. Junko Anzai from Asahi Glass Research Center. References (1) E. Endoh, S. Terazono, H. Widjaja, and Y. Takimoto, Electrochemical and Solid- State Letters, 7 (7)A209-A211,(2004) (2) E. Endoh and H. Kawazoe, Abstract 763, The 207 th Meeting Abstracts of the Electrochemical Society, May 15-20, 2005, Quebec City, Canada (3) E. Endoh, H. Kawazoe and H. Nakagawa, Abstract 1187, The 208 th Meeting Abstracts of the Electrochemical Society, October 16-21, 2005, Los Angeles, CA (4) S. Cleghorn J. Kolde, and W. Liu, Handbook of Fuel Cells Fundamentals, Technology, and Applications, W. Vielstich, A. Lamm, H. A. Gasteiger, Editors, p. 573, figure 13, Chapter 44. Vol. 3, John Wiley & Sons, Chichester, U.K. (2003). 18