Degradation of PEM Fuel Cells

Transcription

1 Degradation of PEM Fuel Cells Dr. Magnus Thomassen SINTEF Materials and Chemistry Materials and Chemistry 1

2 Outline Introduction Overview of PEM fuel cell components Observed long term behaviour of PEM fuel cells Degradation phenomena and methods for investigation Membranes Electrodes Bipolar Plates Sealings Effect of impurities Materials and Chemistry 2

3 PEM Fuel Cell components H 2 2e - O 2 (air) 2e - 2e - H 2 2H + O 2 H 2 O Materials and Chemistry 3

4 Long term PEMFC behaviour F. A. de Bruijn, V. A. T. Dam, G. J. M. Janssen, Fuel Cells 08, 2008, No.1, 3-22 Materials and Chemistry 4

5 Degradation phenomena and methods for investigation H 2 2e - O 2 (air) 2e - 2e - H 2 2H + O 2 H 2 O Materials and Chemistry 5

6 Membrane Mechanical degradation Contamination of ionic species Chemical degradation Proton conductor, electronic insulator and gas separator Materials and Chemistry 6

7 Mechanical degradation Perforations, pinholes, cracks or tears Causes membrane defects from manufacturing or improper MEA assembly In plane tension/compression due to RH cycling Non-uniform compression Leads to early life catastrophic failure Pinhole Exothermic reaction Hot spots Gas crossover Materials and Chemistry 7

8 Chemical Degradation Ionomer attacked by radicals Hydroxyl ( OH) or peroxy ( OOH) attack polymer end groups (eg. Carboxylic end groups )1: Unzipping reaction 1: D.E. Curtin, R.D. Lousenberg, T. J. Henry, P.C. Tangeman, T. Kaz, E. Roundner, Phys Chem Chem Phys. 2004, 6, 2891 Materials and Chemistry 8

9 Chemical Degradation Where do the radicals come from? H 2 O 2 Gas crossover & chemical reaction Electrochemical oxygen reduction H 2 O 2 Oxidizable metal ions are needed (Fe 2+ ) Materials and Chemistry 9

10 Chemical Degradation Direct radical route Floride emissions observed without presence of metal ions Hydroxyl ( OH) and peroxy ( OOH) radicals are short lived intermediates in the oxygen reduction reaction. Pt from cathode deposits as nanoparticles inside the Nafion membrane Leads to radical formation throughout the nafion membrane and increased degradation Considered more important than the H 2 O 2 pathway Pt band in Nafion membrane 50h 100h Wang XP, Kumar R, Myers DJ, Electrochem. Lett. (9) 2006, A225-A227 Materials and Chemistry 10

11 Ionic contamination Nafion has a stronger affinity to all metal ions than for protons (except Li + ) Leads to ion exchange of protons with metal ions Reduction of proton conductivity Reduction of performance and increased heat production Reduction of water transport Drying out of membrane and water management issues Materials and Chemistry 11

12 Characterization of membrane degradation Fluoride Emission Rates Chemical degradation rate Hydrogen Cross Over Membrane thinning Open Circuit Voltage Decay Membrane thinning, pinholes Membrane conductivity (Current Interrupt, High frequency resistance) Ionic contamination Ex situ characterisation SEM, TEM EDX, XRD Materials and Chemistry 12

13 Fluoride Emission Rates Fluoride concentration in outlet water Fluoride selective electrodes SINTALYZER Online fluoride monitoring Materials and Chemistry 13

14 Fluoride Emission Rates Fluoride concentration in outlet water Liquiud Chromatograph Fluoride emissions Sulphate emissions Materials and Chemistry 14

15 Hydrogen Crossover Hydrogen crossover rate proportional to hydrogen partial pressure and thickness of membrane Beginning of life (50 µm membrane) : 1 macm -2 End of life : ~ 13 macm -2 N 2 H 2 N 2 H 2 2e - 2e - 2H + H 2 Materials and Chemistry 15

16 Membrane degradation, summing up F. A. de Bruijn, V. A. T. Dam, G. J. M. Janssen, Fuel Cells 08, 2008, No.1, 3-22 Materials and Chemistry 16

17 Electrodes Platinum dissolution and particle growth GDL and MPL degradation Carbon support oxidation Support structure for catalysts Facilitate optimal access of gas, electrons, protons and removal of water Materials and Chemistry 17

18 Pt-particle dissolution and growth Loss of Pt electrochemical active surface area Pt = Pt e - E 0 =1.188V vs NHE Pt + H 2 O = PtO + 2H + + 2e - PtO + 2H + = Pt 2+ + H 2 O E 0 =0.980V vs NHE Pt dissolves and migrates/diffuses into membrane (loss of catalyst material) Pt redeposits on other Pt particles causing particle growth (loss of active surface area) Materials and Chemistry 18

19 Pt-particle dissolution and growth Conditions causing Pt-dissolution and growth High cathode potentials (OCV) 0.75 V, 2000h: 46% area loss 0.95 V, 2000h: 75% area loss Potential cycling Continous restructuring of Pt-surface cycles V, 100h, 69% area loss P.J. Ferreira et al. J. Electrochem Soc. 2005, 152 A2256 Materials and Chemistry 19

20 Carbon support oxidation The carbon support is susceptible for oxidation: C + 2H 2 O = CO 2 + 4H + + 4e - Due to very slow kinetics, carbon is realtively stable E 0 =0.207V vs NHE Leads to loss of catalyst area (detachment of Pt nanoparticles and loss of electronic conduction paths) Carbon corrosion is prevalent during Start-stop cycles Fuel Starvation Materials and Chemistry 20

21 Start-stop cycling: The mechanism of carbon corrosion ~1.6 V C + H 2 O CO 2 + 4H + + 4e 2H 2 O O 2 + 4H + + 4e 1.0 V O 2 + 4H + + 4e 2H 2 O e H + H + e O 2 + 4H + + 4e 2H 2 O 1.0 V H 2 2H + + 2e 0.0 V Materials and Chemistry 21

22 Degradation of Gas Diffusion Layer Loss of hydrophobicity, PTFE degradation Pore structure change, increased risk of water flooding Reduced mass transport Risk of fuel/oxidant starvation Water management issues Carbon corrosion Decrease in GDL electronic conductivity Pore structure change Materials and Chemistry 22

23 Characterization of electrode degradation Electrochemical Characterisation Cyclic Voltammetry Catalyst area, double layer capactiance Voltage Current characteristics Gives information of performance and catalytic activity Comparison of curves using oxygen and air indicates mass transport effects Impedance Spectroscopy Gives information of proton conductivity (High frequency, > 10kHz), charge transfer (medium frequency, 1kHz ) and mass transport (low frequency, 1 Hz) Materials and Chemistry 23

24 Electrochemical characterisation: Cyclic Voltammetry Hydrogen desorption Pt oxide formation Double layer charging Hydrogen adsorption Pt oxide reduction Materials and Chemistry 24

25 Active Catalyst Area In-situ cyclic voltammetry 25 and 50 mv/s V V V Decrease in performance found proportional with lower active area Materials and Chemistry 25

26 Characterization of electrode degradation Chemical analysis of effluents Gas analysis, detecting CO2 from carbon corrosion GC, FTIR, MS Fluoride emissions Degradation of PTFE in gas diffusion layer Ex Situ Characterisation SEM, TEM Pt particle growth and distribution Electrode thinning NMR, XPS, EDX Chemical composition of ionomer and electrode materials Materials and Chemistry 26

27 Gas analysis: CO 2 from carbon corrosion CO 2 removal from air feed to test station Fuel cell test station NDIR spectrometer for detection of CO 2 in cathode exhaust Fuel Cell under test Water removal from cathode exhaust Possibility of CO 2 trapping in condensed water Materials and Chemistry 27

28 Results: Gore cell Gore: Dry conditions Gore: Humid conditions. Air purge/h 2 flow rates of 400/200 sccm at 80 C C + H 2 O CO 2 + 4H + + 4e Materials and Chemistry 28

29 Electrode degradation, summing up F. A. de Bruijn, V. A. T. Dam, G. J. M. Janssen, Fuel Cells 08, 2008, No.1, 3-22 Materials and Chemistry 29

30 Effect of Impurities Impurity Source Typical contaminant Air N 2, NO x, SO x, NH 3, O 3 Reformate hydrogen CO, CO 2, H 2 S, NH 3, CH 4 Bipolar metal plates Fe 3+, Ni 2+, Cu 2+, Cr 3+ Membranes Na +, Ca + Sealing gaskets Coolants & Water Si, Al, S, K, Fe, Cu, Cl, V, Cr Si Materials and Chemistry 30

31 Effect of Impurities Carbon Monoxide Well known poison for Pt catalysts. Binds strongly to the Pt surface and prevents hydrogen oxidation Significant reduction in performance is seen at CO levels of 10ppm Poisoning effect is reversible Carbon Dioxide Acts mainly as a dilutant of hydrogen Can cause CO poisoning due to WGS Hydrogen Sulphide A severe catalyst poison Significant performance drop has been observed for H2S levels of 0.1 ppm Irreversible effect Materials and Chemistry 31

32 Effect of Impurities Ammonia Significant decrease in performance is observed for NH3 levels of 1ppm (after 1 week). Acts as a poison for hydrogen oxidation and oxygen reduction NH 4 + absorbs in membrane and reduces conductivity R. Halseid, Effects of Ammonia on PEM fuel cells, PhD thesis, NTNU 2004 Materials and Chemistry 32

33 Effect of Impurities Cations Replaces H + in Nafion membrane Reducing proton conductivity Reducing water transport properties Can act as Fenton reagents, causing severe membrane degradation Can reduce catalytic activity Materials and Chemistry 33

34 Effect of chlorine additon: 100h test Materials and Chemistry 34

35 EQCM Cyclic Voltammograms Materials and Chemistry 35

36 EQCM Potential 1.2V vs RHE Pt nanocrystal stability severly reduced by presence of chloride. Materials and Chemistry 36

37 Acknowledgements Thor Anders Aarhaug, Anders Ødegård, Steffen Møller-Holst SINTEF Materials and Chemistry Axel Baumann Ofstad, Svein Sunde NTNU Materials and Chemistry 37

38 References Durability and Degradation Issues of PEM Fuel Cell Components F. A. de Bruijn, V. A. T. Dam, G. J. M. Janssen, Fuel Cells 08, 2008, No.1, 3-22 A review of PEM hydrogen fuel cell contamination: Impacts, mechanisms, and mitigation, X. Cheng et al., J. Power Sources 165 (2007) A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies, J. Wu et al., J. Power Sources 184 (2008) Materials and Chemistry 38