Membrane Degradation in PEMFC : Study of Sulfonated Polyimides

1. 130°C 110°C -42% -50% 2.2 1.8 Fuel cell : 53 hours I Suggests that the degradation in fuel cell is due to hydrolysis reactions C-S R 1.4 1 Wave number (cm-1) 0 100 200 300 400 500 600 700 800 Time (hours) Ex-situ : 360 hours 80°C 110°C • IEC=0.86 at 70°C ■IEC=1.26 at 70°C ●IEC=1.98 at 70°C • IEC=1.98 at 90°C R’ = Heterogeneous degradation process in fuel cell DO(1350 cm-1) DO(1100 cm-1) R = 130°C DO(1500 cm-1) DO(1500 cm-1) Anode : Cathode : Hydrophilic block Hydrophobic block 1/2 O2 + 2 H+ + 2 e- H2O H2 2 H+ + 2 e- Ionic conductivity measured through the Ionic Exchange Capacity (IEC : n(SO3H)/g)) X : between 1 and 10 Y : between 3 and 20 Experimental conditions • 25 cm2 active area single cell • Gas inlet : 27 cm3/min for H2, 42 cm3/min for O2 • Gas pressure : 3 bar absolute • Current density : 200 mA.cm-2 • Cell temperature : between 60 and 90°C • Electrodes provided by Sorapec Membrane 0.28 mm gasket Electrode Aromatic carbons C=O C-O 15 IEC : 1,26 14 13 IEC : 1,98 ln(operating time) 12 11 10 0,0027 0,0028 0,0029 0,003 0,0031 1/Temperature (K-1) 90 °C 80 °C 70 °C 60 °C Similar dependance towards IEC and temperature than in FC experiments The degradation of sPI membranes in FC is mainly due to hydrolysis reactions -40 % at the cathode side -3 % at the anode side Imide band C-S band R’ : The cathode side is more sensitive to degradation processes reference The evolution of the imide function absorbency is characteristic for partial and complete hydrolysis 80 h 100 h Membrane Degradation in PEMFC : Study of Sulfonated Polyimides G. Meyer (1), G. Gebel (1), M. Bardet (2), J.-L. Gardette (3), M. Pinéri (4), D. Marsacq (4), R. Mercier (5), P. Capron (6) (1): CEA - DRFMC/SI3M/PCI – 17 rue des Martyrs – 38054 Grenoble, France (UMR SPrAM 5819) (2): CEA - DRFMC/SCIB – 17 rue des Martyrs – 38054 Grenoble, France (UMR 5046) (3): CNRS/LPMM, BP 187, 63174 Aubière, France (UMR 6505) (4): CEA - DTEN/SCSE/LSEM - 17 rue des Martyrs – 38054 Grenoble, France (5): CNRS/LMOPS, BP 24, 69390 Vernaison, France (UMR 5041) (6): CEA - DMAT/SCF/LMOD - Centre d’Etudes du Ripault – 37260 Monts, France H2/O2 fuel cell operation Objectives : understand the low operating times in fuel cell (FC) experiments and determine the kinetics of the polymer degradation by ex-situ experiments Hypothesis : the loss of mechanical properties is mainly due to chain cuts induced by the hydrolysis of imide functions modification of the chemical and/or physical structure Sulfonated polyimide = ionomer membrane used as proton conductor in H2/O2 fuel cells Chemical structure of the polymer : Fuel cell experiments Study of the degradation products formation Liquid 13C NMR of the residue obtained after hydrolysis of a membrane in water at 90°C 1- Absence of the peak of an ether carbon (hydrophobic block) at 155 ppm The degradation of sulfonated polyimides takes place only in the hydrophilic blocks of the polymer Membranes properties and FC results 2- There are still imide functions in the residue Quantification of these residual imide functionsby FTIR: temperature effect (200 hours, IEC = 0.86) With regard to the hydrophilic block of the initial sPI • 57% at 80°C • 40% at 110°C • 32% at 130°C End of a test = rupture of the membrane (comparaison of the ratio DO (1350cm-1)/DO (1100 cm-1)) The temperature strongly influences the operating time. Linear evolution shows that the degradation process is thermo-activated (EA=70 kJ.mol-1) The sensitivity towards temperature increases with IEC Study of sPI membranes during ex-situ degradation 1H spectrum after 150 hours at 90°C for IEC = 1.26 : Mechanical properties measurements on hydrolysed sPI membranes identical spectrum after fuel cell experiment large peaks oligomers in the residue • Possible molecule for the doublet (detected after a chromatographic separation of the residue components) : • Singlet : symmetric naphtalenic based molecule such as Hydrolysis reactions followed by FTIR measurements H2O, Δ + doublet Signals between 7.3 and 7.6 ppm Typical infra-red spectrum of a sPI membrane : Partial hydrolysis Complete hydrolysis Comparison ex-situ / fuel cell ageings Normalization of the spectra : band from the hydrophobic block (not sensible to hydrolysis) at 1500 cm-1 sPI 0.86 tested 600 hours at 80°C in fuel cell : Similar FTIR spectrum than after ex-situ hydrolysis Membrane sPI 0.86 : evolution of R at 80, 110 and 130°C Measurement of the SO3H amount on each side of the membrane by FTIR-ATR : R value in the hydrophobic block IR spectroscopy (membrane sPI 1.98, T = 80°C) • Complete disappearance of the imide functions from the hydrophilic blocks : • 110°C : 120 hours • 130°C : 40 hours the ageing in fuel cell is faster than in water presence of radicals in the cell ? EA = 60 kJ.mol-1 Conclusion Quantification of the sulfonic groups present in the membrane during hydrolysis • FC experiments show a high dependence of the membrane structure ant temperature on operating time • Ex-situ experiments in water indicated an identical evolution of chemical and physical of sPI properties than in fuel cell. The thermally activated degradation process limits the possibility of high temperature operations for such structures. Fuel cell ageing is faster than ex-situ hydrolysis • The different ex-situ experiments presented in this work allows the determination of the kinetic degradation of sulfonated polyimides with different chemical structures By infra-red spectroscopy By X-ray fluorescence spectroscopy the plateau corresponds to the complete disappearance of the hydrophilic imides 110°C Problem of diffusion ? The flat sulfur profile in the thickness of the membrane showed that it’s not the case