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Membrane Degradation in PEMFC : Study of Sulfonated Polyimides PowerPoint PPT Presentation


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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.

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Membrane Degradation in PEMFC : Study of Sulfonated Polyimides

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Membrane degradation in pemfc study of sulfonated polyimides

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


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