study of the decomposition of the polymers in the active …...study of the decomposition of the...
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Study of the decomposition of the polymers in the active layer in PEFC
M. Schulze, A. Haug
DLR - German Aerospace Center, Institute of Technical Thermodynamics, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
IntroductionThe water management in PEFC is one of the most important parameters for the fuel cell performance. In order to optimize the performance of fuel cells which are related to the transport processes the complex structure of the active layer is important which contains catalyst, ionomer particles or films and in some cases hydrophobic agents. For this purpose Nafion with a PTFE backbone and sometimes PTFE itself is used in the active layer, the latter component for hydrophobizing purposes. Due to the fuel cell operation these polymers partially decompose thereby losing their specific function in the active layer. The alteration of the polymers in the active layer was studied by X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR-ATR). With both methods the alteration of the polymers in the active layer could be observed. The XPS-measurements shows that the decomposition of the polymers in the anode layer is more significant than in the cathode layer. A similiar decomposition of the polymers was also observed in earlier studies on alkaline fuel cell electrodes. For this system also the decomposition of the PTFE in the anode was more significant than in the cathode although the environmental in both fuel cell types, PEFC and AFC is strongly different.
Problems of degradation of hydrophobic components
Investigation of degradation
AcknowledgmentThe authors acknowledge the NRC-Helmhotz Joint Research program for financial support for the project “PEFC Durability” and also the EU commission for financial support of the DECODE project.
Reversible Degradation
Irreversible and Reversible Degradation
Cell voltage as function of time at constant loading
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Change of cell voltage during constant load -2at 500 mAcm operated with H /air at 80°C2
Variation of the transfer resistance related to the anodereaction derived from the impedance measuremenusing the equivialent circuit
Variation of the transfer resistance related to the cathode reaction derived from the impedance measurement using the equivialent circuit
X-ray photoelectron spectra of used and new electrodes (E-Tek);green: used anode, red: used cathode,black: new electrode
X-ray photoelectron spectraof used and new reactionlayers of a CCM (IonPower)
C1s spectra of a DMFC anode before and after fuelcell operation
Carbon black concentration in dryprepared electrodes before andafter mechanical stressing of the surface
Alteration of the current density distribution due to long term operation atpotentiostatic conditions (600 mA/cm2). The alteration of the current densitydistribution is related to changes of the water management inside the cell(alteration of the hydrophobic properties).
FTIR spectra of PEFC reaction layers after two different periods of operation (black 300 h and red 1200 h). left side: Spectra of the CF vibration measured on the anodemiddle: Spectra of the CF vibration measured on the cathoderight side: Spectra of the CO vibration measured on the anode
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