XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells

Typically polytetrafluoro-ethylene (PTFE) is used to make the electrodes hydrophobic, because PTFE has a high chemical stability. Hydrophobicity depends on the concentration of PTFE on the electrode surface. The PTFE concentration, which is related to the hydrophobic character, can be determined by XPS. The changes in the PTFE content and structure of the electrode of a PEFC was investigated by cyclic voltammetry and XPS and correlated with the performance of the cell in long-term operation. With both methods an initial significant increase in free and electrochemically active surface platinum area is observed. This activation is associated with a degradation of the PTFE in the electrode which is responsible for the hydrophobic properties of the electrode. With further operation the performance of the cell decreases because the water management becomes more critical. Generally, it is shown that XPS can be used for the investigation of the hydrophobicity of electrodes prepared by various manufacturing techniques as well as of changes in their hydrophobicity induced by the electrochemical operation.