Proton exchange membrane impedance measurements

The main difference between the AFC and PAFC is the gas-tight solid polymer electrolyte membrane, a sohd proton exchange membrane which has as its main function the transport of protons from anode to cathode. To investigate the physical and electrochemical origins of the performance loss in PEFC—operated at different conditions like high current densities, fuel composition (neat H2, H2 -1- lOOppm CO, H2O), flow rates, temperature, air or pure oxygen, etc.—electrochemical impedance studies on different PEFC systems with different electrodes and membranes were performed, as mentioned in Section 4.5.4.1. First impedance measurements and interpretation of FIS performed to characterize PEFC were reported by Srinivasan et al. [1988], Fletcher [1992], Wilson et al. [1993] and Poltarzewski et al. [1992], With increasing research and development effort to improve the PEFC performance and availability of suitable instrumentation the number of publications has increased. 

The cell was placed in a thermo-controUed humidity chamber to measure the temperature and humidity dependence of proton conductivity. In this method, a fixed AC current is passed between two outer electrodes, and the conductance of the material is calculated from the ac potential difference observed between the two inner electrodes. The method is relatively insensitive to the contact impedance at the current-carrying electrode and is therefore weU suited for measuring proton conductivity. This open cell is also suited for studying the humidity dependence of conductivity for proton exchange membrane because of the low interfacial resistance. 

As to the membrane conductivity, only small losses of protonic conductivity, of the order of 5-10% after 4000 h, have been observed in well-humidified cells during PEFC life tests according to measurements of cell impedance at 5 kHz [42]. The deionized water employed in the humidification scheme [42] had very low levels of metal ions (e.g., Fe " / +, Ca + or Mg +). Such multivalent ions could exchange irreversibly with protons in the PFSA membrane, causing a drop in membrane conductivity. Deionizing the water used for PEFC humidification is therefore required, and appropriate plumbing should also be used in the humidification loop to avoid generation of ionic contaminants by corrosion processes. 

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