Impedance Analysis of Cells

As realised from the above issues in the comparison of test results on the electrodes and on the cells, it is a non-trivial task to break down the total loss measured on a single cell into its components using the results from the electrode studies. Impedance spectroscopy on practical cells is, however, a technique by which a partial break down can be made. Though the impedance spectra obtained in general are difficult to interpret due to the many processes involved, the spectra can at least provide a break down of the total loss into an ohmic resistance (Rj = Rgiyt + Rconnect) and a polarisation resistance reflecting losses due to chemical, electrochemical, and transport processes, as described in more detail in Chapter 9. 

A large localised leak of air into the anode compartment (for instance at the rim of the cell) causes an increase in temperature due to hydrogen combustion  

However, the temperature increase cannot explain more than half of the deviation of the apparent ASR from the true value. Significant variations in the fuel composition due to localised leaks over the cell area can induce internal currents in the ceil, i.e. the parts of the cell in areas with high local Emf are loaded in the fuel cell mode whereas other parts with Emf below the OCV are loaded in the electrolyser mode. When the cell is externally loaded, internal currents decrease with decreasing cell voltage. Temperature and fuel compositions vary accordingly and thus the local internal resistance also varies. 

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