Validation and evaluation of electrochemical impedance spectra of systems with states that change with time

The influence of hindered water removal from a polymer electrolyte fuel cel l under constant load was investigated using electrochemical impedance spec troscopy. The cathodic gas outlet of the cell was closed and impedance meas urements were performed at periodic time intervals. Under the experimental conditions, the investigated system is far from steady-state conditions. It is shown that enhanced mathematical techniques are required for evaluation of the impedance data obtained. These techniques, the time course interpol ation as well as the Z-HIT refinement, lead to quasi-causal spectra which a re well interpreted by means of the porous electrode model.