Estimation of Water Layer Thickness at the Cathode Catalyst Layer Surface of Polymer Electrolyte Fuel Cells from Diffusion Impedance Analysis

We have analysed the diffusion impedance of a polymer electrolyte fuel cell (PEFC) by electrochemical impedance spectroscopy (EIS). As a result, we derive oxygen diffusion distance δ adjacent to the cathode electrode (catalyst layer) assuming planar oxygen diffusion in a thin water layer unlike the conventional flooded-agglomerate model and gas phase diffusion model in a gas diffusion layer. The experimental results and their analyses show that δ agrees with a value reported from our previous ex-situ measurement. Moreover, δ gives reasonable concentration overpotentials. Increasing current density and gas humidification brings about the growth of δ. It supports that δ corresponds to the thickness of the accumulated product water in liquid state at the cathode electrode. Thus EIS analysis of the diffusion impedance can be used to diagnose the water accumulation at the cathode electrode. This result enables the prediction of the flooding and drying-up of PEFCs and assures their stable operation.