Study of oxygen diffusion in the cathode catalyst layer and gas diffusion layer for polymer electrolyte fuel cells with EIS

Abstract

• Electrochemical impedance model for estimation of oxygen diffusion in PEFC. • Development of a cathodic impedance model comprising the CCL and GDL. • EIS and polarisation curves measured in PEFC operated with nitrox and heliox. • Estimation of oxygen diffusion resistance from EIS in PEFC with nitrox and heliox. • Analysis of oxygen diffusion resistance in catalyst layer and gas diffusion layer. In this study, a new electrochemical impedance model to predict oxygen diffusion resistance in the cathode catalyst layer (CCL) and in the gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs) is proposed. The impedance model of the PEFC is validated with electrochemical impedance spectroscopy (EIS) measurements carried out in a single PEFC operated with oxygen diluted in nitrogen (nitrox) and diluted in helium (heliox) at different relative humidity (RH). The PEFC impedance model combines the impedance model for the CCL reported in the authors’ previous study with a diffusion impedance model for the GDL reported in the literature. The EIS measurements in the PEFC represent lumped parameters (total oxygen diffusion resistance R O 2 _ P E F C ) from the different layers comprising the PEFC. The impedance model ignores the effect of oxygen transport in the flow field plates on EIS measurements, as this effect is apparent on EIS measurements carried out in PEFCs with low oxygen stoichiometry. The results from this study demonstrate that the estimated oxygen diffusion resistances in the CCL and GDL are distributed in the total oxygen diffusion resistance R O 2 _ P E F C = R O 2 _ C C L + R O 2 _ G D L . This is demonstrated by analysing the EIS measurements carried out in the PEFC with nitrox and heliox as cathode gas reactants and neglecting oxygen transport resistance in either the CCL R O 2 _ P E F C ≈ R O 2 _ G D L or GDL R O 2 _ P E F C ≈ R O 2 _ C C L from the PEFC impedance model. It is possible to obtain a deeper understanding of the influence of the electrochemical mechanisms from the different layers comprising the PEFC on EIS measurements by combining fundamental theory and experimental measurements in a complimentary manner.