Impact of the Cathode Electrode Platinum Distribution on the Proton Transport Resistance Measurement in a PEMFC

Due to degradation or by design, the cathode electrode of a proton exchange membrane fuel cell (PEMFC) can exhibit an inhomogeneous Pt-distribution that can affect the assessment of electrode properties by electrochemical impedance spectroscopy (EIS), such as proton transport resistivity (Rp). In this study, bilayer cathodes of varying Pt loadings (Low-Low, Low-High, High-Low, and High-High) were fabricated to simulate homogeneous and heterogeneous Pt distributions. Hereby, the Low-High configuration mimics an aged electrode with a Pt-depleted region near the membrane and a Pt-rich area near the gas diffusion layer. Potentiostatic EIS (PEIS) spectra were compared at two bias potentials. At 0.2 V, where the PEIS measurement is strongly influenced by the pseudo-capacity of hydrogen underpotential deposition (Hupd) on the Pt surface, impedance spectra for nonhomogeneous bilayers deviated significantly from homogeneous bilayers. Conversely, at 0.45 V where the double-layer capacitance of the carbon support dominates, the EIS of homogeneous and inhomogeneous samples overlapped. Aging a commercial sample via load/unload accelerated stress tests resulted in a shift to higher values for Rp assessed at 0.2 V, while the spectra at 0.45 V remained constant, matching the Low-High bilayer sample. These findings highlight the importance of considering the Pt distribution when assessing electrode properties.