Impact of the Cathode Platinum Distribution on the Proton Transport Resistance Measurement in a PEMFC
Authors: Mehrazi Shirin , Stuhmeier Bjorn Marcel , Braaten Jonathan , Cheng Lei 
Published: December 2024

The cathode of a proton exchange membrane fuel cell (PEMFC) could have inhomogeneous platinum distribution through its thickness either by design (e.g., graded Pt loading electrodes) or as a result of degradation (e.g., Pt dissolution and migration to the membrane) [1,2]. The assessment of electrode properties such as inherent proton transport resistivity with electrochemical impedance spectroscopy (EIS) could be impacted by this inhomogeneity. Regarding protonic resistivity in the cathode, typically a transmission line model is fit to the corresponding EIS spectrum collected in the H2/N2 environment [3]. However, the model assumes a homogeneous distribution of capacitance and proton resistance through the electrode thickness and does not account for the inhomogeneity in Pt distribution. In this study, a bilayer electrode model system with variation in Pt loading is proposed to investigate the corresponding EIS spectrum as a function of Pt loading distribution in the cathode. 10 and 40wt% Pt on Vulcan were used to make low- and high-loading catalyst layers with a constant ionomer-to-carbon ratio of 0.85, which is supposed to result in similar proton transport resistivity in all designed electrodes. Artificially designed bilayer electrode structures of Low-Low, Low-High, High-Low, and High-High (first term: loading near membrane and second term: loading near the gas diffusion layer) have been successfully fabricated to simulate the cathode layer with homogeneous and heterogenous Pt mass loading through the electrode thickness. Potentiostatic EIS spectra of the bilayers at 0.45V bias potential revealed similar Rp for all samples (Avg. 51.7 Ω.cm2), while the measured Rp at 0.2V deviated by 22% for the High-Low and 55% for the Low-High sample from the corresponding value at 0.45V. Homogeneous bilayers maintained similar Rp at both bias potentials. Considering the cyclic voltammogram of the samples, the total capacitance is highly affected by a pseudo capacitive contribution from the Pt surface at potentials < 0.35V (Hydrogen adsorption-desorption) and > 0.65 (Pt surface oxidation). Even though the total capacitance is similar for High-Low and Low-High samples, the EIS spectra collected at 0.2V suggest that the distribution of capacitance through the electrode thickness can highly impact the spectra and corresponding Rp estimate.

References

[1] Kneer, Alexander, et al. "Correlation of changes in electrochemical and structural parameters due to voltage cycling induced degradation in PEM fuel cells." Journal of The Electrochemical Society 165.6 (2018): F3241-F3250.
[2] Zheng, Zhifeng, et al. "Design of gradient cathode catalyst layer (CCL) structure for mitigating Pt degradation in proton exchange membrane fuel cells (PEMFCs) using mathematical method." Journal of Power Sources 451 (2020): 227729.
[3] Makharia, Rohit, Mark F. Mathias, and Daniel R. Baker. "Measurement of catalyst layer electrolyte resistance in PEFCs using electrochemical impedance spectroscopy." Journal of The Electrochemical Society 152.5 (2005): A970.