Abstract
Electrochemical impedance spectroscopy is an important tool for fuel-cell analysis and monitoring. This study focuses on the low-AC frequencies (2–0.1 Hz) to show that the thickness of the catalyst layer significantly influences the overall resistance of the cell. By combining known models, a new equivalent circuit model was generated. The new model is able to simulate the impedance signal in the complete frequency spectrum of 105–10−2 Hz, usually used in experimental work on polymer electrolyte fuel cells (PEMFCs). The model was compared with experimental data and to an older model from the literature for verification. The electrochemical impedance spectra recorded on different MEAs with cathode catalyst layer thicknesses of approx. 5 and 12 µm show the appearance of a third semicircle in the low-frequency region that scales with current density. It has been shown that the ohmic resistance contribution (Rmt) of this third semicircle increases with the catalyst layer’s thickness. Furthermore, the electrolyte resistance is shown to decrease with increasing catalyst-layer thickness. The cause of this phenomenon was identified to be increased water retention by thicker catalyst layers.
Highlights
The development and optimization of the key components of proton exchange membrane fuel cells (PEMFCs) are necessary for the material cost reduction needed for the adoption of PEMFC technology in everyday life [1,2,3,4]
While a catalyst layer using HiSpec 4000 with 40 wt% Pt on carbon and 0.125 mgPt cm−2 resulted in an average thickness of 4.4 μm, the thickness increased by the factor 2.7 when the catalyst was changed to HiSpec 3000 with 20 wt% Pt on carbon
Cross sectional SEM of prepared catalyst layers used for layer-thickness determination of: (a) membrane electrode assembly (MEA) 40/0.125, MEA 20/0.125
Summary
The development and optimization of the key components of proton exchange membrane fuel cells (PEMFCs) are necessary for the material cost reduction needed for the adoption of PEMFC technology in everyday life [1,2,3,4]. Adding to the difficulty is the multitude of different models presented in literature that often leave researchers puzzled as to their choices for interpreting their data. This is especially true for the description of low-frequency features in the range of 2 to 0.01 Hz, where diffusion-related impedance signals appear [5,6,7,8]. The lower spectrum of this frequency range (0.2–0.01) produces
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