Impact of inhomogeneous catalyst layer properties on impedance spectra of polymer electrolyte membrane fuel cells

A physically-based 1D impedance model of the cathode catalyst layer that accounts for inhomogeneous catalyst layer (CL) properties is developed. The impact of locally changing proton conductivity, volumetric double-layer capacity, ORR rate and effective porosity on the characteristics of impedance spectra is analyzed. It was found that a non-constant conductivity and capacity profile within the CL results in a specific fingerprint in the high frequency range of the spectra where at homogeneous conditions a linear 45° branch would be expected. Furthermore, a well pronounced splitting of a high and low frequency loop can be provoked by introducing non-uniformly distributed CL properties. The simulation results show that electrochemical impedance spectroscopy is a valuable tool to investigate the inhomogeneity of the catalyst layer originated by the electrode preparation itself or by degradation processes

Modeling the liquid water transport in the gas diffusion layer for polymer electrolyte membrane fuel cells using a water path network

In order to model the liquid water transport in the porous materials used in polymer electrolyte membrane (PEM) fuel cells, the pore network models are often applied. The presented model is a novel approach to further develop these models towards a percolation model that is based on the fiber structure rather than the pore structure. The developed algorithm determines the stable liquid water paths in the gas diffusion layer (GDL) structure and the transitions from the paths to the subsequent paths. The obtained water path network represents the basis for the calculation of the percolation process with low calculation efforts. A good agreement with experimental capillary pressure-saturation curves and synchrotron liquid water visualization data from other literature sources is found. The oxygen diffusivity for the GDL with liquid water saturation at breakthrough reveals that the porosity is not a crucial factor for the limiting current density. An algorithm for condensation is included into the model, which shows that condensing water is redirecting the water path in the GDL, leading to an improved oxygen diffusion by a decreased breakthrough pressure and changed saturation distribution at breakthrough

Coupling of a continuum fuel cell model with a discrete liquid water percolation model

An iterative algorithm is developed to directly integrate a discrete liquid water percolation model into a 3D continuum fuel cell model. In the continuum model the thermodynamic processes, most relevant for the water management and fuel cell performance, are calculated. For the discrete liquid water distribution in the porous transport layer (PTL), a water path network model is used, calculating the discrete, injection pressure and condensation scenario dependent saturation distribution. The saturation in the PTL is compared to synchrotron visualization data and a good comparison is found. Using the model, the influence of PTFE content, the application of a microporous layer, PTL perforation and ionomer desorption rate on the water configuration and fuel cell performance are analyzed. The interfacial liquid water is identified as an important parameter for the liquid water transport and the oxygen diffusivity to the active areas