Electrochemical impedance spectroscopy is potentially a powerful diagnostic tool for the investigation of the effects of aging in porous electrodes. A physically based three-electrode model was developed for a LixNi0.8Co0.15Al0.05O2 composite porous electrode with three porous separators and a reference electrode between a current collector and a plane electrode. Two effects of aging were modeled for this particular electrode chemistry, namely, a resistive corrosion layer on the current collector and a contact resistance between the electronic conductor and the active material of the porous electrode. The derivation of an analytical solution for the impedances between each pair of electrodes in this model yielded a computationally fast, versatile, and modular formulation. The solution was used to study the impact of selected components of the physical model on the impedance spectrum of the porous electrode for a physically relevant base case. Approximating the active material particles as spherical or flake-shaped particles, lognormally or Dirac distributed in size, revealed that the distribution has a negligible impact while the shape makes a noticeable difference. The main aging-related parameters were shown to have quite distinct effects on the impedance spectrum, which is essential for the regression of experimental data and the study of aging hypotheses.

High-power positive LixNi0.8Co0.15Al0.05O2 composite porous electrodes are known to be the main source of impedance increase in batteries based on GEN2 chemistry. The impedance of positive electrodes, both fresh and harvested from coin cells aged in an accelerated EUCAR hybrid electric vehicle lifetime matrix, was measured in a three-electrode setup and the results fitted with a physically based impedance model. A methodology for fitting the impedance data, including an optimization strategy incorporating a global genetic routine, was used to fit either fresh or aged positive electrodes simultaneously at different states of charge down to 0.5 mHz. The fresh electrodes had an exchange current density of approximately 1.0 A m(-2), a solid-phase diffusion coefficient of approximately 1.4 x 10(-1)5 m(2) s(-1), and a log-normal active particle size distribution with a mean radius of 0.25 mu m. Aged electrode impedance results were shown to be highly dependent on both the electrode state of charge and the pressure applied to the electrode surface. An aging scenario incorporating loss of active particles, coupled with an increase both in the local contact resistance between the active material and the conductive carbon and the resistance of a layer on the current collector, was shown to be adequate in describing the measured aged electrode impedance behavior.

Recent years have seen the appearance of numerous modelling studies of the polymer electrolyte fuel cell. However, in spite of observations in different studies that a model for a cell operating under single-phase conditions must include nonequilibrium water transport and must spatially resolve the porous electrodes in order to capture the behaviour of the cell correctly, there are only very few models in the literature that simultaneously do both. This paper, however, formulates such a model and considers a one-dimensional version of it in a parameter study. In future work, the model will be used to calibrate model parameters against experiments and study the operation of cells in higher dimensions.