Modelling the water distribution within a hydrophilic and hydrophobic 3D reconstructed cathode catalyst layer of a proton exchange membrane fuel cell

We reconstruct a section of the cathode catalyst layer (CCL) of a Gore PEMFC membrane electrode assembly three-dimensionally with nanometre scale resolution. Subsequently, we present a new modelling method to fill the pore space of this matrix stepwise with water, enabling the description of varying saturation conditions of the CCL. The method is based on a 3D pore size distribution and enables to differentiate between a hydrophilic and a hydrophobic CCL It utilizes a sequence to fill the pores according to their size, going from small to large (hydrophilic) or vice versa (hydrophobic), until a pre-defined value of water saturation is reached. We compare both cases by calculating an effective diffusivity for oxygen in nitrogen in all spatial directions. Both the hydrophilic and the hydrophobic case display a similar ability to transport oxygen up to approximately 50% water saturation of the pore space. At higher water saturation, we calculate larger diffusivity values for the hydrophobic case. Finally, we calculate the specific reaction surface area that is accessible from the gas diffusion layer via unfilled pores for all water saturation conditions. At 50% saturation, the hydrophobic case displays a twenty times larger reaction surface area than the hydrophilic case

FIB/SEM-based calculation of tortuosity in a porous LiCoO2 cathode for a Li-ion battery

We present a new method to quantify tortuosity in the porous, LiCoO2 cathode of a Li-ion battery. The starting point is a previously published 3D reconstruction from FIB/SEM images with three phases, the active material domain, carbon-binder domain and pore space. Based on this geometrical configuration, we compute effective diffusivities, from which we in turn derive tortuosity values for the pore space ranging between 5 and 11.6 for the three spatial directions. In a next step, we compare our approach to an imaging method that employs back-filling material. These methods do not differentiate between the carbon-binder domain and the pore space. Thus we remove the carbon-binder domain from our 3D reconstruction and add its volume to the pore space. As a result of this procedure, the tortuosity is greatly reduced to values between 1.5 and 1.9. Experiments suggest that both results for tortuosity are inaccurate and that the real values lie somewhere between these parameter sets. Hence, based on experimental data, we propose a nanoporous carbon-binder domain and derive intermediate tortuosity values between 4.2 and 6.1. These values are consistent with experimental values for similar Li-ion cathodes reported previously

Three Phase Multiscale Modeling of a LiCoO2 Cathode Combining the Advantages of FIB SEM Imaging and X Ray Tomography

LiCoO2 electrodes contain three phases, or domains, each having specific intended functions ion conducting pore space, lithium ion reacting active material, and electron conducting carbon binder domain CBD . Transport processes take place in all domains on different characteristic length scales from the micrometer scale in the active material grains through to the nanopores in the carbon binder phase. Consequently, more than one imaging approach must be utilized to obtain a hierarchical geometric representation of the electrode. An approach incorporating information from the micro and nanoscale to calculate 3D transport relevant properties in a large reconstructed active domain is presented. Advantages of focused ion beam scanning electron microscopy imaging and X ray tomography combined by a spatial stochastic model, validated with an artificially produced reference structure are used. This novel approach leads to significantly different transport relevant properties compared with previous tomographic approaches nanoporosity of the CBD leads to up to 42 additional contact area between active material and pore space and increases ionic conduction by a factor of up to 3.6. The results show that nanoporosity within the CBD cannot be neglected. Three Phase Multiscale Modeling of a LiCoO2 Cathode Combining the Advantages of FIB SEM Imaging and X Ray Tomography. Available from https www.researchgate.net publication 268579630Three PhaseMultiscaleModelingofaLiCoO2CathodeCombiningtheAdvantagesofFIBSEMImagingandX RayTomography [accessed Dec 11, 2015