Porous composite electrodes transport properties

The complex phase diagrams and rich crystal chemistry of the transition metal-containing oxide systems, and great diversity in the defect chemistry and transport properties of mixed-conducting materials known in these systems, make it impossible to systematize all promising compositions in a brief survey. The primary attention here is therefore centered on the comparison of major families of the oxide mixed conductors used for dense ceramic membranes and porous electrodes of SOFCs and other high-temperature electrochemical devices. 

When the size of a material is reduced to the nanoscale, their physical and chemical properties are dramatically changed. The separated nanostructure of polymer composites is expected to bring important improvements for polymer electronics because the size reduction of materials increases the contact surface area and lowers the interfacial impedance between the electrode and the electrolyte, and decreases the transport pathways for both electrons and ions (Shi et al., 2015). In addition, the mechanical properties for strain accommodation as well as the flexibility will be improved. A variety of nanostructures of polymer composites have been developed including zero-dimensional nanoparticles, one-dimensional nanowires/rods/belts, two-dimensional nanosheets/plates, and three-dimensional porous frameworks/networks. 

In an effective properties model, the porous microstructures of the SOFC electrodes are treated as continua and microstructural properties such as porosity, tortuosity, grain size, and composition are used to calculate the effective transport and reaction parameters for the model. The microstmctural properties are determined by a number of methods, including fabrication data such as composition and mass fractions of the solid species, characteristic features extracted from micrographs such as particle sizes, pore size, and porosity, experimental measurements, and smaller meso- and nanoscale modeling. Effective transport and reaction parameters are calculated from the measured properties of the porous electrodes and used in the governing equations of the ceU-level model. For example, the effective diffusion coefficients of the porous electrodes are typically calculated from the diffusion coefficient of Eq. (26.4), and the porosity ( gas) and tortuosity I of the electrode ... 

Combination of the macrohomogeneous approach for porous electrodes with a statistical description of effective properties of random composite media rests upon concepts of percolation theory (Broadbent and Hammersley, 1957 Isichenko, 1992 Stauffer and Aharony, 1994). Involving these concepts significantly enhanced capabilities of CL models in view of a systematic optimization of thickness, composition, and porous structure (Eikerling and Komyshev, 1998 Eikerling et al., 2004). The resulting stmcture-based model correlates the performance of the CCL with volumetric amounts of Pt, C, ionomer, and pores. The basis for the percolation approach is that a catalyst particle can take part in reaction only if it is connected simultaneously to percolating clusters of carbon/Pt, electrolyte phase, and pore space. Initially, the electrolyte phase was assumed to consist of ionomer only. However, in order to properly describe local reaction conditions and reaction rate distributions, it is necessary to account for water-filled pores and ionomer-phase domains as media for proton transport. 

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