Relationship of Micro-Structure Morphology to Impedance in Heterogeneous Functional Materials

Abstract

In the present work, we are concerned with heterogeneous material systems that have multiple distinct materials and void phases, and associated interfaces, for which the geometric scale and specific morphology at the micro/nano-level play an essential role in the global properties, functional behavior, and material system performance. Such materials are at the heart of revolutionary advances in devices that convert and store energy (e.g., batteries, fuel cells, solar cells, capacitors, and many electro-optical devices), and other membrane-based devices used in chemical and fuel processing, sequestration, and extraction. In recent years, advances in a variety of additive manufacturing methods and techniques have made it possible to design, control, and fabricate specific micro- or nano-structure to achieve prescriptive functional performance of the material systems and devices in which they appear. However, systematic multiphysics analysis methods properly set on field equations that represent the local details is not available, so that first-principles understandings and designs of those materials are not properly founded. Recently, the doe established an energy frontiers research center for physics based nanostructure design and fabrication of heterogeneous functional materials, called the HeteroFoaM center, to address this and related questions. The present paper presents some initial findings of part of that effort related specifically to morphology. For the present study, a finite element model (FEM) was developed Using COMSOL MULTIPHYSICS to predict impedance behavior when the field equations are set on the local features of regular geometric micro-structures The results for are compared with experimental data obtained from impedance spectroscopy of single SOFC fuel cell elements. For the model, equivalent idealized geometric structures were assumed corresponding to the YSZ material morphology of the fuel cell. Continuously aligned pore structures were represented by rectangular extrusions and regular porosity was represented by geometric shapes such as circles, rectangles and triangles. For experimental data, fragments of a button cell were used with silver paste contacts on the electrodes. For the model, the geometric microstructure was varied by using different shapes i.e. circles, rectangles and triangles while keeping the total material quantity constant, and by using equivalent areas for each of the geometric shapes. Impedance response for the frequency range from 1Hz to 1MHz was obtained for both the models and the experiments. Observations and interpretations of morphology effects are presented.