Towards an Impedance-Based Analysis of SOFC-Stacks

Abstract

To understand the performance and durability limitations in SOFC stacks and systems, impedance spectroscopy is becoming increasingly important. However, in the analysis of entire stacks, gradients over the active cell area are averaged out, preventing access to local phenomena.To get better insight into the spatial distribution of performance losses in a stack layer, incremental cells and repeat units with an active electrode area of 1 cm² are investigated in this study.Impedance spectroscopy and the subsequent impedance data analysis by the distribution of relaxation times (DRT) is applied to deconvolute the polarization processes in the Ni/CGO fuel electrode and the LSM air electrode of the investigated electrolyte supported cell. To ensure an appropriate deconvolution of air and fuel electrode processes, next to full cells symmetrical anode and cathode cells are applied. In case of Ni/CGO fuel electrodes charge transfer and gas diffusion overlap in the spectrum and the assignment has to be done indirectly using inert gas mixtures [1].In Figure 1 impedance spectra and DRTs of fuel and air electrode obtained from symmetrical cells are compared with the full cell spectrum. This analysis enables a clear allocation of the processes in the single cell to Ni/CGO fuel electrode and LSM air electrode.Next to the cell testing in an inert, ideal environment namely an Al2O3 flow field with gold and nickel contact grids respectively, the study is expanded to stack like conditions. The additional losses due to contact resistances and gas supply limitations are studied similar to Kornely et al. [2] for single repeat units (single cell + contact layer + metallic interconnects). In this contribution a physio-chemical equivalent circuit model based on these measurements will be presented and the complex impedance behavior of single cells and repeat units will be discussed.