Uncovering the electrochemical processes and understanding the causes of the degradation via EIS-DRT in large-scale solid oxide fuel cell

Abstract

Inducing changes in performance and observation of electrochemical processes is quite easy to do using ‘button cells’, which are the favorite objects in most research studies. Unfortunately, those cells may sometimes provide misleading outcomes or lead to promising results, applicable only on small scale, belittling their technological value. In this study, a large-scale SOFC with a 100 cm² cathode active surface area was subjected to 1700 h working time tests. After the stabilization period, a series of EIS measurements was taken under various changes in operational conditions. Variations in multiple parameters such as pO₂, pH₂, pH₂O, drawn current, or working temperature were introduced to provide an ‘electrochemical fingerprint’ of the large-scale cell. The impedance data was processed via equivalent circuit fitting and DRT analysis. It was possible to assign the electrochemical and transport processes to the corresponding peaks in the DRT spectra. A clear separation of the peaks originating from diffusion and charge transfer-related resistances was obtained. The single-cell tests allowed for the analysis of the actual state of the repeating units in the working 15-cell stack. The material causes of the changes in the performance of the cells were found to be mostly related to changes in the microstructure of the anode via Ni migration and unwanted diffusion of LSC-born elements such as Co segregation or the formation of the SrO-ZrO₂ phases. Furthermore, the degradation issues of the cells in the stack differ depending on the placement due to the formation of the hotspot in the central part.