Determination of Temperature and Fuel Utilization Distributions in SOFC Stacks with EIS

Abstract

Efficient operation of solid oxide fuel cell (SOFC) stacks requires uniform temperature and fuel utilization distributions in the stack. Especially high fuel utilizations (FU) necessitate uniform fuel flow distributions to avoid anode redox of the cell with the highest local FU. A uniform temperature distribution helps to prolong the life-time of the stack. Knowledge about such distributions would aid in both stack design and in monitoring the “state-of-health” of stacks. This work presents a method to determine the fuel utilization and temperature distribution in SOFC stacks with electrochemical impedance spectroscopy (EIS). EIS is a powerful and non-destructive technique that enables an in-depth analysis of the stack in-operando. The equations for relating the measured impedance and fuel utilization and temperature are derived from common equations describing the SOFC stack. Thus, the relations are anchored in physical processes. The method is demonstrated with an ElringKlinger AG 10-cell stack at nominal operation conditions. The difference in fuel flow, i.e. fuel distribution, and temperature between individual cells was determined. The temperature distribution exhibits a stack effect, where the top and bottom-most cells are the coldest, and is in line with the cell voltages. The paper evaluates the method and its usability in SOFC research. The NELLHI (grant agreement no. 621227) and INNO-Sofc (grant agreement no. 671403) projects, which have received funding from the European Union’s Fuel Cells and Hydrogen Joint Technology Initiative, are acknowledged. Additionally, the STEP project, funded by the Finnish Funding Agency for Innovation TEKES and Projekträger Jülich (Germany), is acknowledged.