Analysis of polarization characteristics and optimal operating conditions for industrial-sized SOFC based on comparison with button cells

Abstract

Although button solid oxide fuel cells (SOFCs) have been extensively used in fundamental research, practical commercial applications are developed on industrial-sized SOFCs. As the active area and fuel utilization increase, some issues that are not considered in button cells become essential. Therefore, it is necessary to focus on the differences between button cells and industrial-sized cells, including their polarization characteristics and performance evaluation. In this study, the polarization curves of both types of cells under different hydrogen flow rates are tested. Both types of cells experience a sudden increase in total area-specific resistance (Rtot) under high current density. However, the effects of hydrogen flow rate differs between cell types. Based on the equivalent circuit model (ECM) fitting of electrochemical impedance spectroscopy (EIS) measured under different DC biases and the theoretical calculation of gas diffusion and gas conversion resistances, it is demonstrated that the rapid increase in Rtot in button cells is primarily attributed to gas diffusion through the anode. In contrast, non-uniform gas distribution along the flow channel direction results in a dominant role of gas conversion resistance on Rtot in industrial-sized cells, especially at hydrogen flow rates below 0.4 L min−1. Although electrical efficiency is often overlooked in button cells, it is a crucial performance evaluation index in industrial-sized cells. The impact of operating parameters on power density and electrical efficiency is analyzed, revealing a trade-off between the two. With the critical Nernst electromotive force of Ni oxidation as the voltage lower limit, the optimal operating conditions with high electrical efficiency and prevention of Ni oxidation under different flow rates are obtained. It is investigated that operation should be carried out at a low flow rate below 0.3 L min−1 to achieve high electrical efficiency of over 50%. The selected operating parameters will change when the cell performance or load demand changes. However, the analysis method proposed in this paper can still provide a reference for selecting the operating parameters of a SOFC system.