Development of metal-supported solid oxide fuel cell stack based on post-mortem analysis

Abstract

In recent years, efforts to increase the rate of using renewable energy sources are increasing to reduce CO2 emissions by fossil fuels. Solid oxide fuel cells (SOFCs) receive much attention as eco-friendly power converting system. Metal-supported solid oxide fuel cells (MS-SOFCs) are a type of SOFC and have advantage of potential application in transportation as well as stationary power generation. In order to utilize the SOFCs in real life, several unit cells must be stacked for available level of power output. In the SOFC stack, the stacked unit cells must be electrically connected, and the anode and cathode, and the interior and exterior of the stack must be completely sealed. Currently, the SOFC stack is in the initial stage of commercialization, but most stack suppliers and research institutes only disclose the electrochemical performance, durability and efficiency of the stack and not the core stacking technologies. In this study, core technologies for the development of metal-supported SOFC stack were developed. Metal-based SOFC was proposed by bonding a metal substrate and a conventional ceramic cell using a silver bonding layer. This sinter-joining process has the advantage that sufficient sintering of the cathode is possible at 1,100 °C by performing all cell fabrication processes in an oxidizing atmosphere, which can achieve low area specific resistance. A prototype 3-cells metal-based SOFC stack was successfully developed based on the large-area metal-based SOFC cells fabricated by a sinter-joining process. After evaluation of the stack performance, the causes of low electrochemical performance and degradation were analyzed through post-mortem analysis. Based on the results of post-mortem analysis, the study focused on retaining reliable sealing, and preventing oxidation of metallic interconnect and suppressing chromium volatilization by protective coating of the metallic interconnect. Through the computational analysis based on finite element method (FEM), the stack was designed to achieve stable sealing performance by retaining sufficient compressive stress on the surface of the sealing gasket, and bolt-spring system and hydraulic compression system were analyzed as the stack compression system. In order to maintain the compression and insulation of the stack, a hot box was designed based on various mechanical properties such as thermogravimetric properties and shrinkage behavior of sealing gaskets and ceramic fiber insulators. In addition, Mn-Co spinel oxide was selected as the protective coating layer material for the metallic interconnect. Coating methods and heat treatment conditions were selected to produce a protective coating layer with dense internal structure and high connectivity with the metallic interconnect. Finally, metal-supported SOFC stack was developed by applying the modified design based on the post-mortem analysis. The 1-cell stack with sputtering-based thin film metal-supported SOFC showed an open circuit voltage of 1.1 V and a maximum power density of 80 mW cm$^{-2}$ at 650 °C. The sealing rates of both the anode and the cathode sides were 100% at the initial stage and after 500 h, and cross-leak was not observed. The 5-cells stack with commercial anode-supported SOFC showed an open circuit voltage of 5.5 V and a maximum power density of 430 mW cm$^{-2}$ at 800 °C. As a result, a stack with reliable sealing and improved electrochemical performance was successfully developed based on the design modifications through the post-mortem analysis. Through this study, it is possible to develop a reliable SOFC stack along with a comprehensive understanding of stack design and development.