Fabrication of AAO-Based Four-Stack Thin Film Solid Oxide Fuel Cells for the Low-Temperature Operation Using Sputtering Method

Abstract

Anodic Aluminium Oxide (AAO) based four thin-film solid oxide fuel cell (TF-SOFC) stack was fabricated. The fuel cell stack was tested and analyzed at the low-temperature window. Since the low-temperature operation has many advantages, such as lowering the system cost and widening the material selection, there has been much effort to reduce conductive resistance, mainly at the electrolyte, which is the most problematic issue in low-temperature operation. One of the solutions is TF-SOFC deposited on the various substrates or supporting materials. AAO, a non-conductive and nanoporous substrate, has been widely used in fabricating TF-SOFC as a substrate because of its uniformly arranged pores and highly developed manufacturing process.However, AAO is a poor template for stacking TF-SOFCs because it has lower mechanical strength than metal, has no electrical conductivity, and has a different coefficient of thermal expansion from metal. In this study, therefore, planarly stacked TF-SOFC were fabricated to expand its limits of use as a template for the stacked fuel cell. Nickel-based anode, yttria-stabilized zirconia (YSZ) electrolyte with functional layers, and platinum cathode and current collector were sequentially deposited using sputtering and atomic layer deposition methods. All thin-film components of four cells were deposited on the AAO at the same time, making good connections between each electrode. By effectively setting the deposition process, unnecessary pores of AAO were blocked, and current collecting layers were stably formed in between the structure of the cells with a step difference.Then, the stacked TF-SOFC were electrochemically analyzed, including the current density-voltage-power density curve (i-V-P curve) and electrochemical impedance spectroscopy (EIS) being compared to a single cell. The microstructure of the fuel cell and its electrically connected structures were observed by scanning electron microscopy (SEM) and focused ion beam (FIB) cross-sectional SEM analyses. The cell’s overall voltage was comparable to four times that of a single cell, and the power density was obtained. We revealed the correlation between the electrochemical performance and the microstructure of individual cells and the whole stack. The full stack achieved electrochemical and mechanical stability by forming a complete fuel cell structure and enhanced the applicability of the planar cell stack on the AAO substrate.