(Invited) Effect of Anode Thickness on Thin-Film Solid Oxide Fuel Cells Deposited on Nanoporous Substrates

Abstract

In this study, low-temperature solid oxide fuel cells having different thin-film anode thicknesses were fabricated, and their characteristics were studied. Since the low-temperature solid oxide fuel cells (LT-SOFCs) have the advantages of a low-cost fuel cell system and a wide selection of components’ materials, many researchers have made efforts to lower the operating temperature range. Naturally, thin-film electrolyte technology has been improved to reduce the exponentially increasing ohmic resistance of oxygen conduction when the working temperature is lowered. Various substrates or even no substrate were used to fabricate thin-film solid oxide fuel cells (TF-SOFCs). However, free-standing TF-SOFCs have poor mechanical strength, and the active area was limited to tens of square micrometers. A nanoporous substrate such as Anodic aluminum oxide (AAO) has been a great platform on which TF-SOFCs were stably deposited and operated. AAO has well-arranged and straight pores from its bottom to the top but no electrical conductivity. Due to the resistance characteristics of TF-SOFCs based on AAO, the anode’s thickness and the nanostructure significantly influence the overall performance of the fuel cell. According to previous studies, the performance tends to increase as the thickness increases with well-maintained porosity in the thin-film anode. To confirm this tendency and further improve cell performance, AAO substrates with 250nm-size pores and relatively rough surfaces were used. On the AAO substrate, nickel gadolinium-doped-ceria (Ni-GDC) mixed ionic and electronic conductor (MIEC) was introduced as the anode material, which increased hydrogen oxidation reaction (HOR) reactivity at the anode. In addition, the glancing angle deposition (GLAD) of the sputtering method was applied to maintain porosity better than the conventional method. With this method, thickening the thin-film anode improved the current collecting performance while the pores were well connected to the anode-electrolyte interface. Following that, the yttria-stabilized zirconia (YSZ) electrolyte was densely deposited, and the porous deposited platinum cathode completed the entire cell. The fabricated cells were tested at 500C and electrochemically analyzed by i-V-P curves and the EIS method. Also, the anode’s nanostructures were revealed by various analyzing methods such as surface FESEM, FIB cross-sectional SEM, TEM-EDS, and 4-probe conductivity measurement. Each cell’s stable structure and the uniformity of some fuel cell components such as the electrolyte and the cathode were confirmed. We found that the electrochemical performance increases as the anode thickness increases while its porosity is well-maintained. About 550mW/cm2 was measured as the highest maximum power density, which is more than a 20% increased result than the other fabricated cells. Those investigations also enabled the anode nanostructure to be identified, such as the anode thin-film porosity, conductivity, and three-dimensional morphology. The correlation between the anode nanostructure and the fuel cell performance was discussed, and the optimized cell structure was found. This study enlarged the understanding of nanoporous substrate-based TF-SOFC manufacturing and their expandability.