Thin-Film SOFC Enabling Low-Temperature and Low-Hydrogen-Concentration Operation

Abstract

Solid oxide fuel cells (SOFCs), which have the highest power-generation efficiency among generators, are key in achieving a low-carbon society. To reduce the cost of an SOFC system including mounting cost, thereby, expand its application, we have been developing a thin-film SOFC suitable for low-temperature operations.Since the anode, solid-electrolyte, and cathode layers are formed by sputtered thin films, the internal resistance of the cell decreases; thus, the operating temperature can be lowered [1, 2]. Figure 1 shows a cross-sectional micrograph of the developed SOFC. In this study, nanoporous anodic aluminum oxide (AAO) was used as a support substrate [3, 4, 5]. The pore diameter was 55 nm and thickness was 100 μm. On the AAO substrate, a stack of dense (70 nm) and porous-platinum (80 nm) layers was deposited for the anode. Pores were formed in the dense platinum layer because the surface of the underlying AAO support substrate has 55-nm pores. A dense yttria-stabilized zirconia (YSZ) was used for the solid electrolyte layer (290 nm). The density of the YSZ layer is significant for inhibiting both gas and electron/hole leakage between the anode and cathode sides. Reactive sputtering by using a metal Zr84-Y16 target under a DC bias condition was key to deposit the dense YSZ layer on the porous-platinum anode layer. The interfacial layer (40 nm) of gadolinia-doped ceria (GDC) was deposited on the YSZ layer. The cathode layer (20 nm) of GDC-platinum composite material was then deposited on the interfacial layer. The characteristics of our SOFC was measured at 453°C, as shown in Fig. 2. The air was flowed to the cathode, and wet 3%-hydrogen of nitrogen base was flowed to the anode. An open circuit voltage of 1.049 V and output power density of 203 mW/cm2 were achieved using 3% hydrogen. At the cathode side, the polarization resistance was reduced by forming a gadolinium-doped ceria layer at the interface [5]. In addition, the GDC-platinum cathode was found to reduce the polarization resistance compared to the porous platinum cathode. At the anode side, the hydrogen diffused through the pores of the AAO substrate and reached the anode layer of the SOFC. The triple-phase boundaries formed among gas-, porous-anode-, and solid-electrolyte phases arguably enhanced the performance of the anode. The water vapor produced at the anode side diffused through the pores of the AAO and joined the exhaust gas. The power density at 453°C is expected to increase by using high-concentration hydrogen [6], which we plan to demonstrate for future study. Apart from operation in high-concentration hydrogen, our SOFC has another application. The experimental results indicate that our SOFC can generate high power by using the low-concentration hydrogen from hydrous bioethanol even at low temperatures.