Zirconia Doped Ceria Cathodic Interlayer By Atomic Layer Deposition for Low Temperature Solid Oxide Fuel Cell

Abstract

Solid oxide fuel cell (SOFC) has the advantages of high energy conversion efficiency, fuel flexibility, and environment friendliness. However, conventional SOFC also has limitations in applying to various fields due to the high operating temperature of 800-1000℃. Therefore, low temperature SOFC (LT-SOFC) (≤ 500 ℃), which can be operated at relatively low temperature, are being widely researched these days. Operation losses of LT-SOFCs, however, have to be minimized to maintain the performance even at low temperature; especially, at low temperature, the activation loss at cathode is the dominant loss due to sluggish oxygen reduction reaction (ORR). The adoption of cathode interlayer such as doped ceria has been researched as an effective method to dramatically lower the cathodic activation loss in LT-SOFCs. In this study, we used zirconia-ceria composite cathodic interlayer fabricated by atomic layer deposition (ALD) to improve the performance of SOFC at low temperature. Ceria-based materials with high oxygen ion conductivity and high oxygen surface exchange coefficient at relatively low temperatures have been widely used as cathodic interlayers for LT-SOFCs, but the relatively low chemical stability has been the practical issue. Zirconia doped ceria (ZDC), on the contrary, is known to have better catalytic properties than pure ceria with even improved chemical stability. We show that the performance of LT-SOFC can be significantly enhanced by using ZDC as cathodic interlayer. The ZDC interlayer was deposited on the cathode side surface of yttria stabilized zirconia (YSZ) electrolyte using ALD. ALD is beneficial in depositing ultra-thin film with the capability to precisely control the composition and the thickness of films. Each interlayer of 20 nm-in-thickness was deposited with different ratios of zirconia and ceria; the molar ratio of zirconia was varied at 0, 30, 50, 80, and 100 mol%, respectively. The composition of the interlayer was analyzed by X-ray Photoelectron Spectroscopy (XPS) and the electrochemical performance was measured at 450℃ by using current-voltage characterization and electrochemical impedance spectroscopy (EIS). The maximum power density of the cell with the ZDC interlayer improved by 50% compared to that of the reference cell without the interlayer, which is even higher those of the cells with zirconia-only or ceria-only interlayers. In addition, the activation resistance of the cell with ZDC was decreased by ~50% in comparison with the cell without interlayers as shown in EIS measurements.