Abstract
Low-temperature solid oxide fuel cells (LT-SOFCs), which are SOFCs operating below <500C, face two major challenges in performance due to low operating temperatures: 1) reduced oxygen ion conductivity and 2) sluggish oxygen reduction reaction (ORR). The former is partially solved by introducing a thin-film electrolyte, but the latter is still a major issue. Therefore, the use of metal catalysts is essential to enhance ORR kinetics at the cathode, despite the issue of thermal degradation of such metal catalysts. Various methods for stabilizing metal catalysts at elevated temperatures have recently been reported, including exsolution, infiltration, and vapor-phase deposition. By adopting such a method, the thermal degradations of nanostructured metal catalysts can be effectively suppressed; in some cases, a simultaneous enhancement in the surface activity at the electrode surface has been reported. Among them, atomic layer deposition (ALD), a modified chemical vapor deposition method, can be an effective technique for the surface modification of nanostructured metal catalysts. ALD uses a sequential dose of the precursor and oxidant, and each chemical reacts via a self-limiting reaction on the substrate surface. High-quality films with uniform and conformal surface coverage even on nanostructured surfaces, can be obtained while precisely tailoring the thickness and composition. In this study, we applied an ALD YDC thin-film overcoating onto the Pt cathode of LT-SOFCs. The doping concentration was precisely controlled by varying the ALD cycle ratio between CeO2 and Y2O3 cycles. The effects of the doping concentration of ALD YDC on the ORR activity and thermal stability were analyzed. Electrochemical characterization showed that the ORR activity of the Pt cathode with 19 mol ALD YDC overcoating improved by a factor of five compared to that of the bare Pt cathode. The thermal stability of the low-to-medium-doped ALD YDC-coated Pt cathode was higher than that of the bare Pt cathode or the cathode with excess doping. Therefore, the ORR kinetics were improved by applying the ALD YDC films onto the Pt cathode, which may be due to the enhancement of individual steps at the Pt surface and Pt cathode/electrolyte interface. In addition, the thermal stability of the Pt cathode was enhanced by maintaining fine nanoporous structures even after operation at 450 C. As a result, the ORR kinetics and thermal stability of the cathode were simultaneously improved by ALD YDC overcoating at an optimal concentration. The materials and process solutions for manufacturing active and stable catalysts with metal–oxide interfaces shown in this study could contribute to the development of catalyst/electrode components in energy conversion systems, especially those operated at elevated temperatures.
