Atomic layer deposition(ALD)-functionalized perovskite oxide surface to develop highly active and durable SOFC electrode

Abstract

Solid oxide fuel cells (SOFCs) operating at high temperatures above 700 °C are attracting much attention as potential next-generation electricity producing devices because they are environmentally friendly and energy efficient. However, because the operating temperature is very high, the production cost of the device rises. Maintenance is also difficult, and problems relating to long-term durability are becoming challenging. In order to solve these problems, substantial research is being conducted to try to lower the operating temperature. However, when the operating temperature is lowered, the electrode reactivity rapidly decreases and therefore energy conversion efficiency deteriorates. In addition, the irreversible degradation of performance due to chemical instability of the electrode surface is a key problem that needs to be solved for the efficient operation of SOFCs at low temperatures. In this study, atomic layer deposition (ALD) coating technology was introduced to SOFC electrodes to solve the problems presented above. Electrodes were developed that exhibit high performance/high durability even at low temperatures. ALD is a technology that is actively used in the semiconductor industry. It is possible to precisely control the thickness of the coating layer. It is a very suitable technology for coating SOFC electrodes with a complex 3D structure as it has excellent step coverage. When using ALD, in order to improve the durability and reactivity of LSC, the representative cathode material of SOFCs, surface protective agents of various materials are first coated on the SOFC electrode with precise thickness control. LSCs exposed to high temperatures are known to irreversibly degrade due to Sr separation. In order to suppress this, by coating a protective agent such as Al$_2$O$_3$ or HfO$_2$ on to the surface, durability is improved. A closer look at the literature further affirms the effectiveness of protective agents. Therefore, in this study we fabricated a model thin-film electrode using PLD and introduced a sophisticated coating technology called ALD to precisely control the thickness of the coating layer. We then analyzed the effect of the protective agent on the reactivity and durability of the electrode through ECR. As a result, the oxygen exchange kinetics of the uncoated LSC decreased by 10 times after 50 h. However, when coated with Al$_2$O$_3$ and HfO$_2$ of an appropriate thickness, it showed excellent performance without deterioration for 50 h. Interestingly, SEM, XPS, ToF-SIMS, and TEM analysis showed that the chemical composition of the surface was very different. These observations indicate that there are two different surface stabilization mechanisms reducing the driving force of Sr diffusion and scavenging Sr depending on the coating material. These findings suggest the design principle of a new strategy based on ALD to improve the perovskite surface durability. Secondly, a study was conducted to introduce a metal nanocatalyst for improving the activity of SOFCs. Because metal nanocatalysts have very poor thermal/chemical stability at high temperatures, they do not perform well under SOFC operating conditions. To realize the unique performance of metal nanocatalysts in SOFC electrodes, we used an oxide material coating to stabilize thermally unstable nanocatalysts. In fact, many excellent achievements have been reported by successfully stabilizing metal nanocatalysts using ALD in various chemical catalyst fields, but this thesis presents the first attempt to improve SOFC electrode performance using metal NCSs with ALD coating. To implement this, a porous electrode was fabricated using LSCM, a representative oxide anode, and then nanocatalysts were applied. Subsequently, various thicknesses of Al$_2$O$_3$ were coated using ALD, and the stability and electrochemical properties of the developed electrodes were evaluated. EIS, SEM, TEM, XPS, and FT-IR analyses confirmed that a 1.5 nm Al$_2$O$_3$ coating successfully improves the stability of the nanocatalysts, achieving a performance improvement of approximately 100 times with high durability. This observation implies that ALD can be a new method for developing high-performance/high-durability electrodes. Finally, instead of stabilizing the nanocatalysts by overcoating them, the unique properties of nanocatalysts at elevated temperature tried to be realized through the undercoating method in the SOFC electrode. I introduced an amorphous TiO$_2$ seed layer on a porous Pr$_{0.5}$Ba$_{0.5}$MnO$_3$ electrode using atomic layer deposition. It took less than 1 minute to fabricate well-dispersed Pt NCs, and achieved successful stabilization at 700 °C with a nearly dry methane atmosphere and without carbon deposition. Amorphous TiO$_2$ acts as a nucleation site for Pt NCs and increases dispersibility. As it crystallizes during the infiltration process, Ti and Pt form a strong metallic bond at the interface to improve the durability of Pt NCs. Moreover, button cells within the developed electrode exhibit a dramatic increase in power density of more than 100 % using wet (3 % H$_2$O) H$_2$ fuel, and 500 % using wet (3 % H$_2$O) CH$_4$ fuel, not showing degradation for 120 h at 700 °C. These unique observations provide a new direction for the design of heterogeneous metal-oxide catalysts for high-temperature applications. Thus, ALD is a technology that has excellent step coverage, thereby allowing us to coat SOFC electrodes with complex structures. This study has solved various problems of SOFCs and developed highly durable and active electrodes using ALD. Therefore, based on the results of this study conducted as part of this Ph.D. thesis, ALD can be a method that can be actively used in the SOFC industry and research. Based on the research results, I expect that the introduction of ALD in SOFC fields will contribute to the increase in SOFC application and further research.