Development of Advanced Electrodes for High Temperature Solid-state Energy Conversion Devices

Abstract

Solid oxide cells (SOCs) are powerful electrochemical devices for sufficient and simultaneous produc-tion and storage of eco-friendly energy. The SOCs can operate reversibly in a solid oxide fuel cell (SOFC) mode to generate electricity from chemical fuels, and a solid oxide electrolysis cell (SOEC) mode to convert electrical power into chemical fuels. An SOC is composed of high-density electrolyte, oxygen electrode, and fuel electrode. During operation, the oxygen electrode provides an active site for oxygen reduction or evolu-tion reaction (ORR / OER), while the fuel electrode oxidizes fuel (e.g. H2, CH4 and CO) or produces fuel (e.g., electrolysis of H2O and CO2). Both electrodes must provide a sufficient number of active sites for electrode reactions as well as suitable routes for rapidly transporting species (ions, electrons, and gas molecules, etc.) involved in the continuous electrode reaction. Thus, both electrodes are essential to improve SOC perfor-mance and stability. Currently, nickel/yttria-stabilized zirconia (Ni–YSZ) has been the most widely used fuel electrode material for SOCs due to its high catalytic activity, operational reliability, and acceptable cost. However, the long-term durability of Ni–YSZ is inadequate due to inherent redox instability and severe car-bon deposition during operation. To address this, alternatives to Ni-YSZ electrode materials, including various perovskite oxides, have been developed and investigated, many of which have shown significant catalytic activity and stability. Owing to their superior redox-stability, lanthanum titanium-based perovskite oxides have garnered significant interest as a promising reliable ceramic fuel electrode for SOFCs. Moreover, recent studies demonstrated that A-site deficiency of La-doped SrTiO3 perovskite could effectively and simultane-ously promote conductivity and catalytic activity. Among those, a La0.4Sr0.4TiO3−δ (LST44) material in the LaxSr1−3x/2TiO3−δ series is a promising candidate as a ceramic fuel electrode due to its redox tolerance and phase stability. Therefore, LST44-Ce0.9Gd0.1O1.95 (GDC) composite fuel electrodes were developed, and their electrochemical performance and stability were investigated. (Chapter III) To improve the electrochemical performance, La2NiO4+δ with a Ruddlesden-Popper structure (RP structure, space group I4/mmm or F4/mmm) has been intensively studied for its rapid surface exchange reaction and bulk ion diffusion. In its crystal structure, perovskite (ABO3) and rock salt (AO) layers are alternately stacked along the c-axis. Intersti-tial oxygen is preferentially localized at the pseudo-tetrahedron of La in the rock salt, and has fast ionic con-duction paths through the AO layers, promoting ionic and electronic conductivity. Furthermore, Gd and Mn co-doped layered perovskite fuel electrode materials can be rationally designed in order to promote the elec-trocatalytic activity, mixed conductivity, and high stability. Therefore, we developed novel SrGdNixMn1-xO4±δ (x = 0.2, 0.5, 0.8) materials with a Ruddlesden-Popper structure and used them as SOFC fuel electrodes. (Chapter IV) Meanwhile, practical applications for SOC systems have been limited due to chemical reactivity of ox-ygen electrode with adjacent SOC components, resulting in poor long-term stability of SOCs. As an alterna-tive, due to their high electrical conductivity and good chemical compatibility with SOFC components, some AB2O4-type spinel materials, which are widely used as protective coating layers on metallic interconnects, have been recently investigated as oxygen electrode materials for IT (intermediate temperature)-SOFCs. Therefore, we developed a novel spinel oxygen electrode material of Cu-doped Mn1.5Co1.5O4. The electro-chemical performance of an SOFC with the MCCO-(Sc2O3)0.1(CeO2)0.01(ZrO2)0.89 (ScSZ) composite oxygen electrode was evaluated to demonstrate the feasibility of using the MCCO materials as ORR catalysts for IT-SOFCs. (Chapter V) Moreover, due to sluggish ORR/OER activities at IT region, the development of oxygen electrodes with high catalytic activity and stability is essential for high performance SOCs. For example, Sr-doped LaMnO3 (LSM), which is commonly used for SOC oxygen electrode, lacks ionic conductivity and pos-sesses a highly thermal-activated nature for oxygen reduction/evolution reaction. To overcome this limita-tion, conventional oxygen ion-conducting phase, such as Gd-doped CeO2 (GDC) and Y2O3-stabilized ZrO2 (YSZ), have been combined with LSM, resulting in reduced resistance of SOC electrode. Owing to their excep-tionally high ionic conductivity, Bi2O3-based oxide ion conductors are promising materials for replacing con-ventional oxide ion conductors such as YSZ and GDC. Particularly, Dy and Y co-doped Bi2O3 (DYSB) with low total dopant concentration (12 mol%) required for cubic phase stabilization showed extremely high con-ductivity at IT region. In this study, the electrochemical properties of LSM combined with DYSB (LSM-DYSB) were investigated in SOFC, SOEC, and reversible SOC operating conditions to explore its advantage as oxygen electrode. Finally, we succeeded in achieving great performance and stability in SOC. (Chapter VI) This systematic study suggests a method for rational design of advanced alternative electrodes for applica-tion in SOC.