Fundamental Understanding and Nano-Engineering of Oxygen Electrodes for Solid Oxide Fuel Cells Applications

Abstract

In recent years, the hydrogen has played a unique role in the essential transition from fossil fuels to a renewable energy because of its high gravimetric energy density, and environment-friendly nature. One of the most promising clean energy conversion devices for hydrogen utilization is solid oxide fuel cells (SOFCs), which can directly convert chemical fuel (e.g., hydrogen) to electrical energy with high efficiency and fuel flexibility (if the reactions of SOFCs are operated in reverse, hydrogen can be produced from electricity in electrolysis mode as a storage device, solid oxide electrolysis cells (SOECs)). Such a high operating tempera-ture (> 800 °C) of SOFCs is a key barrier to its wide commercialization because of slow startup, and perfor-mance degradation with mechanical and chemical stabilities. However, at reduced temperatures, the sluggish oxygen reduction reactions (ORRs) of the oxygen electrode critically afftect the cell performance. This remains the greatest obstacle for the development of high-performance and durable SOFCs. Therefore, numerous studies have focused on the development of a highly active and durable oxygen electrode for SOFC applications in the intermediated temperature (IT) regime. Perovskite-based mixed ionic and electronic conductors (MIECs) such as La0.6Sr0.4Co0.2Fe0.8O3-𝛿 (LSCF6428) and La0.8Sr0.2MnO3-𝛿 (LSM) are the state-of-art materials for oxygen electrode. In general, LSCF6428 shows a relatively high conductivity and good electrochemical performance at reduced operating temperature, but suffers from chemical instability over the long-term operation. On the other hand, LSM elec-trodes exhibit good chemical compatibility with conventional electrolyte materials such as yttria-stabilized zirconium (YSZ), gadolinium-doped cerium oxides (GDC), and bismuth-oxides. However, at reduced temperatures, LSM displays a significant drop in performance for ORR activities due to its poor ionic conductivity. Therefore, more work needs to be studied to understand degradation behavior and to enhance the performance and durability of oxygen electrodes for SOFC applications in the IT regime. The formation of Sr segregation at the surface of LSCF6428 is detrimental to the electrochemical performance and durability. However, a quantitative correlation between the degradation of the oxygen surface exchange kinetics and formation of Sr precipitation at the LSCF6428 surface is not yet clearly understood. Herein, the in-situ monitored surface exchange coefficient (kchem) was found to significantly drop by 86% over the 800 h. The estimated coverage of Sr segregation on the LSCF surface was observed to be 15% even after 800 h of aging time, a significant deviation from the kchem degradation rate (∼86%). To further investigate surface chemistry, the clean surface area, which is believed to be electrochemically active, was further analyzed on the nanoscale. The quantified results showed that the Sr elemental fraction of the A-site at the outermost surface of the LSCF6428 samples decreased from 4.0 at 0 h to 0.27 at 800 h of annealing. Interestingly, the time-dependent relationship between kchem degradation and surface Sr fractional changes were highly analogous. Thus, our results suggest that the Sr deficiency on the clean surface region has a greater impact on the degradation process than the electrochemical activity passivation by SrOx precipitation, which has been shown to be a major degradation mechanism of LSCF6428 performance. To improve the stability and performance of La1-xSrxCo1-yFeyO3-𝛿 (LSCF) electrodes, we investigated a new composition of La0.2Sr0.8Co0.8Fe0.2O3-𝛿 (LSCF2882) material as a novel oxygen electrode for reversible solid oxide cells (SOCs) in the IT regime. Unlike the widely used LSCF6428 with a rhombohedrally distort-ed perovskite structure, LSCF2882 possesses a simple cubic perovskite structure with a symmetric BO6 octahedron network. This suggests the possible existence of additional interstitial oxygen transport pathways. The kchem and Dchem values of LSCF2882 as determined by electrical conductivity relaxation (ECR) method were consistently higher by > 2 and 20 times, respectively, compared to those of LSCF6428 at 700 °C, respectively. This result is further supported by a 43% reduction in oxygen vacancy formation energy of LSCF2882 using density functional theory (DFT) calculations. The reversible SOCs with LSCF2882 oxygen electrodes significantly outperforemd in both fuel cell (2.55 W/cm2) and electrolysis modes (2.09 A/cm2 at 1.3 V) at 800 °C, with excellent reversible-cyclic stability. Our findings strongly suggest that the LSCF2882 is a promising candidate as a bifunctional oxygen electrode for high performance reversible SOCs at intermediate temperatures. For the composite oxygen electrodes, the utilization of stabilized-bismuth oxides as the ionic conducting phase is to improve the performance of LSM-based oxygen electrodes at reduced operating temperatures. An approach of using the composite materials to enhace the ORR activity and durability of LSM-ESB, instead of single-phase LSCF as an oxygen electrode, was further investigated using a surface decorated with nano-oxides. The decorated nano-cobalt oxides were able to decrease the polarization resistance by 50% (0.09 Ω-cm2 at 650 °C) compared to bare LSM-ESB (0.19 Ω-cm2 at 650 °C), but had a poor stability for 200 h of operation at 650 °C with particle agglomerations (0.09 Ω-cm2 at 0 h vs. 1.27 Ω-cm2 at 200 h). In contrast, the surface decorated lanthanum-cobalt oxides of LSM-ESB (La-Co/LSM-ESB) increased ORR activity by 40% (0.12 Ω-cm2 at 650 °C) and superior stability for 700 h at 650 °C. The La-Co/LSM-ESB cell consistently yielded high performance (1.65 W/cm2 at 650 °C) without any marked degradation during 100 h of operation at 650 °C. To further improve the performance of the oxygen electrode for SOFC applications at reduced temperatures, the nano-tailoring of the oxygen electrode is designed consisting of LSM and the Hf-doped ESB (HESB) which has superior ionic conductivity (0.14 S/cm at 600 °C) with high stability for 1200 operation at 600 °C. The LSM-HESB mixture exhibited high stability for 200 h at 600 °C without observable degradation under typical SOFC operation conditions. Furthermore, HESB@LSM manufactured nano-LSM infiltrated into porous scaffolds of HESB was fabricated. The HESB@LSM oxygen electrode showed a significantly 70% reduction in polarization resistances (0.02 Ω-cm2 for HESB@LSM vs. 0.07 Ω-cm2, for LSM-HESB, respectively at 700 °C). The HESB@LSM cell yielded 2.78 W/cm2 of maximum power density (MPD). This electrochemical performance is superior to any SOFC cell configurations using YSZ-electrolyte to date. Our results demonstrate that the nano-engineered HESB@LSM material is a promising material with high-performance and durable oxygen electrocatalyst for SOFC applications at intermediate temperatures. In this dissertation, the basic properties, degradation mechanism, and nano-engineering of the oxygen electrode are investigated to assist in the development of high-performance SOFCs at reduced operating temperatures and to provide new insights into designing oxygen electrodes for SOFC applications.