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

Fundamental Understanding and Nano-Engineering of Oxygen Electrodes for Solid Oxide Fuel Cells Applications
Kim, Doyeub
DGIST Authors
Kim, Doyeub; Hong, Seung-Tae; Lee, kang Ta다
Lee, kang Ta다
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solid oxide fuel cell applications, perovskite structure, oxygen electrodes, performance degradation, oxygen transport kinetics, nano-engineering
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.
Table Of Contents
I. Introduction 1.1 Motivation 1 1.2 Objective 2 II. Background 2.1 Solid Oxide Fuel Cells (SOFCs) 5 2.1.1 Operating principles 5 2.1.2 Voltages losses 7 2.1.3 Current state-of-the-art materials 8 Fluorite electrolyte materials 8 Zirconium (Zr)-based oxides 8 Cerium (Ce)-based oxides 9 Bismuth (Bi)-based Oxides 9 Perovskite oxygen electrode materials 10 La1-xSrxCo1-yFeyO3-𝛿 (LSCF) 10 La1-xSrxMnO3-𝛿 (LSM) 11 2.2 The features of oxygen electrodes 12 2.3 Degradation of oxygen electrodes 14 2.3.1 Formation of secondary phases 14 2.3.2 Strontium (Sr) segregation 14 2.3.3 Microstructural change 15 2.4 Nano-structural engineering 15 2.4.1 Nano-scaled structure by infiltration 15 2.5 SOFC applications 16 2.5.1 Solid oxide electrolysis cells (SOECs) 16 III. Correlation of Time-Dependent Oxygen Surface Exchange Kinetics with Surface Chemistry of La0.6Sr0.4Co0.2Fe0.8O3-𝛿 Catalysts 3.1 Introduction 28 3.2 Experimental 30 3.2.1 Sample preparation 30 3.2.2 Electrochemical measurement 31 3.2.3 Surface chemistry characterization 32 3.3 Results and Discussion 33 3.3.1 Time-dependent oxygen transport kinetics 33 3.3.2 Surface-chemical composition analysis 33 3.3.3 Quantification of Sr segregation at the surface 34 3.3.4 Comparison between oxygen surface exchange kinetics and Sr-precipitate coverage 35 3.3.5 Localized chemical composition analysis at the clean surface 36 3.3.6 Comparison between the clean surface chemistry and oxygen surface exchange kinetics 38 3.4. Conclusions 40 IV. A Highly Active Bifunctional Perovskite La0.2Sr0.8Co0.8Fe0.2O3-𝛿 Catalysts as an Ox-ygen Electrodes for Reversible Solid Oxide Cells 4.1 Introduction 54 4.2 Experimental 56 4.2.1 Sample preparation 56 4.2.2 Cell fabrication 57 4.2.3 Characterization 58 Structural analysis 58 Bond valence sum maps 59 Computational details 59 Electrochemical measurement 60 4.3 Results and Discussion 61 4.3.1 Crystallographic analysis 61 4.3.2 High-temperature XRD analysis with 3D BVS calculation 62 4.3.3 Oxygen transport kinetics 63 4.3.4 First-principles study of oxygen vacancy formation 64 4.3.5 Oxygen reduction reaction 65 4.3.6 Cell performances 67 4.4 Conclusions 71 V. Enhancement of Catalytic Activity of LSM-ESB Oxygen Electrode via Surface Deco-rated Nano-dual Oxides for Intermediate Temperature Solid Oxide Fuel Cells 5.1 Introduction 101 5.2 Experimental 102 5.2.1 Materials synthesis 102 5.2.2 Characterization 104 5.3 Results and Discussion 104 5.3.1 Phase analysis 104 5.3.2 Microstructure analysis of oxygen electrodes 105 5.3.3 Oxygen reduction reactions 106 5.3.4 Microstructure analysis after long-term test 108 5.3.5 Electrochemical performances 108 5.4 Conclusions 109 VI. Nano-tailoring of LSM-HESB Oxygen Elector for High-Performance Solid Oxide Fuel Cells at Intermediate Temperature 6.1 Introduction 118 6.2 Experimental 119 6.2.1 Sample preparations 119 6.2.2 Characterization 121 6.3 Results and Discussion 121 6.3.1 Oxygen ion transfer characteristics of HESB 121 6.3.2 Features of LSM-HESB mixture oxygen electrode 122 6.3.3 Dependence of microstructure on different LSM concentration of HESB@LSM oxygen electrode 123 6.3.4 Dependence of ASRs on different LSM concentration of HESB@LSM oxygen electrode 123 6.3.5 Oxygen reduction redactions of HESB@LSM oxygen electrode 124 6.3.6 Microstructure of HESB@LSM single cell 124 6.3.7 Electrochemical performance of HESB@LSM single Cell 125 6.4 Conclusions 125 VII. Summary 134 VIII. References 136 Summary (in Korea)
Department of Energy Science and Engineering
Related Researcher
  • Author Hong, Seung-Tae Battery Materials Discovery Laboratory
  • Research Interests Magnesium, calcium, and zinc ion batteries; lithium all-solid-state batteries, New inorganic materials discovery; Solid state chemistry; Crystallography; Mg, Ca, Zn 이온 이차전지; 리튬 전고체전지; 신 무기재료 합성; 고체화학; 결정화학
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