Oxygen reduction performance of F-doped La1?xSrxCo1?yFeyO3?δ solid oxide fuel cells cathode
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摘要: 固體氧化物燃料電池(SOFC)因其高效、清潔及便攜性等優點被認為是當前最具應用前景的新能源技術之一。傳統SOFC較高的工作溫度（>800 ℃）限制了其商業推廣，降低其工作溫度成為當前研究的熱點。鈣鈦礦陰極材料La1?xSrxCo1?yFeyO3?δ(LSCF)因具有較高的電子離子混合導電性而成為中溫SOFC陰極材料的較佳選擇，同時實驗證明F替代O位能有效提升SOFC穩定性。基于已有實驗報道，本文采用第一性原理計算了F摻雜對LSCF電子結構影響、氧氣分子在(100)表面吸附能的變化、陰極體內氧空位形成能及氧離子遷移活化能的影響。通過與未摻雜材料性能的比較，證明：適量F摻雜LSCF在有效提升陰極表面對氧氣分子吸附能力同時能進一步提高體內氧離子遷移效率，從而提升陰極氧化還原反應能力。Abstract: Solid oxide fuel cells (SOFCs), which are electrochemical devices that generate power with high efficiency, free of pollution, and nonregional restrictions, have attracted extensive attention. A traditional SOFC works at temperatures more than 800 °C, which introduces several severe problems or drawbacks, such as the high possibility of interfacial reaction between the cell components, easy densification of the electrode layer, possible crack formation owing to mismatch in the thermal expansion of cell components, and the requirement for a high-cost LaCrO3 ceramic as the interconnect material. Thus, reducing the operating temperature of SOFCs has become a consensus among researchers for the benefit of long-time operation. On the other hand, the operation of SOFCs at lower temperatures introduces several major issues, such as the increase in electrode resistivities and polarization losses of electrode reactions, particularly the oxygen reduction reaction in the cathode. Presently, perovskite-based oxides with mixed ion-electron conductivity (MIEC) are the most promising cathode materials for intermediate temperature SOFCs. Among the various mixed conducting oxides, cobalt-containing ones usually show excellent ionic conductivity and catalytic activity for oxygen reduction, and therefore, have received particular attention recently. La1?xSrxCo1?yFeyO3?δ(LSCF) is a candidate of SOFC cathodes working below 800 °C, considering its high oxygen reduction reaction activity together with its mixed ionic electronic conducting property. Meanwhile, many experimental results pointed out that doping an F anion into the perovskite cathode can improve its electrochemical performance and stability in addition to the conventional A- and B-site doping. To study the oxygen reduction reaction process of the F-doped perovskite cathode, the electronic structure, oxygen absorption on the (100) surface, the formation energy of oxygen vacancy, and activation energies for oxygen ion migration in the bulk F-doped LSCF were calculated based on the density functional theory. The results reveal that doping F in the LSCF can improve oxygen absorption and oxygen ion migration, further promoting the activity of the cathode.
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圖 1 F在鈣鈦礦體內（a）及(100)表面（b）摻雜后的晶胞結構.
Figure 1. Structures of bulk (a) and (100) surface (b) of the F-doped La0.75Sr0.25Co0.25Fe0.75O3
圖 2 F摻雜前（a）、后（b）鈣鈦礦LSCF (100)表面層的電子局域函數；（c）摻雜前O-2p及摻雜后F-2p態分波態密度
Figure 2. Electron localization function of (100) surfaces (a–b) and particle density of states of O-2p in the LSCF and doped F-2p state (c)
圖 3 氧氣分子吸附在LSCF（a）和LSCFF（b）表面Fe原子位的最優化結構
Figure 3. Optimized structures of the O2 absorbed on the Fe atoms in the LSCF (a) and LSCFF (b)
圖 4 （a）氧氣吸附在LSCFF（上）及LSCF（下）表面后主要表面原子的分波態密度；（b）O2吸附LSCFF體系表面電子局域函數
Figure 4. (a) Partial density of states of atoms on the (100) surfaces of the LSCFF (up) and LSCF (down) after oxygen absorption; (b) electron localization function of the LSCFF after oxygen absorption.
圖 5 LSCFF體系中F近鄰氧空位（vac1 和vac2）（a）和氧離子在空位間遷移示意圖（b）；氧離子在LSCF（c）和LSCFF（d）體系中遷移的活化能
Figure 5. Schematic illustration of the oxygen vacancies (vac1 and vac2) near the F atom (a) in the LSCFF and the related migration path (b); calculated activation energies for oxygen ion migration in the LSCF (c) and LSCFF (d)
表 1 F摻雜前后LSCF鈣鈦礦氧吸附性能及氧空位形成能
Table 1. Absorption energies of oxygen and formation energies of the oxygen cavity in the LSCF before and after F doping
Structure Eads/eV dFe-O1/nm dO1-O2/nm Evac1/eV Evac2/eV LSCF On Fe ?0.557 0.199 0.126(3) 2.262 2.327 On Co ?0.302 0.204 0.126(2) — — LSCFF On Fe ?0.559 0.195 0.1268 2.472 2.468 On Co ?0.326 0.204 0.1262 — — 
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