Valence electronic structure on oxygen reduction reaction kinetics of Fe-based perovskite oxides for intermediate-temperature solid oxide fuel cells

Taeheun Lim^a, Kanghee Jo^a, Woo Jin Kim^a, Yong-Nam Kim^b, Heesoo Lee^a

^aPusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, Korea, Republic of

^bKorea Testing Laboratory

Proceedings of 24th International Conference on Solid State Ionics (SSI24)

Devices for a Net Zero World

London, United Kingdom, 2024 July 14th - 19th

Organizers: John Kilner and Stephen Skinner

Oral, Taeheun Lim, presentation 139
Publication date: 10th April 2024

Cobalt-based perovskite oxides (La_xSr_1−xCo_yFe_1−yO_3−δ, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ, etc.) show excellent electrochemical performance as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs), but it has limited application due to low thermal stability by the easy evaporation and reduction of cobalt [1]. For this reason, cobalt-free cathode materials are being studied, and Ba_0.5Sr_0.5FeO_3-δ (BSF) can have a thermal expansion coefficient (TEC) similar to that of ceria-based electrolyte through doping [2]. BSF has disordered oxygen vacancies and a three-dimensional oxygen diffusion path, which are advantageous for oxygen ion conduction [3].

In our previous study, the thermal stability of BSF was improved by doping Mn into the B-site to reduce the difference in TEC with electrolyte (samarium doped ceria, SDC), and oxygen reduction reaction (ORR) kinetics were slowed as a trade-off [4]. The ORR consists of five steps: adsorption of oxygen molecules, dissociation of oxygen molecules, formation of oxygen ions, lattice mixing of oxygen ions, and migration of oxygen ions to the electrode/electrolyte interface. The ORR kinetics of BSF can be improved by doping other elements such as Cu, Zn, and Sb into the B-site to enhance reactivity with oxygen and promote oxygen ion conduction [2,5].

In this study, we investigated the electrochemical properties and ORR kinetics of Ba_0.5Sr_0.5Fe_1-xCu_xO_3-δ (BSFCux) with the changes in oxygen vacancies and in electronic structure by doping Cu. The BSFCux were synthesized by a solid-state reaction, and all BSFCux was crystallized as cubic perovskite with a Pm-3m space group. In the Fourier transform data of the Fe-K edge obtained through X-ray absorption spectroscopy, the Fe-O peak intensity was decreased, indicating the increase in oxygen vacancy concentration of BSFCux with Cu doping. For the electronic structure of BSFCux, the average oxidation state of Fe was decreased from 3.584 (BSF) to 3.218 (BSFCu0.3), and the valence band maximum was calculated from valence band spectra, yielding −0.128 eV (BSF), −0.159 eV (BSFCu0.1), −0.185 eV (BSFCu0.2), −0.232 eV (BSFCu0.3). The BSFCux|SDC|BSFCux symmetrical cells were examined by electrochemical impedance spectroscopy (EIS), and BSFCu0.2 showed the smallest polarization resistance of 0.037 Ω cm² at 700 ℃. The distribution of relaxation times (DRT) analysis for BSFCux will be discussed with its oxygen vacancy and valence band.

References:

[1] Bu, Y. F., Zhong, Q., Chen, D. C., Chen, Y., Lai, S. Y., Wei, T., ... & Liu, M. (2016). A high-performance, cobalt-free cathode for intermediate-temperature solid oxide fuel cells with excellent CO2 tolerance. Journal of Power Sources, 319, 178-184.

[2] Ling, Y., Lu, X., Niu, J., Chen, H., Ding, Y., Ou, X., & Zhao, L. (2016). Antimony doped barium strontium ferrite perovskites as novel cathodes for intermediate-temperature solid oxide fuel cells. Journal of Alloys and Compounds, 666, 23-29.

[3] Wang, J., Saccoccio, M., Chen, D., Gao, Y., Chen, C., & Ciucci, F. (2015). The effect of A-site and B-site substitution on BaFeO3− δ: an investigation as a cathode material for intermediate-temperature solid oxide fuel cells. Journal of power sources, 297, 511-518.

[4] Lim, T., Yun, S. S., Jo, K., & Lee, H. (2023). Bonding State and Thermal Expansion Coefficient of Mn-Doped Ba0. 5Sr0. 5FeO3− δ Perovskite Oxides for IT-SOFCs. Nanomaterials, 14(1), 82.

[5] Wei, B., Lü, Z., Huang, X., Liu, M., Li, N., & Su, W. (2008). Synthesis, electrical and electrochemical properties of Ba0. 5Sr0. 5Zn0. 2Fe0. 8O3− δ perovskite oxide for IT-SOFC cathode. Journal of Power Sources, 176(1), 1-8.

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