Oxygen Nonstoichiometry of Bulk and Thin Film of Mixed Ionic and Electronic Conductors
Authors: Miyashita Nanaha , Budiman Riyan Achmad , Yamaguchi Mina , Yashiro Keiji , Kawada Tatsuya 
Published: December 2024

One of the most important properties of mixed ionic and electronic conductor is oxygen non-stoichiometry which defines the oxygen reduction reaction (ORR) process in solid oxide fuel cell (SOFC) cathode. Typically, to understand the ORR of SOFC cathode, a thin film electrode is preferred to be used than a porous electrode due to the simpler microstructure and geometry. However, one of the most used SOFC cathodes, La0.6Sr0.4CoO3-δ (LSC64), has been reported to have different oxygen nonstoichiometry in thin films and bulk [1][2]. This has been hypothesized that the discrepancy might be due to the segregation of Sr in the thin film resulting in an inhomogeneous distribution of the composition [2]. The cause of the Sr segregation could be due to the Sr having a larger cation radius than La. Instead of Sr, Ca that has a cation radius closer to the La is expected to have higher stability than Sr that have attracts attention recently [3]. Thus, the oxygen nonstoichiometry of LSC64 and La0.6Ca0.4CoO3
-

δ
(LCC64) was studied to understand the discrepancy factor in bulk and thin film electrodes.
In this study, the oxygen nonstoichiometry of the LCC64 bulk material was measured by a high-temperature gravimetry (TG) (NETZSCH, STA449F1-K21Jupiter, Germany) and Coulometric titration (CT) as a function of temperature and oxygen partial pressure (p(O2). Meanwhile the oxygen nonstoichiometry for LSC64 bulk from reference [4] was used in this study. The thin film LSC64 and LCC64 electrodes were prepared by pulsed-laser deposition on the top of Ce0.9Gd0.1O1.95(GDC) substrate. The thin film electrodes were measured their electrochemical performance by electrochemical impedance spectroscopy as a function of temperature and p(O2). For the AC impedance method, an experimental system combining an impedance analyzer (Solartron, SI1255B) and a potentiogalvanostat (Solartron, SI1287) was used. The chemical capacitance from impedance measurement was used to determine the oxygen nonstoichiometry as reported by Kawada et al [1]. In addition, the compositions were analyzed by SEM-EDX (JEOL, JSM-7001F) and STEM-EDX (Jeol, JEM2100F).
As a result, the oxygen nonstoichiometry in the LSC64 thin film were not significantly different from those in the bulk at low p(O2) (10-3 – 10-4 bar) range for all measured temperature, but at high p(O2) range, a large discrepancy was observed as reported in our earlier paper [1]. Oxygen vacancy formation enthalpy determined for the thin film was higher than the reported value for bulk of LSC64[4]. SEM-EDX and STEM-EDX observations on the LSC64 thin film confirmed that the as-prepared thin film was homogeneously formed on both surface and inside the thin film while the film after EIS showed segregation of Sr on the surface and Co3O4 near the interface with the substrate. The compositional deviation in thin film could be one of the factors of the discrepancy in oxygen nonstoichiometry. In the other side, careful measurement of the bulk nonstoichiometry of LCC64 was performed in its stability range which was narrower than LSC64. It showed that the oxygen vacancy concentration (δ) in the bulk was slightly smaller than that of LSC64. What is noteworthy is that the chemical capacitance of LCC64 thin film at 773 K matched with that estimated from oxygen nonstoichiometry of LCC64 bulk. To confirm the effect of compositional change on the discrepancy of oxygen nonstoishiometry, further study is necessary to determine by EIS at different temperatures with careful observation of compositional change before and after the measurement. The factor that governs the defect equilibrium of bulk and thin film electrode will be discussed.

References

[1] T.Kawada et al., J. Electrochem. Soc.,149(7), E252-E259, (2002).
[2] D.Oi, Master Thesis
Tohoku University, (2019).
[3] J. Qian et al., Energy
Stor. Mat., 42, 470-476, (2021).
[4] M. Kuhn et al., J. Solid State Chem., 197, 38-45 , (2013).