Publication date: 10th April 2024
In this work, thin film SOFC mixed conducting anode material La0.31Sr0.58Ti0.97Ni0.03O3-d (LSTN) microelectrodes were studied with electrochemical impedance spectroscopy (EIS) in three gas environments with different water-hydrogen concentrations (1.6%, 12% and 42% of H2O by volume in mixture with H2) at OCV and under anodic polarization up to + 0.4 V at 650 °C in two-electrode configuration, where counter electrode is highly active porous GDC-Pt electrode [1]. The measurements were performed to complement NAP-XPS measurements performed at the same oxygen partial pressures with thin film LSTN model electrodes.
Impedance spectra of circular LSTN microelectrodes (0.15 -1.8 mm in diameter) had highly depressed shape uncommon when compared with other works using mixed ionic electronic conducting SOFC microelectrodes [2]. Distinct microelectrode size-dependent feature was observed in the high to medium frequency range appearing because of relatively high chemical capacitance to electronic conductivity ratio of the LSTN, the larger the electrode was the larger the in-plane electronic conductivity limitation was. For that reason, low frequency region (1 – 0.01 Hz) of EIS spectra were used for fitting with Rs(Rp-CPE) circuit, where Rp described LSTN|gas surface electrochemical activity and Q of the constant phase element described bulk LSTN chemical capacitance. Rs was determined by the contact taken with microelectrode and was negligible compared to Rp. Polarization dependence showed a minimum of Rp at 0.3 V and increase in Rp upon further anodic polarization. The phenomena is explained by reoxidation of Ni exsolutes, causing sudden increase in polarization resistance and reversibly passivating the surface. Comparison between measurements in 1.6% H2O in the beginning and in the end of the measurement series (40 h between) showed that Q parameter returned to the same values (under polarization and at OCV), indicating stability of the bulk LSTN, while Rp increased by ~30%, showing surface passivation. Activation energy of Rp showed a decrease when going from 1.6% H2O to 12% H2O, which matched with a drop in Rp between these concentrations.
This work was supported by Estonian Research Council grant PRG551 and by the project „Increasing the knowledge intensity of Ida-Viru entrepreneurship“ (ÕÜF2) co-funded by the European Union, and by Estonian Ministry of Education and Research (TK210). Part of the research was conducted using the NAMUR+ core facility supported by the Estonian Research Council (TT 13).
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