| 研究実績の概要 |
Faster oxygen surface exchange on solid oxide fuel cell electrodes is sought to enable higher efficiency at lower temperature. Previous work suggested that electron transfer from the cathode to adsorbed oxygen was rate-limiting in the cathode Sr(Ti,Fe)O3-x; STF, so on that basis, one expected that increasing its electron concentration would increase the surface exchange rate. In this work, I investigated the role of Fermi level/ electron concentration in controlling surface exchange kinetics, by introducing a donor dopant (La) into this model cathode ("LSTF"). Achievements:
1) Fabricated bulk ceramic and thin film LSTF.
2) Developed a defect chemical model, quantifying the impact of donor doping on point defect concentrations, including electron concentration and Fermi level. The model was based on new electrical conductivity and optical absorption measurements of the samples, and prior thermogravimetric measurements.
3) Performed in situ measurements of the surface exchange rate of the thin film cathodes using AC-impedance spectroscopy and, for the first time on thin film STF, optical transmission relaxation (OTR). (Also extended the OTR technique to a new electrode, (Ce,Pr)O2-x.)
4) Concluded that donor doping only slightly raised the Fermi level, but also lowered the concentration of oxygen vacancies and holes. Since the electrode kinetics were not improved, the results showed that electron transfer is likely not rate limiting for these compositions, but rather oxygen vacancy and/or hole concentrations are key.
5) Studied also the chemical expansion behavior of STF cathodes.
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