The Sr segregation
at the surface of a perovskite La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) oxygen
electrode is detrimental to the electrochemical performance and durability
of energy conversion devices such as solid oxide fuel cells. However,
a quantitative correlation of degradation of the oxygen surface exchange
kinetics with Sr precipitation formation at the LSCF surface is not
clearly understood yet. Herein, the correlation of the time-dependent
degradation mechanisms of the LSCF catalysts with respect to Sr segregation
phenomenon at the surface were investigated at 800 °C for a prolonged
annealing time (∼800 h) by combining in situ electrochemical
measurements, and ex situ chemical and structural analyses at the
multiscale. The in situ monitored surface exchange coefficient (k
chem) was found to drastically drop by ∼86%
over the 800 h, and it was accompanied by the formation of Sr-containing
secondary phases on the bulk LSCF surface, as expected. However, the
estimated coverage of Sr segregation on the LSCF surface was only
∼15%, even after 800 h of aging time, showing significant deviation
from the k
chem degradation rate (∼86%).
The surface chemistry evolution at the clean surface area, which is
believed to be electrochemically active, was further analyzed on the
nanoscale. The quantified results showed that the Sr elemental fraction
of the A-site at the outermost surface of the LSCF samples was becoming
deficient from ∼4.0 at 0 h to ∼0.27 at 800 h annealing.
Interestingly, the time-dependent behavioral tendencies between k
chem degradation and surface Sr fractional changes
were highly analogous. Thus, our results suggest that this Sr deficiency
at the clean surface region more dominantly impacts the degradation
process rather than an electrochemical activity passivation by the
SrO
x
precipitates, which has been shown
to be a major degradation mechanism of LSCF performance.
Highly conductive Dy and Y co-doped bismuth oxides combined with La0.8Sr0.2MnO3−δ significantly enhanced the ORR and OER as oxygen electrodes for reversible SOCs.
Fast oxygen-ion conductors for use as electrolyte materials have been sought for energy conversion and storage. Bi2O3-based ionic conductors that exhibit the highest known oxygen-ion conductivities have received attention for use in next-generation solid electrolytes. However, at intermediate temperatures below ~600 °C, their conductivities degrade rapidly owing to a cubic-to-rhombohedral phase transformation. Here, we demonstrate that physical manipulation of the grain structure can be used to preserve the superior ionic conductivity of Bi2O3. To investigate the effects of microstructural control on stability, epitaxial and nanopolycrystalline model films of Er0.25Bi0.75O1.5 were fabricated by pulsed laser deposition. Interestingly, in situ impedance and ex situ XRD analyses showed that the grain boundary-free epitaxial film significantly improved the stability of the cubic phase, while severe degradation was observed in the conductivity of its polycrystalline counterpart. Consistently, the cation interdiffusion coefficient measured by the Boltzmann–Matano method was much lower for the epitaxial thin film compared to the polycrystalline thin film. Furthermore, first-principles calculations revealed that the presence of grain boundaries triggered the structural resemblance between cubic and rhombohedral phases, as evidenced by radial distribution functions. Additionally, phase transition energetics predicted that the thermodynamic stability of the cubic phase with respect to the rhombohedral counterpart is reduced near grain boundaries. Thus, these findings provide novel insights into the development of highly durable superionic conductors via microstructural engineering.
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