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.
We demonstrate quantum dot (QD) formation in three-dimensional Dirac semimetal CdAs nanowires using two electrostatically tuned p-n junctions with a gate and magnetic fields. The linear conductance measured as a function of gate voltage under high magnetic fields is strongly suppressed at the Dirac point close to zero conductance, showing strong conductance oscillations. Remarkably, in this regime, the CdAs nanowire device exhibits Coulomb diamond features, indicating that a clean single QD forms in the Dirac semimetal nanowire. Our results show that a p-type QD can be formed between two n-type leads underneath metal contacts in the nanowire by applying gate voltages under strong magnetic fields. Analysis of the quantum confinement in the gapless band structure confirms that p-n junctions formed between the p-type QD and two neighboring n-type leads under high magnetic fields behave as resistive tunnel barriers due to cyclotron motion, resulting in the suppression of Klein tunneling. The p-type QD with magnetic field-induced confinement shows a single hole filling. Our results will open up a route to quantum devices such as QDs or quantum point contacts based on Dirac and Weyl semimetals.
A perovskite La0.2Sr0.8Co0.8Fe0.2O3−δ catalyst exhibited remarkably high activities for the ORR and OER as a novel bifunctional oxygen electrode for reversible SOCs.
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