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.
A functionally graded BiErO (ESB)/YZrO (YSZ) bilayer electrolyte is successfully developed via a cost-effective screen printing process using nanoscale ESB powders on the tape-cast NiO-YSZ anode support. Because of the highly enhanced oxygen incorporation process at the cathode/electrolyte interface, a novel bilayer solid oxide fuel cell (SOFC) yields extremely high power density of ∼2.1 W cm at 700 °C, which is a 2.4 times increase compared to that of the YSZ single electrolyte SOFC.
Solid
oxide cells (SOCs) are mutually convertible energy devices
capable of generating electricity from chemical fuels including hydrogen
in the fuel cell mode and producing green hydrogen using electricity
from renewable but intermittent solar and wind resources in the electrolysis
cell mode. An effective approach to enhance the performance of SOCs
at reduced temperatures is by developing highly active oxygen electrodes
for both oxygen reduction and oxygen evolution reactions. Herein,
highly conductive Sm3+ and Nd3+ double-doped
ceria (Sm0.075Nd0.075Ce0.85O2−δ, SNDC) is utilized as an active component
for reversible SOC applications. We develop a novel La0.6Sr0.4Co0.2Fe0.8O3 −δ (LSCF)–SNDC composite oxygen electrode. Compared with the
conventional LSCF–Gd-doped ceria oxygen electrode, the LSCF–SNDC
exhibits ∼35% lower cathode polarization resistance (0.042
Ω cm2 at 750 °C) owing to rapid oxygen incorporation
and surface diffusion kinetics. Furthermore, the SOC with the LSCF–SNDC
oxygen electrode and the SNDC buffer layer yields a remarkable performance
in both the fuel cell (1.54 W cm–2 at 750 °C)
and electrolysis cell (1.37 A cm–2 at 750 °C)
modes because the incorporation of SNDC promotes the surface diffusion
kinetics at the oxygen electrode bulk and the activity of the triple
phase boundary at the interface. These findings suggest that the highly
conductive SNDC material effectively enhances both oxygen reduction
and oxygen evolution reactions, thus serving as a promising material
in reversible SOC applications at reduced temperatures.
Composite cathodes comprising nanoscale powders are expected to impart with high specific surface area and triple phase boundary (TPB) density, which will lead to better performance. However, uniformly mixing nanosized heterophase powders remains a challenge due to their high surface energy and thus ease with which they agglomerate into their individual phases during the mixing and sintering processes. In this study, we successfully synthesized La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF)−Gd 0.1 Ce 0.9 O 1.95 (GDC) composite cathode nanoscale powders via an in situ sol−gel process. High-angle annular dark field scanning transmission electron microscopy analysis of in situ prepared LSCF−GDC composite powders revealed that both the LSCF and GDC phases were uniformly distributed with a particle size of ∼90 nm without cation intermixing. The in situ LSCF−GDC cathode sintered on a GDC electrolyte showed a low polarization resistance of 0.044 Ω cm 2 at 750 °C. The active TPB density and the specific two phase (LSCF/pore) boundary area of the in situ LSCF− GDC cathode were quantified via a 3D reconstruction technique, resulting in 12.7 μm −2 and 2.9 μm −1 , respectively. These values are significantly higher as compared to reported values of other LSCF−GDC cathodes, demonstrating highly well-distributed LSCF and GDC at the nanoscale. A solid oxide fuel cell employing the in situ LSCF−GDC cathode yielded excellent power output of ∼1.2 W cm −2 at 750 °C and high stability up to 500 h.
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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