10.1021/acs.accounts.5b00555.s001
Zhenxing Feng
Zhenxing
Feng
Wesley T. Hong
Wesley T.
Hong
Dillon
D. Fong
Dillon
D.
Fong
Yueh-Lin Lee
Yueh-Lin
Lee
Yizhak Yacoby
Yizhak
Yacoby
Dane Morgan
Dane
Morgan
Yang Shao-Horn
Yang
Shao-Horn
Catalytic Activity and Stability of Oxides: The Role
of Near-Surface Atomic Structures and Compositions
American Chemical Society
2016
APXPS
surface oxygen exchange kinetics
ion mass spectrometry
surface Sr segregation
cation segregation
COBRA
cation migration
Sr enrichment
oxygen evolution reactions
MO
La
Bragg rod analysis
contrast Sr segregation
Sr redistribution
CoO
2016-05-05 17:26:53
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Catalytic_Activity_and_Stability_of_Oxides_The_Role_of_Near_Surface_Atomic_Structures_and_Compositions/3258211
ConspectusElectrocatalysts play an important role in catalyzing the kinetics
for oxygen reduction and oxygen evolution reactions for many air-based
energy storage and conversion devices, such as metal–air batteries
and fuel cells. Although noble metals have been extensively used as
electrocatalysts, their limited natural abundance and high costs have
motivated the search for more cost-effective catalysts. Oxides are
suitable candidates since they are relatively inexpensive and have
shown reasonably high activity for various electrochemical reactions.
However, a lack of fundamental understanding of the reaction mechanisms
has been a major hurdle toward improving electrocatalytic activity.
Detailed studies of the oxide surface atomic structure and chemistry
(e.g., cation migration) can provide much needed insights for the
design of highly efficient and stable oxide electrocatalysts.In this Account, we focus on recent advances in characterizing
strontium (Sr) cation segregation and enrichment near the surface
of Sr-substituted perovskite oxides under different operating conditions
(e.g., high temperature, applied potential), as well as their influence
on the surface oxygen exchange kinetics at elevated temperatures.
We contrast Sr segregation, which is associated with Sr redistribution
in the crystal lattice near the surface, with Sr enrichment, which
involves Sr redistribution via the formation of secondary phases.
The newly developed coherent Bragg rod analysis (COBRA) and energy-modulated
differential COBRA are uniquely powerful ways of providing information
about surface and interfacial cation segregation at the atomic scale
for these thin film electrocatalysts. <i>In situ</i> ambient
pressure X-ray photoelectron spectroscopy (APXPS) studies under electrochemical
operating conditions give additional insights into cation migration.
Direct COBRA and APXPS evidence for surface Sr segregation was found
for La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>CoO<sub>3−δ</sub> and (La<sub>1–<i>y</i></sub>Sr<sub><i>y</i></sub>)<sub>2</sub>CoO<sub>4±δ</sub>/La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>CoO<sub>3−δ</sub> oxide thin
films, and the physical origin of segregation is discussed in comparison
with (La<sub>1–<i>y</i></sub>Sr<sub><i>y</i></sub>)<sub>2</sub>CoO<sub>4±δ</sub>/La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>Co<sub>0.2</sub>Fe<sub>0.8</sub>O<sub>3−δ</sub>. Sr enrichment in many electrocatalysts,
such as La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>MO<sub>3−δ</sub> (M = Cr, Co, Mn, or Co and Fe)
and Sm<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>CoO<sub>3</sub>, has been probed using alternative techniques,
including low energy ion scattering, secondary ion mass spectrometry,
and X-ray fluorescence-based methods for depth-dependent, element-specific
analysis. We highlight a strong connection between cation segregation
and electrocatalytic properties, because cation segregation enhances
oxygen transport and surface oxygen exchange kinetics. On the other
hand, the formation of cation-enriched secondary phases can lead to
the blocking of active sites, inhibiting oxygen exchange. With help
from density functional theory, the links between cation migration,
catalyst stability, and catalytic activity are provided, and the oxygen <i>p</i>-band center relative to the Fermi level can be identified
as an activity descriptor. Based on these findings, we discuss strategies
to increase a catalyst’s activity while maintaining stability
to design efficient, cost-effective electrocatalysts.