12 research outputs found
High-Activity, Durable Oxygen Reduction Electrocatalyst: Nanoscale Composite of Platinum−Tantalum Oxyphosphate on Vulcan Carbon
A new oxygen reduction electrocatalyst for proton-exchange membrane fuel cells (PEMFCs) is synthesized by dispersing nanoscale Pt on a nanoscale tantalum oxyphosphate phase on a Vulcan carbon support, designated as Pt/[TaOPO<sub>4</sub>/VC]. Electrocatalytic activity is measured by the thin-film rotating disk electrode methodology in 0.1 M HClO<sub>4</sub> electrolyte. The Pt/[TaOPO<sub>4</sub>/VC] electrocatalyst has a high mass-specific activity of 0.46 A mg<sup>−1</sup><sub>Pt</sub> compared to 0.20 A mg<sup>−1</sup><sub>Pt</sub> for a Pt/Vulcan carbon standard and has met the 2015 DOE goal of 0.44 A mg<sup>−1</sup><sub>Pt</sub>. This tantalum-containing electrocatalyst is twice as durable as the standard Pt/carbon in terms of its loss of Pt electrochemical surface area after aggressive electrochemical cycling
Nanoscale Imaging of Fundamental Li Battery Chemistry: Solid-Electrolyte Interphase Formation and Preferential Growth of Lithium Metal Nanoclusters
The performance characteristics of
Li-ion batteries are intrinsically linked to evolving nanoscale interfacial
electrochemical reactions. To probe the mechanisms of solid electrolyte
interphase (SEI) formation and to track Li nucleation and growth mechanisms
from a standard organic battery electrolyte (LiPF<sub>6</sub> in EC:DMC),
we used in situ electrochemical scanning transmission electron microscopy
(ec-S/TEM) to perform controlled electrochemical potential sweep measurements
while simultaneously imaging site-specific structures resulting from
electrochemical reactions. A combined quantitative electrochemical
measurement and STEM imaging approach is used to demonstrate that
chemically sensitive annular dark field STEM imaging can be used to
estimate the density of the evolving SEI and to identify Li-containing
phases formed in the liquid cell. We report that the SEI is approximately
twice as dense as the electrolyte as determined from imaging and electron
scattering theory. We also observe site-specific locations where Li
nucleates and grows on the surface and edge of the glassy carbon electrode.
Lastly, this report demonstrates the investigative power of quantitative
nanoscale imaging combined with electrochemical measurements for studying
fluid–solid interfaces and their evolving chemistries
Multimetallic Core/Interlayer/Shell Nanostructures as Advanced Electrocatalysts
The fine balance between activity
and durability is crucial for
the development of high performance electrocatalysts. The importance
of atomic structure and compositional gradients is a guiding principle
in exploiting the knowledge from well-defined materials in the design
of novel class of core–shell electrocatalysts comprising Ni
core, Au interlayer, and PtNi shell (Ni@Au@PtNi). This multimetallic
system is found to have the optimal balance of activity and durability
due to the synergy between the stabilizing effect of subsurface Au
and modified electronic structure of surface Pt through interaction
with subsurface Ni atoms. The electrocatalysts with Ni@Au@PtNi core-interlayer-shell
structure exhibit high intrinsic and mass activities as well as superior
durability for the oxygen reduction reaction with less than 10% activity
loss after 10 000 potential cycles between 0.6 and 1.1 V vs
the reversible hydrogen electrode
3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports
Understanding
the mechanisms associated with Pt/C electrocatalyst degradation in
proton exchange membrane fuel cell (PEMFC) cathodes is critical for
the future development of higher-performing materials; however, there
is a lack of information regarding Pt coarsening under PEMFC operating
conditions within the cathode catalyst layer. We report a direct and
quantitative 3D study of Pt dispersions on carbon supports (high surface
area carbon (HSAC), Vulcan XC-72, and graphitized carbon) with varied
surface areas, graphitic character, and Pt loadings ranging from 5
to 40 wt %. This is accomplished both before and after catalyst-cycling
accelerated stress tests (ASTs) through observations of the cathode
catalyst layer of membrane electrode assemblies. Electron tomography
results show Pt nanoparticle agglomeration occurs predominantly at
junctions and edges of aggregated graphitized carbon particles, leading
to poor Pt dispersion in the as-prepared catalysts and increased coalescence
during ASTs. Tomographic reconstructions of Pt/HSAC show much better
initial Pt dispersions, less agglomeration, and less coarsening during
ASTs in the cathode. However, a large loss of the electrochemically
active surface area (ECSA) is still observed and is attributed to
accelerated Pt dissolution and nanoparticle coalescence. Furthermore,
a strong correlation between Pt particle/agglomerate size and measured
ECSA is established and is proposed as a more useful metric than average
crystallite size in predicting degradation behavior across different
catalyst systems
Interfacial Stability of Li Metal–Solid Electrolyte Elucidated via in Situ Electron Microscopy
Despite their different
chemistries, novel energy-storage systems, e.g., Li–air, Li–S,
all-solid-state Li batteries, etc., face one critical challenge of
forming a conductive and stable interface between Li metal and a solid
electrolyte. An accurate understanding of the formation mechanism
and the exact structure and chemistry of the rarely existing benign
interfaces, such as the Li–cubic-Li<sub>7–3<i>x</i></sub>Al<sub><i>x</i></sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (c-LLZO) interface, is crucial for enabling the use
of Li metal anodes. Due to spatial confinement and structural and
chemical complications, current investigations are largely limited
to theoretical calculations. Here, through an in situ formation of
Li–c-LLZO interfaces inside an aberration-corrected scanning
transmission electron microscope, we successfully reveal the interfacial
chemical and structural progression. Upon contact with Li metal, the
LLZO surface is reduced, which is accompanied by the simultaneous
implantation of Li<sup>+</sup>, resulting in a tetragonal-like LLZO
interphase that stabilizes at an extremely small thickness of around
five unit cells. This interphase effectively prevented further interfacial
reactions without compromising the ionic conductivity. Although the
cubic-to-tetragonal transition is typically undesired during LLZO
synthesis, the similar structural change was found to be the likely
key to the observed benign interface. These insights provide a new
perspective for designing Li–solid electrolyte interfaces that
can enable the use of Li metal anodes in next-generation batteries
Formation of the Conducting Filament in TaO<sub><i>x</i></sub>‑Resistive Switching Devices by Thermal-Gradient-Induced Cation Accumulation
The distribution of tantalum and
oxygen ions in electroformed and/or switched TaO<sub><i>x</i></sub>-based resistive switching devices has been assessed by high-angle
annular dark-field microscopy, X-ray energy-dispersive spectroscopy,
and electron energy-loss spectroscopy. The experiments have been performed
in the plan-view geometry on the cross-bar devices producing elemental
distribution maps in the direction perpendicular to the electric field.
The maps revealed an accumulation of +20% Ta in the inner part of
the filament with a 3.5% Ta-depleted ring around it. The diameter
of the entire structure was approximately 100 nm. The distribution
of oxygen was uniform with changes, if any, below the detection limit
of 5%. We interpret the elemental segregation as due to diffusion
driven by the temperature gradient, which in turn is induced by the
spontaneous current constriction associated with the negative differential
resistance-type <i>I</i>–<i>V</i> characteristics
of the as-fabricated metal/oxide/metal structures. A finite-element
model was used to evaluate the distribution of temperature in the
devices and correlated with the elemental maps. In addition, a fine-scale
(∼5 nm) intensity contrast was observed within the filament
and interpreted as due phase separation of the functional oxide in
the two-phase composition region. Understanding the temperature-gradient-induced
phenomena is central to the engineering of oxide memory cells
Unraveling the Effects of Strontium Incorporation on Barite Growthî—¸In Situ and Ex Situ Observations Using Multiscale Chemical Imaging
Impurity ions influence
mineral growth rates through a variety
of kinetic and thermodynamic processes that also affect partitioning
of the impurity ion between the solid and solution. Here, the effect
of an impurity ion, strontium, on Barite (BaSO<sub>4</sub>) (001)
growth rates was studied using a combination of high-resolution in
situ microscopy with ex situ chemical imaging techniques. In the presence
of strontium, ⟨120⟩ steps roughened and bifurcated.
The overall Barite growth rate also decreased with increasing aqueous
strontium-to-barium ratio ([Sr]/[Ba]<sub><i>aq</i></sub>) < 1. Analysis of the reacted solids using chemical imaging techniques
indicated strontium incorporated uniformly across all step orientations
into the Barite growth hillock for [Sr]/[Ba]<sub><i>aq</i></sub> < 1. However, at [Sr]/[Ba]<sub><i>aq</i></sub> > 5, steps with an apparent [010] orientation were expressed
and
growth in the [010] step direction led to an increase in the overall
growth rate of the surface. Strontium became preferentially incorporated
into the [010] step direction, rather than being homogeneously distributed.
The [Sr]/[Ba]<sub><i>s</i></sub> in the newly grown solid
was found to correlate directly with that of solutions at [Sr]/[Ba]<sub><i>aq</i></sub> < 5, but not for higher [Sr]/[Ba]<sub><i>aq</i></sub>. Solid composition analyses indicate that
thermodynamic equilibrium was not achieved. However, kinetic transport
modeling successfully reproduces the shift in growth mechanism
Control of Architecture in Rhombic Dodecahedral Pt–Ni Nanoframe Electrocatalysts
Platinum-based
alloys are known to demonstrate advanced properties
in electrochemical reactions that are relevant for proton exchange
membrane fuel cells and electrolyzers. Further development of Pt alloy
electrocatalysts relies on the design of architectures with highly
active surfaces and optimized utilization of the expensive element,
Pt. Here, we show that the three-dimensional Pt anisotropy of Pt–Ni
rhombic dodecahedra can be tuned by controlling the ratio between
Pt and Ni precursors such that either a completely hollow nanoframe
or a new architecture, the excavated nanoframe, can be obtained. The
excavated nanoframe showed ∼10 times higher specific and ∼6
times higher mass activity for the oxygen reduction reaction than
Pt/C, and twice the mass activity of the hollow nanoframe. The high
activity is attributed to enhanced Ni content in the near-surface
region and the extended two-dimensional sheet structure within the
nanoframe that minimizes the number of buried Pt sites
Unraveling the Effects of Strontium Incorporation on Barite Growthî—¸In Situ and Ex Situ Observations Using Multiscale Chemical Imaging
Impurity ions influence
mineral growth rates through a variety
of kinetic and thermodynamic processes that also affect partitioning
of the impurity ion between the solid and solution. Here, the effect
of an impurity ion, strontium, on Barite (BaSO<sub>4</sub>) (001)
growth rates was studied using a combination of high-resolution in
situ microscopy with ex situ chemical imaging techniques. In the presence
of strontium, ⟨120⟩ steps roughened and bifurcated.
The overall Barite growth rate also decreased with increasing aqueous
strontium-to-barium ratio ([Sr]/[Ba]<sub><i>aq</i></sub>) < 1. Analysis of the reacted solids using chemical imaging techniques
indicated strontium incorporated uniformly across all step orientations
into the Barite growth hillock for [Sr]/[Ba]<sub><i>aq</i></sub> < 1. However, at [Sr]/[Ba]<sub><i>aq</i></sub> > 5, steps with an apparent [010] orientation were expressed
and
growth in the [010] step direction led to an increase in the overall
growth rate of the surface. Strontium became preferentially incorporated
into the [010] step direction, rather than being homogeneously distributed.
The [Sr]/[Ba]<sub><i>s</i></sub> in the newly grown solid
was found to correlate directly with that of solutions at [Sr]/[Ba]<sub><i>aq</i></sub> < 5, but not for higher [Sr]/[Ba]<sub><i>aq</i></sub>. Solid composition analyses indicate that
thermodynamic equilibrium was not achieved. However, kinetic transport
modeling successfully reproduces the shift in growth mechanism
Rational Development of Ternary Alloy Electrocatalysts
Improving the efficiency of electrocatalytic reduction
of oxygen
represents one of the main challenges for the development of renewable
energy technologies. Here, we report the systematic evaluation of
Pt-ternary alloys (Pt<sub>3</sub>(MN)<sub>1</sub> with M, N = Fe,
Co, or Ni) as electrocatalysts for the oxygen reduction reaction (ORR).
We first studied the ternary systems on extended surfaces of polycrystalline
thin films to establish the trend of electrocatalytic activities and
then applied this knowledge to synthesize ternary alloy nanocatalysts
by a solvothermal approach. This study demonstrates that the ternary
alloy catalysts can be compelling systems for further advancement
of ORR electrocatalysis, reaching higher catalytic activities than
bimetallic Pt alloys and improvement factors of up to 4 versus monometallic
Pt