18 research outputs found
A fundamental investigation of process efficiencies in the Laser Engineered Net Shaping (LENS) solid freeform fabrication process
In operando Transmission Electron Microscopy Imaging of SEI Formation and Structure in Li-Ion and Li-Metal Batteries
Tuning Electrodeposition Parameters for Tailored Nanoparticle Size, Shape, and Morphology: An In Situ ec-STEM Investigation
Platinum and Palladium Overlayers Dramatically Enhance the Activity of Ruthenium Nanotubes for Alkaline Hydrogen Oxidation
Templated
vapor synthesis and thermal annealing were used to synthesize
unsupported metallic Ru nanotubes with Pt or Pd overlayers. By controlling
the elemental composition and thickness of these overlayers, we obtain
nanostructures with very high alkaline hydrogen oxidation activity.
Nanotubes with a nominal atomic composition of Ru<sub>0.90</sub>Pt<sub>0.10</sub> display a surface-specific activity (2.4 mA/cm<sup>2</sup>) that is 35 times greater than that of pure Ru nanotubes at a 50
mV overpotential and ∼2.5 times greater than that of pure Pt
nanotubes (0.98 mA/cm<sup>2</sup>). The surface-segregated structure
also confers dramatically increased Pt utilization efficiency. We
find a platinum-mass-specific activity of 1240 A/g<sub>Pt</sub> for
the optimized nanotube versus 280 A/g<sub>Pt</sub> for carbon-supported
Pt nanoparticles and 109 A/g<sub>Pt</sub> for monometallic Pt nanotubes.
We attribute the enhancement of both area- and platinum-mass-specific
activity to the atomic-scale homeomorphism of the nanotube form factor
with adlayer-modified polycrystals. In this case, subsurface ligand
and bifunctional effects previously observed on segregated, adlayer-modified
polycrystals are translated to nanoscale catalysts
Ruthenium-Alloy Electrocatalysts with Tunable Hydrogen Oxidation Kinetics in Alkaline Electrolyte
High-surface-area ruthenium-based
Ru<sub><i>x</i></sub>M<sub><i>y</i></sub> (M =
Pt or Pd) alloy catalysts supported
on carbon black were synthesized to investigate the hydrogen oxidation
reaction (HOR) in alkaline electrolytes. The exchange current density
for hydrogen oxidation on a Pt-rich Ru<sub>0.20</sub>Pt<sub>0.80</sub> catalyst is 1.42 mA/cm<sup>2</sup>, nearly 3 times that of Pt (0.490
mA/cm<sup>2</sup>). Furthermore, Ru<sub><i>x</i></sub>Pt<sub><i>y</i></sub> alloy surfaces in 0.1 M KOH yield a Tafel
slope of ∼30 mV/dec, in contrast with the ∼125 mV/dec
Tafel slope observed for supported Pt, signifying that hydrogen dissociative
adsorption is rate-limiting rather than charge-transfer processes.
Ru alloying with Pd does not result in modified kinetics. We attribute
these disparate results to the interplay of bifunctional and ligand
effects. The dependence of the rate-determining step on the choice
of alloy element allows for tuning catalyst activity and suggests
not only that a low-cost, alkaline anode catalyst is possible but
also that it is tantalizingly close to reality
3D Nanoprinting via laser-assisted electron beam induced deposition: growth kinetics, enhanced purity, and electrical resistivity
We investigate the growth, purity, grain structure/morphology, and electrical resistivity of 3D platinum nanowires synthesized via electron beam induced deposition with and without an in situ pulsed laser assist process which photothermally couples to the growing Pt–C deposits. Notably, we demonstrate: 1) higher platinum concentration and a coalescence of the otherwise Pt–C nanogranular material, 2) a slight enhancement in the deposit resolution and 3) a 100-fold improvement in the conductivity of suspended nanowires grown with the in situ photothermal assist process, while retaining a high degree of shape fidelity
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
Influence of Dioxygen on the Promotional Effect of Bi during Pt-Catalyzed Oxidation of 1,6-Hexanediol
A series of carbon-supported, Bi-promoted
Pt catalysts with various
Bi/Pt atomic ratios was prepared by selectively depositing Bi on Pt
nanoparticles. The catalysts were evaluated for 1,6-hexanediol oxidation
activity in aqueous solvent under different dioxygen pressures. The
rate of diol oxidation on the basis of Pt loading over a Bi-promoted
catalyst was 3 times faster than that of an unpromoted Pt catalyst
under 0.02 MPa of O<sub>2</sub>, whereas the unpromoted catalyst was
more active than the promoted catalyst under 1 MPa of O<sub>2</sub>. After liquid-phase catalyst pretreatment and 1,6-hexanediol oxidation,
migration of Bi on the carbon support was observed. The reaction order
in O<sub>2</sub> was 0 over Bi-promoted Pt/C in comparison to 0.75
over unpromoted Pt/C in the range of 0.02–0.2 MPa of O<sub>2</sub>. Under low O<sub>2</sub> pressure, rate measurements in D<sub>2</sub>O instead of H<sub>2</sub>O solvent revealed a moderate kinetic
isotope effect (rate<sub>H<sub>2</sub>O</sub>/rate<sub>D<sub>2</sub>O</sub>) on 1,6-hexanediol oxidation over Pt/C (KIE = 1.4), whereas
a negligible effect was observed on Bi-Pt/C (KIE = 0.9), indicating
that the promotional effect of Bi could be related to the formation
of surface hydroxyl groups from the reaction of dioxygen and water.
No significant change in product distribution or catalyst stability
was observed with Bi promotion, regardless of the dioxygen pressure