30 research outputs found
3-D Tracking and Visualization of Hundreds of Pt-Co Fuel Cell Nanocatalysts During Electrochemical Aging
We present an electron tomography method that allows for the identification
of hundreds of electrocatalyst nanoparticles with one-to-one correspondence
before and after electrochemical aging. This method allows us to track, in
three-dimensions (3-D), the trajectories and morphologies of each Pt-Co
nanocatalyst on a fuel cell carbon support. The use of atomic-scale electron
energy loss spectroscopic imaging enables the correlation of performance
degradation of the catalyst with changes in particle/inter-particle
morphologies, particle-support interactions and the near-surface chemical
composition. We found that, aging of the catalysts under normal fuel cell
operating conditions (potential scans from +0.6 V to +1.0 V for 30,000 cycles)
gives rise to coarsening of the nanoparticles, mainly through coalescence,
which in turn leads to the loss of performance. The observed coalescence events
were found to be the result of nanoparticle migration on the carbon support
during potential cycling. This method provides detailed insights into how
nanocatalyst degradation occurs in proton exchange membrane fuel cells
(PEMFCs), and suggests that minimization of particle movement can potentially
slow down the coarsening of the particles, and the corresponding performance
degradation.Comment: Nano Letters, accepte
Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer {\alpha}-RuCl3
Layered {\alpha}-RuCl3 is a promising material to potentially realize the
long-sought Kitaev quantum spin liquid with fractionalized excitations. While
evidence of this exotic state has been reported under a modest in-plane
magnetic field, such behavior is largely inconsistent with theoretical
expectations of Kitaev phases emerging only in out-of-plane fields. These
predicted field-induced states have been mostly out of reach due to the strong
easy-plane anisotropy of bulk crystals, however. We use a combination of
tunneling spectroscopy, magnetotransport, electron diffraction, and ab initio
calculations to study the layer-dependent magnons, anisotropy, structure, and
exchange coupling in atomically thin samples. Due to structural distortions,
the sign of the average off-diagonal exchange changes in monolayer
{\alpha}-RuCl3, leading to a reversal of magnetic anisotropy to easy-axis. Our
work provides a new avenue to tune the magnetic interactions in {\alpha}-RuCl3
and allows theoretically predicted quantum spin liquid phases for out-of-plane
fields to be more experimentally accessible
Atomic-resolution spectroscopic imaging of ensembles of nanocatalyst particles across the life of a fuel cell
The thousandfold increase in data-collection speed enabled by
aberration-corrected optics allows us to overcome an electron microscopy
paradox - how to obtain atomic-resolution chemical structure in individual
nanoparticles, yet record a statistically significant sample from an
inhomogeneous population. This allowed us to map hundreds of Pt-Co
nanoparticles to show atomic-scale elemental distributions across different
stages of the catalyst aging in a proton-exchange-membrane fuel cell, and
relate Pt-shell thickness to treatment, particle size, surface orientation, and
ordering.Comment: 28 pages, 5 figures, accepted, nano letter
Identical Location Transmission Electron Microscopy Imaging of Site-Selective Pt Nanocatalysts: Electrochemical Activation and Surface Disordering
We have employed identical location transmission electron microscopy (IL-TEM) to study changes in the shape and morphology of faceted Pt nanoparticles as a result of electrochemical cycling; a procedure typically employed for activating platinum surfaces. We find that the shape and morphology of the as-prepared hexagonal nanoparticles are rapidly degraded as a result of potential cycling up to +1.3 V. As few as 25 potential cycles are sufficient to cause significant degradation, and after about 500–1000 cycles the particles are dramatically degraded. We also see clear evidence of particle migration during potential cycling. These finding suggest that great care must be exercised in the use and study of shaped Pt nanoparticles (and related systems) as electrocatlysts, especially for the oxygen reduction reaction where high positive potentials are typically employed.This work was carried out under financial support of MICINN (project no. CTQ2013-44083-P) and the energy materials center at Cornell (emc2) an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, under award no. DESC0001086. R.M.A.-A. thanks the funding received from MICINN (EEBB-I-13-06184) to carry out a predoctoral stay in a foreign R&D center. This work and made use of TEM facilities of the Cornell Center of Materials Research, an National Science Foundation Materials Research Science and Engineering Center, under award no. DMR-1120296
Identical Location Transmission Electron Microscopy Imaging of Site-Selective Pt Nanocatalysts: Electrochemical Activation and Surface Disordering
Networked and chiral nanocomposites from ABC triblock terpolymer coassembly with transition metal oxide nanoparticles
Nanoscale deformation mechanics reveal resilience in nacre of Pinna nobilis shell.
8 pagesInternational audienceThe combination of soft nanoscale organic components with inorganic nanograins hierarchically designed by natural organisms results in highly ductile structural materials that can withstand mechanical impact and exhibit high resilience on the macro- and nano-scale. Our investigation of nacre deformation reveals the underlying nanomechanics that govern the structural resilience and absorption of mechanical energy. Using high-resolution scanning/transmission electron microscopy (S/TEM) combined with in situ indentation, we observe nanoscale recovery of heavily deformed nacre that restores its mechanical strength on external stimuli up to 80% of its yield strength. Under compression, nacre undergoes deformation of nanograins and non-destructive locking across organic interfaces such that adjacent inorganic tablets structurally join. The locked tablets respond to strain as a continuous material, yet the organic boundaries between them still restrict crack propagation. Remarkably, the completely locked interface recovers its original morphology without any noticeable deformation after compressive contact stresses as large as 1.2 GPa