128 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
Getter sputtering system for high-throughput fabrication of composition spreads
We describe a sputtering system that can deposit composition spreads in an effectively UHV environment but which does not require the high-throughput paradigm to be compromised by a long pump down each time a target is changed. The system deploys four magnetron sputter guns in a cryoshroud (getter sputtering) which allows elements such as Ti and Zr to be deposited with minimal contamination by oxygen or other reactive background gases. The system also relies on custom substrate heaters to give rapid heating and cool down. The effectiveness of the gettering technique is evaluated, and example results obtained for catalytic activity of a pseudoternary composition spread are presented
Getter sputtering system for high-throughput fabrication of composition spreads
We describe a sputtering system that can deposit composition spreads in an effectively UHV environment but which does not require the high-throughput paradigm to be compromised by a long pump down each time a target is changed. The system deploys four magnetron sputter guns in a cryoshroud (getter sputtering) which allows elements such as Ti and Zr to be deposited with minimal contamination by oxygen or other reactive background gases. The system also relies on custom substrate heaters to give rapid heating and cool down. The effectiveness of the gettering technique is evaluated, and example results obtained for catalytic activity of a pseudoternary composition spread are presented
The Oxygen Reduction Pathway for Spinel Metal Oxides in Alkaline Media: An Experimentally Supported Ab Initio Study
Precious-metal-free spinel oxide electrocatalysts are promising candidates
for catalyzing the oxygen reduction reaction (ORR) in alkaline fuel cells. In
this theory-driven study, we use joint density-functional theory in tandem with
supporting electrochemical measurements to identify a novel theoretical pathway
for the ORR on cubic Co3O4 nanoparticle electrocatalysts. This pathway aligns
more closely with experimental results than previous models. The new pathway
employs the cracked adsorbates *(OH)(O) and *(OH)(OH), which, through hydrogen
bonding, induce spectator surface *H. This results in an onset potential
closely matching experimental values, in stark contrast to the traditional ORR
pathway, which keeps adsorbates intact and overestimates the onset potential by
0.7 V. Finally, we introduce electrochemical strain spectroscopy (ESS), a
groundbreaking strain analysis technique. ESS combines ab initio calculations
with experimental measurements to validate proposed reaction pathways and
pinpoint rate-limiting steps
Transistor behavior via Au clusters etched from electrodes in an acidic gating solution: metal nanoparticles mimicking conducting polymers
We report that the electrical conductance between closely-spaced gold
electrodes in acid solution can be turned from off [insulating; I] to on
[conducting; C] to off again by monotonically sweeping a gate voltage applied
to the solution. We propose that this ICI transistor action is due to an
electrochemical process dependent on nanoparticles etched from the surface of
the gold electrodes. These measurements mimic closely the characteristics of
nanoscale acid-gated polyaniline transistors, so that researchers should guard
against misinterpreting this effect in future molecular-electronics
experiments.Comment: 17 pages, 4 figure
Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes
The observed water oxidation activity of the compound class Co4O4(OAc)4(PyâX)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis
Pt-Richcore/Sn-Richsubsurface/Ptskin Nanocubes As Highly Active and Stable Electrocatalysts for the Ethanol Oxidation Reaction
Direct ethanol fuel cells are one of the most promising electrochemical energy conversion devices for portable, mobile and stationary power applications. However, more efficient and stable and less expensive electrocatalysts are still required. Interestingly, the electrochemical performance of the electrocatalysts toward the ethanol oxidation reaction can be remarkably enhanced by exploiting the benefits of structural and compositional sensitivity and control. Here, we describe the synthesis, characterization, and electrochemical behavior of cubic PtâSn nanoparticles. The electrochemical activity of the cubic PtâSn nanoparticles was found to be about three times higher than that obtained with unshaped PtâSn nanoparticles and six times higher than that of Pt nanocubes. In addition, stability tests indicated the electrocatalyst preserves its morphology and remains well-dispersed on the carbon support after 5000 potential cycles, while a cubic (pure) Pt catalyst exhibited severe agglomeration of the nanoparticles after a similar stability testing protocol. A detailed analysis of the elemental distribution in the nanoparticles by STEM-EELS indicated that Sn dissolves from the outer part of the shell after potential cycling, forming a âŒ0.5 nm Pt skin. This particular atomic composition profile having a Pt-rich core, a Sn-rich subsurface layer, and a Pt-skin surface structure is responsible for the high activity and stability.This work has been supported by FundacioÌn Cajacanarias (project BIOGRAF) and the Ministry of Economy and Competitiveness (MINECO) through the projects CTQ2011-28913-C02-02 and ENE2014-52158-C2-2-R (cofunded by FEDER). We acknowledge the SEGAI services of Universidad de La Laguna for important technical assistance, and R.R. acknowledges the funding received from MINECO (EEBB-I-16-11762) to carry out a predoctoral stay in a foreign R&D center. E.P. acknowledges support from an electron microscopy facility supported by the NSF MRSEC program (DMR 1120296) and an NSF MRI grant (DMR 1429155). J.S.G. acknowledges financial support from VITC (Vicerrectorado de InvestigacioÌn y Transferencia de Conocimiento) of the University of Alicante
Tunneling Spectra of Individual Magnetic Endofullerene Molecules
The manipulation of single magnetic molecules may enable new strategies for
high-density information storage and quantum-state control. However, progress
in these areas depends on developing techniques for addressing individual
molecules and controlling their spin. Here we report success in making
electrical contact to individual magnetic N@C60 molecules and measuring spin
excitations in their electron tunneling spectra. We verify that the molecules
remain magnetic by observing a transition as a function of magnetic field which
changes the spin quantum number and also the existence of nonequilibrium
tunneling originating from low-energy excited states. From the tunneling
spectra, we identify the charge and spin states of the molecule. The measured
spectra can be reproduced theoretically by accounting for the exchange
interaction between the nitrogen spin and electron(s) on the C60 cage.Comment: 7 pages, 4 figures. Typeset in LaTeX, updated text of previous
versio
- âŠ