 |
 |
Electron Microscopy Center |
HighlightsTEM of Fuel Cell Electrocatalysts for Reduced Cost and Improved LifetimesTEM of Fuel Cell Electrocatalysts for Reduced Cost and Improved Lifetime Polymer electrolyte fuel cells
(PEFCs) have platinum
nanoparticle electrocatalysts within both electrodes.
It is known that the state-of-the-art Pt
nanoparticle catalysts slowly agglomerate into larger, less
catalytically active
particles during use, yet the fundamental underlying mechanisms remain
unclear.
With the objective of better understanding the mechanism of
Pt particle
growth, we have used TEM and in-situ
small-angle x-ray scattering (SAXS) to measure the Pt catalyst growth
in a
model fuel cell over time. SAXS
patterns
were recorded at the Advanced Photon Source where Pt /carbon
electrocatalysts
were tested for 16 hours in model cells.
After the SAXS experiments were concluded, the Pt/C
electrocatalyst
samples were examined in a TEM at the Electron Microscopy Center. From the TEM images, one
of which is shown in
Figure 1, the diameters of over 200 Pt particles were measured and a
particle
size distribution was calculated.
Figure
2 shows the particle size distribution from both the TEM and SAXS data. Over
time, there is a clear progression of particle size with relatively
little
change in distribution width. As
shown,
the final SAXS particle size distributions fit well to the post-mortem
TEM
analysis. Log-normal
and Gaussian
distributions showed poor agreement with the experimentally-determined
distributions, suggesting that long-range Pt ion transport (Ostwald
ripening)
is a possible mechanism of Pt particle growth.
Significance PEFC Pt electrocatalysts
currently have a lifetime of
only 1000 hrs – significantly below the 5000 hrs DOE target. In order
to reduce
costs, PEFC developers are reducing Pt loadings, whilst maintaining
catalytic
performance by minimizing particle size.
However, this exacerbates the problem; the smaller the
particle, the
greater the driving force for its coarsening. Without addressing
concerns over
electrocatalyst durability the commercial viability of hydrogen fuel
cells
remains doubtful. Using
in-situ SAXS,
with supporting pre- and post-mortem TEM, we have observed the
changing particle size as a function of applied potential and time.
This
information is of great interest to the fuel cell community, as it
impacts the
theoretical understanding of Pt degradation and by corollary, impacts
future
rationale for catalysis design and optimization of fuel cell
operational
parameters. As a very recent advance its significance to the
wider field
is yet to be gauged, however, our analytical focus in this area has
intensified
significantly. Further experiments have been conducted, and
more are
planned. Additionally, this work will form a central
component of a
multi-year funding proposal in response to the recent DOE solicitation
on fuel
cells. This work has been published in J.
Am. Chem. Soc., 130,
8112 (2008). The
lead author has been invited to
participate in a workshop in this area in the fall. Performers 
|
|
||||
|
||||
