Xirong Jiang Ph.D. Oral Examination (Friday February 20, 10am, Physics/ Astrophysics 102/103)

Xirong Jiang xrjiang at stanford.edu
Tue Feb 17 18:10:21 PST 2009


*Title: Study of Catalysts for Solid Oxide Fuel Cells and Direct Methanol
Fuel Cells*

University Oral Examination
Xirong Jiang
Physics Department

Advisor: Prof. Stacey Bent

Date: Friday, February 20, 2009
Time: 10 am (Refreshments served at 9:45 am)
Locations: Physics/Astrophysics 102/103 (connected to the Varian building)

Abstract:
Fuel cells offer the enticing promise of cleaner electricity with lower
environmental impact than traditional energy conversion technologies. Driven
by the interest in power sources for portable electronics, and distributed
generation and automotive propulsion markets, active development efforts in
the technologies of both solid oxide fuel cell (SOFC) and direct methanol
fuel cell (DMFC) devices have achieved significant progress. However,
current catalysts for fuel cells are either of low catalytic activity or
extremely expensive, presenting a key barrier toward the widespread
commercialization of fuel cell devices. In this thesis, atomic layer
deposition (ALD), a novel thin film deposition technique, will be employed
to apply catalytic Pt to both SOFC and DMFC to increase the activity and
utilization levels of the catalysts while simultaneously reducing the
catalyst loading.
For SOFCs, we are exploring the use of ALD for the fabrication of electrode
components, including an ultra-thin Pt film for use as the electrocatalyst,
and a Pt mesh structure for a current collector for SOFCs, aiming for
precise control over the catalyst loading and catalyst geometry, and
enhancement in the current collect efficiency. We choose Pt since it has
high chemical stability and excellent catalytic activity for the O2
reduction reaction and the H2 oxidation reaction even at low operating
temperatures. Working SOFC fuel cells have been fabricated with
ALD-deposited Pt thin films as an electrode/catalyst layer. The measured
fuel cell performance reveals that comparable peak power densities are
achieved for ALD-deposited Pt anodes with only one-fifth of the Pt loading
relative to DC-sputtered counterpart. In addition to the continuous
electrocatalyst layer, a micro-patterned Pt structure has been developed via
the technique of area selective ALD. By coating yttria-stabilized zirconia,
a typical solid oxide electrolyte, with patterned (octadecyltrichlorosilane)
ODTS self-assembled monolayers (SAMs), Pt thin films are grown selectively
on the SAM-free surface regions. Features with sizes as small as 2 mm have
been deposited by this combined ALD-mCP method. The area selective atomic
layer deposited micro-patterned Pt structure has been applied to SOFC as a
current collector grid/patterned catalyst for the fuel cells. An improvement
in the fuel cell performance by a factor of 10 has been observed using the
Pt current collector grids/patterned catalyst integrated onto cathodic
La0.6Sr0.4Co0.2Fe0.8O3-δ. To improve the utilization, stability and
efficiency of catalysts for methanol oxidation for DMFCs, two strategies
have been employed in this thesis. One approach is to use a core-shell
structured catalyst, where ALD Pt is used to decorate dc-sputtered metal (Pd
and Ru) as core-shell catalysts toward methanol oxidation. The activity of
the metal catalysts is enhanced by the Pt shell. In addition, Pt decorated
Ru is found to be more active than Pt decorated Pd, likely because water
dehydrogenation, needed to provide a -OH to oxidize the CO, is more facile
on Ru than on Pd. Another strategy we have employed is to replace or alloy
Pt with Ru where both dc-sputtering and atomic layer deposition have been
employed to fabricate PtRu catalysts of various Ru contents and tested as
catalysts for methanol oxidation. The results indicate that the optimal
stoichiometry of the alloys measured in 1 M MeOH and 16.6 M MeOH is Ru1Pt3
and Ru1Pt1, respectively. Both strategies are shown to reduce the Pt loading
while achieving better utilization of the catalyst.
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