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.
