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Title

MULTIFUNCTIONAL ELECTROCATALYST SUPPORTS AND ELECTROCATALYSTS FOR ELECTROCHEMICAL ENERGY CONVERSION

Creator

Kumar, Amod

Date

2014, 2014-07

Description

Polymer electrolyte fuel cell (PEFC) technology has several underlying problems (lifetime, reliability and cost) that need to be resolved as a...
Show morePolymer electrolyte fuel cell (PEFC) technology has several underlying problems (lifetime, reliability and cost) that need to be resolved as a prerequisite for large-scale commercialization. Amongst these, the lifetime issue is regarded as critical. From the perspective of durability, the current state-of-the-art electrocatalyst, viz. platinum (Pt) supported on high surface area carbon, suffers substantial degradation during fuel cell operation. A key pathway for electrocatalyst degradation is the corrosion of the carbon support. During fuel cell startup and shutdown in automotive applications, the cathode potential undergoes excursions of up to 1.6 V. At these high potentials, the carbon corrosion reaction is significantly accelerated, leading to irreversible carbon loss at the electrode. The best way to eliminate the issue of support corrosion in PEFCs is to develop alternate corrosion-resistant catalyst supports. This dissertation investigates (i) mixed oxides of silica (SiO2) and ruthenia (RuO2) and (ii) tantalum modified titanium oxide (Ta0.3Ti0.7O2) for use as corrosion-resistant electrocatalyst supports in PEFCs, especially for the automotive sector, using detailed microstructure characterization and electrochemical evaluation, including two carefully chosen accelerated stress tests (startstop cycling to simulate fuel cell vehicle start-up and shut-down, and load cycling to simulate full-load to no-load transitions in the vehicle during operation). RuO2–SiO2 (0.5:0.5) displayed outstanding electrochemical stability upon exposure to rigorous accelerated potential cycling tests that mimic automotive drive cycles. The Pt/RuO2–SiO2 (0.5:0.5) electrocatalyst yielded a mass activity of 50 mA/mgPt and an area specific activity of 156.3 μA/cm2 at 0.9 V vs. RHE when tested in a fuel cell operating at 80 ºC and 75% RH. A 20 wt.% Pt/Ta0.3Ti0.7O2 catalyst was compared in terms of activity and stability against benchmark Pt/C catalysts. The importance of a corrosion resistant support in a PEFC was demonstrated by monitoring performance loss during start-stop cycling and load cycling; while load cycling did not cause a performance drop, the corrosion-resistant nature of the Ta0.3Ti0.7O2 support helped prevent catastrophic fuel cell failure observed in carbon-supported catalysts during start-stop cycling. Corrosion-resistant electrode materials can not only be applied to PEFCs but can also be extended to other electrochemical energy storage and conversion devices such as redox-flow batteries and electrolyzers. The results of this work also indicate suitable strategies for incorporation of other possible dopants (such as Nb, W and Zr) within TiO2, as well as the use of other metal oxides (such as SnO2 and HfO2) as alternative catalyst supports.
Ph.D. in Chemical Engineering, July 2014
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