Title

Development of co-tolerant anode electrocatalysts for fuel cell applications

Date of Completion

January 2000

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Proton Exchange Membrane (PEM) Fuel Cells offer much promise as cleaner and efficient alternative energy conversion devices for automobile propulsion and portable power generation applications. A major challenge in using PEM fuel cells for transportation applications is their reduced power density from CO poisoning of the Pt based catalyst when running on hydrogen produced by on-board reforming of hydrocarbons. The best performing oxidation catalyst for hydrogen containing CO is PtRu (1:1 atomic ratio), where Ru is believed to assist in the oxidation of CO through chemisorbed −OH at potentials as low as 0.25V. However, this catalyst does not provide adequate CO tolerance resulting in higher precious metal requirements. ^ Different catalytic solutions are investigated in this work. One approach studied here involves new ternary alloys or intimate ad-mixtures of metals of the form Pt-Ru-X as CO tolerant catalysts. Au, Ag, Rh, and W2C were investigated as component X. CO tolerance was evaluated using polarization studies for the oxidation of H2/104 ppm CO gas and CO stripping voltarnmetry. Pt-Ru-W2C catalyst oxidizes CO at a lower potential than Pt-Ru and shows lower anode polarization losses. ^ A new approach studied here uses organic metal macrocycle complexes as co-catalysts with platinum. The complex is believed to act as a redox mediator in generating −OH species that are essential for CO oxidation. Investigations were carried out with different phthalocyanine, porphyrin, and cyclam complexes. It was observed that CO oxidation potentials were lowered and CO tolerance was enhanced when complexes with lower redox potentials were used. Mo-tetra methoxy phenyl porphyrin and Hexaammine Ru complexes show performance almost as good as Pt-Ru catalyst. Better catalysts could be developed in a systematic approach using redox potential of the complex as guideline. Influences of the catalyst supports and supporting methods on the activity of the complex and CO tolerance were also investigated. ^ “Sulfided” Pt catalysts were studied as another novel catalytic solution for CO poisoning. These catalysts show inhibition for CO adsorption and have activity for CO oxidation at potentials as low as 0.1V. The lowest anode polarization loss for oxidation of H2 containing 104 ppm CO obtained for this catalyst in this study was 0.167V at 400mA/cm 2 which was lower than that for the widely used Pt-Ru catalyst. Further optimization of “sulfided” catalysts could provide a cheaper and CO tolerant catalyst than the widely used Pt-Ru catalyst. ^