The dissertation work focuses on research and development of durable nanoscale catalysts and supports for rechargeable Li-air batteries that use aqueous catholytes. Transition metal oxides, TiO2 and Nb2O5 in particular, were prepared from a sol-gel process in the form of nanocoatings (5~50 nm) on carbon nanotubes (CNTs) and studied as catalyst supports. Carbon doping in the oxides and post annealing significantly increased their electronic conductivity. Pt catalyst on the support with TiO2 (Pt/c-TiO2/CNTs) showed a much better oxygen reduction reaction (ORR) activity than a commercial Pt on carbon black (Pt/C). Negligible loss (\u3c 3%) in ORR activity was found in Pt/c-TiO2/CNTs as compared to more than 50% loss in Pt/C, demonstrating a significantly improved durability in the developed catalysts. However, Pt/c-Nb2O5/CNTs was found to be worse in ORR activity and durability, suggesting that c-Nb2O5/CNTs may not be a good support.
CNTs have fibrous shape and would provide a unique porous structure as electrode. Their buckypapers were made and used to support catalysts of Pt and IrO2 in the cathodes of Li-air batteries with sulfuric acid catholyte. At low Pt loading (5 wt.%) without IrO2 on the buckypaper cathode, the Li-air cell achieved a discharging capacity of 306 mAh/g and a specific energy of 1067 Wh/kg at 0.2 mA/cm2. A significant charge overpotential reduction (~ 0.3 V) was achieved when IrO2 was also used to form a bifunctional catalyst with Pt on the buckypapers. The round trip efficiency was increased from 72% to 81% with the bifunctional cathode, demonstrating a higher energy conversion efficiency --Abstract, page iv
