New Oxygen Reduction Electrocatalysts Based on Oxide Nanosheet Materials

Three types of titania nanosheet-based materials, a H3O+-exchanged rolled nanosheet (nanotube) (H3O+-NT(Ti)), layered titanate (H3O+-RG(Ti)), and restacked titania nanosheets (H3O+-RE(Ti)) were synthesized by soft chemical methods, and their oxygen reduction reaction (ORR) activities were examined by semi-steady-state voltammetry with a rotating ring-disc electrode at 70{degree sign}C in 0.05 M H2SO4. All the samples showed similar onset potentials (Eon) of the ORR current, ~ 0.50 vs. reversible hydrogen electrode, while the efficiencies (Eff4) of the 4-electron reduction of oxygen depended on their nanostructures, i.e. the stacking morphology of nanosheets, specific surface area and kinds of inserted cation between the nanosheets. Both H3O+-RG(Ti) and H3O+-RE(Ti) samples showed higher Eff4 values as compared with Cs+-form layered titanate (Cs+-RG(Ti)) and the H+-form restacked titania nanosheets (H+-RE(Ti)). This reveals that the H3O+ions and the number of the active sites for ORR are related to the ORR activity. The H3O+-RE(Ti) exhibited the best Eff4 value (> 90%), which is comparable to that of a conventional 20 mass% Pt/C catalyst. The effects on the ORR activity are also observed in alkaline (0.1 M KOH) aqueous solution, i.e. the H3O+-RE(Ti) exhibited the best Eon (~0.8 V) and Eff4 values (> 90%).