Titanium Oxide Nano-Particles as Supports of Cathode Catalysts for Polymer Electrolyte Fuel Cells

Abstract

INTRODUCTION Development of carbon-free electro-conductive supports for oxygen reduction reaction (ORR) catalysts are required for a widespread of polymer electrolyte fuel cells (PEFCs) because carbon materials, which are used as a support in the present PEFCs, are easily oxidized at high potential [1]. Many conductive or semi-conductive oxides such as indium tin oxide (ITO) [2], TiOx [3], SnO2 [4] and so on have been studied as catalyst supports or as secondary supports to improve the performance of the cathodes of PEFCs. These metal oxides have shown promising effects on the catalytic activity for the ORR [5,6] and the durability [7,8] under the condition of PEFC cathode. Nb-doped TiO2 shows the high conductivity compared with pure TiO2, suggesting that Nb-doped TiO2 particles have great potential as carbon-free electro-conductive supports. EXPERIMENTAL Nb-doped TiO2 particles were prepared by hydrothermal method. First, titanium butoxide was mixed with niobium ethoxide with a desired Nb/Ti atomic ratio. Second, a deionized water of 30 mL was added to the mixture to obtain complex hydroxide. The precipitation and remained solution were moved into the reactor. Then, the reactor was heated at 180 oC, 20 h to proceed the hydrothermal reaction. Finally, the precipitate was obtained by filtration and dried under vacuum at 90 oC, 12 h and calcined at 380 oC, 3 h in air to remove the organic residue. The Nb-doped TiO2 particles was heated under pure hydrogen gas to further increase the conductivity. RESULTS Fig. 1 shows the XRD pattern of Nb-doped TiO2 prepared by hydrothermal treatment. According to the XRD pattern, the Nb-doped TiO2 was identified as the anatase phase. In addition, there were no niobium oxides peaks such as Nb2O5 or other niobium oxides, indicating that niobium was doped into TiO2 anatase successfully. In order to evaluate the durability of the Nb-doped TiO2, the accelerated durability tests (ADT) was done in 0.1 M HClO4 at 30 oC under N2 atmosphere in the potential range from 1.0 to 1.5 V vs. RHE with a scan rate of 500 mVs-1. After undergoing the 15000 cycles ADT, the mass loss of the support was less than 3%.