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Introduction The development of polymer electrolyte fuel cells (PEFCs) has attracted tremendous interest, because they have many advantages, including high power density, high energy conversion efficiency, and low operating temperatures. Recently, PEFCs has been already commercialized as power supply of fuel cell vehicles and residential use. Generally, platinum-based alloys supported carbon black are used as catalysts in both the anode and cathode of PEFCs. However, Pt is high cost and limited resources. In addition, carbon support is fundamentally unstable under cathode conditions to oxidize at high potential region. The development of alternative catalysts with high stability for oxygen reduction reaction (ORR) is required for wide spread of PEFCs. This catalyst needs two components, precious-metal-free active sites for ORR and carbon-free electro-conductive supports. We have focused on group 4 and 5 metal oxide-based compounds as both precious-metal-free materials with active sites and electro-conductive materials. As an alternative support of carbon , we focused on titanium Magnéli sub-oxides (TinO2n-1; 4≦n≦10), particularly Ti4O7, because of its high chemical stability in acid electrolyte and high conductivity. However, the formation of Ti4O7 needs to reduce at high temperature, 1050 oC, under 4% H2 for 20-60 h1). Such a high temperature caused the drastic particle growth and morphology destruction. In this study, we applied solid-phase reduction at low temperature by using NaBH4 2) to investigate the appropriate condition for the formation of reduced titanium oxides as support. Experimental Rutile type TiO2 powder (ca. 30 nm) was mixed with NaBH4. The mixture was heat-treated under Ar atmosphere at 300 - 400oC for 24 h. After heat-treatment, we washed products with deionized water and 0.1 M hydrochloric acid to obtain the reduced titanium oxide powders. X–ray diffraction spectroscopy (XRD, Rigaku Ultima IV) was performed to determine the crystalline structure of the powder. Results and discussion Figure 1 shows the XRD patterns of the reduced powders heat-treated at various temperatures. The peaks near 27.4o identical to first main peak of TiO2 rutile were gradually decreased from 300 to 360 oC, and the peaks near 23.8 o and 33.0 o identical to Ti2O3 appeared above 350 oC, indicating that the formation of bulk Ti2O3 gradually proceeded above 350 oC. On the other hand, the temperature over 375 oC caused transition to amorphous phase. These results suggested that it was difficult to form the Ti4O7 phase by solid-phase reduction at low temperature. We need to find an appropriate condition to form the Ti4O7 phase. On the other hand, it was reported that the bulk Ti2O3 has much larger conductivity than TiO2 3). However, the all reduced powders showed large resistance measured by two-terminal resistance measurement. The large resistance was caused by the contact resistance due to the passive layer formed on the nano-particles. In order to reduce the contact resistance, we think that the formation of the electro-conductive network structure is effective by control the morphology of the reduced oxides. Acknowledgments The authors thank New Energy and Industrial Technology Development Organization (NEDO) for financial support. The Institute of Advanced Sciences (IAS) in Yokohama National University was supported by the MEXT Program for Promoting Reform of National Universities. References 1) M. Hamazaki, A. Ishihara, Y. Kohno, K. Matsuzawa, S. Mitsusima, and K. Ota, Electrochemistry, 83, 817 (2015). 2) S. Tominaka, Chem. Commun., 48, 7949 (2012). 3) S. Tominaka, Inorg. Chem, 51, 10136 (2012) Figure 1 |
