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For polymer electrolyte fuel cell cathodes, highly durable supports are required to prevent catalyst degradation in supports. In this study, as model Pt catalysts, 2-10 wt% Pt was deposited on Magnéli-phase niobium-doped macroporous Ti 4 O 7 (Nb-Ti 4 O 7 ) mounted on glassy carbon rods using the coaxial arc plasma deposition method. The morphologies of 2, 5, and 10 wt% Pt catalysts showed the hemisphere fine particles, islands with ca. 1.4 nm diameter and ca. 2.4 nm thickness, and films with ca. 3.3 nm thickness, respectively. During start/stop accelerated durability tests (ADTs) of 5000 cycles following the Fuel Cell Commercialization Conference of Japan protocol, Pt was slightly agglomerated; consequently, the morphologies of the 2, 5, and 10 wt% Pt catalysts were island-like with 3.5 nm thickness, chain bead-like with 4 nm thickness, and film-like with 4 nm thickness, respectively. This slight agglomeration led to good durability during the ADTs. Herein, the oxygen reduction reaction (ORR) mass activity (MA) values at 0.9 V vs. reversible hydrogen electrode (RHE) of the 2, 5, and 10 wt% Pt catalysts were 79, 60, and 36 A g À1 Pt after 5000 cycles ADT, respectively, which had declining ratios after 5000 cycles were 32 %, 17 %, and 0 %, respectively. The island-like and film-like Pt/Nb-Ti 4 O 7 presented activity and durability comparable to a Pt/C catalyst, which was 42 A g À1 Pt (0.9 V vs. RHE) with a 12 % of declining ratio after the ADTs. The durability of the MA suggested that the different affinity caused by different crystal faces led to the slight agglomeration of 2, 5, and 10 wt%_Pt/Nb-Ti 4 O 7 catalysts. These catalysts showed electrochemical surface areas (ECSAs) of 36, 27, and 29 m 2 g −1 after the ADTs, with declining ratios as low as 20 %, 6 %, and 0 %, respectively. All Pt/Nb-Ti 4 O 7 catalysts showed higher durability of the ECSAs than the Pt/C catalyst, which was 68 m 2 g −1 with a 30 % declining ratio after the ADT. Different from common Pt nanoparticle catalysts, which agglomerate into large spherical Pt particles, the slight agglomeration was caused by the interconnection of the deposits and supplemented by a limited increase in the diameter or thickness. The island-like morphology of Pt with a limited thickness presented both high durability and activity among the Pt/Oxide catalysts.

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Introduction Due to carbon materials are easily oxidized at high potential[1], it is necessary to develop carbon-free electro-conductive supports with superior stability for oxygen reduction reaction (ORR) catalyst in polymer electrolyte fuel cells (PEFCs). Lots of conductive or semi-conductive oxides, such as indium tin oxide (ITO)[2], Ti4O7[3], Nb doped SnO2[4] and son on have been reported as catalyst supports or as secondary supports for the improvement of the catalytic activity performance for the ORR. Although these oxide supports showed high stability at high potential region, the low specific surface area, which are significantly lower than carbon materials, limited the actual use in PEFC system. In this study, we focused on Nb doped TiO2 nanoparticles. We found that Nb doped TiO2, which was reduced by NaBH4 and further treated via mixed gas of hydrogen and oxygen, showed high conductivity and high specific surface area compared with pure TiO2. We attempted to evaluate the Nb doped TiO2 as carbon-free electro-conductive supports. Experimental 10 atomic% Nb doped TiO2 was prepared by hydrothermal method as a precursor. First, 0.002 mol niobium ethoxide and 0.018 mol titanium butoxide were mixed completely, and then the mixed solution was dropped into 30 mL deionized water. All of product was put into 100 mL Teflon autoclave, and heated at 180 oC for 20 h to proceed the hydrothermal reaction. Reaction products were collected by filtration, and then dried under vacuum at 80 oC for 12 h to obtain a precursor. To improve conductivity, 1g of precursor and NaBH4 were mixed (w/w, 1:8) with 5 mL EtOH, and the mixture was heated under Ar atmosphere for 20 h. Then, the samples washed by 1M HCl 3 times, and dried under vacuum at 80 oC, 12 h. Finally, the samples were treated by mixed gas of pure hydrogen and 0.1% oxygen at given temperature for given time. 20wt%Pt was deposited by a revised bromide anion exchange (BAE) method. Results and Discussion After the heat treatment at 400 oC, 500 oC and 550 oC for 0.25 h under mixed gas of pure hydrogen and 0.1% oxygen, the BET surface areas of the samples reached to 98 m2g-1, 69 m2g-1 and 29 m2g-1, respectively. in addition, the sample heated at 400 oC also had a 143 Ω·cm at 60 MPa, which could be used as a support in PEFCs. Pt/Nb-TiOx was evaluated in 0.1 M HClO4 at 60 oC, the accelerated durability test (ADT) was performed under N2 atmosphere in the potential range from 1.0 to 1.5 V vs. RHE with a scan rate of 500 mVs-1. Figure 1 shows the polarization curves of Pt/Nb-TiOx for the ORR before and after 500 cycles ADT. It could be found that the catalyst showed a good durability after 500 cycles ADT. Acknowledgment This research is supported by the Strategic International Joint Research Program (SICORP) of the Japan Science and Technology Agency (JST). In addition, the authors wish to thank the support of JSPS grants-in-aid for scientific research, Suzuki Foundation and Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation. References [1] W. Q. Han, et al., Appl. Phys. Lett., 92, 2008, 203117. [2] H. Chhina, S. Campbell, and O. Kesler, J. Power Sources, 161, 2006, 893–900. [3] T. Ioroi, Z. Siroma, N. Fujiwara, S. Yamazaki and K. Yasuda, Electrochem. Commun. 7, 2005, 183–188. [4] Senoo, Yuichi, et al., Rsc Advances 4，61, 2014, 32180-32188. Figure 1

Because of low durability of carbon support which is widely used as a matrix of cathode catalyst in polymer electrolyte fuel cells (PEFCs), a novel carbon-free support with high durability is needed. The conductive oxide is one of the promising candidates due to its high durability. Titanium oxide nano-particles were already applied to an alternative material of carbon, with they have high chemical stability in the acid electrolyte and high electronic conductivity. In this study, we prepared a Nb-doped TiO2 by hydrothermal method and it was reduced in a pure hydrogen atmosphere to obtain Nb-TiOx support. Comparing with a commercial Pt/C catalyst, the Pt/Nb-TiOx catalyst behaved higher durability after 5000 cycles of start-stop accelerated durability test.