Pt/TiO_x Cathode Catalysts for Polymer Electrolyte Fuel Cells

Abstract

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