Carbon-free Pt-based electrocatalysts supported on semiconducting SnO 2 ͑Pt/SnO 2 ͒ have been developed for polymer electrolyte fuel cells ͑PEFCs͒ by various electrochemical and microstructural characterizations. Pt/SnO 2 exhibited comparable currentvoltage characteristics to conventional Pt electrocatalysts and, in particular, a considerable tolerance against 10,000 cycles of voltages up to 0.9 and 1.3 V RHE ͑RHE denotes reversible hydrogen electrode͒ vs the RHE. These results indicate that the carbon-free oxide-supported Pt/SnO 2 can be a promising alternative electrocatalyst with long-term durability against voltage cycling up to higher potentials, as a possible fundamental solution to the PEFC degradation caused by carbon support corrosion.Polymer electrolyte fuel cells ͑PEFCs͒ are environmentally compatible promising power sources possibly for automotive, stationary, and portable applications, 1 where Pt-based electrocatalysts supported on carbon black are widely used. While carbon black is the state-of-the-art support material, 2 corrosion of the carbon-based electrocatalyst support is one of the important technological issues that should be solved to improve the long-term durability of the PEFC systems. Oxidation-induced carbon support corrosion in cathode electrocatalysts occurs especially under high potential conditions associated with ͑i͒ start-up and shutdown, ͑ii͒ potential cycling, and ͑iii͒ open-circuit preservation of the PEFCs. 3-9 Anode electrocatalyst layers could be under a higher potential in the start-up situation, 4 which may also cause carbon support corrosion. For automotive applications, frequent voltage cycling during the operation should be tolerated, including high potential conditions beyond 1.0 V RHE ͑RHE denotes reversible hydrogen electrode͒. Tolerance against potential cycling is also essential for stationary PEFC systems probably operated in the daily start-and-stop mode for a decade. 9 Because carbon black ͑graphite͒ is thermochemically unstable under higher potential conditions, 10 alternative carbon-free electrocatalyst support materials are desired to be developed for PEFCs with a long-term durability. Various oxides have been evaluated as possible PEFC electrocatalyst support materials, including TiO 2 , 11 Ti 4 O 7 , 12,13 WO 3 , 14 and SnO 2 . [15][16][17][18][19] Electrocatalyst support materials are used as the mixture with strongly acidic electrolyte materials, e.g., Nafion; thermochemical stability is required under such acidic conditions during the lifetime of the cells. Among various oxides relatively stable under such conditions, we have selected SnO 2 known as a broad-band oxide semiconductor with high electronic conductivity and electronic mobility used, e.g., for transparent conductors and gas sensors, the conductivity of which could be further increased, e.g., by oxygen deficiency and extrinsic impurity doping. [20][21][22] The pH-potential diagram ͑so-called Pourbaix diagram 10 ͒ of the Sn-H 2 O system at 80°C thermochemically calculated indicated that SnO 2 is...

Oxidation-induced carbon support corrosion especially in cathode electrocatalysts for polymer electrolyte fuel cells (PEFCs) is one of the important technological issues to be solved. We focus our attention to SnO2, known as a broad-band oxide semiconductor with a high electronic conductivity, as a possible electrocatalyst support material. In this study, nanostructure, electrochemical properties and durability of carbon-free electrocatalysts, Pt/SnO2, Pt/Nb-SnO2, Pt/Al-SnO2, are investigated.