Abstract
In order to develop noble-metal- and carbon-free cathodes, titanium-niobium oxides were prepared as active materials for oxide-based cathodes and the factors affecting the oxygen reduction reaction (ORR) activity were evaluated. The high concentration sol-gel method was employed to prepare the precursor. Heat treatment in Ar containing 4% H2 at 700–900 °C was effective for conferring ORR activity to the oxide. Notably, the onset potential for the ORR of the catalyst prepared at 700 °C was approximately 1.0 V vs. RHE, resulting in high quality active sites for the ORR. X-ray (diffraction and photoelectron spectroscopic) analyses and ionization potential measurements suggested that localized electronic energy levels were produced via heat treatment under reductive atmosphere. Adsorption of oxygen molecules on the oxide may be governed by the localized electronic energy levels produced by the valence changes induced by substitutional metal ions and/or oxygen vacancies.
Highlights
Polymer electrolyte fuel cells (PEFCs) offer many advantages, including high power density, high energy conversion efficiency, and lower operating temperatures
The crystalline phase of the catalysts prepared by heat treatment in air at temperatures between 600 and 900 °C was identified as anatase TiO2 (JCPDS no. 00-021-1272), indicating that the niobium atoms were incorporated into the TiO2 anatase structure
For samples prepared at higher temperatures, peaks corresponding to the rutile TiO2 (JCPDS no. 00-021-1276) and TiNb2O7 (JCPDS no. 1001270)
Summary
Polymer electrolyte fuel cells (PEFCs) offer many advantages, including high power density, high energy conversion efficiency, and lower operating temperatures. We prepared noble-metal- and carbon-free cathodes comprising niobium-titanium oxides with active sites and titanium oxides with magneli phase Ti4O7 as the electro-conductive material (i.e., TixNbyOz + Ti4O7) [9]. Domen et al demonstrated that Nb-doped TiO2 synthesized by the oxidation of Nb-doped TiN nanoparticles exhibited definite ORR activity and high long-term stability in acidic solutions [11] These catalysts contained carbon residues that functioned to improve the conductivity between the particle aggregates. It is necessary to secure the sufficient electro-conductivity between oxide-based catalysts and conductive oxide support when carbon-free cathodes are prepared. The effects of the preparation conditions, such as the gas atmosphere and heat treatment temperatures, on the ORR activity of the titanium-niobium oxides employing a GC rod are evaluated
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