Preparation of Nano-Sized Doped Ceria for SOFC Anodes
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Preparation of Nano-Sized Doped Ceria for SOFC Anodes Jumpei Kobayashi1,2 , Hitoshi Takamura1,2 , Atsunori Kamegawa1,2 and Masuo Okada1,2 1 Department of Materials Science, Tohoku Univ., 6-6-02 Aramaki Aza Aoba, Sendai 980-8579, Japan 2 CREST, Japan Science and Technology Agency, Kawaguchi, Japan ABSTRACT Nano-particles of Pr and Mn-doped ceria have been prepared by mixing nitrate solutions and hexamethylenetetramine as an oxidizing agent, and their thermal stability has been investigated. The particle size was evaluated by means of XRD, BET and dynamic light scattering techniques. The particle size of Pr and Mn-doped ceria was found to be approximately 3 nm as prepared. Even though the particle size of doped ceria increases with increasing annealing temperature, the size of less than 15 nm was kept after annealing at 600˚C for 10 h. On the other hand, the lattice constant of nano-sized ceria, for example 0.544 nm for 3 nm-size particles, decreases and reaches to the value of bulk material (0.541 nm) as annealing temperature increases. This suggests that the valence state of cerium and the amount of oxygen vacancies strongly depend on the particle size. INTRODUCTION Ceria (CeO2 ) is one of the functional ceramics with a cubic fluorite crystal structure. Pure ceria and cation-doped ones have a wide range of applications such as catalytic supports for automobile exhaust systems and an electrolyte of solid oxide fuel cells (SOFCs) because of their catalytic activity and high oxide-ion conductivity. Recently, some cerium oxides, for example, Pr and Mn-doped ceria showing a certain level of electronic conduction at elevated temperatures, have also attracted attention for use in ceramics-based anode materials for SOFCs and oxygen permeable membranes [1–4]. However, for that purpose, even for highly Pr-doped CeO2 with a maximum total conductivity of approximately 0.1 S/cm at 1000˚C, the conductivity is too low. To improve the electrode properties, it is important to increase the mixed conductivity, especially, the electronic conductivity of the doped ceria. For that purpose, nano-particles of Pr and Mn-doped ceria have been prepared. By reducing the particle size of ceria down to nano-scale, the electronic conductivity is expected to enhance due to a space-charge layer effect [5, 6]. The electronic conductivity of nanoparticle is higher than that of bulk by four orders of magnitude [6]. The purpose of this study is to prepare the nano-sized Pr and Mn-doped ceria as well as to clarify the dependence of reaction conditions on the particle size. Their thermal stability and lattice expansion behavior were also investigated. EXPERIMENTAL PROCEDURES According to reports by Blazevic and Kolbah [7], and Dreyfors et al. [8], the samples of Pr and Mn-doped ceria have been prepared by mixing nitrate solutions and hexamethylenetetramine (HMTA) as an oxidizing agent. The mixed solution was stirred at room temperature. To control pH of solutions, hydrogen peroxide solution (30-35.5 vol% H2 O2 ) was added. The mixed soluti
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