Redox behavior of Dicerium trioxide and the Possible Formation of Sesquioxide- C for Fuel cells
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Redox behavior of Dicerium trioxide and the Possible Formation of Sesquioxide- C for Fuel cells Yang Yue1, K.S.V. Santhanam* 1, K. Reed2 and T. Allston1 1. School of Chemistry and Materials Science, Rochester Institute of Technology, Rochester, NY 14623 2. Cerion Enterprises, Rochester,NY 14610 * Corresponding author [email protected] ABSTRACT Cerium oxide is an important electrode material in fuel cells. It has been interconverted in the gaseous phase at high temperatures or in the presence of oxygen and more recently in the solid state by applying an electric field. In the dynamic change that occurs, the migration of oxygen vacancies have been initiated at a critical potential of 2.8 V. We wish to report here an electrochemical method where the conversion of cerium oxides is brought out in aqueous medium where hydrogen ion is assisting the process of conversion. Keeping dissolved oxygen level negligible, the conversion of Ce2O3 to CeO2 occurs at 0.72 V vs saturated calomel electrode (SCE) with hydrogen ion assisting the process and the reversible conversion at 0.15 V (SCE). The hydrogen ion assisted conversion is compared with the solid state conversion that operates on oxygen vacancy creation. INTRODUCTION Cerium oxides are unique in the class of metal oxides as they exhibit interesting physicochemical properties that enable them to be used in technological applications (1-16). The pronounced catalytic effect observed with this class of oxides led to its application in automobile exhaust (4), solid oxide fuel cells (5), optical displays, production of hydrogen and microelectronic devices where refractive index is important (6). As cerium belongs to the class of lanthanides exhibiting variable electronic structure with the 4f level having the same energy as that of the 6s valence electrons, it enjoys a dual valence states that affords unique redox behaviors. The transition from valence state of three to four which is a consequence of removal of 4f electron results in the ionic radius changing from six coordination to eight coordination (18); the ionic radius changes from 1.01Å to 0.87 Å in this transition (19). The charge localization effect in cerium oxides has been discussed in the literature (20). Cerium oxide exists in two stable compositions; cerium sesquioxide (Ce2O3) [also called dicerium trioxide] and cerium dioxide (CeO2). The crystallographic structures of the two oxides have been well analyzed. They have a pseudocubic lattice type with lattice parameters (a=3.8905 Å, c=6.0589 Å) (7). It contains a sandwich of penta layers and a close packing of these penta layers forms type-A structure with a stable (001) surface. The sesquioxide-C type differs from Type-A in that it has 32 metal ions and 48 oxygen ions which may be considered as a fusion of eight unit cells of cubic cerium dioxide with two oxygen vacancies. The type-C sesquioxide has also been described as sandwich trilayer. The two forms of oxides form solid solutions giving raise to several mixed oxides. There are very few reports in the literature on t
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