Oxygen Anion Diffusion in Doped Ceria M x Ce 1-x O 2-0.5x (M=Gd, Sm and Pr): A Molecular Dynamics Simulation Study
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.165
Oxygen Anion Diffusion in Doped Ceria MxCe1-x O2-0.5x (M=Gd, Sm and Pr): A Molecular Dynamics Simulation Study Neetu Kumari, Uzma Anjum, M. Ali Haider* and Suddhastawa Basu* Renewable Energy Centre (REC) Lab, Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India *Corresponding Author, Email-id: [email protected], [email protected]
ABSTRACT Molecular dynamics simulations were utilized to determine the oxygen anion diffusivity in pure ceria (CeO2) and doped ceria MxCe1-xO2-0.5x (M=Gd, Sm and Pr) with varying level of dopant concentration from 5-30% (x = 0.05-0.3). Doping with Gd showed an improvement in oxygen anion diffusivity value by two order of magnitude (D = 4.67x10-8 cm2/s at 1173 K) as compared to the undoped ceria (D = 1.33x10-10 cm2/s at 1173 K). 10% of doping level was estimated as the optimum concentration of all the dopants at which all of the doped ceria materials showed maximum diffusivity of oxygen anion. Among the three dopants studied, Pr was observed to show maximum diffusivity of oxygen anion in the temperature range of 7731173 K of simulations.
INTRODUCTION Pure ceria and ceria based materials have been studied for catalytic[1,2] electrocatalytic[3] and thermocatalytic[4] properties. This is owing to their high oxygen storage capacity via a vacancy mechanism in which oxygen anions are incorporated or transported within the lattice of the material. Utilizing density functional theory (DFT) calculations, our previous studies have highlighted the mechanism of oxygen incorporation in the ceria lattice while reducing CO2 into CO and methanol[5–7]. In partially reduced ceria, a small fraction of Ce4+ reduces to Ce3+, resulting into the formation of oxygen vacancy. The oxygen vacancy sites present on the ceria surface, are active for the dissociation of CO2 into CO and atomic oxygen. The oxygen atom on the surface could replenish the vacancy site and are subsequently transported through the bulk under applied potential[6]. Formation of oxygen vacancy and diffusion of oxygen ion play a key role in determining the electrocatalytic activity of ceria as observed for CO2 reduction[8]. On doping pure ceria with a trivalent metal cation, oxygen vacancies
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are created inside the anion sublattice, resulting into a relatively higher oxygen anion diffusivity and ionic conductivity in the material[9]. Previous experiment[10–12] and theoretical[13,14] studies have attempted to explain this improvement in diffusivity and ionic conductivity in doped ceria based materials. For example, gadolinium doped ceria (GDC) and samarium doped ceria (SDC) have shown relatively higher oxide ion conductivity, 4.16x10-2 S cm-1 and 9.45x10-2 S cm-1 at 1073 K, respectively than the
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