TY - THES

T1 - Synthesis and Characterization of Mass and Charge Transport Properties of the New Rare Earth Nickelate Pr2Ni0.9Co0.1O4+δ

AU - Strasser, Anna

N1 - embargoed until null

PY - 2017

Y1 - 2017

N2 - The focus of the present study is on the synthesis and characterization of the novel K2NiF4-type oxide Pr2Ni0.9Co0.1O4+δ (PNCO). Nickel in Pr2NiO4+δ is partially substituted with cobalt in order to increase the surface exchange coefficient of oxygen. The crystal structure, as well as mass and charge transport properties, especially oxygen exchange kinetics, of PNCO are studied. PNCO shows promising properties for application in air electrodes of solid oxide fuel cells or electrolyzer cells, electrochemical sensors, oxygen permeable membranes etc. To obtain higher quantities of chemically homogenous, single phase Pr2Ni0.9Co0.1O4+δ powder in one batch, a new freeze drying method based on aqueous acetate precursor solutions was developed. The solutions were mixed, snap-frozen in liquid nitrogen, and freeze-dried. During calcination the complex oxide was formed. X-ray powder diffraction (XRD) and Rietveld refinement verified that the material was single phase, and confirmed the K2NiF4 structure. The Pr2Ni0.9Co0.1O4+δ powder was further characterized by dilatometry, differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). In pure argon, a transition from the orthorhombic to the tetragonal K2NiF4 modification occurred, which was also reported for Pr2NiO4+δ. Based on the dilatometry curve a pellet was sintered at 1120°C to obtain a dense sample for oxygen exchange measurements. However, XRD of the sintered pellet, as well as DSC and TG of powder samples, indicated that phase decomposition of Pr2Ni0.9Co0.1O4+δ into Pr4(Ni,Co)3O10-x and Pr-oxide occurs at T≥750°C and pO2=0.2 bar as previously reported also for Pr2NiO4+δ. Finally, by optimization of the sintering program, it was possible to obtain a phase-pure Pr2Ni0.9Co0.1O4+δ pellet with 97 % of the theoretical density, which was used for studies of the oxygen exchange kinetics by the dc-conductivity relaxation method. In addition to that, a Pr2NiO4+δ pellet was prepared as a reference sample. For all three specimens (phase decomposed Pr2Ni0.9Co0.1O4+δ, phase pure Pr2Ni0.9Co0.1O4+δ, and Co-free reference Pr2NiO4+δ) the electronic conductivity and the surface exchange coefficient were determined and compared with each other in order to evaluate the validity of the results.

AB - The focus of the present study is on the synthesis and characterization of the novel K2NiF4-type oxide Pr2Ni0.9Co0.1O4+δ (PNCO). Nickel in Pr2NiO4+δ is partially substituted with cobalt in order to increase the surface exchange coefficient of oxygen. The crystal structure, as well as mass and charge transport properties, especially oxygen exchange kinetics, of PNCO are studied. PNCO shows promising properties for application in air electrodes of solid oxide fuel cells or electrolyzer cells, electrochemical sensors, oxygen permeable membranes etc. To obtain higher quantities of chemically homogenous, single phase Pr2Ni0.9Co0.1O4+δ powder in one batch, a new freeze drying method based on aqueous acetate precursor solutions was developed. The solutions were mixed, snap-frozen in liquid nitrogen, and freeze-dried. During calcination the complex oxide was formed. X-ray powder diffraction (XRD) and Rietveld refinement verified that the material was single phase, and confirmed the K2NiF4 structure. The Pr2Ni0.9Co0.1O4+δ powder was further characterized by dilatometry, differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). In pure argon, a transition from the orthorhombic to the tetragonal K2NiF4 modification occurred, which was also reported for Pr2NiO4+δ. Based on the dilatometry curve a pellet was sintered at 1120°C to obtain a dense sample for oxygen exchange measurements. However, XRD of the sintered pellet, as well as DSC and TG of powder samples, indicated that phase decomposition of Pr2Ni0.9Co0.1O4+δ into Pr4(Ni,Co)3O10-x and Pr-oxide occurs at T≥750°C and pO2=0.2 bar as previously reported also for Pr2NiO4+δ. Finally, by optimization of the sintering program, it was possible to obtain a phase-pure Pr2Ni0.9Co0.1O4+δ pellet with 97 % of the theoretical density, which was used for studies of the oxygen exchange kinetics by the dc-conductivity relaxation method. In addition to that, a Pr2NiO4+δ pellet was prepared as a reference sample. For all three specimens (phase decomposed Pr2Ni0.9Co0.1O4+δ, phase pure Pr2Ni0.9Co0.1O4+δ, and Co-free reference Pr2NiO4+δ) the electronic conductivity and the surface exchange coefficient were determined and compared with each other in order to evaluate the validity of the results.

KW - Rare earth nickelates

KW - freeze drying

KW - crystal structure

KW - phase stability

KW - oxygen exchange kinetics

KW - electronic conductivity

KW - Seltenerdnickelate

KW - Gefriertrocknung

KW - Kristallstruktur

KW - Phasenstabilität

KW - Sauerstoffaustauschkinetik

KW - elektronische Leitfähigkeit

M3 - Master's Thesis

ER -