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Novel materials with mixed ionic-electronic conductivity for solid oxide fuel cell technologies

Scientific organization
Institute of Solid State Physics
Academic degree
Junior Scientist
Scientific discipline
Chemistry & Chemical technologies
Novel materials with mixed ionic-electronic conductivity for solid oxide fuel cell technologies
One of the important challenges in developments commercially feasible intermediate temperature solid oxide fuel cells (SOFCs) relates to electrode materials and systems proving with sufficient stability and high electrochemical activity at operation conditions. The present work is centered on the studies of stability, oxygen nonstoichiometry, conductivity and ion transference numbers of Ce1-x-yLaxPryO2-δ, where the partial substitution of Pr4+/3+ cations for cerium makes it possible to improve p-type electronic transport under the SOFC cathodic conditions.
SOFC, protective interlayer, conductivity, oxygen permeability, Seebeck coefficient, cation interdiffusion, oxygen nonstoichiometry

Single-phase powders of Ce1-x-yLaxPryO2-δ (x=0.29-0.40, y=0-0.20) with submicron particle size were synthesized by the glycine-nitrate technique. Following firing at 1073 K and ball-milling in ethanol, final annealing of the powders was carried out at 1223 K in air for 4 h. Gas-tight ceramics (relative density >92%) were uniaxially compacted at ~100 MPa and then sintered at 1723 K for 20 h. The materials were characterized by XRD, SEM, and measurements of the total conductivity, Seebeck coefficient, oxygen nonstoichiometry, thermal and chemical expansions, transference numbers and steady-state oxygen permeation as function of the oxygen partial pressure and temperature. In order to evaluate chemical compatibility with solid oxide electrolytes, powder mixtures of (Ce,La,Pr)O2-δ and La0.8Sr0.2Ga0.8Mg0.2O3-δ or 8 mol.% yttria-stabilized zirconia (1:1 weight ratio) were fired at 1473-1623 K for 50-70 h. Model electrochemical cells with La0.8Sr0.2Ga0.8Mg0.2O3-δ solid electrolyte, (Ce,La,Pr)O2-δ interlayers and various perovskite electrodes were fabricated using screen-printing and annealing at 1373-1473 K, and characterized by impedance spectroscopy.

The results showed that, as expected, Pr doping leads to a higher p-type electronic conductivity under oxidizing conditions, whilst the ionic conductivity variations are determined by the [Ce]/([La]+[Pr]) ratio. The steady-state oxygen permeability is limited by the hole transport and, hence, correlates with praseodymium concentration. The variations of n-type electronic conductivity under reducing conditions, when most Pr cations are trivalent, can be described by classical defect models similar to those for ceria. XRD and SEM analyses of the reacted mixtures demonstrated that chemical interaction between (Ce,La,Pr)O2-δ, La0.8Sr0.2Ga0.8Mg0.2O3-δ and zirconia cannot be neglected, at least at temperatures above 1500-1550 K. The electrochemical cell fabrication temperature should therefore be minimized down to 1373-1473 K. Nevertheless, these temperatures are sufficient to produce porous interlayers with high mechanical strength and sufficient adhesion to the solid electrolyte membranes.