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