Influence of Microstructure on Defect Chemistry - Transport - Chemical Strain Coupling of (Pr,Ce)O2-δ Nanoparticles

Abstract

Solid oxide fuel/electrolysis cells (SOFC/SOECs) are promising and environmentally-friendly technologies for meeting ever-increasing electricity and energy storage demands. Nanostructuring of SOFC/SOEC electrodes can enable improvement of the reaction kinetics and reduction of the operating temperature, owing to factors including the enlarged surface area, short diffusion lengths, and unique surface & interface properties. While the kinetic benefit is apparent, the effect of nanostructuring on other device-relevant physico-chemical properties cannot be neglected. Especially, the coefficients of chemical expansion (CCEs) are an important metric for mixed ionic-electronic conductors that can undergo changes in oxygen stoichiometry with variations in the surrounding atmosphere, temperature, and voltages during processing/operation. Chemical strains can cause cracking of monolithic parts during fabrication and delamination of electrodes from electrolytes in operation. CCEs are typically measured on bulk, polycrystalline materials with large particle or grain sizes. Therefore, it is important to understand the coupled transport and chemical expansion behavior in nano-ionic materials with large interface/surface density. This work aims to uncover the role of microstructure and nanoscale effects on the chemical strain and transport properties, through the lens of defect chemistry, with the goal of ultimately developing design principles for zero-strain materials with improved device lifetime and performance.Pr0.2Ce0.8O2- δ (PCO20) was selected as a model system and prepared as nanoparticles by a surfactant-free, low temperature co-precipitation method. (Pr,Ce)O2-δ compositions are valuable model oxide ion conductors to study, since both Pr and Ce can undergo changes of valence states under different temperature and pO2 regimes to accommodate oxygen stoichiometry excursions. Prepared nanoparticle samples were first in-situ sintered in the dilatometer at three different temperatures (600 ⁰C, 725 ⁰C, 850 ⁰C) to induce and evaluate microstructure evolution. The chemical strain and electronic/ionic conductivity were then studied on stable microstructures with different average particle sizes at lower temperatures by simultaneous in-situ dilatometry and electrochemical impedance measurement (EIS) upon changing pO2 at different isotherms (550 to 400 ⁰C). At each isotherm, all the samples expand as pO2 decreases. From the conductivity vs. pO2 measurements, the maxima at certain pO2 indicate the dominant polaron mechanism of the electronic transport, consistent with the measured conductivity activation energy. However, samples with lower sintering temperatures exhibit less pO2 dependence of the conductivity, which implies a transition in the mixed ionic/electronic conduction behavior toward higher ionic transference numbers as particle size decreases. Correspondingly, the chemical strains associated with losing oxygen monotonically decrease with the decrease of the sintering temperature, and hence the particle size. The chemical strain vs. oxygen stoichiometry dependence and corresponding CCEs were quantified as function of particle size and temperature by combining thermogravimetric analysis (TGA) with the dilatometry results and in-situ X-ray diffraction. The particle size, chemical composition, valence states, and morphology of samples after CCEs measurements were also compared using XRD, SEM, and S/TEM-EELS, with the latter indicating the expected differences between bulk and surface defect chemistry. Implications of the results for size-dependent defect chemistry and its influence on chemical strains and redox CCEs will be further discussed. Keywords: redox, chemical expansion, defect chemistry, mixed ionic electronic conduction, ceria, chemo-mechanics, nanoparticle, thermogravimetric analysis, dilatometry, impedance