Transport properties of mixed ionic and electronic conductors - from bulk to nanostructure

Ceria-based materials exhibit mixed ionic and electronic conductivity due to the redox-activity of the cation (Ce4+/Ce3+) and the oxygen ion mobility in the fluorite-type lattice, which intrinsically tends to form a high concentration of Anti-Frenkel defects. Both electrons and ions migrate by an activated hopping mechanism with activation barriers of 0.4 eV for the hopping process of small polarons, 0.8 eV and up to 1.6 eV for the migration and the extrinsic formation and migration of oxygen vacancies, respectively. This leads to a p(O2)-sensitive electrical conductivity, which can be dominated by each process depending on temperature and oxygen activity. Moreover, the material is quite tolerant regarding the substitution of cations. By choosing the type and range of substitution, electrical properties can be adjusted in different ways. Therefore, ceria and its substituted analogues qualify for various applications as solid electrolytes in oxygen membranes, electrode material in solid oxide fuel cells (SOFCs) and in combination with the high oxygen storage capacity for support material in heterogeneous catalysis. The defect chemistry of ceria is already extensively investigated in literature. Thus the material is an ideal model system to study interface effects, in particular the concentration and type of electronic and ionic defects as well as their transport properties in the vicinity of interfaces, complementing the established defect chemical models for bulk material. Within this work we compare the solid solution of ceria and praseodymia (Ce1-xPrxO2-δ) with the solid solution of ceria and zirconia (Ce1-xZrxO2-δ). It is already known, that due to the redox-activity of Pr-ions the combination with praseodymia can lead to an additional transport of polarons, increasing the electronic contribution to electrical conductivity. In contrast, the combination with isovalent zirconia results in an increase of the so-called reducibility of ceria due to the size mismatch of the cations. To gain a deeper understanding of the role of these substitutions on electrochemical transport processes at interfaces, highly ordered mesoporous thin films of Ce1-xPrxO2-δ (CPO) and Ce1-xZrxO2-δ (CZO) have been investigated. The mesoporous samples have been synthesized by a sol-gel process using evaporation induced self-assembly, resulting in a regular pore structure surrounded by a closed packed, interconnected 3D architecture of nanocrystallites. The structural properties were analyzed by SEM, WAXD, XRD, XPS and Raman spectroscopy, confirming the successful synthesis of a mesoporous material of high structural quality, where the surface dominates over the bulk behaviour. The electrical properties were investigated as a function of temperature and oxygen partial pressure using electrochemical impedance spectroscopy. The comparison of the results with previous results of single crystalline samples elucidates the effect of the continuous pore structure on electrical transport properties. The CZO thin films show an unusual p(O2)-dependence at high oxygen partial pressures, which cannot be explained by standard defect chemical models. Furthermore, both mesoporous samples reveal a conductivity plateau under strongly reducing conditions, which is discussed in terms of hopping statistics and defect-defect interaction

DFT+U studies including spin-orbit coupling - a case study for f-electrons in praseodymium-doped ceria

The mixed ionic electronic conductor Ceria exhibits not only a high concentration of Anti-Frenkel defects with high mobility, resulting in ionic conductivity of oxygen ions, but also enables an additional electronic conduction mechanism in form of small polaron hopping between the f-states of the cations. This promotes the reversible exchange of oxygen with the surrounding atmosphere and thus the oxygen storage capacity of the binary oxide CeO2-δ. The material has been established as a model system to describe both ionic and electronic transport processes in bulk material to gain deeper insights into the characteristics of polaron hopping and defect-defect interactions in mixed conductors. By introducing the redox active lanthanide Praseodymium to the Ceria host lattice, both electronic and ionic conductivities are increased in temperature and oxygen partial pressure regions where pure Ceria lacks of good performance. The redox properties of Pr-ions, shifting the equilibrium from Pr4+ to Pr3+ and forming oxygen vacancies, is key to understand the additional contribution to the total electrical conductivity and the enhanced catalytic activity. So far in literature, only the effect of Pr3+-ions in the Ceria host lattice has been investigated by means of density functional theory. To complement these investigations with the impact of Pr-ions in both oxidation states, density functional theory was applied, including a Hubbard-U correction for electronic correlation in the f-states of both cations in Ce1-xPrxO2-δ. A systematic study of spin polarization, antiferromagnetic coupling and spin-orbit interaction of the unpaired 4f-electrons was performed to investigate the influence of magnetic interactions on the description of localized polarons. The preferred localization of the excess electrons on Pr- rather than Ce-ions as well as the defect formation and configuration is discussed by analyzing the resulting energy levels and densities of states of the investigated ideal and defective super cells

Tackling neural architecture search with quality diversity optimization

Neural architecture search (NAS) has been studied extensively and has grown to become a research field with substantial impact. While classical single-objective NAS searches for the architecture with the best performance, multi-objective NAS considers multiple objectives that should be optimized simultaneously, e.g., minimizing resource usage along the validation error. Although considerable progress has been made in the field of multiobjective NAS, we argue that there is some discrepancy between the actual optimization problem of practical interest and the optimization problem that multi-objective NAS tries to solve. We resolve this discrepancy by formulating the multi-objective NAS problem as a quality diversity optimization (QDO) problem and introduce three quality diversity NAS optimizers (two of them belonging to the group of multifidelity optimizers), which search for high-performing yet diverse architectures that are optimal for application-specific niches, e.g., hardware constraints. By comparing these optimizers to their multi-objective counterparts, we demonstrate that quality diversity NAS in general outperforms multiobjective NAS with respect to quality of solutions and efficiency. We further show how applications and future NAS research can thrive on QDO

Mesoporous hollow carbon spheres for lithium-sulfur batteries : distribution of sulfur and electrochemical performance

Hollow carbon spheres (HCS) with a nanoporous shell are promising for the use in lithium–sulfur batteries because of the large internal void offering space for sulfur and polysulfide storage and confinement. However, there is an ongoing discussion whether the cavity is accessible for sulfur. Yet no valid proof of cavity filling has been presented, mostly due to application of unsuitable high-vacuum methods for the analysis of sulfur distribution. Here we describe the distribution of sulfur in hollow carbon spheres by powder X-ray diffraction and Raman spectroscopy along with results from scanning electron microscopy and nitrogen physisorption. The results of these methods lead to the conclusion that the cavity is not accessible for sulfur infiltration. Nevertheless, HCS/sulfur composite cathodes with areal sulfur loadings of 2.0 mg·cm$^{-2}$ were investigated electrochemically, showing stable cycling performance with specific capacities of about 500 mAh·g$^{-1}$ based on the mass of sulfur over 500 cycles

Synthesis and characterization of polyphosphazene electrolytes including cyclic ether side groups

This paper presents the synthesis and detailed characterization of two polyphosphazene based polymers, including different cyclic ether side groups. The final polymers were obtained by a well-known method employing a living cationic polymerization and subsequent nucleophilic substitution. The synthesized polymers Poly [(1,3-dioxane-5-oxy) (1,3-dioxolane-4-methoxy)phosphazene] (DOPP) and Poly[bis(2-Tetrahydro-3-furanoxy)phosphazene] (THFPP) were mixed with varied amounts of lithium bis(trifluoromethane)sulfonamide (LiTFSI) and the interactions between the salt and the polymer chains were studied by Fourier transform infrared (FT-IR) and differential scanning calorimetry (DSC) measurements. Electrochemical characterization was performed by electrochemical impedance spectroscopy (EIS) and direct current polarization in the temperature range of 20–60 °C. These measurements were utilized to calculate the lithium transference number (t+), the lithium conductivity (σ) and its activation energy in order to elucidate the lithium transport behavior. Relatively high lithium transference numbers of 0.6 (DOPP) and 0.7 (THFPP) at 60 °C are found and reveal maximum lithium conductivities of 2.8·10−6 S⋅cm−1 and 9.0·10−7 S⋅cm−1 for DOPP and THFPP at 60 °C, respectively

Off-stoichiometry, Vacancy Trapping and Pseudo-irreversible First-cycle Capacity in LiNiO2

We demonstrate that the ubiquitous off-stoichiometry of LiNiO2 in the form of Li_{1-z}Ni_{1+z}O2 slows the kinetics of the material both by diminishing the number of charge carriers and increasing the length of diffusion paths. Excess Ni in the Li layer, Ni_{Li}, exerts an attractive potential on Li vacancies, lowering their energy with respect to defect-free regions. This attractive field extends over a radius of two lattice sites and also considerably lowers the barrier for a Li vacancy to approach the defect, effectively making Ni_{Li} a sink for lithium vacancies. A similar argument can be made for divacancies, which are split by Ni_{Li} and pinned in the form of single vacancies. In addition to pinning effects, which could vary depending on the state of charge, Ni_{Li} also constitutes an obstacle to Li migration, because it is rather immobile and does not undergo site-exchange with an adjacent Li vacancy