Experimental and Computational Studies of Oxide Ion Conductors

The work in this thesis focuses on the study of oxide ion dynamics with the aim to develop improved oxide ion conductors. As the main techniques used to achieve this were ab initio molecular dynamics (AIMD) and quasielastic neutron scattering (QENS), this combined approach is also the focus of the literature review in Chapter 1. Chapter 2 introduces the methods used for synthesis, characterisation, and further study of the materials studied. Chapter 3 investigates the effect of the dopant on the oxide ion dynamics in two doped $\delta$-Bi$_2$O$_3$ oxide ion conductors: Bi$_{0.852}$V$_{0.148}$O$_{1.648}$ and Bi$_{0.852}$P$_{0.148}$O$_{1.648}$. QENS allowed observation of nanosecond dynamics, corresponding to the diffusion of the oxide ions in the Bi-O sublattice via vacancy-hopping, and picosecond dynamics, corresponding to localised motion within the dopant sublattices. AIMD gave further insight into the different oxide ion dynamics in Bi$_{0.852}$V$_{0.148}$O$_{1.648}$ and Bi$_{0.852}$P$_{0.148}$O$_{1.648}$, showing that the flexibility of the V coordination environment plays an important role, creating additional vacancies in the Bi-O sublattice, consistent with the superior conductivity of the vanadate. Chapter 4 describes the systematic study of conductivity of the complex scheelite-type materials: Bi$_{3}$(BO$_{4}$)(B'O$_{4}$)$_{2}$ (B = Fe, Ga, Fe$_{0.9}$Ti$_{0.1}$; B' = Mo) as well as Bi$_3$(B$_{2}$O$_{8}$)$_{1/2}$(B'O$_{4}$)$_{2}$ (B = Sc, In; B' = Mo). Impedance measurements indicate that interstitial oxide ions are responsible for conductivity in these materials, and the conductivity of Bi$_{3}$(Fe$_{0.9}$Ti$_{0.1}$O$_{4.05}$)(MoO$_{4}$)$_{2}$ was found to be 1.5 $\times$ 10$^{-3}$ S cm$^{-1}$ at 800 $^\circ$C, which is comparable to the scheelite-type oxide ion conductor LaNb$_{0.92}$W$_{0.08}$O$_{4.04}$ Chapter 5 discusses the study of two hexagonal perovskites: Ba$_3$NbMoO$_{8.5}$ and Ba$_7$Nb$_4$MoO$_{20}$. Using variable temperature powder X-ray diffraction, the reversibility of the phase transition in Ba$_3$NbMoO$_{8.5}$ was demonstrated for the first time. QENS showed that oxide ion dynamics in both compounds are too slow to be observed on a nanosecond timescale. In Ba$_7$Nb$_4$MoO$_{20}$, AIMD revealed a continuous oxide ion migration pathway in the $ab$ plane, and moreover showed an important out-of-plane contribution to the long-range diffusion. This allowed suggestion of a new doping strategy to further enhance oxide ion conductivity. Chapter 6 discusses results obtained from the first AIMD simulations on a Dion-Jacobson phase oxide ion conductor, CsBi$_{2}$Ti$_{2}$NbO$_{10-\delta}$, revealing an important contribution of the O2 site to the long-range diffusion. This suggests that oxide ion migration occurs predominantly via an O1-O2-O1 pathway, demonstrating the importance of rotationally flexible octahedra for high ionic conductivity in this new family of oxide ion conductors

Oxide ion dynamics in hexagonal perovskite mixed conductor Ba 7 Nb 4 MoO 20: a comprehensive ab initio molecular dynamics study

Hexagonal perovskite Ba7Nb4MoO20-related materials are very promising solid electrolytes with high oxide ion conductivity and redox stability, making them potentially applicable in solid oxide fuel cells. Optimizing the properties of this family of materials necessitates atomic-level understanding of the oxide ion dynamics leading to high conductivity. Here we report extensive ab initio molecular dynamics simulations of Ba7Nb4MoO20 investigating oxide ion motions, which allowed the observation of a continuous diffusion pathway for oxide ions in the (ab) plane, but also revealed significant contribution of the oxygen atoms from crystallographic sites located outside this plane, to the long-range dynamics. To probe the timescale of oxide ion diffusion, complementary quasielastic neutron scattering experiments were carried out, and showed that oxide ion dynamics in Ba7Nb4MoO20, even at 950 °C, are too slow to be observable on a nanosecond timescale. Based on the atomic-level understanding of structure–property relationships afforded by this detailed computational study, we propose new materials design strategies with potential to significantly increase oxide ion conductivity in Ba7Nb4MoO20-related hexagonal perovskites, which target the simultaneous increase of the number of oxide ion charge carriers and rotational flexibility of the (Nb/Mo)Ox polyhedra

Oxide Ion Mobility in V- and P-doped Bi2O3-Based Solid Electrolytes: Combining Quasielastic Neutron Scattering with Ab Initio Molecular Dynamics

We report the direct observation of oxide ion dynamics on both nano- and picosecond timescales in the isostructural Bi2O3-derived solid electrolytes Bi0.852V0.148O1.648 and Bi0.852P0.148O1.648 using quasielastic neutron scattering. Comprehensive ab initio molecular dynamics simulations allowed us to reproduce the experimental picosecond timescale data by directly simulating the scattering function at various temperatures. Our analysis of the experimental data in conjunction with the simulations revealed the origin of the picosecond dynamics to be localized motions within the V–O and P–O sublattices, while nanosecond dynamics correspond to the diffusion of the oxide ions in the Bi–O sublattice via vacancy-hopping. This combined approach provides insight into the different oxide ion migration pathways and mechanisms in Bi0.852V0.148O1.648 and Bi0.852P0.148O1.648, with the flexibility of the V coordination environment playing an important role, consistent with the superior conductivity of the vanadate

Computational Insights into Dion–Jacobson Type Oxide Ion Conductors

Dion–Jacobson type materials have recently emerged as a new structural family of oxide ion conductors, materials important for applications in a variety of electrochemical devices. While some attempts to improve their ionic conductivity have been reported, a detailed understanding of the underlying oxide ion diffusion mechanisms in these materials is still missing. To explore the structure–property relationships leading to the favorable properties, we carried out ab initio molecular dynamics simulations of oxide ion diffusion in CsBi2Ti2NbO10−δ. Our computational study reveals significant out-of-plane dynamics, indicating that the dominant pathway for oxide ion migration is via jumps into and out of the (ab) crystallographic plane. This suggests that further improvement of oxide ion conductivity relative to CsBi2Ti2NbO10−δ could be achieved by enhancing the rotational flexibility of the coordination polyhedra located in the inner perovskite layer, thereby facilitating faster out-of-plane motions

Oxide Ion Dynamics in Hexagonal Perovskite Mixed Conductor Ba7Nb4MoO20: A Comprehensive Ab Initio Molecular Dynamics Study

Hexagonal perovskite Ba7Nb4MoO20-related materials are very promising solid electrolytes with high oxide ion conductivity and redox stability, making them potentially applicable in solid oxide fuel cells. Optimizing the properties of this family of materials necessitates atomic-level understanding of the oxide ion dynamics leading to high conductivity. Here we report extensive ab initio molecular dynamics simulations of Ba7Nb4MoO20 investigating oxide ion motions, which allowed the observation of a continuous diffusion pathway for oxide ions in the (ab) plane, but also revealed significant contribution of the oxygen atoms from crystallographic sites located outside this plane, to the long-range dynamics. To probe the timescale of oxide ion diffusion, complementary quasielastic neutron scattering experiments were carried out, and showed that oxide ion dynamics in Ba7Nb4MoO20, even at 950 oC, are too slow to be observable on a nanosecond timescale. Based on the atomic-level understanding of structure-property relationships afforded by this detailed computational study, we propose new materials design strategies with potential to significantly increase oxide ion conductivity in Ba7Nb4MoO20-related hexagonal perovskites, which target to simultaneously increase the number of oxide ion charge carriers and rotational flexibility of the (Nb/Mo)Ox polyhedra

A thermoelectrically stabilized aluminium acoustic trap combined with attenuated total reflection infrared spectroscopy for detection of Escherichia coli in water

Acoustic trapping is a non-contact particle manipulation method that holds great potential for performing automated assays. We demonstrate an aluminium acoustic trap in combination with attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) for detection of E. coli in water. The thermal conductivity of aluminium was exploited to thermo-electrically heat and hold the acoustic trap at the desired assay temperature of 37 degrees C. Systematic characterisation and optimisation of the acoustic trap allowed high flow rates while maintaining high acoustic trapping performance. The ATR element serves not only as a reflector for ultrasound standing wave generation but also as a sensing interface. The enzyme conversion induced by alkaline phosphatase-labelled bacteria was directly monitored in the acoustic trap using ATR-FTIR spectroscopy. Sequential injection analysis allowed automated liquid handling, including non-contact bacteria retention, washing and enzyme-substrate exchange within the acoustic trap. The presented method was able to detect E. coli concentrations as low as 1.95 x 10(6) bacteria per mL in 197 min. The demonstrated ultrasound assisted assay paves the way to fully automated bacteria detection devices based on acoustic trapping combined with ATR-FTIR spectroscopy.European CommissionFFG - Österr. Forschungsförderungs- gesellschaft mbH

An Acoustic Trap for Bead Injection Attenuated Total Reflection Infrared Spectroscopy

The final publication is available via https://doi.org/10.1021/acs.analchem.9b00611.Europäische Kommissio