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In this work, local environments of ions in solid oxide fuel cell (SOFC) electrolyte materials with perovskite and perovskite-derived crystallographic structures, i.e. Ba₂In₂O₅, Ba₂(In₁₋ₓGaₓ)₂O₅ and Ba₂In₂O₄(OH)₂, were investigated for their high ionic (O²⁻ and H⁺) mobility at elevated temperatures. Two general methods were employed in this investigation; first, computational methods, such as density functional theory (DFT), gauge including projector augmented wave (GIPAW), cluster expansion (CE) and Monte Carlo simulations (MC); second, experimental methods, such as nuclear magnetic resonance (NMR), X-ray scattering (both powder diffraction and pair distribution function (PDF) analysis) and thermo-gravimetric analysis (TGA). The parent material, Ba₂In₂O₅, has inherent oxygen vacancies which allow for fast O₂₋ ion mobility at elevated temperatures and for hydration of the material needless of doping. We improve a previous NMR study of Ba₂In₂O₅ by Adler et al. [1], assigning all three oxygen crystallographic sites to their relevant NMR peaks and investigate the high temperature structure. We then study the iso-valent doping of Ga into the In site resulting in Ba₂(In₁₋ₓGaₓ)₂O₅. While Yao et al. [2] find that Ga doping levels higher than 20% form a stable cubic structure, our findings indicate that Ga doping results in a phase segregation. However our findings for quenched samples are no different than those of Yao et al. [2]. Lastly we study the hydrated form of the parent material, Ba₂In₂O₄(OH)₂, which has high H⁺ ion mobility above 180°C. We observe at least three possible hydrogen sites with local environments slightly different from the previous neutron diffraction study by Jayaraman et al. [3]. In contrast to the observation by Jayaraman et al. [3] of the hydrogen presence in all O2 layers we find an alternating occupancy of hydrogens in those layers.

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This thesis described an investigation of charge carrier dynamics in dense, flat bismuth vanadate (BiVO4) photoanodes using transient absorption spectroscopy and photoelectrochemical measurements including transient photocurrents. Transient absorption spectroscopy was employed to probe directly the photogenerated charge carrier population change as a function of time from microsecond (μs) to second (s) timescales. Transient photocurrent measurements were used to monitor charge extraction under chopped light conditions. Photo-induced absorption spectroscopy was employed to investigate charge carriers under working photo-electrochemical (PEC) conditions. The transient absorption signals due to photogenerated holes in BiVO4 were determined through using electron/hole scavengers and applied electrical bias in a complete photoelectrochemical cell. In 'un-doped' BiVO4, photogenerated holes were found to absorb from 500 nm to 900 nm. The dynamics of photogenerated holes were studied as a function of applied potential and excitation intensity. The population of long-lived (milliseconds-seconds) holes increased with increasing the width of space charge layer as a function of applied potential. A recombination process in kinetic competition with water oxidation on these long timescales was found to limit the photocurrent amplitude and onset potential in un-doped BiVO4 photoanodes. Using transient photocurrent measurements, this recombination process was identified as recombination of surface-accumulated holes with electrons from the bulk of the semiconductor (back electron/hole recombination). Doping molybdenum (MoVI) in un-doped BiVO4 has been reported to be an effective method to increase photocurrent amplitude. Impedance measurements were carried out to determine the donor density increased by the presence of MoVI doping. The increased donor density limited efficient generation of the space charge layer to retard fast recombination on microseconds to milliseconds timescales, thus limiting the long-lived hole yield under modest applied potentials. MoVI dopants were shown to improve the electron transport determined by front/back side illumination in PEC and transient absorption spectroscopy (TAS) measurements. Cobalt phosphate (CoPi) surface-modified un-doped BiVO4 photoanodes were also studied using transient absorption spectroscopy and transient photocurrent measurements. Transient absorption spectra of CoPi-modified BiVO4 were similar to those of unmodified BiVO4, and the kinetics on milliseconds to seconds did not change in the presence of CoPi surface modification. Both results indicated that photogenerated holes in BiVO4 rather than CoPi species were monitored by transient absorption spectroscopy. However, the negative shift of photocurrent onset and increased photocurrent could be explained by efficient suppression of back electron/hole recombination in BiVO4 photoanodes. In terms of the function of CoPi water oxidation catalysts on BiVO4, photo-induced absorption (PIA) was employed to further study the CoPi/BiVO4 system. CoPi species oxidised by BiVO4 holes were observed in PIA measurements due to the high extinction coefficient of oxidised CoPi and significant hole accumulation generated by continuous illumination. However, these oxidised CoPi species did not appear to drive catalytic water oxidation, as evidenced by results from spectroelectrochemical measurements of CoPi/FTO electrodes; water oxidation still occurred via BiVO4, consistent with the transient absorption results. Therefore, I concluded that in the CoPi/BiVO4 system, CoPi did not act as a catalyst, although hole transfer to CoPi can take place.