No description available.

No description available.

No description available.

In an effort to design systems that harvest solar light and convert this into chemical energy, the primary aim of the work presented in this thesis was to develop complex metal oxide materials that were active photocatalysts under visible light. The existing methods for visible light incorporation into photocatalytically active materials are reviewed. Of these, metal to metal charge transfer (MMCT) between bimetallic surface- grafted assemblies was taken as particular inspiration. It was hypothesised that MMCT between metal centres within a bulk complex metal oxide could be similarly applied to yield photocatalytic ally active charge carriers. This approach takes advantage of the stability of bulk systems and the ability to tune the compositions of complex oxide materials. Moreover, it was proposed that MMCT between metal centres located on crystallographically distinct sites of a bulk material would aid charge separation and migration throughout the extended lattice. The optical properties of the RE2 Ti207 (RE = Y, La, Ce, Pr) and Ba2XTizM3015 (X = La, Ce, Pr, Nd, Bi; M = Nb, Ta) series, which include some novel cerium(III) titanates, revealed systematic changes in the electronic structures of these materials. These were rationalised with respect to the energy of Ln 4f states. The proposed electronic structures present the partial achievement of the bulk MMCT hypothesis, with optical transitions from occupied Ce 4f midgap states to the unoccupied primarily Ti 3d conduction band. These Ce3+ /rr" charge transfer materials were inactive photocatalysts, attributed to the presence the Ce 4f-based midgap states that facilitate charge recombination. The double perovskite CaCu3T40IZ, with A-site Cu2+ and B-site Ti4+ cations and whose dielectric properties have been studied extensively in the past, is an ideal candidate for the two site MMCT strategy. Here, the optical and photocatalytic properties, rationalised with the aid of DFT calculations, present the partial achievement of the bulk MMCT hypothesis. Sol-gel derived Pt-CaCu3 Ti4012 is an active photocatalyst toward the visible light photo-oxidation of model pollutants methyl orange (MO) and 4-chlorophenol (4CP). Optical spectra and product analysis show that these reactions proceed via more selective routes than the typical reaction over TiOz P25 under DV light. Interestingly, the products of 4CP photo-oxidation were shown to be dependent on the wavelength of incident light. Cu-doping of BizTiz07 was found to stabilise the pyrochlore structure with respect to the Aurivillius phase Bi4 Ti3012 and to impart significant visible light absorption. Sol-gel derived Pt-BiI.6Cuo.4 Tiz07 photo-oxidised MO under visible light via conventional band gap excitation, as determined by quantum efficiency measurements. In contrast, sol-gel derived Pt-B4 Ti3012 photo-oxidised MO via the excitation of adsorbed MO, and was also active toward 4CP photo-oxidation under visible light. The excitation method, mechanisms and product distributions have been investigated for each of the photo-oxidation reactions presented in this thesis. In particular, the photo- oxidation of MO over some Pt-modified metal oxides has been shown to proceed via excitation of adsorbed MO and not of the semiconductor. Additionally, the mechanism and products of these processes are far more selective than the related DV reactions over TiOz P25, and have been shown to depend to some extent on the semiconductor support.

This thesis examines the performance of a building-integrated 10X photovoltaic concentrator proposed within the EU Framework-5 CONMAN project. The concentrator behaves as a window or skylight, allowing diffuse light to enter the building, whilst using mirrored slats to capture direct sunlight for conversion to electricity. The underlying logic is that by building integration, the proposed design can be economically and environmentally preferential to standard building-mounted PV panels in situations where the EU Energy Performance of Buildings Directive applies. A survey of building-integrated concentrator systems was conducted and indicated this to be a novel system. A technique of pressing to form the parabolic mirrors amenable to mass production was developed, in addition to a novel tracking system. Tests on BP LSBG cells indicated an approximate cell efficiency of 19% at the concentration levels expected. These design features in combination indicated that the cost of the system could be kept reasonably low, and a detailed design, called the 'Venetian Blind' was developed, constructed and tested. A biaxial model of the collector was developed, and a Visual Basic programme developed to simulate its output. Good correlation was found between the system model and the test data. When used to simulate annual output for the system in climates typical of the south of Spain, the model indicated that the system produced electricity at an approximate module cost of $8/equivalent Wp when not building integrated, and $5/equivalent Wp when building integrated. The system incurred longer energy payback periods than flat plate PV, but not substantially so when building integrated. Overall, 1 the optimised system is projected to produce electricity for 0.3$/kWh at good sites. Note that the system's potential contributions to-the passive solar gain of a building are likely to be very substantial, in addition to its electrical output.

The chemistry of systems intended to produce metal-doped, nanostructured oxide materials containing titaniumffV) has been investigated. Manganese and iron were chosen as cheap, sustainable metal atom dopants with a view to possible use of product materials in electrochemical devices. In particular, the aim was to enhance electronic conductivity and to introduce psudocapacitance for investigation in supercapacitor devices. Previous literature in this area was found to be misleading; products of earlier synthetic studies have been incompletely characterised, with solid products being assumed to be single crystalline phases and metal atom contents being assumed on the basis of hypothetical doping levels rather than being determined analytically. Electrochemical properties of product materials in electrode films were determined in this study via use of EIS, CV and galvanostatic charge- discharge techniques. Conventional sol-gel and micro-emulsion synthetic methods were undertaken with the view to control of product particle sizes and yields for single phase, Mn-doped titaniumfl V) oxide (ideally within the anatase structure). Other than for the pure binary oxides of manganese or titanium, the products obtained were mixtures of oxides rather than single phases, with titaniumfl V) in the rutile polymorph within the I mixed-phase products. "Doped" product materials produced by the emulsion method showed no improvements in conductivity or specific capacitance when compared with a standard anatase (undoped) sample. The mixed-oxide materials produced by the sol-gel method, containing Mn02, had enhanced conductivities but no improvement in specific capacitance. Single-phase, anatase structured, Mn-doped titanium dioxide materials, were prepared using the continuous hydrothermal synthesis (CHS) method. The maximum amount of Mn successfully incorporated into the .anatase structure was found to be approximately 8%wt, as determined by AAS, ICP-MS and SEM-EDX analytical methods; high Mu-content resulted in phase segregation and irreversibility within cyclic voltammograms, as evident in observation of an oxidation peak but no reduction peak. The highest recorded specific capacitance value for these samples was that for the CHSMn008 sample (theoretical composition Tio.92Mno.os02), with a value of 4.5 F g'l (oxide specific surface area, 34.8 m2 g·l). This compared to a similarly prepared anatase (undoped) material with a specific surface area of 48.6 m2 g'l and a specific capacitance of 2.5 F g·l. No correlation between specific surface areas and specific capacitance values was observed, but the BET-determined ·specific surface areas were for powdered samples whereas capacitances refer to composite electrode films. Chapter 1 Introduction • ", The initial eHS product materials displayed no improvement in conductivity, but p H-adjusted synthesis led to samples (denoted CHS MnKOH) with higher conductivities consequent on higher Mn contents, and these samples also had the highest specific capacitance values. In view of the success of the eHS method in producing Mn-doped anatase materials, it was also used to synthesise Fe-doped titaniumffV) oxides. The products were a range of single phase materials, but syntheses aimed at high Fecontents led to mixtures, Ti02 + Fe oxides + Fe2Ti05; the maximum amount of Fe incorporated whilst still maintaining a pure single phase product was approximately 7%wt. The conductivity of these materials showed marked improvement relative to the standard (undoped) anatase sample, comparable to those for eHS MnKOH samples, but the Mn-doped samples had higher specific capacitances than Fe-doped counterparts . . The highest specific capacitance values recorded were between 25 and 32 F g'l, regardless of dopant (MnlFe) and/or phase purity, and were associated with a high specific surface area or a high concentration of "dopant" within the sample. 1

Polymer electrolyte membrane fuel cells (PEMFC) and polymer electrolyte membrane water electrolysers (PEMWE) are promising electrochemical energy conversion devices. This thesis describes research carried out on high temperature PEMFC and PEMWE. High temperature (> 100 QC) operation is one of the most topical research trends of PEMFC and PEMWE, because of the operational and kinetic advantages it can provide. In this research, an anhydrous solid electrolyte, Sb-doped SnP207 was prepared and characterized. The synthesis parameters, microstructure, and conductivity of Sb-doped SnP207 were studied. The Sbo.2Sno.gP207 exhibited good conductivity (0.01-0.l S cm") in the temperature range 100-300 QC, and was initially applied in PEMFCs at intermediate temperatures (200-300 QC). Research into development of polymer acid complex membranes for high temperature PEMFCs, including phosphoric acid and sulphuric acid - doped polybenzimidazole membranes, was also carried out. The maximum power density of high temperature PEMFCs achieved was 0.5-0.7 W cm-2. For high temperature PEMWEs, firstly, Sn and Ir stabilized RU02 nanopartic1es were synthesized and studied as catalysts for the oxygen evolution reaction (OER). Iro.7RUo.302 nanopartic1es were found to be the most stable and active catalysts for OER. Research to develop a catalysts coated membrane (CCM) method for PEMWE membrane electrode assembly (MEA) fabrication is described. A high performance PEMWE was developed by studying fabrication parameters of the CCM method and optimizing electrode compositions of MEAs. High temperature (> 1 00 QC) PEMWE was eventually realized, using a perfluorinated silica composite membrane. The voltage achieved at 1 A cm-2 current density of PEMWE was 1.51 V at 140 QC and 4 bar pressure. The final part of this thesis describes work on catalyst support materials, which are essential for reducing noble metal loading in PEMWEs. Although the costs of PEMFCs and PEMWEs are still high, which is due to noble metal catalysts and expensive membranes, high temperature operation (> 100 QC) can help both devices be more competitive for energy conversion applications.

Using first-principles density functional calculations, LiNiO2-related cathode materials are studied. It is found that in contrast to previous studies, the hole state in Li doped NiO shows predominately Ni character and is accompanied by a local Jahn-Teller distortion. We show that this is consistent with experiments. A new potential ground state LiNiO2 cell is found in which charge disproportionation Ni3+àNi2++Ni4+ occurs. However another cell in which the Jahn-Teller distortions of Ni3+ octahedral are in a zigzag ordering, is close in energy. Therefore we suggest that in real LiNiO2 samples, the two phases coexist. This explains the absence of long range ordering in LiNiO2. Rock-salt LiMO2 compounds crystallise in three different structures depending on the cation ordering. We show that this cannot be explained by the size effect and propose that the exchange interaction between M ions is responsible for the ordering. Both size difference between Li and M and the exchange interaction between nearest-neighbouring M ions favour the layered structure, whereas the exchange interaction between second-nearest-neighbouring M ions destabilises the layered structure. The defect formation energies are low in LiNiO2, consistent with the difficulty to synthesise truly stoichiometric LiNiO2. The tendency for Ni to be present in the Li layers can be explained by super-exchange interactions. Therefore with Co substitution for Ni, the nonmagnetic Co ions screen these interactions and destabilise the presence of Ni in the Li layer. The same effect is found with Al substitution from our calculations. We also show why substitution of Ni by Mn increases the concentration of the interlayer mixing defects worse compared to LiNiO2. In addition, a correlation between the oxygen charge and the defect formation of oxygen vacancy is found. It appears that the lower the effective oxygen charge, the smaller the defect formation energy.

No description available.

No description available.