This research project is focused on the process optimisation and optical enhancement of the hydrogenated amorphous silicon solar cell design, achieved by the incorporation of light scattering plasmonic nano-particles. These treatments consist of a very thin layer of finely tuned silver metal-island films, which preferentially scatter light within a wavelength range tailored to the device absorption characteristic. This serves to increase the optical path length without the need for surface texturing of the semiconductor material. Within this study, the PECVD process is used to explore the parameter space and fabricate silicon thin films with excellent optical and electrical performance, and a P-I-N amorphous silicon device structure is fabricated with a high performance of 6.5% conversion efficiency, 14.04mA/cm2 current density and 0.82V open circuit voltage. The effects of metallic nano-particle arrays is demonstrated by numerical simulation, showing that variations in particle size, shape, position within the structure and surrounding material greatly influence the enhancement of the nano-particles on silicon absorber layers, and that particles positioned at the rear of the device structure adjacent to a back reflector avoid absorption losses which occur below the particle resonance frequency when such structures are positioned at the front surface. It is shown than an improvement in optical absorption of just over 1% is possible using this method. Silicon thin films are fabricated with self-organised nano-particle arrays via means of annealed metal films, positioned at the front or back adjacent to a metallic reflector, and measurements of optical transmittance, reflectance and absorption are taken. The optimum annealing temperature and duration is identified, and it is shown that these variables can greatly affect the absorption of the device stack. To conclude the study, an amorphous silicon P-I-N photovoltaic device is fabricated featuring self-organised nanoparticle arrays within the back reflector, and a modest improvement of energy conversion efficiency is observed with scope for further optimisation and enhancement.

One of the technological problems associated with the lead-acid battery is the self-discharge of the negative plate as a result of the deposition of antimony onto the sponge lead electrode, a process known as 'Antimony Poisoning'. The rate of deposition (III) from aqueous sulphuric acid onto pure lead electrodes was measured using a tracer technique. The deposition rate was found to be independent of the hydrogen overpotential of the electrode and was ascribed to the electrochemical displacement reaction 2Sb0⁺ 3Pb + 4H⁺ + 3SO₄ = 2Sb + 3Pb SO₄ + 2H₂0. The results obtained were assessed in conjunction with published data, and a comprehensive picture of the various antimony (III) and antimony (V) reaction paths occurring in the lead-acid battery has been presented. The placing of a suitable ion-exchange material between the separator and the negative plate, with the object of removing antimony (III), could be a feasible method of limiting the problem of 'Antimony Poisoning'.

Microbial fuel cells (MFC) are devices that convert chemical energy in soluble organic matter into electrical energy. They can be used for wastewater treatment coupled with energy production as well as for sensing, hydrogen production, electrosynthesis and metal recovery. Implementing these technologies is hindered by low current production. Currently, little is known about anodic communities regarding growth, electrode coverage, bacterial composition, biofilm structure, metabolism and how are they affected by operational factors. Such knowledge is needed to engineer MFCs that can overcome current limitations. The subject of the present study is the mixed-community bioanode. The effects of light, anode-tocathode surface ratio (A/C), substrate composition and anode potential on bioanodes were investigated. Two types of substrates were used: the first was based on sodium acetate and the second was a synthetic wastewater which simulated the chemical composition of real wastewater. First bioanodes were studied in presence and absence of light. A different set of bioanodes were grown at 9 different A/C ratios in single-chamber MFCs. Another set of bioanodes were grown in half-cells at 3 different anode potentials (-400 mV, -200 mV and 0 mV vs Ag/AgCl). The development of anodic biofilms and their long-term dynamics were investigated using a multi-anode reactor which allowed for better replication of running conditions. Geobacter was identified in all bioanodes but its abundance was highly variable and dependent on running conditions. Over time the bacterial composition of bioanodes under constant conditions continuously changed during the first 33 days but stabilised by the 67th day. Bioanodes fed on acetate had higher cell counts, Geobacter percentage, and current output than bioanodes fed on synthetic wastewater. Light exposure decreased coulombic efficiency by almost 14 times and favored growth of Rhodopseudomonas species in the detriment of Geobacter. Abundance of Geobacter increased with anode potential when fed on acetate (from 609.98 106 cells/gram at -400 mV to 5212.38 106 cells/gram at 0 mV) but decreased when fed on synthetic wastewater (from 200.6 106 cells/gram at -400 mV to 49 106 cells/gram at 0 mV). Current density and Geobacter density decreased by an order of magnitude when A/C ratio was varied from 1:12 to 1:1 but remained relatively constant when A/C was increased further to 8:1. Uneven biomass coverage on bioanodes and a decrease of biofilm volume with depth inside bioanodes were observed suggesting that anodes were only partially used by electrigenic bacteria. Results reported here have important implications for future reactor designs, on the use of three-dimensional bioanodes and on long-term applications of Microbial Fuel Cells.

Several novel mixed ionic-electronic conductors (MIECs) based on the oxygen interstitial diffusion mechanism have been developed and studied for the past decade. This kind of materials has the potential to be used as the cathode of intermediate temperature solid oxide fuel cell (IT-SOFC) systems. Among them, the La2NiO4+δ Ruddlesden-Popper (RP) and the CeNbO4+δ fergusonite materials attract particular attention due to their interesting properties. These materials are capable to take extra oxygen and become oxygen ion conductor without any aliovalent doping. To understand more about their catalytic and diffusion behaviours, atomistic computer simulations using both classical pair potentials and density functional theory (DFT), as well as classical thermodynamics calculations, have been carried out on these materials. The La2NiO4+δ material has recently been discovered to exhibit a La-only surface. This observation disagrees with previous theoretical study which suggest a Ni-terminated surface and challenges the traditional belief that the oxygen reduction takes place on La2NiO4+δ's exposed Ni sites. Our hybrid DFT surface energy calculation suggests that the most stable (001) La2NiO4 surface is La terminated. Previous theoretical surface study's results may not appropriately reflect the room temperature La2NiO4 sample. To explain the La-only surface observed on the polycrystalline La2NiO4+δ samples, a thermodynamic decomposition based hypothesis is proposed. The evaluation of La2NiO4's stability with respect to the higher ordered RP materials (La3Ni2O7, La4Ni3O10 and LaNiO3) shows that the decomposition of La2NiO4 is thermodynamically favourable and supports our hypothesis. The defective La-Ni-O RP material's decomposition thermodynamics are estimated with the help of hybrid DFT calculations and agree with our hypothesis too. The decomposition hypothesis predicts a Ni-enriched subsurface layer and agrees with the experiments. The La vacancy formation energy and diffusion barriers are calculated with hybrid DFT methods, as these results may provide hints on why the thermodynamically favourable decomposition only limits to the surface of La2NiO4. Before these calculations, different input La2NiO4 phases and magnetic orderings are evaluated to check their impacts on simulation results' reliability. We have shown that the choice of input structure and magnetic ordering will have significant effect on the simulation results. Appropriate choice of the input setups is therefore very important to the quality of the simulation. Based on the evaluation, La2NiO4's room temperature Fmmm phase and gx type anti-ferromagnetic ordering are selected for the DFT calculations. The La vacancy formation energy and diffusion barrier calculated are compared to the oxygen defect formation energy and related diffusion barrier. Possible relations between the La2NiO4 decomposition reaction and the La vacancy's formation/diffusion energetics are also discussed. Similar to the La2NiO4+δ, the CeNbO4+δ fergusonite material has a range of different oxygen enriched phases and is able to conduct oxygen ions. Its characteristic complex modulated structures also attracts much attentions. The details of its modulated oxygen-rich structures remained unclear for a long time. Recently the CeNbO4.25 modulated structure has been fully solved and opens opportunity for further theoretical studies. Molecular dynamics simulations using classical pair potentials are carried out on the stoichiometric CeNbO4 and the modulated CeNbO4.25 materials to study the oxygen diffusion pathways for the first time. The calculated oxygen ions' mean-squared displacements clearly show that the stoichiometric CeNbO4 exhibits no oxygen self-diffusion up to 1773 K. In addition, the oxygen diffusion coefficients in the CeNbO4.25 are calculated and plotted over a range of temperatures. Three regions with different oxygen diffusion behaviours are found from the graph and the oxygen diffusion activation energies are calculated for each of the regions. The differences and similarities between the computed oxygen diffusion behaviours and the experimentally observed oxygen diffusion behaviours are explored and discussed. Finally, the oxygen diffusion trajectory snapshots are taken from the MD simulation. The exact oxygen diffusion pathways and possible diffusion anisotropicities of the CeNbO4.25 material are analysed and discussed based upon the diffusion trajectories.

Conventional electricity generation, transmission and distribution infrastructures are AC systems, which combine to form an AC grid. Supply security can be increased by interconnecting AC grids. Additionally, there is an increasing reliance on remote asyn-chronous distributed generation from bulk renewable energy sources. High Voltage Direct Current (HVDC) systems allow asynchronous AC networks to be connected, and are able to transmit bulk power over longer distances with a lower total cost-than High Voltage Alternating Current (HVAC) transmission, or are perhaps the only option. At both ends of a HVDC link, a converter station is required to convert from AC to DC, or vice versa. Originally, these converters were Current Source Converters (CSCs), and later two- or three-level Voltage Source Converters (VSCs).

The awareness of global climate change by emissions of greenhouse gases from fossil fuel combustion is widely known by current society. Polymer Electrolyte Fuel cell (PEFC) technology has been a very promising clean technology with high efficiency that has been used in a wide range of portable, automotive and stationary applications. The fuel cell research has been developing very rapidly and successfully in the last few years. However, some issues remain largely unresolved, namely water management and high cost of the PEFC component. One of the efficient and cost-effective ways to improve the design of the PEFC and consequently resolve the above mentioned issues is through modelling. However, the built PEFC models need to be fed with accurate transport coefficients to enhance their productivity. One of the most important transport coefficients is the gas permeability of the PEFC porous media which highly affects the convective flow. Therefore, in this thesis, thorough experimental studies have been conducted to investigate the gas permeability of gas diffusion media used in PEFCs. The focus has been on the effects of the following on the gas permeability of the gas diffusion layers (GDLs): (i) type of carbon black used in the microporous layers (MPL) attached to the GDL, (ii) carbon and polytetrafluoroethylene (PTFE) loading, and (iii) the thickness of the MPL. Further, a novel method has been proposed to estimate the penetration of the MPL into the carbon substrate (i.e. the GDL before being coated with the MPL ink). Also, the effect of sintering on the gas permeability of the MPL has been investigated for the first time.

High temperature co-electrolysis of carbon dioxide and steam may provide an efficient, cost effective, and environmentally friendly production of syngas from curtailed wind energy. To achieve cost competitive high performance (e.g. with minimum internal resistance) electrolysis cells, it is critical to develop materials and cell configuration optimal for coelectrolysis. In addition, a cost-effective fabrication procedure is important in allowing broader commercialisation of Solid Oxide Electrolysis Cells (SOECs). The initial part of this work emphasises on the feasibility of SOECs plant for converting curtailed wind energy to syngas to enhance the grid flexibility. We first obtained operating parameters for the conversion plant based on the most recent literature data on the performance of high temperature co-electrolysis for syngas production. In addition, an evaluation of the interaction between variable generation and typical electricity demand patterns was presented; and, limitations in the flexibility of traditional electric generators were considered. Furthermore, in a projection of wind generation made for 2020, we estimated the maximum power value of the curtailment wind profile to be 23.9 GW. It was remarked that the cost increase for constraining wind in future could make SOEC conversion technology more commercially attractive. An estimation of the total investment costs for grid connected electrolysis system was made by considering the share of operating cost. The share of electricity price in the total cost of syngas production was estimated to be 61%. It was shown that using cost effective electricity could significantly reduce the syngas production price. The total investment costs for grid connected electrolysers were projected to be 0.38 M£/MW in 2020. It was highlighted that the scope of electrochemical conversion of CO2 to fuel offers flexible demand that is not yet sufficiently understood. There are still technical barriers that need to be addressed in the field of manufacturing processes, grid integration and system operation. A key factor in operating solid oxide electrolysis cells (SOECs) is the ability to provide a sufficiently high level of oxide ion conduction through the electrolyte in the cell. Commonly, high performance cells use Y-stabilised ZrO2 (YSZ) or Gd-doped CeO2 (GDC10). Whilst GDC10 has higher oxide ion conductivity than YSZ, it suffers from electronic conduction due to the partial reduction of Ce4+ to Ce3+ during operation at high temperature and low oxygen partial pressure environment. Here we describe the fabrication of a bilayer GDC10/8YSZ electrolyte support using tape casting and single step co-sintering. A cost effective fabrication procedure is important in allowing broader commercialisation of Solid Oxide Cells for fuel cell/electrolysis applications. A bi-layer 8YSZ/GDC electrolyte is suggested as an effective solution to avoid ceria reduction in a fuel (reducing) environment, thereby preventing current leakage across the electrolyte, while maintaining high oxide ion conduction. Bilayer zirconia/ceria processing has proven problematic due to thermochemical instability at high sintering temperatures. We first prepared and optimised the slip formulations for tape casting process, this was necessary to achieve high green density and uniform tapes. Furthermore, the shrinkage profile of the two bulk materials in bilayer electrolyte were matched using a Fe2O3 sintering additive. Additions of 5 mol% of Fe2O3 in the GDC layer and 2 mol% of Fe2O3 in the YSZ layer prevents delamination during co-sintering. The addition of Fe2O3 promotes densification behaviour, enabling achievement of a dense bilayer (~90%) at a reduced sintering temperature of 1300 °C; ~ 150 °C below conventional sintering temperatures. Bilayer 8YSZ/GDC10 electrolytes with relative thickness of 73/154 μm was successfully fabricated by tape casting and low-temperature co-sintering at 1300 °C. No significant microstructural defects or delamination were observed after co-firing The effect of the Fe2O3 sintering aid on the crystal structure of two bulk materials used in bilayer electrolyte were investigated by X-ray diffraction. Results showed that, both materials with Fe2O3 additions maintain their fluorite structure. The analysis revealed a reduction in unit cell volume for both Fe2O3-doped samples. While using Fe2O3 sintering aid was found to improve the sinterability of the two bulk materials, increasing the dopant concentration above the solubility limit leads to the formation of an iron rich phase, which was subsequently analysed by energy-dispersive X-ray spectroscopy. Elemental analysis at YSZ/GDC interface revealed asymmetric elemental diffusion behaviour when using Fe2O3 to co-sinter YSZ/GDC bilayers, with lower diffusivity of Zr and Y ions in the GDC layer compared to that of Ce and Gd ions detected in the YSZ layer, showing the positive effect of Fe2O3 on limiting the interdiffusion behaviour. Electrochemical impedance measurements in air revealed the total conductivity of the Fe2O3 containing bilayer electrolytes increased by an order of magnitude compared to Fe2O3-free bilayers. This was attributed to two factors; first, by limiting the overall elemental interdiffusion length from ~15 to ~5μm and, second, by achieving better contact between the YSZ and GDC layers and higher sintered density when using a Fe2O3 additive as a sintering aid. The cost-effective low-temperature processing technique presented in this study is expected to help widen the material selection and resolve the thermochemical issues associated with high-temperature co-sintering allowing a broader commercialisation of SOECs.

Solid oxide fuel cells are a clean, attractive and highly efficient alternative to traditional methods of power generation. However, their high operating temperature prevents their widespread use, as it makes them prohibitively expensive and causes stability issues. In response to this, new materials which exhibit fast oxide ion conduction at low-intermediate temperatures have gained considerable research interest. In this thesis, atomic scale computational modelling techniques have been employed to investigate defects, dopants and oxide ion conductivity in the A2BO4 materials; Cd2GeO4 and Ba2TiO4. Calculations suggest that these materials' parent structures are poor oxide ion conductors due to their highly unfavourable intrinsic defect formation energies of 3.00-10.56 eV defect-1. Results also indicate that oxide ion interstitials can be formed in Cd2GeO4through trivalent doping of the Cd sites. In Ba2TiO4, monovalent and trivalent doping of its Ba and Ti sites respectively induces the formation of oxide ion vacancies. In both materials, strong dopant-oxide ion defect associations are present. Interestingly, only Cd2GeO4 shows enhanced oxide ion migration upon doping, the defects in Ba2TiO4 being effectively immobile. This suggest that the oxide ion vacancies are more intensely associated with their causal dopant ions. With an average migration barrier of ~0.79 eV, oxide ions diffuse in Cd2GeO4 via a 'knock-on' mechanism down the a-axis and a stepwise mechanism along the c-axis. Despite this, defect trapping confines the interstitials to the dopant rich regions of the cell, resulting in poor oxide ion diffusion on the order of 1x10-8 cm2 s-1 at 1273 K. Generally, defects are found to be more stable in the α'-phase of Ba2TiO4, suggesting, in agreement with experiment, that they are likely to stabilise the α'-phase at reduced temperatures. Subsequent investigations, also in accord with experiment, reveal carbonate impurities are likely to be common in pristine and doped Ba2TiO4 systems alike, and that their presence will stabilise the α'-phase. The hydroxide type defects formed upon water incorporation are shown to be low in energy in Ba2TiO4 systems containing oxide ion vacancies or interstitials. Both carbonate and hydroxide type defects are shown to bind aggressively to any oxide ion defects present, reducing their mobility.

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