IT-­PEFC operating at 120°C and not the usual 80°C has many advantages, such as faster chemical reactions. If the gas humidification is reduced, simpler and lighter humidifiers can be used, leading to a reduction in the fuel cell total cost. However, at this condition the current commercial membrane Nafion is not able to hold water and perform satisfactorily. Therefore, in this study the application of multilayer membranes for IT-PEFC was investigated. These membranes were divided into two groups, a first with external layers of Nafion and an inner layer of sulphonated polyindene, and a second with external layers of Nafion and an inner layer of graphene oxide. The membrane preparation method was also investigated. The multilayer membranes were prepared by hot pressing and solution casting. As a result, cast multilayer membranes showed better performance and proton conductivity than hot pressed. Delamination and low interface interaction were the main drawbacks for hot pressed membranes. Cast multilayer sulphonated polyindene membranes showed higher performance than Nafion at 120°C and 20% of relative humidity. In the meantime, cast graphene oxide multilayer membranes showed higher water uptake and open circuit voltage than Nafion.

Fuel cells are becoming increasingly important in the conversion of our society to clean, and renewable energy sources. However, there are some technical, as well as commercial barriers, which remain to be overcome before the fuel cell industry may be counted a success. One such problem is that of nonlinear current fluctuations, which have been observed under quite general conditions, in solid oxide fuel cells. This thesis attempts to elucidate the mechanisms driving this undesirable be- haviour, by developing a rational mathematical model based on fundamental chemical kinetics, and mass transfer effects, which take place within the porous anode of the fuel cell. A system of nonlinear, coupled ordinary differential equations is derived to describe the reaction and transfer processes associated with this fundamental model. This system is then rationally reduced to a planar dynamical system and the cases of weakly and fully humidified fuel streams are considered. Self-sustained, temporal oscillations are shown to arise through Hopf bifurcations in each case, and key parameter regimes leading to oscillatory behaviour are identified. Experiments have been conducted on commercial fuel cells, with results presented in Chapter 5.

This aim of this work is to determine carbon formation when methane is reformed directly inside the SOFC anodes, from thermodynamic equilibrium and kinetic approaches. Two carbon formation determination approaches - carbon-steam equilibrium and carbon activity approaches were then presented, compared and discussed. The consideration of the reversed syngas formation reaction is important to the carbon activity approach, which is expected to be a useful tool for determining carbon formation in the anode recirculation system. The investigation of the combined steam and dry methane reforming, both thermodynamic equilibrium computational modelling approach and kinetic experimental validation were presented in this work. Different ratios among methane, steam and carbon dioxide, leading to different oxygen to carbon atomic were used to examine the methane reforming and the carbon formation prevention abilities. Finally, a kinetic modelling for a methane fed SOFC with anode recirculation system was built based on the integration of different functions for a more detailed investigation. The combination of different fuel current densities (i.e.0.5, 1.0 and 1.4), steam to methane ratios (i.e. 0.25-4.0) in the fuel and different recycling rate (i.e.10%-90%) were given to the model to investigate the effects of fuel conditions on the system operation.

In this thesis testing of solid oxide cells in fuel cell and electrolysis operation have been performed. Attempts to fabricate fuel cells are described, equipment for testing solid oxide electrolysis cells has been constructed and the development process for this described. Cells of a number of different types have been tested in which initial work was performed using microtubular cells. Work on the fabrication of planar solid oxide cells is described, anode supports were prepared by pellet pressing however the application of a suitably dense electrolyte was unsuccessful which resulted in a poor cell OCV. The initial degradation of commercial solid oxide cells has been investigated. During cyclic testing at low current density the cells were found to degrade at twice the rate in electrolysis operation compared to fuel cell operation. This leads to the conclusion that the degradation observed in electrolysis is reversible and that there is a disconnect between the electrolysis and fuel cell degradation processes. During testing at different current densities the cells were found to undergo severe degradation when operated with very low water content supplied to the cells. The degradation was 512 mV kh\(^{−1}\) at 2.5 vol% H2O and reduced to 45mV kh\(^{−1}\) at 50 vol% H2O. Over the timescales investigated in this work and due to the reversible nature of the electrolysis degradation identifying a degradation mechanism was very difficult.

In this PhD thesis, new materials for solid oxide fuel cells have been researched. It focuses on both the cathode and electrolyte components. Two general systems, the perovskite-type ABO3 and apatite-type M10-x(XO4)6O2+y structures, have been investigated. The structural characteristics, conductivity and stability have been examined. The perovskite work for the cathode uses doping strategies to introduce disorder into the system and change the conduction characteristics through a structure change to cubic. It has been shown that only small amounts of dopants are required to cause this structural change and effect the conductivity. In addition, thermal and chemical compatibility tests, along with ASR tests with known fluorite and apatite electrolytes, have been investigated. Their stability in a CO2 containing environment was tested and a full-scale production of a fuel cell was attempted (Chapters 3 and 4). The electrolyte investigations focussed on doping the Ba2Sc2O5 sample to form a perovskite structure that possesses both oxide ion and protonic conductivity. The doping has decreased the amount of scandium present with cheaper elements such as rare earth Yb3+, or transition metals Fe4+ and Ti4+, all in an attempt to form the cubic structure that results in high oxide ion/proton conductivity and increased stability in CO2 environments (Chapter 5). The final chapter focuses on phosphate and rare earth doping of BaPrO3, to form the cubic perovskite structure. These samples were seen to have increased water incorporation and stability in CO2. However, this was at the expense of the ionic conductivity due to vacancy trapping.

Formative time lags (FTL's) have been measured in low pressure hydrogen discharges under both low and high overvoltages. The low overvoltage experiments were conducted in the range 40 (E/p t( 250 V(cm.mm Hg)-1 and were designed to investigate the relative importance of the secondary processes which could be active in the considered discharges. Results from four experimental tubes are presented, the later tubes having evaporated gold electrodes and a variable inter-electrode distance. The hydrogen was prepared by the electrolysis of barium hydroxide solution, and was purified by passing it through two palladium osmosis tubes in series. Davidson's approximate theory of the temporal growth of discharges was used to analyse the experimental results, and it was found that the cathodic photon process was the predominant secondary process under all the experimental conditions considered, accounting for at least 60% of the secondary action. A preliminary curve of the FTL vs. inter-electrode distance, for constant E/p, pd and overvoltage was obtained for one of the tubes. The high overvoltage experiments were quite distinct from the lower overvoltage experiments, FTL's being obtained of the order of nano-seconds, about two orders of magnitude lower than in the lower overvoltage experiments. The primary ionization process is sufficient to explain some very short FTL's but some secondary processes have also to be considered to explain others. The high space charges developed in the initial avalanche was seen, in general, to retard the avalanche growth, and to make the value of the primary ionization coefficient within the gap less certain. Using three experimental tubes having different inter-electrode distances, the effects of varying pressure and inter-electrode distance on the growth of the discharge could be studied at a given E/p; also, the effect of a change in E/p at constant pressure was studied for the three inter-electrode distances.

This thesis is about the processes of creating renewable electricity systems in new locations. Specifically it addresses the challenges and drivers of building-up renewable energy system in a `fast follower' country, Ireland. With increasing scientific, political, civil society and private sector agreement on the need to reduce green-house gas emissions from the provision of electricity, the rolling out of complex, renewable electricity systems from countries in which initial system building activities have taken place to others is an important issue. The primary research question posed is `what explains the growth of the wind electricity system in Ireland?' This question is reflected upon by asking `what does the Irish experience tell us about why and how these systems spread to new locations?' The thesis addresses an innovation studies audience, making a theoretical contribution to the field of sustainability transitions. It contributes to recent research integrating theories from human geography by offering new insights on how location influences building of large scale renewable electricity systems in new jurisdictions. It contributes findings about the rapid development of the Irish wind system that challenge two dominant perspectives; roll-out in Ireland has been driven by EU policy push and the development of the industry is fundamentally about the extent of national subsidies. We find both perspectives are partial; what they omit is likely to be essential to reproducing the Irish experience. This thesis takes as its unit of analysis the wind energy system itself, and using a `technological innovation system' framework, examines and evaluates the structure of the system; the complex arrangement of institutions, actors and technologies; and the dynamic innovation processes or `functions' of the system. An inquiry into the substantive historical contexts of the development of the system make possible insights into the locational characteristics and relations within and between the system; drivers, barriers and influences of direction of the system processes; and the contexts in which decisions are made and technological change takes place. The thesis finds the development of a renewable electricity system in new locations is simultaneously heavily influenced by transnational dimensions of system actors relations and institutions, and shows that while the direction of the emergent technological pathway is influenced at multiple spatial and governance levels, legitimation of the technology is highly localised.

Two general systems, brownmillerite-type Ba₂In₂O₅ and apatite-type silicates have been investigated for potential solid oxide fuel cell electrolyte applications. The combination of powder diffraction, NMR, TGA, Raman and AC impedance spectroscopy indicated the successful incorporation of phosphate, sulphate and silicate into the Ba₂In₂O₅ structure leading to a transition from an ordered brownmillerite-type structure to a disordered perovskite-type, which led to the conductivity enhancement below 800 °C, along with a significant protonic contribution in wet atmospheres. The CO₂ stability was also shown to be improved on doping. This oxyanion doping strategy has been extended to the analogous system, Ba₂Sc₂O₅, which resulted in samples with high conductivity and good stability towards CO₂. Neutron diffraction studies on La₉.₆Si₆O₂₆.₄ indicated that the interstitial oxide ion is located near the channel centre. Further interstitial anions could be accommodated through hydration, which led to displacement of the interstitial site away from the channel centre, with an accompanying swelling of the channel. Although long term annealing of these apatite silicates showed no apparent significant structural change, a reduction in the bulk conductivity was observed, while the grain boundary conductivity was improved, thus resulting in a small enhancement in the total conductivity below 400 °C.

Developing solid oxide fuel cell (SOFC) systems that operate in lower temperature regimes improves system stability, widens materials selection and lowers performance degradation issues previously observed with higher temperature cells. In this work, the development of an intermediate temperature SOFC (IT-SOFC) based on a Ce-doped Scandia-stabilised Zirconia (ScSZ) structure manufactured via screen-printing is outlined. In this thesis we report on the successful manufacture of anode supported 8YSZ baseline cells and cells containing Ni-10Sc1CeSZ anodes supporting 10Sc1CeSZ electrolytes both prepared via die pressing, screen-printing and co-sintering. Commercial 8YSZ and 10Sc1CeSZ reference cells were also tested. This research aims to demonstrate the viability of 10Sc1CeSZ within an IT-SOFC cell structure and examine the effectiveness of 10Sc1CeSZ on lowering the cell operating temperature. Results for the tested cells on H\(_2\) at 800\(^o\)C show the lab grade and commercial grade YSZ cells obtained OCV values of 1.06V and 1.04V and maximum power density values of 392 mW cm\(^-\)\(^2\) and 466 mW cm\(^-\)\(^2\) respectively. The commercial 10Sc1CeSZ cells exhibited the highest OCVs > 1.10V of all tested cells with the lowest area specific resistance of 0.496 Ω cm\(^2\) obtained for the lab grade 10Sc1CeSZ cells. Peak power densities of 68.24 mW cm\(^-\)\(^2\) and 9.12 mW cm\(^-\)\(^2\) at 800\(^o\)C were achieved for the biogas fuelled lab grade YSZ and 10Sc1CeSZ cells respectively.

The two-stage AC to AC direct power converter is an alternative matrix converter topology, which offers the benefits of sinusoidal input currents and output voltages, bidirectional power flow and controllable input power factor. The absence of any energy storage devices, such as electrolytic capacitors, has increased the potential lifetime of the converter. In this research work, a new multi-motor drive system based on a two-stage direct power converter has been proposed, with two motors connected on the same shaft. A vector control scheme is proposed where each motor has an independent current control loop, but shares a single speed control loop. The two-motor on the same shaft solution has applications in aerospace to increase the availability of the system. The two-stage direct power converter therefore offers the possibility of a cost effective multi-motor drive system. The feasibility and performance of the proposed multi-motor drive system is demonstrated through simulation results and validated with experimental results from a laboratory prototype.