A poly(arylene ether sulfone) known as KM180 is investigated as a suitable material for fuel cell membranes when sulfonated. The literature relating to this class of materials, sulfonation and fuel cell applications is reviewed and sulfonation and stabilisation of the polymer selected as areas of interest. The literature relating to the chain extension and crosslinking of polymers to add stability has also been reviewed. Sulfonic acids and sulphur trioxide complexes are used to sulfonate KM180 and chlorosulfonic acid selected as the most suitable. This sulfonation is then optimised to give a repeatable result. End group reactions with acid chlorides, pyromellitic dianhydride and Cymel 350 (a trifunctional crosslinking agent) are investigated in order to strengthen the polymer. It is found that catalysis is necessary. Pyromellitic dianhydride and Cymel 350 are selected for further study. Sulfination, activation by carbonyl diimidazole and interaction with butyltin hydroxide oxide are all attempted to strengthen the polymer through interactions of sulfonic acid sidechains. The interaction with butyltin hydroxide oxide is not seen. Membranes are prepared from KM180 sulfonated using chlorosulfonic acid and modified with pyromellitic dianhydride, Cymel 350, sulfinated KM180 and carbonyl diimidazole. These are compared to membranes prepared from high molecular weight KM polymer and KM180 sulfonated using chlorosulfonic acid and sulfuric acid in terms of stability and proton conductivity. The aggregation of sulfonic acid groups is considered to be the most important factor for proton conductivity and the molecular weight was considered most important for stability. The conflict of these is discussed. Suggestions are made for further work.

Research and development of solid polymer fuel cell (SPFC) systems for the transportation and stationary power generation industries has evolved rapidly over the last decade. This growth has been due to the ever-increasing demand for a cleaner and more efficient technology in these industries. To compete with the existing technology, SPFC systems have to be highly efficient at both full and partial loads, environmentally friendly (in terms of emissions and noise) and competitively priced. For many applications, SPFCs have the potential to deliver a system that can fulfil these criteria. However, a number of system design issues have to be addressed in order to provide a well-integrated and optimised system, which is a practical alternative to conventional modes of energy conversion.

The project has dealt with the synthesis and characterisation of new materials for use in solid oxide fuel cells. In terms of potential electrolytes we have been studying a range of apatite based materials. High oxide ion conductivity has been recently reported in the apatite phases La10-yM6O26+/-z (M = Si, Ge). Preliminary work focused on the preparation and characterisation of single phase samples of previously reported phases, e.g. La9.33(Si/Ge)6O26 and La8A2(Si/Ge)6O26 (A = alkaline earth). Neutron diffraction studies revealed significant disorder within the oxygen channels for systems showing high oxide ion conductivity, namely non-stoichiometric systems containing either cation vacancies, e.g. La9.33Si6O26, or excess oxygen, e.g. La9SrSi6O26.5. Compositions containing excess oxygen showed the highest conductivities, e.g. 0.01 S cm-1 at 800 °C for La9SrSi6O26.5. Conversely, fully stoichiometric systems, e.g. La8Sr2Si6O26, showed poor oxide ion conduction, which appears to be associated with oxygen ordering within the channels. A range of doping studies followed in order to optimise the oxide ion conductivity, i.e. La9.33+x/3Si6-xMxO26 (M = Al, B, Ga) and (La/M)10-xSi6O26+/-y (M = Mg, Ca, Sr, Ba). The sample La9BaSi6O26.5 showed the highest conductivity, with a value of 6 x 10-3 S cm-1 at 500 °C, which is significantly higher than that of YSZ at this temperature (1 x 10-3 S cm-1). This sample is therefore a highly promising candidate material for use as an electrolyte in intemiediate temperature SOFCs (500 °C - 700 °C), as well as other technological applications. Similar studies were performed for samples with germanium in place of silicon. High oxide ion conductivity was observed for these systems, although germanium volatility was identified as a significant problem in these cases. In terms of cathode materials research has involved the preparation of a range of perovskite type phases based on YBa2Cu3O7-x (YBCO). The compositions tested were of formula type YSr2Cu3-xMxO7-y (M = Ga, Co, Fe), and the results indicated that these samples were not promising candidates as replacement cathode materials since, they all showed only moderately high conductivity. Furthermore, the phases were shown not to be chemically compatible with most current or prospective electrolyte materials, with significant impurity phases found to be produced when the electrolyte and cuprate were heated together at SOFC operating temperatures (900 °C - 1000 °C).

The integrated planar solid oxide fuel cell designed by Rolls-Royce pic consists of a fuel electrode and an air electrode separated by an ion-conducting solid electrolyte. The electrolyte must exhibit ionic conductivity and be non-permeable to the cell gases. The other cell components set limits for the firing temperature of the electrolyte, thus restricting its densification. Further, the use of a thin constrained layer, fabricated via screen-printing, makes the separation of the fuel and oxidant much more challenging than in unconstrained planar fuel cell designs. Thus, the aim of this study was to investigate the influence of the screen-printing and sintering processes on the microstructure of the electrolyte and ultimately on the permeability or gas flow rate. The ceramic material used throughout this study was zirconia doped with 3-mol% yttria formulated into a screen printable ink and deposited onto a pre-sintered substrate. The ink was deposited through mesh sizes ranging in thickness from 50 to 215 mum. The printed layer was then dried and additional layers added up to a maximum of four applications. The resulting laminate was sintered using a conventional sintering scheme. The thickness, relative density and grain size of the sintered electrolyte were recorded. Increasing sintered layer thickness showed an associated reduction in relative density and grain size. The conventional sintering profile was modified by changing the heating rate, sintering temperature and sintering time. From this part of the study a baseline fabrication process was established. This baseline used a 165 mesh size with three print-dry applications, a heating rate of 10°C min-1, a sintering temperature of 1450°C and a sintering time of 10 hours. The conventional sintering profile was modified to incorporate coarsening, two-step sintering and a combination of these denoted as 'three-step' sintering. Layer thickness, relative density, grain size distribution and pore size distribution were determined. It was possible to achieve full density with all three types of sintering profile. Grain growth could not be avoided in the constrained thick-films but the pore size distribution was narrower for the modified sintering profiles compared with that for conventional sintering. Gas leakage testing showed that the effect of microstructure on the gas flow through the electrolyte was not a function of a single variable. Rather, it was a combination of at least four parameters (the ratio of grain size to sintered layer thickness, relative density, pore size range and number of pores per unit area). Only if all four parameters were within certain defined boundary conditions was the gas flow minimised. Although the lowest gas flow rate measured was still just outside of the design requirement for an industrial application, the associated sintering schedule used lower temperatures than currently used to densify the electrolyte and there is scope for further optimisation.

The Rolls-Royce Integrated Planar Solid Oxide Fuel Cell (IP-SOFC) features anode, electrolyte and cathode layers of 5-20 pm in thickness, connected in series by highly-conductive precious-metal based current collecting layers of ~10 mum thickness, which are screen-printed and sintered upon a porous substrate. Replacement of the palladium- based cathode current collector material is desirable for increasing the economic and environmental potential of the IP-SOFC system, due to low resource availability, high cost and environmental degradation caused by mining of platinum-group metals. The electrically conductive ABO3 perovskite-structured lanthanum transition-metal oxide ceramics were identified as potential cathode current collector materials, and lanthanum nickel ferrite materials, of compositions LaNi0.6Fe0.4O3 and LaNi0.5Fe0.5O3, were selected for investigation based on the combined favourable properties of electrical conductivity, phase stability and compatible coefficient of thermal expansion with other cell materials. Both compositions were found to be reactive towards the IP-SOFC cathode materials, lanthanum strontium manganite (LSM) and yttria-stabilised zirconia (YSZ), and the lower conductivity of LaNi0.5Fe0.5O3 compared with LaNi0.6Fe0.4O3 meant a thicker layer would be required to meet the conductivity requirements, which negates the advantage of its more suitable coefficient of thermal expansion. It was found that a LaNi0.6Fe0.4O3 layer of ~80 mum was adequate to meet the conductivity target, and could be applied by a single stencil-print and sintered at 1125°C, which is compatible with the screen-printing and firing production line, although the manufacturing method requires optimisation to eliminate layer defects. In addition it is believed that the material can offer significant economic and environmental advantages over the present palladium-based cathode current collector. However the reaction of LaNi0.6Fe0.4O3 (LNF) with LSI'I was found to critically compromise its use in conjunction with an LSM-based cathode. Efforts to incorporate an LNF-based cathode also failed due to reaction of LNF with YSZ and gadolinium-doped ceria (CGO), and it must be concluded that the reactivity of LNF with common solid oxide fuel cell materials severely limits its potential to be used as a cathode current collector layer in the IP-SOFC.

This thesis presents some model developments for the simulation and optimization of the design of fuel cells, in particular for the Solid Oxide Fuel Cell (SOFC) and Proton Exchange Membrane Fuel Cell (PEMFC). However, the approaches and models presented can be basically applied to any type of fuel cell. In this study, the multicomponent diffusion processes in the porous medium of a SOFC anode has been investigated through comparison of the Stefan Maxwell Model, Dusty Gas Model and Binary Friction Model in terms of their prediction performance of the concentration polarization of a SOFC anode to mainly investigate the effect of the Knudsen diffusion on the predictions. The model equations are first solved in 1 D using an in-house code developed in MATLAB. Then the diffusion models have been implemented into COMSOL to obtain 2D and 3D solutions. The model predictions have been evaluated for different parameters and operating condi- tions for an isothermal system and assuming that reaction kinetics are not rate limiting. The results show that the predictions of the models are similar and the differences in the predictions of the models reported previously are mainly due to the definition of the effective diffusion coefficient, i.e. the tortuosity parame- ter, and with a tortuosity parameter fitted for each model, the models that take into account the Knudsen diffusion and that do not predict similar concentration polarization. Moreover, in this research, the application of an Adaptive Neuro- Fuzzy Inference System (ANFIS) to predict the performance of an Intermediate Temperature Solid Oxide Fuel Cell and a Proton Exchange Membrane Fuel Cell (PEMFC) have been presented. The results show that a well trained and tested ANFIS model can be used as a viable tool to predict the performance of the fuel cell under different operational conditions to facilitate the understanding of the combined effect of various operational conditions on the performance of the fuel cell and this can assist in reducing the experimentation and associated costs.

In this research, three different sol-gel synthesis methods by using cost effective materials, such as sugar + pectin, sodium alginate beads or sodium alginate granules, have been investigated for nanopowders production in Solid Oxide Fuel Cells (SOFCs) application. Materials (cerium gadolinium oxide and nickel oxide) have been selected as model materials. Cerium gadolinium oxide (CGO) is one of the most important electrolyte materials in SOFC due to its high ionic conductivity at 500-800 "C, whilst nickel oxide (NiO) can be reduced into nickel in SOFC fuels (H2 or CH4) as catalyst in anode layer. First of all, a novel sol-gel method has been developed for the production of high purity nanopowders of Ce0.8Gd0.201.9 (CG02) solid solution using maltose or sucrose as an organic chelating agent and pectin for gelation. The results of this investigation indicate that the final particle size of approximately 10 nm can be obtained after calcination of the dried gel at , 500°C for 2 hours in ambient air. Powder X-ray diffraction (XRD) shows that all samples are single phase cubic CGO powders. The mean crystallite sizes calculated from XRD analysis using Rietveld refinement method agree with the morphological features observed by transmission electron microscopy (TEM). The nominal composition of CG02 has been found to be in excellent agreement with that determined by energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma - atomic emission spectrometry analysis (ICP-AES). The ionic conductivities of Ceo.8Gdo.201.9 samples are measured by AC-impedance which appears reasonably well with the reference data which will qualify the use of this material for SOFC as solid electrolyte and in the fabrication of composite electrodes. On the other hand, another novel and generic sol-gel method has been developed for the production of high purity metal oxide nanopowders using sodium alginate (Na-ALG). This has been demonstrated successfully employing NiO and CGO CG01 (Ce0.9Gd0.1O1.95) and CGO2 (Ce0.8Gd0.201.9) as model materials in this instance. For NiO, the results of this investigation indicate that the final particle size of -20 nm can be obtained after calcination of the predried beads at 500°C for 3 hours in ambient air. XRD shows that the obtained samples are single phase cubic NiO powders. Furthermore, freeze dried and X-ray micro-tomography (XMT) technologies are applied to observe the inside morphology of the Ni-ALG beads. XMT shows that nickel ions have been uniformly cross-linked in the alginate structure and remained stable after freeze drying evidenced by the bright green color of the freeze dried beads. Finally, NiO nanopowders can also be synthesized using Na-ALG granules. Moreover, this alginate method has also been demonstrated successfully employing CGO in two composites designated as CG01 (Ceo.9Gdo.101.95) and CG02 (Ceo.aGdo.201.9), respectively. The results indicate that the nanopowders having a final particle size of -7 nm can be obtained after calcination of ion-exchanged alginate precursor at 500 degC for 2 hours in ambient' air. The chemical structures of Na-ALG solution and CGO beads are analyzed by Fourier transform infrared spectroscopy (FTIR) which indicates that Ce3+/Gd3+ are ion-exchanged with Na+ after gelation. The nominal compositions of CG01 and CG02 have been found to be in excellent agreement with that determined by EDS and ICP-AES. The ionic conductivities of these two samples are measured by AC-impedance which appears reasonably well with the reference data which will also qualify the use of this material for SOFC as solid electrolyte. All of these new sol-gel methods are simple, environmentally friendly and non-toxic routes for a large scale production of high purity single phase nanopowders in a cost effective manner at significantly low temperatures.

No description available.

Despite three decades of development of solid oxide fuel cells (SOFCs) since the conception of the tubular Siemens–Westinghouse design, no practical alternatives to yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (CGO) electrolytes have been established. However, there have been considerable improvements in the performance of SOFCs, decreasing their specific overall costs, by decreasing operating temperatures, understanding their reaction kinetics, increasing specific surface areas of electrode / electrolyte / reactant three-phase boundaries, establishing new fabrication techniques and employing new geometric designs. So called micro-tubular SOFCs (MT-SOFCs) are one of the most promising geometric designs, though a misnomer, as tube diameters are normally several millimetres, significantly smaller than Siemens–Westinghouse SOFCs with 22 mm tube diameters. This three-year Ph. D. project was aiming to establish the feasibility of, and develop, a novel design of SOFC, fabricated using hollow fibres (HFs) with diameters of hundreds of micrometres, thereby increasing the specific surface area of electrodes, increasing the power output per unit volume/mass, facilitating sealing at high temperatures, and decreasing costs. Collaborators used a spinneret in phase inversion process to produce HFs with non-porous, gas-tight cores and porous outer layers ca. 50-100 μm thick; suspensions of YSZ or CGO particles were used to produce the precursor micro-tubes for electrolyte-supported structures. After sintering the HFs, Ni was deposited electrolessly onto their inner surfaces to form Ni-YSZ anodes, using aqueous nickel (II) solutions and (sodium) hypophosphite (H2PO2-) as the reducing agent. With YSZ electrolyte-supported structures, lanthanum strontium manganite (LSM)-YSZ particles were then coated onto outer surfaces of the HFs to form cathodes; these cells produced only 46-400 W m-2 at 800 oC, compared with ca. 800 W m-2 at 600 oC for CGO-supported cells. Anode-supported structures were also produced using non-conductive, porous NiO-YSZ HFs as anode precursors. YSZ particles were suspended in ethanol and electrophoretically deposited (EPD) onto the external surface of NiO-YSZ HFs, requiring electric fields of ca. 22 kV m-1 between a tubular Cu cathode, placed inside the porous HF precursor, and a tubular platinised titanium mesh anode; this implied they had an effective positive charge. The YSZ-coated NiO-YSZ fibres were then co-sintered at 1500 oC. Mixed (YSZ-LSM) and pure LSM cathode layers, for creating functional layers and enhanced current collector electrodes, were deposited using a paint brush and re-sintered at 1200 oC. The resulting anode-supported HF-MT-SOFCs delivered peak power density of 2 kW m-2 at 800 oC. Collaborators then used a triple orifice spinneret in the phase inversion process to co-extrude CGO/NiO-CGO dual layer-HFs, which were then co-sintered. Dispersions of CGO-LSCF particles were then painted or sprayed onto their outer surfaces, as "graded" LSCF-CGO porous cathode precursors that were then sintered at 1200 oC. HF-MT-SOFC fabrication was completed by winding a silver wire current collector spirally round the cathode. Similar arrangements were used for collecting the current from the HF lumen (anode). The use of functional cathode layers, higher porosity anodes, improved anode and cathode current collectors, and optimizing the thickness of the electrolyte layer and operating parameters, enabled maximum power densities of ca. 25 kW m-2 at ca. 600 oC, believed to be a record for a single MT-SOFC. The effects of electrolyte thickness (100-10 μm), cell length (10-50 mm), and anode morphologies / porosities were also determined. HF-MT-SOFCs were found to be stable to reduction/oxidation and thermal cycling for up to 8 days. Finally, a novel design for stacking individual HF-MT-SOFC in series (voltage scale up) and parallel (current scale up) was studied experimentally; 3 HF-MT-SOFCs in parallel delivered ca. 0.67 W (=3.4 kW m-2) at 7.5 kA m-2, 0.45 V and 600 oC.

No description available.