Structural investigations of some novel doped zirconias with potential as solid oxide fuel cell electrolytes

This research has concentrated on the doping of the already well-studied zirconia system, and the structural changes caused by this doping.These were all considered as potential electrolyte materials for solid oxide fuel cells.Initially, boron doping was investigated as a potential new method of doping that had yet to be studied.Borate loss was found to be a major problem, but this could be worked around using carefully controlled preparations.Results from structural investigations were ambiguous, with XRD showing little change in the structure, but also showing no second crystalline phase.NMR showed the boron to be in a similar environment to the system YBO3 yet IR showed differences between the two systems.Thus it was concluded that boron was inserted into the structure in a manner similar to the YBO3 structure.EXAFS studies were conducted on three doping systems, yttria doping (YSZ), scandia doping (SSZ) and dual doping with both yttria and scandia (YScSZ).These systems were found to have subtle but extremely important differences.In YSZ, vacancies were found to be clustered preferentially around the zirconium ions, until doping levels became so high that yttrium co-ordination also had to decrease.Multi cluster refinements confirmed this theory and suggested the possibility of microdomain formation in this system.In SSZ vacancies were more evenly distributed through the system, with both zirconium and scandium showing a preference for a six fold co-ordination system, which is not possible in the fluorite structure, suggesting a local structure perhaps closer to rutile may be likely.EvidenceThe Case for Fuel CellsThe main benefit of the fuel cell system over conventional turbine systems is that it is far more efficient and, with the correct fuels, far cleaner.The high efficiency of fuel cells in comparison to other methods of power generation arises from the Carnot Limitation of efficiencies in heat engines.The Carnot equation is as follows 12 : Carnot Efficiency = 1 -Tc/Th Thus, the maximum efficiency is dependent solely on the difference in temperature between the hot sink and the cold sink and efficiency only tends towards unity as Th tends towards infinity or Tc tends towards zero.70%, or higher if the waste heat is used in a combined heat and power plant.It has long been known that human activity affects the environment and the land around us.The Victorians complained of the London smog caused by the many coal fires and Edinburgh was called "Auld Reekie" for centuries.However, it is only recently that the true scale of these effects has been fully realised.The hole discovered in the Antarctic ozone layer in the 1970s, its explanation in the 1980s, and more recent debates on global warming are finally bringing acceptance that humans are massively changing the world through their actions, and not necessarily for the good.The issue which is perceived as the most serious is the so-called "Greenhouse effect", where certain heat trapping gases such as carbon dioxide and methane build up 500-1000C Distributed power generation and CHP.2kW to several MW.Alkali Fuel Cell (AFC) Alkali fuel cells are low temperature fuel cells made with an alkaline electrolyte, usually sodium or potassium hydroxide, that conducts OH" ions.The principles for these have been known since at least 1902, 21 but they were first practically demonstrated in the 1940s and 1950s by F. T. Bacon at Cambridge.They were then employed by NASA 22 in the Apollo space missions and have been used by NASA ever since.which the cell would recombine overnight, guaranteeing full-time electricity.This would overcome the problems with CO2.Polymer Fuel Cells/Proton Exchange Membrane fuels cells (PEM/PEMFC) Proton exchange membrane fuel cells were first developed in the 1960s by General Electric for the early manned space missions of NASA, 22 although they were soon superseded by alkali fuel cells.This was because they needed a considerable Recent development of the PEMFC has been made possible by great improvement in catalyst technologies.As far less platinum is required, down from 28 2 2mgcm" in its earliest NASA incarnations to 0.2 mgcm" in a modern system, the cost of the platinum is no longer a limiting factor.Much of the renaissance of the PEMFC is due to the work of Ballard Power systems of Vancouver, who now have contracts to provide several large car manufacturers with fuel cell power systems.Due to its scalability (it has been proposed for uses from laptop computers to factory stack systems) and the backing of several very large car manufacturers, the polymer fuel cell is probably the closest to full commercialisation of any fuel cell system.Molten Carbonate Fuel Cell (MCFC)Molten Carbonate Fuel Cells are medium/high temperature cells made with a carbonate electrolyte that conducts CO3 " ions.They are aimed at stationary power uses, in particular as combined heat and power (CHP) solutions.Each component has to be stable, ideally for at least 50,000 hours, at operating temperature, typically 700C to 1000C.The materials must be chemically compatible with the adjoining components.They must also have similar expansion coefficients to their neighbours, otherwise the cell will crack on thermal cycling.Ideally, the materials will also be relatively cheap and easily fabricated in conjunction with each other.carrier is H rather than O ".This has benefits of not diluting the fuel stream as the reaction take place on the oxidant side of the cell, with the water (from the oxidation of 'coking' of the anode, preventing further use without cleaning.Thus, there has been Solid Oxide Fuel Cell Research GoalsThere are still considerable problems preventing the full introduction of SOFCs into general use.The main problem is the high cost of the electricity produced bySOFCs, especially when one allows for the system costs, and this is being addressed in several ways.A major advance would be to lower the temperature of operation.This would allow the use of cheaper materials in the fabrication of the cell.Currently cells are built from expensive alloys of Lanthanum Chromate but at a temperature of 800C or so it would be possible to use the slightly cheaper yttria containing chromium steels and at 750C stainless steel, which is both cheaper and has better mechanical properties, run on hydrogen, this is currently impractical on a large scale.There is currently insufficient capacity to generate a large amount of hydrogen renewably and generating hydrogen using power from traditional sources is no real solution.Therefore the ideal solution in the near future would be to run on hydrocarbons.This would still be environmentally beneficial, as the conversion to energy and fuel utilisation would still be more efficient, reducing (rather than eliminating) greenhouse gas produced for a given amount of power generation. Y203 + 2ZrZrx + Oox -> 2YZr' + VQ + 2Zr02There is a possible follow-up reaction, in which the vacancies are filled with atmospheric oxygen and two holes are created to preserve electro-neutrality.However, the equilibrium point for this is far to the left, so it can be largely ignored for zirconias.Vo + '/202(g) -> Oo" + 2h'This brings us to the question of the correct structure for yttria stabilised zirconia.It is a fluorite structure with some of the anions absent and is usually described as the defect fluorite structure.The simple fluorite structure is illustrated in Figure 1.7.The defect fluorite structure assumes that the oxygen vacancies are evenly distributed throughout the lattice.In the case of YSZ, it has been suggested that this is not entirely correct.The falling conductivity with increasing levels of aliovalent dopant Other Stabilised Zirconias Several other metals have been used to stabilise the fluorite phase in zirconia, most notably calcium in calcia stabilised zirconia,55 which has been considered as an SOFC electrolyte material.56Other metals that can be used to stabilise zirconia include ruthenium, ytterbium, ^magnesium and many more.While zirconia is an old and well studied system, it is continually surprising.New forms, such as nano-crystalline YSZ or mesoporous YSZ,58 and new uses are still being discovered after a hundred years of work.

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