Conventional oxygen anion conducting yttria-stabilized zirconia (YSZ) based solid oxide fuel cells (SOFCs) operate at high temperatures (800oC-1000oC). SOFCs based on proton conducting ceramics, however, can operate at intermediate temperatures (450oC-750oC) due to low activation energy for protonic defect transport when compared to oxygen vacancy transport. Fuel cells that operate at intermediate temperatures ease the critical materials requirements of cell components and reduce system costs, which is necessary for large scale commercialization. BaCeO3-based perovskite materials are candidates for use as ion conductors in intermediate temperature SOFCs (IT-SOFCs) when doped with trivalent cations in the B-site. B-site doping forms oxygen vacancies which greatly increases the electrical conductivity of the material. The oxygen vacancies are consumed during the creation of protonic defects or electronic defects, depending on the atmosphere and temperature range. High performance IT-SOFCs based on the Y3+ and Yb3+ doped BaCeO3-based system, BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) have been recently reported. High conductivity in O2/H2O atmosphere was reported, however, a more basic understanding of the BZCYYb structure, electrical conductivity, and the portion of the charge carried by each charge carrier under fuel cell conditions is lacking. In this work, the BZCYYb material is fabricated by the solid state reaction method and the crystal structure at intermediate temperatures is studied using HT-XRD. The total conductivity of BZCYYb in H2/H2O, O2/H2O, and air atmospheres in the IT-SOFC temperature range is reported. The activation energy for transport at these conditions is determined from the conductivity data and the transference numbers of protonic defects, oxygen anion defects and electronic defects in the BZCYYb material are determined by the concentration cell - OCV method. BZCYYb is a mixed proton, oxygen anion, and electronic conductor at IT-SOFC temperature ranges (450oC - 750oC), in H2, O2, and H2O containing atmospheres. Ni-BZCYYb/BZCYYb/BZCYYb-LSCF fuel cells were constructed and peak power densities of ~1.2 W/cm2 were reported at 750oC after optimization of the Ni-BZCYYb anode porosity. Decreasing the Ni-BZCYYb anode porosity did not significantly affect the electrical conductivity of the anode, however the peak power densities of the IT-SOFCs based on the anode with less porosity, calculated from I-V curve data, showed dramatic improvement. The fuel cell with the lowest anode porosity demonstrated the highest performance. This finding is in stark contrast to the optimal anode porosity needed for high performance in YSZ-based, oxygen anion conducting SOFCs. Because of significant proton conduction in the BZCYYb material, fuel cell reaction products (water) form at the cathode side and less porosity is required on the anode side. The improvement in performance in the BZCYYb based IT-SOFC is attributed to the unique microstructure formed in the Ni-BZCYYb anode when no pore forming additives are used which may contribute to high electrocatalytic behavior for anode reactions. This work provides a basic understanding of the electrical properties of BZCYYb and clarifies the feasibility of using BZCYYb in each component of the IT-SOFC system as well as in other electrochemical devices. The high performance of the Ni-BZCYYb/BZCYYb/BZCYYb-LSCF IT-SOFC, due to low anode porosity, provides a new understanding for the rational development of high performance IT-SOFCs based on electrolytes with significant protonic conduction.

A fuel cell is an electrochemical power source with advantages of both the combustion engine and the battery. Like a combustion engine, a fuel cell will run as long as it is provided fuel; and like a battery, fuel cells convert chemical energy directly to electrical energy. As an electrochemical power source, fuel cells are not subjected to the Carnot limitations of combustion (heat) engines. Fuel cells bear similarity to batteries, with which they share the electrochemical nature of the power generation process and to the engines that, unlike batteries, will work continuously consuming a fuel of some sort. A fuel cell operates quietly and efficiently and, when hydrogen is used as a fuel, it generates only power and water. Thus, a fuel cell is a so called ‘zero-emission engine’. In the past, several fuel cell concepts have been tested in the laboratory but the systems that are being potentially considered for commercial developments are: (i) Alkaline Fuel Cells (AFCs), (ii) Phosphoric Acid Fuel Cells (PAFCs), (iii) Polymer Electrolyte Fuel Cells (PEFCs), (iv) Solid Polymer Electrolyte Direct Methanol Fuel Cells (SPE-DMFCs), (v) Molten Carbonate Fuel Cells (MCFCs) and (vi) Solid Oxide Fuel Cells (SOFCs). Among the aforesaid systems, PEFCs that employ hydrogen as fuel are considered attractive power systems for quick start-up and ambient temperature operations. Ironically, however, hydrogen as fuel is not available freely in the nature. Accordingly, it has to be generated from a readily available hydrogen carrying fuel such as natural gas, which needs to be reformed. But, such a process leads to generation of hydrogen contaminated with carbon monoxide, which even at minuscule level is detrimental to the fuel cell performance. Pure hydrogen can be generated through water electrolysis but hydrogen thus generated needs to be stored as compressed/liquefied gas, which is cost-intensive. Therefore, certain hydrogen carrying organic fuels such as methanol, ethanol, propanol, ethylene glycol and diethyl ether have been considered for fueling PEFCs directly. Among these, methanol with hydrogen content of about 12.8 wt.% (specific energy = 6.1kWh kg-1) is the most attractive organic liquid. PEFCs using methanol directly as fuel are referred to as SPE-DMFCs. But SPE-DMFCs suffer from methanol crossover across the polymer electrolyte membrane, which affects the cathode performance and hence the fuel cell during its operation. SPE-DMFCs also have inherent limitations of low open-circuit-potential and low electrochemical-activity. An obvious solution to the aforesaid problems is to explore other promising hydrogen carrying fuels such as sodium borohydride (specific energy = 12kWh kg-1), which has a capacity value of 5.67Ah g-1 and a hydrogen content of about 11wt.%. Such fuel cells are called direct borohydride fuel cells (DBFCs). This thesis is directed to studies on SPE-DMFCs and DBFCs

Cerias, appropriately doped with trivalent rare earth ions in particular, can have high oxide ion conductivity and are attractive as both SOFC (solid oxide fuel cell) electrolytes and anodes. Here, four groups of candidate electrolyte materials were synthesised using a low temperature method in order to determine the effect of multiple doping on their microstructure and ionic conductivity. In an initial study, seven compositions of Ce0.8SmxGd[sub]yNd[sub]zO1.9 (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) were synthesised and the properties of multiply-doped materials were compared with the corresponding singly-doped parent materials. The effect of co-doping with Gd and Sm was investigated in more detail by preparing and studying five compositions of Ce1−2xSmxGdxO2−x (where x = 0.125, 0.1, 0.0875, 0.075 or 0.05) and seven compositions of Ce0.825SmxGd0.175−xO1.9125 (where x = 0.175, 0.14, 0.105, 0.0875, 0.07, 0.035 or 0). The effect of additional doping with a divalent ion- Ca2+- was studied in six compositions of Ce[sub](0.825+y)Sm[sub](0.0875-y)Gd[sub](0.0875-y)Ca[sub]yO1.9125 (where y = 0, 0.00875, 0.0175, 0.02625, 0.035 or 0.04375). The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Crystallite sizes were determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities were obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association were calculated. The most promising material for SOFCs operating at intermediate temperatures was found to be Ce0.825Sm0.0875Gd0.0875O1.9125 which had a total conductivity at 600 °C of 2.23 S m−1. Lastly, doped ceria materials, primarily Ce0.8Sm0.2O1.9, were employed as catalytic supports for Pd and PdO nanoparticles and these were investigated as SOFC anode materials.

It is evident that human activity has an important impact on climate. Constantly increasing energy demand is one of the biggest causes of climate change. The fifth assessment report of the Inter-governmental panel on climate change states that decarbonisation of electricity generation is a key component of climate change mitigation. Increased awareness of this fact and escalating concerns around energy security has brought public attention to the energy industry, especially sustainable power generation systems. Future energy systems may need to include hydrogen as an energy carrier in order to achieve necessary levels of CO2 emission reductions, and overcome the challenges renewable energy systems present. Fuel cells could be a corner stone of future hydrogen inclusive energy solutions. New solutions like fuel cells have to compete with existing technologies and overcome the shortcomings of emerging technology. Though these shortcomings are well-recognised, fuel cells also have many advantages which makes continued research and development in the field highly worthwhile and viable. Key to their adoption is the identification of a niche market to utilise their advantages while overcoming their shortcomings with continuous research and development. This thesis aims to evaluate some of the stationary fuel cell applications and determine whether one could become the niche market as an entry point for fuel cells. This is achieved by economic evaluations of real and hypothetical applications. Results of the studies here imply that to decrease the total life cycle impacts of fuel cells to more acceptable levels, resource use in the manufacturing phase and recycling in decommissioning should be shown more attention. Results also present a picture showing that none of the applications investigated are economically feasible, given the current state of technology and energy prices. However, fuel cell-based combined cooling, heating and power systems for data centres show the potential to become the niche market that fuel cells need to grow. A further conclusion is that a broad market, longer stack lifetime, the possibility of selling electricity back to the grid and governmental subsidies are essential components of an environment in which fuel cells can permeate through the niche market to the mainstream markets. / <p>QC 20151210</p>

Direct liquid fuel cells represent an interesting alternative to conventional hydrogen fuel cell technology. A novel analytical method for the monitoring of direct liquid fuel cells is presented. Employing a combination of chronoamperometric, gravimetric and Raman spectroscopic measurements this method allows a straightforward determination of the Faradaic efficiency of a fuel cell. This method was applied in a proof of concept study analysing the operational behaviour of a direct methanol fuel cell. A very low Faradaic efficiency was found for the fuel cell under study. This was attributed to loss of methanol from the fuel mixture due to methanol crossover, stripping of methanol by carbon dioxide as well as evaporation. It is known from the literature that a fuel change from methanol towards higher alcohols and other hydrocarbons can help to mitigate the effects of these loss processes. However, the behaviour of such alternative fuels and their mixtures in an operating fuel cell and hence the performance of the fuel cell depends at least in part on the intermolecular interactions present in those fuel mixtures. Therefore, the intermolecular interactions in binary and ternary mixtures of potential candidates for alternative fuels were investigated in the main part of this thesis. Studies on the intermolecular interactions in binary mixtures of acetone with ethanol and 1-butanol showed a tendency for self association of both compounds albeit in different concentration ranges. It was further found that the alkyl chain length of the alcohols did not have a significant effect on the intermolecular interactions in the binary and ternary mixtures. Further, the behaviour of the ternary mixture was found to closely resemble the effects found in the individual binary mixtures. Finally, binary mixtures of dimethyl sulfoxide (DMSO) and different alcohols did not show self association. It was rather found that alcohol molecules inserted into chains of DMSO molecules eventually leading to the formation of alcohol-DMSO dimers.

This study has focused on the implementation of a stack system for a novel design of solid oxide fuel cell (SOFCRoll). The issues affecting the commercialization of SOFCs are mainly based on durability and cost. The new design offers a number of advantages over the existing designs; it seeks to retain the specific advantages of both the tubular (high unit strength, no sealing problems) and planar arrangements (high power density). This design also aims to achieve low manufacturing cost by utilizing a cheap, easily scalable production technique: tape casting, together with co-firing all components, in one single step. In this study aspects of the design and operation of SOFCRoll stacks were studied particularly those affecting the single cell test reproducibility such as pre test quality control and scale up issues such as bundle and stack gas distribution. Initially the performance of single cells was characterized and the variation of their power output with temperature was observed. The maximum power, 0.7W at 800°C was achieved with a high silver content. The OCV and total resistance of this cell were 0.93V, 0.30Ω respectively. A standard pre-test quality control and current collection technique was introduced. At 800°C reproducible performance of 0.5W power obtained, average OCV was 0.935V and average series and polarization resistances of 0.18Ω and 0.19Ω was achieved respectively. Once single cell reproducibility was achieved, the design and operation of a 5 cell SOFCRoll bundle was investigated. A FLUENT CFD model was used to optimize the gas distribution in the five cell manifold design. The value of the model as a design tool was demonstrated by the comparison of 3 different gas manifold designs. The final manifold design M3 achieved 2.5W which is consistent with the 0.5W per a cell target. This manifold was then used as the basis for the development of a 25 cell stack which was built and tested. The 25 cell stack testing results were down to 0.35W per a cell. The performance drop highlighted the problem of fuel cell manufacturing reproducibility and also the importance of introducing reproducible manufacturing tequniques. That been the case for single cell manufacturing reproducibility issue, the fundamental concern for performance drop remains a design issue. To optimize the SOFCRoll design and to assist with the development program a single-cell CFD model was developed using FLUENT. The model was validated by comparison with data from experimental measurements for the single cell. The model work was used to predict the geometrical effect of the SOFCRoll tubular and the spiral gas channel configuration and current collector configuration. Results indicate the outlet gas flow velocity is higher around the spiral, near the gas inlet (the gas interring the cell preferentially flows around the spiral) therefore, velocity decrease as the gas moves along the cell. The lowest outlet velocity is registered opposite to the gas inlet, thus creating non-uniform gas distribution. The current density distribution is not uniform and is affected primarily by reactant flow distributions along the cell and possible current collection issues particularly around the spiral part of the cell.

Σε αυτή την μελέτη διμεταλλικά ηλεκτρόδια Pt-TiO2 παρασκευάστηκαν και χαρακτηρίστηκαν με περίθλαση ακτίνων Χ (X-ray diffraction - XRD), με ηλεκτρονικό μικροσκόπιο σάρωσης (Scanning Electron Microscopy - SEM), φασματοσκοπία φωτοηλεκτρονίων από ακτίνες Χ (X-ray photoelectron spectroscopy - XPS), ηλεκτροχημικές τεχνικές και πειράματα ρόφησης-οξείδωσης μονοξειδίου του άνθρακα (CO stripping). Ερευνήθηκε η μείωση της περιεκτικότητας σε Pt χωρίς απώλειες της ηλεκτροκαταλυτικής ενεργότητας. Το TiO2 επιλέχθηκε λόγω της χημικής του σταθερότητας και του χαμηλού κόστους. Βρέθηκε ότι περιεκτικότητα σε TiO2 μέχρι 50% οδηγεί σε αύξηση της ηλεκτροχημικά ενεργής επιφάνειας (EAS) του ηλεκτροδίου.Η EAS του ηλεκτροδίου Pt(50%)-TiO2(50%) ήταν σχεδόν μια τάξη μεγέθους μεγαλύτερη από το ηλεκτρόδιο της καθαρής Pt, ενώ για περιεκτικότητα σε Pt χαμηλότερη από 30% η EAS μειώνεται δραματικά. Το παραπάνω συμπέρασμα στηρίχθηκε σε μετρήσεις του φορτίου της αναγωγικής κορυφής του οξειδίου της Pt και σε πειράματα ρόφησης-οξείδωσης του CO (CO stripping). Όλα τα δείγματα χρησιμοποιήθηκαν επίσης και ως άνοδοι κατά την διάρκεια ηλεκτροχημικής οξείδωσης μεθανόλης και αιθανόλης. Και στις δύο περιπτώσεις το ηλεκτρόδιο Pt(50%)-TiO2(50%) παρουσίασε τη μεγαλύτερη ηλεκτροκαταλυτική ενεργότητα. / In this study Pt-TiO2 binary electrodes were prepared by thermal decomposition of chloride precursors on Ti substrates, characterised by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), electrochemical techniques and CO stripping and used as anodes for alcohol oxidation. The minimization of the Pt loading without electrocatalytic activity losses was also explored. TiO2 was chosen due to its chemical stability, low cost and excellent properties as substrate for Pt dispersion. It was found that TiO2 loading up to 50% results in Electrochemically Active Surface (EAS) increase. The EAS of Pt(50%)-TiO2(50%) was found to be almost one order of magnitude higher than that of pure Pt while for Pt loadings lower than 30% the EAS was diminished. The above conclusion has been confirmed both by following the charge of the reduction peak of platinum oxide and by CO stripping experiments. All samples have been evaluated during the electrochemical oxidation of methanol and ethanol. In both cases the Pt(50%)-TiO2(50%) electrode exhibited better electrocatalytic activity than the pure Pt anode. The observed higher performance of the binary electrodes has been attributed to the enhanced Pt dispersion as well as to the formation of smaller Pt particles by the addition of TiO2.

published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy

<p>The direct methanol fuel cell or DMFC is emerging as a promising alternative energy source for many applications. Developed and developing countries, through research, are fast seeking a cheap and stable supply of energy for an ever-increasing number of energy-consuming portable devices. The research focus is to have DMFCs meeet this need at an affordable cost is problematic. There are means and ways of making this a reality as the DMFC is found to be complementary to secondary batteries when used as a trickle charger, full charger, or in some other hybrid fuel cell combination. The core functioning component is a catalyst containing MEA, where when pure platinum is used, carbon monoxide is the thermodynamic sink and poisons by preventing further reactions at catalytic sites decreasing the life span of the catalyst if the CO is not removed. Research has shown that the bi-functional mechanism of a platinum-ruthenium catalyst is best because methanol dehydrogenates best on platinumand water dehydrogenation is best facilitated on ruthenium. It is also evident that the addition of other metals to that of PtRu/C can make the catalyst more effective and effective and increase the life span even further. In addition to this, my research has attempted to reduce catalyst cost for DMFCs by developing a low-cost manufacturing technique for catalysts, identify potential non-noblel, less expensive metallic systems to form binary, ternary and quarternary catalysts.</p>

In this study photoelectrocatalytic technology has been combined with fuel cell technology in an attempt to provide a stand alone water polishing device to be applied to the water purification industry. Tungsten trioxide was chosen as the photoelectrocatalyst to be applied to the fuel cell membrane electrode assembly (MEA). In this thesis two possible WO₃-based photoanodes were studied. Firstly a Nafion-loaded WO₃ photoanode utilising the state of the art proton conductor Nafion in the MEA. The second WO₃-based photoanode was synthesised by a sol-gel method with a view to being directly sintered onto a not yet developed solid state MEA containing a proton conductive glass. In both methods electrochemical studies were undertaken with both WO₃ based photoanodes deposited on fluorine doped tin oxide glass (FTO). The WO₃ catalysts were studied by X-ray diffraction, Raman spectroscopy, Nitrogen adsorption and UV-visible spectroscopy. Electrochemical studies included cyclic voltametry and linear sweep voltametry under illumination to ascertain the photocurrent densities of the photoanodes and hence their ability to degrade water borne contaminants. The underlying materials properties were explored as well as the nature of the deposition to gain insight into the mechanisms responsible for effective photoelectrocatalytic activity. The Nafion-loaded WO₃ was applied to a Nafion membrane based MEA and utilised in a photoelectrocatalytic fuel cell. This was studied for possible application under self sustaining conditions for application in the water industry.