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>

Investigation of nitride fuels and their properties has, for decades, been propelled on the basis of their desirable high metal densities and high thermal conductivities, both of which oer intrinsic advantages to performance, economy, and safety in fast and light water reactor systems. In this time several key obstacles have been identied as impeding the implementation of these fuels for commercial applications; namely chemical interactions with air and steam, the noted diculty in sintering of the material, and the high costs associated with the enrichment of 15N. The combination of these limitations, historically, led to the well founded conclusion that the most appropriate use of nitride fuels was in the fast reactor fuel cycle, where the cost burdens associated with them is substantially less. Indeed, it is within this context that the vast majority of work on nitrides has been and continues to be done. Nevertheless, following the 2011 Fukushima-Daiichi nuclear accident, a concerted governmental-industrial eort was embarked upon to explore the alternatives of so-called \accident tolerant" and \high performance" fuels. These fuels would, at the same time, improve the response of the fuel-clad system to severe accidents and improve the economy of operation for light water reactor systems. Among the various candidates proposed are uranium nitride, uranium silicide, and a third \uranium nitride-silicide" composite featuring a mixture of the former. In this thesis a method has been established for the synthesis, fabrication, and characterization of high purity uranium nitride, and uranium nitride-silicide composites, prepared by the spark plasma sintering (SPS) technique. A specic result has been to isolate the impact of the processing parameters on the microstructure of representative fuel pellets, essentially permitting any conceivable microstructure of interest to be fabricated. This has enabled the development of a highly reproducible technique for the production of pellets with microstructures tailored towards any desired porosity between 88-99.9%TD, any grain size between 6-24 μm, and, in the case of  the uranium nitride-silicide composite, a silicide-coated UN matrix. This has permitted the evaluation of these microstructural characteristics on the performance of these materials, specically with respect to their role as accident tolerant fuels. This has generated results which have tightly coupled nitride performance with pellet microstructure, with important implications for the use of nitrides in water-cooled reactors. / Under artionden har forskning om nitridbranseln och dess egenskaper bedrivits pa grundval av nitridbransletsatravarda egenskaper avseende dess hoga metall tathet och hog varmeledningsformaga. Dessa egenskaper besitter vasentliga fordelar avseende prestanda, ekonomi och sakerhet for metallkylda som lattvatten reaktorer. Genom forskning har aven centrala begr ansningar identierats for implementering av nitridbranslen for kommersiellt bruk. Begransningar avser den kemiska interaktionen med luft och vattenanga, en uppmarksammad svarighet att sintring av materialet samt hoga kostnader forknippade med den nodvandiga anrikningen av 15-N. Kombinationen av dessa begransningar resulterade, tidigare, i en valgrundad slutsats att nitridbranslet mest andamalsenliga anvandningsomrade var i karnbranslecykeln for snabba reaktorer. Detta da kostnaderna forenade med implementeringen av branslet ar avsevart lagre. Inom detta sammanhang har majoriteten av forskning avseende nitrider bedrivits och fortskrider an idag. Dock, efter karnkraftsolyckan i Fukushima-Daiichi 2011, inleddes en samlad industriell och statlig anstrangning for att undersoka alternativ till sa kallade \olyckstoleranta" och \hogpresterande" branslen. Dessa branslen skulle samtidigt forbattra reaktionstiden for bransleinkapsling systemet mot allvarliga olyckor samt forbattra driftsekonomin av lattvattenreaktorer. Foreslagna kandidater ar urannitrid, uransilicid och en tredje \uran nitrid-silicid", komposit bestaende av en blandning av de foregaende. Genom denna avhandling har en metod faststallts for syntes, tillverkning och karaktarisering av uran nitrid av hog renhet samt uran nitrid-silicid kompositer, forberedda med tekniken SPS (Spark Plasma Sintering). Ett specikt resultat har varit att isolera eekten av processparametrar pa mikrostrukturen pa representativa branslekutsar. Detta mojliggor, i princip, framstallningen av alla tankbara mikrostrukturer utav intresse for tillverkning. Vidare har detta mojliggjort utvecklingen av en hogeligen reproducerbar  teknik for framstallningen av branslekutsar med mikrostrukturer skraddarsydda for onskad porositet mellan 88 och 99.9 % TD, och kornstorlek mellan 6 och 24 μm. Dartill har en metod for att belagga en matris av uran nitrid-silicid framarbetats. Detta har mojliggjort utvarderingen av dessa mikrostrukturella parametrars paverkan pa materialens prestanda, sarskilt avseende dess roll som olyckstoleranta branslen. Detta har genererat resultat som ar tatt sammanlankat nitridbranslets prestanda till kutsens mikrostruktur, med viktiga konsekvenser for den potentiella anvandningen av nitrider i lattvatten reaktorer. / <p>QC 20170210</p>

Submitted in the fulfillment of the requirements for the Master of Engineering, Durban University of Technology, Durban, South Africa. 2016. / The effectiveness of using a low cost non - platinum (Pt) material for the catalyst layer of a polymer electrolyte fuel cell (PEMFC) was investigated. A test cell and station was developed. Two commercial Pt loaded membrane electrode assemblies (MEA) and one custom MEA were purchased from the Fuelcelletc store. Hydrogen and oxygen were applied to either side of the custom MEA which resulted in an additional sample tested. An aluminium flow field plate with a hole type design was manufactured for the reactants to reach the reaction sites. End plates made from perspex where used to enclose the MEA, flow field plates, and also to provide reactant inlet and outlet connection points. The developed test station consisted of hydrogen and oxygen sources, pressure regulators, mass flow controllers, heating plate, and humidification units. A number of experimental tests were carried out to determine the performance of the test cells. These tests monitored the performance of the test cell under no-load and loaded conditions. The tests were done at 25 °C and 35 °C at a pressure of 0.5 bar and varying hydrogen and oxygen volume flow rates. The no-load test showed that the MEA’s performed best at high reactant flow rates of 95 ml/min for hydrogen and 38 ml/min for oxygen. MEA 1, 2, 3, and 4 achieved an open circuit voltage (OVC) of 0.936, 0.855, 0.486 and 0.34 V respectively. The maximum current density achieved for the MEAs were 0.3816, 0.284, 15x10-6, and 50x10-6 A/cm2. Under loaded conditions the maximum power densities achieved at 25 °C for MEA’s 1, 2, 3, and 4 were 0.05, 0.038, 2.3x10-6, 1.99x10-6 W/cm2 respectively. Increasing the temperature by 10°C for MEA 1, 2, 3, 4 resulted in a 16.6, 22.1, 1.79, 10.47 % increase in the maximum power density. It was found that increasing platinum loading, flow rates, and temperature improved the fuel cell performance. It was also found that the catalytic, stability and adsorption characteristics of silver did not improve when combining it with iridium (Ir) and ruthenium oxide (RuOx) which resulted in low current generation. The low maximum power density thus achieved at a reduced cost is not feasible. Thus further investigation into improving the catalytic requirements of non Pt based catalyst material combinations is required to achieve results comparable to that of a Pt based PEMFC. / M

Direct methanol fuel cell (DMFC) is considered as a highly promising power source. It is based on polymer electrolytes membrane (PEM) fuel cell technology. It posses a number of advantages such as a liquid fuel, quick refueling, low cost of methanol and the compact cell, design making it suitable for various potential applications including stationary and portable applications. DMFCs are also environmentally friendly. Although carbon dioxide is produced, there is no production of sulfur or nitrogen oxides. The development of commercial DMFCs has nevertheless been hindered by some important issues. The most important are the low power density caused by the slow electrochemical methanol oxidation at the anode and methanol crossover through PEM, which is responsible for inhibiting the activity of the cathode catalyst as well. With the eventual goal of improving the overall performance of the DMFC, this study has been concerned with an investigation of the issues and effect of various parameters on its performance. First of all, the electrode preparation methodology and the effect of the catalyst were investigated. The most efficient membrane electrode assembly (MEA) was prepared with Pt/Ru black at anode and Pt black cathode on either side of a Nafion 117 membrane. Performance was however limited by current oscillations observed at low cell voltage and high current density attributed to carbon dioxide removal. Consequently, the effect of flow rate was investigated. Higher flow rates eliminated these oscillations. Then attention was focused on the management of the two-phase flow that occurs in the diffusion layer of the electrode as well as in the anode bipolar plate flow channels. Removal of carbon dioxide formed during methanol oxidation was thus found to be an important issue in DMFC. There is a competition between methanol diffusion to the catalyst layer and CO2 removal in the opposite direction. The two fluxes needed to be balanced in order to optimize performance. To accomplish this, the ratio of hydrophilic and hydrophobic pores respectively formed in the catalytic layer by Nafion and PTFE (Teflon) was altered. It also had an effect on crossover. The effect of a barrier layer was investigated to reduce crossover. Finally, zirconia and silica nano-composite membranes were tested instead of Nafion and found to reduce crossover. Developing a good understanding of what happens on the catalyst surface is important to develop a strategy on how improve DMFC performance. Thus is why a dynamic model based on a simplified mechanism for methanol electro-oxidation reaction was developed. It shows, amongst other insights, how the intermediate species coverage evolves with time. The mechanism was however too simple to provide an idea of which poisoning species are formed on the catalyst surface. A more exhaustive mechanism is thus being developed using Reaction Route analysis.

Direct methanol fuel cell (DMFC) is considered as a highly promising power source. It is based on polymer electrolytes membrane (PEM) fuel cell technology. It posses a number of advantages such as a liquid fuel, quick refueling, low cost of methanol and the compact cell, design making it suitable for various potential applications including stationary and portable applications. DMFCs are also environmentally friendly. Although carbon dioxide is produced, there is no production of sulfur or nitrogen oxides. The development of commercial DMFCs has nevertheless been hindered by some important issues. The most important are the low power density caused by the slow electrochemical methanol oxidation at the anode and methanol crossover through PEM, which is responsible for inhibiting the activity of the cathode catalyst as well. With the eventual goal of improving the overall performance of the DMFC, this study has been concerned with an investigation of the issues and effect of various parameters on its performance. First of all, the electrode preparation methodology and the effect of the catalyst were investigated. The most efficient membrane electrode assembly (MEA) was prepared with Pt/Ru black at anode and Pt black cathode on either side of a Nafion 117 membrane. Performance was however limited by current oscillations observed at low cell voltage and high current density attributed to carbon dioxide removal. Consequently, the effect of flow rate was investigated. Higher flow rates eliminated these oscillations. Then attention was focused on the management of the two-phase flow that occurs in the diffusion layer of the electrode as well as in the anode bipolar plate flow channels. Removal of carbon dioxide formed during methanol oxidation was thus found to be an important issue in DMFC. There is a competition between methanol diffusion to the catalyst layer and CO2 removal in the opposite direction. The two fluxes needed to be balanced in order to optimize performance. To accomplish this, the ratio of hydrophilic and hydrophobic pores respectively formed in the catalytic layer by Nafion and PTFE (Teflon) was altered. It also had an effect on crossover. The effect of a barrier layer was investigated to reduce crossover. Finally, zirconia and silica nano-composite membranes were tested instead of Nafion and found to reduce crossover. Developing a good understanding of what happens on the catalyst surface is important to develop a strategy on how improve DMFC performance. Thus is why a dynamic model based on a simplified mechanism for methanol electro-oxidation reaction was developed. It shows, amongst other insights, how the intermediate species coverage evolves with time. The mechanism was however too simple to provide an idea of which poisoning species are formed on the catalyst surface. A more exhaustive mechanism is thus being developed using Reaction Route analysis.

Electrochemical impedance spectroscopy (EIS) is a technique consisting of the application of a small perturbing current or voltage to an electrochemical system and measuring the response of the system. The response of the system can be described through the notion of impedance, Z, which is defined as the transfer function between the voltage and the current signal. By describing the impedance, one can gain insight into the interpretation of EIS experiments for the measurement of fundamental physical properties (eg diffusion coefficients). The impedance responses of electrochemical systems have been described in the past as an arrangement of ideal equivalent-circuit elements. Simple lumped-parameter circuits and more complex finite-transmission-line circuits have been used in the past, but the disadvantage of this approach is the difficulty in interpreting the equivalent-circuit parameters in terms of fundamental properties. It is then interesting to determine impedance by describing mathematically the fundamental physical processes that govern the response of the system. By describing and predicting analytically the impedance response induced by the perturbing current signal, one can: (i) gain considerable insight into the electrochemical process of interest, (ii) make explicit use of the modeling approach to address operational issues such as process design optimization, monitoring, diagnostics and control, and (iii) offer an interpretation to carefully designed EIS experiments for the measurement of fundamental physical properties such as diffusion coefficients or surface of active catalyst.

In recognition of the shortcomings inherent to the operating temperature ranges of current fuel cell systems, namely the“temperature gap" between 200C and 600C, an effort to develop an intermediate-temperature molten-salt electrolyte fuel cell (IT-MSFC) was undertaken. In this type of fuel cell, the molten salt electrolyte is supported on a porous support, in a planar or other geometry similar to that used in existing fuel cell technologies, such as phosphoric acid fuel cell (PAFC) and molten carbonate fuel cells (MCFC). Such a fuel cell using a molten hydroxide electrolyte and Pt/C catalyst was constructed and tested using hydrogen and oxygen as fuel. The performance was comparable to that which has been obtained from PEM fuel cells at the low end of the voltage range, reaching 950ma/cm2 at 0.4 V in the highest performing test. Performance was superior to PEM fuel cells at the high end of the voltage range, due to the more favorable kinetics at the higher temperatures, with an open circuit voltage (OCV) of 1.0 V with a linear performance curve between 1.0 V and 0.6 V, which is characteristic of fuel cells with low kinetic overpotentials. Longevity of the fuel cell was very poor, however a number of experiments were undertaken to improve it, enabling extension of operating life from 5 minutes to 30 minutes, which is still far too low for practical use. The key problem was identified as electrolyte retention by the support matrix and possible degradation of the gas diffusion layer and catalyst. Experiments were also conducted using methanol vapor as fuel, and it was found to provide performance close to that recorded with pure hydrogen. Experiments were also conducted using several alternative molten salts, including nitrate and chloride eutectics. Combinations of nitrates with hydroxides added to act as a charge carrier produced a working fuel cell, however performance was greatly reduced. Though preliminary, the work described herein demonstrates the great potential of IT-MSFC, and outlines the work needed to make this type of fuel cell practical.

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