<p>The direct methanol fuel cell (DMFC) is a very promisingpower source for low power applications. High power and energydensity, low emissions, operation at or near ambientconditions, fast and convenient refuelling and a potentiallyrenewable fuel source are some of the features that makes thefuel cell very promising. However, there are a few problemsthat have to be overcome if we are to see DMFCs in our everydaylife. One of the drawbacks is the low performance of the DMFCanode. In order to make a better anode, knowledge about whatlimits the performance is of vital importance. With theknowledge about the limitations of the anode, the flow field,gas diffusion layer and the morphology of the electrode can bemodified for optimum performance.</p><p>The aim of this thesis is to elucidate the limiting factorsof the DMFC anode. A secondary goal is to create a model of theperformance, which also has a low computational cost so that itcan be used as a sub model in more complex system models. Toreach the primary goal, to elucidate the limiting factors, amodel has to be set up that describes the most importantphysical principles occurring in the anode.</p><p>In addition, experiments have to be performed to validatethe model. To reach the secondary goal, the model has to bereduced to a minimum. A visual DMFC has been developed alongwith a methodology to extract two-phase data. This has provento be a very important part of the understanding of thelimiting factors. Models have been developed from a detailedmodel of the active layer to a two-phase model including theentire three-dimensional anode.</p><p>The results in the thesis show that the microstructure inthe active layer does not limit the performance. Thelimitations are rather caused by the slow oxidation kineticsand, at concentrations lower than 2 M of methanol, the masstransport resistance to and inside the active layer. Theresults also show that the mass transfer of methanol to theactive layer is improved if gas phase is present, especiallyfor higher temperatures since the gas phase then contains moremethanol.</p><p>It is concluded that the mass transport resistance lower theperformance of a porous DMFC anode at the methanolconcentrations used today. It is also concluded that masstransfer may be improved by making sure that there is gas phasepresent, which can be done by choosing flow distributor and gasdiffusion layer well.</p><p>Keywords: direct methanol fuel cell, fuel cell, DMFC, anode,model</p>

Direct Methanol Fuel Cells (DMFCs) are electrochemical energy conversion devices that convert chemical energy into electrical energy. The bipolar plate component of the DMFC is required to be fluid impermeable to prevent fuel leakage and electrically conductive to collect the electrons produced within the cell. Graphite possesses the properties of high electrical conductivity, low weight and resistance to corrosion that make it an attractive material for bipolar plates. However, the poor mechanical properties of graphite lead to prohibitive machining costs and increased production times. The objective of this research is to develop an indirect laser sintering (LS) process, involving the laser sintering of graphite powders mixed with a phenolic resin binder which offers the advantage of complex part production and testing of prototype bipolar plates in short times. Due to the nature of the indirect LS process, the as-produced (green part) plates are porous and possess low electrical conductivities (< 0.1 S.cm-1). This research describes a viable method to rapidly fabricate and test multiple graphite bipolar plate designs using indirect LS. This process involved identifying and selecting suitable graphite powder and binder systems based on their thermal and electrical properties and developing a post process heat treatment method for achieving electrical conductivity of 250 S/cm for LS graphite parts which exceeds the DOE target of 100 S/cm for bipolar plate materials. The post processing also covered a method of infiltration using cyanoacrylate which was capable of rendering porous brown parts fluid impermeable and suitable for use in DMFCs. The cyanoacrylate infiltrated LS graphite parts were characterized for flexural strength and electrical and thermal conductivities and bipolar plates were made and evaluated in a DMFC test stand. Various flow field designs including plates with varying channel and rib widths and triangular, elliptical and rectangular flow field cross sections were fabricated using indirect LS and their respective polarization curves were compared to commercially machined graphite plates. The fuel cell tests show the improvement in mass transport performance could be due to improved methanol distribution and water removal characteristics of triangular and elliptical cross sectional channels over rectangular channels of equivalent dimensions. / text

Direct Methanol Fuel Cells (DMFC) provide an exciting alternative to current energy storage technologies for powering small portable electronic devices. For applications with sufficiently long durations of continuous operation, DMFC’s offer higher energy density, the ability to be refueled instead of recharged, and easier fuel handling and storage than devices that operate with hydrogen. At present, materials and manufacturing challenges impede performance and have prevented the entry of these devices to the marketplace. Higher-performing, cost-effective materials and efficient manufacturing processes are needed to enable the commercialization of DMFC. In a DMFC, the methanol-rich fuel stream and the oxidant are isolated from one another by a proton-conducting and electrically insulating membrane. Catalysts in the electrodes on either side of the Membrane Electrode Assembly (MEA) promote the two simultaneous half-reactions which allow the chemical energy carried in the fuel and oxidant to be converted directly into electricity. The goal of this research effort is to develop a continuous manufacturing process for the fabrication of effective DMFC MEAs. Based on the geometry of the electrode and materials used in the MEA, we propose a roll-to-roll process in which electrodes are coated onto a suitable substrate and subsequently assembled to form a MEA. Appropriate coating methods for electrode fabrication were identified by evaluating the requirements of continuous manufacturing processes; an appropriate set of these processes was then reduced to practice on a custom-designed flexible test bed designed explicitly for this project. After establishing baseline capabilities for several candidate methods, a spraying process was selected and a continuous manufacturing process concept was proposed. Finally, key control parameters of the spraying process were identified and their influence tested on actual MEAs to define optimal operating conditions. / text

No description available.

As the world begins to focus more and more on new and more effective means of energy production, fuel cells become increasingly more popular. While different fuel cells are already found in industry today, the direct methanol fuel cell (DMFC) is becoming an increasingly more probable means for portable power production. In such applications a passive air breathing direct methanol fuel cell would be ideal. However, successful use of the passive DMFC in such applications requires that the fuel cell be capable of operating at various temperatures and relative humidities. A passive air breathing direct methanol fuel cell was developed and manufactured for this study. This work studied the effects of varying relative humidity and temperature over a probable range of operating conditions for small scale portable power applications on the performance of the fuel cell, both in relation to power production and fuel consumption. Potentiostatic, electrochemical impedance spectroscopy, and polarization tests were performed in order to characterize the performance of the fuel cell. Additionally, a one dimensional steady state isothermal mass transport model was developed to provide insight to the behavior of the fuel cell. The experimental data and model results show that increasing the fuel cell temperature and decreasing the ambient relative humidity increases the current production capabilities of the fuel cell. Further, the experimental data suggests that the major problem hindering current production in passive air breathing direct methanol fuel cells is flooding of the cathode diffusion layer. / Master of Science

Foram preparados eletrocatalisadores PtRu/C e PtSn/C utilizando feixe de elétrons para a redução dos íons metálicos em solução. Neste procedimento submeteu-se ao feixe de elétrons, sob agitação, soluções de água/etileno glicol (EG) e água/2- propanol, ambas contendo íons dos metais precursores e o suporte de carbono. Foram variadas as razões volumétricas água/2-propanol e água/etileno glicol, a razão atômica entre os metais, o tempo de irradiação e a taxa de dose. Os eletrocatalisadores obtidos foram caracterizados por análise de raios X por energia dispersiva (EDX), por difração de raios X (DRX), por voltametria cíclica (VC) e por espectroscopia Mössbauer. A atividade destes eletrocatalisadores na oxidação eletroquímica do metanol e do etanol foi avaliada por voltametria cíclica e cronoamperometria, utilizando a técnica do eletrodo de camada fina porosa, e pelas curvas de polarização obtidas em células a combustível unitárias operando diretamente com metanol e etanol. Os eletrocatalisadores PtRu/C preparados no meio reacional água/etileno glicol(EG) apresentaram razões atômicas diferentes das razões atômicas nominais. Os resultados sugerem que parte dos íons Ru(III) presentes no meio reacional não foram reduzidos. Os materiais obtidos apresentaram a fase cúbica de face centrada (cfc) da Pt e suas ligas e tamanhos de cristalito na faixa de 2 a 3 nm. Os eletrocatalisadores PtRu/C preparados em água/2- propanol apresentaram razões atômicas Pt:Ru similares às razões nominais. Os materiais obtidos apresentaram as fases cfc da platina e suas ligas e tamanhos de cristalito entre 3 e 4 nm. Os eletrocatalisadores PtSn/C preparados no meio reacional água/EG e água/2-propanol apresentaram razões atômicas Pt:Sn similares às razões nominais. Os materiais obtidos apresentaram as fases cfc da platina com tamanho de cristalitos na faixa de 2 a 4 nm e SnO2 (cassiterita). Os estudos sobre a oxidação eletroquímica de metanol e etanol mostraram que foi possível obter materiais com atividades similares e/ou superiores às atividades dos eletrocatalisadores comerciais PtRu/C (E-TEK) e PtSn/C (BASF), tidos como referência na área. / PtRu/C and PtSn/C electrocatalysts were prepared using electron beam irradiation. The metal ions were dissolved in water/2-propanol and water/ethylene glycol solutions and the carbon support was added. The resulting mixtures were irradiated under stirring. The effect of water/ethylene glycol and water/2-propanol (v/v) ratio, Pt:Ru and Pt:Sn atomic ratios, the irradiation time and dose rate were studied. The obtained materials were characterized by Energy dispersive analysis of X-rays (EDX), X-ray diffraction (XRD), cyclic voltammetry (CV) and Mössbauer spectroscopy. The electro-oxidation of methanol and ethanol were studied by cyclic voltammetry and chronoamperometry using the thin porous coating technique. The electrocatalysts were also tested on the Direct Methanol and Ethanol Fuel Cells. PtRu/C electrocatalysts prepared in water/ethylene glycol showed Pt:Ru atomic ratios different from the nominal ones. The results suggested that part of the Ru(III) ions were not reduced. The obtained materials showed the face-centered cubic (fcc) structure of Pt and Pt alloys with crystallite sizes of 2-3 nm. PtRu/C electrocatalysts prepared in water/2-propanol showed Pt:Ru atomic ratios similar to the nominal ones. The obtained materials also showed the fcc structure of platinum and platinum alloys with crystallite sizes of 3-4 nm. PtSn/C electrocatalysts prepared in water/ethylene glycol and water/2-propanol showed Pt:Sn atomic ratios similar to the nominal ones. The obtained materials showed the platinum (fcc) phase with crystallite sizes in the range of 2 - 4 nm and a SnO2 (cassiterite) phase. The obtained PtRu/C and PtSn/C electrocatalysts showed similar or superior performance for methanol and ethanol electro-oxidation compared to commercial PtRu/C (E-TEK) and PtSn/C (BASF) electrocatalysts.

Foram preparados eletrocatalisadores PtRu/C e PtSn/C utilizando feixe de elétrons para a redução dos íons metálicos em solução. Neste procedimento submeteu-se ao feixe de elétrons, sob agitação, soluções de água/etileno glicol (EG) e água/2- propanol, ambas contendo íons dos metais precursores e o suporte de carbono. Foram variadas as razões volumétricas água/2-propanol e água/etileno glicol, a razão atômica entre os metais, o tempo de irradiação e a taxa de dose. Os eletrocatalisadores obtidos foram caracterizados por análise de raios X por energia dispersiva (EDX), por difração de raios X (DRX), por voltametria cíclica (VC) e por espectroscopia Mössbauer. A atividade destes eletrocatalisadores na oxidação eletroquímica do metanol e do etanol foi avaliada por voltametria cíclica e cronoamperometria, utilizando a técnica do eletrodo de camada fina porosa, e pelas curvas de polarização obtidas em células a combustível unitárias operando diretamente com metanol e etanol. Os eletrocatalisadores PtRu/C preparados no meio reacional água/etileno glicol(EG) apresentaram razões atômicas diferentes das razões atômicas nominais. Os resultados sugerem que parte dos íons Ru(III) presentes no meio reacional não foram reduzidos. Os materiais obtidos apresentaram a fase cúbica de face centrada (cfc) da Pt e suas ligas e tamanhos de cristalito na faixa de 2 a 3 nm. Os eletrocatalisadores PtRu/C preparados em água/2- propanol apresentaram razões atômicas Pt:Ru similares às razões nominais. Os materiais obtidos apresentaram as fases cfc da platina e suas ligas e tamanhos de cristalito entre 3 e 4 nm. Os eletrocatalisadores PtSn/C preparados no meio reacional água/EG e água/2-propanol apresentaram razões atômicas Pt:Sn similares às razões nominais. Os materiais obtidos apresentaram as fases cfc da platina com tamanho de cristalitos na faixa de 2 a 4 nm e SnO2 (cassiterita). Os estudos sobre a oxidação eletroquímica de metanol e etanol mostraram que foi possível obter materiais com atividades similares e/ou superiores às atividades dos eletrocatalisadores comerciais PtRu/C (E-TEK) e PtSn/C (BASF), tidos como referência na área. / PtRu/C and PtSn/C electrocatalysts were prepared using electron beam irradiation. The metal ions were dissolved in water/2-propanol and water/ethylene glycol solutions and the carbon support was added. The resulting mixtures were irradiated under stirring. The effect of water/ethylene glycol and water/2-propanol (v/v) ratio, Pt:Ru and Pt:Sn atomic ratios, the irradiation time and dose rate were studied. The obtained materials were characterized by Energy dispersive analysis of X-rays (EDX), X-ray diffraction (XRD), cyclic voltammetry (CV) and Mössbauer spectroscopy. The electro-oxidation of methanol and ethanol were studied by cyclic voltammetry and chronoamperometry using the thin porous coating technique. The electrocatalysts were also tested on the Direct Methanol and Ethanol Fuel Cells. PtRu/C electrocatalysts prepared in water/ethylene glycol showed Pt:Ru atomic ratios different from the nominal ones. The results suggested that part of the Ru(III) ions were not reduced. The obtained materials showed the face-centered cubic (fcc) structure of Pt and Pt alloys with crystallite sizes of 2-3 nm. PtRu/C electrocatalysts prepared in water/2-propanol showed Pt:Ru atomic ratios similar to the nominal ones. The obtained materials also showed the fcc structure of platinum and platinum alloys with crystallite sizes of 3-4 nm. PtSn/C electrocatalysts prepared in water/ethylene glycol and water/2-propanol showed Pt:Sn atomic ratios similar to the nominal ones. The obtained materials showed the platinum (fcc) phase with crystallite sizes in the range of 2 - 4 nm and a SnO2 (cassiterite) phase. The obtained PtRu/C and PtSn/C electrocatalysts showed similar or superior performance for methanol and ethanol electro-oxidation compared to commercial PtRu/C (E-TEK) and PtSn/C (BASF) electrocatalysts.

Magister Scientiae - MSc / Fuel cells convert chemical energy from a fuel into electricity through chemical reaction with oxygen. This possesses some challenges like slow oxygen reduction reaction (ORR), overpotential, and methanol fuel cross over in a direct methanol fuel cell (DMFC). These challenges cause inefficiency and use of higher amounts of the expensive platinum catalyst.Several binary catalysts with better ORR activity have been reported. In this study we investigate the best catalyst with better ORR and MOR performances and lower over-potentials for PEMFC and DMFC applications by comparing the in-house catalysts (10%Pt/C, 20%Pt/C,30%Pt15%Ru/C, 40%Pt20%Ru/C, 30%PtCo/C, 20%Pt20%Cu/C and 20%PtSn/C) with the commercial platinum based catalysts (10%Pt/C, 20%Pt/C, 20%Pt10%Ru/C, 20%PtCo/C,20%PtCu/C and 20%PtSn/C) using the cyclic voltammetry and the rotating disk electrode to determine their oxygen reduction reaction and methanol tolerance. HRTEM and XRD techniques were used to determine their particle size, arrangement and the atomic composition. It was observed that the 20%Pt/C in-house catalyst gave the best ORR activity and higher methanol oxidation current peaks compared to others catalysts followed by 20%Pt10%Ru/C commercial catalyst. The 20%PtCo/C commercial, 30%PtCo/C in-house and 20%PtSn/C in-house catalysts were found to be the most methanol tolerant catalysts making them the best catalysts for ORR in DMFC. It was observed that the ORR activity of 20%PtCo/C commercial and 30%PtCo/C inhouse catalysts were enhanced when heat treated at 350 0C. From XRD and HRTEM studies, the particle sizes were between 2.72nm to 5.02nm with little agglomeration but after the heat treatment, the particles were nicely dispersed on the carbon support.

>Magister Scientiae - MSc / The purpose of this study was to develop a high performance-lower cost catalyst to be applied in Direct Methanol Fuel Cells (DMFC). The study also aimed to prepare plurimetallic supported platinum (Pt), platinum-ruthenium (PtRu), platinum-ruthenium-vanadium (PtRuV) and platinum ruthenium-vanadium-iron (PtRuVFe) upon multi-walled carbon nanotube (MWCNT) as well as upon multiwalled carbon nanotube-titanium oxide (MWCNT/TiO2) supports. Platinum is very active but prone to poisoning by carbon monoxide (CO), which may be present in the fuel used in fuel cells. The focus on the use of methanol was because of its better reaction kinetics, and better performance in direct methanol fuel cells (DMFC) better than proton exchange membrane fuel cell (PEMFC). When Pt is alloyed with another platinum group metals (PGM) the alloying decreases the over-potential for reactions critical in the fuel cells. Proton exchange membrane fuel cell (PEMFC) performance may be improved at low metal loading, when supported pluri-metallic catalysts are applied since the trimetallic catalysts may promote high catalyst utilisation. In practice, DMFC require electrodes with a Pt loading to achieve acceptance fuel cell (FC) power performance. The aim of this study was therefore the reduction of the catalyst loading through further improvement of mass activity of Pt based catalysts by partial substitution of the noble metal/metals, and the use of a carbon support that will provide high surface area, good electrical conductivity and high stability. MWCNT supported pluri-metallic (PtRuVFe,) and bimetallic (PtRu) nanoparticles possessed characteristic of increased surface area, improved electron transfer rate, enhance electro-catalytic activity and promoted stability.

This thesis deals with modeling of two types of fuel cells:the polymer electrolyte fuel cell (PEFC) and the directmethanol fuel cell (DMFC), for which we address four majorissues: a) mass transport limitations; b) water management(PEFC); c) gas management (DMFC); d) thermal management. Four models have been derived and studied for the PEFC,focusing on the cathode. The first exploits the slenderness ofthe cathode for a two-dimensional geometry, leading to areduced model, where several nondimensional parameters capturethe behavior of the cathode. The model was extended to threedimensions, where four di.erent flow distributors were studiedfor the cathode. A quantitative comparison shows that theinterdigitated channels can sustain the highest currentdensities. These two models, comprising isothermal gasphaseflow, limit the studies to (a). Returning to a two-dimensionalgeometry of the PEFC, the liquid phase was introduced via aseparate flow model approach for the cathode. In addition toconservation of mass, momentum and species, the model wasextended to consider simultaneous charge and heat transfer forthe whole cell. Di.erent thermal, flow fields, and hydrodynamicconditions were studied, addressing (a), (b) and (d). A scaleanalysis allowed for predictions of the cell performance priorto any computations. Good agreement between experiments with asegmented cell and the model was obtained. A liquid-phase model, comprising conservation of mass,momentum and species, was derived and analyzed for the anode ofthe DMFC. The impact of hydrodynamic, electrochemical andgeometrical features on the fuel cell performance were studied,mainly focusing on (a). The slenderness of the anode allows theuse of a narrow-gap approximation, leading to a reduced model,with benefits such as reduced computational cost andunderstanding of the physical trends prior to any numericalcomputations. Adding the gas-phase via a multiphase mixtureapproach, the gas management (c) could also be studied.Experiments with a cell, equipped with a transparent end plate,allowed for visualization of the flow in the anode, as well asvalidation of the two-phase model. Good agreement betweenexperiments and the model was achieved. Keywords:Fuel cell; DMFC; PEFC; one-phase; two-phase;model; visual cell; segmented cell; scale analysis; asymptoticanalysis.