Made available in DSpace on 2014-10-09T12:26:25Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:27Z (GMT). No. of bitstreams: 0 / Os eletrocatalisadores PtRh/C, PtRu/C, PtSn/C, PtRuRh/C e PtSnRh/C (20% em massa de metais) foram preparados pelo método da redução por álcool usando H2PtCl6.6H2O (Aldrich), RhCl3.xH2O (Aldrich) e SnCl2.2H2O (Aldrich) como fonte de metais e o carbono Vulcan XC-72 como suporte. Os eletrocatalisadores foram caracterizados pelas técnicas de EDX, difração de raios X e voltametria cíclica. A eletro-oxidação do metanol e do etanol foram estudadas através das técnicas de voltametria cíclica, cronoamperometria e curvas de polarização obtidas em células a combustível unitárias alimentadas diretamente por metanol ou etanol. As análises por EDX mostraram que as razões atômicas dos diferentes eletrocatalisadores preparados pelo método da redução do álcool são bastante similares às composições nominais de partida. Em todos os difratogramas para os eletrocatalisadores preparados observa-se um pico largo em aproximadamente 2 = 25o o qual é associado ao suporte de carbono e quatro outros picos de difração em aproximadamente 2 = 40o, 47o, 67o e 82o os quais são associados aos planos (111), (200), (220) e (311), respectivamente, da estrutura cúbica de face centrada (CFC) de platina e ligas de platina. PtSn/C e PtSnRh/C além da estrutura cúbica de face centrada apresentaram também fases de óxidos de estanho em 2 = 34o. PtSn/C e PtSnRh/C apresentaram os melhores resultados para o etanol a temperatura ambiente, enquanto que para a eletro-oxidação do metanol os sistemas PtRu/C, PtSn/C e PtRuRh/C apresentaram os melhores resultados. Os testes em células a combustível para o etanol mostraram que o sistema PtSnRh/C foi mais ativo em relação ao sistema PtSn/C. No caso do metanol PtRuRh/C apresenta um desempenho ligeiramente superior ao sistema PtRu/C e PtSn/C. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:53:53Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:09:32Z (GMT). No. of bitstreams: 1 12641.pdf: 5742514 bytes, checksum: 8dbb5287cf99fe54ebb95b5b3406a880 (MD5) / A fabricação e a caracterização de eletrólitos compósitos Nafion - TiO2, e seu uso em células PEM (Proton Exchange Membrane) operando em temperaturas elevadas (~ 130 ºC) foram estudados. A operação em altas temperaturas da célula PEM traz benefícios, como o aumento da cinética das reações eletródicas, o aumento da cinética de transporte difusional nos eletrodos e o aumento da tolerância da célula ao contaminante monóxido de carbono. O Nafion ®, eletrólito polimérico comumente empregado em células PEM, possui condutividade elétrica dependente da quantidade de água contida em sua estrutura. Desta forma, o aumento da temperatura de operação da célula acima de 100 ºC causa a desidratação do polímero diminuindo acentuadamente sua condutividade elétrica. Para aumentar o desempenho dos eletrólitos operando em altas temperaturas, eletrólitos compósitos (Nafion-TiO2) foram preparados pelo método de conformação por evaporação em molde. A adição de partículas higroscópicas de titânia (TiO2) na matriz polimérica visa melhorar as condições de umidificação do eletrólito em temperaturas elevadas. Três tipos de partículas de titânia com diferentes áreas de superfície específica e formas distintas foram investigados. Compósitos à base de Nafion com adição de 2,5 a 15% em massa de partículas de titânia com forma aproximadamente esférica e com área de superfície específica de até ~115 m2g-1 apresentaram maiores valores da temperatura de transição vítrea do que o polímero. Este aumento melhora a estabilidade do eletrólito durante a operação de células a combustível PEM em 130 ºC. Os compósitos formados a partir da adição de nanotubos derivados de titânia apresentaram pronunciado ganho de desempenho e maior estabilidade térmica em operação de células acima de 100 ºC. Neste caso, a elevada área superficial e a forma dos nanotubos de titânia contribuíram significativamente para o aumento da absorção e da retenção de água do compósito. Por outro lado, as curvas de polarização mostraram um aumento na polarização por queda ôhmica com o aumento da concentração das partículas cerâmicas adicionadas. A morfologia do polímero não foi alterada com a adição de partículas inorgânicas, portanto, o desempenho dos compósitos reflete uma competição entre a adição de uma fase isolante, que diminui a condutividade elétrica, e o aumento da estabilidade térmica ou da retenção de água do compósito. Os eletrólitos compósitos testados provaram serem promissores na aplicação em células PEM em temperaturas acima de 100 ºC. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Fuel cells have long been seen as an alternative to combustion powered and diesel powered engines and turbines. Production of energy via a fuel cell conversion method can generate up to 60% efficiency in comparison to 30% using a combustion powered engine, with low co-production of harmful side-products. The polymer electrolyte membrane (PEM) adapted for the fuel cell application is one of the main components that determines the overall efficiency. This research project was focused towards novel PEMs, such as sulfonated poly(etheretherketone) or SPEEK, which are cost-efficient and robust with high proton conductivities under hydrated conditions. The degree of sulfonation (DS) of a particular SPEEK polymer determines the proton conducting ability, as well as the long term durability. For SPEEK with high DS, the proton conduction is facile, but the mechanical stability of the polymer decreases almost proportionally. While low DS SPEEK does not have sufficient sulfonic acid density for fast proton conduction in the membrane, the membrane keeps its mechanical integrity under fully saturated conditions. The main purpose of this work was to address both issues encountered with SPEEK sulfonated to low and high DS. The addition of both solid acids and synthetic cross-links were studied to address the main downfalls of the respective SPEEK polymers. Optimization of these techniques led to increased understanding of PEMs and notably better electrochemical performance of these fuel cell materials. Oxo-acids such as tungsten (VI) oxide (WO3) and phosphotungstic acid (PTA) have been identified as candidate materials for creating SPEEK composite membranes. The chemistry of these oxo-acids is well known, with their use as highly acidic catalyst iv centers adopted for countless homogeneous and heterogeneous, organic and inorganic reactions. Uniform dispersion of WO3 hydrate in SPEEK solution was done by a sol-gel process in which the filler particles were grown in an ionomer solution, cast and allowed to dry. PTA composites were made by adding the solid acid directly to a solution of the ionomer and casting. The latter casting was allowed to dry and Cs+ - exchanged to stabilize the PTA from dissolution and leaching from the membrane. The chemical and physical properties of these membranes were characterized and evaluated using mainly conductometric and X-ray photoelectron spectroscopic methods. Composite SPEEK/ PTA membranes showed a 50% decrease in PEM resistance under hydrogen fuel cell testing conditions, while SPEEK/ WO3 composites demonstrated a ten-fold increase in the membrane's in-plane proton conductivity. The chemical and physical properties of these composites changed with respect to their synthesis and fabrication procedures. This study will expound upon their relations.

Thesis (Ph.D.(Chemistry)) -- University of Limpopo, 2020 / The interest on the use of proton exchange membrane (PEM) fuel cells for vehicle application has increase due to its efficiency, high power density and rapid start up. The on-board reforming process is used to generate hydrogen; however, this process simultaneously produces 1% CO which poisons Pt-based anode catalyst. Previous studies have shown that supported Pd-based catalysts have very good stability on preferential oxidation (PROX) of CO, but these catalysts suffer from lower selectivity. Metal oxides such as Co3O4 and CeO2 are known to have high oxygen vacancy which promotes CO oxidation. Furthermore, the pre-treatment of the catalysts by hydrazine as well as the addition of MnOx species have been shown to improve the surface properties of metal/metal oxides catalysts. The study envisages that the modification of PROX catalysts will improve the CO conversion and its selectivity while maintaining higher stability. In this work, as-prepared (Co3O4) and hydrazine treated cobalt (Co3O4(H)) based catalysts were prepared by precipitation method and investigated at temperature range of 40-220 oC for preferential oxidation (PROX) of CO in excess hydrogen. The FTIR and XPS data of hydrazine treated Co3O4 does not show peak ratio differences, indicating that usual amounts of Co3+ and Co2+ were formed. An improved surface reducibility with smaller crystallite size was noted on Co3O4(H) catalyst, which indicate some surface transformation. Interestingly, the in-situ treatment of standalone Co3O4(H) decreased the maximum CO conversion temperature (T100%) from 160 oC (over Co3O4) to 100 oC. The Co3O4(H) catalyst showed good stability, with approximately 85% CO conversion at 100 oC for 21 h, as compared to fast deactivation of the Co3O4 catalyst. However, the Co3O4(H) catalyst was unstable in both CO2 and the moisture environment. Based on the spent hydrazine treated (CoO(H)) cobalt catalyst, the high PROX is associated with the formation of Co3+ species as confirmed by XRD, XPS, and TPR data. The Pd species was incorporated on different Co3O4 by improved wet impregnation method and this has improved the surface area of the overall catalysts. However, the presence of Pd species on Co3O4(H) catalyst decreased the CO conversion due to formation of moisture. Although, the Pd on Co3O4(H) had lower activity, the catalyst showed better stability under both moisture and CO2 conditions at 100 oC for 21 h. vi The 2wt.% metal oxides (MnO2, CeO2, Cr3O4, TiO2, MgO) on cobalt, and Pd on CeO2- Co3O4 and MnO2-Co3O4 were prepared by co-precipitation method and the structural composition was confirmed by XRD, FTIR, XPS and TPR data. Although, 2wt.%MnO2 on Co3O4(H) showed higher activity at 80 oC, both MnO2 and CeO2 improved the activity of Co3O4(H) at 100 oC. The higher activity of MnO2 is attributed to the higher surface area of the composite catalyst, in relation to ceria composite catalyst. Although the MnO2 species transformed the structure of Co3O4 by lowering the oxidation state to Co2+, the spent catalyst showed transformation from Co2+ to Co3+ during PROX, as confirmed by TPR data. Studies on the effects of CeO2 loading on Co3O4 catalysts, showed an optimum activity over 2wt.%CeO2-Co3O4 as compared to other ceria loadings (i.e., 3, 5, 8, 10, 15, 30wt.%CeO2). However, upon addition of 0.5wt.%Pd species on 2wt.%CeO2- Co3O4(H) composite, the activity of the catalyst decreased slightly at 100 oC, which could be due to a decreased surface area. Although its activity is lower, the catalyst has shown good stability in dry, moisture and CO2 conditions at 100 oC for 21 h. In addition, studies were also undertaken on the effect of MnO2 concentration on Co3O4 catalysts. The data shows that 7wt.%MnO2 species improved the activity of Co3O4 catalyst at 60 oC, however, the catalyst could not improve the activities at higher temperatures. This low activity is associated with a decrease in surface area as concentration increases. The presence of 0.5wt.%Pd species on 7wt.%MnO2-Co3O4 increased the activity at 60 and 80 oC, which could be due to reduction of Co3+ to Co2+ in the presence of Pd, as confirmed by XPS data. The catalyst has shown good stability in dry, moisture, and CO2 conditions at 100 oC for 21 h. The hydrazine treated cobalt-based catalysts in the presence of palladium and manganese oxide is the promising catalysts for proton exchange membrane fuel cells technology. / National Research Foundation (NRF) , Faculty of Science and Agriculture University of Limpopo and School of Physical and Mineral Sciences

Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, the most commonly used platinum electrocatalyst is expensive and the world supply of Pt is limited. In addition, the slow oxygen reduction and methanol oxidation kinetics as well as the poisoning of the Pt catalyst at the cathode resulting from methanol permeation from the anode through the Nafion membrane to the cathode lead to significant performance loss. Also, the electrocatalyst utilization in the electrodes also needs to be improved to reduce the overall cost of the electrocatalysts and improve the fuel cell performance. This dissertation explores nanostructured Pt alloys with lower cost and higher catalytic activity than Pt for oxygen reduction in PEMFC to understand the effect of synthesis and structure on the catalytic activity, methanol tolerant Pt/TiOx nanocomposites for oxygen reduction in DMFC, nanostructured Pt-Ru alloys for methanol oxidation in DMFC, and improvement in the utilization of Pt by optimizing the membrane-electrode assembly (MEA) fabrication. From a systematic investigation of a series of Pt-M alloys (M = Fe, Co, Ni, and Cu), the catalytic activity of Pt-M alloys is correlated with the extent of atomic ordering. More ordered Pt alloys exhibit higher catalytic activity than disordered Pt alloys. The higher activity of the ordered Pt alloys is found to relate to various factors including the Pt-Pt distance, Pt: 5d orbital vacancy, {100} planar density and surface atomic configuration. The catalytic activity of the Pt alloys is also influenced by the synthesis method. Low temperature solution methods usually result in smaller particle size and higher surface area, while high temperature routes result in larger particle size and lower surface area but with a greater extent of alloying. Pt/TiOx/C nanocomposites exhibit higher performance than Pt for oxygen reduction in DMFC. The nanocomposites show higher electrchochemical surface area, lower charge transfer resistance, and higher methanol tolerance than Pt. Pt-Ru alloy synthesized by a reverse microemulsion method exhibits higher catalytic surface area than the commercial Pt-Ru. The higher catalytic activity is attributed to a better control of the particle size, crystallinity, and microstructure. Membrane-electrode assemblies (MEAs) fabricated by a modified thin film method exhibit much higher electrocatalyst utilization efficiency and performance than the conventional MEAs in PEMFC. Power densities of 715 and 610 mW/cm2 are obtained at a Pt loading of, respectively, 0.1 and 0.05 mg/cm2 and 90 oC. The higher electrocatalyst utilization is attributed to the thin catalyst layer and a better continuity of the membrane/catalysts layer interface compared to that in the conventional MEAs. / text

Philosophiae Doctor - PhD / Proton exchange membrane fuel cells (PEMFC) converts chemical energy from the electrochemical reaction of oxygen and hydrogen into electrical while emitting heat, oxygen depleted air (ODA) and water as by-products. The by-products have useful functions in aircrafts, such as heat that can be used for ice prevention, deoxygenated air for fire retardation and drinkable water for use on board. Consequently, the PEMFC is also studied to optimize recovery of the useful products. Despite the progress made, durability and reliability remain key challenges to the fuel cell technology. One of the reasons for this is the limited understanding of PEMFC behaviour in the aeronautic environment. The aim of this thesis was to define a comprehensive non-intrusive diagnostic technique that provides real time diagnostics on the PEMFC State of Health (SoH). The framework of the study involved determining factors that have direct influence on fuel cell life in aeronautic environment through a literature survey, examining the effects of the factors by subjecting the PEMFC to simulated conditions, establishing measurable parameters reflective of the factors and defining the diagnostic tool based on literature review and this thesis finding.

The incorporation of an ejector refrigeration cycle with a high temperature PEM fuel cell (HT-PEMFC) presents a novel approach to combined heat and power (CHP) applications. An ejector refrigeration system (ERS) can enhance the flexibility of a CHP system by providing an additional means of utilizing the fuel cell waste heat besides domestic hot water (DHW) heating. This study looks into the performance gains that can be attained by incorporating ejector refrigeration with HT-PEMFC micro-CHP (mCHP) systems (1 to 5kWe). The effectiveness of the ERS in utilizing fuel cell waste heat is studied as is the relulting enhancement to overall system efficiency. A test rig specially constructed to evaluate an ERS under simulated HT-PEMFC conditions is used to test the concept and verify modeling predictions. In addition, two separate analytical models were constructed to simulate the ERS test rig and a HT-PEMFC/ERS mCHP system. The ERS test rig was simulated using a Matlab based model, while two residential sized HT-PEMFC/ERS mCHP systems were simulated using a Simulink model. Using U.S. Energy Information Administration (EIA) air conditioning and DHW load profiles, as well as data collected from a large residential monitoring study in Florida, the Simulink model provides the results in system efficiency gain associated with supporting residential space cooling and water heating loads. It was found that incorporation of an ERS increased the efficiency of a HT-PEMFC mCHP system by 8 t0 10 percentage points over just using the fuel cell waste heat for DHW. In addition, results from the Matlab ERS test rig model were shown to match well with experimental results. / by Michel Fuchs. / Thesis (Ph.D.)--Florida Atlantic University, 2012. / Includes bibliography. / Mode of access: World Wide Web. / System requirements: Adobe Reader.

Pulsing the flow of reactants in proton exchange membrane fuel cells (PEMFC) is a new frontier in the area of fuel cell research. Although power performance losses resulting from water accumulation also referred to as flooding, and power performance recovery resulting from water removal or purging, have been studied and monitored, the nexus between pulsing of reactants and power performance has yet to be established. This study introduces pulsing of reactants as a method of improving power performance. This study investigates how under continuous supply of reactants, pressure increase due to water accumulation, and power performance decay in PEMFCs. Furthermore, this study shows that power performance can be optimized through pulsing of reactants, and it investigates several variables affecting the power production under these conditions. Specifically, changes in frequency, duty cycle, and shifting of reactants as they affect performance are monitored and analyzed. Advanced data acquisition and control software allow multi-input monitoring of thermo-fluid and electrical data, while analog and digital controllers make it possible to implement optimization techniques for both discrete and continuous modes. / by Aquiles Perez. / Thesis (Ph.D.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

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