Les membranes ionomères (par exemple le Nafion®) sont utilisées en tant qu’électrolyte dans les piles à combustible à membrane échangeuse d’ions (PEMFC) dont les performances dépendent fortement de l’état d’hydratation de cette membrane. Il est donc fondamental de connaître la distribution en eau dans la membrane ainsi que dans la pile à combustible. Les coefficients d'autodiffusion ont été mesurés par Résonance Magnétique Nucléaire (RMN) employant des gradients de champ magnétique statique B0 ou des gradients de champ magnétique radiofréquence B1. Cette seconde méthode permettant de s’affranchir de l’effet des gradients internes, nous avons pu mesurer une diminution du coefficient de diffusion apparent en fonction de l'intervalle de diffusion ce que ne permet pas l’utilisation des gradients B0. L'effet du flux électro-osmotique a pu être mis en évidence dans une membrane soumise à un champ électrique. Après avoir appliqué une tension constante aux bornes de deux électrodes en platine, placées à chaque extrémité de la membrane, la migration de l'eau de l’anode vers la cathode a pu être visualisée par des techniques d’Imagerie par Résonance Magnétique (IRM). L’IRM a finalement été employée pour étudier les phénomènes de transport directement dans une PEMFC en fonctionnement. Ces expériences sont plus délicates à mettre en œuvre, et nécessitent la conception d’une PEMFC optimisée pour l’observation par IRM. Cette cellule élémentaire s'est avérée avoir des propriétés comparables à celles disponibles dans le commerce. Les résultats préliminaires montrent une accumulation progressive de l'eau près de la sortie des gaz tandis que l'admission reste sèche. / PEMFC use perfluorosulfonic acid membranes (Nafion® for example) as solid electrolyte and their performances are strongly dependent on membrane hydration. Therefore, the accurate knowledge of water distribution in the membrane and in the fuel cell is a fundamental issue. First, self diffusion coefficients have been thoroughly measured by Nuclear Magnetic Resonance (NMR) using B0 gradients and/or B1 gradients. The latter method is more suitable in the case of short relaxation times and for avoiding effects of the so-called internal gradients. Indeed, we were able to observe the decrease of the apparent diffusion coefficient as a function of the diffusion interval whereas this feature is totally absent in the data obtained by B0 gradients. Secondly, electro-osmotic flow effect has been detected in a membrane experiencing an electrical field. The setup consists of two platinum electrodes at each extremity of the membrane. We have observed by Magnetic Resonance Imaging the migration of water when a constant tension between the membranes is imposed. Finally, MRI has been used to study these phenomena into a PEMFC under operation. These experiments, carried out with a whole fuel cell, are more difficult to achieve and they require a PEMFC optimized for the MRI observation. This fuel cell proved to have performances comparable to commercially available fuel cells. Preliminary results indicate a progressive accumulation of water close to the gas outlet while the gas inlet remains dry.

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Tese de mestrado integrado. Engenharia Química. Faculdade de Engenharia. Universidade do Porto, Laboratoire des Sciences du Génie Chimique. 2009

The clustered water and interfacial water in Nafion 117 membrane have been quantified by NMR deuterium and proton experiments. The oscillation of intensity profile in 2H CPMG sequence with large duration time is induced by the residual quadrupolar interaction in clustered water. While the intensity profile of 2H CPMG sequence at short duration time followed the rule of exponential decay which is the consequence of the profile-overlapping between clustered water with residual quadrupolar interaction and interfacial water with susceptibility effect. The populations of clustered water and interfacial water in fully hydrated membrane are estimated by quadrupolar echo sequence. The population of former and latter is 70% and 30%,respectively. At an elevated temperature, water in cluster region can transport to the interfacial region and result in change of intensity for proton spectrum. The activation energy of translational diffusion of interfacial water is lower than that of clustered water due to the strong binding energy between sulfonate group and water molecule. The rf-heating effect on the proton spectrum of clustered water also has been explored by 1H CPMG and rotating-frame-relaxation-dispersion experiments. The dielectric response to a time-dependent external electric field provides a source of heat which causes a drift in chemical shift and interfere with transverse relaxation .

Research was compiled in regards to the effect of various changes in the development and fabrication of the ultracapcitor. The binder was changed from Teflon to a mix of Teflon and Nafion. Results demonstrated that an increase in capacitance was seen in the ultracapcitor and is may be attributed to the structure of Nafion. Furthermore water soluble graphene, graphene oxide, and ionic liquid graphene was tested in order to determine the effects of those alterations on the overall capacitance. / text

Stuttgart, Univ., Diss., 2000.

The focus of this research project was to develop a fundamental understanding of the structure-gas transport property relationship in Nafion® to investigate its potential use as a gas separation membrane material for natural gas (NG) applications including carbon dioxide removal from NG, helium recovery, higher-hydrocarbon removal, and nitrogen separation from methane. Separation processes account for ~45% of all energy used in chemical plants and petroleum refineries. As the drive for energy savings and sustainability intensifies, more efficient separation technology becomes increasingly important. Saudi Arabia ranks among the world’s top 5 NG producers. Commercial hydrocarbon-based glassy polymers often lose their gas separation properties in the presence of condensable, highly sorbing NG components such as CO2, ethane, propane, n-butane, and C5+ hydrocarbons. This deterioration in gas separation performance results from penetrant-induced dilation and plasticization of the polymer matrix, leading to significant methane and higher hydrocarbon losses. Polymers that have intrinsically low affinity to high-solubility NG components may be less susceptible to plasticization and therefore offer better performance under actual field conditions. By virtue of their strong carbon-fluorine bonds and chemical inertness, perfluoropolymers exhibit very low affinity for hydrocarbon gases. Nafion®, the prototypical perfluoro-sulfonated ionomer, comprising hydrophilic sulfonate groups phase-separated from a hydrophobic perfluorocarbon matrix, has demonstrated interesting permeability and selectivity relationships for gas pairs relevant to NG applications. Gas transport properties of Nafion® indicated gas solubility behavior similar to rubbery polymers but with sieving properties more commonly observed in low free volume glassy polymers. Nafion® demonstrated very low solubility for CO2 and hydrocarbon gases; the trend-line slope of solubility versus penetrant condensability in Nafion® was almost 2.5 times lower than that of typical hydrocarbon polymers, highlighting Nafion’s® effectiveness in resisting high-solubility induced plasticization. Additionally, Nafion® showed extraordinarily high permselectivities between small gases (He, H2, CO2) and large hydrocarbon gases (C1+): He/CH4 = 445, He/C3H8 = 7400, CO2/CH4 = 28, CO2/C3H8 = 460, H2/CH4 = 84 and H2/C3H8 = 1400 owing to its tightly packed chain domains. These high selectivities could potentially be harnessed for helium recovery and CO2 removal in natural gas applications, and hydrogen recovery from refinery gas streams. Pressure-dependent pure- and mixed-gas permeabilities in Nafion® were determined at 35 °C. Nafion® demonstrated two divergent pressure-dependent permeability phenomena: gas compression and plasticization. In pure-gas experiments, the permeability of the permanent gases H2, O2, N2 and CH4 decreased with increasing pressure due to polymer compression, whereas the permeability of the more condensable gases CO2, C2H6 and C3H8 increased dramatically due to solubility-induced plasticization. Binary CO2/CH4 (50:50) mixed-gas experiments showed reduced performance with up to 2-fold increases in CH4 permeability from 0.075 to 0.127 Barrer, and a 45% drop in selectivity (from 26 to 14), between 2 and 36 atm total pressure as a result of CO2-induced plasticization. At a typical NG CO2 partial pressure of 10 atm, Nafion® exhibited 24% lower CO2/CH4 selectivity of 19, with a 4-fold lower CO2 permeability of 1.8 Barrer relative to a commercial cellulose acetate (CA) membrane. Ternary CO2/CH4/C3H8 (30:50:20) experiments quantified the effect of CO2 and C3H8 plasticization. The presence of C3H8 reduced CO2 permeability further due to a competitive sorption effect causing a 31% reduction in CO2/CH4 selectivity, relative to its pure-gas value of 29, at 16 atm total feed pressure. The strong cation-exchanging sulfonate groups in Nafion® provided an opportunity to tailor the material properties by incorporating metal ions through a simple ion-exchange process. Nafion® neutralized with Fe3+ was investigated as a potential approach to mitigate CO2-plasticization. XRD results demonstrated an increase in crystallinity from 9% in Nafion H+ to 23% in Nafion Fe3+; however, no significant changes in the average inter chain spacing was observed. Raman and FT-IR technique qualitatively measured the strength of the ionic bond between Fe3+ cation and sulfonate anion. The strong crosslinking effect in Fe3+-cation-exchanged membrane demonstrated substantial increase in permselectivity: N2/CH4 selectivity increased by 39% (from 2.9 to 4.0) and CO2/CH4 selectivity increased by 25% (from 28 to 35). Binary CO2/CH4 (50:50) mixed-gas experiments at total feed pressures up to 30 atm quantified the effect of CO2 plasticization on the CO2/CH4 separation performance. Nafion® Fe3+ demonstrated better resistivity to plasticization enduring approximately 30% CH4 permeability increases from 0.033 Barrer at 2 atm to 0.043 Barrer at 15 atm CO2 partial pressure. At 10 atm CO2 partial pressure, CO2/CH4 selectivity in Nafion® Fe3+ decreased by 28% to 28 from its pure-gas value of 39, which was a significant improvement compared to Nafion® H+ membrane that decreased by 42% to 19 from its pure-gas value of 32.

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. / The fabrication and characterization of Nafion - TiO2 composites, and the use of such electrolytes in PEM (Proton Exchange Membrane) fuel cell operating at high temperature (130 °C) were studied. The operation of a PEM fuel cell at such high temperature is considered as an effective way to promote fast electrode reaction kinetics, high diffusional transport, and high tolerance to the carbon monoxide fuel contaminant. The polymer Nafion® is the most used electrolyte in PEM fuel cells due to its high proton conductivity. However, the proton transport in Nafion is dependent on the water content in the polymeric membrane. The need of absorbed water in the polymer structure limits the operation of the fuel cell to temperatures close to 100 °C, above which Nafion exhibits a fast decrease of the ionic conductivity. In order to increase the performance of the electrolyte operating at high temperatures, Nafion-TiO2 composites have been prepared by casting. The addition of titania hygroscopic particles to the polymeric matrix aims at the enhancement of the humidification of the electrolyte at temperatures above 100 °C. Three types of titania particles with different specific surface area and morphology have been investigated. Nafion-based composites with the addition of titania nanoparticles, in the 2.5-15 wt.% range, with nearly spherical shape and specific surface area up to ~115 m2g-1 were found to have higher glass transition temperature than the polymer. Such an increase improves the stability of the electrolyte during the fuel cell operation at high temperatures. The addition of titania-derived nanotubes results in a pronounced increase of the performance of PEM fuel cell operating at 130 °C. In this composite, the high specific surface area and the tubular shape of the inorganic phase are responsible for the measured increase of both the absorption and retention of water of the composite electrolyte. Nonetheless, the polarization curves of fuel cell using the composite electrolytes exhibited an increase of the ohmic polarization associated with the addition of the insulating titania particles. As the chemical structure of Nafion was observed to be insensitive to the addition of the inorganic particles, the high performance of the composite electrolytes is a result of competing effects: the decrease of the electrical conductivity and a higher thermal stability or water absorption/retention capacity. The experimental results suggest that the Nafion-TiO2 composites are promising electrolytes for PEM fuel cells operating at temperatures above ~100 °C.

Este trabalho teve como objetivo a combinação de ânodos e eletrólitos otimizados, para a formação de células a combustível de etanol direto (DEFC), operantes em elevadas temperaturas (130 ºC). Como materiais de ânodo, foram produzidos eletrocatalisadores baseados em PtSn/C, com diversas razões atômicas Pt:Sn, preparados pelo método do poliol modificado, essa metodologia possibilita a produção de eletrocatalisadores auto-organizados com estreita distribuição de tamanhos de partículas e elevado grau de liga. Os eletrocatalisadores foram caracterizados por DRX e stripping de CO. Os resultados mostraram que esses materiais apresentaram elevado grau de liga e Eonset de oxidação de CO em potenciais menores do que os materiais comerciais. Como eletrólito, foram sintetizados híbridos Nafion-SiO2 com a incorporação do óxido diretamente nos agregados iônicos de diversos tipos de membranas Nafion. Os parâmetros de síntese, tais como o solvente em meio solgel, a espessura da membrana, e a concentração do precursor de sílica foram avaliados em termos do percentual de sílica incorporada e da estabilidade mecânica do híbrido. Por fim, ânodos e eletrólitos otimizados foram avaliados em DEFCs nas temperaturas de 80 e 130 ºC. Os resultados mostraram um significativo incremento no desempenho de polarização (122 mW cm-2), resultado da aceleração na taxa de oxidação de etanol devido ao material de ânodo otimizado e do aumento de temperatura de operação, uma vez que o uso de eletrólitos híbridos possibilita o aumento da temperatura sem perdas de condutividade. Nesse sentido, a combinação de eletrodos e eletrólitos otimizados é uma alternativa promissora para o desenvolvimento de tais dispositivos. / This work has as objective to evaluate anodes and electrolytes in direct ethanol fuel cells (DEFC) operating at high temperature (130 ºC). As anode materials, electrocatalysts based on PtSn/C were prepared by Modified Polyol Method with various Pt:Sn atomic ratios. Such methodology promotes selforganized electrocatalysts production with narrow particle size distribution and high alloying degree. The eletrocatalysts were characterized by XRD, and CO stripping. The results showed that these materials presented high alloying degree and Eonset CO oxidation at lower potential as commercial materials. As electrolyte, Nafion-SiO2 hybrids were synthesized by sol-gel reaction, by the incorporation of oxide directly into the ionic aggregates of various kinds of Nafion membranes. The synthesis parameter, such sol-gel solvent, membrane thickness and silicon precursor concentration were studied in terms of silica incorporation degree and hybrid mechanical stability. Finally, the optimized anodes and electrolytes were evaluated in DEFC operating at 80 130 ºC temperature range. The results showed a significant improvement of the DEFC performance (122 mW cm-2), resulted from the acceleration of ethanol oxidation reaction rate due to anode material optimization and high temperature operation once the use of hybrids possibilities the increase of temperature without a significant conductivity loses. In this sense, the combination of optimized electrodes and electrolytes are a promising alternative for the development of these devices.