JAQUELINE DE SOUZA DA SILVA

JAQUELINE DE SOUZA DA SILVA

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Identification of the polymer and electrode polarizations of Nafion dielectric spectrum
2019 - MATOS, B.R.; SILVA, J.S. da; SCHADE, U.; PUSKAR, L.; FONSECA, F.C.
Efeito da relaxação do polímero na cristalinidade, fase iônica e formação de grupos sulfônicos anidridos no Nafion
2017 - MATOS, B.R.; SANTIAGO, E.I.; TOSCO, B.; REY, J.F.Q.; SILVA, J.S. da; SCHADE, U.; PUSKAR, L.; AZIZ, E.F.; FONSECA, F.C.
A relação entre as propriedades estruturais e elétricas de amostras de Nafion preparadas sob tratamento térmico em diferentes temperaturas (220 > T > 100 °C) foi investigada por espectroscopia no infravermelho (FTIR), espalhamento de raios-X em baixo ângulo (SAXS), calorimetria diferencial exploratória (DSC), análise dinâmico-mecânica (DMA) e espectroscopia de impedância (IS). Os resultados combinados de FTIR, SAXS, DSC, DMA revelaram que o tratamento térmico em baixas umidades relativas altera a morfologia do Nafion irreversivelmente devido a três características principais: i) o rearranjo das ligações de hidrogênio; ii) o reordenamento da fase cristalina; e iii) a formação de grupos sulfônicos anidridos. No entanto, a formação dos grupos anidridos é revertida pela reacidificação da membrana em soluções ácidas. O aspecto mais importante da dinâmica dos três processos descritos é que tais alterações são aceleradas acima de uma temperatura crítica: a temperatura de término da transição α do Nafion (Tendα ~ 160 °C). A transição α é atribuída ao enfraquecimento das interações eletrostáticas existentes entre os grupos sulfônicos do Nafion, que permitem a movimentação de longo alcance das cadeias poliméricas [1]. Este resultado indica que a maior dinâmica das cadeias do polímero em T > Tendα é o principal fator envolvido na modificação irreversível da morfologia do Nafion. As alterações da estrutura das ligações de hidrogênio afetaram negativamente as propriedades elétricas do Nafion. O aumento da temperatura de tratamento térmico reduz a condutividade protônica e aumenta a energia de ativação do transporte de cargas no Nafion. Tais resultados são relevantes para a preparação de conjuntos eletrodos-membrana (MEA) e para a operação de células a combustível de eletrólito polimérico (PEFC) em altas temperaturas [1].
The relation between the alpha/beta relaxation dynamics and the shape of ionomer building blocks
2018 - MATOS, B.R.; REY, J.F.Q.; MERINO, D.H.; SILVA, J.S. da; SCHADE, U.; PUSKAR, L.; FONSECA, F.C.
The relation between the alpha/beta relaxations and the shape of the building blocks of ionomer materials is a key factor for programming an important temperaturedependent property: the memory of shape [1,2,3]. However, the morphology of ionomers is indirectly obtained via modeling of small angle X-ray scattering (SAXS) data owing to the hardly accessible image characterization of the nanometric building blocks - micelle-like cylindrical polymeric aggregates (radius ~ 2 - 6 nm and length > 100 nm) [3]. Herein, broadband dielectric spectroscopy (BDS) measurements, free from electrode polarization effects, allowed identifying the time and temperature dependence of the polarization of different length scales of the ionomer matrix, and more importantly, by directly providing the aspect ratio of the radius and length of the polymeric aggregates for each desired temperature. This finding is essential for controlling the shape of ionomer based functional products under several stimuli conditions, thereby advancing remarkable applications, such as four dimensional (4D) printing and polymer electrolyte fuel cells.
Electrostatic interactions of Ionomer Films as Probed by Variable Temperature Synchrotron Infrared Spectroscopy
2018 - SILVA, J.S. da; MATOS, B.R.; SCHADE, U.; PUSKAR, L.; FONSECA, F.C.
The state of the art polymer electrolyte of Proton Exchange Membrane Fuel Cells (PEM), Nafion®, has poor mechanical and electrical properties at T > 100 °C. Specifically, long-term operation leads to irreversible performance losses that are related to an irreversible modification of the ionomer morphology above the temperature of alpha-transition (Talpha ~ 110 °C). Previous characterizations showed that the alpha-transition is dependent on the different states of covalent and ionic interactions among sulfonic acid groups, such as: ionic repulsions among RSO3-, dipolar attractions among RSO3H dipoles, hydrogen bonding of sulfonic groups with bulk and coordinated water molecules. The identification of both the functional groups interactions in the MIR bands and the “ion-hopping bands” in the FIR bands for annealed Nafion samples can give new insights into the role played by the ionic interactions on the alpha of ionomer membranes. Such approach for understanding the relationship between the dynamics of aplha-relaxation and Nafion morphology is missing in the literature, possibly due to the incipient number of FIR studies of Nafion. The central objective of this work is to advance the understanding of the chemical features involved during the aplha-transition of in situ annealed Nafion membranes with the high-resolution mid (MIR) and far (FIR) infrared spectroscopy using the IRIS beamline of BESSY II synchrotron light source. Herein, the effect of short-term annealing of Nafion at RH ~ 0% and selected temperatures below and above the alphatransition is presented. The comparison between SAXS and FTIR data of Nafion membranes annealed in situ revealed that the alpha-transition is due to the long range motion of the ionomer chains via weakening of electrostatic interactions of the ionomer functional groups.
Structure and transport properties of annealed nafion membranes
2016 - MATOS, BRUNO R.; SILVA, JAQUELINE de S. da; FONSECA, FABIO C.
The relationship between electrical properties and the structural features of annealed Nafion samples was investigated by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and impedance spectroscopy. Understanding of the observed effects on the proton conductivity due to annealing at controlled humidity at high temperature is crucial for improving the polymer electrolyte fuel cell (PEFC) at high operating temperatures (T > 100 °C) [1]. The experimental results reveal that membranes submitted to a heat treatment in the 100 – 200 °C temperature interval at low relative humidity (RH), display higher crystallinity and irreversibly shrinking of the ionic domains. However, the electrical properties of the polymer matrix were mostly affected by the irreversible destabilization of the ionic phase. The increase in crystallinity and shrinking of the hydrophilic domains reduce the ionomer conductivity. These features were reflected in the polymer electrolyte fuel cell (PEFC) tests in which the measured performance was lower for Nafion samples annealed at high T compared to pristine membranes. Such results are relevant for the preparation of membrane electrode assemblies as well as for the development of highperformance ionomer membranes.
Properties and defc tests of nafion added functionalized titanate nanotubes prepared by extrusion
2016 - MATOS, B.R.; GOULART, C.A.; ISIDORO, R.A.; SILVA, J.S. da; SANTIAGO, E.I.; FONSECA, F.C.; TAVARES, A.C.
Composite electrolyte membranes based on the incorporation of a second inorganic phase into ionomer matrices such as Nafion revealed to possess enhanced properties such as increased mechanical resistance and reduced permeability of solvents. It has been reported that surface functionalized titanate nanotubes (H2Ti3O7.nH2O) display a proton conductivity of ~ 10-2 Scm-1, which is attractive for the use of such composites in direct ethanol fuel cells (DEFC). Herein, composite membranes based on the addition of sulfonic acid groups functionalized titanate nanotubes into Nafion matrix were prepared by grafting followed by extrusion. These membranes were characterized by infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), acid-base titration, proton conductivity measurements and DEFC tests. FTIR measurements confirmed both the grafting of the titanate nanotubes. BET measurements showed that the functionalized titanate nanotubes possess a high surface specific area. Acid-base titration evidenced that additional sulfonic acid groups are present in the composite membranes compared to the pristine ionomer. The conductivity measurements show that the increase in the titanate nanotube volume fraction into the ionomers has not resulted in a decrease of the proton conductivity. The results show that the addition of functionalized titanate nanotubes into Nafion polymer matrix resulted in an improvement of the electric transport properties, reduction of the fuel crossover and, consequently, a higher DEFC performance for the composites were observed with respect to the pristine Nafion.