Effect of SiO₂ on Relaxation Phenomena and Mechanism of Ion Conductivity of [Nafion/(SiO₂)_x] Composite Membranes

Abstract

This report describes a study of the effect of SiO2 nanopowders on the mechanism of ionic motion and interactions taking place in hybrid inorganic-organic membranes based on Nafion. Five nanocomposite membranes of the formula [Nafion/(SiO2)x] with SiO2 ranging from 0 to 15 wt % were prepared by a solvent casting procedure. TG measurements demonstrated that the membranes are thermally stable up to 170 degrees C but with the loss water it changes the cluster environments and changes the conductivity properties. MDSC investigations in the 90-300 degrees C temperature range revealed the presence of three intense overlapping endothermal peaks indicated as I, II, and III. Peak I measures the order-disorder molecular rearrangement in hydrophilic polar clusters, II corresponds to the endothermic decomposition of -SO3 groups, and III describes the melting process in microcrystalline regions of hydrophobic fluorocarbon domains of the Nafion moiety. ESEM with EDAX measurements revealed that the membranes are homogeneous materials with smooth surfaces. DMA studies allowed us to measure two relaxation modes. The mechanical relaxation detected at ca. 100 degrees C is attributed to the motion of cluster aggregates of side chains and is diagnostic for R-SO3H...SiO2 nanocluster interactions. DMA disclosed that at SiO2/-SO3H (psi) molar ratios lower than 1.9, the oxoclusters act to restrict chain mobility of hydrophobic domains of Nafion and the dynamics inside polar cages of [Nafion/(SiO2)x] systems; at psi higher than 1.9, the oxoclusters reduce the cohesiveness of hydrophilic polar domains owing to a reduction in the density of cross-links. FT-IR and FT-Raman studies of the [Nafion/(SiO2)x] membranes indicated that the fluorocarbon chains of Nafion hydrophobic domains assume the typical helical conformation structure with a D(14pi/15) symmetry. These analyses revealed four different species of water domains embedded inside polar cages and their interconnecting channels: (a) bulk water [(H2O)n]; (b) water solvating the oxonium ions directly interacting with sulfonic acid groups [H3O+...SO3(-)-].(H2O)n; (c) water aggregates associated with H3O+ ions [H3O+.(H2O)n]; and (d) low associated water species in dimer form [(H2O)2]. The conductivity mechanism and relaxation events were investigated by broadband dielectric spectroscopy (BDS). [Nafion/(SiO2)x] nanocomposite membranes were found to possess two different molecular relaxation phenomena which are associated with the alpha-relaxation mode of PTFE-like fluorocarbon domains and the beta-relaxation mode of acid side groups of the Nafion component. Owing to their strong coupling, both these relaxation modes are diagnostic for the interactions between the polar groups of the Nafion host polymer and the (SiO2)x oxoclusters and play a determining role in the conductivity mechanism of the membranes. The studies support the proposal that long-range proton charge transfer in [Nafion/(SiO2)x] composites takes place due to a mechanism involving exchange of the proton between the four water ...

New nanocomposite proton conducting membranes based on a core–shell nanofiller for low relative humidity fuel cells

“…22 The formation of dynamic SiO 2 … HSO 3 -crosslinks in the hydrophilic polar clusters, as indicated by the increase in the elastic modulus, influences the chain dynamics of the hydrophobic fluorocarbon domains of the host material. 22 Other studies were performed on [Nafion/(M x O y ) n ] membranes containing 5 wt% Ti, Zr, Hf, Ta and W oxides. 23,24 These studies indicated that: a) the amount of each of the four water domains detected in the bulk membranes depends on the acidity of M x O y oxocluster; b) the thermal, mechanical and electrical stability of the Nafion 1 host polymer depends on the concentration of dynamic R-SO 3 H … M x O y … HSO 3 -R cross-links in bulk material; and c) the concentration and strength of the dynamic R-SO 3 H … M x O y … HSO 3 -R cross-links occurring inside polar hydrophilic cages of the membranes increase along group IV and decrease along period VI of the periodic exhibits very favorable physicochemical properties in terms of elastic modulus and conductivity, which result from strong acid-base interactions that occur within the ion aggregate domains between HfO 2 and the sulfonic acid groups of host polymer matrix.…”

“…The new ZrO 2 and Ta 2 O 5 -containing nanofiller is prepared with the intent of combining the improvements resulting from fillers with a ''core-shell'' morphology with the intrinsic characteristics of the individual oxides that will allow it to maintain strong R- 3 -R interactions and reduce the water uptake within the hybrid membranes. Of the group IV and period VI oxoclusters previously investigated, [22][23][24]28 32 The preparation of the nanofiller based on ZrO 2 and Ta 2 O 5 , indicated as ZrTa, is part of an effort to produce a ''chemical core-shell'' nanofiller and subsequently elucidate the effect of its chemistry and surface properties on the structure, properties and fuel cell performance of the resulting hybrid inorganic-organic membranes. 30 In addition, the water uptake, the thermal and structural properties will be examined.…”

Relação morfologia-propriedades elétricas de eletrólitos compósitos de Nafion para célula a combustível de alta temperatura

“…Na prática, a adição de nanopartículas cerâmicas em membranas Nafion resulta geralmente no decréscimo da condutividade protônica (σ) e o papel efetivo da segunda fase nas propriedades elétricas do Nafion não é totalmente compreendido [48]. …”

“…Na prática, a adição de nanopartículas cerâmicas em membranas Nafion resulta geralmente no decréscimo da condutividade protônica (σ) e o papel efetivo da segunda fase nas propriedades elétricas do Nafion não é totalmente compreendido [48]. As medidas de condutividade de eletrólitos compósitos reportadas variam com relação à orientação da amostra e o intervalo de temperatura medido.…”

“…É bem sabido que a microestrutura do Nafion é anisotrópica, e diferentes valores de σ são obtidos nas diferentes direções [57]. Além disso, medidas de σ para T > 80 °C (RH = 100%) são raramente encontrados para estes materiais [48]. As variações nas condições das análises de condutividade dificultam a análise comparativa dos fatores que influenciam o desempenho de células a combustível em altas temperaturas.…”