New inorganic–organic proton conducting membranes based on Nafion® and [(ZrO2)·(SiO2)0.67] nanoparticles: Synthesis vibrational studies and conductivity☆
Introduction
Perfluorinated polymer electrolytes such as Nafion®, Aciplex, Flemion and Dow membranes are crucial materials for the development of polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) [1], [2]. In these devices, the hydrated condition of the protonated form of a proton conducting membrane (PEM) is a critical parameter to control in order to obtain PEMFCs and DMFCs with a high performance and durability [3].
Organic-inorganic composite membranes based on Nafion® and inorganic micrometer to nanometer size fillers have been intensively explored and remain one of the most interesting routes in the preparation of promising electrolytes for application in fuel cells [4], [5], [6], [7], [8], [9], [10], [11], [12]. Thus, Nafion® membranes doped with: (a) heteropolyacids, such as phosphotungstic acid (PTA), allowed to obtain fuel cells with high performance at lower relative humidity (RH) and elevated temperature (ca. 120 °C); (b) hygroscopic oxides, such as SiO2, TiO2, ZrO2, Al2O3 and others, were proposed with the aim to increase the water uptake of the membranes and to decrease the humidification requirements of PEMFCs [4], [5], [6], [7], [8], [9], [10], [11], [12].
To date, the influence of chemical and physical properties of the inorganic fillers on the structure of the composite proton conducting polymer electrolytes is not well understood and prompted us to investigate Nafion® composite membranes systematically under varying conditions [4], [13], [14]. Thus, the effects of silica concentration in [Nafion/(SiO2)x] nanocomposite membranes with 0 ≤ x ≤ 15 wt% has been studied demonstrating that the inorganic filler and the four different water species domains embedded in bulk membranes affect the dynamic relaxations of Nafion® and thus, the material conductivity [12]. In addition, it was shown that the formation of dynamic cross-links SiO2···HSO3– in hydrophilic polar clusters influences the chain dynamics of hydrophobic fluorocarbon domains of the host material. Further investigations were performed on the [Nafion/(MxOy)n] membranes with MTi, Zr, Hf, Ta and W and n = 5 wt% [15]. Particularly, by vibrational spectroscopy it was demonstrated that fluorocarbon domains of [Nafion/(MxOy)n] materials consist mostly of chains with helical conformation 157 with a smaller amount of 103 helices [15]. The concentration of 103 helical chains depends on the type of metal oxide used in the preparation of the composite membrane. Furthermore, it was revealed that the amount of each of the four water domains detected in the bulk membranes depends on the acidity of MxOy oxocluster. Particularly, it was observed that the mechanical, thermal and dynamic characteristics of Nafion® host polymer depend on the concentration in bulk material of dynamic cross-links, RSO3H···MxOy···HSO3R, which are responsible for the good thermal, mechanical and electrical stability of the materials. In continuation of our efforts to develop new Nafion®-based nanocomposite materials with improved performance and to elucidate systematically the effect of inorganic fillers on the chemical and physical properties of membranes, here we investigated the influence on membrane structural characteristics of: (a) ZrO2 nanofillers covered with a SiO2 layer; (b) nanofiller concentration; (c) Nafion®-nanofiller interactions.
These studies were carried out by preparing seven nanocomposite membranes of formula with , where [(ZrO2)·(SiO2)0.67] is a ZrO2 nanoparticle core covered with a thin layer of SiO2. This aim was pursued by studying seven different films prepared by solvent casting procedure at 100 °C from a dispersion of Nafion® and [(ZrO2)·(SiO2)0.67] nanoparticles. The nanocomposite polymer electrolyte films were accurately characterized by scanning electron microscopy (SEM) and vibrational FT-IR ATR and micro-Raman laser spectroscopy. Vibrational investigations were performed in order to elucidate: (a) the [(ZrO2)·(SiO2)0.67]-Nafion® polymer interactions; (b) the water domain distributions present in bulk materials; and (c) the effect of [(ZrO2)·(SiO2)0.67]-Nafion® interactions on the secondary structure of fluorocarbon chains in the hydrophobic PTFE domains of membranes. Impedance measurements were performed in order to measure the conductivity dependence on temperature and composition.
Section snippets
Reagents
Nafion® ionomer 5 wt% solution (perfluorosulfonic acid PTFE copolymer solution) with a proton exchange capacity of 0.80 meq g−1 (Alfa Aesar, ACS grade) was used as purchased. MO2 nanometric oxoclusters with MSi and Zr (Aldrich, ACS grade) were purified by standard methods [16]. The SiO2 and ZrO2 oxides were characterized by an average particle size of 7 and 20–30 nm, respectively and by a density of 2.2 and 5.19 g mL−1, respectively. All solvents used were supplied by Aldrich and further purified by
Preparation of [(ZrO2)·(SiO2)0.67] nanofiller
Among the additives MxOy, with M = Si, Ti, Zr, Hf, Ta and W, recently investigated [15], the most promising composite membranes were obtained using as MxOy nanofillers with M = Hf and W. It was reported that MxOy influences the properties of the composite materials owing to the formation of strong dynamic cross-links, RSO3H···MxOy···HSO3R, which improve the mechanical, thermal and dynamic characteristics of Nafion® host polymer [12], [15]. In this report, to promote the interactions between
Conclusions
In this paper is reported the preparation of six membranes with . The nanofiller [(ZrO2)·(SiO2)0.67] was prepared by reacting SiO2 and ZrO2 nanopowders suspended in a DMF solution by a ball milling process. The inorganic nanofiller exhibited a size diameter between 15 and 50 nm and consists of a ZrO2 core covered by a layer of SiO2. Water uptake (WU) values of membranes were: (a) lower than that of pristine Nafion®;
Acknowledgements
Research was funded by the Italian MURST project NUME of FISR2003, “Sviluppo di membrane protoniche composite e di configurazioni elettrodiche innovative per celle a combustibile con elettrolita polimerico”.
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