Elsevier

Journal of Power Sources

Volume 178, Issue 2, 1 April 2008, Pages 561-574
Journal of Power Sources

New inorganic–organic proton conducting membranes based on Nafion® and [(ZrO2)·(SiO2)0.67] nanoparticles: Synthesis vibrational studies and conductivity

https://doi.org/10.1016/j.jpowsour.2007.09.043Get rights and content

Abstract

In this report is described the preparation of six nanocomposite membranes of formula {Nafion/[(ZrO2)(SiO2)0.67]ΨZrO2} with ΨZrO2 ranging from 0 to 1.79 based on Nafion® and [(ZrO2)·(SiO2)0.67] nanofiller. Morphology investigations carried out by SEM measurements indicate that the composition of membranes is asymmetric. Indeed, with respect to the direction of the films after casting procedure, the top side (A-side) and bottom side (B-side) present a different nanofiller concentration. The concentration of nanofiller increases gradually from A to B side. The membranes present thicknesses ranging from 170 to 350 nm and are studied by FT-IR ATR and micro-Raman measurements.

The vibrational investigations permit us to reveal that: (a) the hydrophobic polytetrafluoroethylene (PTFE) domains of Nafion® are composed of a mixture of polymer chains with 157 and 103 helical conformations; (b) the concentration of chains with 103 helical conformation depends on the nanofiller concentration and is much higher in side A; (c) six different water domains are present in bulk membranes which are singled out as I, II, II, III, III and IV; (d) water uptake of membranes is correlated to the conformational transition 103  157 of PTFE chains occurring in hydrophobic domains of Nafion®. The conductivity of {Nafion/[(ZrO2)(SiO2)0.67]ΨZrO2} was determined by analyzing the complex conductivity plots measured in the frequency and temperature range of 10−2 Hz–10 MHz and 5–155 °C, respectively. Interestingly, the {Nafion/[(ZrO2)(SiO2)0.67]ΨZrO2} nanocomposite membranes with ΨZrO2=0.313 and 0.534 showed values of conductivity of 4.3 × 10−2 S cm−1 at 135 °C and of 3.5 × 10−2 S cm−1 at 115 °C, respectively.

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 Mdouble bondTi, 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, Rsingle bondSO3H···MxOy···HSO3single bondR, 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 {Nafion/[(ZrO2)(SiO2)0.67]ΨZrO2} with 0ΨZrO21.79, 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 Mdouble bondSi 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, Rsingle bondSO3H···MxOy···HSO3single bondR, 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 {Nafion/[(ZrO2)(SiO2)0.67]ΨZrO2} membranes with 0ΨZrO21.79. 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 {Nafion/[(ZrO2)(SiO2)0.67]ΨZrO2} 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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Oral contribution presented at the PBFC-2007 conference.

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