Structural characterization of PVdF-HFP/PEG/Al2O3 proton conducting membranes for fuel cells

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Abstract

Composite polymer membranes comprising of poly(vinylidene fluoride-hexafluoro propylene)/alumina (PVdF-HFP/Al2O3) were prepared by phase inversion technique with poly(ethylene glycol) (PEG) as an additive. The prepared membranes were soaked in 6 M H3PO4 and were immobilized for protonic conductivity. The morphological properties of the prepared membranes were examined by scanning electron microscopy (SEM). The infra-red spectroscopic measurements revealed the presence of phosphate anions in the composite membranes. The ionic conductivity of the membranes has also been measured. The protonic conductivity of the membrane has considerably been increased upon the incorporation of ceramic filler (Al2O3). The polymer membrane prepared with a weight ratio of PVdF-HFP (40):PEG (40):Al2O3 (20) showed maximum protonic conductivity due to the combined effect of inert filler and its porous nature.

Introduction

For more than a decade, there has been a progressive interest in the development of ionic conducting membranes. Having excellent properties they replace conventional liquid electrolyte systems leading to favorable substantial improvements in popular electronic devices such as lithium batteries and fuel cells [1], [2], [3], [4]. In particular, extensive research and developmental activities have been addressed to find a new class of polymer electrolytes for non-conventional polymer electrolyte fuel cells [5]. Anhydrous proton conducting polymer electrolytes have been formed by complexing a slurry of acid, e.g. H3PO4 or H2SO4 with a polymer containing a basic function such as poly(ethylene oxide) (PEO) or poly(ethylene imine) (PEM) [6]. Non-aqueous proton conducting gels with H2SO4 and organic solvents in a polymer matrix have also been studied [7].

Proton conducting membranes such as Nafion have been very successful for a number of applications where high proton conductivity is needed. These membranes as well as gel type proton conducting membrane consists of a polymer matrix swollen with water and/or liquid electrolyte solution and the proton transport takes place primarily in the electrolyte embedded in the polymer matrix [8]. However, these membranes suffer from low thermal stability and low methanol selectivity [2]. Recently, Ciuffa et al. [9] reported the ionic conductivity properties of membranes comprising of PVdF-SiO2-EC-PC-H3PO4 for the applications in non-conventional polymer electrolyte fuel cells. In a similar way, other polymer hosts like, poly(silamine) [10], poly(benzimidazole) [11], poly(methyl methacrylate) (PMMA) [12] have also been explored.

In the recent past, Bellcore laboratories successfully launched a new type of porous membranes with dibutylphthalate as an additive for high energy density lithium batteries. In a similar study, Croce et al. [2] reported the ionic conductivity and current–voltage characteristics of PVdF-CTFE copolymer with dispersed ceramic filler. So far, to the best of our knowledge no attempt has been made on PVdF-HFP based membranes which possess amorphous domains capable of trapping large amount of liquid electrolytes as well as the crystalline phase which provides sufficient mechanical strength for the processing of free-standing films. This paper reports the ionic conductivity, surface morphology and crystallinity studies of the polymer membranes prepared by phase inversion technique. Also of importance the ceramic filler, Al2O3 was added which can adsorb large amount of liquid electrolyte thus promoting ionic conductivity of the membrane.

Section snippets

Materials

Poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP) (Kynarflex 2801, Japan), PEG (Aldrich, molecular weight −11,500), alumina with an average particle size of 14–32 nm and surface area of 250 ± 25 m2 g−1 (LECO corporation, USA), acetone (Aldrich), phosphoric acid (Aldrich) were purchased and utilized as such without further purification.

Preparation of the membrane

Composite polymer electrolyte membranes were prepared by phase inversion technique as reported elsewhere [2], [13], [14]. Appropriate amounts of PVdF-HFP, PEG

Surface morphology

The SEM images of PVdF-HFP/PEG/Al2O3 membranes are shown in Fig. 1 (a–d). The membrane which is prepared without PEG Fig. 1(a) shows no porosity. The membranes with less amount of PEG Fig. 1(b and c) show an irregular morphology and lesser porosity. Although the morphology of the membranes Fig. 1(b and c) seem to be similar, the data obtained from the uptake of liquid electrolyte (Table 1) reveal that the content of PEG in the polymer solution has a finite influence on the morphology of the

Conclusion

As a proton conducting membrane, PVdF-HFP/PEG/Al2O3 membrane was prepared by a phase inversion method. The vibration spectroscopy and diffraction studies provided information on the structure and functionality of the PVdF-HFP/PEG/Al2O3 membrane. It is clear that both PEG and Al2O3 significantly improve the proton conductivity. After removal of PEG, the resultant membrane showed well-developed porosity, which can allow it to absorb large amount of acid molecules resulting in an enhanced ionic

Acknowledgement

This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J07501). One of the authors (G.G) thanks Dr. B. Karunagaran, SPRC, Chonbuk National University, Korea, for his help and useful discussions.

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