Fast Proton Conductors from Inorganic-Organic Composites: II. Amorphous Phosphate-PTFE and ZrP-PTFE Composites
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ABSTRACT A high proton-conductivity was observed in the composite of amorphous phosphate and polytetafluoroethylene (PTFE). Incorporation of amorphous phosphate into PTFE emulsion caused a large increase of conductivity to about 4x 10-2S/cm at 23°C. However, the conductivity decreased with increasing heat-treatment temperature, and the fabricated composite showed very low chemical stability. As a chemically stable composite, PTFE-based composite was also synthesized from a- or yzirconium phosphate crystalline powders dispersed in partially polymerized PTFE particles. By addition of zirconium phosphate powders, the proton conductivity jumped up to 2.2x l 0 3S/cm from 10 3S/cm of PTFE.
INTRODUCTION Nowadays, environmental conservation and new energy development have become serious issues for human beings. As the most practical fuel cell candidate, solid polymer electrolyte fuel cells (PEFCs) are receiving attention. Non-production of CO 2and suitability for electric vehicles are advantages of PEFCs. The polymer membrane in PEFC works as both the separator and the electrolyte. Therefore, electrolyte material in PEFC needs both high proton conductivity and high mechanical strength [ 1, 2]. For PEFC application, the inorganic-organic hybrids composites can be one of the most promising electrolyte materials because inorganic-organic hybrids represent a new class of materials that possess both inorganic and organic functionality. These materials have been synthesized by utilizing the low temperature reaction of the sol-gel process. Some kind of these materials fabricated by co-polymerization of metal alkoxides and organic-substituted silicon alkoxides or organic-based oligomers have been reported. Incorporating of other inorganic species can be also used to control the electrochemical and physical properties of hybrid composites. However, almost no attempt to use these inorganic-organic hybrids as a proton-conducting electrolyte has been reported. Here, we report fabrication procedure and properties of
proton-conducting inorganic-organic composites of: (1) amorphous phosphate-PTFE composite (P-PTFE) : PTFE membrane in which an amorphous phosphate phase incorporated via in situ sol-gel reaction for di-isopropyl phosphate. (2) a- or y-zirconium phosphate-PTFE composite (ZrP-PTFE) : PTFE-based hybrid incorporating layerstructured a- or y-zirconium phosphate crystalline (a-ZrP or y-ZrP) in partially polymerized PTFE grain boundaries.
EXPERIMENTAL PROCEDURE Sample preparation Poly-tetrafluoroethlyene (PTFE) emulsion (60wt%, grain size : 0.2ýim-0.4pm) and di-isopropyl phosphite (HPO(0C 3H7 )2) were used as starting materials for fabrication of P-PTFE composite. By mixing the starting materials and adding water at room temperature, fast agglomeration and co-polymerization were occurred resulting in a flexible bulk composite. After drying at room temperature for 4 hrs, the obtained samples were heat-treated at 25°C - 300°C for Ihr.(Fig. I, (a)). a-ZrP-PTFE and y-ZrP-PTFE composites were prepared using the PTFE emulsion, guaran
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