Structurally Composite Membranes of Titanium Oxide and Titanium Phosphorus Oxide for Proton Conduction at Intermediate T
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Structurally Composite Membranes of Titanium Oxide and Titanium Phosphorus Oxide for Proton Conduction at Intermediate Temperatures
Toshinori Tsuru*, Yasuhito Yagi, Yosuke Kinoshita, Tomohisa Yoshioka, and Masashi Asada Department of Chemical Engineering, Hiroshima University Higashi-Hiroshima 739-8527, JAPAN
ABSTRACT Composite membranes of titanium oxide and phosphorus oxide (TiP) were prepared by the sol-gel method and evaluated for use as proton conductive materials at intermediate temperatures. Titanium phosphorus oxide sol solutions were prepared by the hydrolysis of titanium isopropoxide (TTIP) using hydrochloric acid as a catalyst in isopropanol solutions, and the addition of an appropriate amount of phosphoric acid (H3PO4). A new concept for structurally composite membranes is proposed for proton conductive membranes. A composite membrane, Ti/ TiP, where the pores of a porous titanium oxide layer are filled with titanium phosphorus oxide, was found to be effective for high electrical conductivity as well as mechanical strength. Electrical conductivities as high as 0.1 and 0.06 S cm-1 at 100 and 300 °C, respectively, under a partial pressure of water of 50 kPa, was achieved for the Ti/ TiP membranes.
INTRODUCTIONS Materials proposed for proton conduction at higher temperatures (above 100 °C) can be classified into 3 types of materials: organic materials which are stable at high temperatures such as polyimide, inorganic materials, such as glasses [1-8], and fullerene [9], and organic-inorganic composite materials [10, 11]. Recently, amorphous oxides including SiO2, P2O5-SiO2, P2O5-TiO2 and P2O5-ZrO2-SiO2 prepared by sol-gel methods have been reported to show high proton conductivity in the range of 1x10-3~10-1 S/cm at room temperature by Nogami et al [1-4]. Mastuda et al. [5, 6] also reported a high proton conductivity for P2O5-SiO2. Nogami et al. concluded the origin of the high proton conductivity was the dissociation of protons from hydroxyl bonds on the pore surface and proton hopping between hydroxyl groups and adsorbed water. Phosphorus was introduced into silica matrix with the expectation that it would enhance proton conduction. In the present paper, we report on the preparation of titanium-phosphorus oxide membranes, by the sol-gel method, containing different phosphorus compositions and their electrical conductivity at intermediate temperatures TiO2 Ti1P9 of up to 300 °C. A new concept for structurally Figure 1. Structurally composite membrane, composite membranes is proposed for proton Ti/Colloidal TiP composite membrane form a porous Ti/ TiP. TiO2 sols conductive membranes [12]. Figure 1 shows a composite structure, in which the pores of titanium structure and the pores are filled with highly proton conductive material. oxide are filled with titanium phosphorus oxide;
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titanium oxide provides a mechanical strength to the composite membranes, while titanium phosphorus oxide, the mechanical strength of which is not sufficient, is responsible for proton conduction. The electrical
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