Characterisation of intermediate temperature polyphosphate composites

Abstract

High temperature proton conducting solids are useful materials for many electrochemical applications such as high temperature fuel cells, hydrogen sensors, and hydrogen gas separators. However, many protonic conductors decompose at temperatures above 300 ◦C. For high-temperature proton conductors, progress is a matter of combining high mobility with a high concentration of protons, but also of avoiding accompanying contributions from oxygen ions and electrons. The intermediate-temperature (200–600 ◦C) fuel cells are very attractive since they combine the advantages of highand low-temperature fuel cells. A membrane formed from an inorganic polyphosphate composite has good ionic conduction properties at temperatures between 473 and 573 K especially in a humidified atmosphere [1, 2]. The number of fast protonic conductors, organic or inorganic, crystalline and amorphous, have been prepared during the past two decades [3], which are categorized into two types based on their operating temperatures. One is hydrated crystalline compounds [4], and the other is perflurinated ionomers [5] showing high conductivities of ∼10−2 S cm−1 below 100 ◦C, but their chemical degradation limits their use in practical applications. On the other hand, fast proton conducting glasses, if developed, extend beyond the limitation of the above compounds because of their high chemical and mechanical durability and easy formation into films and plates. Solid state proton conductors which show high conductivities in the medium temperature range (100– 200 ◦C) with low humidity have been required as the electrolyte for polymer electrolyte fuel cells and direct methanol fuel cells operating in the temperature range [6]. The operation of fuel cells in the medium temperature range improves the efficiency of energy conversion in the cells and reduces the poisoning of Pt catalysts with CO in the fuel gases. In addition, working of the fuel cells under the condition of low humidity permits the reduction of the weight and volume of humidifiers, so that the fuel cells should have advantages for application in electric vehicles. The situations described