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Sintered zirconium phosphate membranes containing zeolites have significant water absorptive capacities over a temperature range of from ambient to 150°C. This feature makes them sufficiently conductive for fuel cell applications over this temperature range. Such membranes have transverse strengths of 5000 to 6000 psi. They readily gain and lose water vapor in a reversible manner while maintaining good stability. This accounts for promising results obtained to date for regenerative hydrogen--oxygen fuel cells using this membrane. In addition to presenting regenerative hydrogen-oxygen fuel cell data, water absorptivity and conductivity data are given and interpreted in terms of membrane composition and structure.'"phe earliest reported use of zirconium phosphate membranes as a solid A electrolyte for hydrogen-oxygen fuel cells dates back to 1961 (9,10, IS). Astropower Laboratory has been investigating the electrochemical behavior of modified zirconium phosphate structures from both a fundamental as well as a developmental aspect. Significantly, a comprehensive investigation of composition and fabrication techniques, as they are related to membrane strength, conductivity, and hydrolytic stability, has led to deriving useful solid electrolyte structures. This is evidenced by successful hydrogen-oxygen fuel cell tests over the temperature range of 25° to 148°C. (6).Most characteristic of these membranes is an incorporated zeolite component serving a water-balancing function by virtue of its high affinity for water and low rate of desorption. In this manner, the conductivity of the membrane is maintained at a suitable level over wide temperature 17

A new type of solid inorganic membrane for intermediate temperature hydrogen-oxygen fuel cell application is described. This is based on zirconium phosphate sintered with the zeolite material "Zeolon-H." The zeolite material contributes to maintenance of water balance in the membrane which is required especially at temperatures above 25~Under fuel cell operational conditions, the resistivities of this membrane are at the 20-25 ohm-cm level. Stable fuel cell performance over the temperature range of 25~176 was obtained. A maximum performance of 0.790v (64% of the theoretical) at 30 ma/cm 2 was obtained at 130~The principal advantages to be gained from the use of solid membrane electrolytes in fuel ce]ls are compactness, simplicity of design, and minimization of zero gravity limitations. The impetus behind the selection of inorganic membranes in this application is that they are insensitive to oxidation both from a chemical as well as from an electrochemical standpoint, and their dimensional, thermal, and radiation stabilities are high.Since 1962 this laboratory has been conducting a systematic investigation on cationic inorganic ion exchange membrane materials for fuel cell application. Exploratory work with zeolites demonstrated that they were lacking in both ionic conductivity and mechanical strength (1). Utilization of metallic oxidephosphoric acid systems, particularly zirconium dioxide, has yielded unique membranes manifesting transverse strength levels of 5000-9000 psi (2a) and resistivities at the 20-25 ohm-cm level (2b).The original pioneering effort on the use of zirconium phosphate as cation exchange material was performed by Kraus (3), Amphlett (4), and Larsen and Vissers (5). Hamlen (6) studied the conductivity of zirconium phosphate under various conditions of hydration and found that at the highest level of hydration, the mechanism of conductance corresponds to that of an aqueous phase. Hamlen (6) and Bregman in 1961 and 1962 (7, 8) were the first to employ zirconium phosphate membranes in hydrogen-oxygen fuel cells; this was done at ambient temperature. The performances were marginal, and the membranes were weak. More recently, Hamlen and Szymalak (9) have used a specially prepared zirconium phosphate-Teflon matrix with hydrogen and propane fuel cell studies with oxygen over the temperature range 125~176Significantly, it has been found in our laboratory that the use of the zeolite material Zeolon-H 2 in these membranes has permitted the maintenance of the water balance at elevated temperatures with resulting stable fuel cell performance. It appears that the water retention action of the zeolite water balancing agent minimizes dehydration at elevated temperatures.As described by Amphlett (10), zirconium phosphate is ordinarily prepared by two techniques. A salt, such as zirconyl nitrate, is precipitated from acidic solution either by the addition of phosphoric acid or by a soluble phosphate. In the second method, zirconium dioxide is treated directly with phosphoric acid. The nature of the products ...