Inorgano-organic proton conducting membranes for fuel cells and sensors at medium temperatures

Layered zirconium sulfoarylphosphonates of the α- and the γ-type are proton conductors thermally stable up to at least 180°C. In these materials, the sulfophenyl groups are bonded through the phosphorus atoms to an α- or a γ-inorganic framework made of oxygen and zirconium atoms. Compounds where the sulfonic function is attached to a phenyl, benzyl or to a fluorinated benzyl group were characterized for their conductivity as a function of temperature and relative humidity (r.h.). Independent of the layer type, the highest conductivities were found for the sulfophenylphosphonates. The conductivity is strongly affected by the r.h. reaching values of 5×10−2 S cm−1 at 100°C (100% r.h.) and 2×10−2 S cm−1 at 150°C (80% r.h.). Due to their ability to undergo infinite swelling in appropriate solvents, these materials can be incorporated into polymeric proton conducting membranes. The possible advantages of these membranes for increasing the efficiency of indirect or direct methanol fuel cells working at medium temperature are discussed. The use of these membranes in gas sensors working at medium temperatures are also discussed. Preliminary results for the detection of hydrocarbons at 300°C by means of a sensor based on the protonic conductivity of zirconium phosphate are reported.