title: Proton conductivity in doped aluminum phosphonate sponges.
abstract: Proton-conducting networks (NETs) were prepared successfully by the insertion of phosphonated nanochannels into organic-inorganic hybrid materials that contain Al(3+) as the connector and hexakis(p-phosphonatophenyl)benzene (HPB) as the linker. Noncomplexed phosphonic acid groups remain in the framework, which depends on the ratio of both compounds, to yield a proton conductivity in the region of 10(-3) S cm(-1). This conductivity can be further improved and values as high as Nafion, a benchmark proton-exchange membrane for fuel cell applications, can be obtained by filling the network pores with intrinsic proton conductors. As a result of their sponge-like morphology, aluminum phosphonates adsorb conductive small molecules such as phosphonic acids, which results in a very high proton conductivity of approximately 5 × 10(-2) S cm(-1) at 120 °C and 50 % relative humidity (RH). Contrary to Nafion, the doped networks show a remarkably low temperature dependence of proton conductivity from external humidification. This effect indicates a transport mechanism that is different to the water vehicle mechanism. Furthermore, the materials exhibit an activation energy of 40 kJ mol(-1) at 15 % RH that starts to diminish to 10 kJ mol(-1) at 80 % RH, which is even smaller than the corresponding values obtained for Nafion 117.
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz (Germany), Fax: (+49) 6131-379-350.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Wegener
authors.forename: Anke
authors.initials: A
authors.lastname: Kaltbeitzel
authors.forename: Robert
authors.initials: R
authors.lastname: Graf
authors.forename: Markus
authors.initials: M
authors.lastname: Klapper
authors.forename: Klaus
authors.initials: K
authors.lastname: Müllen
mesh_terms: D000327:Adsorption; D000535:Aluminum / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D010757:Phosphorous Acids / Q000737:chemistry*; D011522:Protons
chemical_list: D010757:Phosphorous Acids; D011522:Protons; D000535:Aluminum
keywords: aluminum; fuel cells; mesoporous materials; metal-organic frameworks; self-assembly