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Proton Conductivity in Doped Aluminum Phosphonate Sponges

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Abstract

Proton-conducting networks (NETs) were prepared successfully by the insertion of phosphonated nanochannels into organic–inorganic hybrid materials that contain Al3+ 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.

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