Proton Conductivity in Doped Aluminum Phosphonate Sponges

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. Surf the NET: Acid‐doped inorganic–organic hybrid materials based on aluminum phosphonates have been synthesized and can compete with the conductivity performance of commonly used benchmark polymers as a proton‐exchange membrane for fuel cell applications. As a result of their sponge‐like morphology, they can adsorb intrinsic proton conductors such as phosphonic acids.