Facile proton conduction via ordered water molecules in a phosphonate metal-organic framework

A new phosphonate metal-organic framework (MOF) with a layered motif but not that of the classical hybrid inorganic-organic solid is presented. Zn3(L)(H2O)2 \ub72H2O (L ) [1,3,5-benzenetriphosphonate]6-), henceforth denoted as PCMOF-3, contains a polar interlayer lined with Zn-ligated water molecules and phosphonate oxygen atoms. These groups serve to anchor free water molecules into ordered chains, as observed by X-ray crystallography. The potential for proton conduction via the well-defined interlayer was studied by 2H solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H2 was measured as 3.5 7 10-5 S cm-1 at 25 \ub0C and 98% relative humidity. More interestingly, an Arrhenius plot gave a low activation energy of 0.17 eV for proton transfer, corroborating the solid-state NMR data that showed exchange between all deuterium sites in the D2O analogue of PCMOF-3, even at -20 \ub0C.Peer reviewed: YesNRC publication: Ye