Mechanochemical Synthesis of Phosphonate-Based Proton Conducting Metal-Organic Frameworks

Water-stable metal-organic frameworks (MOFs) with proton-conducting behavior have attracted great attention as promising materials for proton-exchange membrane fuel cells. Herein, we report the mechanochemical gram-scale synthesis of three new mixed-ligand phosphonate-based MOFs, {Co(H(2)PhDPA) (4,4'-bipy) (H2O)center dot 2H(2)O}(n) (BAM-1), {Fe(H(2)PhDPA) (4,4'-bipy) (H2O)(2)center dot 2H(2)O}(n) (BAM-2), and {Cu(H(2)PhDPA) (dpe)(2)(H2O)(2)center dot 2H(2)O}(n) (BAM-3) [where H-2 PhDPA = phenylene diphosphonate, 4,4'-bipy = 4,4'-bipyridine, and dpe = 1,2-di(4-pyridyl)ethylene]. Single-crystal X-ray diffraction measurements revealed that BAM-1 and BAM-2 are isostructural and possess a three-dimensional (3D) network structure comprising one-dimensional (1D) channels filled with guest water molecules. Instead, BAM-3 displays a 1D network structure extended into a 3D supramolecular structure through hydrogen-bonding and pi-pi interactions. In all three structures, guest water molecules are interconnected with the uncoordinated acidic hydroxyl groups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. BAM-1 and BAM-2 showed a gradual increase in proton conductivity with increasing temperature and reached 4.9 x 10(-5) and 4.4 x 10(-5) S cm(-1) at 90 degrees C and 98% relative humidity (RH). The highest proton conductivity recorded for BAM-3 was 1.4 x 10(-5) S cm(-1) at 50 degrees C and 98% RH. Upon further heating, BAM-3 undergoes dehydration followed by a phase transition to another crystalline form which largely affects its performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism, as suggested by their low activation energy.

Automated summary

This study reported the synthesis and proton conductivity of three new mixed-ligand phosphonate-based MOFs. These MOFs have stable structures and high proton conductivity, showing potential applications in proton-exchange membrane fuel cells.