Designs, Synthesis And Properties Of Proton Conductors Based On Polyoxometalates And Ionic Water Clusters

It is well known that the solid proton conductor are widely applied in all kinds of aspects like thefuel cells、hydrogen sensor、separation and purification of hydrogen、or dehydrogenation and so on.Keggin-type heteropolyacids （HPAs）, which possess a unique discrete ionic structure includingheteropoly anions and countercations （H⁺, H₃O⁺, H₅O₂, etc）, are both a class of excellentlow-temperature proton conducting electrolyte and a kind of fine additives as proton conductor for thepreparation of inorganic-organic composite films. In this paper, we succeeded in synthesizing a seriesof inorganic–organic complex solid proton conductors, in which the Keggin structure polyanion andthe ionized water clusters are incorporated into the self-ordered hydrogen-bonded networks or themetal-organic frameworks to form the proton channel or the proton transfer way.First, we selected the fine proton conductive Keggin structure polyanions [PM₁₂O₄₀]^3-（M=Mo/W）,isonicotinic acid N-oxide and alkali-earth metal ions to successfully synthesize eight new organic-inorganic proton conduction materials by the self-assembly of molecular units in the acetonitrile/watersolution at the room temperature, namely compounds1-8. Compounds1-8had been characterizated byelement analyses, spectra analyses, single-crystal X-ray diffraction analyses and so on. Single-crystalX-ray diffraction analyses reveal that the eight compounds present similar three-dimensional （3D）hydrogen-bonded networks with large one-dimensional （1D） channels。Interestingly, Compoundsincluding the Ca²⁺and Sr²⁺ions have no isolated ionized water clusters with a special hydrationnumber and typical poly-Keggin-anion chains, but form decorated poly-Keggin-anion chains, whileother compounds have the isolated ionized water clusters and typical poly-Keggin-anion chains. UVspectroscopy results show that these compounds are sight soluble in water. TG results show that thesecompounds are stable below70°C. Powder diffraction datas show that the proton conductivity ofpowder samples have the same super-molecular skeleton before and after the measurements. ACimpedance test results show that the proton conductivities of compounds1-8increase as thetemperature and humidities increase, they reach high proton conductivities of10^-3S cm^-1in thetemperature100°C under relative humidity98%. These results all show that compounds1-8are good,a new type of good proton conducting materials. Secondly, we selected Keggin structure [PW₁₂O₄₀]^3-anions with nanometer size and high negativecharge,4,4’-bipyridine-N, N’-dioxide（dpdo） and Cu²⁺ions to synthesize compound9in theacetonitrile/water solution. Compound9has been characterizated by element analysis, UVspectroscopy analysis, Fluorescence analysis, TG analysis and single-crystal X-ray diffraction analysis.Single-crystal X-ray diffraction analysis revealed that the compound crystallized in a cubic spacegrouping and had the3D non-interpenetrated skeleton structure, the cubic cavities of skeleton wereoccupied by the anions and protonated water clusters. The UV spectroscopy result shows thatcompound9is sight soluble in water. TG result shows that the skeleton of the compound could keepunder300°C. The fluorescence analysis indicates that the exchange of the protons and the alkali metalions could not occur between the solid of compound9and the salt solution. AC impedance test resultsshow that the compound9is a new type of good proton conducting material.