Design,Synthesis,Crystal Structure And Proton Conduction Properties Of Crystalline Polyoxometalate-based Coordination Compounds

Proton exchange membrane fuel cells(PEMFC)have attracted much attention because they can efficiently convert chemical energy to electrical energy.Its performance and service life are affected by the proton exchange membrane.At present,the Nafion membrane is the proton exchange membrane with excellent performance and wide application,but it still has disadvantages,such as expensive price,complicated process,and high fluorine content.In addition,the complex composition of the Nafion membrane makes it difficult to explore the relationship between the structure and the proton conductivity.Therefore,the search for crystalline proton conducting materials with a clear structure and high efficiency has become a research hotspot.Polyoxometalates are widely used as proton conductors due to their various types,easy to modify structures,abundant oxygen atoms and ability to hold protons.Generally,polyoxometalates are highly soluble in water,which will cause leakage problems when applied in PEMFC,thereby greatly reducing the performance of the fuel cells.greatly reduces the performance of the fuel cell.Improving the stability of polyoxometalates is the basis for improving their proton conductivity.In this paper,a series of stable and insoluble polyoxometalate-based crystalline proton conducting materials were designed and synthesized by the hydrothermal method.The details are as follows:1.The Preyssler type{P₅W₃₀}is selected as the construction unit,and the well coordinated cobalt atom and the 2-methylimidazole(2-MI)ligand for proton transport were introduced to obtain a novel polyoxometalate-based supramolecular crystalline material:H_4.5Na Co₅(2-MI)_15.5(2-MI)₃[Na P₅W₃₀O₁₁₀]·5H₂O(1).The interaction between 2-MI,water molecules and Na⁺,in addition to the continuous hydrogen bond network construct between them and the polyoxometalates make compound 1 have good proton conductivity.At 94?,97%RH,the proton conductivity is 4.42×10^-3 S cm^-1.Through D₂O exchange experiment and Hebb-Wagner polarization method,it was confirmed that compound 1 conducts protons instead of electrons or other ions.2.Sodium molybdate dihydrate is used as the source of molybdenum,mixed with transition metal chloride and 2-methylimidazole,using the"one-pot method"to synthesize two crystalline proton conducting materials based on{P₄Mo₆}units:H₁₄[C₄H₆N₂]₂[M(H₂O)₅][M(H₂O)₂]₂{M[(PO₃)₃(PO₄)Mo₆O₁₅]₂}·4H₂O(M=Co and Fe)(2-3).Different from the common{P₄Mo₆},we use H₃PO₃ to adjust the p H,resulting in two coordination modes of P atoms in the crystal structure.The structure is expanded into three-dimensional network by metal ions.At 75?and 98%RH,the proton conductivity of compounds 2 and 3 are1.33×10^-2 S cm^-1 and 1.03×10^-2 S cm^-1,respectively.The high-efficiency proton conductivity is mainly attributed to the free state of 2-methylimidazole as the proton carrier,which has a faster migration rate.3.Using ethylenediamine as a template,three examples of isostructural metal phosphovanadate(NH₃CH₂CH₂NH₂)₂M(H₂O)₂V₈O₁₂(HPO₄)₄(PO₄)₄(M=Co,Ni and Cu)(4-6)were synthesized by hydrothermal method.These three compounds have cavities with a width of about 6.4?-7.5?.The protonated ethylenediamine can balance the charge of the system and act as a good carrier to transport protons quickly.Under the conditions of 85?and 97%RH,the proton conductivity of compounds 4-6 can reach 10^-2 S cm^-1.