Construction And Proton Conduction Behavior In Series Of Polyoxometalate-based Lanthanide-amino Acid Coordination Polymers

Metal-organic frameworks or coordination polymers served as novel proton conductive materials have inspired researchers with enthusiasm.Furthermore,it's advantageous for the enhancement of proton conductivity if Polyoxometalates functional clusters possessing oxygen-rich surface can be introduced into the above systems by bonding effort to form Polyoxometalate-based coordination polymers,which has been well established.In this work,a simple conventional aqueous solution"one-pot"method was employed to prepare a series of Polyoxometalate-basedlanthanide-aminoacidcoordinationpolymers:[Ln?NH₃CH₂COO?₂?H₂O?₅][Cr?OH?₆Mo₆O₁₈]?10H₂O?Ln=Pr?1?,Nd?2?,Sm?3?,Eu?4?,Gd?5???series I?and their analogs La?6?,Ce?7??series II,without specific single crystal structures?by low-cost and readily available materials of lanthanide salts,glycine,Cr?NO₃?₃?9H₂O and Na₂MoO₄?2H₂O.Owing to the considerable yield in a short period as well as single-phase separation of target products,this synthetic strategy has numerous advantages of cost-effective,facile,environmentally friendly,scalable and highly purified compared with the synthesis approaches of conventional MOFs proton conductors.1.Compounds 1-5?series I?are isomorphic.The noteworthy feature of their structures is the formation of one-dimensional water clusters along c-axis,while[Cr?OH?₆Mo₆O₁₈]^3-layers stacked on both sides of 1D water clusters like a"wall"form hydrogen-bonded interaction with the lattice water molecules on account of abundant surface oxygen atoms,and then promote the oriented arrangement of lattice water molecules,resulting in the constructing of continuous hydrogen-bonding network along this direction successfully.AC impedance measurement exhibited the proton conductivity with a maximum value of 1.56×10^-4 S cm^-1 at 75°C and97%RH in series I compounds?represented by Nd?2??.2.Compounds 6-7?series II?are also isomorphic with each other,but they are nonisomorphic with series I.Unfortunately,we have regulated various possible influencing factors under the similar synthetic conditions with series I,but none of the specific structures of series II were known.However,their IR spectra suggested a similarity between the structures of series I and II,which might be due to the fine-tuning of the coordination modes caused by the lanthanide contraction.The conductivity of series II compounds?represented by La?6??reached up to the highest value of 4.97×10^-3 S cm^-1 at 80°C and 97%RH,which could be comparable to the reported high-performing proton conductors.