Synthesis Based On {MnMo₉}-MOFs And The Performance Of Supercapacitors And Electrocatalytic Water Splitting

Polyoxometalates(POMs)have clear structures and unique properties,so they have broad application prospects in many fields,they can not only become supercapacitor electrode materials with excellent performance,but also can be used to develop efficient and inexpensive electrocatalysts.The Waugh-type structure is one of the six basic structures in POMs,and[MnMo₉O₃₂]^6-({MnMo₉})is a typical representative of the Waugh-type structure.Combining{MnMo₉}with metal-organic frameworks(MOFs)to form polyoxometalate-based metal organic frameworks(POMOFs)can not only retain the advantages of POMs,but also introduce a larger specific surface area of MOFs,which is beneficial to the transport of electrons,thereby improving the electrochemical performance of POMs.In this paper,using{MnMo₉}as the basic unit,transition metal ions and organic ligands were introduced for modification to prepare POMOFs compounds with novel structures,and their electrochemical properties were explored.{MnMo₉},transition metal ions and 1,3,5-benzenetricarboxylic acid(H₃BTC)as raw materials,two POMs@M-BTC(M=Ag(1),Ni(2))compounds were successfully synthesized by a simple grinding method,infrared spectroscopy(IR),X-ray powder diffraction(XRD),scanning electron microscope(SEM)and other analytical methods to determine its composition,structure and morphology.In the carbon cloth current collector three electrode system,the specific capacitances of compounds 1 and 2 were332.04 F g^-1 and 230.01 F g^-1,respectively,and the capacitance retention rates after5000 cycles were 95.24%and 94.67%;in the three electrode system with nickel foam as current collector,the specific capacitances were 573.10 F g^-1 and 421.18 F g^-1,respectively,and the capacitance retention rates were 94.77%and 93.68%after 5000cycles.Both are better than their corresponding{MnMo₉}and M-BTC precursors,indicating that the introduction of transition metal ions and BTC is beneficial to improve the capacitive performance of the materials.The two electrode materials were assembled into symmetric supercapacitors with specific capacitances of 100.19 F g^-1and 87.88 F g^-1,respectively.When the energy densities were 20.04 Wh kg^-1 and 17.58Wh kg^-1,their power densities were 604.58 W kg^-1 and 600.03 W kg^-1,respectively.In addition,compounds 1 and 2 were used as catalysts for the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),the overpotentials of HER were221.16 mV and 237.10 mV;the overpotentials of OER were 378.17 mV and 403.05 mV,respectively,which are slightly worse than the electrocatalyst 20%Pt/C and RuO₂ under the same test conditions,but better than{MnMo₉}and M-BTC precursors,indicating that the synthesized compounds are both electrode materials for supercapacitors and efficient catalytic materials for electrocatalytic water splitting.Four compounds based on{MnMo₉},BTC or terephthalic acid(BDC)and transition metal ions were synthesized by solution method,and their chemical formulas were{MnMo₉}@M-MOFs(MOF=BTC:M=Cu(3),Zn(4),Mn(5);MOF=BDC:M=Mn(6)),the morphologies and structures of compounds 3-6 were characterized.Carbon cloth and nickel foam were used as current collectors,respectively,and in the three electrode system,supercapacitor tests of compounds 3-6 showed that when nickel foam was used as current collector,the compounds showed better capacitance performance,at the current density of 1 A g^-1,the specific capacitances were 469.26,475.97,416.81,570.23 F g^-1,and the specific capacitance retention rates after 5000cycles were 94.64,97.06,93.75 and 96.77%,respectively.In the symmetrical two electrode system,the specific capacitances of compounds 3-6 were 90.79,96.56,79.43,100.04 F g^-1,respectively,and when the energy densities were 12.16,13.41,7.06,8.89Wh kg^-1,the power densities were 500.01,503.69,400.03,410.15 W kg^-1,indicating that compounds 3-6 have certain practical application significance in the field of supercapacitors.Compounds 3-6 were used as electrocatalysts for water splitting,at a current density of 10 m A cm^-2,the overpotentials of HER were 205.37,240.10,191.72and 184.16 mV;the overpotentials of OER were 390.0,409.85,357.76 and 348.32 mV,respectively,four compounds have excellent catalytic activities in the hydrogen evolution and oxygen evolution reactions,indicating their potential applications in HER and OER research.