Synthesis Of MOFs Modified ｛NiMo₆O₂₄H₆｝ Compounds And Their Capacitance And Overall Water Splitting Properties

Growing energy demand and increasing environmental pollution have spurred research into advanced electrochemical devices for low-cost and safe energy storage and conversion.Polyoxometalates（POMs）have been used as electrode materials for capacitive storage as well as electrocatalytic monolithic water splitting.Single-atom catalysts（SACs）have achieved the maximum utilization of atoms in the electrocatalytic hydrogen evolution（HER）and oxygen evolution（OER）processes,and have made remarkable progress in catalysis.However,the SACs reported so far are mainly defect rich carriers doped with a few metal ion junctions in nano-metal alloys or inorganic organic materials.The low active site utilization,poor dispersion and low molecular accessibility of such SACs lead to a major problem of low catalytic efficiency.Anderson{NiMo₆O₂₄H₆}POMs have a disc-like plane structure,high exposure of central heteroatom Ni,and good accessibility.As transition metal-based oxygen cluster,they have great potential in electrochemical energy storage and electrocatalytic integral hydrolysis of electrode materials.Metal-organic framework（MOFs）has a good structure and can regulate its microenvironment during the anchoring of single atomic sites（SASs）.π-d hybridization with strong constraint effect is produced,which improves the structural stability and overall hydrolysis performance.Therefore,in this paper,{NiMo₆O₂₄H₆}and{Mo₇O₂₄}with similar structure were anchored in mof materials to synthesize POMOFs compounds.The important role of central heteroatom Ni in energy storage of supercapacitor and electrocatalysis of integral water decomposition is compared and analyzed.Two examples of POMOFs compounds{Co（NH₄）₂NiMo₆O₂₄H₆}@Co-BDC（1）and{Co₂（NH₄）₂Mo₇O₂₄}@Co-BDC（2）are synthesized by solution method using{NiMo₆O₂₄H₆},{Mo₇O₂₄},terephthalic acid（H₂BDC）,and Co²⁺.And their morphology and composition are determined by infrared spectroscopy（IR）,X-ray powder diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and other characterization methods are used to determine their morphology and composition.In the three-electrode system with carbon cloth as the collector,the specific capacitances of compounds 1 and2 are 326.43 F g^-1 and 285.37 F g^-1,respectively.When the current density is 1 A g^-1,and the capacitance retention after 5000 cycles are 96.08%and 90.16%.In the three-electrode system with nickel foam as the collector,the specific capacitances(when the current density is 1 A g^-1)are 417.32 F g^-1 and 308.08 F g^-1,respectively.The symmetrical and asymmetrical button cell（SSC,ASC）are made from compound 1 with excellent capacitance performance with specific capacitance of 144.63 F g^-1,63.48 F g^-1,energy density and power density of 7.23 Wh kg^-1,22.57 Wh kg^-1,and 300.03 W kg^-1,802.13 W kg^-1,respectively（Asymmetric button cell with compound as the positive electrode material and activated carbon as the negative electrode material）.In addition,when the two compounds were used as catalysts for hydrogen evolution（HER）and oxygen evolution（OER）,compound 1 shows significant catalytic activity with an overpotential of 235.45 m V(η₁₀),386.12 m V(η₁₀₀)and a Tafel slope of 104.66 m V dec^-1.The overpotential of OER is 255.68 m V(η₅₀),468.15 m V(η₁₀₀),and the slope of Tafel is 89.16 m V dec^-1,which is better than compound 2 and the corresponding precursor,but still slightly lower than 20%Pt/C and Ru O₂.The{NiMo₆O₂₄H₆}and{Mo₇O₂₄}are combined with Ni-BTC and HKUST-1 to synthesize{Ni（NH₄）₂NiMo₆O₂₄H₆}@Ni-BTC（3）,{Ni₂（NH₄）₂Mo₇O₂₄}@Ni-BTC（4）,{Cu（NH₄）₂NiMo₆O₂₄H₆}@HKUST-1（5）,{Cu₂（NH₄）₂Mo₇O₂₄}@HKUST-1（6）four POMOFs compounds.The results with carbon cloth and nickel foam as collectors,respectively,show that nickel foam exhibited superior capacitive performance,with specific capacitances of 956.22 F g^-1,617.23 F g^-1,978.50 F g^-1,and 428.26 F g^-1 for compounds 3-6 at a current density of 1 A g^-1,respectively.The assembled symmetrical and asymmetrical coin cell obtain 130.03 F g^-1,162.53 F g^-1（SSC）;157.78 F g^-1,123.05F g^-1（ASC）specific capacitance;46.23 Wh kg^-1,57.79 Wh kg^-1（SSC）,79.61 Wh kg^-1,82.72 Wh kg^-1（ASC）for energy density;800.19 W kg^-1,800.17 W kg^-1（SSC）,956.01W kg^-1,1107.07 W kg^-1（ASC）for power density.When compounds 3 and 5 are used as HER and OER catalysts,the overpotentials of HER are 97.95 m V(η₁₀),201.37 m V(η₁₀₀),Tafel slope 73.59 m V dec^-1（compound 3）;100.03 m V(η₁₀),192.98 m V(η₁₀₀),76.90 m V dec^-1（Compound 5）.the overpotentials of OER are 377.11 m V(η₅₀),421.07m V(η₁₀₀),Tafel slope 68.32 m V dec^-1（Compound 3）;366.53 m V(η₅₀),405.38 m V(η₁₀₀),Tafel slope 65.40 m V dec^-1（Compound 5）.The catalytic activity is even better than that of commercial 20%Pt/C and Ru O₂.comparative analysis reveals that the introduction of the central heteroatom Ni successfully anchored and obtained a large amount of SASs（Ni）,which obtained a great improvement in the supercapacitor and electrocatalytic water cracking performance.