Preparation And Electrochemical Properties Of POM@Fe-MOFs Complexes And Their Derivatives

Metal-organic frameworks compounds?MOFs?,which have the advantages of porosity,tunable structure and performance,can be used as a template to prepare metal oxide or porous carbon with special morphologies and exhibite great potential application in energy storage.However,when using the derivatives of MOFs as a template,the structure of framework easily collapses and the composition is relatively simple.By inducing polyoxometallate?POM?which contains pre-transition metal elements of high-valence state and a large number of oxygen atoms on the surface to make it easy to coordinate and possesses high activity,it can stabilize the structure of POM@MOFs and their derivatives on the one hand and it is also possible to increase the number of high-valence state metal elements in the derivative to enhance its redox ability and promote the improvement of electrochemical performance on the other hand.Organic ligands,as one of the important components of POM@MOFs,have a great influence on the structure and electrochemical performance of POM@MOFs.In this paper,POM@MOFs materials were synthesized by in-situ solvothermal method.By changing the organic ligands,the morphology,structure and chemical bonding of the compounds were studied,and the effect of heat treatment on the morphology and crystal structure of the derivatives was investigated.The electrochemical properties of the derivatives were analyzed.The main research work are as follows:?1?One-step solvothermal method was used to synthesize POM@Fe-bpy-MOFs by adopting the transition metal element Fe as the metal center ion,4,4'-bipyridine as organic ligand and the Keggin type H₃PW₁₂O₄₀ as additives.The results showed that the sample was a cluster-like crystal consisted of micron-sized petal-like tiny crystals,in which Fe ion was the center of the linkage and N element of 4,4'-bipyridine was bonded to O element of the phosphotungstic acid radical forming Fe-N and Fe-O bonds and leading to a spatial network structure.Composite metal oxide derivatives were prepared by decomposition via high-temperature under inert atmosphere-air oxidation at low temperature method with POM@Fe-bpy-MOFs compounds as precursors.The electrochemical test results showed that the samples treated at 800°C had the highest specific capacitance of 68.8 F/g?discharge current density was 0.5 A/g?.The porous carbon materials were obtained by uniformly mixing the compound with melamine which was decomposed under a high-temperature inert atmosphere.The electrochemical test results showed that the sample treated at 600°C had the highest specific capacitance of 70 F/g?discharge current density was 0.5 A/g?.?2?On the basis of?1?,the organic ligand was changed to rigid terephthalic acid,and the POM@Fe-H₂BDC-MOFs compound was synthesized.The results showed that the compound was a strip-like crystal with a needle-point tip.The Fe element in the crystal structure was the center and two different Fe-O bonds were formed by bonding O element on the carboxyl groups of the H₂BDC and the O element of the phosphotungstic acid groups to result in a spatial network structure.Composite metal oxide derivatives were obtained by high-temperature under inert atmosphere-air oxidation at low temperature method with the compounds of POM@Fe-H₂BDC-MOFs as precursors.Electrochemical test results showed that samples treated at 800°C had the highest specific capacitance of 124.3 F/g?discharge current density was 0.5 A/g?.The increased specific capacitance was mainly due to the complete retention of the structure.At the same time,the crystallinity of the metal oxide was greatly improved after high-temperature treatment,which was favorable for the oxidation-reduction reaction to give rise to the increase of pseudocapacitance.The porous carbon materials were obtained by uniformly mixing the compound with melamine and which was decomposed under a high-temperature inert atmosphere.The electrochemical test results showed that the samples treated at 800°C had the highest specific capacitance of 35 F/g?discharge current density of 0.5 A/g?.?3?On the basis of?1?and?2?,H₂BDC and 4,4'-bipyridine were selected as mixed ligands to synthesize POM@Fe-bpy/H₂BDC-MOFs compound.The results showed that the compound was a leaf-like crystal.The Fe element was served as the metal center,and bonded to two organic ligands and phosphotungstic acid to form a spatial network structure.The composite metal oxide derivatives were obtained by high-temperature under inert atmosphere-air oxidation at low temperature method with POM@Fe-bpy/H₂BDC-MOFs compounds as precursors.The material retained its original morphology after heat treatment at 600°C.When the temperature increased to800°C,the surface of the sample became rough.When the temperature increased to1000°C,a large number of holes appeared on the surface of the sample.Electrochemical test results showed that the sample after the 1000°C treatment had the highest specific capacitance of 101.3 F/g?discharge current density was 0.5 A/g?.The porous carbon materials were obtained by uniformly mixing the compound with melamine which was decomposed under in a high-temperature inert atmosphere.Electrochemical test results showed that the samples treated at 1000°C had the lowest internal resistance and charge transfer resistance to get the highest specific capacitance of 75 F/g?discharge current density was 0.5 A/g?.