Study On Synthesis And Oxygen Evolution Properties Of Polyoxometalate(POM)-based Metal–Organic Framework(MOF) Derived Transition Metal Carbides

In recent years,in order to solve problems such as energy supply and demand,environmental pollution,and efficient conversion of new energy,it is possible to utilize excess or hard-to-store electrical energy by electrolyzing water to produce hydrogen.Noble metal catalysts can reduce the cost of hydrogen produced by water electrolysis,but due to its high cost and low stability,noble metal catalysts can be replaced by research of highly efficient non-noble metal catalysts.Transition metal carbides have long been proved to have properties similar to precious metals,and transitional metal carbides have strong conductivity and stability.Polyoxometalate?POM?-based metal–organic framework?MOF?materials have been extensively studied because of their unique host-guest structure,and MOF-derived transition metal carbides can be ternally regulated by Polyoxometalate-metal ion-organic ligands.Common Polyoxometalate in Polyoxometalate-based metal organic frameworks are H₃PMo₁₂O₄₀ or H₃PW₁₂O_40.Transition metals are often used for metal ions because they have more valence states.Organic ligands are often selected from organic materials containing heteroatoms,which are doped with heteroatoms after carbonization to improve conductivity and catalytic activity.Therefore,I chose an imidazole organic substance containing N as an organic ligand,and a cobalt with high electrocatalytic activity as a metallic building blocks to form a ZIF-based organometallic skeleton encapsulating Keggin-H₃PMo₁₂O₄₀,which is carbonized to form a P and N double doped transition metal carbide Co₃Mo₃C/C composites and Co₃Mo₃C/MoC/C composites were prepared by controlling the amount of H₃PW₁₂O₄₀package and metal ion regulation.Co₃Mo₃C/C composites and Co₃Mo₃C/MoC/C composites were prepared and investigated for oxidation or not during oxygen evolution reaction?OER?.The results show that the prepared materials showed good performance for OER,with small overpotential,small transfer resistance,high catalytic activity and high stability.The specific research contents are as follows:1.By encapsulating H₃PMo₁₂O₄₀ in ZIF-67,carbonization is used to obtain Co₃Mo₃C/C composites.The size of precursor,the phase of carbonized product and the oxygen evolution performance of electrolytic water are controlled according to different H₃PMo₁₂O₄₀ encapsulation in order to explore the factors that affect the oxygen evolution performance of electrolytic water.The optimum amount of H₃PMo₁₂O₄₀ is 75 mg.The excellent material Co₃Mo₃C/C composites was obtained.The current density was 10 mA cm^-2 at 1.64 V.and the Tafel slope is 77 mV dec^-1 The performance of the material is close to commercial RuO₂.By studying the current density normalized by the catalyst electrochemically active surface area,we conclude that the key factor affecting the performance of oxygen evolution in electrolytic water is the intrinsic catalytic activity.By studying the TEM images of the materials before and after the OER process,we concluded that the Co₃Mo₃C/C material is oxidized during the OER process and oxidized to Co₃O₄/C.It is concluded that Co₃Mo₃C/C should be called precatalyst.2.By encapsulating the H₃PMo₁₂O₄₀ in ZIF-67/ZIF-8,then carbonization gives the Co₃Mo₃C/MoC/C composite.The composition,conductivity and oxygen evolution of bimetallic carbides were controlled by adjusting the proportion of bimetallic ions in precursors.Obtaining an optimal ratio Co:Mo=4:1 and a Co₃Mo₃C/MoC/C precatalyst material with excellent performance,the precatalyst Co₃Mo₃C/MoC/C with excellent performance can reach a current density of 10 mA cm^-2 at a voltage of 1.59 V and a low Tafel slope of 72 mV dec^-1.T`he intrinsic activity was studied by normalizing the electrochemical active area.By studying the TEM image and XPS analysis of the materials before and after the OER process,it was found that the P and Mo elements were gradually in the electrolyte solution and at the same time,Co₃Mo₃C/MoC/C is oxidized to form Co₃O₄/CoMoO₄/C to continue the OER reaction as a new OER catalyst.