Polyoxometalate\nMetal–Organic Frameworks: Keggin\nClusters Encapsulated into Silver-Triazole Nanocages and Open Frameworks\nwith Supercapacitor Performance

Abstract

To investigate the relationship between the structures\nof polyoxometalate\nhost–guest materials and their energy-storage performance,\nthree novel polyoxometalate-based metal–organic compounds,\n[Ag₁₀(C₂H₂N₃)₈]­[HVW₁₂O₄₀], [Ag₁₀(C₂H₂N₃)₆]­[SiW₁₂O₄₀], and [Ag­(C₂H₂N₃)]­[Ag₁₂(C₂H₂N₃)₉]­[H₂BW₁₂O₄₀] are synthesized by a one-step hydrothermal\nmethod and further confirmed by single-crystal X-ray diffraction analyses\nand other numerous characterization techniques. In compound [Ag₁₀(C₂H₂N₃)₈]­[HVW₁₂O₄₀], the Keggin clusters are intersected into\nchannels formed by a 3D open metal–organic framework. In contrast,\nin compounds [Ag₁₀(C₂H₂N₃)₆]­[SiW₁₂O₄₀] and [Ag­(C₂H₂N₃)]­[Ag₁₂(C₂H₂N₃)₉]­[H₂BW₁₂O₄₀], the Keggin clusters are encapsulated into silver-triazole metal–organic\nnanocages to construct core–shell structures, which are further\nfused together by covalent bonds to form 3D polyoxometalate-based\nmetal–organic frameworks. The electrochemical properties of\nthree compound-based electrodes are estimated by cyclic voltammetry,\ngalvanostatic charge–discharge, electrochemically active surface\narea, and electrochemical impedance spectroscopy. The results of the\nelectrochemical performance tests indicate that these compounds possess\nhigh specific capacitance and cycling stability, especially [Ag₁₀(C₂H₂N₃)₈]­[HVW₁₂O₄₀], showing a specific capacitance of 93.5 F\ng^–1, which is higher than that of many other polyoxometalate-based\nelectrode materials. A possible mechanism of the electrochemical performance\nis explored, which is mainly related to the redox capacity of polyoxometalate,\nthe electrochemically active surface area, the electrochemical impedance\nspectroscopy, and the microstructures of polyoxometalate-based metal–organic\nframeworks.