Molybdenum-functionalized metal–organic framework crystals interconnected by carbon nanotubes as negative electrodes for supercapacitors

Abstract: As the pseudocapacitive material operated in the negative potential window in an aqueous electrolyte, the molybdenum-functionalized MOF-808-CNT nanocomposite can obviously outperform both the molybdenum-functionalized MOF-808 and molybdenum-functionalized CNT. Crystals of a water-stable Zr-based metal–organic framework (MOF), MOF-808, are directly grown on the surface of carboxylic acid-functionalized carbon nanotubes (CNT) to synthesize the nanocomposites with tunable MOF-to-CNT ratios. The crystallinity, morphology, porosity, and electrical conductivity of all nanocomposites are characterized. To install the electrochemically active sites within the highly porous MOF framework, the obtained MOF-808-CNT nanocomposites are further subjected to the functionalization of spatially dispersed Mo(VI) sites by a self-limiting process followed by the electrochemical reduction to generate the molybdenum nanoparticles confined within the MOF pore. Thin films of these Mo-functionalized materials are served as the pseudocapacitive materials in aqueous electrolytes and operated in a negative potential window. By utilizing the electrochemically active molybdenum confined within the highly porous MOF and the electronic conduction between MOF crystals facilitated by CNT, the optimal Mo-functionalized nanocomposite can significantly outperform both the Mo-functionalized MOF and Mo-functionalized CNT. Discussion: MOFs are highly porous materials, which should be attractive candidates for electrochemical energy storage, but their poor chemical stability and low electrical conductivity hinder the practical use of MOFs in supercapacitors.Even though several MOFs have been directly applied for supercapacitors in aqueous electrolytes, most of these reported MOFs are not stable in water (or the alkaline electrolytes tested), which would generate MOF-derived materials.Reported examples of MOF-based materials for supercapacitors that are chemically robust in the tested electrolytes are relatively rare.Pseudocapacitive materials show higher specific capacitances than the double-layer-type materials, but most pseudocapacitive materials can only be operated in the positive potential window. Thus, asymmetric supercapacitors are usually fabricated by serving the double-layer-type material as the negative electrode.Molybdenum-based pseudocapacitive materials can be operated in the negative potential window, which makes it feasible to design the supercapacitors based on all pseudocapacitive materials.