Metal-organic frameworks and composites on their basis: structure, synthesis methods, electrochemical properties and application prospects (a review)

Various types of metal-organic frameworks (MOF) and their structural parameters have been reviewed and their classification has been presented. Most widely used synthesis methods and approaches for MOF and composites on their basis have been discussed. MOF structure is a regular 3D lattice formed by organic linkers and metallic clusters. It has been shown for an example of literary data on MOF synthesis and structural studies that the types of bonds and metals can strongly affect the spatial structure and dimensions of MOF crystals: they can have nano-, micro- and meso-dimensions, be dense or porous, bulk or layered. This variety of structural parameters determines the wide range of their properties and potential applications. Prospects and methods of controlling the shapes of the crystals, their size and spatial bonds between organic components and metal ions have been reviewed. Major attention has been paid to zeolite-like frameworks (ZIF) as the most promising ones from the viewpoint of structure, synthesis and electrochemical current source applications. Discussion has touched on possible modifications and methods of controlling the properties of those MOFs and composites on their basis and introduction of impurities, including those having magnetic properties. Possible synthesis options of complex composites through controlled MOF pyrolysis have been presented, for either small-batch or scalable processes. The effect of heat treatment conditions on the final properties and electrochemical applications of those materials has been demonstrated. ZIF-67 structured MOF doping with one more metal has been presented as variant of modifying the properties of MOF and composites on their basis. For example, the Authors have implemented cobalt MOF synthesis wherein cobalt is partially substituted for manganese at the synthesis stage. Furthermore, a simple water solution co-deposition synthesis technique has been modified with ultrasonication thus reducing the time consumption of the process. Electrochemical studies have shown that the unit electrochemical capacity of pyrolyzed MOF electrodes with partial cobalt substitution for manganese is appreciably higher as compared to the manganese-free materials. The unit capacity and energy density increase with manganese content in the MOF. MOF manganese doping delivers a considerable improvement in the electrochemical parameters of the electrode materials for hybrid supercapacitors on their basis (from 100 to 298 F/g at 0.25 A/g current density). The results suggest that cobalt substitution for manganese is an effective method of improving the electrochemical parameters of MOFs. It has been demonstrated that the development of new approaches to the design of MOF based composite materials and the study of the physicochemical regularities of their interaction with various carriers is an important task.