Chapter 21 - Other nanocomposites of MOFs for supercapacitors

Abstract

Since the first discovery, metal organic frameworks (MOFs), their derivatives, and composites have attracted a wide attention from the scientific community in different domains of applications because of their superior characteristics in terms of porosity, specific surface area (up to 6000m2g−1), and an unlimited possibility to modulate their physical chemical properties by changing either the ligand structures and/or the nature of metallic centers. Fortunately, all the aforementioned characteristics are very beneficial in responding to the development of electrochemical energy storage (EES) systems, in particularly supercapacitors. As a brief introduction, supercapacitors refer to a type of EES devices that store and release energy by reversible adsorption/desorption of ions at the interface electrode/electrolytic solution upon polarization of the electrodes coupling with faradaic charge related to redox reaction and/or rapid 3D intercalation process. Thus, greater the electrochemical active surface area (ECSA), the better the performance of the supercapacitor. Consequently, these devices are considered nowadays as a complementary system to assist batteries during high-power demand (acceleration, start&stop …). Also, their long–life stability compared to batteries makes them a very reliable technology. However, they are still suffering from high self-discharge rate (Diab et al., 2009) as well as low energy density. Considering all the mentioned arguments, the MOF would be attractive in development of SCs in terms of specific surface are and nanostructured channels that can facilitate the diffusion of ions back and forth during the cycling. However, exempted from few rare examples, most of the MOFs up-to-date have their limitation in the matter of electronic conductivity which is ranged from 10−15 to 10−2Scm−1 (Xie et al., 2020). From this standpoint, it is preferable to either develop new family of conductive MOFs (Yan et al., 2016; Sheberla et al., 2017; Xie et al., 2020) or couple poorly conductive MOF with other materials to generate synergetic effect from which the second material tops up the electrical conductivity of the MOF and vice versa, the presence of MOF increase the ECSA of the latter. Within this chapter, we emphasize the use of other materials rather than carbon (CNTs, Graphene, activated carbon, etc.) or metal oxide/hydroxide/sulfide/phosphide-based ones which are abundantly described in previous chapters for generation of MOF composites. Accordingly, recent advancements in generation of MOF composites using metal oxalate, metal particles, MXenes and more importantly, polymeric materials toward application in supercapacitor will be mentioned and summarized. Indeed, the flexibility, malleability of polymeric materials could bring additional beneficial properties to inherent crystalline, brittle and porous MOF solids. Recent studies have demonstrated that polymers could be used as template to modulate the nucleation and growth of MOF, thus, tune the characteristics of prepared MOFs into an unexplored interesting research direction. Furthermore, the rise in using electrically conducting polymer for generation of MOF-based composites has attracted wide attention to enhance the electrochemical storage performance of the composites.