Advances in Rechargeable Batteries and Supercapacitors Based on Metal-Organic Frameworks

Introduction

Metal-organic frameworks or MOFs can be defined as a kind of materials constituted by metal ions and coordination complexes with organic bonds forming networks with one-, two-, or three-dimensional structures containing potential voids that can be chemically exploited (Rosseinsky, 2014). The coordination network can be extending unidimensional through cross-links between coordination entities based on different types of links and moieties (Zhang, 2016). The potential advantage of these frameworks is that their pores can be stabilizing during the removal of the invited molecules, and these can be filling with other chemical moieties, which can be exploited to produce conducting solids in electrochemical storage applications such as rechargeable batteries and supercapacitors. For these applications, mixed electrical conduction and redox activity of the frames can lead to electrochemically active materials. The choice of metal and linker establishes the properties and structure of the framework such as size and shape of the pores, which establish how many bonds can be established with the metal and in which orientation. For metal-ion batteries, the material used, as an electrode must be subjecting to an increasing number of adsorption and desorption cycles during its charge and discharge cycles (Tang, 2016). Three charge storage mechanisms are implicit in a rechargeable battery based on MOFs: 1) modification of metal-containing electrodes by means of oxidation states, 2) use of linkers to develop redox processes, and 3) formation of completely reduced metal and alloy complex through the metal ion by means of conversion reactions. Although the electrical conductivity of the metal-organic frameworks is low, its porosity increases the ionic diffusion ratio that is the main characteristic to be used in battery electrode materials. In addition, the frameworks have a high crystallinity, which gives them a mechanical stability, and their tunable structures increase their physicochemical functionalities. The most desired main performance parameters in a rechargeable battery are a maximum charge capacity, long cycling life, fast charge-discharge rate, and a high current density. Frameworks also have a high surface area and higher thermal stability than pure organic materials, and thus increase the efficiency of the use of metal ions (Xu, 2018). Based on these crystalline materials it is possible to develop positive materials (cathodes), negative (anodes) and electrolytes for rechargeable batteries and supercapacitors. In the case of supercapacitors, these are dividing into two types: 1) electrochemical double-layer capacitor and 2) pseudocapacitors. The use of frames in supercapacitors is basing on high electrical conductivity, higher power densities and energy storage capacity, and large surface area accessible to the electrolyte (Wang, 2018). Frameworks in the supercapacitors can be applying for three purposes:

• 1.

  Pristine frameworks to store electrical charges by means of physical adsorption on their inner surfaces of the electrolyte ions to exploit the reversible redox reactions of the metal centers;

• 2.

  Frameworks to synthesize metal oxides and preserve electrons by means of the transfer of charge between the electrode and electrolyte, and

• 3.

  Frameworks to obtain porous carbon-based materials for increasing the capacitance via enrichment of the electrical conductivity.

Different pathways to design electrode materials for supercapacitors and rechargeable batteries based on conductive MOFs include conductive MOFs (Xu, 2018), MOF-based composites (Chen, 2014; Yang, 2018), and MOF-derived materials (Zhang, 2017; Yang, 2018; Kung, 2019; Zhong, 2019). Metal ion batteries based on metal-organic frameworks make use of monovalent cations such as lithium (Li⁺), potassium (K⁺) and sodium (Na⁺), on divalent cations such as zinc (Zn²⁺), calcium (Ca²⁺) and magnesium (Mg²⁺) or on trivalent cations such as aluminum (Al³⁺). Different metal-organic frames have been proposing as cathodes or anodes for rechargeable batteries and supercapacitors, some examples are: