(Invited) group 4 Metal-Based Metal–Organic Frameworks and Their Roles in Electrocatalysis and Electrochemical Energy Storage
Authors: Kung Chung-Wei 
Published: December 2024

Metal–organic frameworks (MOFs) with ultrahigh surface areas, interconnected porosity and intra-framework chemical functionality have been considered as attractive platforms to incorporate high-density and highly accessible active sites for electrocatalysis and electrochemical energy storage. However, the poor stability in water and electrically insulating nature of most MOFs strongly restrict the design and direct use of structurally robust MOFs in aqueous electrolytes. The development of stable group 4 metal-based MOFs, such as Zr(IV)-based MOFs (Zr-MOFs), has allowed the use of MOFs in aqueous media while preserving their structural integrity, but the poorly conducting nature of these highly robust MOFs still limits their performances in electrochemical applications.

This talk will highlight a few recent examples from our research group on the use of chemically stable Zr-MOFs and their isostructural Ce(IV)-based MOFs in various electrochemical applications, with the main focus on the material-design strategies to overcome the challenges and the roles of such highly porous and stable MOFs in aqueous electrochemical systems. For example, with the redox-active and catalytically active sites, e.g., coordinated iridium ions, immobilized in the rigid Zr-MOF, electrons can be transported through the redox-hopping pathway during electrochemical operations, which is sufficient for electrocatalytic reactions that do not require a high current density such as electrochemical sensors (see panel (a) in the Figure). Further developing the nanocomposite composed of the redox-active group 4 metal-based MOF and conducting nanocarbons such as carbon nanotubes can facilitate the interparticle electronic conduction, allowing the use of such stable MOF-based materials in high-current electrocatalysis and charge storage (see panel (b) in the Figure).

On the other hand, the rigid and porous Zr-MOFs that are electrically insulating and “non-catalytic” can also play a role as the thin-film coating on top of the active electrocatalyst to modulate the microenvironment near the underlying surface of the electrocatalyst and thus adjust the selectivity of complicated electrochemical reactions. The underlying electrocatalytic material with a high electrical conductivity and a high catalytic activity, and the highly porous MOF capable of turning the microenvironment near the electrocatalyst, can thus be designed separately. Following this concept, our recent work demonstrated that by coating a thin film of an anionic sulfonate-grafted Zr-MOF on top of the active electrocatalyst for electrochemical sensing, both the reaction rate for the targeted reactant as well as the selectivity toward the targeted cationic reactant against undesired anionic reactants can be remarkably enhanced (see panel (c) in the Figure). Such highly porous Zr-MOF coatings can even outperform the conventionally used Nafion thin films. In addition, as demonstrated in another example, such anionic and highly stable sulfonate-functionalized Zr-MOFs can also be used to disperse and align the cationic monomers during the oxidative polymerization of aniline in strong acids, which can largely improve the pseudocapacitance of the resulting polyaniline for the use in aqueous supercapacitors, even in the presence of the electrochemically inactive MOF inside (see panel (d) in the Figure). Findings here suggest that the chemically robust and porous Zr-MOFs, even without electrical conductivity and catalytic activity, can still play diverse roles to enhance the performances in electrochemical applications.

Figure 1