(A) study on metal–organic frameworks for electrochemical applications

Abstract

Porous materials have been extensively utilized in various research fields over the past decades. Since the emergence of metal–organic frameworks (MOFs), its highly large surface area and easily designable structure have been advantageous for the various applications compared to traditional porous materials. As a result of the inception of the new chemistry field, the development of new porous material has been no longer a daunting task. Considering the required properties in the target applications, we can choose a specific MOF from a known library or develop a new one. The aim of this thesis is to propose the conceptual solution of the “modifying frameworks to create the properties required for electrochemical applications.” MOFs show geometrically well-defined structure through strong bonds between polynuclear metal clusters and organic linkers. I hypothesized that metal sites and organic linkers sites within MOFs can act as both electroactive and/or non-active (supporting electroactive material) by modifying frameworks. First research highlights the new strategy to simultaneously produce and stabilize electroactive subnanometric particles (SNPs) within MOFs. Although atomic cluster-sized SNPs have shown great promise in many fields such as full atom‐to‐atom utilization, their precise production and stabilization at high mass loadings remain a great challenge. As a solution to overcome this challenge, a strategy allowing synthesis and preservation of SNPs at high mass loadings within multishell hollow MOFs was demonstrated in first research. I assumed that MOFs could transfer and stabilize molecules using their pores, but also be a precursor of SNPs. Based on that assumption, I developed multilayer MOFs with alternating stable and decomposable layers for the simultaneous production and stabilization of SNPs. The step‐by‐step deterioration of a decomposable MOF was caused by the transferred water molecules. The transmission of water molecules via controlled hydrogen bonding affinity through the water‐stable MOF layers was a key step to realize SNPs from various types of alternating water‐decomposable and water‐stable layers. This built up atomic precursor, thus promoting the synthesis of SNPs in the stable MOF layer. Based on this strategy of stacking up to 10 layers, I was able to synthesize SNPs embedded multishell hollow MOFs with up to five shells. Additionally, the multishell stabilized SNPs by π‐backbonding allowing high conductivity to be achieved via the hopping mechanism, and hollow interspaces minimized transport resistance. These features, as demonstrated using SNP‐embedded multishell hollow MOFs with up to five shells, lead to high electrochemical performances including high volumetric capacities and low overpotentials in Li–O2 batteries. The SNPs as electroactive material and hollow MOF as non-active material dramatically enhance the electrochemical performance by increasing the number of active sites. This modifying framework provides a new method to produce and stabilize SNPs from alternating MOF layers so that it could be expanded to fabricate SNPs within various other types of frameworks. Second research focuses on the use of metal sites and organic linker sites within MOFs to produce highly affinitive sites for supporting electroactive materials. For electrocatalysts, using good supports is the most powerful way to enhance the activity and stability. In addition, high affinitive force prevents the leaching of active atoms. Therefore, the development of adhesive excipient materials that can endow electroactive material with improved performance is one of the most important challenges. Although many MOFs have been successfully utilized as excipient materials, their metal sites have weak affinity for the guest material due to blockage by organic linkers.I believe that MOFs can provide high-affinitive sites when MOFs are converted into metal-organic fragments, because each components of MOFs possess many dangling bonds and unsaturated orbitals. In this strategy, the H2 plasma partially damaged the organic linkers while retaining the tetrahedrally coordinated metal nodes. As a result, a multi-pore structure and delocalized electrons were induced. The spatiality prevented site-aggregation and re-crystallization, leading to the extinction of the newly induced properties. Subsequently, tri-metallic electrocatalysts were impregnated. This leads to a low overpotential and fast kinetics in water oxidation reaction. Moreover, the high activity was maintained for at least 45 days. This modifying framework suggests a new approach that realizes remarkable supporting performance for multi-metallic electroactive materials by utilizing the numerous uncoordinated metal sites and organic linker sites within MOFs. This thesis presents novel methodologies for deriving new electroactive and non-electroactive properties into MOFs so that MOFs can be applied to the targeted electrochemical application through “modifying frameworks strategies”.