Imidazolium Covalent Organic Frameworks-derived Electrocatalysts for Oxygen Redox Reactions in Rechargeable Zn-Air Batteries

Abstract

The demand for advanced energy storage and conversion technologies is gradually increasing since global energy consumption has increased along with global warming and fossil fuel depletion. Accordingly, rechargeable secondary batteries have gained a lot of attention in various fields, including portable and wearable electronic devices, and electric vehicles. Li ion batteries have been commercialized and widely employed in various electronic devices and electric vehicles. However, they have intrinsic imitations such as low energy density, explosiveness, and flammability caused by Li metals and organic electrolytes. To address these limitations, recently, aqueous Zn-air batteries have emerged as next-generation secondary battery systems. Rechargeable Zn-air batteries exhibit high energy density, excellent durability, cost-effectiveness, and superior safety, compared with Li ion batteries. Rechargeable Zn-air batteries run through oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the air cathode during the discharge and charge processes. The ORR and OER, however, have very sluggish kinetics requiring high overpotentials, which leads to a decrease in the overall energy efficiency of rechargeable Zn-air batteries. Hence, bifunctional electrocatalysts are required to effectively promote both the ORR and OER on the air cathode of Zn-air batteries. Covalent organic frameworks (COFs) with well-defined chemical and physical structures are promising candidates as carbon-based electrocatalysts to promote the ORR and OER in Zn-air batteries. However, it remains a big challenge to construct the COFs-based electrocatalysts that can effectively promote both the ORR and OER on the air cathode of rechargeable Zn-air batteries. Accordingly, a synthetic strategy to impart the bifunctional electrocatalytic activities to COFs-derived electrocatalysts is essential to utilize them to rechargeable Zn-air batteries. In Chapter 1, novel imidazolium-rich covalent organic frameworks (IMCOFs) with well-defined active sites and 3D-assembled structures were synthesized, and their electronic structures were tuned by Co incorporation, resulting in inducing the bifunctional electrocatalytic activities for the ORR and OER. The Co nanoparticle-incorporated spherical IMCOF (CoNP-s-IMCOF) exhibited superior electrocatalytic activities for the ORR and OER, compared with the atomic Co-incorporated planar IMCOF (Co-p-IMCOF). DFT simulations revealed that the imidazolic carbons of CoNP-s-IMCOF were the active sites for the electrocatalytic ORR and OER under alkaline conditions. In addition, the Zn-air battery composed of CoNP-s-IMCOF displayed excellent performance with enhanced durability, compared with the one bearing Pt/C and RuO2 electrocatalysts. In Chapter 2, chemical modulations of IMCOF were performed to enhance the OER stability of COF-based electrocatalysts while their electrocatalytic activity for the ORR remines same. According to the DFT simulations, the carbon site of the imidazole group of CoNP-s-IMCOF is unstable during the electrocatalytic OER, leading to its poor OER stability. This unstable carbon sites of CoNP-s-IMCOF were protected by a methyl group (MICOF) or a benzyl group (MBICOF). The Co-incorporated MICOF and MBICOF (Co-MICOF and Co-MBICOF) exhibited the enhanced OER stability along with greater electrocatalytic ORR and OER performances, compared with CoNP-s-IMCOF. This approach for control over the chemical, 3D-assembly, and electronic structures of IMCOFs can be extended to the development of diverse catalytic nanomaterials for applications of interest.