Redox-active 2D Covalent Organic Frameworks for Energy Storage and Proton Conduction

Abstract

Covalent Organic Frameworks (COFs) are a new class of crystalline polymers constructed using reticular chemistry. The functionalizable precursors connected by covalent bonds give rise to graphene like 2D layers. These layers p stack to generate a layered 2D structure with the accessible pores. Their distinctive properties include distinguishable crystalline structure, high surface area, structural versatility, easy surface modification, tunable pore size, and excellent thermal and chemical stability. These properties make COF an interesting candidate for various applications such as energy storage, optoelectronic devices, gas separation, heterogeneous catalysis, sensing, etc. COFs can serve as a good alternative to heteroatom-doped graphite materials for charge storage and conduction. The precise installation of heteroatoms in the COF backbone, can boost the performance. In the design, redox active groups are incorporated in the COFs to make them more suitable for supercapacitors and Zn-ion batteries. The presence of redox-active functional groups (carbonyl, hydroxyl, pyridine, triazole, tetrazine, etc.) stores additional charges via Faradaic processes and also helps in its percolation. Additionally, the redox-active supporting electrolytes adsorbed on the COF would provide a significant advantage. In this thesis, Schiff-based COFs strategically synthesized from heteroatoms containing precursors, which can stabilize and facilitate the storage and conduction of charge. In the first part of the work, IISERP-COF25 built by coupling tripodal amine units with tricarbaldehyde stabilized via keto-enol tautomerism, was utilized for supercapacitor application with potassium iodide (KI) as an active-redox electrolyte. Operando spectro-electrochemical measurements reveal the existence of multiple polyiodide species with I3− is the predominantly electroactive species adsorbing on the COF electrode, which is responsible for enhancing the charge storage capacity. Subsequently, we designed IISERP-COF22, consisting of squaric acid and phloroglucinol, based Zn-ion battery. The design of the COF orients nitrogen and oxygen in a way that it can chelate/decelerate the Zn ion during discharging/charging, which is yielding good performance of the Zn-ion battery (ZIB). However, with ZnI2 as an active-redox electrolyte, the overall capacity of ZIB increases drastically. The final part of the work focuses on the synthesis of structurally related IISERP-CON1 and IISERP-COF32 via molecular-level engineering for high proton conductivity. Construction of crystalline phase with improved long-range ordering was obtained through atomic-manipulation, this positively impacts the proton conduction at room temperature.