Molecular Level Tuning of Redox-active 2D-Covalent Organic Frameworks for Energy Storage

Abstract

Covalent Organic Frameworks (COFs) offer a dynamic platform to play with its chemical functionalities as well as with it’s the structure and symmetry. The choice of symmetry of the building units drives the propagation the COFs either in a two-dimensional way or in three dimensional way. The 2D network of the COFs when decorated with multiple redox-active functional groups it resembles like heteroatom doped graphite structure. Additionally, the presence of one-dimensional porous nano-channel inside the 2D layered structure of COFs opens up the possibility for grafting the nano-wall with electron-rich light elements. Along with the interlayer - stacking like graphite these 2D-COFs gain its extra stability from inter and intralayer hydrogen bonding. The plausible tunability of the building units and variation of the redox segments of the 2D-COFs bring the attention for utilising it as electrode materials in the energy storage system, hence to find out a suitable alternative of graphitic materials with lot more advantage in terms of durability and performance. Hereby the designed way we developed the Nitrogen and Oxygen-rich 2D-COFs since these two lightweight atoms are already known to interact with electro-positive charge carriers (Li+, Na+ and H+). Whenever these atoms are introduced into the frameworks in terms of redox-active functional groups, these COFs start acting as an apt candidate for energy storage in Metal ion battery and in supercapacitors. The scope of synthetic development of desired building monomers aids to enhance the number of redox centres per unit of the 2D-COFs. As a result, systematic improvement of the performance of the LIB, SIB and supercapacitors were possible. Moreover, structure-property co-relation to explain the mechanistic pathway has been properly investigated in both experimental and theoretical way. The advantage of the inherent surface area of COFs and enhanced surface accessibility of COF derived Covalent Organic Nanosheets (CONs) assist to understand the double layer capacitive and redox driven battery characteristics of COF based electrodes. The ease of exfoliation of 2D-COFs breaks the kinetic barrier for the diffusivity of the charge carriers inside the network of the COFs. The chance of restacking of the layers like Graphene has been overcome in case of self-exfoliated and chemically exfoliated CONs. As a result, the more exposed surface functionality offers the facile interaction to the guest ions of electrolytes. The clear overview of Li+-CONs interactions has been discussed in Chapter 1 through a detailed analysis of LIB half-cells. Whereas Chapter 2 brings the practical life applicability of CON-electrode based full-cell LIB. But energetically unfavourable sodium ion insertion in the layered structure of graphite as well as in 2D-COF restricts the recent technologies to develop cheaper and user-friendly Sodium-ion battery. In chapter 3 we tried to overcome this thermodynamic barrier by electronic energy level tuning of the highly crystalline COFs. The electronic driving force accelerates the movements of the Sodium ions even inside the buried layers of the COFs. However, the heteroatoms decorated porous 1D nano-channels of COFs always provides a suitable path for percolation of the charge carriers under applied potentials. The lack of disorderness of the highly crystalline COFs also assists for adopting a crisp experimental-computational approach to gain coherent structure-property relation. The chapter 4 brings the impact from a large surface area along with the contribution of redox-active groups present in each of the layers of the COFs. These two features synergistically increase capacitive behaviour when subjected to acid electrolytes. The stability originated from the β-ketoenamine form of the COFs prohibits the degradation and dissolution of the COF derived electrodes even under applied potential. Starting from the trihydroxy moiety, the methodical variation of a number of redox-active hydroxyl groups pointed out the importance of the enamine building units of the COFs in terms of electrochemical stability even during accumulation of the large amount of charge per unit cell of the COFs. Though all the novel COFs were tested for LIB, SIB and supercapacitor-electrodes individually, the best outcome in terms of stored charge per unit weight of the COF-materials has been considered for further studies. Also, the precise engineering of the framework was established from a lot of trials with different COFs. We mainly focused on developing the high performing electrodes which either overcome the performance of the commercially available LIB, SIB and Supercapacitors or at least comparable to those. Through our research did not always bring out the best performance among all the existing literature reports, it certainly went through the meticulous analysis of the results and pave the pathway for futuristic opportunities. Our investigations and insightful analysis undoubtedly prove that COF as an electrode in the energy storage system is not restricted into just to find out another application of the COFs. The potential of COF derived electrode as a suitable substitute for graphitic materials has also been properly justified in our research. Molecular-level designability of the 2D-COFs as well as systematic control on the number of electronically active segments, open up the jackpot to fabricate high performing electrode materials in the field of battery and supercapacitors, something which is not so straight forward in heteroatom doped graphite. The real-time utility of COFs to decrease the charging time of the electronic device was always the key attention of our studies. Hence the COFs were developed and systematically tuned in such a way that it fulfils all electronic demands to be charged rapidly. We also tried our best to address some obvious challenges and grey areas related to the performance of the fabricated devices which is developed from the COF based electrodes. We do agree that the insitu experiments during the charging-discharging of the device could provide strong support to establish the mechanism of the charge carriers’ interactions with the framework. That is something we are focusing on our future works. The wonderful coherence of the experimental in-situ analysis with the theoretical modelling, along with thorough MD-simulation would draw the clear picture for the future development of the redox-active COFs as per requirement.