Design Of Conjugation-Extended Viologen Based Electrolyte And Its Application In Aqueous Flow Battery

Developing clean and renewable energy（such as solar and wind energy）is a strategic support for achieving“dual carbon goals”.Solar and wind power have typical intermittent and fluctuating characteristics,and grid connection in direct way poses a threat to grid security.Energy storage technology must be equipped for their effective digestion.First,aqueous organic flow batteries have the characteristics of power/capacity decoupling,and intrinsic safety advantage in long-term energy storage application.Second,organic molecules as energy storage medium,can be freed from resource dependence on nonferrous metals such as lithium and vanadium.Third,organic molecular structure/energy levels are easily adjustable and thus can realize the regulation of battery voltage,capacity,and stability.Therefore,aqueous organic flow battery is considered as one of the important technologies for efficient and safe long-term energy storage.Among many electroactive molecules,viologen-based compounds can be used in neutral electrolyte environments,featuring of high electrochemical reversibility,as well as low corrosion to equipment and thus low system maintenance cost.They have recently received widespread attention and become a leading hot topic.In this thesis,in response to the problems of low cell voltage,insufficient cycling capability and air intolerance faced by the commercial application of viologen-based flow batteries,the relevant researches on viologen-based electrolytes with low potential,high stability and oxygen tolerance are carried out.Focusing on the key scientific issues of“viologen molecule and its electrolyte structure regulation”,we develop the exploration work on molecular engineering and electrolyte’s thermodynamic regulation,as follows:（1）Design of electron-rich five-membered-heterocycle bridged viologen and the study of high-voltage aqueous flow battery.We develop a“conjugation extension”strategy,which introduces electron-richπbridges（thiophene and furan units）into the center of bipyridinium skeleton,yielding low-potential five-membered-heterocycle bridged viologens（（ATBPy）Cl₄and（AFBPy）Cl₄）.Through density functional theory calculation,nuclear magnetic resonance,Raman spectroscopy,and electrochemical measurements,the regulation effects ofπ-bridge units on the molecular orbital energy levels,as well as the structural evolution and charge-storage mechanism are studied carefully.The flow battery achieves a high standard battery voltage of 1.51 V,by using（ATBPy）Cl₄ as negative electrolyte and pairing with the TEMPO derivative positive electrolyte.（2）Investigation into protonation behavior of reduced viologen and its stabilization mechanism regulated by conjugation-extended structure.It is found that the protonation behavior at N-position of doubly-reduced viologen molecule is an important reason for the low Coulombic efficiency and capacity degradation of flow battery.Inhibiting such a process can effectively improve the performance of viologen-based flow batteries.Based on the“conjugation extension”strategy,the stabilization effect of conjugated bridging groups on the two-electron storage of viologen is systematically studied;and four nitrogen-heterocycle bridged viologen molecules（such as thiadiazole bridged viologen（ATzBPy）Br₄）are designed and synthesized.Density functional theory calculations show that the thiadiazole unit has the strongest charge delocalization ability,reducing Lewis basicity of N-position.Raman tests show that the degree of protonation in doubly-reduced state decreased significantly for（ATzBPy）Br₄.Using（ATzBPy）Br₄ as negative electrolyte,flow battery delivers a high Coulombic efficiency of 99.99%and superior cycling stability（100%capacity retention rate after 100 cycles）.（3）Regulation of oxygen tolerance of viologen-based electrolyte by host-guest interaction.We develop a“supramolecular self-assembly”strategy using hydroxyethyl-β-cyclodextrin（HE-β-CD）as the host and reduced-state conjugation-extended viologen（ATBPy）Br₂ as the guest to regulate the thermodynamic and kinetic processes of the electrolyte,achieving the oxygen tolerance of viologen-based electrolyte.The interaction and assembly behavior of host-guest molecules are studied by means of nuclear magnetic resonance,Raman,and UV-vis spectra.Electrochemical tests（such as cyclic voltammogram,differential pulse voltammetry and linear sweep voltammogram）are used to characterize the electrochemical behavior and kinetic evolution caused by the assembly process.The results indicate that during the flow battery charging,the doubly reduced（ATBPy）Br₂ as the guest is encapsulated by two HE-β-CD molecules through hydrophobic effect.The host HE-β-CD can effectively shield the attack of dissolved dioxygen in space and hinders the occurrence of oxygen reduction reaction kinetically.The flow battery achieves stable operation in air environment,displaying excellent Coulombic efficiency（～99.905%）and cycle stability（97.73%capacity retention rate for 300 cycles）.This thesis innovatively proposes“conjugated extension”and“supramolecular self-assembly” strategies,providing design concept for creating new conjugation-extended viologen molecules with low potential and high chemical stability,and effective thermodynamic/kinetic regulation pathway for designing air-tolerant viologen-based electrolyte,which have important practical significance.