Exploring the Spin Density and Electrochemical Properties of Benzotriazinyl Radicals: Advancement in Functional Materials for Flow Battery Application
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Authors
Onyenwe, kingsley Eberechukwu
Issue Date
2025
Type
Thesis
Language
en_US
Keywords
Benzotriazinyl radiclas , Grid scale energy storage , Redox Flow batteries
Alternative Title
Abstract
Abstract: Redox flow batteries (RFBs) are charge storage devices that are particularly well-suited for grid-scale energy storage due to their long-life span, independent scalability of power and energy, enhanced safety compared to other battery technologies, ability to handle a large number of charge-discharge cycles without significant degradation, and potential for cost-effectiveness at large scales. The dominant material for redox flow batteries is based on inorganic vanadyl salts, which provide large cell voltages and excellent fatigue resistance. Redox-active organic compounds are promising alternatives to traditional inorganic redox-active compounds for redox flow battery (RFB) applications due to their tunable electrochemical properties through molecular design and synthesis, as well as their potential for faster charge-discharge kinetics. Stable organic radicals have come to the forefront of this field, in which fully reversible electrochemical processes are often observed. Benzotriazinyl radicals are unique materials for redox flow battery (RFBs) applications owing to their bipolar redox chemistry which arises from the ability to undergo fully reversible reduction and oxidation processes. Previous work suggests that benzotriazinyls are promising for redox flow batteries, but challenges arise in increasing the cell voltage. In this work, we report the design, synthesis and characterization of a series of 6-position functionalized 1,3-diphenyl-1,2,4-benzotriazinyl radicals (6-X-BTR, where X = H, Cl, CH3, NH2, NO2, and OMe) toward the aim of increasing cell voltage and charge/discharge kinetics. The electronic structures of the 6-X-BTR series were characterized by electronic absorption spectroscopy, electron paramagnetic resonance (EPR), cyclic voltammetry (CV), computational methods, and, for some of the series, single-crystal X-ray diffraction (SCXRD). Cyclic voltammetry confirmed reversible reduction and oxidation processes for all stable radical derivatives, revealing tunable redox potentials and moderate cell voltages (~1.0 V). Electron-withdrawing substituents (e.g., –Cl and –NO₂) were found to stabilize the radical anion forms, as indicated by anodically shifted reduction potentials, as expected. Heterogeneous electron-transfer rates were measured for the series in which it was found that some of the derivatives had a faster electron transfer kinetics compared to previously reported benzotriazinyl derivative. Substituent effects are rationalized based on a combination of computation, electronic absorption spectra and spin densities obtained by room temperature EPR. Attempts to gain structural insight from the 6-amino-benzotriazinyl radical lead to SCXRD crystallographic characterization of an 8-amino-functionalized leuco radical, which suggest that steric interactions may inhibit final oxidation to the benzotriazinyl radical. Electronic absorption spectroscopy (UV-vis) and EPR analyses were used to follow the conversion of the leuco to the radical form which occurs slowly over 14–15 hours in the presence of oxygen as an oxidant. Overall, these results provide insights into the role of steric hindrance and planarity in stabilizing the redox processes in these unique radicals and support the potential of functionalized benzotriazinyl radicals as tunable, bipolar electrolytes for RFB charge storage materials with fast charge-discharge kinetics.