Covalent conjugation of a 'hydroxide-philic' functional group achieving 'hydroxide-phobic' TEMPO with superior stability in all-organic aqueous redox flow batteries

We show that a covalent conjugation of a 'hydroxide-philic' functional group to a TEMPO moiety is the key to effectively mitigating chemical degradation during charging and discharging, which hinders OH- access to oxoammonium sites in the intramolecular regime, leading to a stable TEMPO-based aqueous organic redox flow battery (AORFB) with long-term cyclability. Thorough quantitative analysis of the degradation of 4-OH-TEMPO in its fully discharged and charged states was performed under various pH conditions; it was confirmed that the strong interaction between the oxidized form (i.e., TEMPO+) and OH- is mainly responsible for the molecular decomposition. Based on this analysis, a TEMPO derivative covalently conjugated with a hydroxide-philic functional group was carefully designed and synthesized; in this article, this derivative is denoted as MIMAcO-TEMPO (4-[2-(N-methyl imidazolium) acetoxy]-2,2,6,6-tetramethylpiperidine-1-oxyl chloride). The hydroxide-philic MIMAcO functional group intimately attracts OH- before the neighboring TEMPO moiety is exposed to OH- attack. Once an OH- is combined with the MIMAcO functional group to form the [(OH- & RARR; MIMAcO)-TEMPO+] adduct, the oxoammonium site acquires a high electrostatic resistance to OH-. The change in the sequential binding energy of OH- to the MIMAcO functional group and the TEMPO moiety was elucidated by density functional theory (DFT) simulations. The estimated resilient characteristics of the oxoammonium site in MIMAcO-TEMPO+ against OH- and the resultant structural stability were confirmed by UV-vis, NMR, mass spectroscopy, and electrochemical analyses. In AORFB application, MIMAcO-TEMPO showed a remarkable improvement in the capacity decay rate (0.012% per cycle) over 1000 cycles of the galvanostatic charge-discharge test, indicating that this is one of the most stable TEMPO derivatives. In addition, based on the highly hydrophilic properties of the MIMAcO functional group, a high-concentration AORFB was successfully demonstrated with 2.5 M MIMAcO-TEMPO, delivering a record-breaking discharge capacity of 57.1 Ah L-1.