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

Abstract

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⁻ → 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.