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 TEMPO moiety is the key to effectively mitigating chemical degradation during charging and discharging, which hinders OH–-access to...

Probing Local pH Change during Electrode Oxidation of TEMPO Derivative: Implication of Redox-Induced Acidity Alternation by Imidazolium-Linker Functional Groups

“…Previous work has indicated that Cl − interferes with OH/O adsorption and causes the formation of Pt-chloro complexes (e.g., [PtCl 4 ] 2− and [PtCl 6 ] 2− ) to become competitive with PtO generation. 38 Second, we found in our earlier work 17 that the electrogenerated [MIMAcO-T + − OH − ] adduct repels OH − , which may cause ΔpH local to deviate from the theoretically expected results. These two factors may contribute to the difference between the experimental and simulated results, which are reflected in the relatively large error bars in Figure 5f.…”

“…The molecular structures of 4-OH-T and MIMAcO-T are shown in Figure a. MIMAcO-T is a Bro̷nsted acid, which liberates protons upon hydrolysis to form [MIMAcO-T–OH – ] MIMAcO − normalT + H 2 normalO ⇌ false[ MIMAcO − normalT − OH − false] + H + …”

“…A few studies suggest that the presence of conjugated functional groups on TEMPO influences its interactions with H + and OH – and enhances the chemical stability of its oxidized and reduced forms. Song et al , showed that covalent conjugation of either imidazole-acetyl-amino or trimethylammonium-acetamido groups to the TEMPO moiety altered the electron distribution in the molecule and protected the N–O radical and oxoammonium cation against H + and OH – attack.In addition, our team has found that covalent attachment of the hydrophilic imidazolium-linker functional group to TEMPO captures OH – , which hinders the hydroxide ion access to the oxoammonium site of TEMPO + and significantly mitigates against its chemical degradation . These previous literature studies suggest that functional groups improve the chemical stability of TEMPO by interacting with H + and OH – .…”

“…Song et al 10,32 showed that covalent conjugation of either imidazole-acetyl-amino or trimethylammonium-acetamido groups to the TEMPO moiety altered the electron distribution in the molecule and protected the N−O radical and oxoammonium cation against H + and OH − attack.In addition, our team has found that covalent attachment of the hydrophilic imidazolium-linker functional group to TEMPO captures OH − , which hinders the hydroxide ion access to the oxoammonium site of TEMPO + and significantly mitigates against its chemical degradation. 17 These previous literature studies suggest that functional groups improve the chemical stability of TEMPO by interacting with H + and OH − . However, explanations of these phenomena have relied mostly on computational studies, and the effect of the local pH changes driven by the H + -dependent electrode reaction has not been experimentally examined.…”

“…An increase in the H + concentration accelerates the disproportionation of TEMPO into TEMPOH and TEMPOH 2 + , which are electroinactive within typical aqueous electrochemical windows. In the case of TEMPO oxidation, the OH – reacts with TEMPO + to produce a hydroxyl radical, which initiates ring-opening and other side reactions. − The catalytic selectivity of TEMPO in alcohol oxidations is also influenced by pH. , For example, TEMPO selectively oxidizes primary alcohols under neutral or basic conditions, but its selectivity changes to secondary alcohols in an acidic solution. Several studies have reported the effect of pH on the electrocatalytic activity of TEMPO. − Thus, the pH of the solution surrounding the TEMPO·/TEMPO + redox reaction should be carefully considered in electrochemical applications.…”