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Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties, solubilities and battery performance.

ABSTRACT: Water-soluble 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have been frequently utilized as catholytes for aqueous redox flow batteries to achieve cost-effective renewable energy storage. However, fundamental knowledge of TEMPO derivatives is still largely underdeveloped. Herein, a comprehensive study on the properties of TEMPO derivatives has been conducted in aqueous electrolytes. The results confirm that the redox potential, diffusion coefficient, electron transfer rate constant and solubility are clearly influenced by functional groups of TEMPO derivatives and supporting electrolytes. The charge-discharge cycling performance is evaluated using a symmetric redox flow battery configuration. The capacity decay for TEMPO-based catholytes is mainly derived from the crossover of the oxidized state. The presented study not only advances an in-depth understanding of TEMPO-based RFB applications, but also highlights the challenge of crossover of redox-active TEMPO derivative molecules applied in aqueous RFBs.

SUBMITTER: Zhou W

PROVIDER: S-EPMC9054512 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties, solubilities and battery performance.

Zhou Wenbo W Liu Wenjie W Qin Meng M Chen Zhidong Z Chen Zhidong Z Xu Juan J Cao Jianyu J Li Jun J

RSC advances 20200608 37

Water-soluble 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have been frequently utilized as catholytes for aqueous redox flow batteries to achieve cost-effective renewable energy storage. However, fundamental knowledge of TEMPO derivatives is still largely underdeveloped. Herein, a comprehensive study on the properties of TEMPO derivatives has been conducted in aqueous electrolytes. The results confirm that the redox potential, diffusion coefficient, electron transfer rate constant a ...[more]

PMID: 35516610

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Project description:Application of redox-active polymers (RAPs) in redox flow batteries (RFBs) can potentially reduce the stack cost through substitution of costly ion-exchange membranes by cheap size-exclusion membranes. However, intermolecular interactions of polymer molecules, i.e., entanglements, particularly in concentrated solutions, result in relatively high electrolyte viscosities. Furthermore, the large size and limited mobility of polymers lead to slow diffusion and more sluggish heterogeneous electron transfer rates compared to quickly diffusing small molecules. Although a number of RAPs with varying electrolyte viscosities have been reported in the literature, the relation between the RAP structure and the hydrodynamic properties has not been thoroughly investigated. Herein, hyperbranched 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO)-based polymers intended for application as low-viscosity catholytes for RFBs are presented and the influence of the structure and the molar mass distribution on the hydrodynamic properties is investigated. A new synthesis approach for TEMPO-based polymers is established based on step-growth polymerization of a TEMPO-containing monomer using an aza-Michael addition followed by a postpolymerization modification to improve solubility in aqueous solutions. The compact structure of hyperbranched polymers was demonstrated using size-exclusion chromatography (SEC) with viscometric detection and the optimum molar mass was found based on the results of viscometric and crossover investigations. The resulting RAP revealed a viscosity of around 21 mPas at a concentration corresponding to around 1 M TEMPO-containing units, according to the calculated mass of the repeating unit, showing potential for high capacity polymer-based catholytes for RFBs. Nevertheless, possible partial deactivation of TEMPO units lowered the active TEMPO concentration of the hyperbranched RAPs. A faster diffusion and higher charge transfer rate were observed for the hyperbranched polymer compared to the previously reported linear polymers. However, in RFB tests, a poor performance was observed, which is attributed to the side reactions of the oxidized TEMPO moieties. Finally, pathways for overcoming the main remaining challenges, i.e., high loss of material during dialysis as an indication of being prone to crossover, the partial deactivation of TEMPO moieties, and the subsequent side reactions under battery conditions, are suggested.

Project description:Significance To widen the design space for advanced batteries, developing new electrochemical conversion reactions is challenging yet critical. Primary Li batteries have the highest energy densities among all battery technologies, owing to the successful exploitation of light-weight, non-transition-metal redox centers, and thus represent exemplary systems for maximizing energy storage in chemical bonds. Here, we report a novel class of fluoro-organosulfur catholyte that allows up to 8-e− transfer per molecule and boosts the energy of the leading Li primary battery chemistry (Li−CFx) by 20% based on electrode + electrolyte weight. More broadly, we illuminate an unpreceded electrochemical mechanism exploiting full defluorination of S–F bonds, opening new strategies to tailor S-based redox systems of broader relevance to the battery field. Discovery of new electrochemical redox motifs is essential to expand the design landscape for energy-dense batteries. We report a family of fluorinated reactants based on pentafluorosulfanyl arenes (R-Ph-SF5) that allow for high electron-transfer numbers (up to 8-e−/reactant) by exploiting multiple coupled redox processes, including extensive S–F bond breaking, yielding capacities of 861 mAh·greactant−1 and voltages up to ∼2.9 V when used as catholytes in primary Li cells. At a cell level, gravimetric energies of 1,085 Wh·kg−1 are attained at 5 W·kg−1 and moderate temperatures of 50 °C, with 853 Wh·kg−1 delivered at >100 W·kg−1, exceeding all leading primary batteries based on electrode + electrolyte (substack) mass. Voltage compatibility of R-Ph-SF5 reactants and carbon monofluoride (CFx) conversion cathodes further enabled investigation of a hybrid battery containing both fluorinated catholyte and cathode. The hybrid cells reach extraordinarily high cell active mass loading (∼80%) and energy (1,195 Wh·kg−1), allowing for significant boosting of substack gravimetric energy of Li−CFx cells by at least 20% while exhibiting good shelf life and safety characteristics.

Dataset Information

Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties, solubilities and battery performance.

Publications

Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties, solubilities and battery performance.

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