Non-aqueous redox flow batteries (RFBs) are promising energy storage devices owing to the broad electrochemical window of organic solvents. Nonetheless, the wide application of these batteries has been limited by the low stability and limited solubility of organic materials, as well as the insufficient ion conductivity of the cell separators in non-aqueous electrolytes. In this study, two viologen analogues with poly­(ethylene glycol) (PEG) tails are designed as anolytes for non-aqueous RFBs. The PEGylation of viologen not only enhances the solubility in acetonitrile but also increases the overall molecular size for alleviated crossover. In addition, a composite nanoporous aramid nanofiber separator, which allows the permeation of supporting ions while inhibiting the crossover of the designer viologens, is developed using a scalable doctor-blading method. Paired with ferrocene, the full organic material-based RFB presents excellent cyclability (500 cycles) with a retention capacity per cycle of 99.93% and an average Coulombic efficiency of 99.3% at a current density of 2.0 mA/cm2. The high performance of the PEGylated viologen validates the potential of the PEGylation strategy for enhanced organic material-based non-aqueous RFBs.

A non-aqueous redox flow battery based on all-PEGylated, metal-free compounds is presented. The PEGylation enhances the stability of the redox-active materials, alleviating crossover by increasing the anolyte and catholyte species’ molecular sizes.

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The lack of a suitable ionic exchange membrane has retarded the development of organic nonaqueous redox flow batteries (RFBs). Membrane-free redox stratified batteries, wherein electroactive materials in immiscible nonaqueous and aqueous solvents as anolyte and catholyte, have emerged as a promising strategy to mitigate the high dependence of RFBs on battery separators. Here we report the exploration of the application of immiscible electrolytes, water and dichloromethane, in membrane-free redox stratified batteries. With 0.5 M phenothiazines in dichloromethane as the catholytes and zinc metal in aqueous electrolyte as the anolyte, the aqueous/ nonaqueous stratified battery presents stable long cycling with a capacity retention of 79.1% over 202 cycles under ambient testing conditions. Study of phenothiazines with varying lengths of alkyl chains (C0, C3, C8, and C18) reveals that the hydrophobicity of the phenothiazine molecules greatly affects the solubility in dichloromethane and battery cyclability. Computation on free energy of solvation and molecular dynamics is also performed to elucidate the hydrophobicity effects. The results presented in this work lay a solid foundation for potential development of the membrane-free RFBs.

The effects of primary and second coordination spheres on molecular electrocatalysis have been extensively studied, yet investigations of third functional spheres are rarely reported. Here, an electrocatalyst (ZnPEG8T) was developed with a hydrophilic channel as a third functional sphere that facilitates relay proton shuttling to the primary and second coordination spheres for enhanced catalytic CO2 reduction. Using foot‐of‐the‐wave analysis, the ZnPEG8T catalyst displayed CO2‐to‐CO activity (TOFmax) thirty times greater than that of the benchmark catalyst without a third functional sphere. A kinetic isotopic effect (KIE) study, in conjunction with voltammetry and UV/Vis spectroscopy, uncovered that the rate‐limiting step was not the protonation step of the metallocarboxylate intermediate, as observed in many other molecular CO2 reduction electrocatalysts, but rather the replenishment of protons in the proton‐shuttling channel. Controlled‐potential electrolysis using ZnPEG8T displayed a faradaic efficiency of 100 % for CO2‐to‐CO conversion at −2.4 V vs. Fc/Fc+. A Tafel plot was also generated for a comparison to other reported molecular catalysts. This report validates a strategy for incorporating higher functional spheres for enhanced catalytic efficiency in proton‐coupled electron‐transfer reactions.