Redox flow batteries (RFBs) are a promising technology for large-scale energy storage, and organic molecules are attractive active materials due to their potential for cost reduction and enhanced energy density compared to vanadium-based systems. Viologens (4,4'-bispyridinium species) are particularly promising due to their high aqueous solubility, negative potentials, and good electrochemical stability under neutral conditions. However, a comprehensive understanding of their charge-discharge processes and degradation mechanisms, especially under air exposure, remains lacking. Most bispyridinium-based RFBs have been operated under strict anaerobic conditions, limiting knowledge about their air tolerance. The reactivity of reduced bispyridinium species (mono- and diradicals) with dissolved oxygen is crucial, as this can lead to the formation of reactive oxygen species (ROS), which are detrimental to battery performance and longevity. Previous studies have linked the formation of viologen-based structures like π-dimers, σ-dimers, and charge-transfer complexes to capacity fade, influencing molecular design strategies to avoid these structures. This research aims to systematically investigate the structure-property relationships of a diverse library of bispyridinium electrolytes, utilizing operando techniques to understand their air tolerance and provide design principles for improved RFB electrolytes.

Existing literature highlights the potential of organic redox flow batteries as a cost-effective and high-performance alternative to traditional vanadium-based systems. Studies on viologens have shown their promise as anolytes, demonstrating high aqueous solubility and negative potentials. However, the lack of a systematic understanding of their degradation mechanisms under real-world conditions, particularly concerning their air tolerance, has hindered their widespread application. Previous research has associated the formation of various radical-mediated structures, such as π-dimers and σ-dimers, with capacity fade, prompting designs aimed at avoiding such structures. This work builds upon this foundation by exploring the correlation between molecular structure and electrochemical performance under both anaerobic and aerobic conditions, providing a more nuanced understanding of the role of radical species and their aggregates.

The researchers designed and synthesized a library of extended bispyridinium compounds by varying the central aromatic core between pyridinium rings, leveraging Suzuki-Miyaura coupling for structural diversity. The redox characteristics of these compounds (both viologens and extended bispyridiniums) were systematically investigated. Density functional theory (DFT) calculations were employed to correlate the electronic structure with experimental redox potentials, showing a strong correlation between experimental Hammett constants and calculated potentials. The influence of R-groups was also evaluated, indicating that core chemistry primarily modulates the gap between the first and second redox potentials. Cyclic voltammetry was used to assess electrochemical reversibility across the library, revealing a range of behaviors influenced by core substitution patterns and conjugation. Key parameters were assessed, including the singlet-triplet free energy gap (ΔEST) which was found to directly relate to electrochemical irreversibility. To gain insights into the electrochemical behavior under operating conditions, coupled operando nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies were performed. These studies examined the spectral characteristics of different bispyridinium compounds (with varying ΔEST values) during cell operation, providing information on radical concentrations and their pairing. This allowed for calculation of comproportionation (Kc) and dimerization (Kd) equilibrium constants. Electrochemical performance, including capacity fade and Coulombic efficiency, was assessed through galvanostatic cycling experiments, utilizing varying electrolyte concentrations to probe the effect of π-dimerization on capacity retention. Online electrochemical mass spectrometry (OEMS) was used to monitor oxygen consumption during cell operation, and DFT and coupled cluster calculations [CCSD(T)] were used to confirm observed phenomena. The electrochemical rate constants were evaluated for the electrode to the electrolyte compounds and for oxygen reduction to corroborate the observed suppression of oxygen reduction.

The research identified the singlet-triplet free energy gap (ΔEST) as a powerful physico-chemical descriptor for predicting the electrochemical reversibility of bispyridinium compounds. Compounds with ΔEST values less than -6.0 kcal/mol exhibited reversible redox processes, while those with ΔEST between -6.0 and 0 kcal/mol showed irreversibility, likely due to highly reactive diradical species participating in side reactions. The studies revealed two distinct electrochemical performance regimes defined by ΔEST: wide gap systems demonstrating closed-shell character and enhanced stability, while narrow gap systems exhibited thermally accessible paramagnetic triplet states and increased capacity fade. Capacity fade was strongly correlated with the formation of open-shell structures (mono- or diradicals). Systems with low Kc (comproportionation constant), high Kd (dimerization constant), and high ΔEST values showed improved capacity retention, indicating that processes promoting closed-shell structures and suppressing radical concentrations are beneficial. Surprisingly, π-dimerization was found to mitigate capacity fade, specifically by reducing the reactivity of radical monomers with dissolved oxygen. Higher concentrations of the bispyridinium compounds favored π-dimer formation and led to significantly improved air tolerance. OEMS experiments confirmed the consumption of oxygen during charging, suggesting that reduced bispyridinium species serve as redox mediators for the two-electron reduction of oxygen. The rate constants for electron transfer from the electrode to the bispyridinium compounds were significantly higher than that for direct electron transfer to oxygen, supporting the mediation process.

The findings address the research question by demonstrating a strong correlation between molecular structure, electrochemical properties, and air tolerance in bispyridinium electrolytes. The identification of ΔEST as a predictive descriptor enables the rational design of more stable and reversible RFB electrolytes. The unexpected finding that π-dimerization enhances air tolerance challenges the prevailing view that these structures contribute to capacity fade. The significant improvement in air stability at higher concentrations highlights the importance of considering intermolecular interactions in electrolyte design. The observed mediation of oxygen reduction by reduced bispyridinium species explains the observed pH changes during cycling. The results have significant implications for the field by providing a clear set of design principles for creating high-performance, air-tolerant RFB systems. This advancement paves the way for practical applications where air-free operation might be challenging or costly.

This research established the singlet-triplet free energy gap (ΔEST) as a key descriptor for predicting electrochemical reversibility in bispyridinium electrolytes. It revealed two distinct electrochemical performance regimes and demonstrated that π-dimerization unexpectedly enhances air tolerance, counter to previous assumptions. The ability to operate RFBs in air, achieved through controlling the intra- and intermolecular pairing of radicals, significantly advances RFB technology. Future research could explore broader ranges of core structures and substituents to further optimize electrolyte properties and extend these findings to other organic RFB chemistries.

The study focused primarily on a specific class of bispyridinium compounds. Further investigation is needed to determine the generality of the findings across a wider range of organic RFB electrolytes. While the long-term cycling experiments demonstrate impressive air stability, extended testing over even longer periods is recommended to fully assess the long-term performance and robustness of these systems under realistic operating conditions. The current study largely focused on understanding the fundamental mechanisms of capacity fade. Future research could integrate cost analysis and scale-up considerations to further assess the viability of this approach.