Aqueous-organic redox flow batteries (AORFBs) have emerged as a promising technology class for stationary energy storage. To date, however, synthetic strategies for organic electrolytes have emphasised molecular complexity as a means of targeting desired performance characteristics. Consequently, materials typically exhibit high initial production costs and require large-scale manufacturing to re-establish cost competitiveness, undermining current efforts to achieve ultra-low-cost storage over the coming decade. In the present report, supramolecular complexity is leveraged as an alternative and low-cost design paradigm, that is developed toward augmented device functionality. Such capabilities are demonstrated for pyridinium-based systems and operando spectroscopic techniques are used to gain fundamental insights into structure and mechanisms. From such insights, methods are unlocked to both harness and suppress supramolecular effects towards regimes of performance inaccessible to conventional systems.

In a first demonstration, both the host-guest chemistry and existing tonne-scale production capacity of both cucurbit[n]uril (CB[n]) macrocycles and methyl viologen are leveraged to advance an additive concept for AORFBs that enables stable and tuneable multi electron accessibility – without the need for synthetic modification. By encapsulating methyl viologen in all oxidation states, CB[7] macrocycles improve the solubility of reduced viologen species, suppress parasitic side reactions with trace contaminants, and provide resistance to electrochemical degradation at potentials more negative than any demonstrated for methyl viologen and its analogues to date. Host-guest binding is readily modulated by several independent parameters including temperature, concentration, supporting salt choice and supporting salt concentration. Electrolyte formulation additionally results in the production of industry-grade by-products such as CB[5], CB[6], and CB[8], which can be sold to access average installed electricity costs among the lowest projected to date.

In a second demonstration, operando NMR and EPR spectroscopies, as well as operando mass spectrometry are used to understand and harness the inherent assembly processes of pyridinium radicals towards improved battery performance. Making use of a scalable and versatile reaction chemistry, a synthetic library of extended bispyridinium compounds featuring a diverse array of electronically tuneable aromatic cores is demonstrated. These compounds span the widest potential range demonstrated for pyridinium based redox flow batteries to date and are electronically analogous to classical diradical systems (e.g., Thiele’s and Chichibabin’s hydrocarbons), permitting prediction of electrochemical irreversibility and previously unidentified capacity fade mechanisms. Using coupled operando NMR and EPR spectroscopies, the redox behaviour of these electrolytes is elucidated under representative flow battery conditions and the presence of two distinct regimes (narrow and wide singlet triplet energy gaps) of electrochemical performance is identified. In both regimes, capacity fade is tied to the formation of free radical species, and further show that π-dimerisation plays a decisive role in suppressing reactivity between these radicals and trace impurities such as dissolved oxygen. This result stands in direct contrast to prevailing views surrounding the role of π-dimers in aqueous organic redox flow batteries and enables us to efficiently mitigate capacity fade from oxygen even upon prolonged (days) exposure to air.

In a final demonstration, operando paramagnetic NMR spectroscopy is used to detect and understand the formation of triplet diradical species at high states of charge (SOC) – a completely new capacity fade mechanism within the field that occurs in high voltage systems. It is found that formation of triplet diradicals and their associated structures can be suppressed by careful management of operating temperature and state of charge. Through such operation, full cells made from previously unusable materials can be operated stably under practical conditions. Using such strategies, the materials rendered accessible substantially expand the available design space for future bispyridinium flow battery development towards higher voltage systems. Coupled with the known tendencies of such systems to resist side reactions with oxygen through controlled π-dimerisation, they enable the highest voltage air-stable AORFB demonstrated to date presenting new horizons for achievable energy densities in such systems.

Collectively, these findings provide new insights into the degradation and stabilisation of pyridinium electrolytes during cycling and further unlock libraries of previously unusable materials enabling new material design strategies moving forward. They do so largely by mediating supramolecular assembly processes during cycling, and therefore, do not introduce synthetic modifications which ultimately bear out in system costs. As both π-mediated association and triplet diradical formation are exhibited by a wide range of organic electrolytes, it is anticipated that the insights described herein may enable new design paradigms not only for pyridinium systems, but broad classes of organic electrolyte relevant to redox flow batteries.