Propylene‐Bridged Associative Bis(bipyridinium) Electrolytes for Long‐Lifetime Aqueous Organic Redox Flow Batteries

Abstract Aqueous organic redox flow batteries (AORFBs) represent a potent technology in low‐cost storage and grid‐scale adoption of renewable electricity, whereas bipyridinium derivatives, the most extensively adopted anolyte, undergo diverse side reactions causing severe loss of battery lifetime. Here, we propose a strategy to promote the formation of intramolecular radical π dimers of reduced bipyridinium electrolytes and stabilize the radical intermediates during charge and discharge, resulting in significant improvements in battery lifetime. Density functional theory calculations reveal that proof‐of‐concept electrolytes, M‐bisV and TMAP‐bisV, demonstrate Gibbs free energy change of −101.49 kJ mol −1 and −103.90 kJ mol −1 for the formation of radical π dimers, as opposed to the less favored −14.36 kJ mol −1 for methyl viologen. Electron paramagnetic resonance and UV–vis spectra studies confirmed the significantly facilitated intramolecular π–π stacking of M‐bisV and TMAP‐bisV radicals. This translates to a more than one order of magnitude improvement in electrolyte stability for M‐bisV and TMAP‐bisV in flow cells. Specifically, TMAP‐bisV possesses a theoretical capacity of 66.5 Ah L −1 and exhibits negligible capacity fade at 1.5 M e − , which represents a new record in this area. We believe the concept may open a promising avenue toward the design of capacity‐dense and super‐stable AORFB electrolytes.