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

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.