Whole-Electrolyte Design Enables Concentrated Two-Electron Storage of Viologens

Accessing the second reduction of viologens can double their charge-storage capacity, but at high concentrations the doubly reduced species associate into poorly soluble assemblies. Efforts to address this have focused on covalent modification of the redox active molecule. Here we show that the surrounding electrolyte can instead be designed to regulate this association. Charge-matched aromatic ions, PhSO3− and PhN(Me)3+, suppress capacity loss while leaving viologen redox potentials largely unchanged. Structural screening reveals that this function requires charge compatibility with the viologen, an aromatic framework and an appropriate counter anion. DFT calculations and diffusion measurements indicate that PhN(Me)3+ can interpose between reduced viologens and weaken their direct association, whereas NaCl stabilizes compact assemblies. Using this approach, an existing viologen supports reversible two-electron storage at 1.0 M, delivering a maximum capacity of 39.3 Ah L−1 over 100 cycles. These results establish the electrolyte as an active design parameter for controlling molecular association in concentrated multielectron redox chemistry.