Molecular Mechanism and Electrostatic Effect Enabling Symmetric All-Quinone Aqueous Redox Flow Batteries

Symmetric all-quinone aqueous redox flow batteries (SQA-RFBs), in which the same quinone derivative is used as the electroactive compound in the negative and positive electrolytes, thereby obviating the need for a species-selective membrane, have been pursued as a potentially cost-effective and sustainable technology for stationary-electrical energy storage. Molecular decomposition during redox activity has frustrated all symmetric organic aqueous RFB development attempts. We used in situ/operando spectroelectrochemistry and density functional theory calculations to demonstrate that during the redox reaction of alizarin red S (ARS), a promising quinone for SQA-RFBs, intramolecular electronic oscillations form positively charged intermediates. Electrodes functionalized with net negative charge stabilize these intermediates via a hybrid adsorptive–diffusive electrochemical reaction mechanism, thereby enabling the cycling of the SQA-RFB. To understand the mechanism, spectroelectrochemical studies were performed on a series of electrodes with and without this functionalization. We report the performance of the first membraneless SQA-RFB prototype, containing ARS in the electrolyte storage reservoirs and instrumented with a reference electrode to evaluate the evolution of the half-cell potentials.