High Capacity Catholytes for All-Organic Non-Aqueous Redox Flow Batteries

Abstract

Only about 3% of the electrical energy consumed today is supplied by intermittent energy sources such as solar and wind power. To decrease our reliance on CO2-producing fossil fuels, which currently supply almost 70% of electricity, it is necessary to smooth out the intermittency of renewable energy production. The development of low-cost electrical energy storage (EES) systems could lead to significant integration with the electrical grid. Redox flow batteries (RFBs) are of great interest as a low-cost solution, and systems as large as 20 MWh (check value) have been employed for commercial applications. The commercialization of vanadium-based RFBs has been enabled by the combination of low cost, long lifetime, and high efficiency, and stands out due to the unique ability to scale storage capacity independently of the reactor area. Despite their advantages, current RFBs are limited in charging voltage to ca. 1.5 V due to the electrochemical instability of water above this potential. The replacement of the acidic, aqueous electrolytes with aprotic organic equivalents could allow for higher charging voltages – up to 4-5 V, depending on the identity of the electrolyte solvent.1 Compared to aqueous RFBs, non-aqueous RFBs are still in their infancy due to limited stability and/or solubility of electro-active materials, membrane crossover, and the cost of materials. Recently the performance of organic compounds has been tested in flow systems or stationary mimics. More recently, reports of non-aqueous RFBs containing organic electro-active materials have surfaced. N-oxidanyl amines (e.g. TEMPO), dialkoxybenzenes, and phenothiazines serve as electron-donating electro-active materials, while phthalimide, anthroquinones, quinoxilanes, fluorenone, and viologen act as electron-accepting counterparts. Many of the electron donors have been used as electron-transfer catalysts in other energy storage and collection applications, including redox shuttles for overcharge protection of lithium-ion batteries (LIBs), electron-transfer agents in lithium-air batteries, and redox mediators in dye-sensitized solar cells, among others. Our recent work in protecting LIBs from overcharge has led to the development of several phenothiazine derivatives that exhibit high stability in the neutral and singly oxidized (radical cation) forms,2-6 and some examples exhibit high solubility in organic electrolytes – the combination of which inspired us to evaluate these materials as electron-donating electro-active materials for non-aqueous RFBs.7 Here we will share our recent results in the evaluation of highly soluble phenothiazine derivatives using a variety of electrochemical techniques and – in some cases – in flow batteries. Two highly soluble (miscible) derivatives – one of which is a liquid at room temperature – can be prepared in large scales in one step from commercially available phenothiazine. The persistency of the singly oxidized state has enabled us to synthesize and isolate radical cation salts, which can be used in symmetric flow tests. Additionally, we have found that appropriate identity and positioning of substituents around the periphery of the phenothiazine ring has led to a reversible second oxidation event as measured by cyclic voltammetry. Cycling tests reveal that the dication of these molecules is more stable than versions that do not contain substituents and give an atom economy as low as 150 g/mol e-.