Cation Modified Highly Soluble Active Materials for Redox Flow Batteries

Abstract

Renewables such as solar and wind energy are making increased penetration into modern electrical grids.1 However, the reliability of such grids is challenged by the intermittent nature of renewable sources and variable atmospheric conditions, posing a major roadblock in transition toward carbon-neutral energy sources. Grid level energy storage has long been seen as a solution to the intermittency problem.2 Among various storage technologies, redox flow batteries (RFBs) stand out, due to their ability to accommodate various needs of renewable-powered grids.3 RFBs based on non-aqueous chemistry (NRFBs) have a potential to greatly increase the energy density, however, a major setback in NRFB implementation arises from instability and low solubility of active materials.4,5 Herein, we present a strategy to design a flow battery utilizing anionic active materials which exhibit high solubility and high stability. To this end, we utilized a highly stable bio-inspired complex, vanadium-bis-hydroxyiminodiacetate (VBH) as a catholyte and anthraquinone-sulfonate (AQS) as an anolyte scaffold. Cycling performance of a flow battery based on VBH/AQS active materials was investigated in a full cell and a detailed chemical and electrochemical analysis of electrolytes before and after cycling was performed. Post-cycling analysis of electrolyte revealed that the active materials are highly stable even under deep charge-discharge cycling. However, lack of suitable membrane resulted in modest capacity fade. To overcome the crossover issue, a full cell comprising compositionally symmetric electrolytes, made by mixing anolyte and catholyte, was performed and various performance metrics of such cell were evaluated.