Redox Active Molecules for Non-Aquoes Flow Battery
Authors: Zhang Lu , Huang Jinhua , Burrell Anthony K. 
Published: February 2020

As one promising stationary energy storage system, redox flow battery has been increasingly recognized to have significant potential to integrate renewable energy, such as solar and wind, into electrical grid, in order to promote a more intelligent, efficient, and capable electrical power system. Two reservoirs containing redox active molecules serve as the liquid positive and negative electrodes, refereed as catholyte and anolyte, where the intrinsic reversible redox reactions (anodic for catholyte, and cathodic for anolyte) will be utilized to store the energy during the charging process.1
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As one of the most important components, redox active materials in the electrolyte dictate the overall performance of NRF. Organic redox active or reversible molecules are usually built upon conjugated systems, which could help stabilize the radicals and ions generated during redox process, therefore most of them could go back and forth from difference redox states to their neutral states upon external electrochemical control or stimulus. This unique reversible behavior makes them of extreme interest to electrochemical energy storage and leads to many possible applications in various technologies. Molecular engineer of organic redox active molecules could lead to tunable physical and electrochemical properties, including solubility in organic solvents, molecular mobility, redox potential, and electrochemical reversibility, which are key factors to energy storage applications. Specifically, one family of dimethoxybezene based organic molecules will be discussed in this talk. Dimethoxybezene based molecules have drawn increasing attentions due to their excellent electrochemical activities, especially as redox shuttle additives and possible non-aqueous flow battery catholytes, which both require redox active molecules to be the key functional components. Due to the organic synthesis feasibility, structural improvements could be conducted to intentionally tune the physical or electrochemical properties of dimethoxybezene. For instance, ANL-2 was developed with improved solubility in carbonate based electrolytes and excellent overcharge performance as redox shuttle additive.3 The success of ANL-2 was based on the previous attempts using various design strategies to improve the solubility of DDB, including asymmetric ANL-1, and DBMOEB (1,4-di-tert-butyl-2,5-bis(2-methoxyethoxy)methoxy-benzene). Other examples are ANL-8, ANL-9 and ANL-10 molecules, with focus on further increasing the solubility. With the asymmetrically incorporated ether chains, those molecules not only keep the electrochemical behavior of DBBB, but also exhibit higher solubility in polar electrolyte solutions. What’s more, ANL-8 and ANL-9 turn into liquids at room temperature, which imply a brand new possibility of utilization of neat redox molecules as catholyte for energy storage applications, thus dramatically enhancing the energy density. The molecular property/structure relationship has also been investigated, which could lead to further understanding and novel molecular discovery.