Redox Targeting as a Strategy to Thin the Crowd, Toward High Energy Density Flow Battery Electrolytes

Redox flow batteries (RFBs), in which charge-carrying, liquid electrolytes are pumped through electrochemical cells, are a promising, developing energy storage technology. In these systems, the energy capacity scales with the volume of electrolyte stored in tanks, and the power scales with the size of the cell. This decoupling gives RFBs potential advantages for long- and medium duration grid energy-storage applications and a favorable safety profile compared with integrated systems like lithium-ion batteries. A significant remaining challenge in the advancement of RFBs is that the concentrations necessary for high energy-density often lead to high viscosity. So-called, “crowded electrolytes” often result in poor performance due to pumping losses and slow kinetics.<br/><br/>This presentation reports progress on a promising nonaqueous RFB, comprising a bio-inspired catholyte called vanadium hydroxyiminodiacetate (VBH) and an organic anolyte. This system operates at moderate concentration, to high states of charge, with excellent cyclability. Furthermore, we report progress on a redox-targeting flow battery (RTFB) system, designed to overcome the viscosity challenges associated with high-concentration electrolytes by incorporating solid boosters into the system. The RTFB studies comprise investigation of the interaction between VBH, as a solution-phase mediator, and solid, cobalt hexacyanoferrate (CoHCF) booster. By independently monitoring the redox state of VBH, using electronic spectroscopy, and CoHCF, using infrared spectroscopy, we demonstrate tightly coupled electrochemistry that can be controlled by modifying the intercalation cation.