Stability of Alizarin as a Low-Cost Anthraquinone for Aqueous Redox Flow Batteries

An affordable and sustainable redox-active compound with suitable electrochemical properties is the key for commercializing redox flow batteries. Herein, the stability of the flow battery negolyte, alizarin (1,2 - dihydroxyanthraquinone ), is studied under different cycling conditions with a ferro/ferricyanide posolyte. The developed cell exhibits an open circuit potential of 1.12 V at 50% state-of-charge and capacity fade rate of 0.73% per day with full state-of-charge window (∼0%–99%) cycling. Three capacity fade mechanisms are identified: dimerization of reduced alizarin and a decrease in concentration of alizarin due to precipitation and crossover. Capacity recovery via oxidation of formed dimers is implemented with some success, while leaving precipitation as the dominant capacity fade mechanism.