Stability of Organic Reactants for Aqueous Flow Batteries

Abstract

The transition to non-emitting renewable energy requires cost-effective arbitrage on several timescales to balance the disparity between peak demand and the electricity supplied from intermittent renewables. Among the technologies available for grid scale energy storage, flow batteries have recently gained considerable interest for longer duration timescales because their energy and power costs are decoupled. With the aim of minimizing these energy costs, many researchers have shifted toward designing electrolytes with synthetic redox active organic molecules made from inexpensive and earth abundant materials. Within this trend, it has become evident that the calendar life, rather than the cycle life, of the organic compound is a limiting factor for the viability of many chemistries. We present here some examples connecting flow battery capacity fade to electrolyte stability. An in-depth analysis on the chemical stability and decomposition of one particular molecule, 2,6-dihydroxyanthraquinone, is presented and compared with other mechanisms of capacity fade observed for similar molecules. In most cases, symmetric cell cycling is used to assess the capacity fade and stability of single electrolytes and these results are linked to decomposition of electrolytes stored at elevated temperatures in both reduced and oxidized form.