Enabling Low-Cost Electrolytes and Membranes for Redox Flow Batteries

Abstract

The transition to renewable energy sources like solar and wind (which are intermittent) depends on the availability of low cost energy storage. Redox Flow batteries (RFBs) show promise as an alternative to lithium-ion batteries for long duration storage. Low-cost Organic redox-active molecules can potentially replace costly vanadium in RFBs, but their high rates of degradation prevent their use in practical RFBs. We use a combination of experiments, numerical simulations and data-driven methods to understand the degradation of organic molecules, and membrane crossover of active materials. We develop and validate a zero-dimensional model as a tool to analyze the electrochemical performance of an organic flow cell. The model simulates voltage losses using parameters obtained from independent electrochemical impedance spectroscopy and voltammetry measurements to accurately simulate a symmetric ferro-/ferricyanide cell’s performance across a wide range of current densities and electrolyte flow rates. The model also simulates capacity fade caused by molecular degradation and a considerable variation in the capacity fade rates during cell cycling because of changes in the cycling protocol. We then use ultraviolet-visible spectrophotometry and statistical inference techniques to understand molecular degradation in organic redox-active molecules and elucidate the Michael attack decay mechanism for 4,5-dihydroxy-1,3-benzenedisulfonic acid (BQDS), a once-promising molecule for RFBs. Bayesian inference and multivariate curve resolution is used on the spectroscopic data to derive reaction orders and rates for Michael attack, estimate the spectra of intermediate species and establish a quantitative connection between molecular decay and capacity fade. Several quinoxaline derivatives (despite promising electrochemical properties) demonstrate poor chemical stability (with capacity fade rates > 20%/day). We find the reduced form of 2,3-dimethylquinoxaline-6-carboxylic acid (DMeQUIC) losing redox-activity due to tautomerization in alkaline conditions. Using spectroscopic, electrochemical and data-driven techniques, we estimate the kinetic rate constants for the tautomerization reaction. Density functional theory (DFT) modeling qualitatively explains stability trends for several derivatives, among which quinoxaline-2-carboxylic acid showed a stable performance during mixed symmetric cell cycling. Membrane crossover of redox-active molecules is another significant cause of capacity fade and we explore the use of sodium superionic conductor (NaSICON) membranes as a means to prevent it. NaSICON is stable in neutral and strongly alkaline sodium-containing electrolytes, but degrades rapidly when potassium-ions are present. NaSICON’s resistance decreases sharply on increasing temperature and decreasing thickness, and successfully blocks permanganate and polysulfide crossover. We then demonstrate a stable cycling ferrocyanide | permanganate flow cell for three weeks using a NaSICON membrane. This work therefore presents a framework to enable researchers to understand and mitigate degradation of novel organic redox-active molecules in RFBs.