High-Throughput Electrochemical Characterization of Aqueous Organic Redox Flow Battery Active Material
Authors: Fell Eric Michael , Aziz Michael J. 
Published: July 2021

Aqueous Organic Redox Flow Batteries (AORFBs) have emerged as promising and potentially disruptive technologies for the storage of electrical energy from intermittent renewable sources for use over long discharge durations when the wind isn’t blowing and the sun isn’t shining. AORFBs could become preferred over Li-ion batteries for stationary storage due to their potentially low-cost active materials, their inherent non-flammability, and the intrinsic decoupling of energy and power capacities in the flow battery design. Recent work has demonstrated that calendar life, rather than cycle life, limits molecular lifetimes in AORFBs due to various molecular instabilities that lead to side reactions, thus inhibiting performance. For accurately determining molecular fade rates, the importance has been known since 2018 of potentiostatic holds in order to avoid artifacts caused by drifts in internal resistance; additionally the utility of volumetrically unbalanced, compositionally-symmetric cell configurations to distinguish molecular fade from membrane crossover has been known [1]. However, some reports continue to use purely galvanostatic cycling to characterize molecular stability, leading to unreliable accounts of molecular fade rates.

An in-depth study comparing previously published redox-active organic molecules [2-8] has been performed using the symmetric cell protocol. True capacity fade rates as functions of concentration and pH are evaluated in a high-throughput setup using high-precision coulometry. Collectively, these results highlight the relevance of electrochemical techniques to understand molecular degradation in AORFBs. By accelerating the screening process of candidate molecules for AORFBs and ensuring rigorous assessment of battery performance, these techniques are expected to more rapidly advance good candidates for long lifetime RFB designs, capable of enabling massive grid penetration of intermittent renewable energy.

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