High-Throughput Electrochemical Characterization of Aqueous Organic Redox Flow Battery Active Material

Abstract

The development of redox-active organics for flow batteries providing long-discharge-duration energy storage requires an accurate understanding of molecular lifetimes. Herein we report the development of a high-throughput setup for the cycling of redox flow batteries. Using common negolyte redox-active aqueous organics, we benchmark capacity fade rates and compare variations in measured cycling behavior of nominally identical volumetrically unbalanced compositionally symmetric cells. We propose figures of merit for consideration when cycling sets of identical cells, and compare three common electrochemical cycling protocols typically used in battery cycling: constant current, constant current followed by constant voltage, and constant voltage. Redox-active organics exhibiting either high or low capacity fade rates are employed in the cell cycling protocol comparison, with results analyzed from over 50 flow cells.

Leveraging Temperature-Dependent (Electro)Chemical Kinetics for High-Throughput Flow Battery Characterization

“…Elevated temperature cell setup.-Cells were cycled in a glove box configured for high-throughput RFB testing, previously described in Ref. 24. A Novonix battery cycler equipped with a DCoffset unit was used for all cell cycling.…”

“…We have previously reported the development of a highthroughput setup for the cycling of redox flow batteries, which allows for the benchmarking of capacity fade rates and variations in cycling behavior of nominally identical flow cells. 24 As shown herein, augmentation of this system with elevated temperature cell cycling capabilities allows for increased testing throughput of AORFBs. We demonstrate quantitative results for accelerated lifetime testing of multiple previously published AORFB molecules via elevated temperature cycling of nominally identical volumetrically unbalanced compositionally symmetric cells.…”

“…Even when employing high-precision coulometry to cycle labscale RFBs, it appears that extremely stable aqueous RAOMs already approach the limits of precision for quantifying temporal capacity fade rates in lab-scale flow systems. 24 Rather than developing higher-precision instrumentation for the detection of very slow molecular degradation, or methods in which to decrease flow cell noise that are unlikely to translate to real-world scaled electrochemical systems, the goal of the work reported herein is to increase capacity fade rates to the level where quantification with standard battery cyclers is straightforward and capacity fade rate evaluation is expedited. Promoting rapid degradation can also enable in operando spectroscopic analysis of capacity fade mechanisms.…”

“…30,32,33,35 However, the use of temperature to affect capacity fade in AORFBs is rather underdeveloped in the literature. Borchers et al 42 cycled an aqueous polymeric RFB at 60°C, but the use of constant current cyclingwhich results in uncontrolled state of charge (SOC) restriction 24,43 -prevented any quantitative determination of the thermal stability of the redox-active species in an RFB. Rohland et al 44 improved upon the elevated temperature cell by using a volumetrically unbalanced compositionally symmetric cell, 43 with a defined capacity limiting side (CLS) and non-capacity limiting side (NCLS), to cycle an aqueous anthraquinone derivative using constant current constant voltage cycling, at 60°C.…”

Evaluation of in situ thermal stability assessment for flow batteries and deeper investigation of the ferrocene co-polymer

“…In addition, recent studies highlight the strong impact of temperature fluctuations on the measured capacity fade rates and, thus, emphasize the demand for methodologic developments in this area. 10,11…”