Chromium Aminopolycarboxylate Complexes For Use As Electrolytes In Redox Flow Batteries

Robb, Brian

Graduate Thesis Or Dissertation

Chromium Aminopolycarboxylate Complexes For Use As Electrolytes In Redox Flow Batteries Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/j3860818h

Abstract

As renewable energy becomes more prevalent, there is increased need for grid-scale energy storage due to the intermittent nature of renewables. Aqueous redox flow batteries (RFBs), which store energy in liquids, are a compelling platform for providing long-duration energy storage. However, the low-cost potential of RFBs has yet to be realized, primarily due to the cost and performance of current electrolytes. Using inexpensive metals bound to mass produced chelated agents could be a viable pathway to cost-effective electrolytes, if suitable metal-chelate pairings were discovered.

Chromium complexes utilizing aminopolycarboxylate (APC) chelating agents have been created as early as the 1940s. During the initial research into Cr-APC complexes, a variety of properties were studied, including binding constants, pH dependence, spectroscopic properties, and basic electrochemical properties. However, few Cr-APC complexes were investigated for use in batteries, and the ones that were suffered from low efficiencies. Furthermore, properties that are important for RFB electrolytes, such as solubility, were rarely studied or optimized.

To explore if a Cr-APC complex could be used as a successful RFB electrolyte, we performed an investigation into finding a suitable complex. We synthesized select Cr-chelate complexes and screening them for desired properties including electrochemical properties, solubility, and buffer compatibility. The most promising complex was identified, which utilized 1,3-propylenediaminetetraacetic acid (PDTA) bound to chromium to create a robust complex (CrPDTA). CrPDTA has a highly negative standard reduction potential, which enabled its use as an RFB negative electrolyte (negolyte). We demonstrated CrPDTA in an RFB set-up by pairing it against three different positive electrolytes (posolytes), resulting in two record breaking voltage aqueous RFBs with high performance and high efficiency.

While battery chemistry is part of the equation, optimizing the battery components and operation for that chemistry is also an essential element to enabling a high performance RFB. Building from the initial CrPDTA RFB, we performed an optimization into synthesis, membranes, and electrocatalysts that allowed for record breaking power performance, high solubility and long-duration cycling, and elimination of active species membrane crossover.

Taken together, this work demonstrates the ability to successfully engineer a chelated chromium electrolyte from initial synthesis to full RFB optimization.

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