A group of scientists from the University of Pittsburgh, in the United States, has analyzed challenges and opportunities for the commercial viability of redox flow batteries (RFBs) and has identified the kinetics of electron transfer between electrolytes and electrodes in the charge-discharge stack as the crucial factor for their success.
In the paper Harnessing Interfacial Electron Transfer in Redox Flow Batteries, published in Joule, the researchers explained that vanadium redox flow batteries (VRFBs), which are currently being considered as the most mature and popular RFB technology due to their advanced electrolyte chemistry, have still too-high costs for vanadium precursors and stack components and that, as a result, the research on RFBs has recently refocused on the electrolyte design. “Significant interest is currently centered on organic and organometallic redox couples, due in part to the ability to tune key physicochemical properties such as reduction potential and solubility,” the academics explained. “These electrolytes have shown excellent promise in laboratory studies and are undergoing initial deployment in early-stage commercial devices.”
Efficiency, lifetime, and cost, according to the research team, are the three key factors that must be considered in evaluating the performance of RFBs and their ability to compete with lithium-ion technologies. In terms of efficiency, it went on to say, vanadium redox flow devices have so far shown they can operate at high efficiencies only when operating at low power densities. This trade-off between efficiency and power density is due to kinetic limitations in vanadium redox couples and transport limitations in membrane separators.
Redox flow batteries that are not based on vanadium could represent a cheaper and more efficient alternative due to their lower electrolyte and stack costs. “System cost is often touted as a major advantage for RFBs over established secondary batteries such as li-ion in grid scale energy storage,” the U.S. group stated. “This primarily results from the fact that increasing the capacity of a flow battery simply requires the use of more electrolyte.”
Interfacial electron transfer kinetics is pointed out as the key factor that influences voltage efficiencies in RFBs, which is in turn responsible for lower round-trip energy-conversion efficiencies compared to solid-state storage. “Understanding, and ultimately controlling interfacial electron transfer in RFBs fundamentally depends on the ability to accurately measure reaction rates as a function of applied potential and to interpret the results in the context of RFB operation,” the scientists highlighted, adding that high-quality electroanalysis for RFB performance and robust characterization methods for RFB active materials are needed to help future research gain insight in this matter.
Furthermore, the research team believes that the performance of redox flow storage may be improved by relying on design strategies adopted from electrochemical catalysis or by designing electrolytes with organic and organometallic molecules that exhibit inherently fast, outer-sphere electron transfer reactions, although both techniques are described as suffering from several key constraints. “Research in RFBs will also greatly benefit from a deeper understanding of kinetic stabilization strategies to increase cell voltage,” the academics added. “These will likely be necessary for non-aqueous RFBs that are as stable as li-ion batteries, and they may be equally useful for aqueous systems.”
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