Characterizing the Ce3+/Ce4+ Chemistry for Use in Redox Flow Battery Applications

Abstract

Energy storage technologies will be crucial to meeting rising renewable electricity demand in the U.S., but there is currently not enough storage capacity to meet this demand. As described in Chapter 1, redox flow batteries (RFBs) are a favorable energy storage technology for large scale, long-duration energy storage, but the state-of-the-art all-vanadium RFB (VRFB) is too expensive. Cerium is promising in RFBs because of its higher voltage and cheaper precursor. The economic and environmental performance of a Ce RFB compared to the VRFB has not been assessed in detail, however, and the fundamental processes that control the Ce3+/Ce4+ thermodynamics and kinetics are not understood. To address this, we compare the VRFB and Ce-V RFB storage cost and greenhouse gas (GHG) emissions and study the Ce3+/Ce4+ structures, thermodynamics, and kinetics. The theory behind the economic and environmental modeling and spectroscopy and kinetic measurements is discussed in Chapter 2. In Chapter 3, we develop technoeconomic assessment (TEA) and life cycle inventory (LCI) models and determine that the Ce-V RFB minimum levelized cost of electricity (LCOE) is lower and the two RFBs’ levelized GHG (LGHG) emissions are similar, suggesting Ce should be considered further in RFB applications. The redox potential and exchange current density are identified through a sensitivity analysis to be highly influential to the Ce-V RFB cost and emissions, motivating the need for further work into the fundamental phenomena that control thermodynamics and kinetics. A 194 mV increase in redox potential is equivalent to an increase in kinetics by a factor of two, providing electrolyte and electrode engineering guidance. The Ce3+/Ce4+ redox potential is highly dependent on the electrolyte anion. To determine the link between anions and thermodynamics in Chapter 4, we study the Ce3+ and Ce4+ ionic structures in acids relevant for battery applications. Using UV-Vis spectroscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and density functional theory calculations, we find that Ce3+ is coordinated by nine water molecules and Ce4+ is complexed by at least one anion. The decrease in redox potential is driven by stronger anion complexation of Ce4+. Thus, to maximize thermodynamics for RFB applications, electrolytes with weaker complexing anions should be selected. The cerium electron transfer kinetics must be increased for RFB applications by optimizing the factors that control kinetics, but the Ce3+/Ce4+ charge transfer mechanism is not known. We couple EXAFS and kinetics measurements to propose a two-step mechanism in H2SO4 (Chapter 5). The first step of the mechanism is a chemical step, and the second step is a rate-determining electron transfer described through Marcus theory. We find the electrolyte controls the kinetics and hypothesize that the Ce3+/Ce4+ kinetics will be fastest in weaker complexing electrolytes, e.g., HClO4. Assuming the same mechanism holds in HClO4 and the preexponential factor does not change, we expect the kinetics can increase by a factor of 10,000 in HClO4, whereas the electrode would affect the kinetics up to a factor of nine through electrostatic effects. To control the kinetics in an RFB, a weaker Ce4+-anion complexing electrolyte like HNO3 should be selected, and the electrode surface area should be increased until the increase in electrode costs outweighs the kinetic savings. Since the electrolyte is expected to control both the Ce RFB’s thermodynamics and kinetics future work should optimize the electrolyte for thermodynamics and kinetics through electrolyte engineering (Chapter 6).