(226a) Probing the Mechanisms of Aqueous-Phase V2+/V3+ and Ce3+/Ce4+ Redox Reactions for Redox Flow Batteries

Energy demand continues to increase as the world’s population and average quality of life continue to climb, and this demand is accompanied by increasing levels of CO₂. Utilizing renewable energy sources can mitigate the emissions of CO₂, but the intermittency of renewable systems such as solar and wind poses a challenge in matching energy supply to demand. Although pumped hydroelectric is the most common electrical energy storage technology, it has geographical limitations, necessitating new innovations in other energy storage technologies. One promising energy storage technology is redox flow batteries (RFBs). RFBs have the advantages of convective mass transport, long cycle lifetimes, and decoupled power and energy. However, the current densities and efficiencies of RFBs must be increased to drive down costs and make these batteries economical. Improvements to RFB performance can come from minimizing efficiency losses by fundamentally understanding the mechanism of the charge transfer in RFBs. Efficiency losses of these systems arise from mass transport, ohmic, and kinetic overpotentials. In this talk, we will focus on the fundamentals of what controls the kinetic overpotentials for two redox couples employed in aqueous RFBs.

The all vanadium redox flow battery VO²⁺/VO₂⁺//V²⁺/V³⁺ is the most developed RFB,¹ but still the kinetics of the V²⁺/V³⁺ redox couple are incompletely understood. Here we focus on understanding first the structure of the ions in aqueous solution in a series of different electrolytes, using a combination of experimental spectroscopy and time dependent-density functional theory calculations. We then compare the structures in different electrolytes to carefully measured kinetic parameters (e.g., activation barriers, exchange current densities) on controlled surfaces to extract information about the mechanism of reaction. We observe from both steady state activity measurements and electrochemical impedance measurements that halides can act as bridging agents on carbon electrodes for the charge transfer reaction. These results are similar to reports of the influence of halides on Cr²⁺/Cr³⁺ and other redox couples,^2,3 but have not previously been reported for vanadium RFB applications. These results lead to a better understanding of the V²⁺/V³⁺ mechanism, as well as pathways for further improvements in kinetic performance through electrocatalyst and electrolyte design.

We also show results for a less commonly used, but particularly interesting, redox couple, Ce³⁺/Ce⁴⁺, which has a high positive potential,⁴ enabling higher power densities for RFBs without requiring increases in current densities. We first investigate the role of the electrolyte on the structure of the cerium ions and show that although Ce³⁺ is solely complexed by water, Ce⁴⁺ is prone to complexation with the anion, which significantly impacts the redox potential of the couple. We compare the energy of Ce⁴⁺ complexation (determined from the change in redox potential) to our measured kinetic data to understand the mechanism of the reaction in aqueous phase on different electrocatalyst surfaces. The significant shift in the Ce³⁺/Ce⁴⁺ redox potential with electrolyte⁵ (not observed for V²⁺/V³⁺) also has an influence on the electrode structure during reaction. We explore the influence of the electrolyte on the electrocatalyst structure through the use of in situ X-ray Absorption Spectroscopy (e.g., changes in electrocatalyst oxidation state), and show how this may change the kinetics of the Ce³⁺/Ce⁴⁺ reaction.

References

(1) Sum, E.; Skyllas-Kazacos, M. A Study of the V(II)/V(III) Redox Couple for Redox Flow Cell Applications. J. Power Sources 1985, 15, 179–190.

(2) Ulrich, J. J.; Anson, F. C. Ligand Bridging by Halide in the Electrochemical Oxidation of Chromium(II) at Mercury Electrodes. Inorg. Chem. 1969, 8, 195–200.

(3) Hung, N. C.; Nagy, Z. Kinetics of the Ferrous/Ferric Electrode Reaction in the Absence of Chloride Catalysis. J. Electrochem. Soc. 1987, 134, 2215–2220.

(4) Tucker, M. C.; Weiss, A.; Weber, A. Z. Improvement and Analysis of the Hydrogen-Cerium Redox Flow Cell. J. Power Sources 2016, 327, 591–598.

(5) Morss, L. R. Yttrium, Lanthanum, and the Lanthanide Elements. In Standard Potentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordan, J., Eds.; Marcel Dekker, Inc.: New York and Basel, 1985; pp 619–621.