(518m) Probing the Link between Aqueous-Phase Ce3+/Ce4+ Structure and Redox Kinetics for Energy Storage

The need for energy supply to match the world’s increasing population and improving quality of life has clearly been shown to be accompanied by increasing levels of CO₂ in the atmosphere. Using renewable energy sources can decrease CO₂ emissions without decreasing quality of life, but the intermittency of renewable systems such as solar and wind poses a challenge in matching energy supply to demand. The geographical limitations of pumped hydroelectric mean that for a transition to a renewable energy economy to occur, there must be new innovations in energy storage technologies. One promising energy storage technology is the redox flow battery (RFB). 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 overpotentials related to mass transport (also called concentration overpotential), conductivity, and kinetics. In this talk, we will focus on how understanding the structure of aqueous metal ions used in RFBs can help to understand the overpotentials for redox couples employed in aqueous RFBs.

We will briefly discuss how X-ray absorption and UV-Vis spectroscopy can be used to unravel the structure of metal ions in different acids. This redox couple is used in the well-studied vanadium RFB system. We will also show how X-ray absorption and UV-Vis can be used to study the Ce³⁺/Ce⁴⁺ redox couple. Ce³⁺/Ce⁴⁺ has a high positive potential¹ that makes it useful for applications for indirect oxidation, and for higher power densities for RFBs without requiring increases in current densities. We 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. Using our experimental methods, combined with DFT calculations, we explain the role of the electrolyte on structure and solubility. We compare the energy of Ce⁴⁺ complexation (determined from the change in redox potential) and the structure of the Ce ions in different electrolytes to our measured kinetic data to understand the mechanism of the reaction in aqueous phase on different electrocatalyst surfaces.

References

(1) 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.