Modeling Ce3+ and Ce4+ Structures and Redox Kinetics in Acidic Electrolytes

The Ce³⁺/Ce⁴⁺ redox couple is used in many fields, including organic synthesis, waste remediation, and energy storage. Despite the Ce³⁺/Ce⁴⁺ redox couple’s broad applicability, the structures of complexes in many acidic electrolytes remains unknown. Our recent work using UV-Vis spectroscopy and DFT calculations shows that Ce³⁺ likely forms a complex with nine waters, whereas Ce⁴⁺ favorably undergoes single ligand exchange in seven acidic electrolytes (HCl, H₂SO₄, H₃NSO_3, CH₃SO₃H, HNO₃, CF₃SO₃H, and HClO₄) ^[1]. Here we report new experimental EXAFS and molecular dynamics-based EXAFS of cerium complexes to further elucidate the structures of the cerium complexes in solution. In addition, the Ce³⁺/Ce⁴⁺ redox couple provides a unique opportunity to study how changing ion complexation of a single oxidation state (Ce⁴⁺) affects the electron transfer kinetics, while the other oxidation state (Ce³⁺) remains a constant structure. We apply theoretical charge transfer models based on the structure of the cerium complex to understand what controls the Ce³⁺/Ce⁴⁺ mechanism and compare calculated redox kinetics to experimental kinetic trends. We extract kinetic parameters of an outer sphere redox reaction using DFT calculations within a polarized continuum model framework. This research increases understanding of cerium ions in acidic electrolytes and tests theoretical paradigms for charge transfer.

1. Cailin A. Buchanan, Eunbyeol Ko, Spencer Cira, Mahalingam Balasubramanian, Bryan R. Goldsmith, and Nirala Singh Inorganic Chemistry2020 59 (17), 12552-12563 DOI: 10.1021/acs.inorgchem.0c01645