Structures and Free Energies of Cerium Ions in Acidic Electrolytes.

The Ce3+/Ce4+ redox potential changes with the electrolyte, which could be due to unequal anion complexation free energies between Ce3+ and Ce4+ or a change in the solvent electrostatic screening. Ce complexation with anions and solvent screening also affect the solubility of Ce and charge transfer kinetics for electrochemical reactions involving waste remediation and energy storage. We report the structures and free energies of cerium complexes in seven acidic electrolytes based on Extended X-ray Absorption Fine Structure, UV-vis, and Density Functional Theory calculations. Ce3+ coordinates with nine water molecules as [Ce(H2O)9]3+ in all studied electrolytes. However, Ce4+ complexes with anions in all electrolytes except HClO4. Thus, our results suggest that Ce4+-anion complexation leads to the large shifts in standard redox potential. Long range screening effects are smaller than the anion complexation energies but could be responsible for changes in the Ce solubility with acid.