Unveiling the Cerium(III)/(IV) Structures and Charge-Transfer Mechanism in Sulfuric Acid

Area of Science:

• Electrochemistry and Materials Science
• Inorganic Chemistry

Background:

• The cerium (Ce) redox couple exhibits asymmetric charge transfer (CT) and significant redox potential shifts influenced by electrolyte composition.
• Limited understanding of cerium structures and CT mechanisms hinders optimization of redox kinetics for applications like energy storage.

Purpose of the Study:

• To elucidate the structures of Ce(III) and Ce(IV) and their charge transfer mechanism in sulfuric acid.
• To explain the observed asymmetry in CT and redox potential shifts.

Main Methods:

• Extended X-ray absorption fine structure (EXAFS) spectroscopy to determine cerium coordination.
• Kinetic measurements to analyze electron transfer behavior.
• Density functional theory (DFT) calculations to model reaction energetics.

Main Results:

• EXAFS confirmed Ce(III) is coordinated by nine water molecules, while Ce(IV) is complexed by water and three bisulfates in sulfuric acid.
• Kinetic independence from the electrode suggests outer-sphere electron transfer.
• A two-step mechanism was identified: a chemical step of bisulfate-water exchange followed by a Marcus theory-compliant electron transfer step.

Conclusions:

• The proposed two-step mechanism explains the CT asymmetry and redox potential shifts in the Ce(III)/Ce(IV) couple.
• The findings provide a framework for understanding and improving metal ion charge transfer kinetics in electrochemical systems.
• This mechanistic insight is vital for advancing energy storage technologies.