Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: a DFT + U study

Area of Science:

• Materials Science
• Surface Chemistry
• Computational Chemistry

Background:

• Cerium dioxide (CeO2) is a key material in catalysis and energy applications.
• Understanding oxygen interactions on CeO2 surfaces is vital for optimizing its performance.
• Defects on the CeO2 surface significantly influence its reactivity.

Purpose of the Study:

• To investigate the interactions and reduction mechanisms of O2 on oxidized and reduced CeO2 surfaces.
• To determine the energetic favorability of oxygen reduction on different CeO2 surface states.
• To explore oxygen ion diffusion in ceria for potential applications in solid oxide fuel cells.

Main Methods:

• Periodic density functional theory calculations with on-site Coulomb interactions (DFT + U).
• Analysis of adsorption energies and vibrational frequencies for various O2 species.
• Construction of potential energy profiles using the nudged elastic band method.
• Investigation of oxygen ion diffusion barriers on CeO2 surfaces and bulk.

Main Results:

• Physisorption of O2 on oxidized CeO2 surfaces with weak adsorption energies (-0.05 to 0.02 eV).
• Formation of diverse O2 species (physisorbed, O2a-, superoxide, peroxide) on reduced CeO2 surfaces with varying adsorption energies (-0.01 to -1.86 eV).
• Predicted vibrational frequencies align well with experimental data.
• Reduced CeO2 surfaces demonstrate energetically favorable pathways for O2 reduction.
• Low activation barriers for oxygen ion diffusion in ceria suggest high ionic conductivity.

Conclusions:

• Reduced CeO2 surfaces significantly enhance O2 adsorption and reduction compared to oxidized surfaces.
• The identified O2 species and their vibrational frequencies provide insights into surface reaction mechanisms.
• Ceria's high ionic conductivity at low temperatures makes it suitable for intermediate-temperature solid oxide fuel cell (IT-SOFC) electrolytes.