Understanding Carbon Contamination in Proton Conducting Oxides

TL;DR

This study uses first-principles calculations to compare carbon stability in different proton-conducting oxides, revealing why cerates are more prone to carbon contamination and degradation than zirconates.

Contribution

It provides a detailed mechanistic understanding of carbon contamination in cerates versus zirconates, guiding improved material design for proton conductors.

Findings

Cerates require more carbon-poor environments to prevent carbonate formation.

Interstitial carbon has lower formation energies in cerates, leading to higher concentrations.

Cerates exhibit lower migration barriers for carbon interstitials, increasing degradation risk.

Abstract

Carbon contamination is a significant concern for proton-conducting oxides in the cerate and zirconate family, particularly for BaCeO$_3$. Here, we use first-principles calculations to evaluate carbon stability in SrCeO$_3$, BaCeO$_3$, SrZrO$_3$, and BaZrO$_3$. The cerates require more carbon-poor environments to prevent carbonate formation, though this requirement can be loosened through the use of more oxygen-poor growth conditions. Carbonate formation is not the only concern, however. We find that interstitial carbon has lower formation energies in the cerates relative to the zirconates, leading to higher carbon concentrations that compete with the desired oxygen vacancy formation. We also examine the mobility of carbon interstitials, finding that both migration barriers and binding energies to acceptors are lower in the cerates. As a result, the cerates are likely to degrade when exposed to carbon at operating temperatures. Our results show definitively why the cerates are less stable than the zirconates with respect to carbon and elucidate the mechanisms contributing to their instability, thereby helping to explain why alloying with zirconium will enhance their operational efficiency.