Multiple New Families of Fast Oxygen Interstitial Conductor Discovered by High-Throughput Computational Screening
Authors: Meng Jun , Sheikh Md Sariful , Jacobs Ryan , Liu Jian , Nachlas William O. , Li Xiangguo , Morgan Dane 
Published: December 2024

Discovery and design of highly efficient oxygen-active materials that react with, absorb, and transport oxygen is essential for the development of improved fuel cells, electrolyzers and related applications.Currently, the predominant families of oxygen-active materials used in commercial fuel cell and electrolyzer devices are perovskites and fluorites, which diffuse oxygen via a vacancy mechanism and only have adequate oxygen diffusivity and surface exchange at high temperatures (e.g., ~800 ˚C), impeding their use at lower temperatures desirable for more cost-effective and long-lasting device operation. The development of alternative electrolyte/electrode materials with high oxygen ionic conductivity at low temperatures (e.g., room temperature to 400 °C) is of great interest. Interstitial oxygen ion conductors typically have significantlylower migration barriers than even state-of-the-art vacancy-mediated oxygen conductors, which can provide orders of magnitude kinetic enhancement at low temperatures, but have received far less attention than vacancy-mediated conductors. In our recent work, we combined physically-motivated structure and property descriptors, ab initio simulations, and experiments to demonstrate an approach to discover new fast interstitial oxygen conductors. We first conducted a comprehensive high-throughput screening of the 34k oxide materials from the Materials Project database utilizing a set of screening criteria including the geometric free space, thermodynamic stability, synthesizability, redox-active elements, and presence of short diffusion pathways. This screening winnowed the field of 34k oxides down to just 345 oxides, an approximate 99% reduction in the search space considered. For this reduced set of promising materials, ab initio simulations were performed to calculate the interstitial oxygen formation energy and migration energy. We identified multiple new promising structural classes, including but not limited to double molybdates A2TM2(MoO4)3 (A=alkali metal, TM=transition metal), perrierite/chevkinite RE4TM5Si4O22 (RE=rare earth, TM=transition metal), and germinates RE1TM2Ge4O12 (RE= rare earth, TM=transition metal), and have demonstrated the exceptional performance of one example material, La4Mn5Si4O22+
𝜹 (LMS) with experimental validation. The discovery of a new high-performing interstitial oxygen conductor from the experimental study of just one system, as well as the prediction of multiple families of promising interstitial ion conductors, suggests that many more materials exhibiting fast interstitial oxygen kinetics remain to be found.

In this work, we performed detailed studies on the oxygen interstitial diffusion behavior in ZrMn2Ge4O12 (ZMG) and CeMn2Ge4O12 (CMG), as two representive compounds belong to germanate family. Oxygen ion mobility in ZMG and CMG (in the presence of 2% oxygen interstitials) is examined through ab initio molecular dynamics (AIMD) simulations and the associated migration barrier is studied by Climbing Image Nudged Elastic Band (CI-NEB) method. Our results reveal that the oxygen interstitials diffuse through an interstitial-mediated (interstitialcy) mechanism, with the energy barriers of approximately 0.33 eV for ZMG and 0.54 eV for CMG. To validate the fast oxygen conduction, we synthesized ZMGthrough the solid-state reaction and measured both the experimental total and ionic conductivities. Our experimental activation energy barrier aligns closely with the computational prediction, but the measured conductivities were lower. Further analyses of the microstructure, secondary phases, and impurity phases in the synthesized ZMG are essential to fully understand and leverage the fast oxygen diffusion capabilities in ZMG as indicated by our ab initio studies.