Designing Ionic Highways: Autonomous Exploration and Strain Engineering of Novel Oxygen Conductors for Energy Applications

Climate change is a pressing issue impacting our planet, with global temperatures on the rise and showing no sign of slowing. The continued reliance on fossil fuels, particularly coal and oil, leads to ongoing increases in greenhouse gas emissions, which exacerbates climate change.
Although renewable energy sources offer alternatives, they face challenges such as intermittency and storage, fueling efforts to develop a more integrated electrical grid that can deliver power efficiently and manage byproducts for extra energy. Fuel cells, notably solid oxide fuel cells (SOFCs), provide a promising route by converting stored chemical energy into electricity without combustion, following the principle fuel in, power out.
Oxygen-ion mobility remains a key factor in SOFC performance. Improved ion conduction lowers operating temperatures and extends device lifespans. Oxygen vacancies serve as stepping stones for ion movement, and new vacancies can be introduced by doping with aliovalent cations or by inducing structural distortions in crystal frameworks to facilitate ion hopping. Notably, thin-film heterostructures composed of metallic oxide layers on specific substrates can exhibit unmatched structural mismatches, which can create groundbreaking properties, including heightened ionic conduction.
Recent research has revealed new oxide materials with superior ion mobilities; a prominent example is the hexagonal perovskite (HP) Ba₇Nb₄MoO₂₀, which features inherent vacancies in flexible tetrahedral complexes that reduce the energy barrier for oxygen-ion migration, pointing toward operation at lower temperatures and inviting exploration into the broader chemical landscape of HPs.
In this work, a high-throughput computational screening of 5,400 Ba₇Nb₄MoO₂₀-derived configurations, supported by Density Functional Theory (DFT) and Nudged Elastic Band (NEB) calculations, uncovered 39 candidates with low energy barriers for oxygen-ion migration. We also assessed the essential mechanisms of ion migration and the properties that contribute to effective OICs.
Additionally, we have delved into the migration mechanisms under distinct orientations of strain for the Ba₇Nb₄MoO₂₀ HP and found that compressive strain lowers the energy barriers.
In collaboration with experiments, we also evaluated other materials for their electromechanical properties, including the high electrostriction coefficient in oxygen-deficient Gadolinium-doped Ceria (CGO) and the piezoelectric response of hafnium-based oxides under uniaxial deformation.
Altogether, these findings illustrate the still largely unexplored potential of oxygen-ion conducting materials and pave the way for innovative designs in advanced solid-state ionic conductors and electromechanical devices.