EAGER: Establish a New Paradigm in SOFC/SOEC Surface Segregation Modeling

This EArly-concept Grant for Exploratory Research (EAGER) project conceives and studies a novel computational approach to predicting the surface composition of solid oxide electrochemical devices under long-term use in energy related chemical applications. Solid oxide fuel cells (SOFCs), in particular, are amongst the most energy-efficient devices available for generating electrical energy.

Both SOFC and related solid oxide electrolysis cell (SOEC) technology are positioned to play an important role in the transition to net-zero carbon emissions. However, the devices operate at high temperature, which over time in service can lead to degradation in structural stability and performance. The project will link theoretical and experimental approaches to better understand the degradation mechanisms of solid oxide exchange catalysts, and use the resultant insights to predict more stable, and better performing materials compositions than currently available.

The project will support computational analysis to develop refined predictive models of oxide dopant surface-segregation of strontium (Sr) in ABO3 perovskites under electrical conditions characteristic of Solid Oxide Fuel Cell (SOFC), Solid Oxide Electrolysis Cell (SOEC), and reversible Solid Oxide Cell (rSOC) devices. The study is motivated by the fact that past studies have primarily relied on the size and/or charge mismatch between a dopant ion, and the host ion it is replacing, to capture the driving force for dopant segregation out of bulk perovskite lattice structures.

However, such predictions are in direct opposition to recently published experimental observations showing that - while there is significant Sr surface segregation in La0.6Sr0.4Fe0.2Co0.8O3-x (LSCF) and La0.6Sr0.4Fe0.8Co0.2O3-x during 1000 hours of 650-700oC open-circuit aging in air - there is little to no Sr surface segregation in Sm0.5Sr0.5CoO3-x (SSC) exposed to identical testing conditions. Those observations have triggered follow-on collaborative work by the investigators indicating, by both experimental and theoretical (i.e., Surface Gibbs Free Energy (SGFE)) analyses, that this is because LSCF is largely Sr-terminated under SOC operating conditions, whereas the SSC surface remains largely Co-terminated.

However, SGFE phase diagrams are needed for various members of the (La,Sr,Sm)(Fe,Co)O3-x solid solution family, under an assortment of likely SOFC/SO polarizations, to refine surface segregation predictions and extend them to relevant SOFC/SOE/rSOCs operating conditions. To that end, the investigators will work hand-in-hand to 1) produce DFT-computed SGFE diagrams on a range of (La,Sm,Sr)(Fe,Co)O3-x compositions as a function of temperature (𝑇), oxygen partial pressure (𝑝𝑂2 𝐺𝑎𝑠), and overpotential (𝜂), and 2) compare the predicted results to experimental characterization of the surface structure and oxygen exchange performance obtained at various 𝑇, 𝑝𝑂2 𝐺𝑎𝑠 and 𝜂 conditions.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.