(701f) Design of Non-Stoichiometric Mixed Metal Oxides Toward the Advancement of Intermediate Temperature Solid Oxide Fuel Cells

Oxygen electrocatalysis plays an important role in the efficiency of solid oxide fuel cells (SOFCs), especially at intermediate temperatures. The sluggish oxygen reduction reaction (ORR) kinetics at the SOFC cathodes induce large overpotential losses in these cells, requiring operating temperatures higher than 700 °C. First-series Ruddlesden-Popper (R-P) oxides - with a formula of A₂BO₄ - have emerged as promising electrocatalysts for oxygen electrocatalysis at intermediate temperatures, due to their superior mixed ionic and electronic conductivities properties.^1-2 However, the optimization of their performance has been hindered by the limited understanding of the factors that govern their catalytic activity.

In this work, we will discuss our efforts toward obtaining structure-performance relations that can guide the development of optimal R-P oxides for oxygen reduction at intermediate temperature SOFCs. The activity of nanostructured mixed metal oxides with varying surface morphology and composition (i.e. La₂Ni_1-xM_xO_4+δ, with M= Fe, Co and Cu and 0 ≤ x≤ 0.25) is systematically evaluated using a combined theoretical and experimental approach.^1-2-3 The kinetics of the electrochemical oxygen reduction reaction (ORR) on nanostructured R-P oxides are investigated by means of electrochemical impedance spectroscopy. Two main electrochemical processes governing the polarization resistances during ORR have been identified: the electron transfer/oxygen vacancy healing, and the oxygen ion transfer through the electrocatalyst/electrolyte interface. We find that the nanostructure and composition of R-P oxides significantly effects these processes. Furthermore, we show that the incorporation of optimized nanostructured R-P oxides as SOFC cathode electrocatalysts leads to significant improvement in the cell performance. These findings provide important insights into tuning complex mixed ionic-electronic oxides for enhancing oxygen reduction kinetics in intermediate temperature ceramic–based fuel cells.

References

1. Ma, X. F.; Carneiro, J. S. A.; Gu, X. K.; Qin, H.; Xin, H. L.; Sun, K.; Nikolla, E., Engineering Complex, Layered Metal Oxides: High-Performance Nickelate Oxide Nanostructures for Oxygen Exchange and Reduction. Acs Catal 2015, 5 (7), 4013-4019.
2. Carneiro, J. S. A.; Brocca, R. A.; Lucena, M. L. R. S.; Nikolla, E., Optimizing cathode materials for intermediate-temperature solid oxide fuel cells (SOFCs): Oxygen reduction on nanostructured lanthanum nickelate oxides. Appl Catal B-Environ 2017, 200, 106-113.
3. Gu, X. K.; Nikolla, E., Design of Ruddlesden-Popper Oxides with Optimal Surface Oxygen Exchange Properties for Oxygen Reduction and Evolution. Acs Catal 2017, 7 (9), 5912-5920.