(353f) Design of Mixed Metal Oxides for Efficient Intermediate Temperature Oxygen Electrocatalysis

Oxygen electrocatalysis plays an important role in the efficiency of energy conversion and storage technologies. Among them, solid oxide fuel cells (SOFCs) stand out as promising elevated temperature systems by providing improved fuel to energy conversion efficiencies.¹ However, their operation at an intermediate temperature regime (< 700 °C, IT-SOFCs) is important from a materials stability and economic perspective. To achieve this, improved oxygen reduction (ORR) kinetics at the cathode are required. Therefore, the development of highly efficient ORR electrocatalysts is key for achieving high electrochemical rates using IT-SOFCs.

In this work, we demonstrate promising performance for first-series Ruddlesden-Popper (R-P) oxides - with a formula of A₂BO₄ - as electrocatalysts for oxygen electrocatalysis.^2-5 We further provide a clear understanding of the factors governing their structure−activity relations for surface oxygen exchange, fostering the optimization of their performance. We discuss our efforts toward merging theoretical designed principles and experimental synthetical approaches in developing highly active electrocatalysts for intermediate temperature oxygen electrocatalysis. We demonstrate this via the use of a reverse microemulsion method which allows for nanoengineering of the oxide surface, maximizing the B-site metal sites (responsible for the catalytic activity) exposed to the reactive species. Furthermore, we demonstrate our ability to fine-tune the composition of the B-site in order to regulate the electronic fingerprint of the electrocatalyst surface to achieve optimal electrocatalytic activity. The kinetics of the electrochemical ORR on nanostructured R-P oxides are investigated by means of electrochemical impedance spectroscopy. We identify and show the impact of the nanoengineered R-P oxides on the two main electrochemical processes governing the polarization resistances during ORR: the electron transfer/oxygen vacancy healing, and the oxygen ion transfer through the electrocatalyst/electrolyte interface. 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 enhanced oxygen reduction kinetics in intermediate temperature ceramic–based fuel cells.

References

1. Carneiro, J.; Nikolla, E., Nanoengineering of solid oxide electrochemical cell technologies: An outlook. Nano Res, 1-12.
2. 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.
3. 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.
4. Gu, X.-K.; Carneiro, J. S. A.; Samira, S.; Das, A.; Ariyasingha, N. M.; Nikolla, E., Efficient Oxygen Electrocatalysis by Nanostructured Mixed-Metal Oxides. J. Am. Chem. Soc 2018, 140 (26), 8128-8137.
5. 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.