Enhanced bifunctional oxygen catalysis in strained LaNiO3 perovskites

Abstract: Strain is known to greatly influence low temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity essential for fuel cells and metal-air batteries. However, its catalytic impact on transition metal oxide thin films, such as perovskites, is not widely understood. Here, we epitaxially strain the conducting perovskite LaNiO3 to systematically determine its influence on both the oxygen reduction and oxygen evolution reaction. Uniquely, we found that compressive strain could significantly enhance both reactions, yielding a bifunctional catalyst that surpasses the performance of noble metals such as Pt. We attribute the improved bifunctionality to strain-induced splitting of the eg orbitals, which can customize orbital asymmetry at the surface. Analogous to strain-induced shifts in the d-band center of noble metals relative to Fermi level, such splitting can dramatically affect catalytic activity in this perovskite and other potentially more active oxides.

• The paper shows that compressive strain in LaNiO3 reduces ORR and OER overpotentials by at least 50 mV compared to unstrained films.
• The study integrates experiments and density functional theory to reveal that strain-induced splitting of e₉ orbitals drives enhanced catalytic bifunctionality.
• These findings position strained LaNiO3 perovskites as a promising, cost-effective alternative to noble metal catalysts for energy conversion applications.

Analysis of Enhanced Bifunctional Oxygen Catalysis in Strained LaNiO₃ Perovskites

The paper presents a detailed investigation into the catalytic properties of strained LaNiO₃ (LNO) thin films and their potential application in oxygen reduction (ORR) and oxygen evolution reactions (OER). This study addresses a significant gap in the understanding of strain effects on transition metal oxide (TMO) films, particularly LaNiO₃ perovskites, by examining the impact of epitaxial strain on their bifunctionality in catalytic processes.

Key Findings

The authors demonstrate that compressive strain can significantly enhance the bifunctional ORR and OER activities of LaNiO₃, surpassing the performance of conventional noble metal catalysts like platinum (Pt). The improvement in catalytic properties is attributed to strain-induced splitting of the e₉ orbitals, resulting in enhanced orbital asymmetry at the surface of the perovskite.

Several strong empirical results support these findings:

• The paper reports an overpotential reduction for both ORR and OER by at least 50 mV in strained films compared to their unstrained counterparts.
• The bifunctional potential, quantified by the overpotential required to reach a specific current density, showed more than one order of magnitude enhancement under compressive strain.
• Compared to Pt, compressive strain in LNO resulted in a bifunctionality that exceeds that of the leading noble metal catalysts for oxygen reactions.

Theoretical simulations using density functional theory further illustrate the electronic structure changes induced by strain, corroborating the experimental findings. These changes are pivotal in reducing the energy barrier for oxygen reactions, thus leading to improved catalytic efficiency.

Theoretical Implications

The research offers significant insights into the mechanisms by which strain influences surface electronic properties and catalysis in perovskites. By detailing the effects on d orbital polarization and occupancy, the study extends the understanding of how orbital physics can be harnessed to tune catalytic activity. This aligns with and expands upon concepts traditionally applied to noble metal catalysts.

Practical Implications and Future Directions

The demonstrated enhancement of bifunctional catalytic activities offers compelling evidence for using strained perovskites in energy conversion technologies, such as metal-air batteries and regenerative fuel cells. Capitalizing on TMO perovskites' inherent stability and lower cost compared to noble metals, the findings could significantly impact the development of more sustainable and economical catalyst alternatives.

Furthermore, the methodologies and insights from this study could be adapted to explore other perovskites and similar TMO-based catalysts, for exploiting strain-induced enhancements. Future research could investigate the scalability of this approach and evaluate its commercial feasibility in various electrochemical devices.

Conclusion

The paper successfully elucidates the critical role of strain in modulating the catalytic activity of LaNiO₃ perovskites, providing a robust framework for future investigations into strain engineering of TMO catalysts. The integration of experimental and computational analyses in this research exemplifies a comprehensive approach to understanding and optimizing catalyst performance at the atomic level, paving the way for innovations in the field of energy conversion and storage technologies.