Abstract
Improving the activity of catalysts for the oxygen evolution reaction (OER) requires a detailed understanding of the surface chemistry and structure to deduce structure-function relationships (descriptors) for fundamental insight. We chose epitaxial (100)-oriented La0.6Sr0.4Mn1−δO3 (LSMO) thin films as a model system with high electrochemical activity comparable to (110)-oriented IrO2 to investigate the effect of Mn off-stoichiometry on both catalytic activity and stability. Extensive structural characterization was performed by microscopic and spectroscopic methods before and after electrochemical characterization using rotating ring-disk studies. Stoichiometric LSMO had the highest activity, while both Mn deficiency and excess reduced the catalytic activity. Furthermore, all samples preserved the crystal structure up to the very surface. Mn excess improved the long-term activity, and we hypothesize that excess Mn stabilizes the surface chemistry during catalysis. Our data show that the defect chemistry should be considered when designing catalysts with enhanced activity and rugged stability.
Highlights
With the world facing limited supply of fossil fuels, the water splitting reaction poses an alternative and promising approach towards sustainable energy
The investigations on off-stoichiometric LSMO are connected by thorough structural characterization (Section 2.1), followed by electrochemical investigations via ring-disk electrode (RRDE)
In our previous work [9], we proposed that the changes in surface chemistry observed by XPS during the long-term stability measurements are triggered by the charge imbalance introduced during Sr2+ dissolution
Summary
With the world facing limited supply of fossil fuels, the water splitting reaction poses an alternative and promising approach towards sustainable energy. Promising candidates for OER catalysts are first-row transition metal oxides [5,6,7,8,9,10], as they consist of Earth-abundant elements, such as Mn, Fe, Co and Ni, which can show activities per catalyst area comparable to state-of-the-art catalysts, such as RuO2 and IrO2 [11,12,13,14] Among these catalysts are frequently perovskites with the ABO3 structure, which show a broad range of properties that arise from tuning the valence states of the B-site, B-O bonding length and angle by varying the A-site doping, thereby affecting the catalytic activity for the OER. The complex entanglement of structural and electronic properties represents a key challenge in the experimental deduction of descriptors and the complex dependency of both the mechanism and active site on structural and Catalysts 2017, 7, 139; doi:10.3390/catal7050139 www.mdpi.com/journal/catalysts
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