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Transition-Metal Distribution in Brownmillerite Ca2FeCoO5
Kei Nakayama1, Ryo Ishikawa1,2, Akihide Kuwabara3
1Institute of Engineering Innovation , University of Tokyo , Bunkyo , Tokyo 113-8656 , Japan.
Cobalt distribution in Ca2FeCoO5 brownmillerite catalysts impacts oxygen evolution reaction (OER) activity. Cobalt enrichment at tetrahedral sites in as-synthesized catalysts enhances OER performance.
Area of Science:
- Materials Science
- Catalysis
- Solid-state Chemistry
Background:
- Ca2Fe2-xCoxO5 exhibits excellent oxygen-evolution-reaction (OER) catalytic activity.
- Understanding Fe/Co site occupancy in brownmillerite structures is crucial for optimizing OER performance.
Purpose of the Study:
- To investigate Fe/Co distribution in Ca2FeCoO5 using advanced microscopy and theory.
- To correlate site occupancy with OER activity and thermal stability.
Main Methods:
- Atomic-resolution energy-dispersive X-ray spectroscopy in scanning transmission electron microscopy (STEM-EDX).
- Dynamical image simulations.
- Density functional theory (DFT) calculations.
Main Results:
- No long-range Fe/Co order observed within transition-metal layers.
- Cobalt slightly enriched at tetrahedral (T) sites in as-synthesized (1100 °C) and octahedral (O) sites in 800 °C annealed samples.
- As-synthesized catalysts with T-site Co enrichment show superior OER activity.
Conclusions:
- Co site preference is governed by ionic size and ligand field effects, with a crossover temperature between 800-1100 °C.
- Optimal annealing temperature for enhanced OER activity exceeds this crossover temperature.
- Tailoring Co distribution is key to developing highly active Ca2FeCoO5 OER catalysts.
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