Hidden Hydroxides in KOH-Grown BaNiO₃ Crystals: A Potential Link to Their Catalytic Behavior

The hexagonal perovskite BaNiO3, prepared via nonceramic approaches, is known to act as a good catalyst for the oxygen evolution reaction (OER) in alkaline media. Here, we report our observation that BaNiO3 synthesized via KOH flux growth and high-pressure ceramic synthesis with high O-2 pressure have different magnetic properties. We show that this is because the KOH flux-grown crystals made in open air are actually a hydroxide-containing form of BaNiO3 that can be "dried" upon annealing in an O-2 flow. This work not only unveils a previously unknown aspect of the BaNiO3 OER catalyst and offers some insights into the underlying mechanism, but also suggests that hydroxide ions may be present in other hexagonal perovskite oxides prepared in wet conditions.