  The hexagonal perovskite BaNiO3, prepared via non-ceramic 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 O2 pressure ceramic synthesis 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 O2 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.
