Higher-Valent Nickel Oxides with Improved Oxygen Evolution Activity and Stability in Alkaline Media Prepared by High-Temperature Treatment of Ni(OH)2

Nickel(II) hydroxide is a well-known material for the oxygen evolution reaction (OER) in alkaline media, particularly when iron is incorporated into its lattice. Moderate heat treatment of nickel(II) hydroxide (<= 700 degrees C) leads to the formation of nickel(II) oxide (nano)particles, which exhibit reduced OER activity the higher the heat treatment temperature was. In this work, we report that heat treatment of nickel(II) hydroxide in air at even higher temperatures (60 min at 900 degrees C) results in an oxide material with high OER activity superior to that of the nickel(II) hydroxide. Similarly, the stability of the nickel(II) oxide under electrochemical conditions is increased compared to nickel(II) hydroxide. Electrochemical in situ Raman measurements show the formation of surface nickel oxy-hydroxides (NiOOH) at positive potentials and are significantly affected by the initial heat treatment. From XPS, Raman, and XRD results, it is concluded that a Ni3+-enriched phase, possibly a higher-valent mixed nickel oxide, is present at the surface of the nickel(II) oxide sample treated at 900 degrees C resulting in an increased OER activity compared to NiOOH. This basic understanding of high-temperature-treated nickel oxide may contribute to resolving the present stability issues of OER electrocatalysts and may help to leverage alkaline electrolysis as important key technology for a renewable energy supply.