NiOOH

NiOOH, or nickel oxyhydroxide, is a compound containing nickel in the +3 oxidation state within a layered oxyhydroxide framework. It exists mainly in two polymorphs, alpha-NiOOH and beta-NiOOH, which differ in water content and stacking. NiOOH is typically formed by oxidation of nickel hydroxide Ni(OH)2, either chemically or electrochemically, and serves as the active phase at the positive electrode of many alkaline nickel-based rechargeable batteries, including nickel–cadmium and nickel–metal hydride cells.

During charging in an alkaline electrolyte, Ni(OH)2 is oxidized to NiOOH according to Ni(OH)2 + OH− → NiOOH + H2O + e−. During discharge, the reverse reaction occurs. The NiOOH/Ni(OH)2 redox couple governs the electrode potential and the capacity in these systems, with the nickel center alternating between +2 and +3 oxidation states.

Beyond battery applications, NiOOH is an active catalyst for the oxygen evolution reaction in alkaline media and has been studied in various electrocatalytic configurations. Its redox flexibility and layered structure support surface hydration and proton-coupled electron transfer, which influence catalytic activity and stability.

Stability and phase composition depend on factors such as pH, temperature, and the presence of dopants or intercalants. In practical devices, NiOOH is formed in situ during operation and can transform to other nickel oxide or hydroxide phases with cycling. Doping with elements such as cobalt or iron can modify the electronic structure, kinetics, and overall performance of NiOOH-containing systems.