Nickel(II) hydroxide

Structure and physical properties

Ni(OH)2 typically forms pale green to bluish-green crystalline solids. Its structure is closely related to the brucite-type lattice, in which nickel atoms occupy octahedral sites coordinated by hydroxide ligands, producing stacked sheets. Two common forms are the more stable, simple brucite-like β form and a less stable, hydrated or anion-intercalated α form that can incorporate water and other anions between layers. On heating it decomposes to nickel oxide (NiO) and water.

Preparation and chemical behavior

Nickel(II) hydroxide is most often prepared by precipitating a nickel(II) salt solution with a base such as sodium hydroxide. Conditions of preparation—pH, temperature, and presence of other ions—affect whether α or β phases form. In alkaline electrochemistry Ni(OH)2 undergoes reversible oxidation to nickel oxyhydroxide (NiOOH), a redox transition exploited in rechargeable batteries; consult preparation notes and electrochemical behavior at this source.

Uses and applications

Ni(OH)2 is best known as the active material in the positive electrodes of nickel–cadmium and nickel–metal hydride rechargeable cells. Other applications include:

• precursor for nickel oxide catalysts and ceramic materials,
• component or intermediate in electrocatalysts for oxygen evolution and other reactions,
• pigments and coatings where nickel compounds provide color or corrosion resistance.

Its role in energy storage arises from the Ni(II)/Ni(III) redox couple and the material's ability to accept and release charge reversibly. Further background on material properties can be found here.

History, occurrence and distinctions

A natural mineral form of Ni(OH)2 exists as the rare mineral theophrastite. Industrial interest expanded with the development of rechargeable nickel-based batteries in the 20th century. Distinctive points include the layered sheet structure, the existence of α and β polymorphs, and the compound's conversion to NiOOH under charging conditions.

Safety and environmental considerations

Like many nickel compounds, Ni(OH)2 should be handled with care: nickel compounds can be skin sensitizers and may pose health risks on prolonged exposure. The material is sparingly soluble in water but can release nickel ions under acidic conditions; waste and exposure controls are used in industrial settings. For detailed safety data consult material safety resources and regulatory guidance linked at chemical hazards.