Nickel(II) hydroxide is the inorganic compound with the formula Ni(OH)<sub>2</sub>. It is a lime-green solid that dissolves with decomposition in ammonia and amines and is attacked by acids. It is electroactive, being converted to the Ni(III) oxy-hydroxide, leading to widespread applications in rechargeable batteries.
Nickel(II) hydroxide has two well-characterized polymorphs, ñ and ò. The ñ structure consists of Ni(OH)<sub>2</sub> layers with intercalated anions or water. The ò form adopts a hexagonal close-packed structure of Ni<sup>2+</sup> and OH<sup>âÂÂ</sup> ions. In the presence of water, the ñ polymorph typically recrystallizes to the ò form. In addition to the ñ and ò polymorphs, several ó nickel hydroxides have been described, distinguished by crystal structures with much larger inter-sheet distances.
The mineral form of Ni(OH)<sub>2</sub>, theophrastite, was first identified in the Vermion region of northern Greece, in 1980. It is found naturally as a translucent emerald-green crystal formed in thin sheets near the boundaries of idocrase or chlorite crystals. A nickel-magnesium variant of the mineral, had been previously discovered at Hagdale on the island of Unst in Scotland.
Nickel(II) hydroxide is frequently used in electrical car batteries. Specifically, Ni(OH)<sub>2</sub> readily oxidizes to nickel oxyhydroxide, NiOOH, in combination with a reduction reaction, often of a metal hydride (reaction 1 and 2).
Reaction 1
Reaction 2
Net Reaction (in H<sub>2</sub>O)
Of the two polymorphs, ñ-Ni(OH)<sub>2</sub> has a higher theoretical capacity and thus is generally considered to be preferable in electrochemical applications. However, it transforms to ò-Ni(OH)<sub>2</sub> in alkaline solutions, leading to many investigations into the possibility of stabilized ñ-Ni(OH)<sub>2</sub> electrodes for industrial applications.
The synthesis entails treating aqueous solutions of nickel(II) salts with potassium hydroxide. When the same reaction is conducted in the presence of bromine, the product is .
The Ni<sup>2+</sup> ion is a carcinogen when inhaled.