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Nickel(II) hydroxide
Chemical compound From Wikipedia, the free encyclopedia
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Nickel(II) hydroxide is the inorganic compound with the formula Ni(OH)2. 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.[6]
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Properties
Nickel(II) hydroxide has two well-characterized polymorphs, α and β. The α structure consists of Ni(OH)2 layers with intercalated anions or water.[7][8] The β form adopts a hexagonal close-packed structure of Ni2+ and OH− ions.[7][8] In the presence of water, the α polymorph typically recrystallizes to the β form.[7][9] In addition to the α and β polymorphs, several γ nickel hydroxides have been described, distinguished by crystal structures with much larger inter-sheet distances.[7]
The mineral form of Ni(OH)2, 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.[10] A nickel-magnesium variant of the mineral, (Ni,Mg)(OH)2 had been previously discovered at Hagdale on the island of Unst in Scotland.[11]
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Reactions
Nickel(II) hydroxide is frequently used in electrical car batteries.[8] Specifically, Ni(OH)2 readily oxidizes to nickel oxyhydroxide, NiOOH, in combination with a reduction reaction, often of a metal hydride (reaction 1 and 2).[12][13]
Reaction 1 Ni(OH)2 + OH− → NiO(OH) + H2O + e−
Reaction 2 M + H2O + e− → MH + OH−
Net Reaction (in H2O) Ni(OH)2 + M → NiOOH + MH
Of the two polymorphs, α-Ni(OH)2 has a higher theoretical capacity and thus is generally considered to be preferable in electrochemical applications. However, it transforms to β-Ni(OH)2 in alkaline solutions, leading to many investigations into the possibility of stabilized α-Ni(OH)2 electrodes for industrial applications.[9]
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Synthesis
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 Ni3O2(OH)4.[14]
Toxicity
The Ni2+ ion is a carcinogen when inhaled.
See also
References
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