- Nickel(II) hydroxide (Ni(OH)₂) is an inorganic compound consisting of divalent nickel ions (Ni²⁺) and hydroxide ions (OH⁻).
- It typically appears as a fine, pale green powder or crystalline solid that is insoluble in water but readily dissolves in acids to form corresponding nickel salts. Ni(OH)₂ is an important material in electrochemistry, particularly as an active component in rechargeable battery systems. It exists in two main polymorphic forms: the α-form and the β-form, which differ in structure and stability, and these variations influence its electrochemical behavior.
- The β-form of Ni(OH)₂ adopts a brucite-type layered structure, where nickel ions are octahedrally coordinated by hydroxide ions in sheets that are held together by hydrogen bonding. This form is relatively stable in alkaline conditions but converts to the α-form under certain conditions, such as exposure to oxidizing agents or during battery cycling. The α-form includes intercalated anions or water molecules between the layers, giving it a higher capacity for ion exchange and oxidation, but it is less stable and tends to revert to the β-form over time.
- Nickel(II) hydroxide can be synthesized by the precipitation reaction of a soluble nickel salt (e.g., NiCl₂ or NiSO₄) with a strong base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). The resulting green precipitate is washed and dried for further use. Modifications in synthesis conditions, including temperature, pH, and additives, allow for tuning of particle size, crystallinity, and polymorphic form, which is particularly important for optimizing performance in electrochemical devices.
- The primary application of Ni(OH)₂ is in nickel-cadmium (Ni-Cd) and nickel-metal hydride (NiMH) rechargeable batteries, where it functions as the positive electrode material. During charging and discharging, it undergoes reversible redox cycling between Ni(OH)₂ and nickel oxyhydroxide (NiOOH). Its high specific capacity, stability in alkaline electrolyte, and relatively low cost make it a preferred choice in battery systems. It is also used in catalysts, electrochromic devices, and sometimes as a pigment or in ceramic formulations.
- Despite its usefulness, nickel(II) hydroxide poses health and environmental risks. Like other nickel compounds, it is classified as a potential carcinogen and can cause skin sensitization or respiratory irritation upon prolonged exposure. It is not highly mobile in soil due to low solubility, but disposal and handling are strictly regulated to prevent nickel contamination of ecosystems. Research into surface modification, nanostructuring, and doping of Ni(OH)₂ continues to enhance its functional properties while reducing toxicity and improving environmental compatibility.
