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Energy storage will be an essential part of a shift from fossil fuels to a low-emissions future. One key challenge is to store large amounts of energy in small volumes. For this purpose nickel, with its high discharge capacity (mAh/g) and good stability, is being considered as the positive active material for batteries like nickel-metal hydride systems.
Nickel hydroxide (Ni(OH)2) is a complex green crystalline material that is solid at room temperature, nonhygroscopic and soluble in acid and alkaline solutions. It has two oxidation states – +2 in the reduced form and +3 in the a-form of nickel hydroxide. The a-Ni(OH)2 phase has a higher theoretical discharge capacity than the b-Ni(OH)2 form.
When the a-Ni(OH)2 is converted to b-Ni(OH)2 in a battery, oxygen evolution reactions are initiated on a titanium anode, which leads to a decrease in cell voltage and a drop in the discharge capacity. This is due to a reduction of the number of nickel atoms that are exposed to water and oxygen molecules, thus limiting their electrochemical activity.
Several studies have been undertaken to improve the discharge capacity of a-Ni(OH)2 and b-Ni(OH)2 through various methods such as chemical precipitation, hydrothermal treatment, ageing, etc. However, a complete understanding of the structural disorders that exist in both b-Ni(OH)2 and a-Ni(OH)2 remains to be obtained. These structures play a major role in the electrochemical properties of nickel hydroxide. It is therefore essential to know the structure and morphology of these nickel hydroxides in order to optimise their performance in rechargeable batteries.