Determination of the possibility of obtaining ballastless Ni(II)-Ni(III) layered double hydroxides as promising active substances for supercapacitors and electrocatalysis

This study is aimed to investigate the synthesis of Ni2⁺-Ni3⁺ LDH (layered double hydroxide) in the presence of sodium hypochlorite. Ni2⁺-Ni3⁺ LDH is a promising active material for use in supercapacitors and electrocatalysis. The syntheses were carried out at a temperature of 60°C by coprecipitation under conditions of high supersaturation and at constant pH values (8, 10, and 12). To obtain the guest metal cation Ni3⁺ from the initial Ni2⁺ during synthesis, sodium hypochlorite was introduced as an oxidizing agent into the alkaline sodium carbonate solution. As control samples, Ni-Al-carbonate LDH were synthesized using the same methods and conditions. The formation of Ni3⁺ hydroxo compounds during synthesis was visually confirmed by a color change. The crystal structure of the samples was investigated by X-ray diffraction analysis, and the total Ni and Ni3⁺ contents were determined by trilonometric and iodometric titration. The samples synthesized in the presence of hypochlorite exhibited a black color, confirming the successful formation of Ni3⁺ hydroxo compounds. All control samples corresponded to Ni-Al LDH. The nickel hydroxide sample synthesized by coprecipitation at high supersaturation was identified as β-Ni(OH)2, with a total Ni content of 59.5% and a Ni3⁺ fraction of 12.2%. The transformation of Ni2⁺ → Ni3⁺ occurred in the surface layer of the formed β-Ni(OH)2 particles because the oxidation rate was lower than the hydroxide formation rate. The samples synthesized by coprecipitation at constant pH exhibited a layered structure consisting of β-Ni(OH)2 and Ni2⁺–Ni3⁺ LDH with an α-type lattice, suggesting that the oxidation rate exceeded the hydroxide formation rate. Thus, the possibility of synthesizing Ni2⁺-Ni3⁺-carbonate LDH was experimentally demonstrated. Under the optimal conditions (pH 8), the proportion of the Ni-Ni LDH phase reached 55.9%. The synthesized Ni2⁺-Ni3⁺ LDH shows potential for application in supercapacitors and electrocatalysis, provided that its specific electrochemical characteristics are determined in further studies