Nickel hydroxide is widely used as an active substance of power sources (secondary cells and supercapacitors). For application as the active substance of the power sources as directly Ni(OH)2 (alkaline secondary cells and supercapacitors), as NiO (Li secondary cells and supercapacitors). Well known, that cobalt addition increases the specific capacitance of nickel hydroxide as an active substance of alkaline secondary cells. Besides, mixed Ni-Co oxides are used as an effective cathodic substance of Li power sources and faradaic electrode of supercapacitors. Power sources parameters depend on properties of active material, but these properties base on synthesis parameters.
One of the methods for synthesis of the nickel hydroxide for supercapacitors is two-stage high-temperature synthesis with the formation of “pseudo-single” particles with fractal geometry. In the first stage, a concentrated solution of nickel perchlorate was added dropwise to a concentrated solution of NaOH and storage under vigorous stirring and 140ºС during 24 hours. As a result precipitate of sodium nickelate if formed. In the second stage, most of the mother liquor is extracted and the same water volume is added, hydrolysis is carried out under vigorous stirring and 170ºС during 24 hours for transformation the sodium nickelate to nickel hydroxide. The extracted mother liquor is added to the high volume of distilled water for cold hydrolysis for 24 hours.
For nickel-cobalt compound obtaining this synthesis has been modified. For this ain nickel perchlorate was added to the initial solution. Easy oxidation Co(OH)2 to CoOOH has been previously described in the articles. Therefore initial solution has ration Co:Ni=2:1 and we have a hypothesis about formation nickel cobaltate NiCo2O4.
The initial solution has been obtained according to the following scheme:
1) precipitation of base carbonate (with 30% access to stoichiometry);
2) filtration and rinsing from sulfate;
3) reaction base carbonate with concentrated chlorate acid (according to stoichiometry);
5) evaporation to required concentration (controlled by mass).
Using prepared initial solution, two-stage high-temperature synthesis has been carried out.
Synthesized substances have been characterized by PXRD, DTG, DSC, SEM, Electrochemical properties have been investigated by the following methods: 1) cycling voltammetry; 2) galvanostatic charge-discharge cycling. In both case samples mixed with graphite and PTFE and pasted on the nickel foam working electrode.Electrolyte – 6M KOH solution, reference electrode – saturated silver chloride, the counter electrode – Ni mesh. In the cycling voltammetry potential range (0-500 mV) was used. Galvanostatic charge-discharge cycling has been carried out under following current densities: 20, 40, 60, 80, 120 mA/cm2.
Synthesized samples (as cold as hot synthesis) are ultrafine powders with black color.
By voltammogram and charge-discharge cycling, the high electrochemical activity of obtained nickel cobaltate has been shown. Sample of hot hydrolysis has specific capacity 150-183 F/g, and under discharge current density increasing, specific capacities don’t decrease and slightly increase.
Figure 1 − Specific capacity (F/g) of NiCo2O4 sample (hot hydrolysis) via discharge current density under discharge to potential 0 V